Winter Storm in Wyoming, May 1 to 2, 2008

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE BILLINGS MT
359 AM MDT THU MAY 1 2008

.SHORT TERM...VALID FOR TDY AND FRI...

STRONG SPRING STORM SPINNING UP TODAY OVER THE PLAINS WITH DEEP
MOISTURE FLOW AROUND THE EASTERN SIDE OF THIS SYSTEM EXPECTED TO
BRING A SNOW EVENT TO PORTIONS OF EASTERN MONTANA AND NORTHERN
WYOMING. DEEP UPPER LEVEL LOW PRESSURE MOVING FROM UTAH INTO
WESTERN COLORADO EARLY THIS MORNING WITH SOUTHEASTERLY FLOW ALOFT
BRINGING MOISTURE AND PRECIPITATION TO PORTIONS OF SOUTHEAST
MONTANA. SURFACE LOW PRESSURE DEEPENING OVER NEBRASKA WITH AN
INVERTED TROUGH EXTENDING INTO SOUTHEAST MONTANA WITH THE AIRMASS
WEST OF THIS TROUGH COOLER AND DRIER WITH CLEAR SKIES PERMITTING
ADDITIONAL COOLING. AS THE UPPER LOW THE SURFACE LOW TOWARDS
NEBRASKA IT KEEPS THE MOIST FLOW ALOFT OVER THE REGION TODAY AND
TURNS IT EASTERLY WHILE A LOW AT 700 MB STRENGTHENS OVER WESTERN
SOUTH DAKOTA. STRONG MID LEVEL CONVERGENCE AND WEAK UPPER LEVEL
DIVERGENCE WILL COMBINE TO PROVIDE A POTENT PERIOD OF DYNAMIC LIFT
OVER SOUTHEAST MONTANA TODAY WITH THIS BAND EXPECTED TO ELONGATE
TOWARDS CENTRAL MONTANA AND THE BIG HORN MOUNTAINS THIS EVENING.
FOR TODAY HEAVY PRECIPITATION IS EXPECTED OVER AN AREA INCLUDING
CUSTER...CARTER...POWDER RIVER...FALLON AND SOUTHERN ROSEBUD
COUNTIES. CONCERN IS THAT AS THE LIFT CONTINUES THORUGH THE DAY IS
THAT TEMPERATURES IN THE SOUTHERN COUNTIES WILL SUPPORT A CHANGE
TO SNOWFALL WITH THE COOLER AND DRY AIR ADVECTING IN AT THE LOW
LEVELS ON NORTHWEST FLOW HELPING HASTEN THIS CHANGE. THE NORTHERN
COUNTIES WILL BE UNDER THE INFLUENCE OF A EASTERLY LOW LEVEL FLOW
WHICH WILL ADVECT WARMER AIR AND SLOW THE CHANGE TO SNOW. WILL
ISSUE A WINTER STORM WARNING FOR SOUTHERN ROSEBUD...POWDER RIVER
AND CARTER COUNTIES AND SOME OF THESE AREAS SHOULD SEE 6 TO 8
INCHES OF HEAVY WET SNOW TODAY WITH ADDITIONAL ACCUMULATIONS
TONIGHT. STILL NOT CERTAIN ON THE WINTER POTENTIAL FOR AREAS NORTH
AND WEST OF THIS AREA AND HAVE DELAYED THE WINTER STORM WATCH
UNTIL 00Z WHEN COOLER TEMPERATURES WILL PREVAIL OVER CUSTER AND
FALLON COUNTIES AND PRECIPITATION HAS A CHANCE TO EXPAND WESTWARD
INTO SHERIDAN AND SOUTHERN BIG HORN COUNTIES. OF PARTICULAR
CONCERN FOR THE SHERIDAN AREA WOULD BE WINDS CHANNELING ALONG THE
BIG HORN MOUNTAINS PRODUCING BLIZZARD CONDITIONS BUT NOT SURE THE
SNOW IS GOING TO BE THAT HEAVY THAT FAR WEST. MODEL TRENDS HAVE
BEEN TO SHIFT THE WESTWARD EDGE OF PRECIPITATION TO THE EAST SO
LESS CONFIDENT OF THE CHANCE FOR SNOW IN BILLINGS AREA BUT ANY
PRECIP THAT DOES COME THIS FAR WEST WILL EXPERIENCE EVAPORATIVE
COOLING AND WILL LIKELY BE ALL SNOW. BIG HORN MOUNTAINS COULD SEE
HEAVY SNOW LATER THURSDAY NIGHT INTO FRIDAY AS THE FLOW ALOFT
BECOMES UPSLOPE BUT STILL A QUESTION ON HOW FAR WEST THE
PRECIPITATION GETS. THE STORM SYSTEM MAKES SLOW PROGRESS TONIGHT
AND BEGINS ACCELERATING EASTWARD ON FRIDAY SO PRECIPITATION WILL
PERSIST INTO FRIDAY MORNING HELP BRING TOTAL ACCUMULATIONS FOR
HIGHER TERRAIN IN SOUTHEAST MONTANA IN EXCESS OF A FOOT. 06Z WRF
CAME IN LINE WITH THE EARLIER GFS AND ECWMF WITH A MORE EASTERLY
POSITION TO THE UPPER LOW SO CONFIDENCE IN PRECIPITATION BULLSEYES
IS A BIT INCREASED. FRIDAY WILL SEE PRECIPITATION DECREASE ACROSS
THE AREA AS THE UPPER LOW TRACKS TOWARDS IOWA AND AN UPPER LEVEL
RIDGE MOVES INTO THE AREA. TEMPERATURES ON FRIDAY WILL BE LARGELY
DEPENDENT ON SNOWFALL AND HAVE KEPT MUCH BELOW NORMAL TEMPERATURES
OVER SOUTHEAST MONTANA. BORSUM

