Issue Date: Thursday, June 23^rd, 2022
Issue Time: 3PM MDT
Valid Dates: 6/24 – 7/8

It has certainly been an interesting June so far for Colorado’s weather. And fortunately, the main story continues to center on precipitation chances, as opposed to dryness and wildfire issues that have plagued the state over the past few years and even earlier this spring. To put it briefly, given the moisture we have seen in place, it appears that Colorado (especially the southwest corner) has been somewhat lucky in avoiding flooding thus far into our heavy rainfall season. Of course, we are not complaining, but let us quickly recap why this may have been the case before proceeding with the rest of the Outlook.

Shown below is the 500-mb height anomaly over the past week or so (although this is quite similar if we expand to include all of June). This essentially shows the pressure anomaly estimated in the middle of the troposphere (roughly 20,000 feet above sea level). The primary feature that we see is the large ridge centered over central North America. With the clockwise circulation around this ridge paired with a modest low-pressure anomaly over the west coast, we deduce that the mid-level flow into Colorado thus far in June has been generally from the south.

Now, knowing that water vapor generally increases as one moves southward, we can deduce that there has been above average moisture over the southern Four Corners region, right? As shown in the left map, below, this is sort of true. There has definitely been above normal moisture over AZ and NM, but the signal dissipates by the time we get to Colorado. Instead, what we can deduce is that there has been a stronger that normal moisture gradient between Colorado and New Mexico. Accordingly, New Mexico has picked up copious amounts of rain, especially by June standards, whereas Colorado has been only slightly wetter than normal (of course, no complaining about that!). Meanwhile, the below map on the right shows that low-level moisture (a proxy for what is available for fueling thunderstorms) has actually been below normal for most of Colorado! Comparing the two images below, this suggests that the majority of the above normal moisture (as measured by PW) has actually been too high (above the boundary layer) where it is less effective at signaling heavy rainfall potential.

So does any of this have relevance for the current Outlook? In short, yes. Taking a look at this afternoon’s water vapor image, below, we see that the upper-level ridge that has been over the central US has shifted a bit further south. To the west, a cut-off low pressure of modest size and strength is noted over California. Pairing the two together shows that Colorado is again under extensive southerly flow originating in the subtropics/tropics. In fact, there is basically an unabated streamline linking soon-to-be Hurricane Celia all the way to Colorado. Over the next 3-5 days, until the upper-level ridge slowly dissipates and moves westward, Colorado will see continued above normal precipitation chances. Initially, this will be mainly over the western half of the state (Event #1), with limited rainfall intensity. But by Saturday, the approach of a moist Pacific cool front will allow precipitation to overspread most of the state (Event #2). Only considering model QPF at face value, one would think Event #2 could be of serious concern. Yet, a closer look at dynamics instead suggests limited instability both in space and time, instead conveying more of a prolonged, moderate-intensity rainfall for most locations with only a low-end Elevated flood threat anticipated.

By Monday, the forecast PW plumes, below, show a brief post-frontal drying that will reduce precipitation coverage. However, this is only expected to be a brief drying, as moisture should continue to remain above normal through Independence Day weekend. At this time, it is too premature to identify a flood threat after Event #2, as those details will likely be better deduced in the daily Bulletins and next Outlook.

The identified precipitation events are described in more detail below.

Event #1: Friday (June 24)

No Apparent Flood Threat As Mainly Higher Terrain Storms Race Across The State

Marginal remaining subtropical moisture and limited instability will produce scattered showers and storms across mainly the Northern Mountains, Central Mountains and Front Range. Quick storm motion should limit point rainfall to about 0.5 inches, though most of this could fall in a quick 20-30 minute downpour.

Event #2: Saturday – Sunday (June 25 – June 26)

Elevated Flood Threat For Southern High Terrain, While Steady Rainfall Piles Up Further East

A moist cool front will enter the state Saturday morning. The key to the day’s heavy rainfall potential will be the cloud cover ahead of the cool front. At this time, the timing of the front suggests there will be limited instability outside of a small pocket over the Southeast Plains, Palmer Ridge, Southeast Mountains and Raton Ridge (possibly spilling westward into the San Juan Mountains). At least isolated heavy rainfall potential will exist where instability is highest, so an Elevated Flood Threat is warranted for the potential of rainfall exceeding 0.8 inches/30 minute and 1 inch/hour over the higher terrain.

