Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 231712
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
1112 AM MDT THU APR 23 2015

.SHORT TERM...THURSDAY THROUGH SATURDAY NIGHT

DEFINITELY A SPRING LIKE FORECAST ACROSS THE WESTERN AND CENTRAL
SECTIONS OF THE COWBOY STATE TODAY AS IT LOOKS FAIRLY ACTIVE THROUGH
THE SHORT TERM. ON THE RADAR THIS MORNING THERE ARE SOME SHOWERS
MOVING INTO THE WEST FROM IDAHO WITH AREAS EAST OF THE DIVIDE
LARGELY DRY WITH JUST SOME SPRINKLES. THINGS WILL GET MORE ACTIVE
THIS AFTERNOON AS A SHORTWAVE MOVES INTO THE WEST TO PROVIDE SOME
EXTRA LIFT ALONG WITH A WEAK FRONT THAT MAY DEVELOP. THE BIG X
FACTOR TODAY...HOWEVER...IS CLOUD COVER. IF THERE CAN BE SOME DECENT
DAYTIME HEATING TODAY MORE WIDESPREAD SHOWERS AND THUNDERSTORMS
COULD DEVELOP. THE NAM IS MORE BULLISH WITH UP TO 600 J/KG OF CAPE
OVER EASTERN PORTIONS OF THE COUNTY WARNING AREA. MEANWHILE THE GFS
IS NOT AS IMPRESSED. AS FOR THE CHANCE OF STRONGER STORMS...THERE IS
NOT A TREMENDOUS AMOUNT OF SHEAR AND WINDS THROUGH THE ENTIRE
ATMOSPHERIC COLUMN ARE NOT VERY IMPRESSIVE. HOWEVER...WITH THE
FAIRLY LOW WET BULB ZERO LEVELS OF APRIL THERE COULD BE SOME HAIL
WITH THESE. THERE COULD ALSO BE GUSTY WINDS BUT THIS DESCRIBES EVERY
THUNDERSTORM IN WYOMING. SO AS OF NOW...WE EXPECT MAYBE AN ISOLATED
STRONGER STORM BUT NOTHING WIDESPREAD. BUT AGAIN...A LOT DEPENDS ON
HOW MUCH SUNSHINE THERE IS. MOST SHOWERS SHOULD END LATER TONIGHT
WITH THE LOSS OF DAYTIME HEATING THOUGH A FEW COULD
LINGER...ESPECIALLY IN THE WEST.

AS FOR FRIDAY...IT LOOKS LIKE ANOTHER DAY WITH A SCATTERING OF
SHOWERS AND THUNDERSTORMS. THE MODELS ARE A BIT MORE STABLE WITH
LESS FORCING SO THERE COULD BE SOMEWHAT LESS COVERAGE OF
SHOWERS...ESPECIALLY EAST OF THE DIVIDE.

A DEEPER LONGWAVE TROUGH WILL BEGIN TO PUSH TOWARD THE AREA FOR
SATURDAY. WE DID MAKE A FEW TWEAKS THE FORECAST THOUGH. BOTH THE NAM
AND THE GFS HAVE SLOWED DOWN THE EASTWARD PROGRESSION OF THE SYSTEM.
AS A RESULT...WE LOWERED POPS EAST OF THE DIVIDE FOR THE DAYTIME
HOURS ON SATURDAY. BOTH MODELS ARE ACTUALLY SHOWING A DRY DAY FOR
MOST AREAS EAST OF THE DIVIDE SO MAYBE AT LEAST ONE DAY OF THE
WEEKEND COULD BE DECENT FOR THOSE AREAS. SHOWERS WILL SPREAD INTO
THE WEST HOWEVER ALTHOUGH EVEN THERE A DRY MORNING COULD BE A
DISTINCT POSSIBILITY. THE MODELS STILL SHOW A GOOD SWATH OF MOISTURE
SPREADING ACROSS THE AREA FOR SATURDAY NIGHT. AS EXPECTED THIS FAR
OUT PINPOINTING THE HEAVIEST PRECIPITATION IS LIKE TRYING TO HERD
CATS WITH THE MODELS ALL OVER THE PLACE. FOR NOW...CONTINUITY LOOKED
PRETTY GOOD SO WE MADE NO CHANGES.

.LONG TERM...SUNDAY THROUGH THURSDAY

SYNOPSIS...A PACIFIC STORM SYSTEM WILL BRING MUCH COOLER
TEMPERATURES AND WIDESPREAD RAIN AND MOUNTAIN SNOW ACROSS SOUTHERN
AND CENTRAL WYOMING ON SUNDAY.  PRECIPITATION WILL DECREASE
NORTHWEST TO SOUTHEAST MONDAY MORNING.  MAINLY DRY WEATHER WITH A
WARMING TREND WILL FOLLOW TUESDAY THROUGH THURSDAY UNDER A RIDGE OF
HIGH PRESSURE.

