Winter Storm in Wyoming, December 22 to 23, 2014

AREA FORECAST DISCUSSION FOR WESTERN SD AND NORTHEASTERN WY
NATIONAL WEATHER SERVICE RAPID CITY SD
824 PM MST SUN DEC 21 2014

.UPDATE...
ISSUED AT 808 PM MST SUN DEC 21 2014

ONLY MINOR ADJUSTMENTS NECESSARY TO THE FCST THIS EVENING. FIRST
ROUND OF UPSLOPE PCPN IS NOW REACHING THE NRN BLKHLS. LATEST
GUIDANCE SHOWING THERE WILL BE BREAKS IN THE PCPN DURING THE DAY
MONDAY IN BOTH THE NRN BLKHLS AND THE PLAINS...THEN BECOME MORE
WDSPRD MONDAY NIGHT AS NEXT DISTURBANCE MOVES IN. WIND STILL LOOKS
TO BE A BIG ISSUE LATER TONIGHT AND THROUGH MONDAY...STRONGEST
AROUND THE EAST SIDE OF THE BLKHLS. OF COURSE...ANY SNOW COMBINED
WITH THE WIND COULD CREATE VERY POOR VSBYS. CURRENT HIGHLIGHTS
COVER THINGS WELL.

&&

.SHORT TERM...THIS EVENING THROUGH MONDAY NIGHT
ISSUED AT 354 PM MST SUN DEC 21 2014

CURRENT SURFACE ANALYSIS SHOWS LOW PRESSURE OVER SOUTHWEST
SK...WITH FRONTAL BOUNDARY STRETCHING SOUTHEASTWARD INTO THE
WESTERN DAKOTAS. MAIN COLD FRONT LIES SOUTH AND WEST OF THE LOW
FROM NORTHEAST MT TO CENTRAL WY. UPPER LEVEL ANALYSIS SHOWS ZONAL
FLOW OVER THE REGION...WITH A VERY STRONG UPPER JET...150
KT+...STRETCHING ACROSS THE PACIFIC NW INTO THE ROCKIES. UPPER
LEVEL ENERGY IS MOVING QUICKLY EAST ACROSS THE PACIFIC NW AND
SOUTHWEST CANADA. A SHORTWAVE TROF IS STRENGTHENING AND DIGGING
SOUTH ACROSS MT RIGHT NOW. SKIES HAVE BECOME MOSTLY CLOUDY ACROSS
MUCH OF THE AREA...WITH SCATTERED RAIN SHOWERS NOW OVER NORTHEAST
WY AND WESTERN SD. TEMPS ARE MOSTLY IN THE 40S TO AROUND 50
DEGREES.

A UNSETTLED BEGINNING TO THE WEEK CAN BE EXPECTED AS A STORM SYSTEM
DEVELOPS AND INTENSIFIES OVER THE REGION TONIGHT AND MONDAY. VARIOUS
SHORT RANGE MODELS ARE IN GOOD AGREEMENT ON GENERAL DEVELOPMENT...
BUT STILL HAVE DIFFERENCES WITH REGARD TO TRACK AND STRENGTH BETWEEN
TONIGHT AND TUESDAY. THE TRACK OF THE STORM WILL BE IMPORTANT WITH
REGARD TO WHERE AREAS OF HEAVIER PCPN MAY DEVELOP OVER THE
PLAINS...WITH STRONG WINDS ALSO COMPLICATING THINGS. THE GFS AND NAM
DIG THE DEVELOPING LOW FURTHER SOUTH AND QUICKER THAN THE ECMWF AND
GEM. WILL LEAN MORE TOWARD A BLEND OF THE GFS AND ECMWF FOR THIS
FORECAST.

FOR TONIGHT...PCPN WILL EXPAND ACROSS THE AREA FROM WEST TO EAST AS
THE FRONT PASSES AND THE NEXT DISTURBANCE ALOFT CROSSES. THE CHANGE
OVER TO SNOW SHOULD BE GRADUAL FROM WEST TO EAST AS THE AIR BEHIND
THE FRONT IS NOT THAT COLD. PCPN WILL CHANGE OVER THE QUICKEST OVER
THE BLACK HILLS...DURING THE EVENING HOURS...WITH A MORE GRADUAL
CHANGE OVERNIGHT ACROSS THE PLAINS. SNOWFALL AMOUNTS OF UP TO AN
INCH OR SO IS EXPECTED ON THE PLAINS...WITH 2 TO 4 INCHES POSSIBLE
BY MORNING OVER THE TERRAIN FAVORED AREAS OF THE BLACK HILLS.
NORTHWESTERLY WINDS WILL GRADUALLY INCREASE BEHIND THE FRONT
OVERNIGHT...BECOMING BREEZY TO WINDY IN MOST AREAS BY EARLY MORNING.
LOWS WILL MOSTLY BE IN THE UPPER 20S AND LOWER 30S...A BIT COLDER
OVER THE HIGHER BLACK HILLS.

