Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 270528
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
1127 PM MDT SUN APR 26 2015

.SHORT TERM...TONIGHT THROUGH MONDAY

QUITE THE POT POTPOURRI OF ONGOING WX ATTM. WE ARE CURRENTLY IN THE
THROWS OF A COMPLEX WEATHER SYSTEM WITH THE INVERTED SFC TROUGH
EXTENDING NORTH FROM THE SFC LOW WAY DOWN IN NEW MEXICO INTO EAST
CENTRAL WYOMING WITH THE H7/H5 TROUGH OVER WESTERN WY. AHEAD OF
THE SFC TROUGH...CONVECTION HAS FORMED IN THE FAR SOUTHEASTERN
REACHES OF OUR CWA ACROSS EASTERN SWEETWATER COUNTY IN THE PSEUDO
DRY SLOT WHERE WE FIND SOME CAPE/NEGATIVE LIFTED INDICES. THE REST
OF THE CWA IS UNDERNEATH A VAST STRATIFORM FIELD OF RAIN AND
MOSTLY HIGHER ELEVATIONS SNOWFALL...EXCEPT FOR THE FAR WEST. SOME OF
THE PRECIP IN THE NORTHERN TIER COULD BE ASSOCIATED WITH Q VECTOR
FORCING ASCT WITH THE NORTHERN STREAM ENERGY. A LOT OF IT WAS LIKELY
DUE TO WRAP AROUND ISENTROPIC LIFT ACROSS THE REST OF THE AREA. THE
WET SNOW THAT FELL IN MANY LOWER ELEVATIONS EARLIER TODAY HAS
SWITCHED OVER TO RAIN AFTER 2 INCHES OF LESS OF SNOW FELL...A LOT OF
WHICH HAS SINCE MELTED. WE WILL LET THE WINTER WEATHER ADVISORIES
RIDE THROUGH MIDNIGHT FOR THE WESTERN AND NORTHERN MTNS.
UNFORTUNATELY...THE SNOTEL DATA IS NOT AVAILABLE ATTM BUT WE HAVE
GOTTEN REPORTS OF 5 INCHES OF SNOW IN THE TETONS AT AREA SKI
RESORTS. WAS ON THE FENCE ON ISSUING A WINTER WEATHER ADVISORY FOR
NORTHEAST JOHNSON COUNTY WITH THE IDEA OF MOISTURE STREAMING NORTH
ALONG THE EASTBOUND PASSING UPPER TROUGH COMING THROUGH FOLLOWED BY
A COL REGION DURING THE COOLER NOCTURNAL HOURS...SOME MOISTURE OF
WHICH WOULD BE ORIGINATING FROM THE CONVECTION OVER EASTERN
SWEETWATER COUNTY...BUT THERE IS A LACK OF ANY ONE SINGULAR WEATHER
FEATURE TO SINK OUR TEETH INTO SUCH AS A SEPARATE CIRCULATION MOVING
NORTH...SO WE DECIDED TO JUST ISSUE AN SPS TO COVER THE POSSIBILITY
OF 2 TO 3 INCHES OF SNOWFALL THAT POTENTIALLY COULD ACCUMULATE ON
THE HIGHER ELEVATIONS OF NORTHEAST JOHNSON COUNTY THROUGH MIDNIGHT
TONIGHT. QPF SPITS OUT A STORM TOTAL QPF OF AROUND AN INCH OVER THE
BIG HORN MTNS AND ADJACENT JOHNSON COUNTY. LATER TONIGHT...PRECIP
WILL END FROM NORTHWEST TO SOUTHEAST AND COULD LINGER IN OUR EASTERN
ZONES AND POSSIBLY ALONG THE EAST SLOPE OF THE WIND RIVER MTNS FOR A
WHILE UNTIL AROUND NOON MONDAY,. OTHERWISE...A POSITIVELY TILTED
RIDGE WILL MOVE INTO THE AREA FROM THE WEST AS A 1029 SFC HIGH
BUILDS IN FROM THE NORTHWEST RESULTING IN DRIER WEATHER BY MONDAY
AFTERNOON. LOWERED TEMPS IN SOME AREAS EAST OF THE DIVIDE BY ABOUT 4
DEGREES OR SO THINKING THE MORE DRAMATIC WARM UP WILL TAKE PLACE
AFTER MONDAY.

