FXUS65 KRIW 232108
AFDRIW
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
208 PM MST WED DEC 23 2009
.SHORT TERM...TONIGHT THROUGH FRIDAY NIGHT
NORTHERLY FLOW ALOFT...WITH AN 120 KT JET OVER WESTERN MONTANA...
EXTENDING FROM THE YUKON SOUTH ACROSS ALBERTA HAS PUSHED A COLD
ARCTIC SURFACE HIGH PRESSURE CENTER SOUTH INTO NORTH CENTRAL
MONTANA WITH A PRESSURE OF 1035 MB AND AFTN TEMPERATURES AROUND
-8F. SOUTH OF THE HIGH EAST NORTHEAST SURFACE WINDS HAVE BROUGHT
LOW STRATUS...FOG AND SNOW TO MOST AREAS EAST OF THE CONTINENTAL
DIVIDE. STRONG NORTH WINDS ARE BLOWING ACROSS THE EASTERN PLAINS
OF JOHNSON AND NATRONA COUNTIES AND THE BIGHORN BASIN. SNOW WILL
CONTINUE THROUGH THE EVENING THEN TAPER OFF TOWARDS MIDNIGHT. A
WINTER WEATHER ADVISORY WILL REMAIN IN EFFECT UNTIL 11 PM MST FOR
JOHNSON...NATRONA...FREMONT AND NORTHERN SWEETWATER COUNTIES.
SNOWFALL OF 2 TO 5 INCHES WITH LOCAL HIGHER AMOUNTS TO 8 INCHES IN
THE HIGHER ELEVATIONS IS EXPECTED. COLDER TEMPERATURES WITH VERY
LOW SUN ANGLES AND THE SHORTEST DAYS OF THE YEAR WILL BRING COLD
TEMPERATURES TO THE AREA TONIGHT THROUGH FRIDAY NIGHT. AS THE HIGH
PRESSURE CENTERS IN OVER THE AREA THE WIND WILL DIMINISH AND THE
SNOW WILL END. STRATUS AND FOG WILL BECOME TRAPPED IN THE BASINS.
WITH VERY COLD AIR ALOFT AND VERY LIMITED MOISTURE...BANDS OF
LIGHT SNOW SHOWERS IN NORTH NORTHWEST FLOW ALOFT WILL CONTINUE
THURSDAY INTO THURSDAY EVENING. THEN DRY BUT COLD CONDITIONS ARE
EXPECTED AS A HIGH PRESSURE RIDGE ALOFT OFF THE WEST COAST SHIFTS
EAST. FORECAST THEN BECOMES LARGELY DOMINATED BY
TEMPERATURES...MAINLY HOW COLD WILL IT GET. THERE IS A FAIR DEGREE
OF SPREAD IN THE MODEL FORECAST TEMPERATURES. THE CURRENT FORECAST
IS LIKELY TOO WARM...ESPECIALLY AT NIGHT. ONCE A TREND IS
ESTABLISHED MORE CONFIDENCE WILL RESULT IN FORECASTING COLDER
TEMPERATURES. AT THIS TIME DAILY HIGHS ARE GENERALLY IN THE SINGLE
DIGITS AND TEENS WITH LOW TO -20 IN THE UPPER GREEN RIVER BASIN.
.LONG TERM...SATURDAY THROUGH WEDNESDAY
MEDIUM RANGE MODELS BEGIN IN REASONABLE AGREEMENT BUT BY THE END
OF THE PERIOD A WIDE RANGE OF SOLUTIONS EXIST. PATTERN IS VERY
COMPLEX AND VERY MUCH WINTER LIKE WITH THE MODELS CHANGING EACH
RUN. THE GFS RECHARGES THE RIDGE TO OUR WEST EARLY NEXT WEEK AND
ACTUALLY HAS A SHORTWAVE DROPPING DOWN FROM THE SOUTH ON TUESDAY.
