Severe Weather & Northwest Flow

Warm and dry conditions returned to the Front Range Tuesday,  following an active period of severe weather last week and weekend. Several tornadoes touched down over eastern Colorado, along with numerous reports of large hail and heavy rainfall. 

Photo courtesy of 9News

This photo shows a UPS truck damaged by a tornado near Byers last Friday. The driver of the truck was reportedly taken to an area hospital, but luckily did not sustain major injuries. 

Severe weather can strike Colorado’s Front Range any time during the spring, summer, and fall periods. According to NOAA’s Storm Prediction Center (SPC), Denver’s severe weather season spans May 3rd-September 5th. Severe weather occurrences typically peak during the second-half of May through the first-half of June. 

Although thunderstorms are a relatively-small, short-lived, and local phenomenon, large-scale weather patterns certainly contribute to a higher likelihood of severe storm occurrence. Last week/weekend’s storms were largely the result of high pressure positioned W of Colorado, and an upper-level disturbance arriving under northwest flow.

The synoptic pattern depicted above can be a classic setup for severe weather outbreaks over the northern and central plains. So, what is it about northwest flow that stirs the atmosphere so convincingly? 

Instability

Instability is at the core of any discussion concerning thunderstorms, severe weather, or even cloud formation. Very simply, stability is the resistance of an air parcel to rise vertically into the atmosphere. Under favorable conditions, vertically-rising air expands, cools, and condenses into clouds, thunderstorms, and eventually precipitation. However, stability can vary significantly in the atmosphere. 

We know that warm air rises.

During the warm season, the air at the surface is typically warmer than the air above it. This is why we see more surface-based convection and thunderstorms this time of year. Warmer temperatures also yield higher moisture levels, which further contributes to storm development. The vertical profile of the atmosphere (temperature and moisture) dictates instability on any given day. Sometimes, there is resistance to rising air which can suppress storm formation. A good example of this would be a layer of warm air aloft, or a temperature inversion at the surface or mid-levels. 

Similarly, unusually-cold air above the surface can increase instability, leading to faster ascent, expansion, and condensation. 

Colder air aloft typically accompanies atmospheric disturbances – like centers of low pressure, troughs, and shortwave troughs. These upper-air disturbances are an effort to equalize differential heating from the equator to the poles. Some disturbances are stronger and colder than others. Disturbances arriving from the north or northwest typically feature colder air aloft, which can contribute to higher instability at the surface. 

Image result for unstable atmosphere

Figure courtesy of North Carolina Climate Office, NCSU

In the case of last weekend, an upper-level trough dropped out of the Northern Rockies into Colorado, which helped destabilize the vertical profile of the atmosphere. Furthermore, colder, sub-freezing air closer to the surface also helped contribute to hail growth in the thunderstorms that did form. 

Wind Shear

The second ingredient accompanying northwest flow is enhanced vertical wind shear. Vertical wind shear is either defined by a turning in wind direction with height (directional shear), or a change in wind speed with height (speed shear). Both are important considerations for severe thunderstorm development.

Directional wind shear can be common with upper-level disturbances, especially with moist SE surface winds in place. With the arrival of the disturbance from the northwest, the result is a clockwise-turning of wind direction with height – known as “veering.” In contrast, counterclockwise-turning of wind with height is known as “backing.”

Figure courtesy of Learn Weather

Directional shear helps thunderstorm cells develop a rotating updraft, which is the first stage in formation of a mesocyclone and eventual funnel cloud or tornado. Severe thunderstorms can develop in the absence of directional shear, but true tornadoes are not possible without this detail. 

Speed shear is of equal importance for severe thunderstorm formation. Increasing wind speed with height can produce a tilted updraft. This detail effectively keeps different parts of the thunderstorm separate  – e.g. the updraft vs. the downdraft. This allows for stronger, longer-lived, inflow-dominant thunderstorms that can mature to supercell status. 

Image result for directional wind shear

Image courtesy of the National Weather Service 

Inflow-dominant thunderstorms with strong, tilted updrafts will produce the largest hail. These storms also have the best chance of persisting for long periods of time and impacting large areas. 

In the case of last week, vertical speed shear was strong over Colorado due to a passing upper-level wind maximum (jet streak). The result was long-lived, severe thunderstorms over a wide swath of the northern and central plains. 

Enhanced instability and wind shear can push garden-variety storm cells into severe, supercellular status. The result can be impressive, destructive, and downright scary weather phenomenon.

Severe weather can result from a variety of large-scale weather patterns, and we didn’t discuss other required factors for strong thunderstorm formation – e.g. lifting/trigger mechanisms and moisture. However, northwest flow can simultaneously put several favorable-ingredients on the table, quickly increasing the threat for high-impact severe weather along the Front Range.  