.LONG TERM...MONDAY NIGHT THROUGH SUNDAY

HEIGHTS BUILDING QUICKLY BEHIND OUR CURRENT SYSTEM TO START THIS
PERIOD. SOGGY GROUND BEHIND THIS SYSTEM AND WARMING ALOFT WILL SET
THE STAGE FOR LATE NIGHT/MORNING FOG ACROSS MUCH OF THE AREA MONDAY
NIGHT AND TUESDAY MORNING...ESPECIALLY LOW LYING AREAS AND AREAS
WITH LIGHT WIND WHICH IS MUCH OF CENTRAL AND SW WYOMING.
OTHERWISE...WARMING TEMPS TUESDAY THROUGH WEDNESDAY AHEAD OF OUR
NEXT SHORTWAVE WEDNESDAY NIGHT AND THURSDAY. THIS SYSTEM IS ACTUALLY
NEAR THE DATELINE RIGHT NOW AND RACES EWD AND THROUGH OUR CURRENT
ERN PACIFIC TROUGH. MODELS ARE STRUGGLING RUN TO RUN WITH HOW MUCH
ENERGY WILL BE THIS FAR SOUTH. 12Z GFS IS A LITTLE FURTHER NORTH
THAN THE 06Z RUN. 00Z EURO WAS FURTHER SOUTH BUT NOW ITS TRENDED
FURTHER NORTH ALSO. LONG WAVE ANALYSIS SHOWS OUR MEAN RIDGE
GRADUALLY BEING REPLACED BY A FLAT LONG WAVE TROUGH AS WE GET
FURTHER INTO THE EXTENDED PERIOD. ODDS ARE THAT THIS THURSDAY WAVE
COULD TREND A LITTLE STRONGER THIS FAR SOUTH...SO WE`LL NEED TO
WATCH IT. THE NEXT WEEKEND SYSTEM WHICH APPEARS TO BE A COLD TROUGH
OVER NERN ASIA WILL MOVE INTO THE PACNW FRIDAY AND THEN THE LARGE
DISCREPANCIES START SHOWING UP. THE 12Z GFS IS PROGRESSIVE AND MOVES
THE MAJORITY OF THE ENERGY WITH THIS SYSTEM ACROSS SRN CANADA WITH A
WEAKER SRN EXTENSION OF THIS TROUGH SWINGING EAST ACROSS OUR AREA
SATURDAY NIGHT. THE GFS HAS A SECONDARY SYSTEM IN THE NW FLOW
CLIPPING THE NERN ZONES LATER SUNDAY INTO SUNDAY NIGHT. THE 00Z
ECMWF HAD A STRONGER LATE THURSDAY/THU NIGHT SHORTWAVE AND THEN A
MORE SLOWING DOWN...DIGGING TROUGH OVER THE WEEKEND LEADING TO A
CLOSED LOW OVER IDAHO AT THE END OF THIS PERIOD. STRONGER RIDGING
UPSTREAM IS THE MAIN REASON FOR THIS DIFFERENCE. THE 12Z ECMWF HAS
TRENDED STRONGLY TOWARD THE 12Z GFS WITH A SUNDAY TROUGH STAYING
WELL TO THE NORTH IN SRN CANADA/MONTANA AREA. WEAK FRONT THIS FAR
SOUTH MAY HELP FOCUS SOME CONVECTION WITH LIMITED MOISTURE IN THE
WLY FLOW WITH POSSIBLE WEAK EMBEDDED WAVE.

