Severe Thunderstorm Warning 53, NWS Riverton, WY

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
120 PM MDT MON JUL 27 2009

.SHORT TERM...TONIGHT THROUGH THURSDAY
STORMS FIRING AROUND AND IN ADVANCE OF THE CLOSED CIRCULATION
NEARING WEST YELLOWSTONE ATTM. DYNAMICS OF THE LOW WILL CONTINUE
TO INCREASE INTO THE EVENING HOURS AS IT SLOWLY MOVES SE ACROSS
THE AREA. POTENTIAL FOR SEVERE THUNDERSTORMS WILL EXIST WELL INTO
THE EVENING...ESPECIALLY IN OUR CENTRAL ZONES EAST OF THE DIVIDE.
PRESSURE GRADIENT WILL CONTINUE TO INCREASE WITH LOWERING
PRESSURES IN THE NRN GREAT BASIN AND AHEAD OF THE SYSTEM IN
CENTRAL WYOMING. ASSOCIATED FRONTAL PUSH WILL MOVE SOUTH INTO THE
EVENING HOURS WITH SATURATED AIRMASS EXPECTED BY MORNING AS LOW
CLOUDS AND AT LEAST SCATTERED SHOWERS...ISOLATED THUNDERSTORMS
LINGER INTO THE MORNING HOURS. BUMPED UP POPS IN THE CENTRAL ZONES
OVERNIGHT AND INTO TUESDAY WITH MOIST NORTHEAST FLOW. NEXT SHORTWAVE
MOVES QUICKLY SOUTH INTO MONTANA TUESDAY...CLIPPING OUR NERN ZONES
BY TUESDAY EVENING WITH A ROUND OF SHOWERS AND ISOLATED
THUNDERSTORMS. INCREASING UPSLOPE WILL DEVELOP EAST OF THE DIVIDE
BEHIND THIS SYSTEM WITH A COOL TO COLD UPSLOPE DAY EXPECTED EAST
OF THE DIVIDE ALONG WITH AREAS OF LIGHT RAIN OR DRIZZLE...
ESPECIALLY NEAR AND IN THE MTNS. TEMPERATURES WILL BE ON THE
DOWNTREND THROUGH WEDNESDAY WITH WEDNESDAY EXPECTED TO BE THE
COLDEST DAY. HIGHS WILL LIKELY STRUGGLE THROUGH THE 50S EAST OF
THE DIVIDE WEDNESDAY WITH SOME AREAS NOT MAKING IT...ESPECIALLY
WHERE LIGHT RAIN CONTINUES OFF AND ON. HAVE CONTINUED THE TREND OF
LOWERING MAX TEMPS ON WEDNESDAY WITH COOLER TEMPS STILL POSSIBLE.
SNOW LEVEL WILL GRADUALLY LOWER TUESDAY NIGHT AND WEDNESDAY TO
AROUND 9000 FEET IN THE WIND RIVER MTNS AND NEAR 7500 FEET IN THE
BIG HORNS AND NRN ABSAROKAS. COOL...CYCLONIC NNW FLOW CONTINUES
ON THURSDAY WITH SOME AFTERNOON SHOWERS OR ISOLATED STORMS POSSIBLE.
THE GFS SHOWS ANOTHER SHORTWAVE MOVING THROUGH WITH THE NAM DOES
NOT. BOTH ARE RATHER LIMITED WITH THEIR MOISTURE AND SHOW NO
CONVECTION FORMING. WILL LEAVE THE 20-30 POPS IN...ESPECIALLY FOR
THE FAVORED NW FLOW AREAS.

