Area Forecast Discussion
Issued by NWS Rapid City, SD

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000
FXUS63 KUNR 102036
AFDUNR

AREA FORECAST DISCUSSION FOR WESTERN SD AND NORTHEASTERN WY
NATIONAL WEATHER SERVICE RAPID CITY SD
236 PM MDT WED SEP 10 2014

.SHORT TERM...THIS EVENING THROUGH THURSDAY NIGHT
ISSUED AT 235 PM MDT WED SEP 10 2014

ANOMALOUSLY DEEP NORTHERN STREAM UPPER TROUGH WILL
SHIFT SE INTO THE REGION...BRINGING UNSEASONABLY COLD TEMPS AND SNOW
ACCUMULATIONS TO THE AREA. IMPRESSIVE SOUTHERN PERIPHERY IMPULSE
WITH AN ASSOCIATED 75+ KNOT MID LEVEL SPEED MAX QUICKLY ADVECTING SE
ACROSS MT THIS AFTERNOON. THIS SYSTEM WILL SUPPORT STRONG DEEP LAYER
ASCENT WITH REFLECTED LL FGEN AND ASSOCIATED LL FORCING...SUPPORTING
INCREASING CHANCES FOR PRECIP ACROSS THE REGION THIS EVENING. VERY
TRICKY FORECAST GIVEN EXPECTED RAIN/SNOW MIX...WITH WARM SFC
CONDITIONS LIKELY HAMPERING SNOW ACCUMS...ESP ON THE PLAINS...MAKING
HEADLINES A TOUGH DECISION. THE OTHER SIGNIFICANT IMPACT FROM THIS
SYSTEM WILL BE THE GROWING POTENTIAL FOR AN EARLY SEASON HARD
FREEZE ACROSS A LARGE PORTION OF THE AREA FRIDAY NIGHT. GIVEN
CLEARING SKIES/LIGHT WINDS/WITH THE SFC HIGH CENTER SHIFTING SOUTH
OF THE AREA...STRONG RADIATIONAL COOLING AND SIG T FALLS ARE
EXPECTED.

TONIGHT...POWERFUL IMPULSE WILL SUPPORT STRONG LSA WITH POTENT LL
FGEN PER REFLECTED SOUTHERLY FLOW. APPRECIABLE COLUMN MOISTURE WITH
PW IN EXCESS OF ONE HALF INCH WILL BE MORE THAN AMPLE TO SUPPORT A
PERIOD OF MODERATE RAIN/SNOW. STEEP MID LEVEL LAPS RATES WILL ALSO
BE IN PLACE THROUGH  PORTION OF THE THE DGZ...SUPPORTING CONDITIONAL
INSTABILITY...WITH THE CHANCE FOR SOME THUNDER EARLY THUR
MORNING...GIVEN THE AMOUNT OF CONDITIONAL INSTABILITY PRESENT. THE
DIFFICULT PART THIS FORECAST IS THE DETERMINATION OF SNOW ACCUMS.
TEMPS ARE ALREADY BEHIND THE CURVE...WITH LOW TO MID 30S IN THE HIGH
ELEVATIONS OF THE HILLS. ONCE PRECIP BEGINS IN THE HIGHER PORTIONS
OF THE BLACK HILLS IT WILL LIKELY BEGIN AS SNOW INSTEAD OF RAIN. QPF
VALUES IN EXCESS OF 0.5 INCHES IN MEAN MODEL SOLUTIONS SHOULD
SUPPORT 5 INCHES OR MORE OF WET HEAVY SNOW IN AREAS ABOVE 5 KFT.
HAVE DECIDED TO ISSUE A WINTER STORM WARNING FOR THE NORTHERN AND
CENTRAL BLACK HILLS...ESP GIVEN THE VERY EARLY NATURE OF THIS SYSTEM
WITH POTENTIAL FOR TREE DAMAGE AND POWER INTERRUPTIONS FROM WET
HEAVY SNOW. OTHERWISE...PRECIP WILL BEGIN AS RAIN ON THE WY
PLAINS...TRANSITIONING TO ALL SNOW BY EARLY THUR MORNING. SNOW
AMOUNTS WILL BE ESP TRICKY THERE AND INTO PORTIONS OF THE WESTERN
SD PLAINS...INCLUDING THE BH FOOTHILLS. WARM SFC CONDITIONS WILL
SUPPORT SIG MELTING AS SNOW FALLS...ESP AT FIRST. HOWEVER...GIVEN
THE STRENGTH OF THE VERTICAL MOTION FIELDS...A PERIOD OF HEAVY SNOW
WILL BE POSSIBLE...WHICH GIVEN TEMPS AROUND FREEZING MAY OVERWHELM
MELTING MAKING FOR ESP SLICK TRAVEL. THIS IS ALSO TRUE IN AND AROUND
THE SPEARFISH AREA. HAVE DECIDED TO EXPAND THE WINTER WEATHER ADV TO
THESE AREAS...ESP GIVEN THE EARLY SEASON NATURE TO THIS SYSTEM.
EXPECTING 1 TO 4 INCHES ON THE WY PLAINS...WITH THE HIGHEST AMOUNTS
ON RIDGES AND IN THE WESTERN BH FOOTHILLS. FURTHER EAST ON THE SD
PLAINS...A LATER CHANGEOVER WITHE SLIGHTLY WARMER TEMPS WHEN
COMPARED TO THE WY PLAINS SHOULD SUPPORT GENERALLY LIGHT SNOW
ACCUMULATIONS IF ANY. THE BEST CHANCE FOR AROUND AN INCH OR SLIGHTLY
MORE WILL BE ON THE EASTERN AND NORTHERN BLACK HILLS FOOTHILLS.
FORECAST MODELS HAVE COME IN COOLER PER TEMPS TONIGHT THROUGH THUR
NIGHT AND ADJUSTED TEMPS FOR THIS. TEMPS WILL BE AN IMPRESSIVE 30 TO
40 DEGREES BELOW SEASONAL NORMS...A TESTAMENT TO HOW COLD THIS
AIRMASS IS. WIDESPREAD FROST IS EXPECTED FRIDAY MORNING MOST PLACES
GIVEN LOWS IN THE MID TO UPPER 20S TO AROUND 30. SFC HIGH SHIFTING
SOUTH OF THE FA/LIGHT WINDS/DRY AIR/AND CLEAR SKIES WILL SUPPORT
EXCELLENT RADIATIONAL COOLING...ESP OVER ANY RESIDUAL SNOW COVER.
TEENS ARE EXPECTED IN THE BLACK HILLS. HAVE ADDED WIDESPREAD FROST
TO THE GRIDS FRIDAY NIGHT.

