Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 062043
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
243 PM MDT WED MAY 6 2015

.SHORT TERM...TONIGHT THROUGH THURSDAY

WV IMAGERY SHOWING LARGE DIGGING TROF ACROSS THE WRN CONUS...AND
SMALLER UPR SW TROF CURRENTLY MOVING TO THE NE ACROSS THE FA. AT THE
SFC...LEE CYCLONE ERN CO...TO SERN WY...COLD FRONT CUTTING WY IN
HALF FROM NE TO SW...AND HIGH P BEHIND AND OVR WRN MT. WIDESPREAD
SHOWERS ACROSS THE CNTRL MOUNTAINS AND BASINS.

MORE CONVECTIVE INSTABILITY ACROSS THE WRN AND SRN FA THIS
AFTERNOON...WITH A COUPLE OF EMBEDDED TSRA JUST NOW BEGINNING.
HEAVIEST SHOWERS HAVE BEEN STAYING OVR SERN FREMONT COUNTY THIS
MORNING...WITH A LOCAL MAX JUST WEST OF JEFFERY CITY. BEST CHANCE
FOR THE STRONGEST STORM OF THE DAY MOST LIKELY IN SWEETWATER COUNTY
FROM MID AFTERNOON THRU EARLY EVENING AS SOME SUNSHINE PUSHES CAPE
VALUES UP TO 200 TO 400 J/KG BY MID AFTERNOON WHILE 0 - 6KM SHEAR
INCREASES TO 20-25 KTS. HEAVY RAIN...GUSTS TO 30 MPH...WITH SMALL
HAIL POSSIBLE THRU 7PM...WITH HEAVY RAIN STILL POSSIBLE THRU ABOUT
10 PM. LOCALIZED FLOODING WILL BE POSSIBLE HERE.

OTHERWISE...NEXT UPR WAVE AND ADDITIONAL MOISTURE MOVES INTO THE SWRN
FA TONIGHT...SPREADING ACROSS THRU THE MORNING HRS. SNOW LEVELS
SHOULD DROP TO AROUND 9.5K FT BY AROUND MIDNIGHT...CONTINUING TO
FALL TO AROUND 7 TO 7.5K FT WITH RAIN SNOW MIXES POSSIBLE BETWEEN
6.5 AND 7K FT AROUND SUNRISE. LIGHT SNOW SHOULD CONTINUE THROUGH
THURSDAY ABOVE 7500 FEET. NEW MOUNTAIN SNOWFALL OF BETWEEN 1 TO 4
INCHES WILL BE POSSIBLE BY THURSDAY EVENING...WITH ISOLATED AREAS
ABOVE 9500 FEET PICKING UP TO 6 INCHES. LOWER ELEVATIONS NORTH OF
THE FRONT WHERE PRECIPITATION OCCUR WILL SEE THE POSSIBILITY OF A
RAIN SNOW MIX OCCURRING BY/NEAR SUNRISE...WITH LITTLE TO NO
ACCUMULATION EXPECTED...ESPECIALLY ON ROADWAYS. WINTER SPS OUT FOR
THE CENTRAL AND NORTHERN MOUNTAINS...AND ALSO THE REGION FROM THE
UPPER GREEN BASIN...EAST TO SOUTH PASS...GREEN...RATTLESNAKE
MOUNTAINS AND PORTION OF NATRONA COUNTY. OTHERWISE...LIGHT RAINFALL
FROM MOST OF THE CWA THURSDAY WITH SOME REPRIEVE PROVIDED BY A SW
RIDGE MID-DAY...AND SFC HIGH PRESSURE ENTERING THE NRN FA.

.LONG TERM...THURSDAY NIGHT THROUGH WEDNESDAY

AS FOR THE LONG TERM...WE KNOW ONE THING...THAT IS THAT ONE OF THOSE
PAIN IN THE BUTT UPPER LEVEL LOWS THAT SEEMS TO HAVE A MIND OF ITS
OWN WILL INFLUENCE THE WEATHER INTO THE WEEKEND. WE ALSO KNOW THE
WEATHER LOOKS UNSETTLED THROUGH THAT PERIOD. HOWEVER...AS FAR AS THE
DETAILS...THERE REMAINS A LOT OF UNCERTAINTY. SO...HERE IS WHAT WE
THINK WILL HAPPEN ALTHOUGH WITH THE DIFFERENCES IN OPINION IN THE
MODELS WE DID NOT MAKE SWEEPING CHANGES.

THE SHOWERS OF THURSDAY SHOULD BEGIN TO WANE ON THURSDAY NIGHT. 700
MILLIBAR TEMPERATURES FALL TO AROUND MINUS 3 SO SOME SNOW COULD DROP
TO AROUND 7000 FEET BUT HEAVY AMOUNTS ARE NOT EXPECTED. AS FOR
FRIDAY...WHAT HAPPENS DEPENDS ON WHAT MODEL YOU BELIEVE. THE NAM IS
THE DRIEST AND KEEPS MANY OF THE BASIN AREAS LARGELY DRY FOR THE DAY
WITH CONVECTION OVER THE MOUNTAINS. MEANWHILE...THE GFS AND NAM KEEP
THE BEST POPS OVER THE WEST AND SOUTHERN AREAS. CONTINUITY HANDLED
THIS FAIRLY WELL SO FEW CHANGES WERE MADE. AS FOR THE CHANCE OF THE
WHITE STUFF...NOT A GREAT CHANCE AS WITH SOUTHERLY FLOW ANY SNOW
WOULD BE CONFINED TO THE HIGHEST ELEVATIONS.