ON MONDAY...AREAS OF SNOW CAN BE EXPECTED IN THE MORNING...WITH THE
MOST PERSISTENT SNOW OVER THE NORTHERN BLACK HILLS AREA. AS THE
STACKED LOW PRESSURE SYSTEM MOVES INTO EASTERN SD/EASTERN NEB DURING
THE DAY...WINDS WILL FURTHER INCREASE AND PCPN WILL EXPAND ACROSS
THE AREA ON THE BACK SIDE OF THE SYSTEM AS SHORTWAVE ENERGY WRAPS
AROUND THE SYSTEM...ESPECIALLY DURING THE AFTERNOON AND EVENING.
PERIODS OF SNOW AND BLOWING SNOW ARE LIKELY ACROSS A GOOD PORTION OF
THE PLAINS THROUGH MONDAY NIGHT. THE EXCEPTION WILL LIKELY BE
DOWNSLOPED AREAS OF SOUTHWESTERN SD...FROM NEAR RAPID CITY TO THE
SOUTHERN BLACK HILLS...HOT SPRINGS...AND EDGEMONT...WHERE THIS
SHOULD MOSTLY BE A WIND EVENT.

A MYRIAD OF WARNINGS AND ADVISORIES HAVE BEEN ISSUED FOR MOST OF THE
CWA...POSSIBLY SUBJECT TO SOME CHANGE OVER THE NEXT 12 TO 24 HOURS
AS CONDITIONS WARRANT. SOUTH CENTRAL SD WILL LIKELY NEED TO BE ADDED
TO THE WINTER WX HEADLINES FOR LATER MONDAY AND MONDAY NIGHT IF THE
CURRENT TRENDS HOLD...BUT WILL LET THE NIGHT SHIFT HAVE ANOTHER LOOK
AT THE POTENTIAL THERE. FORECASTED SNOWFALL TOTALS FOR NOW RANGE
FROM 2 TO 5 INCHES ACROSS MUCH OF FAR NORTHEAST WY AND THE WESTERN
AND SOUTH CENTRAL SOUTH DAKOTA PLAINS TO 6 TO 12+ OVER THE NORTHERN
BLACK HILLS. UP TO AN INCH OR TWO IS POSSIBLE ACROSS DOWNSLOPED
AREAS OF SOUTHWESTERN SD.

.LONG TERM...TUESDAY THROUGH SUNDAY
ISSUED AT 354 PM MST SUN DEC 21 2014

UPPER TROF MOVES EASTWARD INTO MN-MISSOURI VALLEY BY EARLY TUES...
WITH NORTHERLY FLOW ASSOCIATED WITH 120KT JET OVER FRONT RANGE.
MODELS IN GOOD AGREEMENT WITH GENERAL SURFACE LOW PLACEMENT OVER IA
AND STRONG PRESSURE GRADIENT OVER FORECAST AREA MEANS GUSTY WINDS
WILL PERSIST EARLY TUES...AS WELL AS SOME SNOW LINGERING OVER SOUTH
CENTRAL SD AND NORTHERN BLACK HILLS...THEN GRADIENT DECREASES AS
HIGH PRESSURE BUILDS OVER THE GREAT BASIN. NEXT UPPER TROF COMING
ONSHORE PACIFIC NORTHWEST FORMS A BROAD RIDGE OVER THE ROCKIES AND
HIGH PLAINS WED...BUT TROF MOVES EASTWARD ONTO THE PLAINS. GFS HAS
SURFACE LOW WITH TRAILING COLD FRONT DROPPING ACROSS AREA CHRISTMAS
DAY...WHICH WOULD KEEP BETTER CHANCE OF SNOW OVER ND...AND MOSTLY
LIGHT POST-FRONTAL SNOW SHOWERS OVER SD...WHILE ECMWF FORMS LOW OVER
THE CENTRAL PLAINS...AND BAND OF SNOW IS FARTHER SOUTH OVER SOUTHERN
SD AND NE. CYCLONIC NORTHWEST FLOW REMAINS OVER THE AREA BEHIND THE
TROF...WITH OCCASIONAL WAVES DROPPING INTO MAIN TROF PRODUCING
CHANCES OF SNOW. ARCTIC SURFACE HI PRESSURE WILL KEEP TEMPERATURES
COLD.

&&

.AVIATION...(FOR THE 06Z TAFS THROUGH 06Z MONDAY NIGHT)
ISSUED AT 808 PM MST SUN DEC 21 2014

MAINLY VFR CONDITIONS EARLY THIS EVE WILL CONTINUE TO DETERIORATE
LATE TONIGHT AS SNOW DEVELOPS FROM NW TO SE. IFR/LIFR CONDITIONS
POSSIBLE IN THE NRN BLKHLS DUE TO UPSLOPE SNOW. WDSPRD MVFR
CONDITIONS AND LOCALIZED IFR CONDITIONS (MAINLY IN THE NRN BLKHLS) EXPECTED
THROUGH THE DAY MONDAY. GUSTY NW WINDS TO 50KTS EXPECTED LATER
TONIGHT AND MONDAY...STRONGEST ALONG THE ERN SLOPES OF THE BLKHLS.

&&

.UNR WATCHES/WARNINGS/ADVISORIES...
SD...WINTER STORM WARNING UNTIL NOON MST TUESDAY FOR SDZ024.