By Sunday, the instability shifts slightly south and west with the primary area of concern being the San Juan Mountains. Once again, early cloud cover could make or break the heavy rainfall potential, but sufficient instability and slow storm motion currently warrants another day with an Elevated Flood Threat. Further east, steady moderate rain could produce up to 2 inches of rainfall over parts of the Raton Ridge and Southeast Plains by late Sunday. To have this occur in late June without a flood threat is downright amazing!

Event #3: Monday – Thursday (June 27 – June 30)

No Apparent Flood Threat, But Daily Scattered Showers And Storms Expected

Moisture will decrease a bit after Event #2, though enough of it will remain to fuel daily instability of 400+ J/kg of CAPE over the higher terrain. Coupled with slow storm motion under a weak upper-level ridge, this suggests at least brief heavy rainfall will be possible but it is premature to issue an Elevated threat.

Event #4: Friday – Sunday (July 1 – July 3)

No Apparent Flood Threat At This Time, But Bears Watching

Heading into Independence Day weekend, there are some indications of another Pacific cool front that will approach Colorado from the north. Thus, there may be an increase in precipitation chances over the Northeast and Southeast Plains. However, there is too much uncertainty regarding the timing and instability to warrant an Elevated flood threat. The next Outlook should have a much better look at this possible event.

Issue Date: Monday, June 20^th, 2022
Issue Time: 3PM MDT
Valid Dates: 6/21 – 7/5

By the time we enter this late into June, it is quite unusual to receive significant amounts of rainfall with limited flooding. But that is exactly what occurred over the past 72-hours (see today’s State Precipitation Map discussion) over southwest Colorado. Fortunately, in this Outlook, we will see that this event was not just a one-off event. After a brief lull today, a plethora of moisture will re-enter and hang around Colorado over the entirety of this Outlook, setting the stage for widespread precipitation chances across a majority of our state!

As seen in the water vapor image, below, the key features dictating our atmospheric pattern over the next week or so will be (1) the increasing importance of the upper-level ridge (currently to our east), (2) the continuous import of subtropical moisture, including indirectly through tropical disturbances Blas and Celia, and (3) synoptic-scale forcing coming from the eastern Pacific Ocean. The first round of precipitation, Event #1, will be driven mainly from a the subtropics, where as the second event will be associated with a cool and very moist Pacific frontal passage sometime this week.

As shown in the forecast GEFS PW plumes, below, moisture content is expected to stay well above normal beginning late Tuesday through the foreseeable future. In fact, PW may exceed 1.0 inch by this weekend over eastern Colorado, whereas normal values for this time in June are in the 0.6-0.8 inch range. The other major factor supporting the threat of at least isolated heavy rainfall will be slow steering flow, a change from the recent event which had very quick steering flow. On the contrary, the major inhibiting factor in the potential for flooding in both Event #1 and #2 will be the limited instability. With generally warm mid-level temperatures expected due to the upper-level ridge moving back westward towards Colorado, only marginal to possibly moderate instability will be available. Nonetheless, the ability of our terrain to generate its own circulation suggests the possibility of slow-moving storms with at least short-term (1 hour duration) heavy rainfall potential. Thus, Events #1 and #2 have an Elevated flood threat.

The other matter of importance is the increasing role of saturated soil in raising the chances of mud flows, debris slides and overall runoff. Given Colorado’s steep terrain, typical Flash Flood Guidance (FFG) for 1-hour rainfall is in the 0.75-1.5 inch range, depending on the exact steepness as well as soil type and land cover. As an aside, note that FFG can be as high as 3.0+ inches/hour over sandy, flat parts of the South Platte and Arkansas River valleys of eastern Colorado. The mountainous 0.75-1.50 inch thresholds will be substantially lower over locations that have received significant rainfall. By “significant”, a general rule of thumb is 0.25 inches per day (especially over large areas), which greatly reduces the ability of the soil to uptake more rainfall the following day.

The identified precipitation events are described in more detail below.