Monsoon pattern brings heavy rain, flash flooding to Arizona and Utah

In our last blog post, we discussed the start of the monsoon season across the Southwest U.S.  Over the past week, this pattern has brought a significant increase in moisture to the canyon regions of Arizona and Utah, and as a result significant rainfall to many areas.  Much of the drought-stricken region has experienced above-average rainfall over the past 7 days, as evidenced in the map below.  Farther north into the Wasatch of Utah, and farther east into Colorado, rainfall has been less impressive.

 

 

While moisture in general is welcome news to the Southwest right now, as is often the case with monsoon season rainfall, too much rain in a short amount of time also causes problems.  Flash flooding has become a common occurrence with thunderstorms in recent days, and on Thursday July 12, a major flash flood occurred at a popular backcountry destination – Havasupai Falls in Grand Canyon National Park.  Backpackers who had hiked down to the popular falls noticed the turquoise waters began to turn muddy, and shortly thereafter, water levels quickly rose.  The many hikers in the vicinity were forced to quickly scramble up the hillsides to escape the rising torrent.  A total of 200 people ended up being rescued by helicopter from this area, but fortunately no injuries or fatalities occurred.

In Utah, a recent burn scar from the Black Mountain Fire that occurred earlier this summer received heavy rainfall and significant runoff and flooding late this past week.  Burn scars are highly susceptible to flash flooding and mudslides since the recently burnt soil can’t absorb much water.  Portions of Zion National Park and surrounding areas also received heavy rainfall and flooding that forced road and trail closures and home evacuations.  The heavy rains extended farther west into Nevada as well, with portions of the Las Vegas metro area receiving heavy rain and flash flooding.

Monsoon season getting started across the Southwest

The North American monsoon is getting underway across the Four Corners region as upper level easterly and southerly winds transport moisture Mexico northward into the southwestern U.S.  This signals a shift toward a period in which afternoon thunderstorms become very common across the Four Corners states, along with an increased potential for heavy rainfall with thunderstorms.  Of course the monsoon ebbs and flows depending on the location of the subtropical High, and is influenced by a variety of moisture sources such as tropical activity in the eastern Pacific.

The start of this year’s monsoon is looking quite active, especially across Arizona and Utah.  We’ve already seen an influx of moisture this weekend into this region.  One “unofficial” way the monsoon is measured is when the average dewpoint in Tucson, Arizona is 54 degrees or higher for 3 consecutive days.  We are almost there right now.  Over the past three days, the dewpoint has averaged 51, 52, and 58.  Two more days of dewpoints above 54, and this loose criteria for the start of the monsoon will be met.  The image below was produced by NWS Tucson.

 

 

High pressure centered over Western Colorado this week is resulting in anti-cyclonic (clockwise) flow of moisture around the periphery of the High that will favor moisture in Utah and western Arizona the most early this week.  Nevada and the mountains of California will see enough of this moisture influx for some thunderstorm activity as well, as will the Wind River Mountains in .  The image below (courtesy of weathermodels.com) is the ECMWF Model projected precipitable water anomaly for Monday – a good depiction of the early week pattern, with significant moisture available to fuel thunderstorms over Arizona and Utah.  This is the type of pattern that could easily result in flash flooding in the canyon regions such as Zion, Grand Canyon, etc.

 

 

Later this week, high pressure will begin to shift farther east into the Central Plains.  This will allow for monsoonal moisture to shift farther east.  While Arizona/Utah will continue to see good moisture, we should start to see an uptick in thunderstorm activity across Colorado over the second half of the week, including the Front Range on Thursday and Friday.  Check out the moisture anomaly projected by the ECMWF model on Thursday over Colorado, compared to the much drier values from earlier in the week. 

 

 

It looks like a generally active pattern will continue through next weekend, with some models projecting healthy amounts of rainfall across portions of Colorado, the Southern/Central Rockies, and Desert Southwest through next weekend.  After that, longer range projections are more difficult.  There are some indications a trough of low pressure could develop across the West, which could act to either scour out the moisture for a period of time, or enhance southwesterly flow of moisture into the region.  Too early to tell how that will play out, but at any rate, the outlooks is beginning to turn wetter across the Southwest and southern half of the Rockies over the next week.  Welcome news for this drought and fire stricken region!

Potential for Heavy Rainfall Across the Front Range of Colorado Today

While drought conditions and wildfires have been the big story across Colorado early this summer, today’s outlook is for widespread thunderstorms with heavy rainfall potential across the eastern mountain ranges and adjacent foothills and plains of Colorado.  This wet pattern will only be temporary as conditions will dry out again over the weekend, but as the main ridge of high pressure shifts from the Southeast U.S. to the Southwest U.S., southeasterly low level winds around the center of high pressure are transporting significant amounts of moisture into Eastern Colorado.

