FXUS65 KRIW 142249
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
349 PM MST MON DEC 14 2009

.SHORT TERM...TONIGHT THROUGH THURSDAY
SHORT TERM IS VERY ACTIVE WITH TWO MAIN WEATHER CONCERNS:
HIGH WINDS IN THE WIND CORRIDOR AND CODY FOOTHILLS AND SNOW
IN THE FAR WEST. FIRST THE HIGH WIND POTENTIAL: PRETTY WINDY
ALREADY TODAY IN THE WIND CORRIDOR EVEN WITH LESS THAN FAVORABLE
SURFACE GRADIENT AND MID LEVEL WINDS. BY LATER TONIGHT...THE
ARCTIC AIR GETS REPLACED ALL BUT MAYBE THE NERN CORNER OF JOHNSON
COUNTY AS PRESSURE FALLS RETURN TO MONTANA. THE DEVELOPING LEE
TROUGH WILL INCREASE THE GRADIENT ACROSS THE WIND CORRIDOR AND
ABSAROKA MOUNTAINS. THE MODELS ARE SHOWING A DEVELOPING 50-60 KT
WSW H7-H8 FLOW BY TUESDAY AFTERNOON ACROSS THE WIND CORRIDOR WITH
A MODERATE SFC GRADIENT OF 10-13 MB FROM RKS TO GCC. GFS TIME
HEIGHT SHOWS GOOD SINKING ACROSS THE WIND CORRIDOR INCLUDING ONE
AREA RIGHT OVER CASPER LATE TUESDAY AFTERNOON WHEN STRONGEST H7-H8
WINDS DEVELOP. 50KTS WINDS WERE FORECAST DOWN TO 775MB WITH 60KTS
JUST ABOVE THAT. HIGH WINDS IN THE WINDIER SPOTS SEEMS LIKE A SURE
BET WITH MODERATE CONFIDENCE THAT IT WILL BE A MORE WIDESPREAD
EVENT. I'D LIKE TO SEE A BETTER FORECAST PRESSURE GRADIENT...BUT I
SUSPECT IT WILL BE STRONGER THAN 10MB. THE H7 FLOW COULD BE A
LITTLE MORE SW BUT IT STILL FORECAST TO BE ABOUT 250. THE OTHER
AREA OF HIGH WIND CONCERN IS THE CODY FOOTHILLS AND ABSAROKAS
WHERE POTENTIAL MOUNTAIN WAVE EVENT IS FORECAST BY THE WRF.
COMBINATION OF FAVORABLE 30-40 KT FLOW INTO THE BARRIER...FORECAST
MTN TOP INVERSION AND TIME HEIGHT SHOWING SOME INDICATION OF WAVE
BREAKING. THE NEW FROUDE TOOL INDICATES FAVORABLE NUMBERS AROUND 1
TUESDAY AND TUESDAY NIGHT. ALL THE ABOVE IS ENOUGH TO HIT THE AREA WITH
A HIGH WIND WATCH FOR TUESDAY AND TUESDAY NIGHT. IF EVERYTHING
COMES TOGETHER...CLARK COULD SEE A MAJOR MOUNTAIN WAVE EVENT. THE
EVENING CREW CAN EXAMINE AT LEAST ONE MORE MODEL RUN. WRF ISN'T
SHOWING A LOT OF GAP FLOW SO CLARK MAY BE ALL OR NOTHING AS IS
TYPICAL WITH A MOUNTAIN WAVE EVENT.

OUT WEST...THE CONCERN IS MORE SNOW. SYSTEM MOVING TOWARDS THE
WEST COAST WILL SPREAD INLAND TONIGHT WITH INCREASING ISENTROPIC
UPGLIDE IN THE WEST BY AFTERNOON...CONTINUING THROUGH TUESDAY
NIGHT AND POSSIBLY WEDNESDAY. INCREASED THE POPS TO CATEGORICAL
TUESDAY AFTERNOON AND TUESDAY NIGHT AND AT LEAST LIKELY THROUGH
WEDNESDAY IN THE FAVORED AREAS. SMALL RISK OF WINTER STORM
CRITERIA FROM THE STAR VALLEY TO THE TETONS AND SW PART OF YNP BUT
OPTED FOR THE SOLID ADVISORY FOR NOW WITH LOCALIZED WARNING CRITERIA.
WITH THE ARCTIC AREA BEING REPLACED...TEMPS WILL BE MORE MODERATE
IN THE NORTH TUESDAY AND WEDNESDAY.  

