Area Forecast Discussion
Issued by NWS Riverton, WY

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298
FXUS65 KRIW 151733
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
1133 AM MDT FRI MAY 15 2015

.SHORT TERM...FRIDAY THROUGH SUNDAY NIGHT

QUITE AN ACTIVE DAY YESTERDAY AS THUNDERSTORMS DEVELOPED NEAR THE
WIND RIVER RANGE AND TRACKED NORTHEASTWARD AND BLOSSOMED INTO A
COMPLEX OF THUNDERSTORMS THAT PRODUCED HAIL AS WELL AS SOME HEAVY
RAIN IN SPOTS. A FEW SPOTS IN THE BIG HORN RANGE PICKED UP AROUND 2
INCHES OF RAIN LAST EVENING. EVEN LATE AT NIGHT...SHOWERS ARE
ONGOING WITH SOME THUNDER IN THE EAST AS A LOW LEVEL JET MOVES UP IN
EASTERN WYOMING. ANOTHER WAVE OF MOISTURE IS CROSSING THE AREA AS I
WRITE THIS BUT SHOULD BE ENDING BY MORNING.

EXPECT ANOTHER WET DAY TODAY AS THE SLOW MOVING UPPER LEVEL LOW
CROSSES THE GREAT BASIN. SOME LEE CYCLOGENESIS WILL DEVELOP ACROSS
COLORADO TODAY AND THEN MOVE NORTH-NORTHEASTWARD INTO THE WESTERN
HIGH PLAINS. YET ANOTHER SHORTWAVE WILL LIFT ACROSS THE AREA AND
BRING ANOTHER ROUND OF SHOWERS AND THUNDERSTORMS. THE DIFFERENCE
THIS TIME IS IT MAY BE AN EARLIER SHOW AS BOTH THE NAM AND GFS SHOW
A DRY SLOT WORKING ACROSS MUCH OF THE AREA THIS EVENING AND ACROSS
MOST OF THE AREA BY LATE TONIGHT. THE EXCEPTION WILL BE NORTHWESTERN
WYOMING WHERE SOME MOISTURE WILL WRAP AROUND INTO THAT AREA. A
LITTLE TRICKY TODAY WITH SOME THINGS. ONE IS INSTABILITY AS A LOT OF
THE AREA MAY START CLOUDY. MODELS DO SHOW DECENT CAPE VALUES
HOWEVER...ESPECIALLY ACROSS NORTHERN WYOMING. ANOTHER IS FOR MAYBE
LOCALIZED FLOODING. AREAS NEAR THE BIG HORN RANGE AS WELL AS
JOHNSON COUNTY SAW A LOT OF RAIN YESTERDAY. PRECIPITABLE WATER
VALUES ARE DECENT..APPROACHING AN INCH...SO HEAVY RAIN IS A
POSSIBILITY. HOWEVER...THE MODELS HAVE NOT BEEN DOING A GOOD JOB
PICKING OUT THE AREAS OF THE BEST THUNDERSTORMS. SO WE KEPT THINGS
GENERIC ONCE AGAIN WITH THE HIGHEST POPS IN THE NORTH AND EAST.

