Area Forecast Discussion
Issued by NWS Rapid City, SD

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000
FXUS63 KUNR 211734
AFDUNR

AREA FORECAST DISCUSSION FOR WESTERN SD AND NORTHEASTERN WY
NATIONAL WEATHER SERVICE RAPID CITY SD
1134 AM MDT THU MAY 21 2015

.UPDATE...
ISSUED AT 951 AM MDT THU MAY 21 2015

LATEST OBSERVATIONS SHOW HIGH PRESSURE OVER WESTERN SOUTH DAKOTA
RESULTING IN SUNNY SKIES. SOUTHERLY WINDS HAVE PUSHED AN AREA OF
LOW STRATUS INTO PARTS OF NORTHEASTERN WYOMING...WHICH IS EXPECTED
TO DISSIPATE OVER THE NEXT COUPLE OF HOURS DUE TO INCREASED MIXING
WITH DIURNAL HEATING. FORECAST REMAINS ON TRACK SO NO UPDATES
PLANNED ATTM.

&&

.SHORT TERM...TODAY THROUGH FRIDAY NIGHT
ISSUED AT 300 AM MDT THU MAY 21 2015

A RATHER COMPLEX UPPER PATTERN EXISTS WITH A LARGE-SCALE RIDGE
COVERING WRN CANADA BUT WITH A TROUGH UNDERCUTTING IT OVER THE
SWRN CONUS. A DEEP HUDSON BAY LOW WAS ASSOCIATED WITH A MEAN
TROUGH OVER ERN NOAM. THE RESULT OVER THE BLKHLS AREA WAS CON-
FLUENT UPPER FLOW AND SUBSIDENCE...WITH A DEFORMATION ZONE OVER
THE SWRN CWA...AND A SURFACE HIGH OVER WRN NEB THAT WAS ASSOCIATED
WITH SOME FROSTY CONDITIONS OVER SWRN SD/NERN WY. THE INTERPLAY
BETWEEN THE UPPER LOWS OVER THE SWRN CONUS AND HUDSON BAY WILL BE
THE LARGE-SCALE DRIVER OF OUR WEATHER THROUGH AT LEAST EARLY
SATURDAY.

FOR TODAY...THE SFC HIGH WILL MOVE SOUTH WITH RETURN SOUTHEAST SFC
FLOW DEVELOPING. 850-MB TEMPS WILL RISE 4-6C AND WILL HELP TEMPS
RISE BACK INTO THE 60S FOR A CHANGE. THE CONFLUENT UPPER FLOW AND
DEFORMATION ZONE WILL KEEP FAIR SKIES OVER MOST OF THE CWA...EXCEPT
FOR THE SWRN THIRD OR SO. FOR TNGT...SOUTHEAST SFC FLOW CONTINUES
WITH A 1O-KT STRENGTHENING OF THE 850-MB WINDS. A SHORT-WAVE TROUGH
IS PROGGED TO EJECT NORTHEAST ACROSS CO TO NEAR KDEN BY 12Z THU.
MOISTURE AND LIFT WILL REMAIN CONFINED MOSTLY TO THE SOUTHWEST OF
THE CWA THROUGH 12Z. 850-MB TEMPS OF 8-10C AND BOUNDARY LAYER FLOW
SHOULD PRECLUDE ADDITIONAL FROST.

FOR FRIDAY...THE SHORT-WAVE TROUGH WILL PROGRESS SLOWLY ACROSS THE
NEB PANHANDLE...BEING SLOWED BY THE UPPER-LEVEL CONFLUENT ZONE. A
NARROW ZONE OF MOISTURE/LIFT WILL FAVOR THE HIGHEST POPS ACROSS THE
SRN CWA...AND THIS WILL STRUGGLE TO MAKE IT NORTH AGAINST THE DRY
AIR AND DEFORMATION ZONE. CAPE AND INSTABILITY PROGS ARE QUITE
MEAGER...SO HAVE CUT BACK ON THE THUNDER POTENTIAL FOR FRIDAY. ON
FRIDAY NIGHT...THE SHORT-WAVE TROUGH SHOULD PROGRESS ACROSS THE
CWA. SOME ISENTROPIC LIFT AT LOW-LEVELS IS NOTED...BUT THE WAVE
WILL ENCOUNTER DRY AIR TO THE NORTH...SO HAVE TRIMMED BACK POPS
FROM THE PREVIOUS FORECAST. CAPE ALSO IS MINIMAL...THUS HAVE CUT
BACK ON COVERAGE OF THUNDER.