The image below (courtesy of weathermodels.com) is from the European (ECMWF) Model, showing the projected percent of average precipitable water anomaly for this afternoon.  Precipitable water is essentially a measure of total moisture in the atmosphere from the surface to the upper levels, and is a good general indicator of heavy rainfall potential with thunderstorms during the summertime.  For this afternoon, we will see well above average precipitable water values – as much as 150% of average across Eastern Colorado! 

 

 

In addition, weak upper level winds are going to result in slow storm motions with thunderstorms this afternoon.  With significant moisture in place and the potential for strong, slow moving thunderstorms, there is going to be an elevated threat for heavy rainfall and flash flooding across the Front Range today.  Recent fires across portions of the Southern Front Range will result in elevated concerns for flash flooding as well, since recently burned areas can absorb very little moisture in a short period of time before excessive runoff occurs.  It’s the caveat often experienced in Colorado and the Western U.S. when moisture is needed during periods of drought and fire activity, but too much too quickly can also cause big problems.

Our brief period of high moisture and heavy rainfall potential will come to an end this weekend with a return to drier weather and hot temperatures.  The North American Monsoon is expected to get going this weekend and early next week, but initially the deepest monsoon moisture will be located farther west across Arizona and Utah with less moisture over Colorado.  Portions of western Colorado should start to see a gradual increase in moisture heading into next week, but it may take a bit longer for a more favorable and consistent fetch of monsoonal moisture to take hold across the Front Range.

Fourth of July Weather Outlook for Colorado

The first few days of July have started out much the same way June ended – with hot and dry conditions, and now smoky skies across much of the state due to numerous wildfires.  Heading into the Fourth of July holiday, fire danger is very high state-wide, so it is prudent to be cautious with fireworks and campfires and to obey fire restrictions in place.  Despite the dry start to the week, changes are on the way for the Fourth, with potentially active weather developing across the Front Range and Eastern Colorado.  

A trough of low pressure swinging across the Northern Plains will send a backdoor cold front into Northeast Colorado during the day Wednesday.  At the same time, a subtropical dome of high pressure over the Southeastern U.S. will be retreating westward, allowing for a deeper easterly flow to advect low level moisture into Eastern Colorado behind this cold front.  Dewpoints in the 30s on Tuesday will rise into the mid 50s behind the front on Wednesday – plenty high enough to result in heavy rainfall potential with thunderstorms.  The image below (courtesy of the College of DuPage) shows the projected moisture increase behind the cold front on Wednesday afternoon.

 

 

The increase in low level moisture along with cooler air aloft will also result in a significant increase in atmospheric instability, setting the stage for greater thunderstorm potential across Northeast Colorado.  The biggest question marks will have to with how early in the afternoon the front arrives, and also the potential for a stable layer (known as a “cap”) to develop behind the front.  These stable layers can often inhibit or at least delay thunderstorm development until the cap weakens to the point at which storms can quickly develop, or another outside trigger (such as outflow from distant thunderstorms or terrain forcing) allows the cap to break.

Areas of elevated terrain will have the best chances of seeing thunderstorms on Wednesday afternoon, such as along/near the Front Range foothills up to the Continental Divide, as well as along the Palmer Divide and around the Pikes Peak region.  However, the Denver, Boulder, Colorado Springs, and Ft. Collins areas will all stand a good chance of seeing a round of thunderstorms Wednesday afternoon or evening, with the potential for a strong thunderstorm or two as well.  Any rainfall will certainly be welcome, but you’ll want to keep your guard up if you have outdoor plans for the Fourth of July, especially with the “prime time” for thunderstorms likely to occur between about 4pm-sunset when most of the barbeques and festivites are happening, and there will be some potential for later evening storms as well.

 

Fourth of July Outlook for Western Colorado and Surrounding States

Across the Southern and Central Rocky Mountain Region, the Front Range really will be the prime spot for moisture and thunderstorms.  Along the Continental Divide, there will be a good chance of thunderstorms as well, so keep that in mind if you plan to go hiking or summit a 14er on Independence Day, and get below treeline before the afternoon.

Farther west, the western slope of Colorado including some of the western mountain ranges (such as the San Juans), thunderstorm chances will be pretty minimal.  However, wildfires and smoke will continue to be an issue with very dry conditions in place.  Beyond Colorado, most of Utah and Arizona are likely to stay dry as well with similarly high fire danger.  Up north in Wyoming and Montana, thunderstorm risk is looking minimal as well, and fortunately these areas are in good shape in terms of moisture with limited fire danger to worry about.