.LONG TERM...VALID FOR SAT...SUN...MON...TUE...WED...

EXTENDED FORECAST APPEARS IN GOOD CONDITION OVERALL...ONLY A FEW
CHANGES TO POPS AS A FEW PERIODS NOW LOOK DRYER WITH ZONAL FLOW
SETUP. MODELS APPEAR IN GOOD AGREEMENT THROUGH MOST OF THE
EXTENDED PERIOD.

A SHORTWAVE RIDGE MOVES INTO THE AREA ON FRIDAY NIGHT AS STRONG
SPRING WEATHER PRODUCER PUSHES OFF TO THE EAST. THIS BEGINS A
PROGRESSIVE PATTERN OF SHORTWAVE THROUGH GENERALLY ZONAL FLOW AS
SHALLOW PROGRESSION OF RIDGES AND TROFS CAUSE LITTLE PERTURBATION
OF THE FLOW. SHORTWAVES PRIMARY IMPACT IS CONFINED TO THE
MOUNTAINS AND FOOTHILLS WITH OROGRAPHICS BECOMING THE MAIN SOURCE
OF LIFT...AS DRY AIR ALOFT AND GENERALLY STABLE CONDITIONS
DOMINATE THE PLAINS. THE PATTERN CONTINUES UNTIL WEDNESDAY NIGHT WHEN
A STRONGER UPPER LEVEL PACIFIC TROUGH MAKES ITS WAY THOUGH THE
AREA. PRECIP WITH THIS SYSTEM IS ALSO MAINLY CONFINED TO THE
MOUNTAINS...BUT DOES LOOK LIKE IT COULD SPREAD SOME PRECIP INTO THE
PLAINS. HOT ON THE TRAIL OF THIS SYSTEM IS ANOTHER STRONG PACIFIC
TROF...PROGGED TO MOVE INTO THE AREA RIGHT AT THE END OF THE
EXTENDED...AND BRING PRECIP ACROSS THE AREA. GENERALLY RAN CLIMO
POPS FOR THIS SYSTEM...AS IS SO FAR OUT.

AS FOR TEMPERATURES...MADE LITTLE IN THE WAY OF CHANGES FOR TEMPS
OTHER THAN BLEND TO NEIGHBORS AS THEY MADE CHANGES. ZONAL FLOW
POINTS TO A GRADUAL WARM UP TO CLIMO TEMP RANGE AND LITTLE
FLUCTUATION UNTIL MORE ORGANIZED WEATHER SYSTEMS HEAD OUR WAY AT
THE END OF THE PERIOD. GILSTAD

&&

.AVIATION...

VFR CONDITIONS ARE EXPECTED TO PREVAIL WEST OF KBIL...BUT ARE
EXPECTED TO DETERIORATE TO MVFR AND LOCALIZED IFR THROUGH THE DAY TOMORROW.
CONDITIONS EAST OF KBIL WILL DETERIORATE MORE QUICKLY WITH MVFR
CONDITIONS ALREADY DOMINATING EASTERN MOST ZONES WITH LOCALIZED
IFR AND LIFR CONDITIONS IN SNOW AND RAIN SHOWERS. MOUNTAINS ARE
ALSO EXPECTED TO BE OBSCURED THROUGH MUCH THE DAY THURSDAY.

ADDITIONALLY...WINDY CONDITIONS WILL DEVELOP BY MORNING.
WIDESPREAD NORTHWEST WINDS OF 20 TO 30 KTS GUSTING TO 40 KTS ARE
ANTICIPATED FOR THURSDAY. GILSTAD

&&

.PRELIMINARY POINT TEMP/POPS...

