Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 041815 AAA
AFDRIW

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE RIVERTON WY
1215 PM MDT MON MAY 4 2015

.SHORT TERM...TODAY THROUGH WEDNESDAY NIGHT

THIS EVENINGS CONVECTION HAS LONG SINCE DIED OFF. A CLOSED LOW OVER
THE PAC NW WILL CONTINUE TO CHURN WEAK VORT LOBES NORTHEAST ACROSS
WYOMING TODAY AND TUESDAY. THE BACKDOOR COOL FRONT THAT SWEPT
ACROSS AREAS EAST OF THE DVD SUNDAY MORNING AND WAS PUSHED BACK TO
THE NORTHEAST WITH MONDAYS CONVECTION WILL AGAIN SWEEP SOUTHWEST
ACROSS AREAS EAST OF THE DIVIDE THIS MORNING. TODAY`S VORT LOBE WILL
PRODUCE MORE LATE DAY THUNDERSTORMS MOVING IN FROM THE SOUTHWEST
TODAY WITH REDEVELOPMENT ALONG THE DIVIDE AS THEY CROSS THE DIVIDE
AND BISECT THE OLD SFC FRONT THAT WILL BE LODGED AGAINST THE EAST
SIDE OF THE DVD ONCE AGAIN. THESE STORMS WILL AGAIN PUSH THE FRONT
BACK TO THE EAST WELL AWAY FROM THE CWA THIS TIME. TUESDAY LOOKS TO
BE THE MOST ACTIVE CONVECTIVE DAY FROM NOW THROUGH MID WEEK. THE
STABLE AIR BEHIND THE OSCILLATING SFC FRONT WILL FINALLY BE WELL TO
OUR EAST. LIFTED INDICES WILL BE BETWEEN -2 AND -3 IN MANY AREAS
ALONG WITH LOW BUT WIDESPREAD CAPE. THESE STORMS WILL LIKELY PERSIST
WELL INTO THE NIGHT TUESDAY NIGHT. THE WRF ADVERTISES  THE STRONGEST
STORMS OCCURRING JUST EAST OF THE DIVIDE FROM THE WIND RVR BASIN TO
CODY TUESDAY EVENING. ON WEDNESDAY...THE SOUTHERN STREAM ENERGY
BEGINS TO HAVE A HAND IN INFLUENCING OUR WX. THE WEAK CIRCULATION
OFF THE SOUTHERN CA COAST WILL WOBBLE TOWARD SOUTHEAST WY BY WED.
THE ASCT WEAK H7 CIRCULATION WILL SET UP SHOP OVER SE WY AS WELL PER
THE GFS WITH THE EURO PLACING IT OVER NATRONA COUNTY. THE GFS PLACES
THE SFC LOW OVER NATRONA/JOHNSON COUNTY WED. THIS WOULD PLACE US ON
THE BACKSIDE OF THE WEAK VERTICALLY STACKED LOW...MORE CONDUCIVE FOR
MORE WIDESPREAD RAINFALL. ALSO...WITH THE STRONGER NORTHERN STREAM
LOW STILL OVER THE PAC NW AND THE WEAK LOW OVER SE WY...BOTH LOWS
WILL CONTRIBUTE TO A SOUTHWESTERLY DIFLUENT FLOW ADDING TO THE LIFT
WED. THE SNOW LEVEL SHOULD DROP TO BETWEEN 8 AND 10K THROUGH WED NT.
MOISTURE WILL INCREASE BUT INSTABILITY WILL WEAKEN SO WE ARE LOOKING
AT MORE NUMEROUS SHOWERS AND ONLY ISOLATED THUNDERSTORMS WED.

