Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 242024
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
224 PM MDT WED SEP 24 2014

.SHORT TERM...TONIGHT THROUGH THURSDAY

LARGE AMPLIFYING RIDGE ACROSS THE CNTRL/NRN ROCKIES PRODUCING
LARGELY CLEAR SUNNY SKIES AND KEEPING PRECIP CHCS WELL TO THE NORTH
OR WEST OF THE FA. THIS SET-UP WILL LAST THRU THURSDAY.

HOWEVER...THERE WILL BE A COUPLE OF IMPACTS FROM ALL THE NICE WX IN
THE SECOND PERIOD OF THE FCST. THE FIRST WILL COME AS POSSIBLE RED FLAG
CONDITIONS ACROSS FIRE WEATHER ZONES 280 281 AND PARTS OF 289
THURSDAY AFTERNOON. ALTHOUGH CONDITIONS WILL LARGELY BE
MARGINAL...IT IS JUST TOO CLOSE TO CALL AT THIS POINT SO HAVE ISSUED
A FIRE WEATHER WATCH FOR THESE THREE ZONES FROM 18Z THURSDAY THRU
01Z FRIDAY. ATTM...IT APPEARS THAT THE MOST LIKELY AREA TO HIT RED
FLAG CONDITIONS WILL BE ACROSS ZONE 281 ODDLY ENOUGH IN AN AREA THAT
CAN TAKE ADVANTAGE OF A SLIGHTLY BETTER SURFACE PRESSURE GRAD ALONG
WITH LARGELY UNIDIRECTIONAL SOUTHERLY FLOW THRU DEPTH UNDER THE
INFLUENCE OF INCREASINGLY STRONG LAPSE RATES PEAKING BETWEEN 8.5 AND
11 DEG C/KM IN THE AFTERNOON. THE OTHER IMPACT WILL CONCERN THE
APPROACHING...TYING...OR EXCEEDING OF HIGH TEMPERATURE RECORDS FOR
THE DAY ACROSS THE FORECAST AREA...ESPECIALLY EAST OF THE DIVIDE.

.LONG TERM...THURSDAY NIGHT THROUGH WEDNESDAY

THURSDAY NIGHT...A LARGE PACIFIC LOW PRESSURE TROUGH MOVING INTO THE
WEST COAST WILL BRING INCREASED SOUTH SOUTHWEST FLOW INTO WESTERN
AND SOUTHERN WYOMING AS A LARGE AREA OF HIGH PRESSURE SHIFTS EAST
INTO THE PLAINS.  SCATTERED SHOWERS ARE EXPECTED TO MOVE INTO THE
WESTERN MOUNTAINS WITH ISOLATED EVENING THUNDERSTORMS.

FRIDAY AND FRIDAY NIGHT  THE WEST COAST TROUGH WILL CONTINUE MOVING
INLAND AND START TO CUT OFF IN CENTRAL CALIFORNIA WITH THE SOUTH
SOUTHWEST FLOW SPREADING EAST ACROSS WESTERN AND CENTRAL WYOMING.
MOISTURE AND INSTABILITY WILL INCREASE WITH INCREASING COVERAGE OF
SHOWERS ACROSS THE WEST SPREADING EAST...AND ISOLATED THUNDERSTORMS
DEVELOPING IN THE AFTERNOON AND CONTINUING INTO THE EVENING HOURS.
TEMPERATURES WILL COOL WITH INCREASED CLOUD COVER AND PRECIPITATION
WHILE THE MAIN COLD AIR FROM THE TROUGH REMAINS TO THE WEST OF THE
AREA.

SATURDAY AND SATURDAY NIGHT A THE LOW CUTS OFF OVER THE GREAT
BASIN...THE FLOW ALOFT OVER WESTERN AND CENTRAL WYOMING WILL BECOME
INCREASINGLY DIVERGENT WITH STRONG UPPER LEVEL SUPPORT FOR THE
CONVECTION SPREADING OVER THE AREA.  SHOWERS WILL BECOME SOMEWHAT
WIDESPREAD WITH EMBEDDED AFTERNOON AND EVENING THUNDERSTORMS.  SOME
SNOW CAN BE EXPECTED IN THE WESTERN AND NORTHERN MOUNTAINS. THE COLD
AIR FROM THE TROUGH WILL BEGIN TO MOVE OVER THE AREA WITH
SIGNIFICANT COOLING.  THIS FORECAST COOLER THAN PREVIOUS AND TREND
WILL LIKELY CONTINUE THE NEXT FEW DAYS AS BETTER AGREEMENT OCCURS
BETWEEN THE MODELS.  OVERALL...THE MODELS ARE IN FAIR AGREEMENT
WITH THE LOCATION AND STRENGTH OF THE CUT OFF LOW.

SUNDAY AND SUNDAY NIGHT CUT OFF LOW AROUND ELY NEVADA HEADING
TOWARDS SALT LAKE CITY.  UPPER LEVEL DIVERGENCE CONTINUES TO
INCREASE WITH SIGNIFICANT SUPPORT FOR CONVECTION OVER THE AREA.
SHOWERS WILL REMAIN WIDESPREAD AND UPPER DYNAMICS SHOULD HELP
SUPPORT MID LEVEL DEFORMATION AND DEEPENING MID LEVEL LAPSE RATES
FOR SOME THUNDERSTORM ACTIVITY IN THE AFTERNOON AND EVENING.  COLDER
AIR CONTINUES TO SPREAD OVER THE AREA WITH SNOW IN THE MOUNTAINS.
MODELS
REMAIN IN FAIR AGREEMENT WITH THE DETAILS OF THE UPPER LEVEL LOW.

