Issue Date: Thursday, June 9th, 2022
Issue Time: 3PM MDT
Valid Dates: 6/10-6/24

The middle part of June marks an interesting milestone in Colorado’s summer, which given our semi-arid climate, one that can be too subtle for even long-time residents to fully perceive. As shown below, over the next 10 days or so, we will see the summertime minimum in climatological precipitation chances across essentially the entire state. For example, Grand Junction’s climatological chance of precipitation is about 11% over the next few days, but will quickly jump up to 23% over the next month. Stark swings are seen at basically every long-term station that you can find. The back of the envelope explanation for this is the transition from mainly jet-stream driven springtime dynamics to slower, subtler monsoonal dynamics.

Interestingly enough, such a transition from jet-stream dominated flow to monsoonal is exactly what we are likely to see over the course of this Outlook. As shown in this afternoon’s water vapor imagery, below, another elongated jet is seen across the central and eastern Pacific Ocean. In association, a deep trough is positioned over the Gulf of Alaska. Over the next 3-5 days, several disturbances (Event #1 and Event #2) will detach from this parent trough and race eastwards towards Colorado. However, due to the combination of gradual drying, limited instability and fast storm motion, only isolated to widely scattered showers and thunderstorms are expected with limited rainfall (less than 0.5 inches at this time). Instead, the big story over the next week will be the building heat, first to the west of the Continental Divide, but then mainly over central and eastern Colorado. Temperatures up to 15F above normal will yield afternoon heat exceeding 100F over the state’s lower elevations. Over this time period, most of the remaining snowpack is expected to melt, with generally little fanfare although a few full smaller streams can be expected within the Elk River basin north of Steamboat Springs.

A strong cool front will swing across the state on Tuesday of next week. After a few days of quiet weather, that is when things become interesting. A strongly amplified pattern is expected to develop across North America with a strong ridge over the Northern Plains expected to support a multi-day early summer heat wave across the eastern two-thirds of our country. Associated with this ridge will be a strong southerly flow across the intermountain west, one that will open the gates to the season’s first pulse of monsoonal moisture as shown in the forecast GEFS PW plumes below. A multi-day period of active weather is expected mainly over western Colorado, however, it is typical to see at least some “spillover” of storms to the east of the Continental Divide. An Elevated Flood Threat is expected through the course of this monsoonal pulse as plenty of moisture will support at least isolated heavy rainfall.

The identified precipitation events are described in more detail below.

Event #1: Saturday – Sunday (June 11 – June 12)

Isolated Showers And Storms Expected Across The State But No Apparent Flood Threat

Isolated to widely scattered afternoon showers and storms are expected over mainly the higher terrain of Colorado, though rainfall is expected to stay below 0.5 inches.

Event #2: Monday – Tuesday (June 13 – June 14)

Showers And A Few Storms To Graze State Borders, But No Apparent Flood Threat

The approach of the aforementioned cool front will lead to a slight increase in rainfall chances, though dry air aloft and quick storm motion is expected to limit point rainfall. The best chances of rainfall appear to be over northwest Colorado on Monday, and then over northeast Colorado on Tuesday. At this time, precipitation looks to be limited to 0.5 inches or less. However, any increase the moisture along the NE/KS borders could introduce a low-end flood threat over the eastern Plains. Stay tuned to the next Outlook for updates.

Event #3: Friday – Monday (June 17 – June 20)

Monsoon Surge To Fuel Scattered Storms For A Multi-Day Stretch; Elevated Flood Threat But Spatial Extent Uncertain

Monsoonal moisture looks to enter the state in full by Friday, with scattered to possibly numerous storms expected at least for the preferred southwest part of the state. The degree of moisture availability will determine the northward and eastward spread of rainfall chances by Saturday and especially Sunday/Monday. At this time, each day of this 4-day period looks to have the possibility of 1.0 per hour or greater rainfall rates, which warrants an Elevated Flood Threat for mainly the western portion of the state. Should this materialize, it will be great news for the area given the early snow meltout and serious concerns about an active wildfire season.

Issue Date: Monday, June 6th, 2022
Issue Time: 3PM MDT
Valid Dates: 6/7-6/21

As you have probably heard us say many times, moisture plays a surprisingly large role in dictating our heavy rainfall chances here in Colorado. As shown in the forecast GEFS PW plumes, below, over the next 5 days or so, there will be abundant moisture in place over central and eastern Colorado. PW is expected to range in the 0.7 – 1.2 inch range over the eastern half of the state, which is significantly above normal for early June. An Elevated flood threat is expected, mainly for eastern Colorado, through at least Thursday (Event #1).

