Winter Storm Warning 16, NWS Riverton, WY

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
351 PM MST WED DEC 24 2008

.SHORT TERM...TONIGHT THROUGH SATURDAY
STRONG STORM SYSTEM DIGGING SOUTH SOUTHEAST ALONG THE BC COAST THIS
AFTERNOON...AND IS PROGGED TO TAKE AN EAST TURN OVERNIGHT.  THIS
WILL BRING A SIGNIFICANT WINTER STORM TO WESTERN WYOMING CHRISTMAS
EVE...DAY AND NIGHT.  IT WILL ALSO BRING HIGH WINDS TO SOUTHEAST
FREMONT COUNTY AND NATRONA COUNTY...AND POSSIBLY EASTERN SWEETWATER
COUNTY ON THURSDAY.  ISENTROPIC LIFT IS BRINGING VERY LIGHT SNOW TO THE
WESTERN MOUNTAINS THIS AFTERNOON.  THIS WILL INTENSIFY THIS EVENING
AS WINDS AND MOISTURE INCREASE WITH FLOW BACKING TO THE SOUTHWEST.
RRQ JET DYNAMICS INCREASE OVER WESTERN WYOMING TONIGHT AS WELL WHICH
WILL LIKELY RESULT IN SNOWFALL RATES OF AN INCH OR TWO PER HOUR OVER
THE WEST.  THESE DYNAMICS CONTINUE THROUGH MUCH OF CHRISTMAS DAY
WITH SOME LFQ DYNAMICS BUILDING IN THE AFTERNOON.  WILL ALSO GET
SOME INSTABILITY AS WELL SO THERE SHOULD BE A CONVECTIVE ELEMENT.
HEAVIEST SNOWFALL WILL LIKELY FALL OVER SALTS AND WYOMING RANGE
UNDER SOUTHWEST FLOW WITH 1 TO 2 FEET WITH LOCALLY HIGHER AMOUNTS TO
3 FEET.  THE TETONS...GROS VENTRES...WEST WIND RIVERS AND SOUTHERN
YELLOWSTONE SHOULD GET 1 TO 2 FEET AS WELL.  THE SOUTHERN ABSAROKAS
ARE LOOKING TO GET 10 TO 15 INCHES.  EAST SLOPES OF THE WINDS WILL
SEE SIGNIFICANTLY LESS...BUT STRONG WINDS WILL CREATE NEAR BLIZZARD
CONDITIONS OVER SOUTH PASS.  SOUTH TO SOUTHWEST WINDS WILL INCREASE
AS WELL WITH SUMMITS AND PASSES OF THESE RANGES SEEING STRONG WINDS
WITH GUSTS TO 65 MPH POSSIBLE.  THIS WILL LEAD TO SIGNIFICANT
BLOWING AND DRIFTING SNOW.  HAVE UPGRADED THE WINTER STORM WATCH TO
A WARNING FOR ALL ZONES.  HIGH WINDS ARE EXPECTED WITH NAM12 SHOWING
SOUTHWEST FLOW OF 50KT+ AT H7 OVER THE GREENS AND NATRONA COUNTY
THROUGH THE DAY TOMORROW.  EASTERN SWEETWATER COUNTY GETS THESE
WINDS TOMORROW.  WILL ALSO HAVE A DECENT GRADIENT IN THESE AREAS AS
WELL.  HAVE ISSUED A HIGH WARNING TO COVER THIS THROUGH 00Z FRIDAY.
ALSO HAVE A HIGH WIND WATCH FOR EASTERN SWEETWATER COUNTY 12Z THU TO
00Z FRI.  LOOKED INTO THE POSSIBILITY OF THE LANDER
FOOTHILLS...THOUGH FROUDE NUMBER TOO LOW...AROUND 27 AND THERE IS A
LACK OF STABLE LAYER NEAR THE MOUNTAIN TOP.  THE WIND RIVER BASIN
COULD ALSO SEE SOME WINDS BUT DONT SEE IT REACHING HIGH WIND
CRITERIA.

COLD FRONT MOVES THROUGH JUST AHEAD OF THE TROUGH AXIS THURSDAY
NIGHT WITH SNOW SPREADING EAST OF THE DIVIDE.  MODELS DIFFER HERE
WITH NAM12 BRINGING IN A FAVORABLE NORTH TO NORTHEAST UPSLOPE
FLOW...WHILE GFS KEEPS IT MORE NORTHWEST.  HAVE INCREASED POPS TO
LIKELY OVER JOHNSON AND NATRONA COUNTY...BUT LEFT THEM AS CHANCE
OVER THE BIG HORN BASIN AND WIND RIVER BASIN WITH THE HOPE MODELS
WILL COME TO AN AGREEMENT.

DOUBTFUL THE SNOW WILL TOTALLY STOP IN THE WESTERN MOUNTAINS FRIDAY
WITH TROUGH AXIS STILL MOVING THROUGH.  ISENTROPIC LIFT INCREASES
BEHIND THE TROUGH AXIS AS FLAT RIDGE BUILDS IN.  THE ISENTROPIC LIFT
AND MOISTURE INCREASE FRIDAY NIGHT AND SATURDAY.  COULD SEE A FEW
INCHES OUT OF THIS BUT NOT LIKELY TO BE IN NEED OF ANY HIGHLIGHTS.

