AREA FORECAST DISCUSSION

 
 FXUS65 KRIW 251709
 AFDRIW
 
 AREA FORECAST DISCUSSION
 NATIONAL WEATHER SERVICE RIVERTON WY
 1009 AM MST WED NOV 25 2009
 
 .UPDATE...
 
 HAVE MADE A FEW CHANGES TO THE FORECAST...MAINLY FOR THIS
 AFTERNOON. LOW OVERCAST OVER THE JACKSON HOLE THIS MORNING APPEARS
 AS IF IT WILL BE VERY DIFFICULT TO LIFT THIS AFTERNOON...GIVEN THE
 SHARP RIDGING ALOFT. INCREASED CLOUD COVER LATE THIS MORNING AND
 THIS AFTERNOON OVER THE JACKSON HOLE AND LOWERED HIGH TEMPERATURES
 4-7 F ACROSS THE AREA. OTHERWISE...DECREASED CLOUD COVER ACROSS
 EASTERN PORTIONS OF CENTRAL WY...AS SHORTWAVE IS MOVING RAPIDLY
 SOUTHEASTWARD AWAY FROM THE REGION. UPDATED PRODUCTS WILL BE
 ISSUED BY 10:15 AM MST.  
 
 &&
 
 .PREV DISCUSSION... /ISSUED 245 AM MST WED NOV 25 2009/ 
 
 SHORT TERM...TODAY THROUGH FRIDAY NIGHT
 EARLIER THIS EVENING THE ALBERTA CLIPPER AND ASSOCIATED BACKDOOR 
 COLD BROUGHT A QUICK SHOT OF STRONG NORTH NORTHWEST WINDS TO JOHNSON 
 COUNTY ALONG WITH RAIN THAT TURNED TO WET SNOW WITH LIGHT 
 ACCUMULATIONS WITH MORE SNOW AT THE ADJACENT HIGHER ELEVATIONS AND 
 BIG HORN MTNS. SOME LIGHTER RAIN AND SNOW SHOWERS EVEN FELL ACROSS 
 PORTIONS OF THE BIG HORN BASIN. MOST OF THE Q VECTOR FORCING TRACKED 
 WELL TO THE NORTHEAST ACROSS THE NORTHERN PLAINS. CLEARING WILL 
 CONTINUE IN THE EASTERN ZONES AS NEVA QUICKLY MOVES IN AND SURFACE 
 HIGH PRESSURE BUILDS INTO THE AREA THIS MORNING. A 1035MB HIGH OVER 
 SOUTHWEST WY WILL BECOME MORE ESTABLISHED THIS MORNING. BASINS AND 
 VALLEYS WILL REMAIN TRAPPED IN INVERSIONS THROUGH THURSDAY WITH 
 HIGHS MAINLY IN THE 40S IN THE CENTRAL BASINS AND IN THE 30S IN THE 
 WESTERN BASINS AND VALLEYS. TEMPS ABOVE THE INVERSIONS EAST OF THE 
 DIVIDE WILL BE JUST AS WARM AT 95K AS THEY WILL BE ON THE BASIN 
 FLOORS. TEMPS AT THE HIGHER ELEVATIONS AND ALONG THE SOUTHERN WIND 
 CORRIDOR FROM THE RED DESERT TO CPR AND THROUGH JOHNSON COUNTY WILL 
 EXPERIENCE A NICE WARMING TREND THROUGH THANKSGIVING WITH HIGHS WELL 
 UP INTO THE 50S WITH SOME LOW 60S IN JOHNSON COUNTY WITH H7 TEMPS 
 REACHING 4.5 TO 5C BY THURSDAY. LOWS IN THE SINGLE DIGITS WILL BE 
 FAIRLY COMMON IN THE WESTERN VALLEYS AND BASINS. HIGH PRESSURE WILL 
 WEAKEN ON FRIDAY. THE NEXT UPSTREAM TROUGH WILL APPROACH THE AREA 
 FROM THE WEST RESULTING IN A TIGHTENING PRESSURE GRADIENT. WEST 
 SOUTHWEST WINDS WILL GRADUALLY INCREASE DURING THE LATTER PORTION OF 
 THE WEEK WITH WINDS FINALLY MIXING DOWN TO THE BASINS AND VALLEYS BY 
 FRIDAY WITH INVERSIONS FINALLY MIXING OUT. FRIDAY SHOULD BE THE 
 WARMEST DAY OF THE WEEK IN THE BASINS AND VALLEYS BUT OTHER AREAS 
 WILL LIKELY EXPERIENCE THE HIGHEST TEMPS ON THURSDAY BEFORE COOL AIR 
 ADVECTS INTO THE MID LEVELS BY FRIDAY. WILL KEEP IDEA OF SNOW 
 BREAKING OUT IN WESTERN WY FRIDAY AFTERNOON WITH INCREASING POPS 
 FRIDAY NIGHT AS WESTERLY UPSLOPE FLOW INCREASES AND THE GFS SOLN OF 
 THE FIRST SHORTWAVE ON THE FRONT SIDE OF THE APPROACHING LONG WAVE 
 TROUGH TRACKS ACROSS THE AREA FRIDAY NT AND THEN DIFLUENT FLOW 
 INCREASES OUT AHEAD OF THE NEXT SHORTWAVE ON THE BACK SIDE OF THE 
 LONG WAVE TROUGH. THE SNOWFALL WILL BE LIGHT AS THE ENERGY WITH THIS 
 TROUGH WILL BE SPLITTING.    
 
