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FXUS65 KBOU 182326 AFDBOU

Area Forecast Discussion National Weather Service Denver/Boulder CO 526 PM MDT Sat May 18 2024

KEY MESSAGES

- Isolated to scattered high based showers and storms continue into this evening with gusty outflow winds

- Slight chance of a severe storm or two northeast plains Sunday afternoon

- Cooler, wetter pattern late Monday into Tuesday as a couple weak systems track across the region

- Warming/drying trend for the second half of next week

SHORT TERM /Through Sunday/
Issued at 225 PM MDT Sat May 18 2024

Scattered high based showers and a couple storms have developed, a bit earlier than originally anticipated on the plains with one batch shifting to the east across the Palmer Divide. And that's despite the relatively cool/stable boundary layer in place across the lower elevations. That stability was evidenced in the latest ACARS soundings taking around KDEN, with near 100 J/kg of CIN still noted.

Behind that first wave of showers, there will still be additional chances of shower/storm development given sufficient moisture and cloud breaks for further solar insolation and destabilization.
We've already seen isolated gusts of 40-45 mph with the first showers, and similar rounds of gusty winds/light rain can be expected with any of the additional showers or storms that develop. Eventually later this evening, most of these will work to the east with the best QG forcing headed that way. By midnight, most of the precipitation is expected to have ended with weak subsidence building in and gradual stabilization.

On Sunday, the main concern will shift to the northeast plains.
That's where low level moisture is expected to increase with dewpoints pushing into the lower 50s in deeper moist southerly flow. At the same time, a dryline will organize in the stronger westerly flow pushing off the Front Range, and shift eastward in the late morning to mid afternoon hours. That feature along with weakening CIN during max heating should be enough to get a few stronger storms going, with the highest probability generally along and east of a line from Sterling to Akron. MLCAPE near 1500 J/kg are expected, along with sufficient deep layer shear profiles supporting mainly a large hail and wind threat. SPC's Slight Risk comes right up to the border, with a Marginal Risk farther west toward Sterling. This looks like a good starting point for now.

Meanwhile, the immediate Denver area looks drier Sunday behind the dry line, with mostly virga if anything. Farther to the north, there's still a decent mid level moisture plume - enough to keep isolated to scattered showers and high based storms in the forecast. This would include the northern tier of mountains, Fort Collins, and the northern border area on the adjacent plains.
Temperatures will warm back into the lower 80s for most off the plains considering the downslope flow and slight warm advection aloft.

LONG TERM /Sunday night through Saturday/
Issued at 225 PM MDT Sat May 18 2024

Main changes in the long-term forecast focus on Monday-Tuesday system precipitation chances across the mountains to the plains.
Starting Sunday night, lingering showers are possible mainly in the northern tier of counties. A positive tilted upper level trough enters Utah Monday. It appears there is a slight chance of scattered showers and storms west of the Divide and along the WY- CO border late Monday afternoon with model guidance favoring a swath of moisture extending to these areas before the trough axis enters Colorado. Although these areas are favored across guidance, this will critically depend on if the trough axis can deepen. If this outcome doesn't occur, showers and storms would likely be delayed until overnight Monday into Tuesday morning. As additional forcing enters the region, ensembles indicate hefty QPF amounts especially along the eastern plains Tuesday morning through late evening. NBM QPF fields between 0.30-1 inch seem reasonable across the foothills and plains. 700mb temperatures drop between 1C to -2C throughout the high country Tuesday afternoon. Snow levels will likely sit near 8500 ft. Given QPF fields displaying 0.30-0.60 inches Tuesday morning through Tuesday evening, light snowfall accumulations between 2-5 inches are possible for areas above 9 thousand feet.

By Wednesday, drier air enters the region but there is enough lingering moisture for an isolated shower or two in the foothills and plains late Wednesday afternoon. Afternoon highs remain generally warm Thursday through next weekend with isolated/scattered PoPs focused for the higher elevations.

AVIATION /00Z TAFS through 00Z Sunday/
Issued at 522 PM MDT Sat May 18 2024

Still will see a few higher based showers moving across through 7 pm with a brief period of gusty winds up to 40 mph possible.
Otherwise will see VFR conditions thru the period. Winds will remain from the south with gusts up to 30 mph until 02z. Overnight winds will remain south but become more SSW by 13z. Winds will then switch to SW by 17z and then more NW by 20Z.

BOU WATCHES/WARNINGS/ADVISORIES
None.

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