Although we have a good grasp on moisture remaining above normal, the forecast for the atmosphere’s circulation is unfortunately another matter. As shown in this afternoon’s water vapor image, below, while there is a trail of disturbances across the Pacific Ocean, the steering flow is weaker than in recent weeks. This is consistent with summertime dynamics, as we are entering that time of year where the jet stream begins weakening and moving further north. Thus, the strong trough of low pressure over the Gulf of Alaska is actually expected to remain rather stationary over the next 3 – 5 days. Instead, Event #1 will be more dictated by subtler features within the flow over the Rocky Mountains. With plenty of instability daily, at least isolated heavy rainfall and possibly severe weather is a good bet with Event #1.

After Event #1, a few weak fronts are expected to skirt by the eastern border (Event #2), although any meaningful rainfall is expected to stay east of our border. Thereafter, a strong ridge is expected to build over west-central North America, putting Colorado in a favorable position for above normal temperatures. Possibly, much above normal. The prospects of precipitation will depend squarely on moisture availability. With sufficient moisture, our topographically-induced daily circulation can drive its own storm activity. However, at this time, moisture unfortunately looks to be marginal, and meaningful precipitation is not expected through the end of this Outlook at this time.

Before going into the precipitation events, a quick look at the snowpack. Snowmelt has essentially ended for central and southern Colorado. For the North Platte and northern South Platte basins, there is still pockets of high SWE remaining. Melt rates are expected to hover near normal through this week. Melt rates may increase next week with the expected heat wave, but it does not appear that there will be enough remaining SWE to warrant any flooding concerns. Most of the state is expected to be snow free by the end of next week, about 1-2 weeks sooner than normal.

The identified precipitation events are described in more detail below.

Event #1: Tuesday – Thursday (June 7 – June 9)

Elevated Flood Threat, along with severe weather, mainly for southeast Colorado

With plenty of moisture available, daily rounds of scattered storm activity is expected over the eastern Plains. On Tuesday, activity should remain focused along and south of I-70 where max 1-hour rainfall up to 2.0 inches is possible. There is a potential for storm training on Tuesday, with max 3 hour rainfall up to 4 inches possible where storm training occurs. Additionally, with favorable shear profiles, large hail and damaging winds gusts are expected to accompany the strongest storms. A tornado threat will also exist, though mainly along the KS border.

By Wednesday and Thursday, a cool front passage will generally suppress activity over the Northeast Plains. Instead, the focus will shift towards the Southeast Plains, Palmer Ridge and Raton Ridge where adequate instability will fuel scattered storms and support hourly rainfall intensity up to 1.5 inches. Severe weather chances look to diminish somewhat, although the instability will be high enough to warrant at least a slight threat of large hail and possibly damaging. Overall, an Elevated flood threat is expected through this event.

Event #2: Saturday – Sunday (June 11 – June 12)

No Apparent Flood Threat as storms expected to graze Colorado’s eastern border

Western Colorado will begin to dry out and heat up, while a couple of weak cool fronts are possible over the eastern Plains. With moisture generally dropping and ridging overhead favoring subsidence, storm coverage is expected to stay in the isolated to widely scattered range. However, some storms could skirt by the far eastern counties especially over the Northeast Plains. At this time, the flood threat associated with these storms is expected to remain on the Kansas side of the border, and with rainfall expected to stay below 0.5 inches, no precipitation map is warranted.

Issue Date: Thursday, June 2nd, 2022
Issue Time: 3PM MDT
Valid Dates: 6/3 – 6/17

Quite the wave train is shown in the water vapor imagery below, which will translate to a very active rainfall pattern for this outlook, as mentioned on Tuesday. The Low pressure, shown below, will shift slightly east today and spin off the NW coast over the next several days. This is expected to release several shortwaves of various strengths into the westerly flow, which will help produce some stronger thunderstorms and drop a couple cold fronts across the state. Event #1 begins tomorrow with afternoon and evening thunderstorms forecast for the eastern plains. There is a shot for an isolated thunderstorm or two on Saturday afternoon/evening over the far eastern plains, but otherwise it should remain dry to start the weekend. As the upper Low continues to spin over the same area into next week, a larger and slower moving shortwave will be ejected. In tandem with slightly better dynamics, an Elevated flood threat has been issued for Event #2. Finally, by Thursday (start of Event #3), northwesterly flow aloft will return when a strong ridge begins to build over the Great Basin. This setup tends to send plenty of weak disturbances across eastern Colorado with on and off rainfall, but at this time there is No Apparent flood threat.