Hot, dry weather continues to fuel wildfires in Colorado and Utah

Following a drier than average winter and spring across the Southern Rockies, it comes as little surprise that fire season is off to an active start. Several large fires have been burning across Colorado and Utah over the past month, with a more noticeable uptick occurring over the last week of June as hot and dry conditions prevailed.  The first week of July is starting out similar to the way June ended, with high fire danger and challenging fire behavior due to low relative humidity, dry fuels, hot temperatures, breezy conditions, and dry thunderstorms.  Below is an image produced by the Rocky Mountain Area Coordination Center, outlining the fire concerns for Monday July 2nd across the Central Rockies and High Plains.

 

 

Today’s conditions will be challenging both for ongoing fires and for the potential for new fires to be ignited.  Utah is not included in the map above, but fire danger remains elevated there as well, with most of the state under a Red Flag Warning today.  Below is a quick summary of some of the larger wildfires currently burning in Colorado, Utah, and Wyoming.

 

416 Fire (San Juan Mountains, north of Durango, CO)

The 416 Fire started on June 1, and has been burning for a month now and is only 37% contained. The cause of the fire remains unknown. The fire is located about 13 miles north of Durango and has burned 49,301 acres so far. The popular Durango-Silverton Scenic Railroad was forced to close due to the fire earlier in June, but has since opened back up with limited operations. Much of the San Juan Mountains have been impacted significantly by smoke from this fire.

 

Spring Creek Fire (Sangre de Christo area, near Ft. Garland, CO)

The Spring Creek Fire started on June 27, and has rapidly grown to 56,820 acres in less than a week with only 5% containment. This is currently the largest fire burning in Colorado. Unfortunately, this fire was human-caused and a suspect has been arrested and charged with arson. Extreme fire behavior in recent days has fueled rapid growth, with hot, dry, unstable, and windy conditions present. Gusty and erratic outflow winds from “dry” thunderstorms have impacted the area recently, and continue to be a concern during the first few days of July.

 

Badger Creek Fire (Wyoming/Colorado border in the Medicine Bow Mountains)

The Badger Creek Fire is located near the Colorado/Wyoming border in the Medicine Bow Mountains, and currently stands at 21,190 acres with all of this remaining on the Wyoming side of the border at this time. The fire started on June 10, but is now 80% contained. The cause of the fire remains unknown. Periodic hot, dry, and breezy conditions have continued to result in challenging fire behavior at times.

 

Trail Mountain Fire (SW of Price, UT)

The Trail Mountain Fire started on June 6, as prescribed burn that escaped its perimeter and ran out of control due to high winds.  The fire has grown to 17,811 acres and is now 85% contained.  However, hot, dry, and windy conditions across Utah this week will not help firefighters.

 

West Valley Fire (North of St. George, UT)

The West Valley Fire started on June 27 as a result of an unattended campfire.  The fire has quickly grown to 10,510 acres and is only 5% contained.  Hot and dry conditions over the upcoming week along with occasional gusty winds will result in challenging fire behavior.  There may be some hope on the horizon as longer range models project an increase in monsoonal moisture over Southern Utah during the second week of July.

 

North American Monsoon Outlook

With widespread drought conditions and place along with a significant amount of wildfire in the Southern Rockies, the seasonal North American Monsoon represents our best hope of seeing more meaningful moisture, as well as improving fire danger.  Long range models have consistently been projecting a more active than normal monsoon this summer.  However, the onset of a monsoonal is not happening quickly, and it may still take a while to really get going for some areas.

The first week of July is starting out dry region-wide, but there are some signs of increasing moisture in Arizona and Utah by the second week of July.  Initially, the Front Range of Colorado will see a temporary increase in moisture on Wednesday and Thursday as a backdoor cold front slides across the plains with increasing low level moisture along the eastern slopes of the Continental Divide.  The image below (courtesy of Tropical Tidbits) shows GFS-model projected precipitable water anomalies for Wednesday afternoon, with increased moisture pooling up against the Front Range and across Eastern Colorado.

 

 

However, upper level high pressure will quickly shift from the southeastern U.S. back west to the Four Corners Region heading into the weekend.  This pattern will cut off the flow to significant moisture across much of central and eastern Colorado.  The good news is that clockwise upper level flow around this high pressure center will begin to direct subtropical moisture into Arizona and Utah in about a week from now, which may very well signal the start of the monsoon.  The image below shows this ECMWF-model projected upper level high pressure center over Utah this upcoming weekend (image courtesy of Tropical Tidbits).

 

 

Some of this moisture may impact Western Colorado early next week as well, but as long as high pressure remains situated farther west, then much of the state will remain on the drier side.  There is some hope that by mid-month, the center of high pressure could become more favorably established over Arizona, New Mexico, or the Southern Plains, which would open the door for monsoonal moisture to stream into Colorado.

