Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 312025
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
225 PM MDT SUN MAY 31 2015

.SHORT TERM...TONIGHT THROUGH MONDAY

HIGH PRESSURE RIDGE AXIS ALOFT REMAINS WEST OF WYOMING WITH
WIDESPREAD SUBSIDENCE FIGHTING AFTERNOON HEATING AND OROGRAPHIC
LIFT. SO FAR CONVECTION LIMITED TO MOUNTAIN AREAS AND GENERALLY
WEAK. THUNDERSTORMS OVER THE UINTA MOUNTAINS DRIFTING EAST INTO
NORTHWEST COLORADO MAY CLIP EXTREME SOUTHERN SWEETWATER COUNTY.
MAIN FOCUS WILL BE LIMITED SHOWERS AND ISOLATED THUNDERSTORMS
OVER AND NEAR THE WESTERN AND NORTHERN MOUNTAINS EARLY THIS
EVENING...WITH A FEW LINGERING SHOWERS IN THE NORTHWEST OVER NIGHT.
TEMPERATURES WILL CONTINUE TO WARM WITH RIDGE TO THE WEST BUILDING
AS PACIFIC LOW PRESSURE TROUGH DEEPENS OFF THE WEST COAST.
RIDGE AXIS SHIFTS WEST INTO EAST CENTRAL WYOMING BY MONDAY AFTERNOON
WITH SOUTHWEST FLOW ALOFT.  MOISTURE WILL BE LIMITED AT FIRST
AHEAD OF THE PACIFIC TROUGH THAT BEGINS TO MOVE INLAND.  SUBSIDENCE
WILL MOVE TO THE EAST ALLOWING CONVECTION DUE TO HEATING AND OROGRAPHIC
LIFT SUSTAIN BETTER CONVECTIVE COLUMNS.  OVERALL INCREASE IN SHOWERS
AND ISOLATED THUNDERSTORMS WITH A BETTER SHEAR ENVIRONMENT.  LACK
OF MOISTURE WILL LIMIT THE COVERAGE. THE SOUTHWEST SHOULD BE DRY
SLOTTED WITH DEW POINTS DECREASING AS TEMPERATURES CONTINUE TO WARM.
MONDAY SHOULD BE THE WARMEST OF THE YEAR WITH BIGHORN BASIN
WARMING INTO THE UPPER 80S AND MOST LOWER ELEVATIONS REACHING LOWER
TO MID 80S MID 60S TO 70S IN THE MOUNTAINS WITH ANOTHER NIGHT OF
ABOVE FREEZING MOUNTAIN TEMPERATURES SHOULD ACCELERATE SNOW MELT
WITH RIVERS ON THE RISE. UPPER WIND NEAR DUBOIS AND SOUTH FORK OF
THE SHOSHONE RIVER AT VALLEY IN ACTION STAGE AREA AND COULD CLIMB
CLOSER TO FLOOD STAGE. LOCALIZED HEAVY RAINFALL MAY PUSH THEM OVER
INTO MINOR FLOOD STAGE. WILL NEED TO BE MONITORED MONDAY AFTERNOON
AND EVENING. DO NOT SEE WIDESPREAD SIGNIFICANT RAINFALL AND STORMS
SHOULD MOVE AT AROUND 25 MPH.  TRAINING CELLS WOULD BE THE BIGGEST
THREAT TO CAUSE FLOODING.

.LONG TERM...MONDAY NIGHT THROUGH SUNDAY

THE THUNDERSTORMS AND CONVECTION OF MONDAY AFTERNOON SHOULD WANE
LATER INTO THE NIGHT WITH THE LOSS OF DAYTIME HEATING LATER AT
NIGHT. MOST AREAS SHOULD BE MAINLY DRY BY TUESDAY NIGHT.

THE REST OF THE PERIOD SUMS UP TO WHAT WE KNOW AND WHAT WE DO NOT
KNOW. BOTH THE GFS AND EUROPEAN HAVE A FAIRLY STATIONARY FRONTAL
BOUNDARY IN THE AREA WITH A TROUGH MOVING INTO THE WESTERN UNITED
STATES AND EVENTUALLY CLOSE OFF UNTIL INTO AN UPPER LEVEL LOW...VERY
CONSISTENT WITH THE PATTERN OVER THE PAST FEW WEEKS. THIS WILL
RESULT IN A RATHER UNSETTLED PATTERN WITH JUST ABOUT DAILY CHANCES
OF CONVECTION ACROSS THE AREA...MAINLY DIURNAL IN NATURE.
ALSO...TEMPERATURES WILL COOL SOMEWHAT DURING THE PERIOD WITH THE
SHOWERS AND FLOW TURNING MORE EASTERLY.