     WINTER WEATHER ADVISORY FROM MIDNIGHT TONIGHT TO 5 AM MST
     TUESDAY FOR SDZ001-002-012>014-025-031-032-042>044-072-073.

     WIND ADVISORY FROM 8 AM TO 6 PM MST MONDAY FOR SDZ027-030-041-
     074.

     WINTER WEATHER ADVISORY UNTIL NOON MST TUESDAY FOR SDZ028.

     HIGH WIND WARNING FROM 8 AM TO 8 PM MST MONDAY FOR SDZ026.

WY...WINTER STORM WARNING UNTIL NOON MST TUESDAY FOR WYZ057.

     WINTER WEATHER ADVISORY FROM MIDNIGHT TONIGHT TO 5 AM MST
     TUESDAY FOR WYZ056-071.

&&

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

HAZARDOUS WEATHER OUTLOOK...UPDATED
NATIONAL WEATHER SERVICE RAPID CITY SD
235 PM MST SUN DEC 21 2014

SDZ001-002-012>014-024>032-041>044-046-047-049-072>074-WYZ054>058-
071-221400-
HARDING-PERKINS-BUTTE-NORTHERN MEADE CO PLAINS-ZIEBACH-
NORTHERN BLACK HILLS-NORTHERN FOOT HILLS-RAPID CITY-
SOUTHERN FOOT HILLS-CENTRAL BLACK HILLS-SOUTHERN BLACK HILLS-
CUSTER CO PLAINS-PENNINGTON CO PLAINS-HAAKON-FALL RIVER-SHANNON-
JACKSON-BENNETT-MELLETTE-TODD-TRIPP-STURGIS/PIEDMONT FOOT HILLS-
SOUTHERN MEADE CO PLAINS-HERMOSA FOOT HILLS-NORTHERN CAMPBELL-
SOUTHERN CAMPBELL-WESTERN CROOK-WYOMING BLACK HILLS-WESTON-
NORTHEASTERN CROOK-
235 PM MST SUN DEC 21 2014 /335 PM CST SUN DEC 21 2014/

...WINTER STORM WILL BRING SNOW AND STRONG WINDS TONIGHT THROUGH TUESDAY
MORNING...

THIS HAZARDOUS WEATHER OUTLOOK IS FOR WESTERN SOUTH DAKOTA AND
NORTHEASTERN WYOMING.

.DAY ONE...THIS AFTERNOON AND TONIGHT

A STORM SYSTEM WILL MOVE FROM THE ROCKIES INTO THE DAKOTAS
TONIGHT. RAIN THIS AFTERNOON WILL CHANGE TO SNOW OVERNIGHT WITH
1 TO 3 INCHES OF SNOW POSSIBLE OVER THE NORTHERN BLACK HILLS AND
BEAR LODGE MOUNTAINS BY EARLY MONDAY MORNING...WITH LITTLE
ACCUMULATION EXPECTED OVER THE PLAINS.

.DAYS TWO THROUGH SEVEN...MONDAY THROUGH SATURDAY

SNOW WILL CONTINUE ACROSS THE AREA MONDAY INTO TUESDAY MORNING AS
THE STORM SYSTEM STALLS OVER THE NORTHERN PLAINS. PERSISTENT UPSLOPE-
ENHANCED SNOWFALL OVER THE NORTHERN BLACK HILLS AND BEAR LODGE
MOUNTAINS WILL PRODUCE 6 TO 12 INCHES OF SNOW FROM FOUR CORNERS...
TO ONEIL PASS AND CHEYENNE CROSSING...WITH LOCALLY HIGHER AMOUNTS
POSSIBLE. AN AREA OF 4 TO 7 INCHES OF SNOW MAY ACCUMULATE OVER
SOUTH CENTRAL SOUTH DAKOTA...FROM PINE RIDGE TO WINNER. TWO TO 5
INCHES OF SNOW ARE EXPECTED OVER NORTHWESTERN SOUTH DAKOTA AND
ONLY 1 TO 2 INCHES ACROSS THE SOUTHEASTERN FOOTHILLS AND EXTREME
SOUTHWESTERN SOUTH DAKOTA.

STRONG NORTHWEST WINDS WILL DEVELOP MONDAY AS A TIGHT PRESSURE
GRADIENT STRENGTHENS OVER THE REGION...WITH THE STRONGEST WINDS OVER
NORTHWESTERN AND WEST CENTRAL SOUTH DAKOTA. WINDS WILL GUST TO
OVER 50 MPH DURING THE DAY. THE COMBINATION OF STRONG WINDS AND
LIGHT SNOW MAY PRODUCE AREAS OF BLOWING SNOW.

ANOTHER STORM SYSTEM MAY PRODUCE SNOW ON CHRISTMAS DAY.

.SPOTTER INFORMATION STATEMENT...

SPOTTER ACTIVATION WILL NOT BE NEEDED TODAY.

&&

STAY TUNED TO NOAA WEATHER RADIO FOR FURTHER DETAILS OR UPDATES...
OR CHECK OUR WEB SITE AT WEATHER.GOV/RAPIDCITY