Overnight Heat Burst in Julesburg, Colorado

A rare meteorological phenomenon known as a “heat burst” struck the town of Julesburg in the far northeast Corner of Colorado last night, sending temperatures soaring from 78F to 97F in only 40 minutes shortly before 2:00am this morning.  This type of event is uncommon but does happen on the Great Plains occasionally in the late spring or early summer, and typically happens in the middle of the night once temperatures have already initially cooled from afternoon highs. 

The graph below shows the temperature graph over the previous 24 hours (ending at noon today, June 28).  Notice the rapid spike that occurred in the middle of the night.  Amazingly, the high temperature in Julesburg “only” peaked in the low 90s on the afternoon of the 27th, so the temperature of 97 that occurred at 1:42am was the 24-hour high temperature for the period ending at 7am this morning.

 

 

So what caused this bizarre weather event to occur?  Essentially, heat bursts involve dying nocturnal high-based thunderstorms with a deep layer of extremely dry air at the low and mid levels of the atmosphere as well as very warm air aloft.  Precipitation from these higher-based storms that are dissipating has a longer way to travel before reaching the ground than usual, and so it evaporates long before reaching the surface.  When air evaporates, it becomes cooler and denser, which causes it to accelerate toward the surface – this is how downbursts occur, as the accelerating air means strong outflow winds once this air reaches the surface.

However, when what we think of as a typical downburst occurs, the air is still in its evaporative cooling phase, and so as a result you normally get strong winds along with much cooler temperatures.  However, in the case of a heat burst, the dry layer is so deep that all of the water evaporates before reaching the surface.  At this point, the air begins to warm due to compression – similar to how dry, sinking air in downslope Chinook winds off the Front Range lead to rapid warming near the base of the foothills.  

By the time this warmer, drier, descending air reaches the surface, temperatures can quickly rise 15-20 degrees in a short amount of time.  The other impact is that strong wind gusts occur (similar to what you would expect in a downburst or strong outflow winds), and dewpoints and relative humidity values plummet.  In the case of last night’s storms – these storms formed in a drier atmosphere whereas low level moisture had increased farther east.  Prior to the heat burst, the dewpoint in Julesburg was a sultry 67, before rapidly falling to 31 as the temperature rose from 78 to 97.  The relative humidity obviously experienced a similar plummet, falling from 69% to 10% in a matter of 40 minutes. 

The chart below illustrates when the heat burst occurred, as you can clearly see the upward and downward spikes where temperature rose and moisture dropped.  Also, notice the spike in wind speeds with a gust of 49mph occurring as the temperature was rising.

 

 

And finally, the reason this was more likely to occur in the middle of the night was because temperatures had already cooled significantly.  If this happened during the daytime hours, you wouldn’t have much of a temperature increase because daytime temperatures would already be hot.   

Heat wave looms for Colorado, Southern Rockies this week

On Sunday, a powerful trough of low pressure swept across Colorado, bringing a round of strong thunderstorms to the I-25 corridor and Eastern Colorado, along with much cooler temperatures.  Thunderstorm development occurred early in the day, which sent early afternoon temperatures down into the low 50s.  The high for the day only topped out around 70 in Denver.

 

Today, the low pressure system has shifted well east of Colorado, bringing more active weather to the Midwestern states where portions of Iowa and Missouri are under an elevated threat for severe thunderstorms.  The satellite image below (courtesy of the College of DuPage) depicts this system very well.  You can also see some moisture over the Pacific Northwest, associated with another trough of low pressure over this region.  This feature is expected to result in some stronger thunderstorms across western and central Montana this afternoon.

 

GOES 16 water vapor June 25 2018

 

Colorado and the southern and central Rockies are experiencing beautiful weather today as the atmosphere has dried out behind the weekend system.  The average high for Denver today is 86, and temperatures should be right around this mark across the I-25 corridor this afternoon.  Unfortunately, the comfortable temperatures will be short-lived as high pressure strengthens over the Southern Rockies heading into the middle part of the week.  This will result in a heat wave across the Four Corners states, with temperatures soaring into the mid to upper 90s in Denver on Tuesday.  The image below depicts the GFS model-projected ridge of high pressure over the Southern Rockies on Tuesday (image courtesy of Tropical Tidbits).

 

 

The heat wave will continue on Wednesday and Thursday with even hotter temperatures expected.  In fact, forecasted highs for Denver on Wednesday and Thursday are right around 100 degrees.  Other “usual” hot spots in the region could see temperatures approach or exceed triple digits as well, such as Pueblo, Grand Junction, and Salt Lake City.  Moisture will be sparse through the middle of the week as well, resulting in an increasing wildfire threat as the week progresses.  Thursday will likely see the highest fire danger across the Southern Rockies as winds increase ahead of the next approaching trough from the Pacific Northwest.

