FXUS65 KRIW 250215
AFDRIW
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
715 PM MST THU DEC 24 2009
.UPDATE...
SATELLITE AND RADAR DATA ARE SUGGESTING THAT WIDELY SCATTERED
AREAS OF LIGHT SNOW WILL MOVE ACROSS NORTHWESTERN WY THIS EVENING.
APPEARS THAT LEFT EXIT OF H3 JET IS CAUSING LIGHT SNOW ACROSS
PORTIONS OF SOUTHWEST MT EARLY THIS EVENING. AS JET DYNAMICS
PROGRESS SOUTHWARD TONIGHT ACROSS EASTERN ID/WESTERN WY...WIDELY
SCATTERED AREAS OF LIGHT SNOW ARE EXPECTED ACROSS ALL OF WESTERN
WY...WITH THE EXCEPTION OF SOUTHERN LINCOLN COUNTY. INCLUDED
SLIGHT CHANCE POPS IN NORTHWESTERN WY THIS EVENING AND ACROSS MOST
OF WESTERN WY TONIGHT.
&&
.PREV DISCUSSION... /ISSUED 234 PM MST THU DEC 24 2009/
SHORT TERM...TONIGHT THROUGH SATURDAY NIGHT
NORTHERLY FLOW ALOFT WILL CONTINUE OVER THE AREA TONIGHT THROUGH
SATURDAY WITH A HIGH PRESSURE RIDGE TO THE WEST AND A MAJOR LOW
PRESSURE CENTER OVER THE MIDWEST. COLD MODIFIED ARCTIC AIR WILL
BE PRESENT IN THE VALLEYS AND BASINS WITH COLD TEMPERATURES AND
TRAPPED MOISTURE PRODUCING NIGHT TIME AND MORNING FOG AND STRATUS
CLOUDS. INVERSIONS WILL BE STRONG WITH LOW MIXING HEIGHTS AND
LIGHT WIND. IN THE MOUNTAINS ABOVE THE INVERSIONS...TEMPERATURES
WILL BE WARMER WITH LOWER HUMIDITY AND LOCAL BREEZY CONDITIONS.
MOISTURE TO THE EAST WILL RETURN TO JOHNSON AND NATRONA COUNTIES
TONIGHT AND FRIDAY WITH INCREASED CLOUDS AND AREAS OF LIGHT SNOW
LINGERING THROUGH SATURDAY. STRATUS AND FOG DEVELOPMENT...OR LACK
OF...WILL HAVE A LARGE IMPACT ON LOW TEMPERATURES. AREAS WHERE
DEVELOP OCCURS WILL HAVE SIGNIFICANTLY WARMER TEMPERATURES THAN
AREAS WITH CLEAR SKIES WHERE RADIATIONAL COOLING WILL ALLOW
TEMPERATURES TO COOL AN AVERAGE OF 15 DEGREES COOLER THAN
CLOUDY/FOGGY AREAS. THE AREAS WITH THE BEST SHOT OF FOG AND
STRATUS ARE IN THE WIND RIVER BASIN...THE EASTERN AND SOUTHERN
BIG HORN BASIN...THE STAR VALLEY AND THE YELLOWSTONE PLATEAU.
JACKSON HOLE MAY ALSO SEE ENOUGH LOW LEVEL MOISTURE FOR SOME FOG
DEVELOPMENT. SNOWFALL OVER JOHNSON AND NATRONA COUNTIES IS
EXPECTED TO BE AN INCH OR TWO TONIGHT THROUGH SATURDAY WITH 2 TO 4
INCHES IN THE BIGHORN MOUNTAINS AND ON CASPER MOUNTAIN. WITH EACH
MODEL RUN THE EXTENT AND AMOUNT OF PRECIPITATION GROWS IN THE EAST
AND MAY END UP WITH LOW END ADVISORY SNOWFALL. WOULD LIKE TO SEE
MORE DEVELOPMENT IN THE TRANSITION OF THE NORTHERN LOW
INTENSIFICATION BEFORE INCREASING SNOWFALL AMOUNTS AT THIS TIME.