&&

.LONG TERM...FRIDAY THROUGH WEDNESDAY
ISSUED AT 235 PM MDT WED SEP 10 2014

MAINLY DRY WEATHER AND A WARMING TREND CAN BE EXPECTED
FROM FRIDAY THROUGH EARLY NEXT WEEK AS A RIDGE BUILDS ACROSS THE
WESTERN STATES AND EVENTUALLY MOVES INTO THE PLAINS. TEMPERATURES
WILL RISE BACK TO NEAR AVERAGE LEVELS BY MONDAY. A WEAK SHORTWAVE IS
PROGGED BY A COUPLE OF THE MEDIUM RANGE MODELS TO SLIDE THROUGH THE
AREA ON SUNDAY AND COULD BRING A FEW SHOWERS.

&&

.AVIATION...FOR THE 00Z TAFS THROUGH 00Z THURSDAY EVENING
ISSUED AT 235 PM MDT WED SEP 10 2014

WIDESPREAD MVFR/IFR CONDITIONS WILL PERSIST ACROSS THE
FORECAST AREA THROUGH THIS EVENING. SCT -SHRA WILL CONTINUE THROUGH
THE DAY...THEN MORE WIDESPREAD RAIN WILL DEVELOP OVER WESTERN AREAS
THIS EVENING AND SPREADING EAST OVERNIGHT. RAIN WILL CHANGE OVER
TO SNOW OVER THE BLACK HILLS AND OVER NE WY/NW SD OVERNIGHT INTO
THURSDAY MORNING...RESULTING IN SOME LIFR CONDITIONS AS WELL.

&&

.UNR WATCHES/WARNINGS/ADVISORIES...
SD...WINTER STORM WARNING FROM 9 PM THIS EVENING TO NOON MDT THURSDAY
     FOR SDZ024-028.

     WINTER WEATHER ADVISORY FROM MIDNIGHT TONIGHT TO NOON MDT
     THURSDAY FOR SDZ025-029.

WY...WINTER STORM WARNING FROM 9 PM THIS EVENING TO NOON MDT THURSDAY
     FOR WYZ057.

     WINTER WEATHER ADVISORY FROM MIDNIGHT TONIGHT TO NOON MDT
     THURSDAY FOR WYZ054>056-071.

&&

$$

SHORT TERM...JC
LONG TERM...13
AVIATION...13






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