High Wind in Montana, December 13, 2001

SOUTH CENTRAL AND SOUTHEAST MONTANA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE BILLINGS MT
207 AM MST THU DEC 13 2001

.SHORT TERM...VALID FOR TDY...FRI...SAT...
HAVE ALREADY ISSUED HIGH WIND WARNING FOR PARK AND SWEETGRASS COUNTIES
AS THE SHORTWAVE OVER BC WILL MOVE EAST AND CAUSE PRESSURES TO FALL
OVER ALBERTA TODAY. MODEL PRESSURE GRADIENTS BETWEEN LWT AND IDA
STARTED AT 14 MB AND THEY ARE EXPECTED TO INCREASE TO 17 MB BY 18Z THUS
EXPECT THE VERY STRONG WINDS TO BE SUSTAINED AND EVEN INCREASE. WINDS
AND MIXING WILL BE THE PRIMARY FORECAST INFLUENCES TODAY AS MOST SITES
ARE ALREADY EXPERIENCING THE WINDS AND EXPECT THE WINDS TO REACH ALL
ELEVATIONS DURING THE LATER MORNING HOURS. THE SHORTWAVE WHICH HELPS
REINFORCE THE PRESSURE GRADIENTS ACTUALLY DIGS INTO NORTHCENTRAL
MONTANA THIS AFTERNOON BUT VERTICAL MOTION ASSOCIATED WITH THIS WAVE
STAYS NORTH OF THE FORECAST AREA. THIS WAVE WEAKENS AS IT REACHES THE
EAST TONIGHT AS A VERY STRONG WAVE BEGINS TO MOVE ONTO THE WEST COAST.
THIS WAVE WILL SPLIT AND SEND THE STRONGEST ENERGY INTO NORTHER
CALIFORNIA BY 12Z WHILE A SERIES OF WEAKER WAVES ROTATE ACROSS
NORTHWEST MONTANA ON FRIDAY. THIS CONTINUES TO MAINTAIN THE DOWNSLOPE
GRADIENTS BUT THE SOUTHERN PIECE OF ENERGY ERODES THE SNAKE RIVER
VALLEY HIGH PRESSURE AND LIMITS THE STRENGTH OF THE GRADIENT...THUS
WINDS SHOULD BE DOWN SOME ON FRIDAY. HOWEVER...TEMPS SHOULD BE UP A
FEW DEGREES FROM TODAYS HIGHS ESPECIALLY WITH A MILDER START. MODELS
DIVERGE RAPIDLY IN THEIR SOLUTIONS AFTER 18Z ON FRIDAY AND THIS IS
CAUSING CONSIDERABLE FORECAST UNCERTAINITY. THE AVN BEGINS TO DEEPEN A
WAVE OVER THE PANHANDLE OF IDAHO WHICH EVENTUALLY BECOMES A CLOSED
CIRCULATION OVER WEST CENTRAL MONTANA BY 12Z SATURDAY. THE ETA DOES NOT
INDICATE AS MUCH DEVELOPMENT AND THE 06Z ETA DOES NOT SHOW ANY CHANGES.
AS A RESULT...THE AVN SURFACE PATTERN IS MUCH DEEPER ACROSS MONTANA WITH
A SURFACE WAVE MOVING ACORSS DEVELOPING OVER SOUTHEAST MONTANA WHEREAS
THE ETA DOES NOT SHOW NEARLY AS STRONG A FEATURE. THE ETA IS A BIT
SLOWER ALOFT AND DOES NOT BRING THE FRONT THROUGH THE BILLINGS AREA
UNTIL MIDNIGHT FRIDAY NIGHT WHILE THE AVN IS INDICATING A 00Z FROPA. DUE
TO THE UNCERTAINITY...WILL NOT BE MAKING ANY CHANGES LATER PERIODS OF
FORECAST. PRIMARY CONCERN IS THAT THE ETA INDICATES A LESS THREATENING
SCENARIO AND THE AVN HAS SHOWN LESS RUN TO RUN CONSITENCY. BORSUM

.LONG TERM...VALID FOR SUN...MON...TUE...WED...
MODELS CONTINUE TO BE IN FAIRLY GOOD AGREEMENT THROUGH THE EARLY PART OF
THE EXTENDED. COLD AIR MOVING IN SUNDAY NIGHT LEAVES THE WEST QUICKLY
AND LINGERS A LITTLE IN THE EAST AS UPPER TROUGH MOVES OFF. GOOD WARM
AIR ADVECTION MOVES IN BEHIND THE TROUGH RESULTING IN WARMING
TEMPERATURES FOR BOTH SUNDAY AND MONDAY...WILL WARM TEMPERATURES
SLIGHTLY TO REFLECT THIS. ANOTHER SYSTEM DEVELOPS OFF THE PACIFIC
NORTHWEST AND TRACKS EAST...WELL NORTH OF THE CANADIAN BORDER ON MONDAY.
THE MODELS AGREE THIS SYSTEM WILL SPLIT WITH THE FURTHEST SOUTH OF THE
SURFACE LOWS TRACKING WELL NORTH OF MONTANA. THE BULK OF THE ENERGY
FROM THIS SYSTEM REMAINS TO THE NORTH AS WELL AND THE GREATEST
EFFECT FROM THIS CONTINUES TO BE IN THE EASTERN SLOPES WHERE WINDY
CONDITIONS LOOK TO REMAIN...WITH DRY WEATHER HOLDING ACROSS THE REGION
THROUGH MONDAY AS WELL. MODELS CONTINUE INCONSISTENCY WITH WEAK WAVE
MOVING THROUGH THE AREA ON TUESDAY...THOUGH CURRENT MENTION OF COOLING
TREND AND PRECIP LOOKS TO HAVE THE SYSTEM WELL HANDLED. MACZKO

    TDY FRI     SAT     SUN     MON     TUE     WED
-------------------------------------------------------
BIL 042 031/045 018/027 015/037 023/041 020/032 016/030
    0/N 00/E    33/J    00/B    00/B    22/E    22/E
LVM 040 027/040 016/025 018/035 023/039 019/030 014/028
    0/N 00/N    33/J    00/N    00/N    22/E    22/E
HDN 044 030/047 017/027 014/038 021/042 018/034 014/032
    0/N 00/E    33/J    00/B    00/B    22/E    22/E
MLS 040 025/042 019/025 009/030 016/035 018/029 014/027
    0/B 00/E    44/S    00/B    00/B    22/E    22/E
4BQ 040 023/043 018/027 010/030 015/035 017/030 013/028
    0/B 00/E    44/S    00/B    00/B    22/E    22/E
BHK 038 022/040 015/023 008/028 014/034 016/028 012/026
    0/B 00/E    44/S    00/B    00/B    22/E    22/E
SHR 039 023/041 015/029 007/035 013/039 015/032 011/030
    0/N 00/E    33/J    00/B    00/B    22/E    22/E

.BYZ...HIGH WIND WARNING UNTIL 5 PM MT ZONES 040-041.