MONDAY AND MONDAY NIGHT THE UPPER LEVEL LOW BEGINS TO OPEN UP AND
EJECT NORTHEAST ACROSS THE AREA.  MODELS ADVERTISE A DRY SLOT FROM
THE SOUTHWEST INTO CENTRAL WYOMING WHICH WOULD COMPLICATE THE
FORECAST WITH THE LOCATION AND INTENSITY OF PRECIPITATION. TOO FAR
OUT AT THIS TIME TO INDICATE POTENTIAL MINIMA OF PRECIPITATION. AS
THE LOW CONTINUES OPENING UP AND MOVING NORTHEAST IS EXPECTED TO END
UP IN EASTERN MONTANA WITH COLD CYCLONIC WEST FLOW OVER WESTERN AND
CENTRAL WYOMING.  THIS REMOVES THE FAVORABLE AREA OF UPPER
DIVERGENCE AND THE FLOW LOOKS TO BECOME MOISTURE STARVED...WHILE
OROGRAPHIC PRECIPITATION SHOULD DOMINATE THE PRECIPITATION PROCESS
OVER THE WESTERN MOUNTAINS WITH AREAS EAST OF THE DIVIDE BECOMING
LESS FAVORABLE FOR SIGNIFICANT PRECIPITATION.  TEMPERATURES CONTINUE
TO DROP WITH SNOW LIKELY IN THE WESTERN MOUNTAINS.

TUESDAY AND TUESDAY NIGHT FLOW ALOFT TRANSITION INTO CYCLONIC WEST
FLOW WITH A DEEP LONG WAVE TROUGH OVER THE AREA FROM ALBERTA AND
SASKATCHEWAN SOUTH INTO THE DESERT SOUTHWEST.  THIS SHOULD KEEP
THINGS ON THE COLD SIDE WITH THE PRECIPITATION REMAINING MAINLY OVER
THE WESTERN MOUNTAINS WITH MOISTURE REMAINING LIMITED.

WEDNESDAY A SHORT WAVE TROUGH FROM THE PACIFIC NORTHWEST WILL MOVE
SOUTHEAST DEEPENING THE LONG WAVE TROUGH OVER THE REGION AND SHOULD
SEE AND ENHANCEMENT TO PRECIPITATION IN THE WEST WITH UNSETTLED
SHOWERY COLD WEATHER EAST OF THE CONTINENTAL DIVIDE.  MODELS SHOW
MORE AND MORE SPREAD IN STRENGTH AND LOCATION OF FEATURES ASSOCIATED
WITH THE MAIN LONG WAVE TROUGH AND THE SHORT WAVE TROUGH MOVING
THROUGH THE FLOW.  NO CLEAR WINNER AT THIS TIME AND NOTHING APPEARS
TO MAKE SIGNIFICANT DIFFERENCES IN THE FORECAST.  LIKELY TO BE
COLDER THAN CURRENTLY FORECAST.

&&

.AVIATION.../00Z ISSUANCE/

VFR CONDITIONS WILL PREVAIL OVER THE REGION THROUGH THE FORECAST
PERIOD WITH WARM...DRY...AND BREEZY CONDITIONS.

&&

.FIRE WEATHER...

WARMER AND DRIER TODAY...BUT FIRE DANGER STILL REMAINING BELOW
CRITICAL LEVELS. THURSDAY...HOWEVER...THE HEAT IS ON WITH NEAR OR
RECORD HIGHS EXPECTED ACROSS THE FORECAST AREA AND MINIMUM RH VALUES
DROPPING ACCORDINGLY TO AT OR BELOW 15 PERCENT ACROSS THE EASTERN
AND SOUTHEASTERN ZONES...SPECIFICALLY FIRE WEATHER ZONES 280...281
AND 289. THIS WILL CERTAINLY ENHANCE FIRE DANGER ACROSS THESE AREAS FOR
A SHORT PERIOD IN THE AFTERNOON WHEN WINDS WILL BECOME GUSTY OUT OF
THE SOUTH OR SOUTHWEST ACROSS THESE ZONES. A FIRE WEATHER WATCH HAS
BEEN ISSUED FOR THESE ZONES...AS CRITICAL LEVELS WITH RESPECT TO
MARGINAL RED FLAG CONDITIONS WILL BE TOO CLOSE TO CALL AT MANY
LOCATIONS AND MAY EXCEED CRITICAL VALUES FOR A BRIEF PERIOD. FRIDAY
WILL CONTINUE THE LOW HUMIDITY FOR ANOTHER DAY...HOWEVER WINDS ARE
EXPECTED TO FALL OFF AS DEVELOPING SURFACE LOW PRESSURE SHIFTS
EASTWARD.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
FIRE WEATHER WATCH FROM THURSDAY AFTERNOON THROUGH THURSDAY
EVENING WYZ280-281-289.

&&

$$

SHORT TERM...BRAUN
LONG TERM...C.BAKER
AVIATION...BRAUN
FIRE WEATHER...BRAUN





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