&&

.AVIATION.../06Z ISSUANCE/

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

AREAS OF LIGHT PRECIPITATION ONGOING ALONG AND EAST OF A
KLND-KWRL-KBYG WILL GRADUALLY TAPER OFF AROUND 18Z...LASTING THE
LONGEST OVER THE MOUNTAINS AND NATRONA COUNTY...WITH SOME ISOLATED
ACTIVITY LINGERING INTO THE AFTERNOON. THIS PRECIPITATION AND THE
HIGH BOUNDARY LAYER MOISTURE IS EXPECTED TO KEEP MVFR/IFR CONDITIONS
IN FOG/LOW CEILINGS THROUGH MUCH OF MONDAY MORNING EVEN OUTSIDE
WHERE PRECIP IS FALLING. THE CASPER TERMINAL COULD HANG ONTO LOW
CLOUDS THROUGH MUCH OF THE DAY MONDAY...TO LOSE IT FOR A FEW HOURS
LATE IN THE AFTERNOON/EARLY EVENING...BEFORE A MOIST EASTERLY FLOW
BRINGS BACK LOW CLOUDS OR FOG. FOG WILL ALSO BE POSSIBLE ACROSS MUCH
OF THE AREA MONDAY NIGHT...ESPECIALLY IN AREAS THAT RECEIVED A GOOD
AMOUNT OF PRECIPITATION.

PLEASE SEE THE AVIATION WEATHER CENTER FOR THE LATEST INFORMATION ON
ICING AND TURBULENCE FORECASTS.

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


LINGERING PRECIPITATION WILL GRADUALLY COME TO AN END IN MOST AREAS
BETWEEN 09Z-12Z FROM WEST TO EAST...BUT SOME LIGHT PRECIP COULD
LINGER THROUGH ABOUT 15Z ACROSS SWEETWATER COUNTY. THE TAF CHALLENGE
WILL BE THE FORMATION OF FOG AND MVFR/IFR CEILINGS LATER TONIGHT.
DUE TO THE ABUNDANT PRECIPITATION OVER THE PAST COUPLE OF DAYS...AND
HIGH BOUNDARY LAYER...WOULD EXPECT ONGOING CEILINGS TO GRADUALLY
LOWER OR IF CLOUDS BEGIN TO THIN...THEN FOR FOG TO DEVELOP. THE LOW
CLOUDS/FOG WILL PROBABLY LIFT/SCATTER OUT BETWEEN 16Z AND 18Z...BUT
SOME WILL PROBABLY HOLD TOUGH UNTIL EARLY AFTERNOON. SKIES MIGHT
THEN CLEAR OFF BY AROUND 00Z...BUT ANOTHER ROUND OF FOG COULD OCCUR
MONDAY NIGHT.

&&

.FIRE WEATHER...

MANY LOCATIONS HAVE HAD AND WILL CONTINUE TO RECEIVE A WETTING
RAIN OR SNOW THROUGH THIS EVENING AS A LARGE SYSTEM AFFECTS THE
ENTIRE REGION. IT WILL CLEAR OUT BY TOMORROW HOWEVER WITH HIGH RHS
AND COOLER TEMPERATURES HANGING AROUND ON MONDAY. TUESDAY WILL BE
WARMER AND DRIER BUT WITH NO WIND CONCERNS. WEDNESDAY AND THURSDAY
WILL BE WARMER AS WELL BUT THERE COULD BE ANOTHER...MUCH WEAKER
AND QUICKER...SYSTEM GRAZING THE AREA WHICH COULD INCREASE WINDS
TO ABOVE 25 MPH ON THURSDAY WITH SOME HUMIDITIES POTENTIALLY INTO
THE MID TEENS ACROSS RAWLINS AND PORTIONS OF CASPER DISPATCH...BUT
IT IS STILL A BIT EARLY AND UNCERTAIN TO PINPOINT ANY AREAS OF
CONCERN...IF ANY.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
WINTER WEATHER ADVISORY UNTIL MIDNIGHT MDT TONIGHT
WYZ002-008-009-012-014-015-024.

&&

$$

SHORT TERM...LIPSON
LONG TERM...SKRBAC
AVIATION...WM
FIRE WEATHER...LIPSON








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