.LONG TERM...THURSDAY NIGHT THROUGH TUESDAY
MAIN TREND OF GLOBAL MODELS IN THIS PERIOD IS TO SLOW DOWN OVERALL
PROGRESSION OF HIGHLY AMPLIFIED PATTERN OVER NORTH AMERICA...UPPER
RIDGE ALONG WEST COAST AND DEEP TROUGH OVER CENTRAL/EASTERN U.S.
WYOMING TO REMAIN IN N-NW FLOW THROUGH THE PERIOD WITH MAIN
FORECAST PROBLEM IN TIMING OF INDIVIDUAL SHORTWAVE CLIPPERS AND
ASSOCIATED FRONTS...OF WHICH OPS GFS AND ECMWF HAVE BEEN SHOWING
POOR RUN-TO-RUN CONSISTENCY. MAIN CHANGES TO FORECAST TONIGHT HAS
BEEN TO SLOW DOWN WARMING TREND OVER WEEKEND...KEEPING
TEMPERATURES 5-10F BELOW NORMAL ACROSS THE CENTRAL AND EAST.

ON THURSDAY AND FRIDAY UPPER FLOW BACKS BETWEEN WEDNESDAYS SYSTEM
AND UPSTREAM CLIPPER DIVING SOUTH ACROSS CENTRAL CANADA.
INSTABILITY WILL BE BEST ALONG OLD FRONTAL BOUNDARY ALONG THE
CONTINENTAL DIVIDE WITH STORMS TRACKING SSE ALONG FRONTAL
BOUNDARY.

OVER THE WEEKEND...ECMWF AND GFS SHOW INCREASING SPREAD IN BOTH
THE NORTHERN STREAM CLIPPER SYSTEMS AND UPPER LOW OFF OF THE WEST
COAST UNDERCUTTING RIDGE. FOR WEST AND CENTRAL WYOMING...FORECAST
KEEPS MEAN FRONTAL BOUNDARY AGAINST EAST SLOPES OF THE DIVIDE WITH
BEST CHANCES OF THUNDERSTORMS ALONG THE FRONT...NEAR OR SLIGHTLY
ABOVE NORMAL TEMPERATURES WEST OF THE DIVIDE AND BELOW NORMAL
TEMPERATURES EAST OF THE DIVIDE WITH GFS POSSIBLY TOO STRONG WITH
INDIVIDUAL FRONTS AND ECMWF ON WARM SIDE.

FOR EARLY NEXT WEEK...EXPECT MODELS TO CONTINUE TO TREND SLOWER
WITH PROGRESSION OF THIS HIGH AMPLITUDE BLOCKING PATTERN WITH
TEMPERATURES EAST OF THE DIVIDE EXPECTED TO REMAIN AT LEAST
SLIGHTLY BELOW NORMAL WITH LATE DAY CONVECTION FAVORABLE ALONG
EAST SLOPES OF DIVIDE AND FAVORED AREAS IN NW FLOW.

&&

.AVIATION...
STRONG UPPER LEVEL LOW PRESSURE WILL MOVE ACROSS THE REGION
TONIGHT...AS A SURFACE COLD FRONT MOVES INTO SOUTHERN WYOMING THIS
EVENING. SCATTERED SHOWERS AND THUNDERSTORMS WEST OF THE DIVIDE WILL
PRODUCE LOCALIZED MVFR CONDITIONS THROUGH AROUND 03Z. SCATTERED TO
NUMEROUS SHOWERS AND THUNDERSTORMS WILL PRODUCE LOCALIZED MVFR
CONDITIONS EAST OF THE DIVIDE THROUGH 06Z. STRONG TO SEVERE WIND
GUSTS AND LARGE HAIL WILL BE POSSIBLE IN THUNDERSTORMS THIS EVENING
ACROSS THE ENTIRE REGION. WINDS TONIGHT ARE EXPECTED TO BE GENERALLY
LESS THAN 12 KT...EXCEPT ACROSS SOUTHERN WYOMING...WHERE NORTHEAST
TO EAST WINDS COULD GUSTS TO AROUND 20 KT. SURFACE HIGH PRESSURE
WILL MOVE ACROSS WYOMING TUESDAY WITH LIGHT WINDS. RESIDUAL MOISTURE
AND WEAK INSTABILITY IN WESTERN WYOMING COULD PRODUCE ISOLATED
SHOWERS AND THUNDERSTORMS TUESDAY AFTERNOON AND EVENING.

