Winter Storm Warning 3, NWS Riverton, WY

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
233 AM MST SAT JAN 19 2008

.SHORT TERM...TODAY THROUGH MONDAY NIGHT
VIGOROUS UPPER LOW VERY APPARENT ON IR AND WATER VAPOR SATELLITE IMAGERY
THIS MORNING. THIS FEATURE IS PROGGED TO CONTINUE TO MOVE SOUTH-
SOUTHEAST OVER WASHINGTON STATE DURING THE DAY. WARM AIR ADVECTIVE
SNOW ALREADY BEGINNING OVER THE NORTHWEST AND NORTH-CENTRAL
PORTIONS OF THE STATE IN AN INCREASING WEST-NORTHWEST FLOW AT H7.
AS A HIGH AMPLITUDE UPPER-LEVEL RIDGE OVER THE GULF OF ALASKA
BEGINS TO POSITIVELY TILT...THE VIGOROUS UPPER LOW BEGINS TO SPLIT
DURING THE DAY SUNDAY WITH THE MAIN PORTION RETROGRADING WESTWARD
OVER THE NORTHERN CALIFORNIA COAST AND A SECOND...WEAKER...PORTION
MOVING INTO SOUTHERN IDAHO. ENOUGH ISENTROPIC UPGLIDE PRESENT
TODAY AND Q-G FORCING TONIGHT WITH THIS FEATURE TO BRING
WIDESPREAD SNOW TO THE WEST AND NORTH. AS THE UPPER RIDGE IN THE
GULF OF ALASKA TILTS EASTWARD TODAY...IT PUSHES DOWN A DEEP ARCTIC
AIRMASS ALONG THE LEE OF THE MONTANA ROCKIES TODAY AND THROUGH
CENTRAL WYOMING SUNDAY. THE ARCTIC AIR THEN MOVES THROUGH
SOUTHERN AND WESTERN AREAS OF THE STATE SUNDAY NIGHT INTO MONDAY.
FAIRLY DEEP UPSLOPE FLOW IN COMBINATION WITH THE UPPER LOW TO THE
WEST WILL ALSO BRING WIDESPREAD SNOWFALL TO AREAS EAST OF THE
DIVIDE SUNDAY AND SUNDAY NIGHT IN THE NORTH...ALONG WITH AREAS OF
BLOWING AND DRIFTING. MAIN SNOW EVENT FOR THE SOUTH INCLUDING SWEETWATER
COUNTY EXPECTED TO HOLD OFF UNTIL SUNDAY NIGHT AND CONTINUE INTO
MONDAY MORNING. DECIDED TO HOLD OFF ON WINTER HIGHLIGHTS FOR NOW
WITH ADVISORY LEVEL SNOW AMOUNTS EXPECTED. PLENTY OF WIND AND VERY
COLD TEMPERATURES WILL MAKE THINGS QUITE NASTY BUT MOISTURE
ADVECTION FROM THE PACIFIC LIMITED AND SNOW RATIOS WILL QUICKLY
FALL OFF INTO THE 10 TO 1 RANGE ONCE THE ARCTIC FRONT MOVES
THROUGH. VERTICAL TEMPERATURE PROFILE ( -20C AT H7 AND  -30C AT
H5) BY SUNDAY EVENING BECOMES INDICATIVE OF LOW SNOW RATIOS.

