Area Forecast Discussion
Issued by NWS Rapid City, SD

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609
FXUS63 KUNR 191600
AFDUNR

AREA FORECAST DISCUSSION FOR WESTERN SD AND NORTHEASTERN WY
NATIONAL WEATHER SERVICE RAPID CITY SD
1000 AM MDT TUE MAY 19 2015

.UPDATE...
ISSUED AT 947 AM MDT TUE MAY 19 2015

WATER VAPOR SATELLITE SHOWS PLUME OF MOISTURE MOVING NORTHWARD
TOWARD FORECAST AREA AHEAD OF UPPER LOW OVER GREAT BASIN. BLACK
HILLS AND NORTHEASTERN WY HAVE HAD LGT RAIN/SNOW/SLEET FOR THE
LAST FEW HOURS AS A SMALL BAND OF PRECIPITATION HAS DEVELOPED
AHEAD OF MAIN AREA STILL OVER NEBRASKA. HEIGHT OF WET BULB ZERO
DECREASES DURING THE DAY...SO WOULD NOT EXPECT SNOW TO COMPLETELY
CHANGE TO RAIN THIS AFTERNOON. RAIN WILL BE SLOWER TO DEVELOP
OVER THE SD PLAINS AS AIR JUST ABOVE THE SURFACE IS STILL
DRY...BUT SHOULD BECOME SATURATED BY AFTERNOON...THEN CONTINUE
THROUGH THE NIGHT. ENOUGH RAIN MAY FALL OVER NORTHERN NE PANHANDLE
AND SOUTHWESTERN SOUTH DAKOTA...STREAMS AND ESPECIALLY THE WHITE
RIVER WILL LIKELY RISE AGAIN...SO HAVE ISSUED A FLOOD WATCH FOR
TONIGHT THROUGH THURSDAY MORNING. SURFACE HIGH REMAINS OVER
EASTERN SD WITH COOL AIR ADVECTION OVER THE AREA... COUPLED WITH
CLOUD COVER WILL KEEP TEMPERATURES FROM WARMING MUCH TODAY AND
HAVE DECREASED MAX TEMPS.

&&

.SHORT TERM...TODAY THROUGH WEDNESDAY NIGHT
ISSUED AT 257 AM MDT TUE MAY 19 2015

08Z SURFACE ANALYSIS HAD HIGH PRESSURE OVER THE NORTHERN PLAINS
WITH TEMPERATURES BELOW FREEZING OVER EASTERN PORTIONS OF THE CWA.
WATER VAPOUR LOOP HAD UPPER LOW OVER SOUTHERN NV WITH 100KT JET
STREAK POWERING SHORTWAVE OVER THE FOUR CORNERS AREA. THIS
SHORTWAVE WILL PUSH NORTH TODAY SPREADING FORCING ON NOSE OF JET
INTO FAR SOUTHWEST REACHES OF THE CWA THIS MORNING AND THEN OVER
THE SOUTHERN THREE QUARTERS OF THE AREA THIS AFTERNOON INTO
TONIGHT. SHORTWAVE WILL WEAKEN AS IT BREAKS DOWN UPPER RIDGE IN
PLACE OVER THE CENTRAL/SOUTHERN PLAINS AND ENTERS AN AREA OF
CONFLUENCE ALOFT. DRIER SURFACE AIR WILL BE FUNNELED INTO CWA FROM
AFOREMENTIONED SURFACE RIDGE...WHICH WILL HAVE DELETERIOUS EFFECT
ON PRECIPITATION INVASION THIS MORNING...BUT WILL DEPRESS WET BULB
ZEROES. RESULT WILL BE A SLOWER ONSET OF PRECIPITATION TODAY. SNOW
WILL DEVELOP OVER THE HIGHER ELEVATIONS THIS AFTERNOON AND SPREAD
ACROSS SOUTHWEST SD TONIGHT WITH 850MB TEMPERATURES DROPPING TO 0C.
A FEW INCHES OF SNOW OVER THE BLACK HILLS LIKELY WITH 1-2 INCHES
ON THE ADJACENT PLAINS SOUTH/SOUTHEAST OF THE BLACK
HILLS...DEPENDING ON HOW WARM GROUND AFFECTS ACCUMULATION.
TEMPERATURES WILL BE COOLER THAN GUIDANCE IN THE SOUTH TODAY AND
NEAR GUIDANCE IN THE NORTH.

