FXUS65 KRIW 042227
AFDRIW
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
327 PM MST FRI DEC 4 2009
.SHORT TERM...TONIGHT THROUGH MONDAY
THE BANK OF MID TO LOW LEVEL CLOUDS MOVED THROUGH CENTRAL AND
WESTERN WYOMING THROUGH THE MORNING HOURS...FINALLY PASSING
THROUGH RIVERTON AROUND 1130 AM THIS MORNING. CLEARING OF THESE
CLOUDS ALLOWED TEMPERATURES TO WARM SIGNIFICANTLY THIS
AFTERNOON. CLOUD COVER CONTINUES TO INCREASE OVER CENTRAL IDAHO
AND WESTERN MONTANA. THESE CLOUDS ARE ASSOCIATED WITH AN AREA OF
LOW PRESSURE...WHICH IS EXPECTED TO IMPACT THE CWA TONIGHT THROUGH
THE WEEKEND.
CLOUD COVER OVER NORTHWESTERN WYOMING...ESPECIALLY YELLOWSTONE
NATIONAL PARK WILL INCREASE THIS EVENING...WITH LIGHT SNOW
DEVELOPING AS THE AFOREMENTIONED AREA OF LOW PRESSURE MOVES INTO
EASTERN MONTANA AND CENTRAL IDAHO. SNOWFALL ACROSS THE WEST WILL
INCREASE OVERNIGHT AS THE AREA OF LOW PRESSURE BEGINS TO MOVE
FURTHER SOUTH. SNOWFALL OVER THE WESTERN MOUNTAINS AND VALLEYS
WILL BECOME WIDESPREAD AFTER 0500 AM SATURDAY MORNING AS THE AREA
OF LOW PRESSURE FINDS ITSELF CENTERED OVER NORTHEASTERN WYOMING
AND SOUTHERN IDAHO.
WITH THE ARRIVAL OF THIS SYSTEM ACROSS THE WEST...LOWERING H7
TEMPERATURES WILL BE FOUND WITH COOLER DAYTIME HIGHS EXPECTED ON
SATURDAY. THE AREA OF LOW PRESSURE ON SATURDAY AFTERNOON AND
EVENING WILL SLOW DOWN ITS SOUTHWARD MOVEMENT...STILL REMAINING
OVER SOUTH CENTRAL IDAHO BY SATURDAY EVENING. SNOWFALL OVER THE
AREA SATURDAY MAY BECOME HEAVY AT TIMES...WITH SNOWFALL
ACCUMULATIONS POTENTIALLY REACHING ADVISORY LEVELS BY THE EARLY
EVENING HOURS. THIS TRANSLATES TO YELLOWSTONE AND WESTERN
MOUNTAINS SEEING POTENTIAL SNOWFALL ACCUMULATIONS OF 5 TO 8 INCHES
AND THE VALLEYS SEEING 2 TO 3 INCHES BY SATURDAY EVENING. SINCE
MODELS CONTINUE TO WEAKEN THE EVENT SLIGHTLY...OPTED TO FORGO ANY
HIGHLIGHTS WITH THIS SHIFT TO ALLOW FUTURE SHIFTS THE FINAL CALL
ON ADVISORY ISSUANCE.
THE AREA OF LOW PRESSURE WILL THEN STUMBLE WESTWARD AND JOIN WITH
ANOTHER DEVELOPING AREA OF LOW PRESSURE OVER THE PACIFIC WEST. AS
THIS SYSTEM MOVES FURTHER WEST...AN INCREASED SOUTHWESTERLY FLOW
WILL DEVELOP OVER THE CWA...INTRODUCING INCREASED SNOW SHOWERS
ACROSS SOUTHERN WYOMING BY SUNDAY MORNING. MUCH COOLER TEMPERATURES
WILL IMPACT THE ENTIRETY OF THE CWA BY SUNDAY...WITH HIGH
TEMPERATURES ONLY REACHING THE MID SINGLE TO LOW TEEN VALUES.
EXPECT FRIGID OVERNIGHT TEMPERATURES SUNDAY NIGHT WITH AREAS OF
YELLOWSTONE PARK REACHING TO 20 TO 25 BELOW ZERO...AND MANY OTHER
LOCATIONS ALSO SEEING SUBZERO TEMPERATURES.
MONDAY APPEARS TO BE A MORE FAVORABLE WEATHER SYSTEM FOR CENTRAL
WYOMING AS MODELS CONTINUE TO INDICATE THE DEVELOPMENT OF UPSLOPE
FLOW WHICH MAY BRING SIGNIFICANT SNOWFALL AMOUNTS TO THE LANDER
FOOTHILLS...WIND RIVER MOUNTAINS...AND THE GREEN RIVER AND
RATTLESNAKE RANGE. THIS SYSTEM WILL CONTINUE TO BE MONITORED
THROUGH THE WEEKEND.
.LONG TERM...MONDAY NIGHT THROUGH FRIDAY
AT THE BEGINNING OF THE LONG TERM...A STRONG UPPER LOW OVER
OREGON/NORTHERN GREAT BASIN AREA WILL PUSH EAST AS A STRONG
TROUGH ACROSS THE ROCKIES MONDAY NIGHT AND TUESDAY. AT THE
SURFACE...A STRONG PRESSURE GRADIENT IS EXPECTED BETWEEN A 1040MB
HIGH OVER NORTHWEST MONTANA...AND A DEEPENING SURFACE LOW OVER THE
GREAT BASIN/UTAH. THIS TIGHT PRESSURE GRADIENT WILL REMAIN OVER
THE REGION THROUGH MONDAY NIGHT. LOW LEVEL NORTHEAST UPSLOPE FLOW
IS EXPECTED MONDAY...AS WELL AS SOME ISENTROPIC LIFT. BOTH THE 00Z
GFS/NAM ARE SHOWING DECENT ISENTROPIC LIFT AT 285K SURFACE MONDAY.
