FXUS65 KRIW 290919
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
219 AM MST SUN NOV 29 2009

.SHORT TERM...TODAY THROUGH TUESDAY NIGHT

RIDGE BUILDING IN THIS MORNING BEHIND THE DEPARTED DEFORMATION 
AXIS.  GOOD CLEARING AND WARMING ALOFT ALLOWING FOR GOOD RADIATIONAL 
COOLING CONDITIONS.  AREAS OF FOG DEVELOPING AS A RESULT FROM THE 
BIGHORN BASIN SOUTH INTO THE WIND RIVER BASIN...WESTERN NATRONA 
COUNTIES AND INTO THE EASTERN HALF OF SWEETWATER COUNTY.  EXPECT THE 
FOG TO BURN OFF BY LATE MORNING.  IT WILL BE MOSTLY SUNNY TODAY OVER 
MOST THE AREA.  THE EXCEPTION WILL BE OVER THE NORTHERN MOUNTAINS  
WHERE LIMITED LOW LEVEL MOISTURE IN THE FLOW WILL COMBINE WITH SOME 
WEAK ISENTROPIC LIFT TO PRODUCE SOME CLOUDS AND POSSIBLY SOME LIGHT 
SNOW OR FLURRIES OVER THE HIGHER PEAKS.  WILL HAVE LIMITED WARMING 
TODAY WITH MID 30S TO MID 40S EXPECTED EAST OF THE DIVIDE...WITH 20S 
TO MID 30S WEST OF THE DIVIDE.  THE CENTRAL BASINS WILL LIKELY BE ON 
THE LOWER END OF THE RANGE AS INVERSIONS WILL LIKELY HOLD.  TONIGHT 
WILL BE MOSTLY CLEAR AS WELL WITH ANOTHER NIGHT OF RADIATIONAL 
COOLING.  MOUNTAIN AND RIDGE TOP TEMPERATURES WILL REMAIN WARM AS H7 
TEMPERATURES CLIMB TO NEAR 0C BY 12Z MONDAY.  THE RIDGE WILL BEGIN 
TO BREAK DOWN MONDAY WITH THE RIDGE AXIS SHIFTING EAST BY 
AFTERNOON.  THE FLOW BACKS TO THE WEST TO NORTHWEST AS THE NEXT 
TROUGH APPROACHES.  IT WILL BECOME QUITE WINDY MONDAY NIGHT BETWEEN 
EASTERN SWEETWATER...SOUTHERN FREMONT AND NATRONA COUNTIES...AS WELL 
AS ALONG THE EAST SLOPES OF THE ABSAROKAS AND WIND RIVERS.  THE 
WINDS SHOULD TAPER TUESDAY AS THE TROUGH MOVES THROUGH.  
HOWEVER...MODELS NOT SHOWING GOOD CONSISTENCY OR AGREEMENT WITH THIS 
TROUGH.  THE TROUGH IS LOCATED OVER AND SOUTH OF ALASKA THIS 
MORNING.  THE TROUGH WILL REMAIN MOSTLY OVER CANADA...UNTIL MONDAY 
NIGHT...WHEN IT SWINGS A MAJOR COLD FRONT INTO THE WESTERN HIGH 
PLAINS.  THE NAM IS THE MOST AGGRESSIVE WITH IT BRINGING THE VERY 
COLD AIR OVER WYOMING.  THE GFS KEEPS THE COLDEST AIR EAST OF THE 
AREA WITH A WEAK BACKDOOR FRONT INTO WYOMING.  BOTH MODELS BRING 
SOME LIGHT SNOW THROUGH THE AREAS EAST OF THE DIVIDE TUESDAY INTO 
TUESDAY EVENING.  HAVEN'T COOLED THE TEMPERATURES DOWN FOR TUESDAY 
AND TUESDAY NIGHT AT THIS TIME.  WILL WAIT FOR BETTER CONSISTENCY.  