 

The good news is that a break from the heat is expected this weekend as the previously mentioned trough of low pressure swings across the Central Rockies.  Friday will likely feature another day (though not as hot as Thursday) across Colorado, while temperatures farther west in Utah and Idaho begin to cool off.  By Saturday, the cooler air will spread into Colorado as well with highs in Denver projected in the low 80s on both Saturday and Sunday.  Thunderstorm chances will also increase over the weekend as well.  Northern and eastern Colorado look to be favored for moisture once again with the weekend system.  It’s still a good ways out, but anytime you have a strong upper level trough crossing the Central Rockies at this time of year, the threat for severe weather is probably going to be elevated across Eastern Colorado.  The image below depicts the GFS model projected trough of low pressure on Saturday (courtesy of Tropical Tidbits).

 

 

Now that we’re getting toward the end of June, many people are starting to wonder when the North American Monsoon will get going.  This is especially true this year, given the widespread severe drought conditions across the Four Corners region, along with ongoing wildfires across portions of Utah and Western Colorado.  The start of the monsoon varies every summer, but on average it usually begins during the first or second week of July as moisture from the higher terrain in Mexico spreads northward into Arizona and New Mexico, and then into Utah and Colorado.

There are no real signs of the monsoon at the moment, likely due to the active jet stream over the Western U.S. with frequent troughs crossing the Northern and Central Rockies.  This is suppressing the persistence of a subtropical ridge that allows for moisture to spread northward into the Southwest U.S.  However, longer range models do show an upper level pattern reminiscent of the seasonal monsoon taking hold in the July 5-10 timeframe.  A rough estimate of the second week of July for the start of the monsoon seems like a good call for now.

Severe Thunderstorms Strike The Front Range Again

Severe thunderstorms impacted much of the Front Range on Tuesday June 19th, less than 24 hours after Monday’s severe weather outbreak over Colorado. Tuesday’s storms brought heavy rainfall, tornadoes, and hail up to 3.00″ in diameter to several portions of eastern Colorado. Severe hail was reported as far north as the Fort Collins area, while additional storms further south pounded portions of Littleton, Parker, Centennial, and Aurora. Sadly, some areas in E Boulder County and SW Weld County experienced severe hail for the 2nd consecutive day. 

Here are a few maps of preliminary hail reports across the Front Range. The Fort Collins and Greeley area experienced severe hail between 1.25-1.75″ in diameter (click image to enlarge)
Further south, severe hail struck the Niwot, Longmont, Firestone, and Hudson areas with hail diameter between 1.25-2.00″ reported. The largest hail on Tuesday was reported over southern and eastern portions of the Denver Metro area, which saw hail diameter size between 1.25-3.00″. Here are a few pictures of hailstones just shy of tennis ball sized (2.5″), sent to us from a Skyview Weather friend in S Aurora. 

While consecutive days of severe hail are generally rare over the Front Range Urban Corridor, hail of this magnitude is certainly not unheard of. Regardless, these communities are certainly still reeling from damages incurred over the past 48-hours. The best news we received was there were no reports of serious injury or loss of life associated with Monday or Tuesday’s severe thunderstorms. Though, the financial burden of damages and impact on local communities will certainly be felt for months to come. 

Severe Storms Slam Northern Colorado with Heavy Rainfall and Large Hail

A round of severe storms developed over northern Colorado during the evening and overnight on Monday 6/18 into the early am hours on Tuesday.  Storms initiated over northern Jefferson County and Boulder County spreading out to the NE over the plains.  Hail and accumulating hail up to 3″ in Diameter pounded the NE plains with the worst hit areas being Weld and Morgan Counties.  Heavy rainfall also spawned Flash Flood Warnings from the NWS at 2-5″ or more of rain was reported.  There was heavy damage from flooding and the hail under the stronger storms some of which impacted the northern Denver Metro area and Boulder County but to a lesser degree than Weld and Morgan Counties.  Above is a hail map of some of the reports and below is is the text version of the storm reports.  This information and more can be found on the NWS Boulder page at:  https://www.weather.gov/bou/HailJune18th    

PRELIMINARY LOCAL STORM REPORT...SUMMARY
NATIONAL WEATHER SERVICE DENVER CO
534 AM MDT TUE JUN 19 2018

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

1135 PM     FLASH FLOOD      2 W KEENESBURG          40.11N 104.57W
06/18/2018                   WELD               CO   EMERGENCY MNGR

            FRONTAGE ROAD AT I-76 AND WELD COUNTY RD 53
            HAS WATER FLOWING OVER ROAD AND PART OF ROAD
            IS COMPLETELY WASHED OUT. IMPASSABLE
            ROADWAYS AT WELD COUNTY RD 8 AND 45, WELD
            COUNTY RD 16 AND 63, AND HIGHWAY 52 AND WELD
            COUNTY RD 49. PARTIAL FLOODING AT I-76 AND
            WELD COUNTY RD 55.