.LONG TERM...THURSDAY NIGHT THROUGH MONDAY 
THE ECMWF SHOWS A TROUGH MOVING ACROSS THE AREA THURSDAY INTO
THURSDAY NIGHT. ECMWF IS SHARPER WITH THE TROUGH THAN THE
OPERATIONAL GFS AND CANADIAN MODEL...AND PREFERRED DUE ITS
CONSISTENCY. THUS INCREASING CHANCE OF SNOW FOR THE MOUNTAINS AND
WEST THURSDAY...AND THEN EAST OF THE DIVIDE ON THURSDAY NIGHT. THE
ECMWF KEEPS A NORTHERLY FLOW OVER THE AREA FRIDAY WITH THE
POSSIBILITY OF SNOW SHOWERS LINGERING MUCH OF THE DAY. THE
GFS/CANADIAN ARE DRIER SINCE THEY EXIT THE SYSTEM INTO THE PLAINS
QUICKER. WILL KEEP SOME MOUNTAIN LIGHT SNOW GOING ON FRIDAY. AN
UPPER RIDGE IS FORECAST OVER THE WESTERN U.S. SATURDAY AND SUNDAY.
THE ECMWF AND SOME GFSENSEMBLE MEMBERS ARE MORE AMPLIFIED WITH THE
RIDGE THAN THE 00Z OPERATIONAL GFS. LEANING TOWARD THE MORE
AMPLIFIED SOLUTION WITH A NNW FLOW OVER THE AREA SAT AND SUN. THIS
WOULD LEAD TO MOSTLY DRY WEATHER SAT AND SUN OVER MOST OF THE
AREA. HAVE KEPT LOW PRECIP CHANCES OVER THE NW MTS FOR NOW WHICH
IS A REDUCTION IN COVERAGE FROM LAST NIGHT THINKING. WILL SEE HOW
THINGS LOOK TONIGHT AND MAY NEED TO LOWER PRECIP CHANCES EVEN
MORE. NEXT MONDAY MAY SEE AN INCREASE IN MOISTURE INTO THE NW MTS
WITH DRY WEATHER ELSEWHERE. TEMP FORECAST WILL BE A CHALLENGE
DURING THE EXTENDED. COOLER HIGHS THURS AND FRI WITH A SLIGHT
MODERATION SAT. BACKDOOR COLD FRONT SAT NIGHT INTO SUNDAY FOR THE
EAST ZONES WOULD LEAD TO SLIGHTLY COOLER HIGHS THERE ON
SUNDAY...WITH LITTLE CHANGE IN HIGHS CENTRAL AND WEST. THEN
SLIGHTLY WARMER HIGHS NEXT MONDAY.

&&

.AVIATION...
THE ARCTIC FRONT WILL ERODE THIS EVENING AS WARMER AIR BEGINS TO 
MOVE IN.  LIGHT SNOW WILL CONTINUE OVER THE HIGHER TERRAIN OF 
WESTERN WYOMING WITH MTNS OBSCURATIONS.  LOCAL MVFR CIGS ARE 
POSSIBLE OVER KAFO AND KJAC TONIGHT.  VERY GUSTY SOUTHWEST TO WEST 
WINDS ARE EXPECTED BETWEEN KRKS AND KCPR.  THESE WINDS WILL INCREASE 
TUESDAY TO 30G40KTS WITH LOCAL WINDS OF 40G50KTS.  ANOTHER SYSTEM 
WILL BRING ANOTHER ROUND OF SNOW INTO WESTERN WYOMING TUESDAY 
AFTERNOON.  CIGS AND VSBYS WILL BECOME MVFR TO LCL IFR IN THE LIGHT 
SNOW.

&&

.FIRE WEATHER...
ARCTIC AIR WILL RETREAT ACROSS THE NORTHERN PORTION OF THE
DISTRICT TUESDAY TUESDAY MORNING. STRONG SOUTHWEST WINDS WILL
INCREASE AND SPREAD THROUGHOUT MOST OF THE WIND CORRIDOR FROM
JEFFREY CITY TO CASPER AND IN THE CODY FOOTHILLS. SNOW WILL
INCREASE AGAIN IN THE FAR WEST TUESDAY AFTERNOON AND CONTINUE
THROUGH WEDNESDAY.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
HIGH WIND WATCH FROM LATE TONIGHT THROUGH TUESDAY EVENING FOR 
WYZ019.

HIGH WIND WATCH FROM TUESDAY THROUGH TUESDAY EVENING FOR WYZ020.

HIGH WIND WATCH FROM TUESDAY THROUGH LATE TUESDAY NIGHT FOR
WYZ002-003.

&&

$$

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