Area Forecast Discussion

KEY MESSAGES

- Cool today (Saturday) with a few light, low-impact rain and snow showers, then dry with a strong warming trend Sunday through Wednesday with highs approaching 90 degrees by Wednesday across most South Dakota counties.

- Multi-day stretch of near-critical to critical fire weather conditions today through at least Wednesday due to dry fuels, low relative humidities, and gusty winds; RED FLAG WARNING today for far southwest South Dakota.

- An unsettled pattern returns for mid to late week into next weekend, with the first storm system potentially bringing showers and thunderstorms starting as early as Wednesday evening.

UPDATE

Issued at 933 PM MDT Sat Apr 18 2026

No significant changes to the previous forecast for the evening package. High pressure will traverse the region Sunday. Expect slightly breezy ESE winds over western SD with warmer temps into the 50s and 60s.

DISCUSSION

(This Evening Through Saturday) Issued at 245 PM MDT Sat Apr 18 2026

On the synoptic scale...northeastern Wyoming and western South Dakota look to reside on the back side of a departing trough today (Saturday), with fairly strong, deep-layer northwest flow supporting diurnally breezy conditions across many of our plains areas (gusts of 30-45 MPH), and enough instability (impressive cold pool aloft) to support a few light, low-impact, rain and snow showers, currently in-progress on radar/satellite. We increased and broadbrushed PoPs in our forecast through this evening over the NBM blend based on radar trends (some of the CAMs are not handling today's activity well at all and provided little value), but activity will remain somewhat light and cellular with no support in the guidance for more than a couple hundredths of QPF. A longwave ridge of high pressure is then expected to move in and slowly shift eastward Sunday through Wednesday, supporting dry conditions and a strong warming trend. High temperatures are likely to exceed 80 degrees across portions of our South Dakota plains by Monday, approaching 90 degrees across most South Dakota counties by Wednesday! Currently, the probability of exceeding 90 degrees is 20-30% Wednesday afternoon from just east of Rapid City, over to Pierre and Chamberlain, and down to the Nebraska border...supported by strong SW flow. Multiple days of near- critical to critical fire weather conditions will result from this setup...more on that in the FIRE WEATHER section below. A few disturbances look to ride over this ridge, but this storm track will remain too far north to affect us here locally. Agreement between the various models in the evolution of the ridge is high, as also supported by 500mb height ensemble cluster analysis.

By Wednesday, a large "bowling ball" of a low pressure system will be crashing onshore across the western states and will quickly track east and cross the Rockies, bringing our next chance of precipitation as early as late Wednesday afternoon or Wednesday evening. Given the time of year, hot temperatures in place, and even a slight negative-tilt to the overall longwave troughing associated with this incoming low, hard to believe there won't be a solid chance of thunderstorms with this as well (despite NBM probabilities not currently supporting it). An early glance at long-range GFS model forecast soundings currently reveals strong inverted-V profiles (up to above 600mb) with very steep low- to mid-level lapse rates and strong DCAPE, along with potentially adequate mid-level moisture, supportive of a risk of dry-leaning thunderstorms with gusty winds. Still a hint of "split flow" with this overall incoming trough, although as of this morning, all deterministic models maintain a strong southern branch/component with a closed low pressure center. Potential for meaningful QPF seems low through Wednesday night. This main low then looks to slow down and meander around the northern Plains/southern Canada from Thursday into next weekend as several other embedded waves/disturbances rotate around it. Unsurprisingly, agreement in cluster analysis starts to really break down around this time, with most attempted "clusters" initially just expanding the general troughing signature to encompass most of the western and central US, then diverging into a variety of solutions with EOF variance (uncertainty) split between both timing and amplitude...but of which exact feature/disturbance? Lots of details yet to be resolved. Broadly speaking, this should spell cooler temperatures, periods of breezy conditions, and unsettled weather with daily chances of precipitation. The NBM seems to capture all of these trends well for now. In particular, Thursday looks like it could be quite windy out of the northwest (tight pressure gradient associated with the main low, assuming track/timing of that lead feature doesn't change too much between now and then). It's actually pretty unusual for the NBM to come in as windy as it is this far in advance especially given some of it's model inputs at that timeframe (although the flow aloft doesn't look extreme. Stay tuned. Also Thursday...1 height ensemble cluster (about 30% of the ensemble space, GFS-heavy) leans drier than the mean, but 3 clusters (about 70% of the ensemble space) lean wetter than the mean with over 0.25 inches of QPF across at least northern portions of our CWA. So for now...watch Thursday for wind and the best chance of some moisture to provide some relief from fire risk.

AVIATION

(For the 18Z TAFS Through 18Z Monday) Issued At 1127 AM MDT Sun Apr 19 2026

VFR conditions are expected at both TAF sites throughout the forecast period. Winds will diminish through tonight as a high pressure builds over our area.

FIRE WEATHER

Issued At 245 PM MDT Sat Apr 18 2026

We're sorry to say, we don't have good news to share on the fire weather side of things. A strong warming and drying trend is forecast from today right through Wednesday. The resultant combination of dry fuels, low relative humidities (trending lower day by day), periodically gusty winds, and eventually very warm temperatures will support a multi-day stretch of near-critical to critical fire weather conditions. Today winds will be the strongest regionwide while RH values will be the most marginal. An overlap of 15-20% minimum RHs with wind gusts to 45 MPH looks to exist this afternoon across the Fall River County and Pine Ridge areas of far southwestern South Dakota, where a RED FLAG WARNING remains in effect until 8 PM. Winds have already easily materialized while RH is lagging a bit per sfc obs, but confidence certainly isn't there to cancel the warning as we continue to watch trends this afternoon. Sunday, RHs continue drying to 10-18% across northeastern Wyoming and southwestern South Dakota, but winds slacken with the best chance of gusts approaching 25 MPH shifting into the Plains east and northeast of the Black Hills...it might be enough of a disconnect between the two elements to give as one cautious "breather" day with no warnings. Monday through Wednesday...nearly the entire CWA will be critical for RHs with large swaths of wind gusts exceeding 25 MPH, so additional RED FLAG WARNINGS are looking likely. Our next chance of rain showers and thunderstorms arrives late Wednesday afternoon or evening, but wetting rains do not appear possible over any decent areal coverage until more significant moisture POTENTIALLY arrives Thursday. In fact, an early look at long-range model forecast soundings suggests any storms Wednesday afternoon/evening could lean quite dry with lightning and gusty wind risks, which will only exacerbate the fire risk.

UNR WATCHES / WARNINGS / ADVISORIES

SD...Red Flag Warning from 11 AM MDT /noon CDT/ to 8 PM MDT /9 PM CDT/ Monday for SDZ321-322-325-326-332>335. WY...Red Flag Warning from 11 AM to 8 PM MDT Monday for WYZ315-317.