Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 262229
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
429 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. ALOT 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...ALOT
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 OPF 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.../00Z ISSUANCE/

A MAJOR SPRING STORM WILL RESULT IN WIDESPREAD AND PERSISTENT
IFR/MVFR CONDITIONS THROUGH THIS EVENING. SNOW LEVELS WILL DROP TO
5000 FEET MSL THIS EVENING. HOWEVER...THE PRECIPITATION SHIELD WILL BE
DECREASING LATER THIS EVENING FROM NORTHWEST TO SOUTHEAST...BUT
ENOUGH PRECIPITATION COVERAGE AND CLOUD COVER TO KEEP MVFR/IFR
CONDITIONS EAST...MVFR CONDITIONS CENTRAL AND LOW VFR CONDITIONS
WEST LATER TONIGHT AND INTO MONDAY MORNING. ALSO...BREEZY TO WINDY
NORTH TO EAST WINDS WILL OCCUR IN MOST LOCATIONS...ESPECIALLY IN
SWEETWATER COUNTY INCLUDING KRKS THIS EVENING. MOUNTAINS WILL BE OBSCURED
THROUGH THE FORECAST PERIOD THROUGH AT LEAST 06Z TONIGHT. VSBYS AND
CIGS WILL CONTINUE TO IMPROVE FROM WEST TO EAST MONDAY AFTERNOON.

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

&&

.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...LIPSON
FIRE WEATHER...LIPSON




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