574
FXUS63 KUNR 201749
AFDUNR
AREA FORECAST DISCUSSION FOR WESTERN SD AND NORTHEASTERN WY
NATIONAL WEATHER SERVICE RAPID CITY SD
1149 AM MDT TUE OCT 20 2020
DISCUSSION
(TODAY THROUGH MONDAY)
ISSUED AT 236 AM MDT TUE OCT 20 2020
UPPER AIR ANALYSIS AND WATER VAPOR IMAGERY SHOW A COMPACT SHORTWAVE
PIVOTING THROUGH THE FORECAST AREA WITH ANOTHER WAVE FURTHER
UPSTREAM OVER EASTERN MT. WARM AIR AND THETA-E ADVECTION AHEAD OF
THIS TROUGH CONTINUES TO SUPPORT SCATTERED RAIN/SNOW/WINTRY MIX
ACROSS THE NORTHERN HALF OF THE FORECAST AREA THIS MORNING. THE WAVE
LOCATED OVER EASTERN MT WILL QUICKLY PIVOT THROUGH THE REGION THIS
MORNING AND THE MID-LEVEL BAROCLINIC ZONE WILL SETTLE SOUTH AND EAST
OF THE AREA, ALLOWING FOR A LULL IN PRECIPITATION THIS AFTERNOON.
EXPECT TEMPERATURES FROM THE MID 30S IN THE BLACK HILLS TO LOW 40S
ACROSS NE WY AND NW SD WITH 50S SOUTH AND EAST OF RAPID CITY. A
QUICK SHOT OF COLD AIR ADVECTION COUPLED WITH MODEST PRESSURE RISES
BEHIND THE WAVE WILL RESULT IN BREEZY NW WINDS FROM NW SD AND EAST
OF THE BLACK HILLS. WILL HOLD OFF ON A WIND ADVISORY GIVEN MARGINAL,
SHORT-LIVED SETUP.
LOOKING EVEN FURTHER UPSTREAM, A STRONG SHORTWAVE IS LOCATED OVER
THE GULF OF ALASKA, EAST OF NORTHERN BRITISH COLUMBIA. THIS WAVE
WILL CONTINUE TO DIVE SOUTHEASTWARD, ARRIVING IN THE NORTHERN
ROCKIES/NORTHERN PLAINS OVERNIGHT WEDNESDAY INTO THURSDAY. WHILE
SOME MINOR DETAILS AS TO THE EXACT TRACK AND SPEED OF THE WAVE
REMAIN, OVERALL FAVORABLE DYNAMICS WILL BE IN PLACE FOR AREAS OF
MODERATE TO HEAVY SNOW, ESPECIALLY ALONG THE SD/ND BORDER. A VERY
STRONG JET (140+ KNOTS) WILL LIFT NORTHEAST OF THE FORECAST AREA BY
EARLY THURSDAY MORNING, RESULTING IN WIDESPREAD JET LEVEL DIVERGENCE
OVER MUCH OF THE NORTHERN PLAINS. A STRONG BAROCLINIC ZONE AND
ASSOCIATED FRONTOGENESIS WILL BE IN PLACE ACROSS FAR SOUTHERN ND AS
THE 600-700MB LOW FORMS OVER THE REGION. FAVORABLE LARGE-SCALE
ASCENT COUPLED WITH AREAS OF FOCUSED ASCENT ALONG THE AXIS OF FGEN
WILL RESULT IN AREAS OF A HEAVY SNOW, POSSIBLY PUSHING INTO HARDING
AND PERKINS COUNTIES WHERE AT LEAST SEVERAL INCHES OF SNOW IS
LIKELY. POTENTIAL FOR BANDING SNOW/HEAVIEST AMOUNTS WILL BE HIGHLY
DEPENDENT UPON THE EXACT LOCATION OF BEST FGEN; CERTAINLY A FACTOR
TO WATCH. EPS/GEFS PROBABILITIES ARE CONSISTENT PAINTING A FAIRLY
HIGH PROBABILITY (>50%) OF AT LEAST 3 INCHES OF SNOW ALONG AND JUST
SOUTH OF THE SD/ND BORDER THROUGH THURSDAY. ELSEWHERE, GIVEN THE
BROAD ASCENT, MOST OF THE FORECAST AREA WILL SEE SNOW, PARTICULARLY
ACROSS THE NORTHWESTERN 2/3RDS OF THE FORECAST AREA. A MIX OF RAIN
AND SNOW IS LIKELY ACROSS SOUTH-CENTRAL SD, LIMITING SNOWFALL
POTENTIAL.
LOOKING AHEAD TO THE POTENTIAL WEEKEND STORM...STILL LOW CONFIDENCE
IN EXACTLY HOW THIS EVENT WILL PLAY OUT. RUN-TO-RUN CONSISTENCY
AMONG DETERMINISTIC GUIDANCE HAS BEEN LACKING, ALTHOUGH THE RECENT
EURO/GFS 0Z RUNS HAVE TRENDED A BIT FURTHER SOUTH WITH THE AXIS OF
HEAVIEST PRECIPITATION. EITHER WAY, A ROBUST VORT MAX WILL ROTATE
THROUGH THE ROCKIES AND INTO THE PLAINS BY SUNDAY, RESULTING IN
STRONG, BROAD ASCENT. LATEST RUNS HAVE THE JET A BIT FURTHER SOUTH
ALONG WITH THE MOST FAVORABLE UPPER DIVERGENCE. SUBSEQUENTLY, THE
ENSEMBLE QPF/SNOWFALL SPREAD IS STILL CONSIDERABLE ACROSS THE AREA
AS WELL, FURTHER LOWERING CONFIDENCE. THERE IS HIGHER CONFIDENCE IN
MUCH COLDER AIR FILTERING IN WITH THIS SYSTEM, ESPECIALLY IF THE
SOUTHERN TRACK VERIFIES AND COLD 850/700 MB AIR STREAMS INTO THE
REGION, RESULTING IN POTENTIALLY RECORD COLD HIGHS AND LOWS.
AVIATION
(FOR THE 18Z TAFS THROUGH 18Z WEDNESDAY)
ISSUED AT 1137 AM MDT TUE OCT 20 2020
A COLD FRONT WILL CONTINUE CROSSING THE AREA THIS AFTERNOON WITH
A PERIOD OF GUSTY NORTHWEST WINDS. AREAS OF MVFR CIGS WILL PERSIST
BEHIND THE FRONT BUT MANY AREAS WILL REMAIN VFR. A CHANCE OF RAIN
AND SNOW WILL RETURN LATE THIS EVENING, WITH AREAS OF MVFR/IFR
CONDITIONS OVERNIGHT.
UNR WATCHES/WARNINGS/ADVISORIES
SD...NONE.
WY...NONE.
DISCUSSION...EAGAN
AVIATION...JOHNSON
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