FXUS65 KRIW 051006
AFDRIW
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
306 AM MST SAT DEC 5 2009
.SHORT TERM...TODAY THROUGH MONDAY NIGHT
WATER VAPOR IMAGERY SHOWS A NEARLY CLOSED OFF UPPER LOW OVER CENTRAL
AND WESTERN MT EARLY THIS MORNING. GFS AND NAM INDICATE A 100+ KT
H25 JET ALONG THE SOUTHERN COAST OF BRITISH COLUMBIA EARLY THIS
MORNING. AS THIS FAIRLY STRONG H25 JET MOVES SOUTHWARD ALONG THE
COAST OF THE PAC NW TODAY...THE UPPER LOW ACROSS WESTERN MT WILL
CLOSE OFF AND DIG SOUTHWARD ALONG THE WY/ID BORDER. A SFC COLD FRONT
AHEAD OF THE DEEP COLD POOL OF AIR ASSOCIATED WITH THE UPPER LOW
WILL MOVE ACROSS NORTHERN WY BETWEEN 15Z AND 18Z AND THEN PUSH INTO
NORTHERN CO/NORTHERN UT AROUND 00Z SUNDAY. THE SURFACE COLD FRONT
WILL HAVE VERY LIMITED MOISTURE...BETWEEN 0.05 AND 0.15 OF AN INCH
PRECIPITABLE WATER...TO INTERACT WITH...AS IT MOVES SOUTHWARD ACROSS
WY TODAY. LIGHT SNOW CAN BE EXPECTED IN THE VICINITY OF THE FRONT
THIS MORNING ACROSS NORTHERN WY AND THIS AFTERNOON ACROSS SOUTHERN
WY. THE UPPER LOW WILL MOVE SOUTHWARD ACROSS SOUTHERN ID/NORTHERN UT
TONIGHT WITH UPSLOPE FLOW...DIFLUENT FLOW ALOFT AND LIMITED MOISTURE
COMBINING TO PRODUCE PERIODS OF LIGHT SNOW ACROSS SOUTHERN
WY...FREMONT COUNTY AND NATRONA COUNTY. NORTHEAST FLOW BETWEEN H85
AND H7 WILL FAVOR THE LANDER FOOTHILLS...THE EAST SLOPES OF THE WIND
RIVER MOUNTAINS...THE GREEN MOUNTAINS AND THE RATTLESNAKE RANGE
TONIGHT THROUGH 18Z SUNDAY...BUT LOW PRECIPITABLE WATER VALUES WILL
STILL SUPPORT ONLY LIGHT TO MODERATE SNOW. LIGHT SNOW WILL GRADUALLY
END FROM WEST TO EAST ACROSS SOUTHERN WY AND SOUTH CENTRAL WY SUNDAY
AFTERNOON...AS UPPER LOW BECOMES AN OPEN WAVE AND LIFTS QUICKLY TO
THE NORTHEAST ACROSS EASTERN WY. THE BEST CHANCE FOR ADVISORY
CRITERIA SNOW AMOUNTS SATURDAY NIGHT AND SUNDAY WILL BE ACROSS THE
LANDER FOOTHILLS AND IN THE EASTERN SLOPES OF THE WIND RIVER
MOUNTAINS. ANOTHER DEEP UPPER LOW WILL MOVE SOUTHWARD OVER
OR/NORTHERN CA SUNDAY NIGHT...AS ZONAL FLOW ALOFT AND A VERY COLD
POOL OF COLD AIR MOVES ACROSS WY. MID AND UPPER LEVEL CLOUDS WILL
MOVE ACROSS WY SUNDAY NIGHT AHEAD OF THE UPPER LOW TO THE WEST.
THESE CLOUDS WILL PREVENT STRONG RADIATIVE COOLING...HOWEVER THE
DEEP COLD AIR MASS ACROSS WY WILL STILL PRODUCE LOWS BELOW 0 F
ACROSS THE ENTIRE STATE. THE UPPER LOW OVER NORTHERN CA WILL BECOME
AN OPEN WAVE AND MOVE ACROSS THE NORTHERN GREAT BASIN MONDAY AND
MONDAY NIGHT. GFS AND NAM ARE BOTH INDICATING MODERATE ISENTROPIC
UPGLIDE DEVELOPING ACROSS SOUTHERN WY MONDAY MORNING AND ACROSS
SOUTHERN AND SOUTH CENTRAL WY MONDAY AFTERNOON THROUGH TUESDAY
MORNING. PAC MOISTURE WILL MOVE NORTHEASTWARD WY MONDAY AND MONDAY
NIGHT...INCREASING THE AVAILABLE MOISTURE TO NEAR 0.3 OF AN INCH.
THE EVOLUTION AND TRACK OF THIS LOW PRESSURE WILL HAVE TO BE WATCHED
CLOSELY...AS IT WILL HAVE THE POTENTIAL TO PRODUCE SIGNIFICANT
SNOWFALL ACROSS SOUTHERN AND SOUTH CENTRAL WY MONDAY THROUGH
TUESDAY.
