708 FXUS65 KRIW 151811 AFDRIW AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE RIVERTON WY 1211 PM MDT MON APR 15 2013 .SHORT TERM...TODAY THROUGH WEDNESDAY NIGHT WITH EARLY START OF SNOW UPGRADED WIND RIVER BASIN TO WINTER STORM WARNING. STARTED WARNING FOR LANDER...EAST SLOPES OF WINDS AND GREEN/RATTLESNAKE ZONES. EXPANDED WINTER WEATHER ADVISORIES TO JOHNSON COUNTY...HOT SPRINGS COUNTY...WASHAKIE COUNTY AND BIGHORN MOUNTAINS AS WELL AS WEST SLOPES OF THE WIND RIVER MOUNTAINS AND THE WYOMING AND SALT RIVER RANGES AND MOST OF SUBLETTE COUNTY THIS MORNING. SNOWFALL OF 4 TO 8 INCHES REPORTED AT NOON AROUND LANDER TO SINKS CANYON. SNOW WILL EXPAND THROUGH THE AFTERNOON AND EVENING. INCREASED SNOWFALL AMOUNTS...ESPECIALLY IN FREMONT AND NATRONA COUNTIES. THE SNOW EVENT THAT HAS BEEN NOTED THE PAST FEW DAYS IS NOW BEGINNING TO TAKE SHAPE. SNOW HAS BROKEN OUT ACROSS SOUTHERN WYOMING WITH ROCK SPRINGS NOW REPORTING SNOW. THIS IS ASSOCIATED WITH THE INITIAL WARM ADVECTION MOVING OUT OF COLORADO AND UTAH. THIS WILL SPREAD NORTHWARD THROUGH THE MORNING HOURS. THE MAIN EVENT LOOKS TO BEGIN TO TAKE SHAPE LATER TONIGHT. UPPER LEVEL DIVERGENCE WILL INCREASE ACROSS MUCH OF THE AREA AFTER MIDNIGHT. ALSO..SOME OF THE AREA...ESPECIALLY EASTERN PORTIONS OF THE AREA WILL BE UNDER THE FAVORABLE LEFT FRONT QUADRANT OF THE JET TO HELP FACILITATE MORE UPWARD MOTION. IN ADDITION...ADDITIONAL MOISTURE WILL MOVE IN AS A WARM CONVEYOR BELT DEVELOPS INTO EASTERN WYOMING. STEADIER AND HEAVIER SNOW WILL SPREAD ACROSS MUCH OF THE AREA THAT IS UNDER HIGHLIGHTS AT THIS TIME BY DAYLIGHT ON TUESDAY. WITH THE UPPER LEVEL DYNAMICS AND AND INCREASING MOISTURE TUESDAY INTO TUESDAY NIGHT STILL LOOKS TO BE TIME OF THE HEAVIEST AND STEADIEST SNOW. AS FLOW TURNS MORE NORTHEASTWARD AS LEE CYCLOGENESIS OCCURS ACROSS COLORADO...THE UPSLOPE COMPONENT WILL INCREASE ACROSS THE FAVORED AREAS FROM THE WIND RIVER RANGE THROUGH CASPER. THE AREA OF HEAVIEST SNOW STILL LOOKS GOOD MAINLY ACROSS THE FAVORED UPSLOPE AREAS FROM THE WIND RIVER RANGE AND LANDER FOOTHILLS THROUGH CASPER. THE OTHER FACTOR IS INCREASING WIND...ESPECIALLY ACROSS AREAS FROM CASPER THROUGH ROCK SPRINGS AND ACROSS THE WIND RIVER RANGE. WINDS SHOULD NOT BE AS STRONG AS THE STORM OF EARLY LAST WEEK...BUT STILL STRONG ENOUGH TO BLOW AND DRIFT THE SNOW AND A NEW INTERESTING WRINKLE IS THE INCREASE IN QPF ACROSS AREAS FROM AROUND THE BIG HORN RANGE AND EAST. THE GFS IS OVER A HALF AN INCH OR PRECIPITATION TUESDAY NIGHT INTO WEDNESDAY MORNING. THIS MODEL IS THE EXCEPTION THOUGH AS THE NAM AND EUROPEAN HAVE LIGHTER AMOUNTS. THEY ARE HIGHER THAN EARLIE TODAY SO WE DID BUMP AMOUNTS UP A BIT. THIS COULD WARRANT HIGHLIGHTS FOR JOHNSON COUNTY OR THE BIG HORN RANGE BUT SINCE THIS WOULD NOT BEGIN UNTIL TUESDAY MORNING WE WILL HOLD OFF FOR NOW. BY WEDNESDAY THE SNOW SHOULD BEGIN TO TAPER OFF FROM WEST TO EAST AS LOW PRESSURE MOVES INTO THE CENTRAL PLAINS STATES. STEADY SNOW SHOULD END BY WEDNESDAY NIGHT. HOWEVER...COLD NORTHWEST FLOW WILL CONTINUE AND THAT MEANS SOME SNOW SHOWERS COULD CONTINUE. .LONG TERM...THURSDAY THROUGH MONDAY COLD NLY FLOW TO START THE PERIOD ON THE BACK SIDE OF OUR BIG EARLY WEEK SYSTEM. BRIEF RIDGING THURSDAY NIGHT AND FRIDAY BUT THE NEXT SHORTWAVE IN THE NW FLOW IS ALREADY POISED TO MOVE INTO THE WEST DURING THE DAY FRIDAY AND THEN MOVE SE ACROSS THE AREA FRIDAY NIGHT