471
FXUS63 KUNR 201112
AFDUNR
AREA FORECAST DISCUSSION FOR WESTERN SD AND NORTHEASTERN WY
NATIONAL WEATHER SERVICE RAPID CITY SD
412 AM MST THU FEB 20 2020
DISCUSSION
(TODAY THROUGH WEDNESDAY)
ISSUED AT 248 AM MST THU FEB 20 2020
VERY QUIET NIGHT ACROSS THE AREA AS UPPER RIDGING BEGINS TO NOSE
INTO THE AREA. SKIES ARE GENERALLY CLEAR, ASIDE FROM LOCALIZED LOW
CLOUDS/FOG IN HIGHER ELEVATIONS OF THE BLACK HILLS AND A STRATUS
DECK EXITING S CNTRL SD. WARMEST TEMPS RESIDE OVER THE FOOTHILLS AS
A RESULT OF LOW-TO-MIDLEVEL WAA. THIS WAA HAS ALSO SHUNTED THE SFC
HIGH AND LOW-LEVEL THERMAL TROF TOWARD THE ERN DAKOTAS AND UPPER
MIDWEST, WHERE WIDESPREAD SUBZERO TEMPS CONTINUE. WARMER AIR WILL
CONTINUE TO BUILD INTO THE REGION TODAY, WITH GOOD AGREEMENT IN 850
MB TEMPS ABOVE ZERO ARRIVING BY 00Z. THIS AIR MASS WILL ALSO BE VERY
DRY, WITH PWATS IN THE LOWEST 1 PERCENT OF NAEFS CLIMO. THUS, GOES
WITHOUT SAYING THAT PRECIP IS NOT A CONCERN FOR THE NEXT FEW DAYS.
MAIN FORECAST ADJUSTMENT IN THE NEXT 24 HOURS WAS TO RAISE LOW TEMPS
TONIGHT OVER THE BLACK HILLS/FOOTHILLS AND INCREASE WINDS TODAY
THROUGH TONIGHT OVER NERN WY, THE BLACK HILLS, AND ESPECIALLY THE
FOOTHILLS. LOW-LEVEL INVERSION STRENGTHENS RAPIDLY IN THE 00-06Z
TIME FRAME TONIGHT AS 850-750 MB TEMPS INCREASE, WHICH SHOULD
MAINTAIN MILD TEMPS OVER HIGHER ELEVATIONS. NEAR THE BASE OF THIS
INVERSION, NAM FCST SOUNDINGS SUGGEST WINDS OF 30-40 KT. MEANWHILE,
A 3-4 MB PRESSURE GRADIENT OVER NERN WY EVOLVES INTO A 5-6 MB
GRADIENT ACROSS THE HILLS AFTER 00Z, MAXIMIZED FROM APPROXIMATELY
03Z TO 09Z. TAKEN TOGETHER, THIS COULD LEAD TO SOME GUSTY DOWNSLOPE
WINDS FOR A HANDFUL OF HOURS ACROSS THE FOOTHILLS. 00Z HREF MEAN
AGREES, WITH AN AREA OF 30-40 KT GUSTS OVER THE HILLS AND FOOTHILLS
AROUND THE SAME TIME. THIS MAY ULTIMATELY BRIEFLY RISE TO ADVISORY
CRITERIA ACROSS THE NRN OR SRN/HERMOSA FOOTHILLS, BUT FOR NOW, LACK
THE CONFIDENCE TO PINPOINT A TIME OR LOCATION.
DISTINCT SPLIT FLOW PATTERN DEVELOPS FRIDAY, WITH THE SRN STREAM
FEATURING A CLOSED/NOT QUITE CUT-OFF LOW OFF THE CA COAST AND THE
NRN STREAM WELL TO OUR NORTH IN CN. UPPER-LEVEL RH INCREASES
SLIGHTLY FRIDAY AND SATURDAY, WHICH COULD TEMPER HIGHS SOMEWHAT.
HOWEVER, STILL EXPECT WELL ABOVE AVERAGE TEMPS TO ARRIVE FRIDAY AND
PERSIST SATURDAY AS LOW-LEVEL THERMAL RIDGE BECOMES ESTABLISHED OVER
THE HIGH PLAINS. SRN STREAM BECOMES MORE PROGRESSIVE BY SATURDAY AS
UPPER LOW OFF THE CA COAST BEGINS TO CROSS THE SRN HALF OF THE
CONUS. FAIRLY GOOD AGREEMENT BETWEEN MODELS THAT THIS WAVE WILL
REMAIN LARGELY TO OUR SOUTH AS IT CROSSES THE PLAINS ON SUNDAY.
MEANWHILE, NRN STREAM SAGS SWD INTO THE NRN CONUS AS A WEAK WAVE AND
ATTENDANT FRONT CROSS THE NRN PLAINS, DROPPING US BACK TO NEAR
NORMAL TEMPS TO START NEXT WEEK.
PREDICTABILITY DECREASES EARLY NEXT WEEK AS DIFFERENCES ARISE IN
EURO/GFS/CMC ENSEMBLES. GFS BRINGS A MORE ROBUST CLIPPER THROUGH THE
DAKOTAS ON MONDAY, FOLLOWED BY THE DEVELOPMENT OF AN AMPLIFIED
LONGWAVE TROF TO OUR EAST BY MIDWEEK. EURO/CMC SUGGEST A WEAKER
CLIPPER AND DEVELOP A BROADER, ROUNDED TROF OVER THE CNTRL CONUS BY
MIDWEEK. EITHER WAY, LOOKS BREEZY AS THE CLIPPER PASSES AND COOLER
FOR A DAY OR TWO AS LOW-LEVEL THERMAL TROFING RETURNS TO THE PLAINS
COINCIDENT WITH THE DEVELOPING MID-UPPER TROF. BEST CHANCES FOR
APPRECIABLE QPF (OVER 0.1 INCH) DURING THIS TIME FRAME WILL BE OVER
THE NRN BLACK HILLS AND BEARLODGE MTNS, WITH FAIRLY HIGH CONFIDENCE
BASED ON AGREEMENT IN GEFS/EPS/GEPS (80-100 PERCENT).
AVIATION
(FOR THE 12Z TAFS THROUGH 12Z FRIDAY MORNING)
ISSUED AT 410 AM MST THU FEB 20 2020
VFR CONDITIONS WILL CONTINUE TODAY AND TONIGHT. WINDY
SOUTHWESTERLY WINDS ARE EXPECTED OVER PARTS OF NORTHEASTERN
WYOMING TODAY.
UNR WATCHES/WARNINGS/ADVISORIES
SD...NONE.
WY...NONE.
DISCUSSION...SHERBURN
AVIATION...10
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