.LONG TERM...THURSDAY THROUGH MONDAY

SYNOPSIS...AN UPPER LEVEL LOW PRESSURE SYSTEM WILL SLOWLY WOBBLE
EAST ACROSS THE GREAT BASIN THURSDAY AND FRIDAY...AND LIFT ACROSS
THE CENTRAL ROCKIES INTO THE NORTHERN/CENTRAL PLAINS OVER THE
WEEKEND.  THIS SYSTEM WILL BRING COOLER CONDITIONS AND SCATTERED TO
NUMEROUS SHOWERS ACROSS THE AREA THURSDAY THROUGH SATURDAY WITH
SHOWERS DECREASING ON SUNDAY.

DISCUSSION...THE MAIN FORECAST PROBLEM DURING THIS PERIOD WILL BE AN
UPPER LOW...NEAR 50N/135W EARLY THIS MORNING...PROGGED TO MOVE DOWN
THE WEST COAST...AND INTO THE WESTERN GREAT BASIN AT THE BEGINNING
OF THIS PERIOD.  MEDIUM RANGE GLOBAL MODELS IN GOOD OVERALL
AGREEMENT MOVING THIS SYSTEM SLOWLY EAST ACROSS NEVADA AND UTAH
THURSDAY NIGHT THROUGH FRIDAY NIGHT WITH UPPER LOW JUMPING THE
DIVIDE INTO NE COLORADO ON SATURDAY.

ON THURSDAY AND FRIDAY...UPPER LEVEL SOUTHERLY DIFLUENT FLOW ALOFT
WILL SETUP OVER SURFACE FRONT...LOW LEVEL UPSLOPE FLOW ALONG AND
EAST OF THE DIVIDE.  STRONGER THUNDERSTORMS AND HEAVIER
PRECIPITATION IS EXPECTED NEAR THE FRONTAL BOUNDARY ACROSS SOUTH AND
CENTRAL WYOMING WITH A MORE STABLE AIRMASS ACROSS NORTH CENTRAL AND
NORTHEAST WYOMING.  SNOW LEVELS ARE EXPECTED TO BE MAINLY BETWEEN
8-9KFT...SO SOME HEAVY SNOW SHOWERS MAY BE OCCUR AT PASS LEVEL.

FRIDAY NIGHT AND SATURDAY...CHANCES FOR HEAVIER MORE WIDESPREAD
PRECIPITATION INCREASE ACROSS CENTRAL AREAS AS UPPER LOW CROSSES THE
CENTRAL ROCKIES.  GFS ENSEMBLE MEAN HAS BEEN CONSISTENT IN
PROJECTING THE HEAVIEST AREA OF PRECIPITATION FROM SOUTHEAST WYOMING
BACK ACROSS MAINLY NATRONA AND FREMONT COUNTIES DURING THIS PERIOD
WHERE WCB PRECIPITATION IS MOST LIKELY FRIDAY NIGHT FOLLOWED BY
COLDER UPSLOPE FLOW ON SATURDAY.  GFS IS TRENDING COLDER ON BACKSIDE
OF SYSTEM ON SATURDAY WITH H7 TEMPS -5C TO -6C EAST OF THE DIVIDE
WHICH WOULD PUT SNOW LEVELS 5.5-6KFT.  ECMWF HAS ALSO TRENDED
COLDER...BUT NOT QUITE AS COLD AS GFS.

SUNDAY AND MONDAY....UPPER LOW WILL OPEN UP AND MOVE ACROSS THE
NORTHERN/CENTRAL PLAINS ON SUNDAY WITH UNSETTLED NW FLOW BEHIND THIS
SYSTEM LIKELY TO GENERATE SHOWER ACTIVITY THROUGH THE DAY.  NEXT
PACIFIC TROUGH IS EXPECTED TO PUSH INTO THE WESTERN U.S. ON MONDAY
BRINGING CHANCES OF SHOWERS MAINLY ACROSS THE WEST.