LONG TERM...SUNDAY THROUGH THURSDAY
MODELS BEGINNING TO CONVERGE ON A SOLUTION THAT MAY ACTUALLY
WORK OUT IN THE MEDIUM RANGE PERIOD. THE DETAILS STILL VARY WIDELY
BUT AT LEAST THE SOLUTIONS ARE MUCH CLOSER THAN LAST NIGHT. AT THE
BEGINNING OF THE PERIOD...IT APPEARS WE WILL SEE THE HIGH OVER LOW
SCENARIO AS CLOSED UPPER HIGH SITS OVER SRN CANADA AND A NEW LOW
DIGS INTO THE DESERT SW EARLY NEXT WEEK. IT APPEARS THIS SYSTEM
WILL BE TOO FAR SOUTH FOR ANY SNOW THIS FAR NORTH. BY THE MIDDLE
OF NEXT WEEK...THE MODELS ARE CONSOLIDATING THE FLOW SOME AND
BRINGING A DISTURBANCE INTO THE STATE FOR WEDNESDAY. THE GFS SHOWS
MORE OF A SMALL CLOSED LOW DIGGING INTO THE AREA WHILE THE ECMWF HAS
A MOIST NW FLOW WITH AN EMBEDDED DISTURBANCE. THE GEM AT 144 HOURS
FAVORS THE ECMWF. LEANED TOWARDS THE ECMWF TONIGHT AND BEGAN
INTRODUCING CHANCE POPS IN THE WEST WEDNESDAY AND A SLIGHT
CHANCE EAST OF THE DIVIDE WEDNESDAY NIGHT. FORECAST LOOKS DRY AND
SEASONABLY COLD SUNDAY AND MONDAY WITH SOME MOUNTAIN FLURRIES OR
LIGHT SNOW POSSIBLE AS EARLY AS TUESDAY IN THE NW MTNS.
AVIATION...
LARGE SCALE STORM SYSTEM HAS MOVED INTO THE CENTRAL PLAINS...WITH
WRAP AROUND NORTH NORTHWEST FLOW OVER WYOMING TODAY AND TOMORROW.
SNOW COVER VIRTUALLY EVERYWHERE TODAY...WITH LOW CLOUDS AND/OR FOG
IN PORTIONS OF WIND RIVER BASIN AND STAR VALLEY THIS MORNING.
UNSTABLE NORTHWEST FLOW EXPECTED TO PRODUCE LIGHT SNOW SHOWERS
OVERNIGHT...MAINLY IN CENTRAL AND NORTHERN WYOMING. KBYG TO KCPR TO
SEE BRIEF MODERATE SNOW AROUND SUNRISE CHRISTMAS MORNING AND RESULT
IN IFR CONDITIONS. WITH HIGH PRESSURE MOVING IN FROM WEST...FOG
LIKELY OVERNIGHT AND INTO THE MORNING HOURS FROM KAFO TO KJAC...AND
IN BIG HORN AND WIND RIVER BASINS. GUSTS OVER 25 KNOTS EXPECTED LATE
THIS AFTERNOON AROUND KBPI AND KPNA...WITH 20 TO 35 KNOT GUSTS FROM
KRKS TO KCPR. PERSISTENT WEST WIND OF 20 KNOTS IN SWEETWATER COUNTY
OVERNIGHT.
FIRE WEATHER...
NORTHERLY FLOW ALOFT WILL REMAIN OVER WESTERN AND CENTRAL WYOMING
THROUGH SATURDAY. COLD MODIFIED ARCTIC AIR WILL BE PRESENT IN THE
VALLEYS AND BASINS WITH COLD TEMPERATURES AND TRAPPED MOISTURE
PRODUCING NIGHT TIME AND MORNING FOG AND STRATUS CLOUDS.
INVERSIONS WILL BE STRONG WITH LOW MIXING HEIGHTS AND LIGHT WIND
PRODUCING POOR SMOKE DISPERSAL. IN THE MOUNTAINS ABOVE THE
INVERSIONS...TEMPERATURES WILL BE WARMER WITH LOWER HUMIDITY AND
LOCAL BREEZY CONDITIONS WITH DAYTIME MIXING HEIGHTS AROUND 2500
FEET. THE BREEZES AND HIGHER MIXING HEIGHTS DURING THE AFTERNOONS
WILL PRODUCE GENERALLY FAIR SMOKE DISPERSAL. MOISTURE TO THE EAST
WILL RETURN TO JOHNSON AND NATRONA COUNTIES TONIGHT AND FRIDAY
WITH INCREASED CLOUDS AND AREAS OF LIGHT SNOW LINGERING THROUGH
SATURDAY.
&&
.RIW WATCHES/WARNINGS/ADVISORIES...
NONE.
&&
$$
UPDATE...SMITH
SHORT TERM...C.BAKER
LONG TERM...SKRBAC
AVIATION...MCDONALD
FIRE WEATHER...C.BAKER