THE ECMWF HAS MORE OF AN INFLUENCE FROM THE NEXT DESERT SW UPPER
LOW. THE GEM AT 144HRS IS PROBABLY CLOSER TO A WEAKER VERSION OF
THE ECMWF. THE UKMET HAS A STRONGER DESERT SW TROUGH LIKE THE
EURO. THE DESERT TROUGH SOLUTIONS HAVE MOST OF THE ENERGY TOO FAR
SOUTH FOR US BUT THE EURO DOES LIFT THE TROUGH NE TUESDAY NIGHT
ALLOWING A LITTLE BIT OF QPF INTO OUR FAR SRN ZONES. AT THE SAME
TIME...THE GFS HAS A LITTLE QPF DROPPING SOUTH WITH THE CLIPPER
TYPE SYSTEM. KEEPING A DRY FORECAST FOR NOW AS NEITHER SOLUTION
WOULD PRODUCE MUCH IF ANY SNOW ACROSS OUR AREA. WE MAY STILL BE
UNDER THE INFLUENCE OF THE MONSTER UPPER LOW BY SATURDAY WITH COLD
NLY FLOW BUT PCPN WILL LIKELY BE JUST EAST OF US BY THEN. WE
SHOULD THEN SEE A COUPLE DAYS OF WARMING ALOFT WITH STEEPENING
BASIN AND VALLEY INVERSIONS. HOLDING OFF ON ANY SIGNIFICANT FOG
YET AS RIDGE WEAKENS RATHER QUICKLY AND SOME CLOUD COVER MAY
HAMPER FORMATION. OTHERWISE...COLD PERIOD FOR THE TRAPPED BASIN
AND VALLEYS WITH SOME MODERATION WHERE BREEZES KICK UP. COULD BE A
COUPLE BITTER COLD MORNINGS OVER THE WEEKEND BEFORE SOME
MODERATION OCCURS.
&&
.AVIATION...
LOW VSBY AND LOW CLOUDS ASSOCIATED WITH SNOWFALL FROM A STORM
CENTERED WELL TO THE SOUTH OVER NEW MEXICO WILL BE REPLACED LATE
THIS AFTERNOON BY LOW CLOUDS AND LOW VSBYS FROM AN ARCTIC COLD FRONT
MOVING SOUTH ACROSS WY PIVOTING AROUND A LOW PRESSURE CENTER THAT
WILL BE TRACKING SOUTH FROM MONTANA AND INTO THE CENTRAL PLAINS BY
THURSDAY NT. THIS WILL RESULT IN WIDESPREAD IFR CONDITIONS ALONG WITH
MTN OBSCURATIONS THAT WILL TRANSITION TO MVFR CONDITIONS AND MTN
OBSCURATIONS AFTER MIDNIGHT EAST OF THE DIVIDE. SOME LOCAL BLOWING
AND DRIFTING SNOW CAN BE EXPECTED FROM NORTH WINDS EAST OF THE
DIVIDE WHICH WILL GUST TO SPEEDS OF 30 MPH IN OPEN AREAS ALONG WITH
45 MPH WIND GUSTS IN JOHNSON COUNTY WHERE IFR CONDITIONS DUE TO
CONSIDERABLE BLOWING SNOW WILL TAKE PLACE LATE THIS AFTERNOON. VFR
CONDITIONS WILL PREVAIL THURSDAY ALTHOUGH A MID CLOUD DECK AS WELL
AS AS CLOUD DECK AROUND 5K SHOULD PERSIST THROUGH THURSDAY OVER AND
NORTH OF THE OWL CREEK MTNS AS WELL AS ALONG AND EAST OF THE BIG
HORN MTNS INCLUDING CASPER. SOME LINGERING MVFR CONDITIONS FROM
LINGERING LIGHT SNOW OVER THE FAR WEST VALLEYS SHOULD CLEAR UP EARLY
THIS EVENING. OTHERWISE AREAS WEST OF THE CONTINENTAL DIVIDE SHOULD
REMAIN VFR TONIGHT THROUGH THURSDAY.
&&
.FIRE WEATHER...
COLD ARCTIC HIGH PRESSURE OVER ALBERTA AND NORTH CENTRAL MONTANA
WILL CONTINUE TO DRIVE SOUTH OVER WESTERN AND CENTRAL WYOMING
TONIGHT BRINGING WIDESPREAD LOW STRATUS CLOUDS...LIGHT SNOW AND
FOG TO AREAS WEST OF THE CONTINENTAL DIVIDE. AREAS OF BRISK NORTH
NORTHEAST WIND WILL OCCUR ACROSS THE EASTERN PLAINS. AREAS OF
LIGHT SNOW WILL BE ENCOUNTERED IN THE WESTERN MOUNTAINS. LOW
MIXING HEIGHTS BELOW 1000 FEET WITH INVERSIONS ABOVE THE VALLEYS
AND BASINS WILL KEEP THE AIR TRAPPED WITH POOR SMOKE DISPERSAL.
TEMPERATURES WILL BE COLDER THROUGH SATURDAY.
&&
.RIW WATCHES/WARNINGS/ADVISORIES...
WINTER WEATHER ADVISORY UNTIL 11 PM MST THIS EVENING FOR WYZ010-
011-017>020-022.
&&
$$
SHORT TERM...C.BAKER
LONG TERM...SKRBAC
AVIATION...LIPSON
FIRE WEATHER...C.BAKER