.LONG TERM...SATURDAY NIGHT THROUGH WEDNESDAY
GFS AND ECMWF BOTH AGREE THAT A STRONG W TO NW FLOW WILL DOMINATE
THE MEDIUM RANGE PERIOD. THE RESULT WILL BE PERIODS OF SNOW WEST OF
THE DIVIDE AND WINDY EAST OF THE DIVIDE. RIDGE FLATTENS SATURDAY
BEHIND OUR BIG CHRISTMAS STORM ALLOWING THE FIRST GOOD SHOT OF SNOW
OUT WEST ESPECIALLY BY SATURDAY NIGHT AND CONTINUING THROUGH SUNDAY.
AFTER THAT...THE SNOW SPIGOT WILL BE HARD TO TURN OFF OUT WEST WITH
SNOW OFF AND ON...MORE ON...THROUGH THE REMAINDER OF THE PERIOD.
GENERAL LIGHT SNOW WILL PREVAIL WITH PERIODS OF MODERATE OR GREATER
SNOW WITH EVERY PASSING DISTURBANCE WHICH WILL BE HARD TO TIME THIS
FAR OUT. AREAS EAST OF THE DIVIDE SHOULD SEE PLENTY OF WIND...
ESPECIALLY THE WIND CORRIDOR AND CODY FOOTHILLS BUT EVEN THE WIND
RIVER BASIN WILL LIKELY MIX OUT IN FAVORABLE WNW TO NW MID LEVEL
FLOW. TEMPERATURES WILL THUS BE TRICKY DEPENDING ON HOW MUCH WIND
OCCURS VS THE BASIN INVERSIONS THAT WILL TRY TO STAY IN PLACE. HAVE
PUT THE LEAST AMOUNT OF WIND IN THE BIG HORN BASIN ALONG WITH THE
COOLEST TEMPS. IF THE HIGHS ERROR...IT WILL LIKELY STILL BE ON THE
COOL SIDE...ESPECIALLY IF MORE AREAS SEE THE WIND OFF AND ON. H7
TEMPS VARY QUITE A BIT AT TIMES BETWEEN THE GFS AND THE EURO BUT ARE
GENERALLY IN THE -4 TO -8C RANGE EAST OF THE DIVIDE WITH THE
EXCEPTION OF A SHORTWAVE THAT THE MODELS HAVE PASSING BY TO THE
NORTH EITHER TUESDAY...GFS...OR WEDNESDAY...ECMWF...WHEN H7 TEMPS
DROP TO BELOW -10C ACROSS THE NORTH HALF. WESTERN VALLEY TEMPS WILL
ALSO BE TRICKY AS MEX GUIDANCE IS GENERALLY IN THE SINGLE DIGITS
EVERY NIGHT EVEN WITH SNOW FALLING. HAVE SIDED WITH WARMER TEMPS
THAN GUIDANCE WITH PLENTY OF CLOUD COVER AND OFF AND ON
SNOW...SIMILAR TO WHAT HAPPENED RECENTLY WITH RELATIVELY MILD
OVERNIGHT LOWS IN JAC.

&&

.AVIATION...
CIGS AND VISIBILITIES WILL GRADUALLY LOWER WEST OF THE CONTINENTAL
DIVIDE WITH THE ONSLAUGHT OF A POWERFUL PACIFIC WEATHER SYSTEM FROM
THE WEST. VFR CONDITIONS WILL GIVE WAY TO MVFR CONDITIONS BY THIS
EVENING...THEN TO IFR AND LIFR CONDITIONS BY 06Z TONIGHT AS A RESULT
OF LOW CEILINGS AND MODERATE TO HEAVY SNOWFALL. KJAC...KPNA AND KBPI WILL
ALL BE AFFECTED BY THESE WEATHER CONDITIONS. IFR CONDITIONS CAN BE
EXPECTED AT KRKS BY 17Z THURSDAY.

EAST OF THE DIVIDE...STRONG SOUTHWEST WINDS CAN BE EXPECTED AHEAD OF
THE ASSOCIATED TROUGH AXIS FROM EASTERN SWEETWATER COUNTY TO CASPER
TODAY THROUGH THURSDAY WHERE WINDS WILL BLOW FROM 35 TO 45 MPH WITH
GUSTS TO 60 MPH.

&&

.FIRE WEATHER...
A STRONG STORM SYSTEM WILL BRING HEAVY SNOW TO THE WEST TONIGHT
THROUGH FRIDAY MORNING. NUMEROUS WINTER STORM WARNINGS ARE IN
EFFECT TONIGHT THROUGH FRIDAY MORNING. IT WILL ALSO BRING VERY WINDY
CONDITIONS SOUTH AND CENTRAL...WHERE A FEW HIGH WARNINGS AND
WATCHES ARE IN EFFECT. A COLD FRONT MOVES THROUGH FRIDAY
NIGHT...SPREADING THE SNOW EAST OF THE DIVIDE.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
HIGH WIND WATCH FROM LATE TONIGHT THROUGH THURSDAY AFTERNOON FOR
WYZ030.

HIGH WIND WARNING UNTIL 5 PM MST THURSDAY FOR WYZ019-020.

WINTER STORM WARNING UNTIL 5 AM MST FRIDAY FOR WYZ001-002-
012>015-023>027.

&&

SHORT TERM...TWB
LONG TERM...SKRBAC
AVIATION...LIPSON
FIRE WEATHER...TWB