WEDNESDAY...SYSTEM CONTINUES TO WEAKEN AS IT PUSHES EAST. 00Z
NAM/GFS AT ODDS AS TO HOW MUCH PRECIPITATION WILL BE LEFT
BEHIND...SO SPLIT THE DIFFERENCE. TEMPERATURES WILL BE TRICKY WITH
AMOUNT OF SNOW ON THE PLAINS AFFECTING MAXIMUM TEMPERATURES FOR THE
SD PLAINS.

.LONG TERM...THURSDAY THROUGH MONDAY
ISSUED AT 257 AM MDT TUE MAY 19 2015

MEDIUM RANGE MODELS STILL SHOWING UNSETTLED WEATHER THROUGH THE
PERIOD AS TROF ALONG THE WEST COAST CONTINUES TO EJECT SYSTEMS OUT
ACROSS THE ROCKIES AND THE NORTHERN/CENTRAL PLAINS. WEAKER SHORT-
WAVE TROUGHS IN SOUTHWEST FLOW ALOFT WILL BRING MOSTLY LOW CHANCES
FOR SHOWERS AND THUNDERSTORMS LATE THURSDAY INTO FRIDAY ...WITH
THE BEST CHANCES ACROSS THE SOUTHERN CWA. A STRONGER SYSTEM IS
PROGGED TO MOVE INTO THE REGION OVER THE WEEKEND. THERE ARE SOME
DIFFERENCES BETWEEN THE GFS...ECMWF...AND GEM...ON
TIMING...TRACK... AND STRENGTH OF THE SYSTEM...WITH THE GFS AND
GEM STRONGER... SLOWER...AND FURTHER NORTH AND WEST WITH THE
SYSTEM COMPARED TO THE ECMWF. CONFIDENCE IS RELATIVELY LOW AT THIS
POINT ON WHERE AND WHEN HEAVIEST RAIN WOULD FALL WITH THE SYSTEM.
WILL KEEP MOSTLY BROADBRUSHED HIGHER CHANCE POPS FOR SHOWERS AND
STORMS OVER THE WEEKEND. SATURDAY LOOKS TO BE THE DAY WITH THE
HIGHEST POTENTIAL FOR A FEW STRONG TO SEVERE STORMS...BUT WILL
ULTIMATELY DEPEND ON HOW THE SYSTEM EVOLVES. WITH REGARD TO
TEMPERATURES...THEY SHOULD STAY BELOW AVERAGE THROUGH THE
WEEKEND...WITH HIGHS MOSTLY IN THE MID 50S TO MID 60S.
TEMPERATURES SHOULD FINALLY RISE TO NEAR AVERAGE LEVELS DURING THE
FIRST PART OF NEXT WEEK AFTER THIS SYSTEM EXITS.

&&

.AVIATION...FOR THE 12Z TAFS THROUGH 12Z WEDNESDAY MORNING
ISSUED AT 542 AM MDT TUE MAY 19 2015

LOW PRESSURE OVER THE GREAT BASIN WILL APPROACH THE AREA
TODAY...BRINGING LOWERING CLOUDS AND PCPN TO MAINLY NORTHEAST WY AND
SOUTHERN SD TODAY AND TONIGHT. MVFR CIGS WILL SPREAD ACROSS
NORTHEAST WY AND SOUTHWEST SD THIS MORNING...WITH AREAS OF IFR
CONDITIONS DEVELOPING OVER THESE AREAS BY 18Z. CONDITIONS SHOULD
STAY MOSTLY VFR TO THE NORTH OF THESE AREAS THROUGH THE DAY...WHERE
PCPN CHANCES WILL TAPER OFF SHARPLY ACROSS NORTHWEST SD. RAIN WILL
INCREASE IN COVERAGE THROUGH THE MORNING AND EARLY AFTERNOON HOURS
ACROSS NORTHEAST WY AND SOUTHERN SD....REACHING KGCC BY 15Z AND KRAP
BY 18Z. SOME SNOW CAN BE EXPECTED ACROSS THE BLACK HILLS THIS
AFTERNOON AND TONIGHT...WITH A MIX OR CHANGE OVER TO LIGHT SNOW
TONIGHT ACROSS MUCH OF NORTHEAST WY AND THE FAR WESTERN SD PLAINS.

&&

.UNR WATCHES/WARNINGS/ADVISORIES...
SD...FLOOD WATCH FROM 6 PM MDT THIS EVENING THROUGH LATE WEDNESDAY
     NIGHT FOR SDZ042-043.

WY...NONE.
&&

$$

UPDATE...55
SHORT TERM...HELGESON
LONG TERM...26
AVIATION...26



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