AS FOR SATURDAY...MUCH OF THE AREA WILL START IN THE DRY SLOT SO IT
LOOKS LIKE A DRY START TO THE DAY. AGAIN...THE EXCEPTION WILL BE IN
THE NORTHWEST. THE NAM EVEN HINTS THAT THE SOUTHEASTERN HALF OF THE
AREA WILL BE LARGELY DRY. AS A RESULT...WE RAISED TEMPERATURES EAST
OF THE DIVIDE A FEW MORE DEGREES. HOWEVER...COLDER AIR WILL BEGIN TO
WRAP INTO NORTHWESTERN WYOMING AS A COLD FRONT MOVES IN LATER IN THE
DAY. THE MODELS SHOW 700 MILLIBAR TEMPERATURES FALLING TO MINUS 2 TO
MINUS 3 WHICH COULD LOWER SNOW LEVELS TO AROUND 7500 FEET BY THE END
OF THE DAY. THE COLD FRONT THEN SWEEPS ACROSS THE AREA SATURDAY
NIGHT AND USHERS IN COLDER TEMPERATURES FOR MANY AREAS. AS FOR THE
DREADED S WORD...SNOW...THE NAM IS COOLER WITH 700 MILLIBAR
TEMPERATURES FALLING TO MINUS 5 IN THE NORTHWEST AND AROUND MINUS 3
IN EAST OF THE DIVIDE. THIS WOULD PUT THE LEVELS AROUND 7000 FEET
EAST OF THE DIVIDE SO LOOK FOR MAINLY RAIN FOR THE LOWER ELEVATIONS
ALTHOUGH SEVERAL INCHES OF SNOW ARE POSSIBLE FOR THE HIGHER
ELEVATIONS. THERE IS STILL SOME DISAGREEMENT ON THE PLACEMENT OF THE
BEST QPF SO WE DID NOT MAKE A LOT OF CHANGES. EXPECT A GUSTY NORTH
TO NORTHWEST WIND TO DEVELOP FOLLOWING THE FRONTAL PASSAGE WITH
STRONG WINDS POSSIBLE IN AREAS FAVORED WITH STRONG NORTHWEST COLD
ADVECTION...LIKE THE BUFFALO AREA.

BY SUNDAY...IT LOOKS COOL AND UNSETTLED. AGAIN...SOME DIFFERENCES IN
THE MODELS WITH THE GFS HANGING MORE MOISTURE BACK WHILE THE NAM
DRIES THINGS OUT. AGAIN...WITH THE DETAILS STILL MUDDY...WE MADE FEW
CHANGES. BUT IT DOES LOOK LIKE A 3C`S KIND OF DAY...CLOUDY...COOL
AND CRUDDY. MOISTURE SHOULD SHIFT OUT OF THE AREA SUNDAY NIGHT AS
SOME SHORT WAVE RIDGING BUILDS ACROSS THE STATE.

.LONG TERM...MONDAY THROUGH FRIDAY

AN UPPER LOW/TROUGH WILL PUSH INTO CALIFORNIA MONDAY. SOME
DYNAMICS COULD IMPACT THE WEST/SOUTHWEST...BUT THE EAST SHOULD
REMAIN DRY UNDER UPPER LEVEL RIDGING AND RELATIVELY STABLE
CONDITIONS. THIS WILL BEGIN TO CHANGE MONDAY NIGHT...AS THE UPPER
LEVEL SYSTEM PULLS INTO THE GREAT BASIN WITH CHANCES OF PRECIP
GRADUALLY INCREASING ACROSS THE ENTIRE AREA AS THE DIFFLUENT FLOW
ALOFT OVERSPREADS THE REGION. THIS UPPER LOW IS EXPECTED TO
GRADUALLY PUSH ACROSS THE NORTHERN/CENTRAL ROCKIES TUESDAY AND
WEDNESDAY. BY WEDNESDAY...MODELS ARE SHOWING THIS UPPER
LOW/TROUGH FILLING IN...AND GETTING SHEARED OUT AS IT PUSHES INTO
THE PLAINS. TUESDAY AND TUESDAY NIGHT...LOOK TO BE THE BEST TIME
FOR WIDESPREAD PRECIPITATION...AS MODELS ARE SHOWING ABOVE AVERAGE
PRECIPITABLE WATER VALUES IN A MOIST EASTERLY SURFACE FLOW OVER
MUCH OF THE AREA. BOTH THE ECMWF AND GFS KEEP A RELATIVELY STABLE
ATMOSPHERE OVER THE NORTH AND EAST...WHILE THE WEST AND SOUTH
STILL SHOW SOME INSTABILITY. WILL KEEP MENTION OF THUNDER CONFINED
MAINLY TO THE WEST AND/OR SOUTH BOTH DAYS...AS THE GFS AND ECMWF
HAVE HINTED AT A STABLE...BUT RAINY ATMOSPHERE NORTH AND EAST.