COULD BE SETTING THE STAGE FOR SIGNIFICANT SNOW ACCUMULATIONS OVER
THE SOUTH...AND CENTRAL PARTS EAST OF THE DIVIDE. SNOW CHANCES
WILL CONTINUE...ESPECIALLY FOR THE SOUTH/EAST AS THE FULL LATITUDE
TROUGH MOVES THROUGH MONDAY NIGHT AND TUESDAY. HOWEVER...BY MONDAY
NIGHT AND TUESDAY...MEDIUM RANGE MODELS IN DECENT AGREEMENT WITH
MOST OF THE SIGNIFICANT PRECIPITATION SOUTH OF THE AREA. ANOTHER
CONCERN MONDAY AND TUESDAY WILL BE THE FRIGID AIR OVER THE
REGION...AS HIGHS OVER THE LOWER ELEVATION WILL MAINLY BE IN THE
SINGLE DIGITS WITH BELOW ZERO READINGS AT NIGHT.
IN WAKE OF THIS TROUGH...SURFACE HIGH PRESSURE IS EXPECTED TO
PUSH INTO THE REGION. THIS SHOULD ALLOW WINDS TO BECOME LIGHT. IN
ADDITION...THERE IS A GOOD SHOT AT CLEARING SKIES TUESDAY NIGHT.
THESE FACTORS COMBINED WITH SNOW COVER COULD RESULT IN THE COLDEST
NIGHT WITH MOST AREAS PROBABLY BELOW ZERO. ACTUALLY WENT BELOW
GUIDANCE TUESDAY NIGHT...AND ABOVE GUIDANCE ON MONDAY
NIGHT...SINCE CLOUD COVER AND WINDS SHOULD KEEP THEM ABOVE WHAT
MEX MOS IS SUGGESTING MONDAY NIGHT. CHILLY NORTHWEST FLOW WILL
PREVAIL ON WEDNESDAY...BEFORE THE NEXT...BUT WEAKER SYSTEM BEGINS
TO AFFECT THE AREA LATE NEXT WEEK. HOWEVER...THERE IS A LARGE
SPREAD IN MODELS AND ENSEMBLES. THUS HAVE KEPT POPS TO A MINIMUM
FOR NOW FOR THIS NEXT SYSTEM. MODELS ALSO SHOW A GRADUAL WARMING
TREND LATE NEXT WEEK...BUT HAVE GONE ON THE COOL SIDE...ESPECIALLY
IN THE BASINS WHERE THE COLD AIR COULD EASILY BE TRAPPED
UNDERNEATH A STEEP INVERSION.
&&
.AVIATION...
SOUTHWEST WINDS WILL DIMINISH TO 15 TO 25 MPH BETWEEN ROCK SPRINGS
AND CASPER AS WELL AS OVER THE HIGHER TERRAIN OF CENTRAL AND WESTERN
WYOMING THIS EVENING AND WILL CONTINUE THROUGH MUCH OF THE NIGHT. A
LOW PRESSURE CENTER WILL BE SLIDING SOUTH ALONG THE WYOMING IDAHO
BORDER SATURDAY AND WILL DRAG A COLD FRONT DOWN WITH IT. AREAS OF
MVFR AND LOCAL IFR CONDITIONS WILL ACCOMPANY THIS WEATHER FEATURE AS
CIGS AND VSBYS LOWER DUE TO SNOWFALL FROM NORTH TO SOUTH DURING THE
DAY SATURDAY. ANOTHER AREA OF MAINLY LIGHT SNOW WILL MOVE IN FROM
THE SOUTHWEST ACROSS SOUTHERN SWEETWATER COUNTY LATE SATURDAY
AFTERNOON HELPING TO FILL IN MORE WIDESPREAD AREAS OF MVFR AND LOCAL
IFR CONDITIONS BY LATE SATURDAY IN SOUTHERN WYOMING.
&&
.FIRE WEATHER...
WIND IS EXPECTED TO WEAKEN THIS EVENING AS SNOWFALL IS EXPECTED TO
BE PREVALENT OVER THE WESTERN MOUNTAINS AND VALLEYS TONIGHT
THROUGH SATURDAY NIGHT. THIS SNOWFALL IS THE RESULT OF AN AREA OF
LOW PRESSURE EXPECTED TO MOVE OVER SOUTH CENTRAL IDAHO AND
NORTHWESTERN WYOMING. SOME SNOWFALL OVER THE WEST MAY BECOME
HEAVY...AND WILL WEAKEN SUNDAY MORNING AS THE AREA OF LOW PRESSURE
BEGINS TO MOVE WESTWARD. THIS AREA OF LOW PRESSURE WILL MAINTAIN
INFLUENCE OVER THE AREA THROUGH SUNDAY AND INTO THE EARLY PORTION
OF THE WORK WEEK. MUCH COLDER TEMPERATURES WILL DEVELOP...
ESPECIALLY ON SUNDAY AND SUNDAY NIGHT. ANOTHER POTENTIALLY
SIGNIFICANT SNOWSTORM MAY DEVELOP MONDAY OVER CENTRAL WYOMING. THE
DEVELOPMENT OF THIS LATER SYSTEM WILL CONTINUE TO BE MONITORED.
&&
.RIW WATCHES/WARNINGS/ADVISORIES...
NONE.
&&
$$
SHORT TERM...BRANHAM
LONG TERM...WM
AVIATION...LIPSON
FIRE WEATHER...BRANHAM