After a quiet April and even May (until the last week or so), it looks like June is going to start off much more active. PW over Denver (left) remains at or above average minus a small dip on Friday night into Saturday morning. While there is still some disagreement between ensemble members (gray lines), it is likely PW will remain elevated into next week, which is another reason that Event #2 received an Elevated flood threat. Even over western Colorado, there is a fairly sharp increase in available moisture as the first shortwave moves across the state, and a cold front helps keep that moisture elevated through mid-week. It’s important to note that there is a strong north to south gradient (high to low) with the moisture return, which indicates that storms will likely be confined to central and northern western Colorado during Event #2. On and off fire danger is expected to continue over the southwest corner through this weekend, although the lack of stronger upper-level winds over the state should make it a less widespread critical fire threat.

Streamflow

Forecasts and observations indicate that the large releases from the Flaming Gorge Reservoir have ended, so expect flows along the Green River to return to normal levels over the next day or two. Outside of a couple isolated events over the next week or so, rainfall isn’t expected over the mountains, so there will be some time for some melt out of the snowpack up north. Not looking at any prolonged period of well above average temperatures over the next week, so this should help temper the melting and subsequent runoff. With the base flows currently on the lower end across northern Colorado, there are not any streamflow highlights for this FTO.

Event #1: Friday – Sunday (6/3 – 6/5)

Elevated flood threat for Friday with severe weather possible out east.

Lee troughing tomorrow and south/southeast surface winds should help pull ample moisture north over eastern Colorado. Storms are expected to initiate over the eastern mountains and elevated ridges by the afternoon. As they move east into this deeper moisture, paired with slower steering flow, the potential for localized flooding issues increase, so an Elevated flood threat has been issued. CAPE, shear and helicity values indicate a couple severe storms could be possible over the plains and eastern Palmer Ridge. These stronger storms may produce large hail, strong outflow winds and perhaps a weak tornado. It’ll be a bit calmer on Saturday behind the passing shortwave, but a stronger storm may clip the far Northeast and Southeast Plains. Lower confidence in this solution, but we’ll be tracking this in the FTB so be sure to check back. Weaker and more isolated storms are forecast over the eastern plains on Sunday, so there is No Apparent flood threat to round out this event.

Event #2: Monday – Wednesday (6/6 – 6/8)

Elevated flood threat as a slower and strong shortwave with aid from upper-level dynamics moves through the state.

It likely won’t be a 3-day period with an Elevated flood threat, but just want to mark a much stronger shortwave and jet moving overhead during this period. These elements should increase the coverage of rainfall each day with measurable rain likely returning to the central and northern mountains as well. Heaviest rainfall is still expected over the far eastern plains, but there’s quite a bit of uncertainty in the location and timing. Depending on when the mid-level energy and an accompanying cold front look to drop through the state (Tuesday into Wednesday), high rain rates and severe weather maybe be possible along Kansas/Colorado border.

Event #3: Thursday – Tuesday (6/9 – 6/14)

No Apparent flood threat as upper-level flow becomes northwesterly.
Bit far out for accurate details, which is why no map has been drawn, but it looks like eastern Colorado will be under the influence of northwesterly flow aloft beginning on Thursday. Through the weekend, a strong ridge begins to build over the Great Basin. This pattern will likely push some shortwaves and cold fronts across the state, which may help initiate some PM thunderstorms. Unsure about moisture during this period, so there will likely be on and off precipitation. The GEFS has also been recently hinting at a large subtropical moisture surge by mid-month, so we’ll be keeping our eye on this as well.

Issue Date: Tuesday, May 31^st, 2022
Issue Time: 3PM MDT
Valid Dates: 6/1 – 6/15

The atmosphere is technically a fluid, and in today’s water vapor image, below, that is certainly on display. Much like waves in the ocean, a pronounced wavy pattern is seen in the middle and upper atmosphere over the eastern Pacific Ocean. The strong elongated Pacific jet has briefly become dormant, but will reappear in the next 3-5 days. The overall result will be to keep sending disturbances and moisture into our state, which is a welcome change from the alarmingly dry conditions that most of the state endured during April.

