FXUS65 KRIW 220956
AFDRIW
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
256 AM MST TUE DEC 22 2009
.SHORT TERM...TODAY THROUGH THURSDAY NIGHT
SATELLITE DEMONSTRATING WIDESPREAD INCREASED CLOUD COVER THIS
MORNING...WITH THE RADAR SHOWING SOME WEAK RETURNS OVER CENTRAL
IDAHO. APPROACHING TROUGH AND LOW PRESSURE WILL CONTINUE MOVING
THROUGH THE INTERMOUNTAIN WEST THIS MORNING...AND WILL DIG FURTHER
SOUTH THROUGH TONIGHT. LIGHT SNOWFALL IS EXPECTED ACROSS WESTERN
WYOMING THIS MORNING...WHICH WILL CONTINUE INTO THE EVENING HOURS.
MODELS DEMONSTRATE SNOWFALL REACHING AREAS EAST OF THE DIVIDE THIS
EVENING...THOUGH MOST AREAS EAST OF THE DIVIDE WILL ONLY SEE
AROUND AN INCH OR TWO THROUGH THE OVERNIGHT HOURS.
MODELS IN FAIRLY GOOD AGREEMENT WITH THE ARRIVAL OF ANOTHER AREA
OF LOW PRESSURE MOVING SOUTH FROM CANADA THIS EVENING. THIS
SECONDARY SYSTEM WILL MOVE SOUTH TONIGHT AND WILL BEGIN TO HAVE
ITS IMPACT ON THE AREA WEDNESDAY AFTERNOON. THE ARCTIC FRONT
ASSOCIATED WITH THIS SECONDARY SYSTEM WILL MOVE THROUGH THE AREA
TONIGHT AND WEDNESDAY MORNING...WITH COOLER DAYTIME TEMPERATURES
EXPECTED.
SNOWFALL WILL BE WIDESPREAD EAST OF THE DIVIDE WEDNESDAY AND
WEDNESDAY NIGHT. MODELS DEMONSTRATE THE SECONDARY WAVE MOVING
SOUTH THROUGH CENTRAL WYOMING...WHICH WILL BE FAVORABLE IN THE
CREATION OF SNOWFALL...ESPECIALLY THE DEVELOPMENT OF UPSLOPE
SNOWFALL ACROSS THE CENTRAL BASINS. MUCH COLDER H7 TEMPERATURES
WILL ALSO ACCOMPANY THIS STRONGER PUSH ON WEDNESDAY EVENING WHICH
WILL CAUSE THURSDAY HIGH TEMPERATURES TO BE SIGNIFICANTLY LOWER
THAN THOSE EXPERIENCED WEDNESDAY AFTERNOON. BY THURSDAY
AFTERNOON...THE CENTRAL AND EASTERN BASINS WILL SEE THE POTENTIAL
FOR 2 TO 4 INCHES...WITH THE HIGHER ELEVATIONS SEEING 4 TO 7
INCHES. THESE SYSTEMS WILL BE MONITORED FOR THE POTENTIAL OF
WINTER WEATHER HIGHLIGHTS.
AS MENTIONED PREVIOUSLY...MUCH COLDER TEMPERATURES EXPECTED BY
THURSDAY AFTERNOON...WITH DAYTIME HIGHS BARELY REACHING INTO THE
TEENS FOR MUCH OF THE AREA. THE PASSAGE OF THESE TWO SYSTEMS
THURSDAY NIGHT WILL ALLOW SNOWFALL TO WEAKEN AND WILL BRING
WARMING TEMPERATURES.
.LONG TERM...FRIDAY THROUGH TUESDAY
PRE-CHRISTMAS STORM SYSTEM WILL BE OVER IOWA BY FRIDAY MORNING
BUT SOME WRAPAROUND MOISTURE COULD LINGER IN THE FAR ERN ZONES
CHRISTMAS MORNING BASED ON CURRENT MODEL RUNS. WILL LINGER THE
SLIGHT CHANCES IN JOHNSON AND NATRONA COUNTIES JUST FOR THE
MORNING HOURS. OTHERWISE...THE MEDIUM RANGE PERIOD LOOKS DRY
UNLESS THE SWRN ZONES GET SOME SNOW FROM THE NEXT SYSTEM THAT
DIGS INTO THE DESERT SW EARLY NEXT WEEK. THE 00Z ECMWF IS
SHOWING A STRONGER UPPER LOW WITH SWRN WYO ON THE EDGES OF THIS
SYSTEM. THE GFS IS WEAKER WITH MORE OF A SHEARED LOOKING LOW.