ON THURSDAY...DESPITE BEING IN BETWEEN SYSTEMS...WITH WEAK RIDGING
OVER THE AREA...EXPECT ISOLATED TO LOCALLY SCATTERED CONVECTION
MAINLY OVER THE WEST. DRIER CONDITIONS ARE EXPECTED OVER THE
EAST...ESPECIALLY IN BASINS AWAY FROM THE MOUNTAINS.

MEDIUM RANGE MODELS THEN SHOW ANOTHER UPPER LOW OVER THE DESERT
SOUTHWEST/SOUTHERN ROCKIES BY FRIDAY. ALTHOUGH THE TRACK IS A BIT
FAR SOUTH...A RELATIVELY MOIST AND UNSTABLE ATMOSPHERE AND THE
FACT THE TRACK COULD CHANGE IS ENOUGH TO HAVE SLIGHT VALLEY/CHANCE
MOUNTAIN POPS FOR NOW.

TEMPERATURES WILL BE BELOW TO MUCH BELOW AVERAGE...ESPECIALLY FOR
HIGHS THROUGH AT LEAST WEDNESDAY...WITH TEMPS BECOMING MORE SEASONAL
THURSDAY AND FRIDAY.

&&

.AVIATION.../18Z ISSUANCE/

ISOLATED SHOWERS OVER THE REGION WILL SEE A GENERAL INCREASE IN
COVERAGE WITH A TRANSITION TO ISOLATED EMBEDDED THUNDERSTORMS THIS
AFTERNOON. THIS CONDITION WILL CONTINUE UNTIL 00Z-03Z...WHEN SHOWERS
AND ISOLATED THUNDER WILL DECREASE ACROSS MOST OF THE FORECAST AREA
EXCEPT FOR NORTHERN TERMINALS SUCH AS KCOD AND KWRL WHERE
-SHRA/VCSH/VCFG WILL PERSIST THROUGH THE FORECAST PERIOD. EXPECT
AREAS TEETERING BETWEEN VFR/MVFR THROUGH EARLY EVENING...WITH BRIEF
IFR CONDITIONS IN LOW CIGS DUE TO LOCALLY HEAVY RAIN. ALSO...EXPECT
GUSTY WIND AT TIMES AT MOST TERMINAL SITES TODAY IN HEAVIER SHOWERS.
THEN MOST TERMINAL SITES EAST OF THE DIVIDE SHOULD SEE IMPROVEMENT
TO VFR CONDITIONS...EXCEPT AGAIN FOR KCOD AND KWRL WHICH WILL
CONTINUE TO FLIRT WITH MVFR. ADDITIONALLY...MOUNTAINS WILL BE
OCCASIONALLY OBSCURED THROUGH THE FORECAST PERIOD.

&&

.FIRE WEATHER...

FIRE DANGER RELATIVELY LOW THROUGH THE WEEKEND AS SEASONAL TO COOL
AND RELATIVELY MOIST CONDITIONS PERSIST. HOWEVER...IT WILL BE
UNSTABLE AGAIN FRIDAY AND SATURDAY...OFFERING UP CHANCES FOR MORE
THUNDERSTORMS AND ASSOCIATED LIGHTNING. LATE SPRING SNOWS LIKELY TO
RETURN TO THE HIGHER ELEVATIONS AGAIN THIS WEEKEND...WHILE SNOW
LEVELS MAY EVEN FALL TO THE VALLEY AND BASIN FLOORS OF THE
AREA...MIXING WITH RAINFALL AT TIMES SATURDAY NIGHT AND EARLY SUNDAY.


&&

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

$$

SHORT TERM...HATTINGS
LONG TERM...WM
AVIATION...BRAUN
FIRE WEATHER...HATTINGS





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