.LONG TERM...TUESDAY THROUGH SATURDAY
THE MAIN PESKY TROUGH WILL FINALLY PUSH EAST INTO THE PLAINS DURING
THE DAY ON TUESDAY. SOME SNOW WILL LIKELY BE ONGOING TUESDAY
MORNING OVER THE SOUTH AND EASTERN SECTIONS OF THE FORECAST AREA.
QG FORCING AND FRONTOGENESIS REMAINS STRONG OVER THE AREA
TUESDAY...ESPECIALLY IN THE MORNING. IN WAKE OF THIS
SYSTEM...MAINLY DRY CONDITIONS ARE EXPECTED. HOWEVER...A WEAK
DISTURBANCE RIGHT BEHIND THE TROUGH COULD GIVE THE NORTHWEST SOME
SNOW SHOWERS. TUESDAY NIGHT...COULD BE THE COLDEST NIGHT OF THIS
EARLY WINTER SEASON...AS SURFACE HIGH PRESSURE BUILDS INTO THE
AREA...WHICH SHOULD RESULT IN RELATIVELY LIGHT WINDS. A COUPLE
LIMITING FACTORS ARE THAT MODELS ARE A BIT SLOWER IN CLEARING THAN
LAST NIGHT MODEL RUNS...AND 700MB FLOW WILL BE STRENGTHENING DURING
THE NIGHT. MEDIUM RANGE MODELS FROM WEDNESDAY NIGHT THROUGH FRIDAY
ARE SHOWING NORTHWEST FLOW OVER THE NORTHERN ROCKIES...WITH A MOIST
WESTERLY FLOW DOMINATING THE CENTRAL AND SOUTHERN ROCKIES. THE MAIN
CHALLENGE IS WHAT FLOW WILL HAVE MORE INFLUENCE ON THE FORECAST
AREA. PREFER MORE OF A NORTHWEST FLOW AND A DRIER FORECAST THAN
WHAT THE ECMWF IS CURRENTLY INDICATING. HAVING SAID THAT...HAVE
KEPT CHANCE POPS UNCHANGED WEDNESDAY NIGHT THROUGH FRIDAY OUT
WEST...AS DISTURBANCES IN THE NORTHWEST COULD STILL ALLOW FOR SOME
SNOWFALL...AND SEE WHAT LATER MODELS RUNS SHOW.
TEMPERATURES WILL REMAIN WELL BELOW AVERAGE TUESDAY AND
WEDNESDAY...WITH A TEMPERATURES MODERATING THURSDAY AND FRIDAY...AS
700 MB TEMPS REBOUND TO -5 TO -12 DEG CELSIUS. HOWEVER...INVERSION
TRAPPED...SNOW COVERED BASINS WILL STRUGGLE TO WARM UP. AREAS THAT
CAN MIX OUT...SUCH AS FAVORED WINDY AREAS SHOULD SEE NEAR SEASONAL
TEMPERATURES BY FRIDAY.
&&
.AVIATION...
AN UPPER LEVEL LOW PRESSURE SYSTEM WILL MOVE SOUTH ALONG WYOMING'S
WESTERN BORDER TODAY. WIDESPREAD MVFR/LOCAL IFR CONDITIONS WILL
SPREAD SOUTH ACROSS THE AREA ALONG AN ASSOCIATED COLD FRONT...WITH
WIDESPREAD IFR AND LOCAL LIFR CONDITIONS DEVELOPING VCNTY AND SOUTH
OF A KRKS-KLND-KRIW-KCPR LINE BTWN 22Z SAT-00Z SUN...THESE
CONDITIONS PERSISTING THROUGH 12Z SUNDAY. SOME IMPROVEMENT TO
PREVAILING VFR WITH LOCAL IFR/MVFR CIGS AND VSBYS IN -SN WILL OCCUR
VCNTY AND WEST OF A KJAC-KCOD LINE AFTER 03Z SUNDAY.
&&
.FIRE WEATHER...
VERY COLD AIR MASS WILL DOMINATE THE WEATHER ACROSS WYOMING TODAY
THROUGH SUNDAY. AN UPPER LOW MOVING SOUTHWARD ALONG THE
WYOMING/IDAHO BORDER WILL BRING PERIODS OF LIGHT SNOW TO NORTHERN
WYOMING THIS MORNING AND SOUTHERN WYOMING THIS AFTERNOON. AS THE
UPPER LOW MOVES INTO NORTHERN UTAH TONIGHT...LIGHT SNOW WILL DEVELOP
ACROSS SOUTHERN AND SOUTH CENTRAL WYOMING TONIGHT AND SUNDAY.
ACCUMULATIONS WILL BE GENERALLY LIGHT...EXCEPT IN THE LANDER
FOOTHILLS...THE EAST SLOPES OF THE WIND RIVER MOUNTAINS...THE GREEN
MOUNTAINS AND THE RATTLESNAKE RANGE...WHERE MODERATE SNOWFALL
AMOUNTS ARE POSSIBLE.
&&
.RIW WATCHES/WARNINGS/ADVISORIES...
NONE.
&&
$$
SHORT TERM...SMITH
LONG TERM...WM
AVIATION...AEM
FIRE WEATHER...SMITH