AND SATURDAY WITH ANOTHER ROUND OF MOUNTAIN SNOW SHOWERS AND A MIXTURE OF RAIN AND SNOW SHOWERS ACROSS THE LOWER ELEVATIONS. SNOW AMOUNTS COULD BE SIGNIFICANT IN THE NW AND NRN MOUNTAINS FRIDAY NIGHT AND SATURDAY. CHALLENGING NW FLOW WILL LIKELY CONTINUE THROUGH THE MEDIUM RANGE PERIOD WITH THE NEW EURO MORE AGGRESSIVE WITH THE SYSTEM EARLY NEXT WEEK THAN THE GFS. HOWEVER...THE GFS FLOW HAS FLATTENED CONSIDERABLY FROM ITS 18Z TO 00Z RUN. NW FLOW SYSTEM WILL DEFINITELY IMPACT AT LEAST THE NRN HALF LATER SUNDAY INTO MONDAY BUT OVERALL IMPACT IS STILL UNCERTAIN WITH LARGE MODEL VARIANCES. FOR EXAMPLE...AT 00Z TUESDAY THE 00Z GFS HAS THE H7 TEMPERATURE AT +1.5C AT RIW WHILE ON THE ECMWF ITS -14C. AS ALWAYS...NW FLOW HAS TO BE WATCHED CLOSELY. && .AVIATION.../18Z ISSUANCE/ EAST OF THE DIVIDE...KCPR/KCOD/KLND/KRIW/KWRL ROUTES. PERIODS OF MVFR/IFR CONDITIONS ARE EXPECTED TO OCCUR ACROSS MUCH OF THE REGION THROUGH THE FORECAST PERIOD IN SNOW AND LOW CEILINGS...WITH THE WORST AND MOST WIDESPREAD IFR CONDITIONS EXPECTED AFTER 06Z WEDNESDAY. THE BIG HORN BASIN SHOULD HAVE MAINLY VFR CEILINGS THROUGH ABOUT 06Z WEDNESDAY...BEFORE CONDITIONS THERE DETERIORATE IN LOW CEILINGS AND SOME SNOW. KCPR COULD SEE PERIODS WITH VFR CONDITIONS THIS AFTERNOON AND EVENING...BEFORE THE STEADY SNOW RETURNS IN EARNEST AFTER 06Z WEDNESDAY. WINDS WILL BE NORTHERLY...INCREASING IN STRENGTH LATE TONIGHT. MOUNTAINS WILL BE OBSCURED. WEST OF THE DIVIDE...KJAC/KRKS/KBPI/KPNA ROUTES. SCATTERED SNOW SHOWERS WILL RESULT IN MVFR/IFR CONDITIONS MAINLY ACROSS THE SOUTH/SOUTHWEST...WITH THE SNOW SHOWER ACTIVITY BECOMING MORE ISOLATED FARTHER NORTH TOWARD KJAC. MVFR CEILINGS ACROSS THE INTERSTATE 80 CORRIDOR...INCLUDING KRKS COULD BRIEFLY LIFT TO LOW VFR CONDITIONS THIS AFTERNOON AND EARLY EVENING...BEFORE AN INCREASING NORTHEAST FLOW TONIGHT WILL BRING A RETURN TO DETERIORATING CONDITIONS ACROSS THE INTERSTATE 80 CORRIDOR WITH PREVAILING IFR/LIFR CONDITIONS AFTER 06Z TUESDAY IN SN/BLSN. THIS SAME FLOW WILL BRING DOWNSLOPE DRYING ALONG KPNA- KJAC ROUTES WITH PREVAILING VFR CONDITIONS AFTER 06Z TUESDAY...ISOLATED TO SCATTERED -SHSN MAINLY OVER THE MOUNTAINS. PLEASE SEE THE AVIATION WEATHER CENTER FOR THE LATEST INFORMATION ON ICING AND TURBULENCE FORECAST. && .FIRE WEATHER... AN ACTIVE WEATHER PERIOD THROUGH WEDNESDAY AS A STORM DEVELOPS AND SPREADS SNOW MUCH OF SOUTHERN AND CENTRAL WYOMING. SNOW WILL SPREAD NORTHWARD ACROSS SOUTHERN WYOMING TODAY AND THEN NORTH OF THE WIND RIVER RANGE TONIGHT. THE HEAVIEST SNOW LOOKS TO BE ON TUESDAY WITH THE HEAVIEST ACCUMULATIONS FROM THE WIND RIVER RANGE EASTWARD TO CASPER. SNOW SHOWERS WILL ALSO OCCUR OUT WEST ALTHOUGH ACCUMULATIONS WILL BE LIGHTER. SNOW WILL END FROM WEST TO EAST ON WEDNESDAY. TEMPERATURES WILL REMAIN WELL BELOW NORMAL WITH HUMIDITY VALUES RISING TODAY INTO TUESDAY. && .RIW WATCHES/WARNINGS/ADVISORIES... WINTER WEATHER ADVISORY UNTIL 7 AM MDT WEDNESDAY WYZ028>030. WINTER STORM WARNING UNTIL NOON MDT WEDNESDAY WYZ015-017>019. WINTER WEATHER ADVISORY FROM 6 PM THIS EVENING TO NOON MDT WEDNESDAY WYZ005>011. WINTER WEATHER ADVISORY UNTIL NOON MDT WEDNESDAY WYZ014-016-024-026. WINTER WEATHER ADVISORY UNTIL 6 PM MDT TUESDAY WYZ027. WINTER STORM WARNING FROM 6 PM THIS EVENING TO NOON MDT WEDNESDAY WYZ020-022. && $$ SHORT TERM...C.BAKER/HATTINGS LONG TERM...SKRBAC AVIATION...WM FIRE WEATHER...HATTINGS