272 FXUS65 KRIW 211055 AFDRIW AREA FORECAST DISCUSSION...RESENT NATIONAL WEATHER SERVICE RIVERTON WY 328 AM MDT TUE MAY 21 2013 .SHORT TERM...TUESDAY THROUGH THURSDAY NIGHT RADAR IS STILL SHOWING A COUPLE OF AREAS OF SHOWERS ACROSS EASTERN AND SOUTHERN PORTIONS OF THE COUNTY WARNING AREA ON THE OUTER EDGE OF THE UPPER LEVEL LOW NOW SPINNING OVER SOUTH DAKOTA. A FEW SHOWERS WILL LINGER ACROSS EASTERN AREAS EARLY THIS MORNING BUT A VAST MAJORITY OF THE DAY LOOKS TO BE RAIN FREE WITH WARMER TEMPERATURES THAN RECENT DAYS. THE DRIER WEATHER IS DUE TO A SHORTWAVE RIDGE THAT WILL BE MOVING OVER THE AREA. THE AREA WILL NOT BE COMPLETELY DRY AS THERE IS JUST ENOUGH INSTABILITY IN THE AIR FOR A COUPLE OF AFTERNOON AND EVENING SHOWERS AND THUNDERSTORMS TO FIRE OVER THE HIGHER ELEVATIONS OF THE MOUNTAINS WITH THE HIGH ELEVATION HEAT SOURCE. ALMOST ALL AREAS SHOULD BE DRY FROM LATER TONIGHT INTO WEDNESDAY MORNING. THE WEATHER MAY GET A BIT MORE INTERESTING ON WEDNESDAY AFTERNOON AND EVENING...ESPECIALLY EAST OF THE DIVIDE. FLOW LOOKS TO TURN MORE SOUTHERLY BRINGING IN WARMER TEMPERATURES. AT THE SAME TIME...AN APPROACHING SHORTWAVE WILL HELP TO DECREASE HEIGHTS AND DECREASE INSTABILITY A BIT. EAST OF THE DIVIDE...LOW LEVEL FLOW LOOKS TO TURN MORE SOUTHEASTERLY WHICH WILL DRIVE PRECIPITABLE WATER VALUES UP FROM AROUND 0.35 TO AROUND 0.5 TO 0.6 INCHES ON WEDNESDAY AFTERNOON. MODELS ALSO SHOW LIFTED INDICES TO AROUND MINUS 3 ACROSS MUCH OF THE AREA WITH CAPE VALUES OVER 300 J/KG WITH THE HIGHEST VALUES IN THE ABSAROKAS WITH THE UPSLOPE SOUTHEASTERLY FLOW. WITH SOME DECENT DIRECTIONAL SHEAR AS WELL WITH SOUTHWEST WINDS AT 500 MILLIBARS...ANY THUNDERSTORMS THAT FIRE COULD BECOME STRONG WITH GUSTY WINDS OR HAIL. THESE STORMS LOOK TO END WEDNESDAY EVENING WITH LEFTOVER SHOWERS IN NORTHWESTERN WYOMING FOR LATER WEDNESDAY NIGHT. COVERAGE OF SHOWERS AND THUNDERSTORMS WILL LIKELY BE LESS ON THURSDAY AS MUCH OF THE COUNTY WARNING AREA WILL BE IN A DRY SLOT WITH LOW PRESSURE OVER NORTHWESTERN WYOMING. ANY CONVECTION THAT DAY WILL LIKELY BE CONFINED TO AREAS NORTH OF THE OWL CREEK RANGE AND EAST OF THE BIG HORN RANGE. SOUTH TO SOUTHWEST FLOW WILL KEEP ABOVE NORMAL TEMPERATURES ACROSS MOST OF THE STATE. .LONG TERM...FRIDAY THROUGH TUESDAY A SOUTHWEST FLOW WILL PREVAIL THROUGH THIS PERIOD THAT WILL BRING CHANCES OF AFTERNOON AND EVENING THUNDERSTORMS TO PRIMARILY THE FAR WEST AND NORTH WHILE A DRIER AIRMASS PREVAILS ACROSS SOUTHERN