HOWEVER...WHAT WE DO NOT KNOW AT THIS TIME ARE THE NUANCES AND
DETAILS OF THE SYSTEMS AS THE MODELS STILL HAVE SOME DIFFERENCES IN
THE SOLUTION...MAINLY PAST THURSDAY. ALSO...TOWARD AND END OF THE
PERIOD WITH THE UPPER LEVEL LOW AND THE SHORTWAVES PINWHEELING
AROUND IT. THESE FEATURES ARE VERY FICKLE IN THEIR MOVEMENT AND THE
MODELS USUALLY DO A TERRIBLE JOB IN THEIR TIMING AND
MOVEMENT...ESPECIALLY SEVERAL DAYS OUT. SO FURTHER OUT...CONFIDENCE
IN THE DETAILS DROPS SUBSTANTIALLY.

FOR TUESDAY...A FRONT WILL STILL BE IN THE AREA...BUT EVEN HERE
PLACEMENT IS DIFFERENT ON THE MODELS. THE NAM IS FURTHER NORTH AND
KEEPS MUCH OF THE AREA DRY WHILE THE EUROPEAN HAS CHANCES ACROSS
MOST OF THE AREA WITH THE GFS SOMEWHERE IN THE MIDDLE. FOR NOW...WE
PLAYED THE MIDDLE GROUND WITH SLIGHT CHANCE TO CHANCE POPS ACROSS
THE NORTH AND DRY CONDITIONS IN THE SOUTH. WE ALSO RAISED
TEMPERATURES A FEW DEGREES...ESPECIALLY IN AREAS SOUTH OF THE FRONT.
THE STORM PREDICTION CENTER STILL HAS A MARGINAL RISK AREA IN
PORTIONS OF THE AREAS SO A FEW STORMS COULD TURN FEISTY. LIKE OTHER
DAYS...CONVECTION SHOULD DIE OFF WITH THE LOSS OF DAYTIME HEATING.
IT IS MUCH THE SAME STORY FOR WEDNESDAY...THE NAM IS VERY DRY WITH
ONLY ISOLATED STORMS AND THE EUROPEAN THE WETTEST AGAIN. AGAIN...WE
COMPROMISED WITH MAINLY SLIGHT POPS IN MOST AREAS. WE ALSO RAISED
HIGH TEMPERATURES A FEW DEGREES ON THIS DAY AS WELL.

THERE IS FAIRLY GOOD AGREEMENT FOR THURSDAY WITH SOME DRYING MOVING
INTO SOUTHERN WYOMING SO WE DRIED THINGS OUT IN THAT VICINITY WITH
SLIGHT CHANCE POPS IN OTHER AREAS AND THE BEST CHANCE OF CONVECTION
ACROSS THE NORTH. SOMEWHAT DEEPER MOISTURE MAY MOVE IN FOR FRIDAY SO
ME MADE POPS ABOUT 10 PERCENT HIGHER THAN FRIDAY. ALSO...WITH
INCREASED CLOUD COVER THIS SHOULD BE A COOLER DAY.

TOWARD NEXT WEEKEND THE MODELS DIVERGE A BIT ON PLACEMENT OF THE
CLOSED UPPER LEVEL LOW. THE GFS HAS THE CENTER ACROSS NORTHERN
NEVADA WITH THE EUROPEAN A BIT FURTHER SOUTH CLOSER TO LAS VEGAS. WE
KNOW THERE WILL BE A CONTINUATION OF SHOWERS...THE QUESTION IS WHERE
AND WHEN. AS OF NOW...VERY DIFFICULT TO DETERMINE. SO AS A
RESULT...FEW CHANGES TO CONTINUITY.

&&

.AVIATION.../00Z ISSUANCE/

VFR CONDITIONS ARE EXPECTED AT ALL AIRPORTS THROUGH 00Z TUESDAY.
SHOWERS AND THUNDERSTORMS OVER THE HIGHER MOUNTAINS OF THE WEST
AND NORTH WILL LIKELY PRODUCE LOCAL TO ISOLATED MVFR TO IFR
CEILINGS AND VISIBILITY REDUCED IN RAIN THIS EVENING AND AGAIN
MONDAY AFTERNOON.

&&

.FIRE WEATHER...

WARMER AND DRIER WEATHER WITH INCREASED SOUTHWEST WINDS EXPECTED
ON MONDAY WITH MAINLY MOUNTAIN SHOWERS AND ISOLATED THUNDERSTORMS.
THE NORTH HAS THE BEST CHANCE FOR SHOWERS AND THUNDERSTORMS IN THE
LOWER ELEVATIONS.  MAIN IMPACT OF WARMING WILL BE HIGHER RIVER
LEVELS AS SNOW MELT INCREASES SIGNIFICANTLY AFTER A RATHER WARM
NIGHT.  LOCAL FLOODING MORE OF A THREAT THAN FIRE AT THIS TIME...
ESPECIALLY IN THE WESTERN AND NORTHERN MOUNTAINS.

&&

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

$$

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






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