Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 110838
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
238 AM MDT FRI APR 11 2014

.SHORT TERM...FRIDAY THROUGH SUNDAY NIGHT

TODAY THE PHRASE WILL BE THE RELATIVE CALM BEFORE THE STORM. I USE
THE TERM RELATIVE SINCE THERE WILL BE SOME WINDY CONDITIONS IN SOME
AREAS AS THE PRESSURE GRADIENT TIGHTENS AHEAD OF THE APPROACHING
FRONT THAT WE WILL TALK ABOUT IN DETAIL BELOW. THE WINDIEST AREAS
WILL BE ACROSS THE SOUTHWESTERN WIND CORRIDOR FROM ROCK SPRINGS
THROUGH NATRONA COUNTY. WINDS SHOULD STAY BELOW HIGH WIND CRITERIA
THOUGH AS 700 MILLIBAR WINDS DO NOT LOOK STRONG ENOUGH. OTHER THAN
THE WIND...IT WILL BE A MILD DAY WITH A MIXTURE OF SUNSHINE AND MID
AND HIGH LEVEL CLOUDS. ALL IN ALL A DECENT DAY FOR THE FIRST PART OF
APRIL.

HOWEVER...IT WILL NOT LAST. A COLD FRONT WILL DROP SOUTHWARD OUT OF
MONTANA AND USHER IN SOME MUCH COLDER AIR. THE MODELS HAVE TRENDED
SLOWER WITH THE FRONT HOWEVER...SO WE RAISED HIGH TEMPERATURES A BIT
FOR SATURDAY. PRECIPITATION WILL ALSO BEGIN TO SPREAD INTO NORTHERN
AREAS LATER IN THE MORNING BUT MANY AREAS SHOULD STAY MAINLY DRY FOR
MOST OF THE DAY. IN ADDITION...WITH THE COLDER AIR COMING IN SLOWER
MOST OF THE PRECIPITATION DURING THE DAY SHOULD FALL AS RAIN EXCEPT
IN THE HIGHER ELEVATIONS. ACROSS AREAS WEST OF THE DIVIDE...THERE
COULD EVEN BE ENOUGH INSTABILITY AND UPPER LEVEL DIVERGENCE FOR
ISOLATED THUNDERSTORMS SO WE CONTINUED THOSE INTO THE AFTERNOON.

THINGS LOOK TO REALLY GET GOING SATURDAY NIGHT...ESPECIALLY LATER AT
NIGHT AND INTO SUNDAY MORNING AS UPPER LEVEL DIVERGENCE AND MORE
FAVORABLE JET DYNAMICS MOVE IN. IN ADDITION...SOME LEE CYCLOGENESIS
OVER NORTHEASTERN COLORADO WILL BEGIN TO TURN 700 MILLIBAR WINDS
MORE NORTHEASTWARD LATER AT NIGHT. THIS WILL INCREASE THE UPSLOPE
COMPONENT IN FAVORED AREAS FROM THE LANDER FOOTHILLS THROUGH NATRONA
COUNTY. WITH COLDER AIR DRAINING IN BEHIND THE FRONT RAIN SHOULD
CHANGE TO SNOW EVERYWHERE AFTER MIDNIGHT AND CONTINUE INTO SUNDAY
MORNING AS WELL. STILL SOME QUESTION FOR TOTAL ACCUMULATIONS THOUGH.
ONE FACTOR KEEPING THINGS DOWN WILL BE THE RECENT WARM WEATHER THAT
HAS WARMED GROUND TEMPERATURES THAT COULD INITIALLY MELT THE SNOW.
IN ADDITION...WITH THE STRONG APRIL SUN SOME OF THE SNOW MAY MELT
DURING THE DAY ON SUNDAY...ESPECIALLY ON PAVED SURFACES. SO AT THIS
POINT...I WOULD LEAN MORE TOWARD ADVISORIES WITH THE EXCEPTION OF
POSSIBLY THE LANDER FOOTHILLS. THERE COULD OF COURSE BE LOCALLY
HIGHER AMOUNTS WITH POSSIBLE BANDING...BUT THE MODELS DO NOT HANDLE
THIS WELL AT ALL...ESPECIALLY THIS FAR OUT. THE HEAVIEST SNOW SHOULD
END DURING SUNDAY AFTERNOON AS THE BEST FORCING MOVES AWAY. SNOW
SHOULD END FOR EVERYONE SUNDAY NIGHT AS HIGH PRESSURE BUILDS IN FROM
THE NORTH.

