FXUS65 KRIW 300943
AFDRIW
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
243 AM MST MON NOV 30 2009
.SHORT TERM...TODAY THROUGH WEDNESDAY NIGHT
POSITIVELY TILTED RIDGE OVER THE WESTERN CONUS PUSHING INTO THE
NORTHERN ROCKIES REGION. HIGH CLOUDS SPILLING INTO CENTRAL AND
EASTERN WYOMING THIS MORNING WHICH SHOULD EXIT EAST OF THE AREA BY
LATE MORNING. AREAS OF FOG DEVELOPED THIS EVENING OVER THE WIND
RIVER BASIN WITH PATCHY FOG OVER THE BIG HORN BASIN. THE FOG OVER
THE WIND RIVER BASIN WAS QUITE DENSE IN SPOTS WITH VISIBILITY
DOWN TO LESS THAN 100YDS. HOWEVER...THE FOG DISSIPATED TO PATCHY
FOG AS THE HIGH CLOUDS MOVED IN.
ITS SHAPING UP TO BE A RATHER NICE DAY ACROSS THE CWA TODAY WITH A
SUNNY SKY AND MILD TEMPERATURES. GOOD WARM AIR ADVECTION AT H7 WITH
+1C TO +2C EAST OF THE DIVIDE BY THE AFTERNOON. SHOULD SEE MANY
AREAS IN THE 50S THIS AFTERNOON EAST OF THE DIVIDE...APPROACHING 60
OVER JOHNSON AND NATRONA COUNTIES. CENTRAL BASINS WILL BE TRAPPED
UNDER INVERSIONS SO UPPER 30S TO MID 40S AT BEST. 30S AND 40S FOR
THE WEST AND MOUNTAINS. RIDGE AXIS TO MOVE THROUGH THIS AFTERNOON
WITH WINDS INCREASING AS LEE TROUGH STRAIGHTENS. IT WILL BECOME
QUITE WINDY OVER THE CENTRAL WIND CORRIDOR AS WELL AS OVER THE
ABSAROKAS ESPECIALLY OVERNIGHT AS THE TROUGH DIGS INTO MONTANA
TIGHTENING THE GRADIENT. MODELS HAVE COME ALONG TO A CONSENSUS ON
THE TRACK OF THE TROUGH...WHICH IS SIMILAR TO THE NAM SOLUTION
YESTERDAY. THE MAIN CIRCULATION OF THE TROUGH WILL MOVE INTO THE
NORTHERN PLAINS STATES TONIGHT INTO TUESDAY. A SHARP NNW-SSE
ORIENTED CYCLONICALLY CURVED JET WILL DIG SOUTH OVER WESTERN MONTANA
INTO CENTRAL WYOMING TUESDAY. AN ASSOCIATED COLD FRONT WILL MOVE
INTO NORTHERN WYOMING AROUND 12Z TUE MOVING INTO THE CENTRAL AFTER
18Z...THEN INTO SWEETWATER COUNTY BY 00Z WED. H7 TEMPS WILL FALL TO
-16C...-35C AT H5 BY 00Z. THE STRONG BAROCLINIC AND JET
DYNAMICS...WILL PRODUCE AN AREA OF LIGHT SNOWFALL EAST OF THE
DIVIDE. THE SNOW WILL DEVELOP IN THE NORTH AFTER 15Z AND PERSIST
THROUGH THE LATE AFTERNOON. THE SNOW THEN SHIFTS INTO THE CENTRAL
FOR THE MID AFTERNOON AND EVENING HOURS. THE DYNAMICS EXIT THE AREA
BY 06Z WITH SNOW QUICKLY ENDING AND THE SKY CLEARING. A GENERAL
SNOWFALL OF 1 TO 3 INCHES CAN BE EXPECTED EAST OF THE DIVIDE...WITH
LOCALLY HIGHER AMOUNTS LIKELY IN THE ABSAROKAS AND BIGHORNS. THE
COLD NORTH CYCLONIC FLOW PERSISTS WEDNESDAY AND WEDNESDAY NIGHT.
LOOKING PRETTY CLEAR...THOUGH SOME HIGH CLOUDS COULD SPILL OVER THE
INCOMING RIDGE TO THE WEST. MINIMUM TEMPS TUESDAY NIGHT TO BE IN
THE SINGLE DIGITS AND TEENS...WITH LOCALLY BELOW ZERO READINGS IN
THE WEST AND MOUNTAINS. WEDNESDAY NIGHT WILL SEE MORE SUB ZERO
READINGS...BUT AGAIN MAINLY IN THE WEST AND MOUNTAINS. MAX TEMPS ON
WEDNESDAY TO THE BE IN THE 20S TO LOWER 30S.
