Mid and upper level moisture is on the increase today from the SW resulting in partly to mostly cloudy skies through early afternoon then mostly cloudy to cloudy through sunset. An area of showers over central and southwest Colorado currently will move northeastward and introduce a chance for isolated to widely scattered rain showers and possibly a weak thunderstorm later this afternoon and evening. Best chances for rain showers/thunderstorms will be between 4-7pm although a slight chance will linger through about 9pm. Typical rainfall amounts will be in the TR-0.10” range but if a thunderstorm is observed up to 0.25” cannot be ruled out. Winds are a little tricky to forecast today as they may become variable in or near shower activity but overall winds expected to be from the E to SE most of this afternoon and early evening, generally under 10mph. Higher wind gusts may be observed near storms. Temperatures today are being held down due to the cloud cover but will spend quite a bit of time in the 60s through 6pm then mainly in the 50s through the game. GO BRONCOS!!!
Hurricane Florence will make landfall late tonight or early Friday morning in the Carolina’s. Currently Florence is a Category 2 Hurricane with sustained winds of 110mph. Storm surge and damaging winds will heavily impact coastal areas. As Florence moves inland it will rapidly weaken but copious amounts of rain will impact the SE US initially and then as the storm takes a turn to the NE it will impact the mid Atlantic states and well as the NE US. More about Hurricane Florence can be found on the web at the National Hurricane Center.
According to the Climate Prediction Center there is a “60% chance of El Nino in the Northern Hemisphere fall 2018 (Sept-Nov), increasing to 70% during winter 2018-2019”. The image above shows a typical winter El Nino pattern. The Winter El Nino pattern map and more can be found on the web at: https://www.climate.gov/
A hot and dry summer across a large portion of Western North America has resulted in widespread fire activity from California to Idaho to British Columbia. Many of these fires have been very large, and over the past month smoke has been persistent across much of the West. However, the seasons are beginning to change as we near the end of August, and there are some clear signs that fall is right around the corner.
In general, the Western U.S has already been experiencing cooler temperatures over the past 1-2 weeks compared to a hot first half of August west of the Continental Divide. Today, a more substantial change is occurring as a deep trough of low pressure is pushing into the Northern Rockies, resulting in unseasonably cool temperatures along with much needed moisture. The image below shows the height anomalies at the upper levels of the atmosphere, which depict this fall-like system well over Montana and Wyoming (source: weathermodels.com).
Even though the calendar says late August, this system has a wintery side as well, and snow is falling at the higher elevations of the Northern Rockies, including Grand Teton, Yellowstone, and Glacier National Parks. The webcam image below shows fresh snow near the top of Jackson Hole Ski Area in Wyoming.
The moisture, cold temperatures, and snow with this system should help out with the wildfire the smoke situation. While this system won’t totally extinguish wildfires in the Northern Rockies, it will certainly help to reduce fire behavior, and most areas should see significant improvement with regards to smoke and air quality this week as a result. The exceptions will be in areas such as California and Nevada, which are missing out on the moisture.
Farther south, Colorado is on the dry side of the trough and jet stream today, and as a result hot, dry, and windy conditions are present across the Front Range instead. The fire danger is elevated here today as a result, but fortunately a cold front will arrive tonight with much cooler temperatures on Tuesday. Some parts of Colorado could see some light showers on Tuesday, but in general the state will be missing out on significant moisture with this system. Upslope flow pooling east of the Continental Divide could bring some more smoke to the Front Range on Tuesday, but then improvements are expected for the remainder of the week.
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 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.
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.
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 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.
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.
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!
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.
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.
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.