In this Outlook, we have identified a rare four separate precipitation events, all of which will mainly affect areas east of the Continental Divide. Event #1 will be a minor event on Wednesday for mainly the Northeast Plains (tail end of today’s widespread precipitation). Event #2 will arise from a couple of small disturbances that look to break off from a large-scale trough over the Gulf of Alaska. However, with plenty of solar insolation, isolated to scattered storms are possible mainly over the Southeast Plains. Isolated heavy rainfall will be likely both Thursday and Friday, though coverage will likely be higher on Thursday. In addition, there will likely be a severe weather threat with the stronger storms. After brief and subtle drying, a strong, moist Pacific cold front will usher in much above normal moisture and rainfall potential (Event #3). This event looks to be the most notable of the four, both from the perspective of heavy rainfall potential as well as overall precipitation totals. Fortunately, the relatively dry eastern Plains look to experience some widespread moderate to heavy rainfall right in time for their growing season to really begin. An Elevated Flood Threat is currently warranted, but hopefully rainfall will be spread out enough in time to curb significant (i.e. riverine) flooding concerns. Finally, after another very brief break, above normal moisture looks to keep precipitation chances elevated heading into next weekend (Event #4), though details on location are still highly uncertain.

To close out today’s discussion, let’s take a look at the snowpack conditions across the state. Snowmelt is its waning stages over most of southern and south-central Colorado. But with below normal temperatures and some precipitation, the snowpack has held up very well over remaining northern and central areas. As suggested in the previous Outlook (5/26), the average SWE over the North Platte/Laramie and South Platte basins has actually gone back up above its normal value for the end of May. The North Platte/Laramie average SWE trace is shown below, along with a range of projections that encompass the future change in SWE from the last ~40 years of data. With the cool and wet/snowy conditions outlined in this Outlook, it is quite possible that a week from today, the snowpack will actually grow in the North Platte/Laramie and South Platte basins! Looking at historical percentiles, this will begin to place us in the 70^th percentile if not higher. Little to no significant melt is expected over at least the next week. Thereafter, we will certainly have to keep a close eye on temperatures as the chances of high runoff can rapidly increase with even slightly above average June heat.

The identified precipitation events are described in more detail below.

Event #1: Wednesday (June 1)

Light To Moderate Precipitation Ending Over The Northeast Plains; No Apparent Flood Threat

Very cool temperatures will be accompanied by light to moderate rainfall through at least mid-day Wednesday mainly over the Northeast Plains. Max 1-hour rain rates up to 0.3 inches are possible and up to 0.5 inches of rainfall could occur over isolated parts of the region, but flooding is not expected.

Event #2: Thursday – Friday (June 2 – June 3)

Residual Moisture To Fuel Isolated/Scattered Storms; Low-End Elevated Flood Threat

Although upper-level ridging and subsidence will be present on Thursday and Friday, residual boundary layer moisture will support instability up to 1,200 J/kg and fuel isolated to widely scattered thunderstorms over the Southeast Mountains, Raton Ridge and Southeast Plains. Max 1-hour rain rates up to 1.25 inches are possible, as well as severe weather in the form of large hail and gusty winds. A low-end Elevated flood threat looks warranted especially for the Raton Ridge area where flash flood guidance is closer to 1.0 inch per hour over the steeper terrain.

On Friday, isolated heavy-rainfall producing storms (possibly severe) could additionally graze the far northeast portion of the Northeast Plains. A flood threat appears unlikely at this time, as the heavy rainfall should stay just east of our border.

Event #3: Sunday – Wednesday (June 5 – June 8)

Early Elevated Threat Over the Northeast/Southeast Plains, Then Cooler

Isolated to scattered storms are first expected on Sunday towards the KS border, with max 1-hour rain rates up to 1.6 inches possible. By Monday, a strong cold front will move south into Colorado. Pre-frontal convection could support a 3-6 hour period of heavy rainfall over mainly the eastern parts of the Northeast and Southeast Plains. The heavy rainfall threat further west appears limited, at least for now, though at least some light to moderate precipitation is expected west of I-25 and perhaps even west of the Continental Divide.

At this time, max 1-hour rain rates up to 2.0 inches look possible over eastern areas, though the bigger threat appears to be the 3 hour totals of up to 3.3 inches. Over parts of that terrain, these expected intensities are just above flash flood guidance so an Elevated flood threat is warranted.

By Tuesday and possibly continuing into Wednesday, cooler weather will overtake all of the state along with more stratiform precipitation of up to 1.0 inches possible over eastern Colorado. There is currently No Apparent flood threat by Tuesday and into Wednesday.

Event #4: Friday – Sunday (June 11 – June 13)

Moisture And Precipitation Chances Return, But Where Is The Big Question; No Apparent Flood Threat

Above normal PW values look to return quickly after Event #3, though the atmospheric circulation is uncertain at this time. Isolated to widely scattered showers and thunderstorms look most favorable over the eastern Plains for now. The next Outlook should provide more details on this.