Event #1: Tuesday – Thursday (June 21 – June 23)

Low-end Elevated Flood Threat For Scattered Heavy Rainfall Amidst More Stratiform Precipitation

Another subtropical moisture surge will arrive into Colorado late Tuesday afternoon. On Tuesday, most of the precipitation with this is expected to be in the form of a decaying convective complex from earlier storms in central New Mexico. However, enough instability will exist over the southern San Juan Mountains to cause slow-moving storms capable of 1.0 inch per hour of rainfall.

By Wednesday and Thursday, an increase in precipitation chances is expected for most of southern Colorado. However, it remains to be seen how much instability will be available. At present, it appears that a Low-end Elevated flood threat will exist both of the San Juan and Southeast Mountains, where 1.0 inch per hour rainfall intensity could occur with stronger storms. Further east, instability looks to be limited and only weak storms are expected, along with more general synoptic scale precipitation.

Event #2: Friday – Monday (June 24 – June 27)

Elevated Flood Threat Though Location And Potential Intensity Look Uncertain Due To Limited Instability

Friday looks to be a brief transition day between Event #1 and the aforementioned Pacific cool front set to arrive early Saturday morning. Thus, only isolated to widely scattered showers and storms with limited rainfall intensity are currently expected, mainly over the higher terrain.

By Saturday, the timing of the cool front passage will dictate which parts of the state will still have instability. Earlier guidance suggested enough instability for widespread showers and storms over southern and central Colorado. However, the latest guidance shows lower instability, meaning more of a lighter, stratiform rainfall. Nonetheless, areas further south will still see higher instability and a greater chance of heavy rainfall. By Sunday and Monday, typically post-frontal residual moisture over the Southeast Mountains, Raton Ridge and San Juan Mountains should continue to support scattered storm activity with heavy rainfall potential.

Event #3: Tuesday – Thursday (June 28 – June 30)

Well Above Normal Moisture To Maintain Precipitation Chances, But Intensity Uncertain; No Apparent Flood Threat

Without a mechanism to dry out the atmosphere, plenty of moisture will continue to persist under a wobbling upper-level ridge. With expected slow steering flow, this kind of pattern favors mainly higher terrain shower and storm potential. However, at this time, coverage and intensity remain too uncertain to create a precipitation map.

Issue Date: Thursday, June 16th, 2022
Issue Time: 2:30PM MDT
Valid Dates: 6/17 – 7/1

There is much higher confidence in today’s FTO that widespread rainfall will return to western Colorado and the mountains this weekend. An upper-level Low will move towards the coast of California and dig in place, while the High over the south begins to build back to the west. This pattern will create strong southerly flow over the Desert Southwest and pull a subtropical moisture plume northward into western Colorado. Likely adding to this moisture plume will be Hurricane Blas, which is also marked in the water vapor imagery below. Widespread rainfall is forecast to begin tomorrow and should last through the weekend causing an Elevated flood threat to be issued for Event #1. This flood threat includes recent burn areas, so be sure to track those daily flood threat forecasts in the Fire Burn Forecast table.

Below are the moisture plumes for Grand Junction (right) and Denver (left). Added to the time series for today’s FTO are the 90^th percentile PW values for this time of year to show just how strong this plume of moisture is relative to climatology. While the moisture is notable over Denver, terrain driven storms with southerly steering flows should help to limit the flood threat over the adjacent eastern plains. Over western Colorado, it is possible that Grand Junction’s PW may reach the maximum moving average, which is around 1 inch (red star). It’s impressive how far north this moisture plume is reaching in the models. With the jet stream setting up just to our east and Colorado under the influence of some stronger upper-level winds, does seem likely that higher moisture values will reach the northern border.

For the most part, moisture looks to decrease at the start of next week, but it is likely that residual moisture will linger across the southern border, especially over the southwest quadrant of the state. Thus, we expect daily storms to continue to fire over the central/southern high terrains through next week (Event #2). It will also be possible for some storms to develop over the eastern plains on Tuesday behind the departing trough.

Finally, worth mentioning Event #3. There’s not much of a break between systems this week as another subtropical plume potentially moves northward at the start of next weekend. Right now, there is fairly good model consensus that this will occur. Really hoping this push of moisture continues to show up in the model runs as west-central and southwest Colorado are experiencing severe to extreme drought conditions. Be sure to tune back into the FTO on Monday for the latest.

