051
FXUS63 KUNR 182311
AFDUNR
AREA FORECAST DISCUSSION FOR WESTERN SD AND NORTHEASTERN WY
NATIONAL WEATHER SERVICE RAPID CITY SD
511 PM MDT SAT APR 18 2026
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.
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 00Z TAFS THROUGH 00Z SUNDAY EVENING)
ISSUED AT 508 PM MDT SAT APR 18 2026
BREEZY CONDITIONS WILL DIMINISH THIS EVENING LEADING TO LIGHT,
VARIABLE WINDS. VFR CONDITIONS WILL BE PRESENT THROUGH THE TAF
PERIOD.
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 UNTIL 8 PM MDT THIS EVENING FOR SDZ322-326.
WY...NONE.
DISCUSSION/KEY MESSAGES...KSMITH
AVIATION...SCHULTZ
FIRE WEATHER...KSMITH
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