.LONG TERM...MONDAY THROUGH FRIDAY

GFS MODEL HAS NW FLOW FOR MONDAY AND DRY WEATHER. TUESDAY WILL BE
DRY WITH UPPER RIDGE AXIS MOVING THROUGH THE STATE. GFS MODEL HAS AN
UPPER LEVEL LOW MOVING INTO CALIFORNIA TUE NIGHT AND MOVES IT INTO
NEVADA ON WEDNESDAY. THE GFS TAKES THIS UPPER LOW INTO THE FOUR
CORNERS BY LATE THURSDAY AND ACROSS COLORADO THURSDAY NIGHT...THEN
INTO THE PLAINS NEXT FRIDAY. THIS TRACK WOULD SUPPORT SCATTERED
SHOWERS ACROSS THE SOUTHERN HALF OF THE REGION THURS AND THURS
NIGHT...ENDING FRIDAY MORNING. IN THE NORTHERN STREAM A MOIST WEST
SOUTHWEST DEVELOPS OVER THE NW U.S. THURS AFTERNOON THROUGH FRIDAY
WITH SOME OF THIS MOISTURE MOVING INTO NW WY. THUS A CHANCE OF
PRECIP IN THE NW MOUNTAINS THURS AFTERNOON AND NIGHT...THEN A CHANCE
OF PRECIP OVER MOST OF THE WEST NEXT FRIDAY AND ISOLATED ACTIVITY
EAST OF THE DIVIDE. THE ECMWF MODEL IS DRY MONDAY THROUGH TUESDAY.
BY CONTRAST...THE ECMWF MODEL SHOWS NO SIGN OF AN UPPER LOW FOR WED
THROUGH THURS. INSTEAD THE ECMWF HAS A WEST NORTHWEST FLOW TUE NIGHT
THROUGH THURSDAY NIGHT WITH A SERIES OF WEAK WEATHER SYSTEMS MOVING
THROUGH THE FLOW. THIS WOULD BRING A CHANCE OF PRECIP TO THE WESTERN
MOUNTAINS TUE NIGHT THROUGH THURSDAY NIGHT. LESSER COVERAGE OF
PRECIP OVER THE WESTERN VALLEYS. THEN THE ECMWF HAS A BRIEF BREAK
LATE THURS NIGHT WITH MORE PRECIP FOR FRIDAY. EAST OF THE
DIVIDE...THIS ECMWF PATTERN WOULD BRING MOSTLY DRY CONDITIONS WITH
ONLY ISOLATED SHOWERS AT TIMES AND BREEZY CONDITIONS IN THE FAVORED
AREAS. THE GEM MODEL IS EVEN MORE INTERESTING AS IT DIGS A TROUGH
INTO THE NORTHERN ROCKIES TUESDAY AND INTO OUR AREA WEDNESDAY INTO
WED NIGHT. THEN THE GEM KEEPS A NW FLOW NEXT THURS AND FRI WITH
WEATHER SYSTEMS IN THE FLOW. LITTLE FAITH IN ANY OF THESE MODEL
SOLUTIONS BEING CORRECT WED THROUGH FRIDAY. THUS THE BLENDER
APPROACH MAY HAVE TO OCCUR WHICH GIVES EACH MODEL A THIRD OF A
CHANCE IN BEING CORRECT. THEREFORE... WILL BRING IN A CHANCE OF
PRECIP TO THE WESTERN MOUNTAINS TUE NIGHT AND WED WITH ISOLATED
PRECIP ELSEWHERE IN THE WEST. THEN A BETTER CHANCE OF PRECIP OVER
THE WESTERN MOUNTAINS WED NIGHT THROUGH THURSDAY...A SLIGHT BREAK
THURS NIGHT AND THEN MORE PRECIP POSSIBLE NEXT FRIDAY. FOR THE FAR
WEST VALLEYS...A CHANCE OF PRECIP WED PM THROUGH THURS...THEN A
BREAK THURS NIGHT...CHANCE OF PRECIP FRI. WILL HAVE A LOW CHANCE OF
PRECIP OVER THE SOUTH AND CENTRAL WED NIGHT THROUGH THURSDAY. A
LITTLE BETTER CHANCE OF PRECIP OVER THE NORTH. WENT MOSTLY DRY NEXT
FRIDAY EAST OF THE DIVIDE. AS FOR TEMPS...MONDAY WILL WARM UP BEHIND
THE EXITING SYSTEM AS MORE SUNSHINE IS EXPECTED. WARMER HIGHS ON
TUE. THEN LITTLE DAY TO DAY CHANGE IN HIGHS NEXT WED THROUGH FRI
WITH NEAR SEASONAL TEMPS FOR HIGHS AND LOWS.

&&

.AVIATION.../12Z ISSUANCE/

A DRY WESTERLY FLOW WILL BRING PREVAILING VFR CONDITIONS ACROSS THE
REGION THROUGH 12Z SATURDAY WITH MAINLY SCT-BKN MID AND HIGH
CLOUDS.  A WEAK UPPER DISTURBANCE WILL RACE EAST ACROSS MONTANA
DURING THE DAY FRIDAY KICKING UP SURFACE WEST WIND AT 15-20 KNOTS
ACROSS MOST AREAS FROM EARLY AFTERNOON UNTIL AROUND SUNSET. THESE
WINDS WILL BE A BIT STRONGER ALONG THE INTERSTATE 80
CORRIDOR...VICINITY KRKS...WHERE WIND GUSTS OF 30-40K KNOTS ARE
LIKELY.

&&

.FIRE WEATHER...

IT WILL BE QUITE DRY THIS AFTERNOON WITH MINIMUM RELATIVE HUMIDITY
IN THE 12 TO 17 PERCENT RANGE FOR MANY AREAS EAST OF THE DIVIDE
AND DOWN INTO SWEETWATER COUNTY. THIS LOW RELATIVE HUMIDITY AND
GUSTY WIND WILL INCREASE THE FIRE POTENTIAL. A COLD FRONT WILL
DROP SOUTHWARD ACROSS THE AREA ON SATURDAY WITH COOLER
TEMPERATURES AND HIGHER RELATIVE HUMIDITY. PRECIPITATION IS LIKELY
ACROSS MANY AREAS FROM SATURDAY AFTERNOON INTO SUNDAY...ESPECIALLY
EAST OF THE DIVIDE.

&&

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

&&

$$

SHORT TERM...HATTINGS
LONG TERM...AR
AVIATION...AEM
FIRE WEATHER...HATTINGS




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