Event #1: Friday – Sunday (6/17 – 6/19)

Elevated flood threat as the first subtropical moisture plume arrives to western Colorado.

Finally, some decent rainfall is forecast, which should help somewhat relieve the drought situation over western and southwestern Colorado. Rainfall begins tomorrow and will likely peak on Saturday as far as intensity. Initially, it may take some time for the boundary layer to moisten considering the Grand Junction dew point was measured at 18F this morning. This just means less rainfall accumulation initially at the lower elevation valleys on Friday. However, plenty of rainfall is still forecast on Saturday and Sunday, so southerly flow should help produce some meaningful accumulations at the lower elevations as storms move off the mountains. Heaviest rainfall is anticipated over the south facing slopes of the southern San Juan Mountains.

Due to the ongoing nature of the rainfall this weekend, with plenty of mid-level lift moving through the flow and the diurnal cycle, soils should slowly begin to saturate by late Saturday/Sunday. This will likely enhance runoff, which could cause some rises in local streams over the San Juan and Central Mountains. One limiting factor to the flash flood threat this weekend could be the inability to build instability for convection. However, if pockets of instability could develop over any area between shortwave impulses and breaks in cloud cover, embedded convection would likely help to increase the chance for mud flows and debris slides over the steeper terrains under the stronger storm cores. This is especially true over the south facing slopes of the San Juan Mountains where the soils are most likely to become saturated first. Thus, the Elevated flood threat being issued.

With mostly southerly steering flows during this event, unsure about very much moisture spillover into the Front Range and adjacent eastern plains. There will likely be a bit more of an easterly component to the flow on late Saturday into Sunday, so expect an uptick in storm chances over the Front Range during this period.

Be sure to tune into the FTB for your daily flood threat update as subtle features will likely indicate when and where the flood threat will occur.

Event #2: Monday – Thursday (6/20 – 6/23)

No Apparent Threat flood threat as residual moisture continues showers over the south/central mountains.
Moisture mixes out over north and central western Colorado early next week as dry westerly flow picks back up. However, some residual moisture looks to hold on over south and central Colorado, which should help drive the diurnal storm pattern. At this time there is No Apparent flood threat, but the continued rainfall and cooler conditions should help temper critical fire weather over the area. We will keep an eye out for dry lightning conditions in the daily FTB. Isolated storms may also return to eastern Colorado on Tuesday in association with a stalled-out front behind the departing trough. Less certainty in this outcome as far as location and the ability for storms to break the cap. There is also an outside shot for storms over the far eastern plains on Wednesday. As far as high temperatures, it starts to heat up by mid-week.

Event #3: Friday – Sunday (6/24 – 6/26)

No Apparent Threat as another potential plume of subtropical moisture is pulled northward.

Less confidence in the models this far out, but it seems there may be another surge of subtropical moisture northward for another round of widespread showers over western Colorado to end June. At this time, it looks like there should also be a better chance for storms to develop over the eastern mountains next weekend as flow aloft also gains an easterly component. With such low confidence, no map has been drawn. Hot temperatures also look to return to eastern Colorado with highs likely reaching 100F+ over the far eastern plains by Friday. Details are likely to change, so be sure to tune back into the FTO next Monday.

Issue Date: Monday, June 13^th, 2022
Issue Time: 12PM MDT
Valid Dates: 6/14-6/28

Only three weeks removed from a late spring snowstorm, we are now setting heat records across Colorado. Such are the vagaries of the spring to summer transition in our high elevation climate. This Outlook continues such a roller coaster theme, but with some decidedly positive news from the standpoint of precipitation chances. As shown in the water vapor image, below, a strong trough currently over the central Pacific Ocean will combine with a deep plume of subtropical moisture to fuel a multi-day widespread rainfall event (Event #2) across most of the western slope beginning Friday of this week. This is excellent news for the drought-stricken region, especially the San Juan Mountains, were an early snowmelt caused justified concerns about a potentially dangerous wildfire season. For the time being, those concerns should abate.