.AVIATION.../18Z ISSUANCE/

EAST OF THE DIVIDE...KCOD/KCPR/KLND/KRIW/KWRL ROUTES

ANOTHER ROUND OF SHOWERS AND THUNDERSTORMS ARE EXPECTED WITH PEAK
HEATING AND ARE ALREADY BEGINNING TO POP OVER THE
MOUNTAINS...EXPECT TERMINALS NEAR THE MOUNTAINS TO SEE A FLASH OF
LIGHTNING OR TWO BY 20Z. THERE IS NOT MUCH WIND ALOFT TO MOVE THE
STORMS ALONG SO MOST OF THE ACTIVITY WILL OCCUR OVER/NEAR THE
MOUNTAINS...AND STORMS WILL QUICKLY WEAKEN AS THEY MOVE OFF OF THE
HIGHER TERRAIN. THUS HAVE KEPT THE TERMINALS AWAY FROM THE
MOUNTAINS DRY SUCH AS KRIW AND KWRL...BUT KEPT MENTION OF EITHER
VCSH OR VCTS AT THE REST. WINDS WILL GENERALLY BE EASTERLY IN
MANY LOCATIONS...BECOMING MORE ERRATIC AND GUSTY DUE TO
NEARBY SHOWERS AND STORMS AFTER ABOUT 20Z THROUGH 03Z. EXPECT MORE
SHOWERS AND THUNDERSTORMS ON TUESDAY...GENERALLY AFTER 18Z...WITH
A BIT MORE COVERAGE THAN WHAT IS EXPECTED TODAY.

WEST OF THE DIVIDE...KBPI/KJAC/KPNA/KRKS ROUTES

ANOTHER ROUND OF SHOWERS AND THUNDERSTORMS ARE EXPECTED WITH PEAK
HEATING AND ARE ALREADY BEGINNING TO POP OVER THE
MOUNTAINS...EXPECT TERMINALS NEAR THE MOUNTAINS TO SEE A FLASH OF
LIGHTNING OR TWO BY 19Z WITH ALL TERMINALS EXPECTED TO HAVE A
SHOWER OR STORM IN THE VICINITY BY 21Z. SOME SHOWERS AND STORMS WILL
LAST INTO THE EVENING ACROSS THE SOUTHWEST AS A WEAK DISTURBANCE
MOVES THROUGH...MAINLY AFFECTING KBPI AND KPNA. WIND FIELDS WILL
GENERALLY BE LIGHT...EXCEPT NEAR CONVECTION. EXPECT MORE
SHOWERS AND THUNDERSTORMS ON TUESDAY...GENERALLY AFTER 18Z...WITH
A BIT MORE COVERAGE THAN WHAT IS EXPECTED TODAY.


.FIRE WEATHER...

ANOTHER SURGE OF MOISTURE WILL INITIATE SHOWERS AND THUNDERSTORMS
ACROSS MUCH OF THE AREA TODAY. TEMPERATURES WILL FLUCTUATE UNDER
THE INFLUENCE OF THESE MOISTURE PUSHES...WITH EASTERN WYOMING
SEEING A SLIGHT COOL DOWN TOMORROW WHILE MOST OF THE AREA WILL SEE
A COUPLE DEGREES OF WARMING. GENERALLY CALM WIND EXPECTED THIS AFTERNOON
..OTHER THAN JOHNSON AND EASTERN NATRONA COUNTY WHICH MAY SEE
PERIODS OF GUSTY ACTIVITY. TUESDAY WILL LIKELY FEATURE SLIGHTLY
STRONGER THUNDERSTORMS. ON WEDNESDAY EXPECT COOLER WEATHER WITH
NUMEROUS SHOWERS BUT ONLY ISOLATED THUNDERSTORMS. THE SNOW LEVEL
WILL LIKELY DROP TO BETWEEN 8000 AND 10000 FEET WEDNESDAY THROUGH
WEDNESDAY NIGHT.
&

&&

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

$$

SHORT TERM...LIPSON
LONG TERM...AEM
AVIATION...ALLEN
FIRE WEATHER...LIPSON




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