 LONG TERM...SATURDAY THROUGH WEDNESDAY
 MEDIUM RANGE MODELS ARE IN DECENT AGREEMENT IN DIGGING A SHORTWAVE 
 TROUGH SOUTHEAST OUT OF BRITISH COLUMBIA/PACIFIC 
 NORTHWEST...SOUTHEAST ACROSS IDAHO ON SATURDAY AND THEN QUICKLY 
 SOUTH INTO UTAH/ARIZONA SATURDAY NIGHT...WITH DEEP NORTHERLY FLOW 
 OVER THE AREA ON SUNDAY.  THIS SYSTEM PASSING TO OUR WEST...SHOULD 
 GIVE A DECENT SHOT AT SOME SNOW WEST OF THE DIVIDE SATURDAY/SATURDAY 
 EVENING...BEFORE DRIER CONDITIONS PREVAIL.  HAVE KEPT SLIGHT POPS 
 GOING EAST OF THE DIVIDE SATURDAY...TO RESPECT THE ECMWF SOLUTION 
 THAT HAS MORE MOISTURE LEFT OVER FROM THE INITIAL TROUGH...AND THE 
 WEAK UPSLOPE FLOW THAT DEVELOPS EAST OF THE DIVIDE SATURDAY. HOWEVER
 THE BULK OF THE SNOW IS EXPECTED WEST OF THE DIVIDE.  TOTAL SNOW 
 SHOULD BE IN THE 2-5 INCH RANGE IN THE MOUNTAINS...WITH 1-2 IN THE 
 FAR WESTERN VALLEYS BY SATURDAY EVENING.  A RIDGE IN THE A EASTERN 
 PACIFIC WILL PUSH EAST INTO WEST/ROCKIES SUNDAY NIGHT/MONDAY.  THIS 
 WILL RESULT IN MAINLY DRY CONDITIONS AND A WARMING TREND. A TROUGH 
 OVER THE GULF OF ALASKA SUNDAY...IS PROJECTED TO PUSH EAST SOUTHEAST 
 INTO BRITISH COLUMBIA/PACIFIC NORTHWEST ON MONDAY.  THIS TROUGH WILL 
 CONTINUE EAST SOUTHEAST FLATTENING THE RIDGE MONDAY NIGHT AND 
 TUESDAY.  THE ASSOCIATED COLD FRONT WITH THIS TROUGH SHOULD MOVE 
 ACROSS THE AREA MONDAY NIGHT/TUESDAY.  THE 00Z ECMWF IS INDICATING 
 MORE ENERGY AFFECTING THE FORECAST AREA THEN THE GFS.  HAVE MAINLY 
 CHANCE MTN POPS AND SLIGHT VALLEY POPS TO HANDLE THIS SYSTEM FOR 
 NOW.  TEMPERATURES ARE EXPECTED TO BE SEASONAL TO SLIGHTLY BELOW 
 AVERAGE SATURDAY AND SUNDAY...WITH TEMPERATURES REBOUNDING TO ABOVE 
 AVERAGE ON MONDAY AND THEN BELOW AVERAGE TEMPERATURES RETURN BY 
 WEDNESDAY...AND POSSIBLY ON TUESDAY DEPENDING UPON TIMING OF THE 
 COLD FRONT WITH THE SYSTEM ON TUESDAY.
 