Until we get to Event #2, we will first see a strong cool front cross the state later today and into the overnight hours (Event #1). Between its Pacific origin and the fact that we are in summer, the front will have a hard time fully crossing Colorado and will instead stall out somewhere across the Southeast Plains. This will lead to some precipitation chances to the north of the cool front as a weak low pressure system spins up across this strong baroclinic zone (region with a strong temperature gradient). With very limited instability, only light to moderate showers and perhaps a weak storm are expected mainly over the Northeast Plains on Tuesday. Breezy or even windy conditions will cause an increase in the wildfire threat, as humidity values will plummet in the wake of the cool front. Thereafter, hot temperatures will return mainly over eastern areas as we wait for the deep moisture plume.

As shown in the forecast PW plumes below, moisture will re-enter the state in earnest by later on Thursday though precipitation will likely be limited due to the loss of daytime heating by that point. By Friday, however, PW will rise into the 0.8-1.0 inch range across central and western Colorado, fueling widespread shower and storm activity. This will continue Saturday through Monday, though it is too early to discuss the finer details. At this time, we are labeling Event #2 with an Elevated flood threat over the Friday – Monday period. The primary factor that should preclude a higher threat is the expected strong steering flow that would limit point rainfall into the 60 – 90 minute duration. There is also some notable model uncertainty: the GFS model system has the main PW plume considerably west of the European model. A further west solution would limit the heavy rainfall threat towards the Utah border, while the European model solution would push the heavy rainfall threat eastwards, possibly as far as the Sangre de Cristos. The latter solution could also warrant a higher threat, as storm training will be more of a concern. Nonetheless, there is strong consensus that some portion of the San Juan Mountains will experience a significant amount of rainfall over the 96 hours period of Event #2.

Lastly, a look at snowmelt progress (and corresponding streamflow) suggests peak flows over the next few days for most of the Front Range and Northern Mountain streams. Flows are expected to drop quickly over the next week or so as only small pockets of “hard to melt” snow remain.

The identified precipitation events are described in more detail below.

Event #1: Tuesday (June 14)

Showers And A Weak Storm Possible For Northeast Plains; No Apparent Flood Threat

A cool front will stall across eastern Colorado leading to the formation of a low pressure by Tuesday. Most of the state will experience significant drying, but some moisture will remain over the Northeast Plains (mainly along NE border) and support scattered showers and perhaps a weak storm by late afternoon. Max 1-hour rainfall will be limited to 0.25 inches so flooding is NOT expected.

Event #2: Friday – Monday (June 17 – June 20)

Elevated Flood Threat For Western Colorado As Prolonged Stretch Of Afternoon Storms Expected

Widespread showers and thunderstorms are expected Friday through Sunday across mainly the western slope. Highest coverage currently appears to be over the San Juan Mountains. Max 30-minute rainfall up to 0.8 inches can be expected with max 1-hour rainfall up to 1.2 inches. Swift storm motion appears to be the main factor precluding higher rain intensity, however, storm training could become a problem and warrant a higher threat. This will be better known in the next Outlook as well as the daily Bulletins.

By Monday, the main axis of the moisture plume should be north of Colorado, though at least isolated to scattered storms will still be possible over the San Juan, Central and Northern Mountains.

Overall, widespread rainfall above 1.0 inch is expected across most over the San Juan and Central Mountains. Preferred southward facing slopes of the San Juan Mountains will see up to 3.0 inches of rainfall over the course of this event! With this many consecutive days of heavy rainfall, the threat of mud flows and debris slides will increase every day over steep terrain that accumulates multiple storm passages.

Significantly more uncertainty exists about the rainfall chances to the east of the Continental Divide. With this magnitude of moisture, it is quite likely that at least some of the precipitation chances will “spill over” into the Palmer Ridge and/or Southeast Plains and Northeast Plains. However, this looks a bit less likely than in the previous Outlook, so flooding is not expected over these regions at this time.

Event #3: Tuesday – Thursday (June 21 – June 23)

Moisture And Rainfall Chances To Remain Though Amount And Location Uncertain; No Apparent Flood Threat

In the wake of Event #2, at least some residual moisture looks to remain in place over Colorado. At this time, the preferred region for shower and thunderstorm activity looks to stay over the San Juan Mountains. However, precipitation is not expected to exceed 0.5 inches at this time. The next Outlook should have more insight into whether this event will become more threatening.