Area Forecast Discussion
Issued by NWS Rapid City, SD

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000
FXUS63 KUNR 212019
AFDUNR

AREA FORECAST DISCUSSION FOR WESTERN SD AND NORTHEASTERN WY
NATIONAL WEATHER SERVICE RAPID CITY SD
219 PM MDT TUE APR 21 2015

.SHORT TERM...THIS EVENING THROUGH WEDNESDAY NIGHT
ISSUED AT 215 PM MDT TUE APR 21 2015

20Z SURFACE ANALYSIS HAD BACK DOOR COLD FRONT FROM CENTRAL MT TO
NORTH CENTRAL NE EVIDENCED BY PRESSURE RISE BUBBLE AND LINE OF
CU/SC PER 1KM VISIBLE IMAGERY. COLD FRONT WILL SLOW DOWN AND STALL
ACROSS THE FAR WESTERN REACHES OF THE CWA TONIGHT. IT WILL THEN
MOVE BACK NORTHEAST SLOWLY WEDNESDAY AN UPPER LEVEL RIDGE OVER THE
ROCKIES SHIFTS EASTWARD. SURFACE HIGH SHIFTS SOUTHEAST OF THE AREA
WITH RESULTANT RETURN FLOW...ALTHOUGH NOT IN A STRONG FASHION.
BETTER WARM AIR ADVECTION SETS UP WEDNESDAY NIGHT WITH WEAK LOW
LEVEL JET OVER THE SD PLAINS. TEMPERATURES WILL BE NEAR GUIDANCE
THROUGH THE PERIOD.

.LONG TERM...THURSDAY THROUGH TUESDAY
ISSUED AT 215 PM MDT TUE APR 21 2015

SHOWERS AND THUNDERSTORMS ARE LOOKING MORE PROBABLE FOR THURSDAY
AFTERNOON AND EVENING ACROSS SOUTHWEST SOUTH DAKOTA AND
NORTHEASTERN WYOMING. MODELS PROG A WEAK SHORTWAVE TO IMPACT THE
AREA DURING THURSDAY AFTERNOON...AND WHILE CAPE IS GENERALLY AT OR
UNDER 1000J/KG AND 0-6KM SHEAR UNDER 30KTS...THE COMBINATION OF
WEAK INSTABILITY AND WIND SHEAR COULD SUPPORT SOME ORGANIZED
CONVECTION. CURRENT THINKING IS THAT ANY SEVERE CONVECTION WILL
BE SOUTH OF THE CWA WHERE HIGHER INSTABILITY AND STRONGER SHEAR
EXISTS.

ON FRIDAY...AN UPPER LEVEL LOW CROSSES INTO THE CENTRAL PLAINS AND
WILL GENERATE MORE PRECIPITATION IN OUR CWA. FORECAST SOUNDINGS FROM
THE GFS AND NAM DO SHOW SOME WEAK INSTABILITY IN THE WESTERN HALF OF
THE CWA...SO WILL INCLUDE CHANCE OF THUNDERSTORMS IN THE FORECAST.
WIND SHEAR IS EVEN WEAKER FOR FRIDAY COMPARED TO THURSDAY...SO DO
NOT EXPECT MUCH IN THE WAY OF ORGANIZED CONVECTION.

FOR THE WEEKEND AND EARLY WORK WEEK...MODELS HAVE SEVERAL SHORTWAVES
CROSSING INTO THE AREA AND BRINGING CHANCES FOR SHOWERS NEARLY EVERY
DAY. THERE IS A GENERAL CONSENSUS THAT AN UPPER LEVEL RIDGE WILL
DEVELOP IN THE WESTERN CONUS EARLY NEXT WEEK...BUT THE ECMWF AND GFS
HAVE DIFFERING SOLUTIONS ON THE STRENGTH AND LOCATION OF THE RIDGE.
HAVE USED A BLEND OF THE TWO SOLUTIONS FOR THE FORECAST.

&&

.AVIATION...FOR THE 18Z TAFS THROUGH 18Z WEDNESDAY
ISSUED AT 215 PM MDT TUE APR 21 2015

VFR CONDITIONS WILL CONTINUE THROUGH WEDNESDAY MORNING.

&&

.UNR WATCHES/WARNINGS/ADVISORIES...
SD...NONE.
WY...NONE.
&&

$$

SHORT TERM...HELGESON
LONG TERM...MCKEMY
AVIATION...MCKEMY


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