DISCUSSION...THE MAIN WEATHER MAKER DURING THIS PERIOD WILL BE AN
UPPER LEVEL LOW PRESSURE CENTER DIGGING SSE OFF THE COAST OF BRITISH
COLUMBIA EARLY THIS MORNING.  MEDIUM-RANGE MODELS HAVE ALL TRENDED
FURTHER SOUTH WITH THIS SYSTEM AS IT DIGS ACROSS THE WESTERN GREAT
BASIN AND TURNS EAST ACROSS THE SOUTHERN ROCKIES ON SUNDAY AND
SUNDAY NIGHT.  THE MODELS HAVE THE UPPER LOW CLUSTERED OVER SE NEW
MEXICO BY 12Z MONDAY.  BOTH THE GFS AND ECMWF EJECT SECONDARY UPPER
CIRCULATION(S) NORTH OUT AHEAD OF THE MAIN UPPER LOW ACROSS CENTRAL
AND EASTERN WYOMING THAT LINGER THROUGH THE DAY SUNDAY BEFORE
GETTING EITHER CAUGHT UP IN TRAILING NORTHERN STREAM SHORTWAVE
MOVING INTO THE NORTHERN HIGH PLAINS SUNDAY NIGHT...OR GETTING
DRAGGED SOUTH ON BACKSIDE OF UPPER LOW.  THE MAIN CONSEQUENCE OF THE
OVERALL FURTHER SOUTH TRACK OF THE MAIN UPPER LOW WILL BE A WARMER
VERTICAL PROFILE OVER WYOMING...SO SNOW LEVELS ARE EXPECTED TO BE
BETWEEN 6.5KFT AND 7.5KFT ACROSS MOST OF THE AREA EXCEPT POSSIBLY
ALONG THE CODY FOOTHILLS AND WEST OF BUFFALO/I-90 WHERE SOME COLDER
AIR MAY FILTER SOUTH AND DROP SNOW LEVELS CLOSER TO 6KFT.  AT THIS
TIME...HEAVIEST PRECIPITATION AMOUNTS ARE EXPECTED IN THE N-NE
UPSLOPE FAVORED AREAS FROM CODY/MEETEETSE TO THE LANDER FOOTHILLS
AND CASPER/CASPER MTN.

NORTHERN STREAM SHORTWAVES PASSAGE MONDAY MORNING WILL CLEAR OUT
CLOUDS AND PRECIPITATION FROM NW-SE WITH UPPER RIDGE BUILDING INTO
THE AREA.  GFS SHOWS HIGH AMPLITUDE RIDGE TRANSLATING THE DIVIDE ON
WEDNESDAY WHILE THE ECMWF FLATTENS THIS RIDGE...DEVELOPING FAST
ZONAL FLOW ACROSS THE NW U.S./NORTHERN PLAINS.  GFS SHOWING MORE
CONSISTENCY AND IS PREFERRED.  AFTER MONDAY MORNING...PRECIPITATION
DOES NOT ENTER THE FORECAST UNTIL THURSDAY WHEN WEAK PACIFIC
SHORTWAVES IN SW FLOW WILL BRING CHANCES OF SHOWERS AND
THUNDERSTORMS TO THE NW MTNS.

&&

.AVIATION.../18Z ISSUANCE/

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

SCATTERED SHOWERS AND ISOLATED THUNDERSTORMS OVER AND NEAR THE
MOUNTAINS WILL GRADUALLY SPREAD OVER THE LOWER ELEVATIONS AND
CONTINUE THROUGH 06Z. TERMINAL SITES MAY BE IMPACTED BY SHOWERS AND
OUTFLOW WIND GUSTS AROUND 30 KNOTS. ISOLATED SHOWERS WILL OCCUR
THROUGH 10Z FRIDAY. VFR CONDITIONS WILL PREVAIL TODAY AND TONIGHT
WITH LOCAL MVFR CONDITIONS ASSOCIATED WITH HEAVIER SHOWERS OR
STORMS. EXPECT OCCASIONAL MOUNTAIN OBSCURATION DURING THE
AFTERNOON AND EVENING HOURS. SHOWERS WILL DEVELOP AGAIN OVER THE
MOUNTAINS FRIDAY WITH MORE ISOLATED COVERAGE ACROSS THE LOWER
ELEVATIONS LATE FRIDAY.

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

SCATTERED SHOWERS AND THUNDERSTORMS WILL CONTINUE OVER AND NEAR THE
MOUNTAINS AND GRADUALLY SPREAD THROUGHOUT THE LOWER ELEVATIONS THIS
AFTERNOON AND CONTINUE THROUGH 03Z. TERMINAL SITES MAY BE IMPACTED
BY SHOWERS OR STORMS ALONG WITH OUTFLOW WIND GUSTS AROUND 30
KNOTS. ISOLATED SHOWERS WILL OCCUR THROUGH 10Z FRIDAY. EXPECT
OCCASIONAL MOUNTAIN OBSCURATION THIS AFTERNOON AND EVENING. VFR
CONDITIONS WILL PREVAIL AT THE TERMINAL SITES THROUGH THE PERIOD.
THERE MAY BE LOCAL MVFR CONDITIONS OVER AND NEAR THE MOUNTAINS
DURING THE AFTERNOON AND EVENING HOURS. SCATTERED SHOWERS AND
ISOLATED THUNDERSTORMS WILL MOVE BACK INTO THE WEST FRIDAY
AFTERNOON WITH ISOLATED SHOWERS POSSIBLE IN THE SOUTHWEST FRIDAY MORNING.

&&

.FIRE WEATHER...

AN UPPER LEVEL DISTURBANCE WILL MOVE THROUGH THE AREA TODAY WITH A
SCATTERING OF MAINLY AFTERNOON AND EVENING THUNDERSTORMS. ALTHOUGH
MOST WILL BE IN THE MOUNTAINS...ALL AREAS WILL HAVE A CHANCE OF ONE
THIS AFTERNOON AND EVENING. EXPECT ANOTHER CHANCE FOR FRIDAY BUT
WITH A BIT LESS COVERAGE. A SLOW MOVING LOW PRESSURE AREA COULD
BRING A WETTING RAIN FOR SOME AREAS FOR SUNDAY BUT PINPOINTING THE
AREA IS DIFFICULT AT THIS POINT.
&&

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

&&

$$

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








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