 AVIATION...
 TAIL END OF LOW PRESSURE MOVING THROUGH THE NORTHERN PLAINS NUDGED A 
 COLD FRONT SOUTH INTO JOHNSON AND NATRONA COUNTIES LATE TUESDAY 
 EVENING AND EARLY THIS MORNING. LIGHT SNOW AND IFR/MVFR CONDITIONS 
 HAVE BEEN SEEN WITH THE FRONTAL PASSAGE. IFR CONDITIONS DUE TO LOW
 CIGS SHOULD IMPROVE TO VFR CONDITIONS BY 12Z THIS MORNING IN KCPR. LIGHT
 SNOW AND WIDESPREAD MOUNTAIN OBSCURATIONS WILL LINGER IN AND NEAR
 THE BIGHORN MOUNTAINS UNTIL 15Z-18Z WEDNESDAY BEFORE MOISTURE
 BEGINS TO PULL EAST. ELSEWHERE...MAINLY SCT-BKN CIRRUS CLOUDS WITH
 BKN070-090 DECK AT KCOD AND KWRL. THE EXCEPTION AGAIN IS KJAC
 WHERE HIGH BOUNDARY LAYER MOISTURE HAS ALLOWED FOR BKN DECK TO
 AGAIN FORM AROUND 050 FEET AGL WITH LOCAL MVFR CEILINGS AROUND 015
 FEET AGL. EXPECT LOW CLOUDS TO ERODE AROUND 17Z-18Z WEDNESDAY WITH
 A REPEAT OF THIS SCENARIO SET TO OCCUR BY LATE WEDNESDAY EVENING.
 SOUTHWEST WIND WILL INCREASE TO 15-25KTS BY LATE THIS MORNING AT
 KCPR WHILE MOST OTHER TERMINAL SITES WILL BE LESS THAN 08KTS
 THROUGH WEDNESDAY EVENING. VFR CONDITIONS TO PREVAIL AT ALL
 TERMINALS FROM 18Z/WED TO 06Z/THU.
 
 FIRE WEATHER...
 CLEARING WILL TAKE PLACE IN JOHNSON AND NORTHERN 
 NATRONA COUNTIES EARLY THIS MORNING ALONG WITH DECREASING NORTH 
 WINDS AS AN ALBERTA CLIPPER EXITS TO THE EAST OF THE AREA. STRONG 
 INVERSIONS WILL BE PREVALENT ACROSS THE BASINS AND VALLEYS THROUGH 
 THURSDAY. SOUTHWEST WINDS WILL INCREASE THROUGH THURSDAY OVER THE 
 SOUTHERN WIND CORRIDOR FROM THE RED DESERT TO CASPER. A WARMING 
 TREND WILL TAKE PLACE OVER THE HIGHER TERRAIN AND FROM THE RED 
 DESERT TO BUFFALO THROUGH THURSDAY WITH THURSDAY BEING THE WARMEST 
 DAY EXPECTED WITH LOW 60S FORECAST FOR JOHNSON COUNTY. THE APPROACH 
 OF A PACIFIC TROUGH WILL CAUSE WINDS TO MIX DOWN INTO THE VALLEYS 
 AND BASINS FRIDAY FOR SOME TEMPERATURE RECOVERY TO OCCUR IN THOSE 
 AREAS. LIGHT SNOW SHOULD BREAK OUT ACROSS WESTERN WYOMING FRIDAY 
 AFTERNOON AND NIGHT IN RESPONSE TO THE APPROACHING TROUGH.
 
 &&
 
 .RIW WATCHES/WARNINGS/ADVISORIES...
 NONE.
 &&
 
 

 
 SHORT TERM...LIPSON
 LONG TERM...WM
 AVIATION...CNJ
 FIRE WEATHER...LIPSON