.LONG TERM...WEDNESDAY THROUGH SUNDAY
DIFFICULT MEDIUM RANGE FORECAST WITH WIDELY VARYING MODEL SOLUTIONS 
AND NO CLEAR CUT CONSENSUS. MAIN FORECAST PROBLEM IS WHETHER RIDGE 
WILL HOLD OVER THE WRN U.S. OR WILL A STRONG SHORTWAVE CRASH THROUGH 
IT WHILE THE RIDGE REDEVELOPS FURTHER WEST. THE 00Z GFS CONTINUES TO 
SUPPORT A SIGNIFICANT SHORTWAVE MOVING THROUGH THE RIDGE THURSDAY 
WITH THE GEM AND UKMET BEING ABOUT A 12-18 HOURS SLOWER ON THIS
IDEA. THE 00Z ECMWF MAINTAINS THE RIDGE INTO THE WEEKEND. NCEP
PREFERRED THE GFS IDEA IN THE PMDEPD BUT OVERNIGHT DISCUSSIONS
FAVORED THE ECMWF. AFTER COLLABORATION WITH OUR NEIGHBORS...WE
GENERALLY LEFT THE FORECAST UNCHANGED WHICH GENERALLY FOLLOWS THE
ECMWF SOLUTION. CONFIDENCE IS LOW WITH RAPID CHANGES AND FAST FLOW
ACROSS THE PACIFIC. SEVERAL LARGE PIECES OF THE SIBERIAN COLD POOL
BREAK OFF AND TRAVEL ACROSS THE PACIFIC WITH THE POTENTIAL FOR A
LARGE UPPER LOW OVER THE CENTRAL PACIFIC WHICH COULD REDEVELOP THE
UPPER RIDGE FARTHER WEST SUPPORTING THE GFS/GEMS/UKMET IDEA BUT
ITS A TOUGH CALL. WILL LOOK AT THE HEMISPHERIC PATTERN AGAIN
TONIGHT AND SEE IF WE CAN COME UP WITH THE BEST SOLUTION AND
ADJUST AS NECESSARY. THE GFS IS OBVIOUSLY MUCH COLDER FRIDAY AND
SATURDAY WITH H7 TEMPS OF -16C TO -20C WHILE THE ECMWF IS MORE IN
THE -3C TO -8C RANGE. AND OF COURSE...RIDGE CRASHING MODELS WOULD
SUPPORT SOME LIGHT SNOW IN AT LEAST THE NRN HALF FRIDAY INTO
SATURDAY WHILE THE EURO WOULD NOT.

&&

.AVIATION...

VFR CONDITIONS WILL PREVAIL TODAY AND TONIGHT UNDER A RIDGE OF HIGH 
PRESSURE ALOFT...SURFACE HIGH PRESSURE BUILDING IN FROM THE WEST.  
LOCAL IFR/LIFR FOG WILL PARTIALLY OBSCURE TERRAIN IN THE BIG HORN 
BASIN VCNTY KGEY/KWRL...THE WIND RIVER BASIN VCNTY KRIW/KLND AND 
OVER THE SOUTH PASS AREA THROUGH 18Z.  SURFACE WINDS WILL GENERALLY 
BE LIGHT...10KTS OR LESS...THROUGH TONIGHT. 

&&

.FIRE WEATHER...

HIGH PRESSURE WILL BRING A WARMING AND DRYING TREND TO THE AREA 
TODAY AND MONDAY.  IT WILL BE MOSTLY CLEAR WITH LOWERING RH'S.  THE 
RIDGE WILL BREAK DOWN BY MID DAY MONDAY...WITH WINDS ON THE 
INCREASE.  IT WILL BECOME QUITE WINDY MONDAY NIGHT BETWEEN EASTERN 
SWEETWATER...SOUTHERN FREMONT AND NATRONA COUNTIES...AS WELL AS 
ALONG THE EAST SLOPES OF THE ABSAROKAS AND WIND RIVERS.  IT WILL BE 
DRIEST AND WARMEST MONDAY AS A COLD FRONT WILL MOVE IN 
TUESDAY...WITH A CHANCE OF LIGHT SNOW MAINLY EAST OF THE DIVIDE. 

&&

.RIW WATCHES/WARNINGS/ADVISORIES...NONE.

&&

$$

SHORT TERM...TWB
LONG TERM...SKRBAC
AVIATION...AEM
FIRE WEATHER...TWB