1100 PM     HEAVY RAIN       1 ENE KEENESBURG        40.11N 104.50W
06/18/2018  M2.47 INCH       WELD               CO   COCORAHS

            2.47 INCHES OF RAIN IN PAST 40 MINUTES.

1054 PM     HAIL             4 NW BURDETT            40.40N 103.03W
06/18/2018  M1.75 INCH       WASHINGTON         CO   TRAINED SPOTTER



1033 PM     HAIL             12 S ATWOOD             40.38N 103.27W
06/18/2018  M1.75 INCH       WASHINGTON         CO   TRAINED SPOTTER

            HAIL DEPTH OF 2 INCHES ON THE HIGHWAY.

1033 PM     TSTM WND GST     12 S ATWOOD             40.38N 103.27W
06/18/2018  E60 MPH          WASHINGTON         CO   TRAINED SPOTTER



1013 PM     HAIL             WELDONA                 40.35N 103.97W
06/18/2018  M1.00 INCH       MORGAN             CO   TRAINED SPOTTER

            1.5 INCHES OF RAIN.

0957 PM     HAIL             3 ENE HILLROSE          40.34N 103.47W
06/18/2018  M1.75 INCH       WASHINGTON         CO   TRAINED SPOTTER



0939 PM     HAIL             3 NW SNYDER             40.35N 103.62W
06/18/2018  M1.50 INCH       MORGAN             CO   TRAINED SPOTTER

            HOUSE WINDOWS BROKEN.

0921 PM     HAIL             LOCHBUIE                40.01N 104.72W
06/18/2018  M1.75 INCH       WELD               CO   PUBLIC

            REPORT VIA TWITTER.

0915 PM     TSTM WND GST     FORT MORGAN AIRPORT     40.33N 103.80W
06/18/2018  M71 MPH          MORGAN             CO   AWOS



0905 PM     HAIL             1 W BRIGHTON            39.99N 104.82W
06/18/2018  M2.75 INCH       ADAMS              CO   PUBLIC

            REPORT VIA TWITTER.

0849 PM     HAIL             3 NW HENDERSON          39.95N 104.92W
06/18/2018  M2.50 INCH       ADAMS              CO   NWS EMPLOYEE



0848 PM     HAIL             3 NE NORTHGLENN         39.94N 104.93W
06/18/2018  M1.75 INCH       ADAMS              CO   TRAINED SPOTTER



0847 PM     HAIL             3 NNE NORTHGLENN        39.94N 104.95W
06/18/2018  M2.00 INCH       ADAMS              CO   NWS EMPLOYEE



0846 PM     HAIL             1 SSW NORTHGLENN        39.89N 104.99W
06/18/2018  M1.00 INCH       ADAMS              CO   TRAINED SPOTTER



0835 PM     HAIL             1 ENE WESTMINSTER       39.89N 105.03W
06/18/2018  M1.00 INCH       ADAMS              CO   NWS EMPLOYEE



0834 PM     HAIL             1 NW BROOMFIELD         39.93N 105.08W
06/18/2018  M1.50 INCH       BROOMFIELD         CO   NWS EMPLOYEE



0831 PM     HAIL             1 W BROOMFIELD          39.92N 105.09W
06/18/2018  M1.00 INCH       BROOMFIELD         CO   NWS EMPLOYEE



0827 PM     HAIL             2 W WESTMINSTER         39.88N 105.10W
06/18/2018  M1.00 INCH       JEFFERSON          CO   NWS EMPLOYEE



0823 PM     HAIL             2 S ROCKY FLATS         39.87N 105.20W
06/18/2018  M1.75 INCH       JEFFERSON          CO   TRAINED SPOTTER

            REPORT VIA TWITTER.

0800 PM     HAIL             1 N ERIE                40.04N 105.04W
06/18/2018  M1.00 INCH       WELD               CO   PUBLIC

            REPORT VIA TWITTER.

0757 PM     HAIL             1 S LAFAYETTE           39.99N 105.10W
06/18/2018  M1.50 INCH       BOULDER            CO   TRAINED SPOTTER

            REPORT VIA TWITTER.

0752 PM     HAIL             1 N LAFAYETTE           40.01N 105.10W
06/18/2018  M2.50 INCH       BOULDER            CO   TRAINED SPOTTER

            SHATTERED VEHICLE WINDSHIELDS.

0746 PM     HAIL             1 NNW LOUISVILLE        39.99N 105.15W
06/18/2018  M2.00 INCH       BOULDER            CO   TRAINED SPOTTER

            REPORT VIA TWITTER.

