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FXUS65 KBOU 231129 AFDBOU

Area Forecast Discussion National Weather Service Denver/Boulder CO 529 AM MDT Tue Apr 23 2024

KEY MESSAGES

- Cooler today with a few showers/isolated storms, but then above normal temperatures again for Wednesday and Thursday.

- Stormier weather pattern in the works, starting late Thursday and peaking next weekend, with accumulating snowfall in the mountains.

- Stronger storms possible northeast plains Thursday afternoon/evening.

SHORT TERM /Through tonight/
Issued at 311 AM MDT Tue Apr 23 2024

Surface cold front pushed across northeast Colorado in these wee morning hours, with cooler air filtering in. The airmass is gradually drying behind the front, so despite the developing upslope component this morning it will be hard to develop much if any stratus. Farther west over the mountains and stretching into northern Utah, there was a modest amount of mid level moisture, cloudiness, and isolated light showers. Those may continue this morning with weak QG lift noted, and a few of these are now spreading onto the adjacent plains. There could be a little more sun this morning, and just enough to help us warm a couple degrees more than earlier anticipated - but that's still a good 10-15 degrees cooler than yesterday. That puts most of the plains and I-25 Corridor at near normal temperatures with highs mostly in the lower 60s. Over the mountains and high valleys, the airmass is relatively unchanged since yesterday, outside of the increase in moisture, so mild temperatures will continue until clouds and evaporative cooling become more widespread later in the day.

With daytime heating and destabilization, we expect scattered showers to develop over the high country by early afternoon, gradually spreading eastward onto the I-25 Corridor and northeast plains late this afternoon and evening. They will be relatively high based, but can't rule out a couple rumbles of thunder in/near the mountains this afternoon with MLCAPE near 200 J/kg.

There is weak QG forcing noted through the night, while the best mid level frontogenesis lifts northeastward overnight. Those features would favor the higher PoPs transitioning from the mountains this afternoon to the northern border area and northeast corner of the state later this evening into the early morning hours. Gradual clearing will occur after that, with low temperatures near or slightly above normal.

LONG TERM /Wednesday through Monday/
Issued at 311 AM MDT Tue Apr 23 2024

A ridge of high pressure will bring warmer temperatures back to the region on Wednesday. The flow aloft will be westerly Wednesday, then southwesterly Wednesday night as the ridge axis shift to the east. 700 mb temperatures will warm up to 8C on Wednesday, with breezy southerly winds across the plains. Max temperatures will range from the mid to upper 70s. Higher dewpoints in the low 40s will be possible over the far northeast plains. Model surface based CAPES of 200-400 j/kg support a slight chance of thunderstorms in the afternoon/evening mainly along the northern border. On Thursday, the flow aloft increases from the southwest with a next short wave trough expected to move across northeast CO either Thursday afternoon or Thursday evening as light to moderate mid/upper level QG ascent spreads across the forecast area. GFS surface based CAPES over the northeast plains are progged to be in 500-900 j/kg range which could produce one or two strong thunderstorms. Another warm day with highs again in the 70s. Southeasterly winds over the plains will help advect higher dewpoints into the northeast plains in the afternoon, which should result in a better chance of thunderstorms especially to the north and east of Denver. In the mountains, there will be fairly deep layer of moisture around with the passage of the trough with west/northwest ridge tops winds of 35 kts with the snow level dropping to around 8500 feet Thursday night. The moisture does appear to decrease overnight. The best chance of snow in the mountains appears to be late Thursday afternoon and Thursday night. There could a brief window of elevated fire weather conditions for eastern Elbert and Lincoln counties Thursday afternoon with min rh around 15 percent and wind gusts to 30 mph.

By midday Friday, the trough will shift into central NE and a short wave ridge will move into central CO. While there will still be snow showers in the high country, it should be lighter and primarily orographic with decreasing west/northwest ridgetop winds. Across the plains, shower activity will persist especially over the northeast plains where wrap around moisture associated with the exiting trough still impacting that area. A colder airmass with north/northeast post frontal winds will allow for showers to continue and develop further west near the foothills/Palmer Divide Friday afternoon/Friday night. Mid/upper level moisture will be increasing from the west ahead of the another approaching trough. Basically the models are still leaning towards a cool and unsettled period, with snow showers in the mountains, and rain showers across the plains. The most active day would appear to be Friday night and Saturday for the entire forecast area. By Sunday afternoon, maybe another period of lighter orographic snowfall in the mountains, with wrap around moisture impacting the northeast plains, as the next trough lifts to the northeast of CO. The best window for accumulating snowfall in the mountains will be from Friday Night into Sunday morning where maybe a Winter Weather Advisory may be necessary at some point. By Monday it appears to be drier and warmer as another synoptic scale ridge spreads across CO.

AVIATION /12Z TAFS through 12Z Wednesday/
Issued at 529 AM MDT Tue Apr 23 2024

VFR expected to prevail for all terminals this period. Another frontal surge moved through between 08Z and 09Z, but now winds are settling back to a northwesterly component and should hold through about 15Z. Then daytime heating/mixing would support a more northeast and eventual easterly wind to persist most of the 15Z-03Z period. There is still potential for enough low level moisture for mainly SCT clouds around 3,000 ft AGL, but the upstream observations are not very moist so the odds of a ceiling at that level are still quite small (20%).

We'll see scattered but mainly high based showers develop in the area late in the afternoon and evening (21Z-05Z), and there's enough chance to warrant a PROB30 in the TAFs. The ceilings are expected to hold mostly around 8,000 ft AGL, but with the scattered showers around local ceilings down to 5,000 ft are possible with Instrument Meteorological Conditions (IMC). The showers and potential cyclone or shear zone could also bring some variable winds - about a 30-40% chance of that 23Z-04Z. Then winds slowly transition to south/southwest 06Z-12Z, with clearing skies through that time.

BOU WATCHES/WARNINGS/ADVISORIES
None.

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