Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 252039
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
239 PM MDT MON MAY 25 2015

.SHORT TERM...TONIGHT THROUGH TUESDAY

AS THE UPPER LEVEL LOW PRESSURE TROUGH CENTERED OVER THE NEBRASKA
PANHANDLE MOVES EAST...THE COLD MOIST NORTHERLY FLOW OVER WESTERN
AND CENTRAL WYOMING WILL BECOME NORTHWESTERLY WITH A DECREASE IN
MOISTURE.  THE AIR MASS WILL BE SLIGHTLY UNSTABLE EARLY THIS EVENING
WITH SCATTERED SHOWERS AND THUNDERSTORMS...MOST NUMEROUS OVER THE
SOUTHWEST.  PARTIAL CLEARING WILL OCCUR OVER NIGHT...BUT WITH CLEARING
AREAS OF FOG WILL DEVELOP OVER WT GROUND FROM RECENT RAINS.  TONIGHT
WILL BE COOL...THEN WARMING WILL OCCUR ON TUESDAY. A PACIFIC LOW
PRESSURE TROUGH MOVING ACROSS THE PACIFIC NORTHWEST WILL APPROACH
THE WEST INCREASING INSTABILITY BY AFTERNOON WITH REDEVELOPMENT OF
SHOWERS AND THUNDERSTORMS. FLOODING CONCERNS CONTINUE ON THE
POWDER RIVER FROM KAYCEE TO THE SHERIDAN COUNTY BORDER. VARIOUS
FLOOD WARNINGS CONTINUE THERE. WHILE CLEARING OF DEBRIS FLOWS IN THE
WIND RIVER CANYON KEEP THE ROAD CLOSED...ADDITIONAL FLOODING IS
NO LONGER A THREAT...THIS IS ALSO TRUE OF THE FLOODING ON THE LITTLE
POPO AGIE RIVER IN LYONS VALLEY. THESE FLOOD PRODUCTS EXTEND
THROUGH 630 PM MDT THIS EVENING AND THEY SHOULD BE ALLOWED TO
EXPIRE AT THAT TIME...LEAVING THE FLOOD WARMING ON THE POWDER RIVER
AS THE MAIN HYDROLOGICAL PROBLEM AREA.

.LONG TERM...TUESDAY NIGHT THROUGH MONDAY

AN APPROACHING AREA OF LOW PRESSURE FROM THE PACIFIC NORTHWEST WILL
CONTINUE TO RESULT IN SHOWERS AND THUNDERSTORMS OVER MUCH OF THE
AREA TUESDAY EVENING. THE AREA OF LOW PRESSURE WILL CONTINUE TO DIG
SOUTH THROUGH THE OVERNIGHT HOURS. SHOWER COVERAGE WILL WEAKEN WITH
LOSS OF DAYLIGHT TUESDAY NIGHT...BUT SHOWER COVERAGE WILL AGAIN IN
CREASE WEDNESDAY AFTERNOON AND EVENING. THIS DIURNAL TREND WILL
CONTINUE...ESPECIALLY AS THIS SYSTEM CONTINUES TO DIG SOUTH THROUGH
FRIDAY.

AS THE AREA OF LOW PRESSURE MOVES INTO THE CENTRAL PLAINS... A
TRANSITORY RIDGE WILL WORK TO BUILD OVER THE AREA. THIS WILL BRING A
REPRIEVE FROM THE AFTERNOON SHOWERS AND THUNDERSTORMS ON SATURDAY
ACROSS MOST OF THE AREA EAST OF THE CONTINENTAL DIVIDE. THE RIDGE
AXIS WILL MOVE QUICKLY THROUGH WESTERN WYOMING...WITH AN APPROACHING
WAVE OF LOW PRESSURE HELPING TO FLATTEN THE FLOW. THIS WILL PERMIT
SOME ADDED MOISTURE TO MOVE OVER WESTERN WYOMING BY SATURDAY
AFTERNOON AND EVENING...RESULTING IN SOME SHOWER COVERAGE OVER
MAINLY NORTHWESTERN WYOMING.

MODELS THEN BOTH AGREE ON THE PRESENCE OF ANOTHER RIDGE LATE SUNDAY
AND INTO MONDAY...BUT THE GFS MAKES THE RIDGE DIRTY WITH BOUTS OF
MOISTURE MOVING ALONG IT. THE ECMWF DELAYS THE ONSET OF SHOWER
ACTIVITY UNTIL MONDAY EVENING...BUT THERE IS STILL SOME MODEST
AGREEMENT WITH THE PROGRESSION OF THE SYSTEMS EXPECTED THROUGH THE
FORECAST PERIOD.

&&

.AVIATION.../00Z ISSUANCE/

AREAS OF MVFR CEILINGS AND VISIBILITY REDUCED IN RAIN SHOWERS AND
PATCHY FOG WILL LINGER THROUGH THE NIGHT WITH SOME EVENING
THUNDERSTORMS. OVER THE MOUNTAINS...AREAS OF IFR CEILINGS WILL
OBSCURE THE TERRAIN. WITH PARTIAL CLEARING TONIGHT...AREAS OF FOG
WILL DEVELOP OVER WET GROUND WITH A THREAT OF LOCAL IFR CONDITIONS
AT THE AIRPORTS. AT THIS TIME THE LIKELIHOOD OF THIS IS TOO LOW TO
IMPACT THE TAFS AT THIS TIME. MORE SHOWERS AND THUNDERSTORMS WILL
DEVELOP TUESDAY FROM 18Z THROUGH 00Z TUESDAY.

&&

.FIRE WEATHER...

SCATTERED SHOWERS AND THUNDERSTORMS WILL BE OVER THE AREA THIS
EVENING WITH PARTIAL CLEARING LEADING TO AREAS OF FOG DEVELOPING
OVER WET GROUND FROM RECENT RAINFALL.  TEMPERATURES WILL BE COOL
WITH VERY HIGH RELATIVE HUMIDITY AND LOW MIXING HEIGHTS TONIGHT.
WARMING WILL OCCUR ON TUESDAY WITH MORE SHOWERS AND THUNDERSTORMS
DEVELOPING ESPECAILLY OVER AND NEAR THE MOUNTAINS. RELATIVE
HUMIDITY WILL REMAIN FAVORABLE. WITH DEVELOPING CONVECTION...
MIXING HEIGHTS WILL INCREASE WITH GENERALLY GOOD SMOKE DISPERSAL
EXPECTED. AREAS OF SHOWERS OVER THE MOUNTAINS WILL CAUSE POOR
SMOKE DISPERSAL WITH SIGNIFICANT COOLING IN THE VICINITY OF
PRECIPITATION.

&&

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

$$

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






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