.LONG TERM...TUESDAY THROUGH SATURDAY
LONG RANGE MODELS SEEM TO AGREE ON OVERALL PATTERN THROUGH NEXT
SATURDAY. HOWEVER...THE MOVEMENT OF WEATHER SYSTEMS ACROSS THE
REGION VARIES AND THIS MAKES THE FORECAST MORE CHALLENGING. ON
TUESDAY SOME SORT OF UPPER LOW FINDS ITS WAY TO CALIFORNIA WITH A
DEFORMATION AXIS LYING ACROSS SOUTHERN WY OR JUST SOUTH OF THE
BORDER. MEANWHILE A SHORTWAVE IS FORECAST TO MOVE INTO NORTHERN
MONTANA BY 00Z WED PER THE GFS. THE ECMWF IS SIMILAR AT THAT TIME.
MOST OF THE REGION SHOULD BE DRY ON TUE. DID LEAVE SLIGHT CHANCE
OF SNOW ALONG THE SOUTHERN WY BORDER OF OUR CWA. SOME MODERATION
OF HIGH TEMPS TUE. HOWEVER THE SNOW COVERED BASINS WILL NOT SEE AS
MUCH WARMING. THE SHORTWAVE DIVES SOUTH SOUTHEAST INTO WY LATE TUE
NIGHT AND THROUGH WY WED. WILL KEEP PRECIP CHANCES LITTLE CHANGED
DURING THIS TIME FRAME. LITTLE WARMER WED DUE TO MIXING WITH THE
TROUGH MOVING THROUGH THE AREA. GFS/ECMWF WISH TO TAKE THE UPPER
LOW INTO THE SOUTHWEST U.S. WED NIGHT. THEN THEY MOVE IT ACROSS
UTAH THURSDAY AND INTO COLORADO ON THURSDAY NIGHT AS AN OPEN
TROUGH. IF THIS DOES OCCUR SOME PRECIP MAY NEEDED TO BE ADDED
ACROSS THE SOUTH WED NIGHT THROUGH THURS NIGHT. WILL SEE WHAT THE
MODELS LOOK LIKE TONIGHT. FLOW ALOFT BECOMES NW FOR FRIDAY WITH A
SHORTWAVE APPROACHING WESTERN WY AT 00Z SAT. HAVE ADDED SOME
PRECIP CHANCES TO THE WESTERN MTS AND FAR WEST VALLEYS FRI. THE
TROUGH MOVES ACROSS WY FRIDAY NIGHT SPREADING ITS SNOW MAINLY OVER
THE MOUNTAINS. NEXT SATURDAY MAY BE UNSETTLED OVER THE WEST WITH A
MOIST WEST FLOW ALOFT INDICATED. HAVE KEPT A CHANCE OF SNOW OVER
THE WEST ZONES WITH DRY WEATHER EAST OF THE DIVIDE. LITTLE DAY TO
DAY CHANGE IN HIGHS WED THROUGH SATURDAY WITH HIGHS IN THE TEENS
AND 20S. LOWS WILL VARY WIDELY DUE TO CLOUD COVER AND SNOW
COVER...RANGING FROM 10 ABOVE TO 10 BELOW ZERO...WITH COLDER READINGS
IN THE FAR WEST VALLEYS.

&&

.AVIATION...
LIGHT SNOW AND PREVAILING MVFR WITH LOCAL IFR CIGS AND VSBYS WILL
SPREAD ACROSS MUCH OF FAR WEST WYOMING AND THE WESTERN MOUNTAINS INCLUDING
JAC BY 15Z TODAY AND CONTINUE INTO SATURDAY NIGHT. VFR CIGS BETWEEN
100-120KFT MSL WILL PREVAIL EAST OF THE DIVIDE THROUGH SATURDAY
MORNING WITH THIS DECK BECOMING SCT-BKN IN THE AFTERNOON.
SOUTHWEST WIND WILL INCREASE TO 20-30KTS THROUGH CPR AND VICINITY WITH
LOCAL BLSN AND LIMITED SFC VSBY.

&&

.FIRE WEATHER...
A SIGNIFICANT WINTER STORM IS EXPECTED TO BRING WIDESPREAD SNOW
BOTH WEST AND EAST OF THE DIVIDE TODAY THROUGH MONDAY MORNING. AN
ARCTIC AIRMASS WILL MOVE FROM NORTHEAST TO SOUTHWEST THROUGH THE
DISTRICT ON SUNDAY INTO MONDAY MORNING FOR WINDY AND VERY COLD
CONDITIONS ALONG WITH AREAS OF BLOWING AND DRIFTING SNOW.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...NONE.

&&

SHORT TERM...CHURCH
LONG TERM...AR
AVIATION...AEM
FIRE WEATHER...CHURCH

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