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 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 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:    

534 AM MDT TUE JUN 19 2018

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

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

            COUNTY RD 16 AND 63, AND HIGHWAY 52 AND WELD
            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


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


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


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


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


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


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


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


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


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


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


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


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


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


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

Hot and dry start to June for Colorado

Following a relatively active May for Northeast Colorado, the pattern has become very warm and dry across the entire state over the first 12 days of June.  The drought and fire situation has turned dire for many areas, which suffered from a below average winter snowpack.  The San Juan Mountains in Southwest Colorado experienced one of their worst winters for snowfall on record, and not surprisingly this has been a focal point for wildfire activity so far.  The 416 Fire near Durango started on June 1st, and so far has burned over 22,000 acres and is only 10% contained.  The situation in the San Juans has resulted in a closure of the entire San Juan National Forest to humans due to the wildfire threat.

The maps below show the departure from average precipitation and departure from average temperature over Colorado through the first half of the month.  Certainly not a situation that helps the fire and drought concerns.  Interestingly enough, portions of Southern Colorado and the San Luis Valley are showing an above average precipitation anomaly so far for June, but keep in mind that this area receives very little precipitation on average in June, so it doesn’t take much to be “above average” there at this time of year.


June 1-10, 2018 Departure from Average Precipitation (Source: Western Regional Climate Center)


June 1-10, 2018 Departure from Average Temperatures (Source: Western Regional Climate Center)


In addition to the 416 Fire near Durango, additional fires have recently started in or near Colorado.  On Sunday, the Badger Creek Fire started in the Snowy Range of Southern Wyoming, near the Colorado border.  Closer to home, the Buffalo Mountain Fire started just outside of Silverthorne in Summit County this morning, and is burning beetle kill forest and threatening numerous structures.  

Colorado and the Southwest U.S. is in dire need of moisture right now.  The image below shows the current drought situation in Colorado.  Southern Colorado in particular is experiencing an exceptional drought, but drought conditions are projected to expand to include much of Northern Colorado over the next month as well.


There is both good and bad news with regards to the weather outlook for moisture over the next couple of weeks.  In the short-term, the pattern looks to remain mostly dry across much of the state with hot temperatures and only some isolated thunderstorm chances.  Fortunately, there are no widespread strong winds expected during this time.

As we head into the weekend, we will finally start to see some better moisture potentially arrive as a trough of low pressure deepens across the Western U.S. and moisture from Hurricane Bud in the Pacific Ocean becomes entrained into the flow and likely reaches Colorado.  This should start to bring better chances for rainfall across Western Colorado on Saturday, before spreading into the Front Range and Eastern Colorado on Sunday.  Good chances for rain and thunderstorms could persist well into next week, although it’s too early to say which parts of Colorado will be favored more than others.  This pattern will be watched by many with great interest given the current dry spell.

Monsoon Preview?

With the hot and dry conditions we’ve seen over the past several days in Colorado, the wetting rains of the North American Monsoon pattern may seem like a long-lost, distant memory. July and August are our most active months for monsoon-enhanced precipitation, but it’s not unusual to see the first glimpses of pattern emergence during the month of June. This summer is no exception, as it looks like we’ll get our first taste of subtropical moisture this weekend. But first, here is a little background information to put things into perspective. 

North American Monsoon Pattern (NAM)

Most simply, the NAM is a prolonged, intra-seasonal shift in the wind pattern over the southwestern United States during the mid and late-summer months. This wind shift allows moisture-rich subtropical air to stream north out of the Gulfs of Mexico and California, and move over the Desert Southwest and eventually Colorado. The influx of moisture can contribute to showers and thunderstorms producing heavy rainfall over areas with typically-arid climates. 

The key to diagnosing the NAM lies in the presence and strength of large-scale weather features over the southwestern United States. Here’s a nice schematic of the pattern, courtesy of Mike Baker (NWS Boulder). 

During the summer months in the northern hemisphere, subtropical high pressure positions over the middle part of the United States. Clockwise flow around the high efficiently transports moisture northward over a good portion of the country. The second key feature typically seen with the NAM is an area of low pressure known as a “heat low” or “thermal low.” This feature arises as temperatures hit their seasonal peak over the Desert Southwest. Counterclockwise flow around the low also ushers moisture-rich subtropical air northward. The result is (2) persistent large-scale weather features working in concert to pump deep moisture into our region. 

One of the keys to understanding the NAM is recognizing that it is an intra-seasonal process, and its spatial scope is regional. The NAM is not a storm, nor is it an exceptionally-rainy afternoon. In this sense, the NAM is more similar to a process like ENSO (El Nino Southern Oscillation). Nothing occurs very quickly with the NAM. It takes weeks for large-scale features to strengthen and position correctly. Similarly, the process does not arrive overnight. Typically, the influx of monsoonal moisture slowly ramps up during late June/early July, peaks for a few weeks, then slowly relaxes as we move into September. 

And just because the NAM pattern is in place, it doesn’t mean you’re going to get rain. Thunderstorm coverage is highly-sensitive to the position of the subtropical moisture plume. This plume can position as far west as California/Oregon/Washington, or as far east as the southern plains. Typically, the moisture plume slowly oscillates from east-to-west as the large-scale feature’s shape and strength evolve. 

So, how do we know know when the NAM officially arrives? 

Monsoon daily dewpoint tracker for Tucson

The NAM pattern is declared once the daily-average dewpoint in Tuscon, Arizona reaches 54°F or higher for three-consecutive days. The 54°F threshold is relatively arbitrary, but it does typically signal a large-scale influx of subtropical moisture into the mid latitudes associated with the NAM. As we would expect this time of year, surface moisture is slowly increasing over Arizona, but we still have a lot of work to do to reach the NAM threshold. As the red curve indicates, NAM conditions typically arrive during early-to-mid July and persist for about two months. 

We’ll need to wait a few more days to iron out the finer details, but it looks like we’ll see a preview of NAM conditions over Colorado by this weekend. 

This is GFS model imagery, courtesy of TropicalTidbits. The map shows precipitable water normalized anomaly. Basically, a map of integrated moisture in the atmosphere, and how unusual the values are for this date in June. Anomalous moisture streaming north is clearly evident, and the plume appears to position over much of Colorado this weekend. We’ll get an extra boost from Hurricane Bud in the E Pacific (not pictured), which will move north and phase with the large-scale circulation. 

This is not the North American Monsoon per se, but it is a very monsoon-esque pattern concerning moisture trajectory and magnitude. 

So, is the weekend ahead a wash for outdoor activities in Colorado? Not necessarily. We still need the same daily ingredients at the local scale for thunderstorms to form and eventually capitalize on the increased moisture. 

For now, we’ll keep our eye on the weekend ahead, recognizing that a return to wetter conditions appears likely. We’ll also watch the dewpoints and storm activity over Arizona during the coming weeks as evidence the NAM has arrived and is here to stay.