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FXUS65 KBOU 200552 AFDBOU

Area Forecast Discussion National Weather Service Denver/Boulder CO 1152 PM MDT Sun May 19 2024

KEY MESSAGES

- Strong to severe storms are expected Monday afternoon and evening across most of the eastern plains.

- 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

UPDATE
Issued at 902 PM MDT Sun May 19 2024

Minor convection continues to decrease across the CWA this evening. Most of the plains and lower foothills have northerly winds in place currently.

Models continue to show a decent wind shift after midnight and a cooler airmass moving into the CWA Monday's highs over the plains should be 2-4 C cooler than today's highs were. As far as tonight's updates go, will make a few adjustments to winds and sky cover grids overnight.

SHORT TERM /Through Monday/
Issued at 300 PM MDT Sun May 19 2024

Scattered storms, a couple severe with one already having a landspout, were moving east across the plains. Surface analysis shows a ribbon of 1500-2000 J/kg MLCAPE extending across areas east of a Sterling to Akron line where dewpoints were in the mid 50s. There was some CIN noted farther east, but that is eroding and therefore the existing storms should continue to strengthen or become more numerous as the dryline works eastward. There could even be a lone severe storm in Lincoln County where MLCAPE is holding close to 1000 J/kg, and the dryline progression has been slower than expected.

Most of the storms will diminish or move east of the area by 6 pm.
That occurs as weak subsidence is noted in the QG fields, and indeed, the convection around Denver and points west and southwest over the mountains is quite shallow. Thus, mostly dry conditions are expected after dark. A cold front then arrives overnight with a push of north/northeast winds back into the Front Range. This should set the stage for a more active weather day on Monday.

Low level moisture increases back toward the Front Range on Monday, and this should allow MLCAPE to increase to near 1000 J/kg. It will be a little cooler, and the amount of stratus (if any) could play into destabilization. There is also considerable uncertainty with regard to the frontal location and where it could stall out - adding to the necessary lift required for convective initiation.
Forecaster experience would say it aligns closer to where the NAM suggests, stretching out across the Palmer Divide/South metro eastward onto the adjacent plains. Hodograph curvature is impressive along this boundary, with potential surface-1km helicity near 100-150 m2/s2 and surface-3km over 350 m2/s2! But....that's just one model and others are not nearly as strong and updraft helicity is limited per some of the HRRR members.
Instability could also be limited to less than 1000 j/kg. We coordinated with SPC on the Day 2 outlook, bringing the Slight Risk area into northeast Colorado and Marginal back to the I-25 Corridor. Depending on how the instability and shear profiles appear, further adjustments may occur. Stay tuned for the latest updates. Large hail, damaging winds, and tornadoes would be possible should the higher end instability/shear profiles develop.

LONG TERM /Monday night through Sunday/
Issued at 300 PM MDT Sun May 19 2024

The threat of severe weather will carry into Monday evening and night as storms push east into the plains. Large hail, wind gusts up to 60 mph, a possible tornado, and brief heavy rainfall could occur until 11 PM MDT. Late Monday night, overcast skies and a lack of instability will likely lead to stratiform rain across the region. The threat of flooding should remain low due to rainfall intensity becoming lighter and soil moisture values are slightly drier in the eastern plains. QPF fields display 0.50-1.50 inches total Tuesday along the foothills and plains. With rainfall continuing into early Wednesday morning, expect overcast skies and lower temperatures Tuesday afternoon and night. West of the Divide, light snow will occur for areas above 8k ft. Snowfall totals between 2-5 inches are possible with the highest totals mainly along the Park Range through Wednesday morning.

As this system exits, lingering moisture and weak MLCAPE values between up to 100 J/kg may lead to an isolated storm or two mainly near the WY/NE/CO border. Short-lived ridge pattern enters northeast Colorado starting Thursday evening which could limit severe weather. An isolated shower or two could occur along the foothills and plains but the likelihood of severe remains low. A front develops late Thursday which will slightly cool afternoon temperatures Friday through the weekend. Near normal temperatures are likely for northeast Colorado. It is possible an additional shortwave could bring additional moisture to the region each afternoon Friday through this weekend leading slight chance of showers and storms

AVIATION /06Z TAFS through 06Z Tuesday/
Issued at 1141 PM MDT Sun May 19 2024

Weak drainage winds are expected for the overnight hours, with a push of northerly winds forecast to arrive by around 10-12z. There could be a brief period of MVFR cigs (SCT-BKN 020-030) along with this push but this should be brief. Still not a clear signal for how widespread any AM cloud cover will be.

Winds should turn to the east through the day, with gradually increasing chances for thunderstorms by mid-afternoon. Most guidance keeps storms just east of I-25 but that's close enough to the terminals to continue with the PROB30s during the afternoon.
As a surface low moves off to the east, stronger northerly winds are anticipated by Monday evening. Expect more widespread stratus with this and several hours of MVFR to IFR ceilings Monday night.
There may also be a few showers around the terminals but confidence in that is low.

BOU WATCHES/WARNINGS/ADVISORIES
None.

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