AND CENTRAL WYOMING. A SHORTWAVE WILL ROTATE NORTH ACROSS THE AREA EARLY FRIDAY DOWNSTREAM OF UPPER LOW OVER THE NW U.S. THE GFS IS FASTER AND FURTHER WEST WITH THIS SHORTWAVE...PUTTING IT OVER SW MONTANA 12Z FRIDAY AND LIFTING IT INTO SOUTHERN CANADA BY 00Z SATURDAY. THE ECMWF AND CANADIAN ARE SLOWER AND HAVE TRENDED FURTHER EAST WITH THIS FEATURE...LIFTING IT ACROSS THE NW HALF OF WYOMING 12Z-18Z FRIDAY. FORECAST WEIGHTED TOWARD THIS LATTER...SLOWER SCENARIO AND MAINTAINS CHANCES OF THUNDERSTORMS ACROSS THE NORTH AND FAR WEST. THE ASSOCIATED COLD FRONT WILL RESULT IN TEMPERATURES 5 TO 10 DEGREES COOLER THAN ON THURSDAY...ALONG WITH BREEZY TO WINDY CONDITIONS BEHIND THE FRONT. UPPER RIDGE STRENGTHENS OVER THE SOUTHERN AND CENTRAL PLAINS THROUGH THE WEEKEND WITH REMNANTS OF UPPER LOW OVER THE NW U.S. LIFTING INTO SW CANADA. SW FLOW WITH EMBEDDED WEAK SHORTWAVES WILL INTERACT WITH MEAN SURFACE TROUGH ACROSS NORTHERN AND EASTERN WYOMING WITH HIGHER DEWPOINTS TO THE NORTH AND EAST OF THIS BOUNDARY...PRIMARILY ALONG AND NORTH OF A JACKSON-MEETEETSE-BUFFALO LINE. ISOLATED TO SCATTERED AFTERNOON AND EVENING THUNDERSTORMS WILL FIRE OFF NEAR THIS SURFACE BOUNDARY AND MOVE OFF TO THE NE WITH SHORTWAVES MOVING THROUGH DURING MAX HEATING RESULTING IN BETTER ORGANIZATION...THOUGH WE DO NOT HAVE MUCH CONFIDENCE IN ANY ONE MODEL'S TIMING WITH THESE IMPULSES. A WARMING TREND WILL OCCUR OVER THE WEEKEND WITH CENTRAL BASINS SEEING MOSTLY 80S. A SLIGHT COOLING TREND WILL FOLLOW EARLY NEXT WEEK THOUGH WE WILL REMAIN IN ABOVE NORMAL TERRITORY FOR THE MOST PART. && .AVIATION.../12Z ISSUANCE/ VFR CONDITIONS WILL PREVAIL OVER THE AREA TODAY AND TONIGHT. ISOLATED SHOWERS ARE POSSIBLE UNTIL 16Z FROM THE BIGHORN MOUNTAINS SOUTH TO CASPER MOUNTAIN. ISOLATED SHOWERS AND THUNDERSTORMS WILL OCCUR OVER AND NEAR THE WESTERN MOUNTAINS FROM 18Z UNTIL 03Z. LOCAL MVFR CONDITIONS ARE POSSIBLE IN STORMS ALONG WITH GUSTY WIND TO 25 KNOTS. && .FIRE WEATHER... A RIDGE BUILDING ACROSS THE AREA WILL BRING A WARMER AND DRIER DAY TO THE AREA TODAY. THERE COULD STILL BE ISOLATED SHOWERS AND THUNDERSTORMS BUT THESE SHOULD BE CONFINED TO THE HIGHER ELEVATIONS OF THE MOUNTAINS. ANOTHER STORM SYSTEM WILL BRING A CHANCE OF THUNDERSTORMS TO THE NORTHWEST FIRE ZONES ON WEDNESDAY WITH GUSTY SOUTH TO SOUTHEAST WINDS ACROSS THE AREA. && .RIW WATCHES/WARNINGS/ADVISORIES... NONE. && $$ SHORT TERM...HATTINGS LONG TERM...AEM AVIATION...AR FIRE WEATHER...HATTINGS |