THINGS GET SOME INTERESTING FOR LATER FRIDAY NIGHT THROUGH SATURDAY
NIGHT AS LEE CYCLOGENESIS DEVELOPS AND FLOW TURNS MORE EASTERLY TO
NORTHEASTERLY. FOR FRIDAY NIGHT...THINGS SHOULD STAY WARM ENOUGH TO
KEEP MOST PRECIPITATION AS RAIN OR WET SNOW. COOL AIR THEN WRAPS IN
MORE IMPRESSIVELY FOR SATURDAY AFTERNOON AND ESPECIALLY SATURDAY
NIGHT. 700 MILLIBAR TEMPERATURES TO GET COLD ENOUGH TO SUPPORT SNOW
FOR EVEN SOME OF THE LOWER ELEVATIONS SATURDAY NIGHT. THE PREVIOUS
FORECAST HAD SNOW FOR THAT TIME FRAME AND WE KEPT THAT.
HOWEVER...THE MODELS HAVE SHIFTED THE HEAVIER QPF FURTHER EAST SO WE
DID TRIM AMOUNTS A BIT...ESPECIALLY WEST OF THE BIG HORN RANGE. AS
FOR DURING THE DAY SATURDAY...WE DO NOT THINK IMPACTS WILL BE THAT
GREAT. GIVEN THE RECENT WARM WEATHER AND WARM GROUND TEMPERATURES
AND STRONG MAY SUN ANGLE...NOW BACK TO EARLY AUGUST STRENGTH...ANY
SNOW THAT FALLS SATURDAY DURING THE DAYLIGHT WILL HAVE A HARD TIME
STICKING IN THE LOWER ELEVATIONS...ESPECIALLY ON PAVED SURFACES.
EVEN AT NIGHT...MOST OF THE ACCUMULATION MAY END UP BEING ON THE
GRASS. AS FOR WHO WILL GET THE HEAVIEST PRECIPITATION...A LOT MAY
HAVE TO DO WITH MESOSCALE BANDING THAT IS IMPOSSIBLE TO NAIL DOWN
THIS FAR OUT SO AGAIN...WE MADE FEW CHANGES.

UNSETTLED WEATHER WILL CONTINUE INTO SUNDAY AS THE LOW MOVES INTO
THE PLAINS STATES. THE HEAVIEST PRECIPITATION WILL LIKELY SHIFT EAST
OF THE AREA INTO EASTERN WYOMING...BUT SOME RAIN AND SNOW WILL
CONTINUE. AGAIN...HOW MUCH SNOW STICKS ON THE ROADS DURING THE DAY
IS QUESTIONABLE THOUGH. THE STEADY PRECIPITATION THEN SHIFTS OUT AT
NIGHT.

AS FOR EARLY NEXT WEEK...IT DOES LOOK MILDER. HOWEVER...THE MODELS
ARE ONLY AGREEING TO DISAGREE WITH COVERAGE AND TIMING OF
PRECIPITATION. WITH THE MORE IMPORTANT WEATHER OCCURRING EARLIER IN
THE LONG TERM PERIOD...WE AGREED ALONG WITH OTHER NEIGHBORING
OFFICES TO LEAVE CONTINUITY AS IS UNTIL WE GET CLOSER. THE MOST
ACTIVE AREA WOULD BE OUT WEST BUT AGAIN...DETAILS AND TIMING ARE
HIGHLY UNCERTAIN.

&&

.AVIATION.../00Z ISSUANCE/

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

AREAS OF -RA/-SHRA WILL CONTINUE EAST OF THE CONTINENTAL DIVIDE WITH
WIDESPREAD MVFR TO OCCASIONAL IFR CONDITIONS THROUGH 18Z. RAIN WILL
BE ON THE DECREASE AFTER 00Z BEFORE INCREASING ONCE AGAIN AFTER 09Z.
HOWEVER...MVFR.IFR IN LOW CIGS WILL CONTINUE AT MOST TERMINALS.
GENERAL IMPROVING CONDITIONS  EXPECTED AFTER 18Z.

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

WEST OF THE DIVIDE...VFR TO OCCASIONAL MVFR AND VCNTY SHRA AND WITH
A FEW ISOLATED SHOWERS AND PERHAPS A -TSRA OR TWO EARLY THIS
EVENING...ESPECIALLY NEAR KRKS. BETWEEN 00Z AND 04Z...SHRA ACTIVITY
WILL INCREASE FROM THE S WITH MVFR TO OCCASIONAL IFR CONDITIONS
BECOMING MORE PREVALENT. POSSIBLE RAIN MIXING WITH SNOW AT KBPI AND
KPNA AFTER 08Z. BETWEEN 09Z AND 13Z...-SHRA ACTIVITY WILL
DECREASE WITH SKIES CONTINUING UNDER MAINLY MVFR CONDITIONS IN LOW
CIGS UNTIL 16Z...WITH SOME IMPROVEMENT THEREAFTER.

&&

.FIRE WEATHER...

FIRE DANGER LOW TODAY...REMAINING LOW THROUGH THE
WEEKEND. COOL RAINY CONDITIONS TODAY...WITH SLOW TRANSITION TO LATE
SPRING WINTER-LIKE CONDITIONS THIS WEEKEND. WIDESPREAD PRECIPITATION
AFFECTING NEARLY ALL DISPATCH ZONES AT ONE TIME OR ANOTHER
TODAY...WITH MOST LOCATIONS RECEIVING A WETTING RAINFALL. SATURDAY
AND SUNDAY...SNOW LEVELS MAY FALL TO THE VALLEY AND BASIN FLOORS OF
THE AREA.

&&

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

$$

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





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