    TDY FRI     SAT     SUN     MON     TUE     WED
-------------------------------------------------------
BIL 047 032/046 034/061 037/060 037/061 038/061 040/063
    3/R 74/O    00/B    12/R    22/W    21/B    12/W
LVM 044 030/048 029/059 033/055 031/055 032/056 033/058
    2/O 54/O    00/B    23/R    22/W    22/W    12/W
HDN 044 028/045 028/062 035/064 036/063 035/063 038/063
    6/R 84/O    00/B    01/B    22/W    21/B    12/W
MLS 041 026/041 030/056 034/060 037/060 038/061 040/062
    +/O +2/O    00/B    00/B    12/W    21/B    12/W
4BQ 037 027/034 028/051 031/061 035/059 036/061 038/062
    +/O +6/O    10/B    00/B    12/W    21/B    22/W
BHK 039 025/034 027/049 033/059 034/058 037/059 037/063
    +/O +3/O    00/B    00/B    12/W    21/B    12/W
SHR 037 026/039 023/056 031/057 032/058 032/059 035/061
    7/S 96/O    00/B    12/R    22/W    32/W    22/W

&&

.BYZ WATCHES/WARNINGS/ADVISORIES...
MT...WINTER STORM WATCH IN EFFECT FROM THIS EVENING THROUGH FRIDAY
      MORNING FOR ZONES 32-33-38.
     WINTER STORM WARNING IN EFFECT UNTIL 12 PM MDT FRIDAY FOR
      ZONES 36-37-58.
WY...WINTER STORM WATCH IN EFFECT FROM THIS EVENING THROUGH FRIDAY
      MORNING FOR ZONES 98-99.

&&

WEATHER.GOV/BILLINGS

HAZARDOUS WEATHER OUTLOOK...CORRECTED
NATIONAL WEATHER SERVICE BILLINGS MT
319 PM MDT WED APR 30 2008

MTZ028>042-056>058-063>068-WYZ098-099-012130-
SOUTHERN WHEATLAND-MUSSELSHELL-TREASURE-NORTHERN ROSEBUD-CUSTER-
FALLON-NORTHERN STILLWATER-YELLOWSTONE-POWDER RIVER-CARTER-
SOUTHERN BIG HORN-EASTERN CARBON-NORTHERN PARK-
NORTHERN SWEET GRASS-GOLDEN VALLEY-RED LODGE FOOTHILLS-
NORTHERN BIG HORN-SOUTHERN ROSEBUD-JUDITH GAP-PARADISE VALLEY-
LIVINGSTON AREA-BEARTOOTH FOOTHILLS-ABSAROKA/BEARTOOTH MOUNTAINS-
CRAZY MOUNTAINS-NORTHEAST BIG HORN MOUNTAINS-SHERIDAN FOOTHILLS-
319 PM MDT WED APR 30 2008

THIS HAZARDOUS WEATHER OUTLOOK IS FOR PORTIONS OF CENTRAL
MONTANA...SOUTH CENTRAL MONTANA...SOUTHEAST MONTANA AND NORTH
CENTRAL WYOMING.

.DAY ONE...TONIGHT

ISOLATED RAIN AND SNOW SHOWERS WILL BE OVER THE AREA THIS EVENING.
A DEVELOPING STORM SYSTEM OVER WYOMING WILL BRING AN INCREASING
CHANCE OF PRECIPITATION TO SOUTHEAST MONTANA LATE TONIGHT. THE
PRECIPITATION WILL BE A MIX OF RAIN AND SNOW.

.DAYS TWO THROUGH SEVEN...THURSDAY THROUGH TUESDAY

THE STORM SYSTEM DEVELOPING SOUTHEAST OF THE AREA WILL BRING THE
POSSIBILITY OF HEAVY RAIN AND SNOW TO SOUTHEAST MONTANA FOR
THURSDAY THROUGH FRIDAY AFTERNOON. RAIN IS EXPECTED TO CHANGE TO
ALL SNOW FROM BAKER TO ALZADA BY THURSDAY AFTERNOON...SPREADING
WEST THROUGH LAME DEER...LODGE GRASS AND THEN EXTENDING SOUTH TO
SHERIDAN WYOMING BY THURSDAY NIGHT. SNOW AMOUNTS FROM 6 TO 10
INCHES WILL BE POSSIBLE OVER THESE AREAS THROUGH NOON FRIDAY. IN
ADDITION TO THE POSSIBLE HEAVY SNOW...GUSTY NORTHWEST WINDS WILL
CREATE AREAS OF BLOWING AND DRIFTING SNOW. PERSONS SHOULD MONITOR
THE LATEST FORECASTS FOR UPDATES TO THIS POSSIBLE SIGNIFICANT
SPRING STORM. RANCHERS AND FARMERS SHOULD BEGIN PLANNING FOR THE
PROTECTION OF YOUNG LIVESTOCK DUE TO THE POTENTIAL FOR MUCH COOLER
TEMPERATURES AND POSSIBILITY OF HEAVY WET SNOW.

.SPOTTER INFORMATION STATEMENT...

WEATHER SPOTTERS ARE ENCOURAGED TO REPORT SIGNIFICANT WEATHER
CONDITIONS ACCORDING TO STANDARD OPERATING PROCEDURES.