Area Forecast Discussion

KEY MESSAGES

- Widespread critical fire weather conditions expected Wednesday afternoon.

- Scattered thunderstorms over south central South Dakota Wednesday afternoon and evening may produce strong wind gusts.

- Cool, breezy unsettled weather expected Thursday through the weekend.

UPDATE

Issued at 938 PM MDT Tue Apr 21 2026

Previous forecast largely on track. Upper trough/low complex lifts through the northern Rockies into the plains over the next couple of days. It will drive a dry-line/trough through the CWA tomorrow with gusty southerly winds ahead of it and very dry gusty westerly downslope flow behind it. Temperatures well into the 80s to mid 90s will combine with the dry air to produce some single digit MinRHes. Recipe for large fire growth if ignition occurs. Given forecast very deep boundary layer mixing along the dry- line/trough, enough CAPE (500+J/kg) will be present for some TS, some of which will contain strong, gusty winds (marginal risk of severe) and potential dry lightning. Red Flag Warning looks good. Secondary wave rotates through the region for Thursday night/Friday potentially bringing a stripe of sustained isentropic lift/frontogenesis. Wet bulb zero heights suggests snow for the higher elevations of the CWA. How much is hard to say at this point given wide spread in ensemble guidance, but we'll take any QPF in any form to assist green-up this spring.

DISCUSSION

(This Evening Through Tuesday) Issued at 110 PM MDT Tue Apr 21 2026

Water vapor analysis shows upper ridging across the central CONUS, while a large stacked low just offshore the CA/OR coast slowly moves eastward. At the surface...departing surface high is bringing S/SE return flow to western SD. Temps are rising through the 70s into the 80s across the plains. Quiet weather is expected for the rest of today and tonight, with overnight lows in the 40s/50s. The large Pacific system will slowly continue eastward, eventually affecting our area for the second half of the week. The upper low is progged to break up through Thursday as a second wave digs down the backside of the trough toward Friday and the weekend. With lots of changing dynamics, there is low confidence and consistency with many details/forecast elements over the next several days. Backing up to Wednesday, warm air advection will continue overnight through the daytime hours tomorrow bringing very warm air to the region. Temperatures across the western/central SD plains are expected to reach the 90s...mainly east of the Black Hills. Some locations could even see temps creep into the mid 90s, which would be approaching daily (and even monthly) records. Southerly flow will persist across central SD, with dewpoints in the low 40s. Then models show a dryline pushing into SW SD during the afternoon with SW winds behind it. The main non-fire weather concern tomorrow will be the development of thunderstorms along this line. Some storms may initiate somewhere in the SCentral SD vicinity in the later afternoon/early evening, then ramp up a little as a cold front pushes in from the west later Wed evening (after 03z). With how dry the boundary layer will be, LCLs will be well above 10kft. Instability above that level, however, will be limited to 500-800 J/kg. Hail is not expected to be much of a concern, but inverted V soundings suggest strong wind gusts will be possible. Then a second surge of (elevated) convection with the front itself could bring better chances for stronger winds, especially as the lower atmosphere mixes with the cold air advection. Will keep an eye on this. For Thursday, models continue to back off QPF amounts and winds as the system trends farther north and the CWA sits within the dry slot. Snow amounts across the Black Hills late Thursday/Friday should be limited to an inch. Winds on Thursday have dropped to within advisory criteria, but this could be further decreased with future runs. Mostly cloudy skies, cooler temperatures, and chances of rain/snow continue through the rest of the week into the weekend as the secondary shortwave digs down the backside of trough, keeping it over the region.

AVIATION

(For the 06Z TAFS Through 06Z Wednesday Night) Issued At 1037 PM MDT Tue Apr 21 2026

VFR conditions are expected through the TAF period. Gusty southwest winds will develop across NE WY tonight, spreading eastward overnight as a trough pushes through. Behind the trough, gusty westerly winds are expected Wednesday with wind gusts up to 30-40kts. Ahead of the trough, isolated/scattered TS may develop after 22/21z over south- central SD with erratic gusty winds.

FIRE WEATHER

Issued At 110 PM MDT Tue Apr 21 2026

Widespread critical fire weather conditions are expected for Wednesday. Temperatures will climb into the 80s to 90s across much of western SD, with humidities as low as 10%. Southerly winds will also pick up across the region ahead of an approaching cold front (gusts of 30-45 mph). Thunderstorms are possible Wednesday afternoon. These storms aren't expected to produce much rain but will be capable of lightning and strong, erratic wind gusts of 50+ mph.

A cold front will cross the region early Thursday bringing much cooler temps, rising humidities, and breezy northwest winds (gusts of up to 50 mph across NW SD).

UNR WATCHES / WARNINGS / ADVISORIES

SD...Red Flag Warning from 10 AM MDT /11 AM CDT/ to 8 PM MDT /9 PM CDT/ Wednesday for SDZ319>335. WY...Red Flag Warning from 10 AM to 8 PM MDT Wednesday for WYZ314>318.