.LONG TERM...SATURDAY THROUGH WEDNESDAY
ISSUED AT 300 AM MDT THU MAY 21 2015

MEDIUM RANGE MODELS ARE IN GOOD AGREEMENT ON UNSETTLED WEATHER
THROUGH THE HOLIDAY WEEKEND AS UPPER-LEVEL SYSTEM SLOWLY TRACKS
ACROSS THE CENTRAL ROCKIES INTO THE NORTHERN PLAINS. HOWEVER...THE
VARIOUS MODELS STILL DIFFER ON SPECIFICS SUCH AS TIMING AND
COVERAGE OF BEST PCPN CHANCES SATURDAY AND SUNDAY. MAJORITY OF 00Z
MODELS LEANING MORE TOWARD THE LATE AFTERNOON AND NIGHTTIME HOURS
FOR ANY HEAVIER AND MORE WIDESPREAD RAINFALL ON SATURDAY...WITH
THE BEST INSTABILITY OVER FAR SOUTHERN PORTIONS OF THE CWA DURING
THE AFTERNOON AND EVENING. THIS WOULD LIKELY LIMIT THE THREAT FOR
ANY STRONGER STORMS TO MOSTLY AREAS SOUTHEAST OF THE BLACK
HILLS...AND ESPECIALLY OVER NEB...AS SOUTHERLY LOW-LEVEL JET
INTENSIFIES THERE IN THE EVENING. THE THREAT FOR HEAVY RAINFALL
LOOKS MORE LOCALIZED AT THIS POINT...MOSTLY TO THE EAST OF THE
BLACK HILLS. SUNDAY LOOKS SIMILAR WITH REGARD TO ANY THREAT FOR
STRONGER STORMS. AS THE UPPER LOW MOVES INTO THE NORTHERN HIGH
PLAINS ON SUNDAY...THE BETTER COVERAGE FOR RAINFALL WOULD MOST
LIKELY SHIFT TO NORTHERN PORTIONS OF THE CWA.

AS THE SYSTEM WEAKENS AND EXITS THE AREA SUNDAY NIGHT INTO
MONDAY...WEST-SOUTHWEST FLOW ALOFT WILL BRING WEAKER DISTURBANCES
ACROSS THE AREA EARLY NEXT WEEK...WITH ISOLATED TO SCATTERED SHOWER
AND THUNDERSTORM ACTIVITY EXPECTED MONDAY THROUGH WEDNESDAY.
TEMPERATURES WILL CONTINUE TO BE BELOW AVERAGE OVER THE WEEKEND...
WITH A GRADUAL WARMING TREND TO NEAR AVERAGE TEMPERATURES BY
TUESDAY AND WEDNESDAY.

&&

.AVIATION...FOR THE 18Z TAFS THROUGH 18Z FRIDAY
ISSUED AT 1107 AM MDT THU MAY 21 2015

RAIN SHOWERS WITH MVFR CIGS WILL PUSH INTO NORTHEAST
WY...SOUTHWEST SD...AND THE BLACK HILLS AREA EARLY FRIDAY MORNING.
OTHERWISE VFR CONDITIONS ARE EXPECTED THROUGH THE FORECAST PERIOD.

&&

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

$$

UPDATE...10
SHORT TERM...BUNKERS
LONG TERM...26
AVIATION...POJORLIE



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