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FXUS65 KBOU 020005 AFDBOU

Area Forecast Discussion National Weather Service Denver/Boulder CO 605 PM MDT Wed May 1 2024

KEY MESSAGES

- Scattered showers and a few thunderstorms this afternoon and evening, with light snow showers across the mountains.

- An isolated strong to severe thunderstorm or two remains possible across far eastern Colorado this evening

- Another round of showers and storms late Friday/early Saturday.

- Warmer and breezy over the weekend.

SHORT TERM /Through Thursday/
Issued at 253 PM MDT Wed May 1 2024

The forecast is still going according to plan this afternoon. The surface cold front has advanced into the I-25 corridor over the past hour or so, and should continue to sink southeastward through the evening hours. Meanwhile, there's been a decent burst of rain and snow showers across the high country, associated with the mid-level front with some minor snow accumulations across the higher elevations.

As the better lift arrives and the low-level wind gradually turns to a more favorable upslope component, we should see widespread showers and a few thunderstorms develop across the urban corridor and plains. There's still a chance of a stronger/severe storm or two out in the far eastern plains where modest instability exists... but chances of this remain low.

Thursday will be mostly dry, with cool northwesterly flow aloft.
Warmer air does try to nudge back into the region by the afternoon, but high temperatures are expected to stall out in the low 60s across the plains. There's just enough moisture for a few weak showers during the afternoon, but impacts from these would be minimal.

LONG TERM /Thursday night through Wednesday/
Issued at 253 PM MDT Wed May 1 2024

A broad trough continues to move east across the northern Plains early Friday, with a short wave trough broken off of the main trough well to the west over Idaho. Colorado will be under pretty strong west flow aloft. There is some moisture in the flow aloft, and combined with relatively steep lapse rates, a few snow showers should occur along and north of I-70 as soon as midnight Friday morning. The better chance of precipitation across much of our area occurs when the lingering, broken off part of the trough swings across the CO/WY border Friday afternoon and evening.
There is a fair amount of QG forcing associated with the trough that precipitation is likely (>50%) for much of the northern third of Colorado. In addition to the QG lift, a robust cold front is expected to move across the plains sometime in the afternoon.
There will be some instability (300-700 J/kg) to support showers and thunderstorms along and behind the front during the afternoon.
700 mb temps don't drop well below 0 deg until the cold front sweeps across the area, meaning snow levels throughout the day should remain above 8,000 ft MSL, dropping down to 7,000 ft MSL early Saturday morning. However, significant subsidence on the backside of the fast moving trough will spread over the area after midnight, greatly dropping PoPs Saturday morning across all but the west facing slopes of the northern mountains. The air on the backside of the trough is also dry, so by sunrise all precipitationacross our area should be over. QPF amounts are not overly impressive mountains or plains, though can't rule out a few slick spots across the highest, less traveled mountain passes.
Before the cold front arrives temperatures should warm into the mid 60s across the plains, upper 40s to lower 50s across the mountain valleys.

Saturday should be dry across most of the area as a ridge builds in from the west ahead of a strong/deep trough off the West Coast.
Afternoon convective clouds are expected to form over the mountains and move east, but measurable precip is unlikely other than across the higher ridges and east slope foothills. The winds at and above 10 kft MSL are SSW, not great for upslope in our mountains. An inch or two is possible above 9,000 ft with no travel impacts anticipated on Saturday.

Sunday through Tuesday our weather will be dominated by a long wave trough, with embedded shortwave troughs, in place to our north.
Details are quite uncertain this far out, but at a high level it looks like the main trough will pass over Colorado on Monday, with the best lift remaining north of us across Wyoming. The main trough axis looks to move across the state from west to east during the latter half of Monday. Ahead of the trough axis, scattered afternoon showers and thunderstorms are possible Sunday evening to kick of the chances of precipitation associated with the main trough. In the southwest flow, snow levels will start out very high, at or above 10 kft MSL Sunday afternoon. At some point early Monday after the passage of the trough axis, a cold front is reasonably likely to move across the area as well, lowering snow levels to about 7500 ft MSL. However, there is a strong signal for strong subsidence post-trough axis passage the last half of Monday, which would result in strong winds across most of the area. So the impacts from this broad trough, and embedded short wave troughs are uncertain. A few things that stick out: -> Severe potential Sunday. There will likely be a dry line that sets up along or east of an Akron to Limon line. East of the dryline strong moisture advection with southeast winds will result in strong instability (SBCAPE > 1500 J/kg), and with 50+ kts of 0-6 km shear, supercells would be the dominant storm type with any storms that can break the cap. The best chance for initiation would be along the northern tier of counties nearest the QG lift and cold pool aloft.
-> Winds/Fire Wx. A strong pressure gradient across the plains will result in strong SE winds Sunday afternoon, gusting 35-40 mph, but the air will be moist enough to preclude the need for fire wx concerns. Winds should be less in the foothills and mountains until winds aloft turn more west late Sunday. With increasing west flow aloft, and pre-trough passage, ingredients are in place for a mountain wave enhanced flow late Sunday into early Monday, with 70 kts flow are ridge top and a decent inversion in place. The unfavorable thing is flow will be southwest. In this kind of set up South Park may see the strongest winds early Monday. After trough passage around the middle of Monday, strong subsidence will drive winds toward the surface across the area. Winds across the area should be out of the west, and gusting 20-35 mph across much of the area. Tuesday will likely see a repeat of elevated west-northwest winds. With deep downslope flow east of the Divide, conditions will be dry, probably in between 15-20% across most of the plains.

Overall will keep the NBM forecast as is in the Sunday-Tuesday timeframe given the uncertainty in timing of embedded shortwaves.
The trends look okay, i.e. the warmest day by far will be Sunday with temps approaching 80 across the plains and urban corridor, followed by seasonal highs in the 60s Mon-Wed given the proximity to the cool air aloft with the broad trough to the north. PoPs will remain near climatology given dry downslope flow east of the Divide, but >50% each afternoon for the mountains given the persistent west flow and steep lapse rates. So far though, this does not yet look like any significant amounts of precipitation (rain or snow, looking like a few tenths of liquid) and available ensemble output from the EC and GEFS agrees.

AVIATION /00Z TAFS through 00Z Thursday/
Issued at 605 PM MDT Wed May 1 2024

VFR will prevail through the TAF period, although there will be a period of lower ceilings around 5000-6000 feet AGL in the Denver area TAF sites with scattered showers this evening. Those ceilings will then lift and break closer to 06Z.

Winds are currently from the northeast, and should generally remain that way through about 06Z although scattered showers could still interrupt this wind regime. Eventually we expect to settle to more of an east/southeast component that will last through Thursday, leaning more east/northeast after 18-20Z. Gusts up to 20 knots possible this evening and then again Thursday afternoon.

BOU WATCHES/WARNINGS/ADVISORIES
None.

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