0746 PM     HAIL             1 NW LAFAYETTE          40.00N 105.11W
06/18/2018  E2.00 INCH       BOULDER            CO   TRAINED SPOTTER



0745 PM     HAIL             2 WNW LAFAYETTE         40.00N 105.13W
06/18/2018  M1.00 INCH       BOULDER            CO   NWS EMPLOYEE



0743 PM     HAIL             1 NNW LOUISVILLE        39.99N 105.15W
06/18/2018  M1.75 INCH       BOULDER            CO   TRAINED SPOTTER



0740 PM     HAIL             1 NNW LOUISVILLE        39.99N 105.15W
06/18/2018  E1.25 INCH       BOULDER            CO   TRAINED SPOTTER



0739 PM     HAIL             1 NNW LOUISVILLE        39.99N 105.15W
06/18/2018  M1.00 INCH       BOULDER            CO   TRAINED SPOTTER



0731 PM     HAIL             SUPERIOR                39.93N 105.15W
06/18/2018  M3.00 INCH       BOULDER            CO   TRAINED SPOTTER



0730 PM     HAIL             1 ESE FREDERICK         40.10N 104.93W
06/18/2018  M1.50 INCH       WELD               CO   TRAINED SPOTTER



0728 PM     HAIL             2 N MARSHALL            39.98N 105.24W
06/18/2018  M1.00 INCH       BOULDER            CO   TRAINED SPOTTER



0720 PM     HAIL             3 NW MARSHALL           39.98N 105.26W
06/18/2018  M1.00 INCH       BOULDER            CO   NWS EMPLOYEE



0718 PM     HAIL             3 NNW FIRESTONE         40.16N 104.96W
06/18/2018  E1.75 INCH       WELD               CO   TRAINED SPOTTER



0711 PM     HAIL             5 NE MILTON RESERVOIR   40.29N 104.56W
06/18/2018  E1.00 INCH       WELD               CO   TRAINED SPOTTER



0700 PM     HAIL             1 NE DACONO             40.09N 104.92W
06/18/2018  M2.50 INCH       WELD               CO   NWS EMPLOYEE



0655 PM     HAIL             1 NE DACONO             40.09N 104.92W
06/18/2018  M1.75 INCH       WELD               CO   NWS EMPLOYEE



0651 PM     HAIL             FREDERICK               40.10N 104.94W
06/18/2018  M1.75 INCH       WELD               CO   TRAINED SPOTTER



0648 PM     HAIL             1 NW DACONO             40.09N 104.96W
06/18/2018  M1.00 INCH       WELD               CO   TRAINED SPOTTER



0643 PM     HAIL             1 W MEAD                40.24N 105.00W
06/18/2018  M1.75 INCH       WELD               CO   TRAINED SPOTTER

            REPORT VIA TWITTER.

0635 PM     HAIL             4 N ERIE                40.09N 105.06W
06/18/2018  M1.00 INCH       BOULDER            CO   TRAINED SPOTTER

            REPORT VIA TWITTER.

0632 PM     HAIL             1 NNW PLATTEVILLE       40.23N 104.83W
06/18/2018  M1.00 INCH       WELD               CO   TRAINED SPOTTER



0625 PM     HAIL             2 N LONGMONT            40.20N 105.11W
06/18/2018  M1.75 INCH       BOULDER            CO   TRAINED SPOTTER



0619 PM     HAIL             3 SSW MEAD              40.20N 105.01W
06/18/2018  M1.25 INCH       WELD               CO   TRAINED SPOTTER

            REPORT VIA TWITTER.

0610 PM     HAIL             1 NE BOULDER            40.04N 105.24W
06/18/2018  E1.75 INCH       BOULDER            CO   PUBLIC

            REPORTED AT 3450 MITCHELL LANE, BOULDER, CO.

0610 PM     HAIL             1 NE BOULDER            40.04N 105.24W
06/18/2018  E1.75 INCH       BOULDER            CO   PUBLIC

            REPORTED AT NCAR FOOTHILLS LAB, BOULDER, CO.

0610 PM     HAIL             1 ENE BOULDER           40.00N 105.26W
06/18/2018  E1.25 INCH       BOULDER            CO   TRAINED SPOTTER



0608 PM     HAIL             2 SSW BOULDER           40.00N 105.27W
06/18/2018  M1.50 INCH       BOULDER            CO   TRAINED SPOTTER



0600 PM     HAIL             2 SSW BOULDER           40.00N 105.26W
06/18/2018  M1.00 INCH       BOULDER            CO   EMERGENCY MNGR



0558 PM     HAIL             4 WNW BOULDER           40.04N 105.32W
06/18/2018  M1.00 INCH       BOULDER            CO   PUBLIC
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