THE GEM AT 144 HOURS SEEMS TO BE MORE OF A COMPROMISE. WILL GO
WITH A DRY FORECAST FOR NOW AND SEE HOW THE TRENDS LOOK THE NEXT
COUPLE NIGHTS. OTHERWISE...WITH RIDGE MOVING IN CHRISTMAS DAY
THROUGH THE WEEKEND...STEEP VALLEY AND BASIN INVERSIONS WILL MAKE
FOR A COLD TO VERY COLD PERIOD FOR THE TRAPPED AREAS AND POSSIBLE
FOGGY PERIOD. GRADUAL MODERATION WILL OCCUR ELSEWHERE. COMPLEX
FLOW BY EARLY NEXT WEEK BUT BOTH THE GFS AND EURO ARE TRYING TO
SNEAK A WEAK COLD FRONT DOWN INTO THE NERN ZONES ON MONDAY WITH
SOME COLDER AIR EAST OF THE DIVIDE. LOWERED TEMPS SOME ACROSS THE
BASIN/VALLEY AREAS WITH LITTLE CHANGE ELSEWHERE. COLD CHRISTMAS
DAY WITH HIGHS STRUGGLING INTO THE TEENS IN MOST AREAS AND
POSSIBLY STRUGGLING THROUGH THE SINGLE DIGITS IN PLACES LIKE THE
WIND RIVER BASIN ...UPPER GREEN AND JACKSON VALLEYS WITH GREYBULL
AREA NOT FAR BEHIND. VERY COLD CHRISTMAS NIGHT WITH MOST CLEAR
SKY...LIGHT WINDS AND STEEPENING TEMPERATURE INVERSIONS AS WARMING
ALOFT OCCURS. 10 TO 20 BELOW ZERO WILL BE COMMON IN THE NORMALLY
COLD CENTRAL AND WRN BASINS/VALLEYS. COULD BE EVEN COLDER
DEPENDING ON FRESH SNOW COVER AND OVERALL TIMING OF TROUGH EXIT.
&&
.AVIATION...
EAST OF THE CONTINENTAL DIVIDE...VFR CONDITIONS WILL PREVAIL THROUGH
00Z WED. CLOUD HEIGHTS WILL LOWER AFTER 00Z WED WITH LOCAL MVFR
CONDITIONS OVER THE NORTH INCLUDING KCOD AIRPORT. WIDELY SCATTERED
AREAS OF LIGHT SNOW WILL DEVELOP EAST OF THE DIVIDE AFTER 00Z WED WITH
LOCAL MVFR CONDITIONS AND MOUNTAIN OBSCURATION.
WEST OF THE CONTINENTAL DIVIDE SCATTERED LIGHT SNOW WILL OCCUR
THROUGH 18Z WITH LOCAL MVFR CONDITIONS AND MOUNTAIN OBSCURATION.
THIS WOULD INCLUDE THE KJAC AIRPORT. AFTER 18Z SNOW WILL INCREASE IN
AREAL COVERAGE WITH CEILINGS AND VISIBILITIES LOWERING INTO THE MVFR
TO IFR CATEGORIES...INCLUDING THE KJAC AIRPORT...FROM 21Z TODAY
THROUGH 06Z WED.
&&
.FIRE WEATHER...
ANOTHER APPROACHING STORM SYSTEM BEGINS TO IMPACT THE WESTERN
VALLEYS AND MOUNTAINS TODAY. LIGHT WIND EXPECTED ACROSS MUCH OF
THE AREA FOR THE NEXT COUPLE DAYS. THE ARRIVAL OF THIS SYSTEM WILL
ALLOW SHALLOW INVERSIONS TO DEVELOP WEST OF THE DIVIDE TODAY AND
TONIGHT. AS THIS SYSTEM MOVES FURTHER EAST...ASSOCIATED INVERSIONS
WILL IMPACT AREAS EAST OF THE DIVIDE BY WEDNESDAY EVENING.
LINGERING INVERSIONS WILL CAUSE A DECREASE IN SMOKE DISPERSAL...THOUGH
CONDITIONS SHOULD IMPROVE THURSDAY EVENING AS THE SYSTEMS MOVE
FURTHER EAST.
&&
.RIW WATCHES/WARNINGS/ADVISORIES...NONE.
&&
$$
SHORT TERM...BRANHAM
LONG TERM...SKRBAC
AVIATION...AR
FIRE WEATHER...BRANHAM