.LONG TERM...THURSDAY THROUGH MONDAY
MEDIUM RANGE REMAINS A CHALLENGE WITH THE ECMWF HOLDING THE RIDGE
OVER THE AREA INTO THE WEEKEND WHILE THE GFS/GEM AND UKMET BRING
A SIGNIFICANT SHORTWAVE THROUGH THE WEAKENING RIDGE AROUND THE
FRIDAY TIMEFRAME. ANALYSIS OF THE HEMISPHERIC PATTERN REVEALS THE
PROBLEM. A LARGE PIECE OF THE SIBERIAN COLD POOL IS FORECAST TO
BREAK OFF BY ALL THE MODELS AND DEVELOP A STRONG CLOSED LOW NEAR
THE END OF THE ALEUTIAN CHAIN OVER THE WEEKEND WHICH PUMPS UP A
LARGE RIDGE OVER THE ERN PACIFIC. THE MAIN PROBLEM WITH THE ECMWF
IS THAT IT MAINTAINS THE RIDGE OVER THE ROCKIES AT THE SAME TIME.
THE OTHER MODELS ALLOW THE RIDGE OVER THE PACNW/ROCKIES TO WEAKEN AS
RETROGRESSION OCCURS WHICH RESULTS IN THE SIGNIFICANT SHORTWAVE
ENERGY DIVING INTO THE PACNW OR NRN ROCKIES ON FRIDAY OR FRIDAY
NIGHT. THIS IDEA SEEMS TO BE MORE REALISTIC AND WAS FOLLOWED
TONIGHT. WITH THAT IDEA...INTRODUCED POPS ACROSS MUCH OF THE AREA
BEGINNING FRIDAY AND INCREASING THROUGH THE WEEKEND. WILL HIT THE
AREA HARDER TONIGHT IF THESE TRENDS CONTINUE. THE GFS IS
PARTICULARLY BULLISH FOR OUR AREA LATE IN THE WEEKEND AND INTO
EARLY NEXT WEEK AS ARCTIC AIRMASS BECOMES ENTRENCHED EAST OF THE
DIVIDE WHILE DYNAMICS OVERRUN THIS IN FAVORABLE ISENTROPIC
PATTERN. HAVE ALSO TRENDED TEMPS DOWN OVER THE WEEKEND INTO MONDAY
BUT NOT AS COLD AS THE GFS YET WHICH IS DOWNRIGHT BITTER.
&&
.AVIATION...
LOCAL IFR/LIFR FREEZING FOG WILL OBSCURE TERRAIN IN THE BIG HORN
BASIN VCNTY KGEY/KWRL...THE WIND RIVER BASIN VCNTY KRIW/KLND THROUGH
18Z MONDAY. OTHERWISE VFR CONDITIONS WILL PREVAIL THROUGH 12Z
TUESDAY. SOUTHWEST WIND WILL INCREASE AHEAD OF AN APPROACHING
CANADIAN COLD FRONT MONDAY WITH WIND SPEEDS OF 20-30KTS BTWN
KRKS-KCPR THIS AFTERNOON AND OVERNIGHT MONDAY. AREAS OF LLWS WILL
BE ENCOUNTERED ALONG THE EAST SLOPES OF THE WIND RIVER AND ABSAROKA
MTNS MONDAY NIGHT...OR VCNTY OF A KCOD-KLND LINE. THE COLD FRONT
WILL PLUNGE SOUTH INTO NORTHERN WYOMING 9Z-12Z TUESDAY WITH IFR/LIFR
CONDITIONS IN -SN/SN EXPECTED TO FOLLOW A FEW HOURS AFTERWARDS
DURING THE DAY.
&&
.FIRE WEATHER...
HIGH PRESSURE WILL HANG ON FOR ONE MORE DAY. WINDS WILL INCREASE
OVER THE TYPICAL LOCATIONS TODAY...BECOMING QUITE WINDY TONIGHT. IT
WILL BE MILD WITH RH'S WILL DIPPING INTO THE 20 PERCENT RANGE MANY
AREAS...THOUGH THE CENTRAL BASINS WILL REMAIN MOIST AND COOL AS THEY
REMAIN TRAPPED UNDER INVERSIONS. A TROUGH WILL MOVE THROUGH TUESDAY
BRINGING SOME LIGHT SNOW TO AREAS EAST OF THE DIVIDE. IT WILL
BECOME QUITE COLD BEHIND THE TROUGH. IT WILL REMAIN QUITE COLD
THROUGH THURSDAY MORNING.
&&
.RIW WATCHES/WARNINGS/ADVISORIES...NONE.
&&
$$
SHORT TERM...TWB
LONG TERM...AEM
AVIATION...SKRBAC
FIRE WEATHER...TWB
