FXUS63 KUNR 080311 RRA
AFDUNR

AREA FORECAST DISCUSSION FOR WESTERN SD AND NORTHEASTERN WY
NATIONAL WEATHER SERVICE RAPID CITY SD
810 PM MST MON DEC 7 2009

.DISCUSSION...WATER VAPOR IMAGERY SHOWS A DEEP TROUGH MOVING INLAND 
FROM THE PACIFIC COAST WITH INCREASING SOUTHWESTERLY FLOW ALOFT OVER 
THE NORTHERN PLAINS. REGIONAL RADAR MOSAIC SHOWS LIGHT SNOW 
INCREASING IN COVERAGE AGAIN OVER THE FORECAST AREA.

A STRONG VORT MAX WILL SWING AROUND THE BASE OF THE TROUGH ACROSS 
THE SOUTHERN ROCKIES TONIGHT BEFORE LIFTING INTO THE CENTRAL PLAINS 
TUESDAY. A SURFACE LOW WILL SLIDE ACROSS SOUTHERN COLORADO WITH 
SOUTHEASTERLY WINDS INCREASING OVERNIGHT. ISENTROPIC LIFT WILL ALSO 
INCREASE OVERNIGHT WITH LIGHT SNOW CONTINUING ACROSS MUCH OF THE 
AREA. ACCUMULATIONS OF AN INCH OR SO CAN BE EXPECTED OVERNIGHT. LOWS 
TONIGHT WILL RANGE FROM TEN BELOW TO 5 ABOVE ZERO. ISENTROPIC LIFT 
WILL CONTINUE TUESDAY WITH THE INCREASING SOUTHEASTERLY SURFACE 
WINDS BACKING TO THE EAST THROUGH THE DAY. THE HIGHEST SNOW AMOUNTS 
WILL BE SEEN ACROSS THE FAR SOUTHERN EDGES OF THE CWA...PARTICULARLY 
ACROSS SOUTH CENTRAL SOUTH DAKOTA WHERE SOME AREAS WILL SEE TWO TO 
FOUR INCHES OF SNOW. SO WILL ISSUE A WINTER WEATHER ADVISORY FOR 
BENNETT...TODD...AND TRIPP COUNTIES. OTHERWISE ANOTHER INCH OR SO OF 
ACCUMULATION IS EXPECTED ACROSS FAR WESTERN SOUTH DAKOTA INTO 
NORTHEAST WYOMING. THE EASTERLY WINDS SHOULD BRING SOME UPSLOPE 
ENHANCEMENT TO THE SNOW ACROSS THE EASTERN FOOTHILLS...SO SOME 
LOCATIONS NEAR...OR SOUTH OF RAPID CITY COULD SEE AMOUNTS CLOSER TO 
TWO INCHES BY THE END OF TUESDAY. AS A STRONG WINTER STORM CROSSES 
THE CENTRAL PLAINS TUESDAY AFTERNOON...WINDS WILL CONTINUE TO BACK 
TO THE NORTH. ISENTROPIC LIFT WILL END ACROSS THE CWA TUESDAY 
EVENING WITH PRECIPITATION QUICKLY DISSIPATING. VERY COLD 
TEMPERATURES ARE EXPECTED ACROSS THE AREA TUESDAY NIGHT WITH LOWS 
ZERO TO 15 BELOW. DRIER BUT STILL COLD WEATHER WILL CONTINUE 
WEDNESDAY AS THE TROUGH AXIS SHIFTS EAST OF THE REGION. A SERIES OF 
WAVES EMBEDDED IN THE NORTHWEST FLOW WILL BRING A SLIGHT CHANCE FOR 
SNOW TO NORTHWEST SOUTH DAKOTA AND NORTHEAST WYOMING...BUT NO 
SIGNIFICANT PRECIP IS EXPECTED.

EXTENDED...THE BIG CENTRAL NOAM TROUGH WILL VERY SLOWLY START TO 
BREAK DOWN DURING THE LAST HALF OF THE WEEK AS MIDLEVEL HEIGHTS 
CONCURRENTLY RISE. THE UPPER FLOW THUS WILL TRANSITION TO A MORE 
ZONAL PATTERN BY THE WEEKEND (WHICH HAS BEEN PUSHED BACK FROM THE 
FORECAST YESTERDAY). A SHORTWAVE TROUGH WILL MOVE THROUGH THE 
NORTHWEST FLOW ON FRIDAY...BUT GIVEN THE DRY AIR MASS DO NOT SEE 
MUCH POTENTIAL FOR PRECIPITATION. THERE IS CONSIDERABLE SPREAD IN 
THE ENSEMBLE DATA FOR THE WEEKEND REGARDING THE POTENTIAL FOR 
SHORTWAVE RIDGING AND WARMING...THUS HAVE TRIMMED BACK ON HIGH 
TEMPERATURES AS THEY WERE FAIRLY OPTIMISTIC.

&&

.AVIATION...UPPER-LEVEL STORM SYSTEM WILL MOVE ACROSS CENTRAL PLAINS 
TONIGHTBRINGING LIGHT SNOW TO MUCH OF WRN SD/NERN WY...ESPECIALLY 
SOUTH OF I-90. GOOD MODEL AGREEMENT SUGGESTS FAIRLY HIGH CONFIDENCE. 
THE RESULT WILL BE WIDESPREAD IFR CIGS/VSBYS IN SNOW AS CONDITIONS 
CONTINUE TO DETERIORATE TONIGHT. KGCC WILL HAVE LESS SNOW THAN 
KRAP...AND CIGS/VSBYS MAY BEGIN IMPROVING AT KGCC TUESDAY 
MORNING...BUT REMAIN POOR AT KRAP THROUGH MUCH OF THE DAY.

&&

.UNR WATCHES/WARNINGS/ADVISORIES...
SD...WINTER WEATHER ADVISORY FROM 6 AM MST /7 AM CST/ TO 6 PM MST /7 
     PM CST/ TUESDAY FOR BENNETT-TODD-TRIPP.

WY...NONE.
&&

$$