&&

.FIRE WEATHER...
AN UPPER LEVEL LOW PRESSURE SYSTEM WILL BRING WIDESPREAD
SHOWERS AND THUNDERSTORMS TO MUCH OF THE AREA INTO THE EVENING
HOURS. SOME OF THESE STORMS WILL BE QUITE STRONG TO EVEN SEVERE.
THE HIGHEST PROBABILITY OF SEVERE WEATHER WILL BE EAST OF THE DIVIDE.
A GOOD CHANCE OF SHOWERS AND ISOLATED THUNDERSTORMS WILL CONTINUE
THE NEXT COUPLE DAYS...ESPECIALLY ACROSS THE NORTH...CENTRAL AND
EASTERN SECTIONS. THE SOUTHWEST WILL BE THE DRIEST DURING THIS PERIOD.
TEMPERATURES WILL BE ON THE DOWNTREND THROUGH MIDWEEK WITH INCREASING
MINIMUM RELATIVE HUMIDITIES.

&&

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

&&

SHORT TERM...SKRBAC
LONG TERM...AEM
AVIATION...SMITH
FIRE WEATHER...SKRBAC

HAZARDOUS WEATHER OUTLOOK
NATIONAL WEATHER SERVICE RIVERTON WY
500 AM MDT MON JUL 27 2009

WYZ001>020-022>030-281100-
ABSAROKA MOUNTAINS-BIGHORN MOUNTAINS SOUTHEAST-
BIGHORN MOUNTAINS WEST-CASPER MOUNTAIN-CODY FOOTHILLS-
EAST SWEETWATER COUNTY-FLAMING GORGE-
GREEN MOUNTAINS AND RATTLESNAKE RANGE-JACKSON HOLE-LANDER FOOTHILLS-
NATRONA COUNTY LOWER ELEVATIONS-NORTH BIG HORN BASIN-
NORTHEAST JOHNSON COUNTY-OWL CREEK AND BRIDGER MOUNTAINS-
ROCK SPRINGS AND GREEN RIVER-SALT RIVER AND WYOMING RANGES-
SOUTH LINCOLN COUNTY-SOUTHEAST BIG HORN BASIN-
SOUTHEAST JOHNSON COUNTY-SOUTHWEST BIG HORN BASIN-STAR VALLEY-
TETON AND GROS VENTRE MOUNTAINS-UPPER GREEN RIVER BASIN-
UPPER GREEN RIVER BASIN FOOTHILLS-UPPER WIND RIVER BASIN-
WIND RIVER BASIN-WIND RIVER MOUNTAINS EAST-WIND RIVER MOUNTAINS WEST-
YELLOWSTONE NATIONAL PARK-
500 AM MDT MON JUL 27 2009

THIS HAZARDOUS WEATHER OUTLOOK IS FOR WESTERN AND CENTRAL WYOMING.

.DAY ONE...TODAY AND TONIGHT

A STRONG COLD FRONT WILL MOVE NORTH TO SOUTH ACROSS WYOMING TODAY.
NUMEROUS SHOWERS AND THUNDERSTORMS ARE EXPECTED ACROSS THE NORTH
AND CENTRAL...WITH MORE ISOLATED TO SCATTERED ACTIVITY OVER THE
SOUTH AND FAR WEST.  HEAVY RAIN...SMALL HAIL...FREQUENT LIGHTNING
AND WIND GUSTS TO 50 MPH ARE LIKELY.  A FEW STRONG THUNDERSTORMS MAY
PRODUCE LARGE HAIL AND WIND GUSTS TO 60 MPH.

.DAYS TWO THROUGH SEVEN...TUESDAY THROUGH SUNDAY

ISOLATED TO SCATTERED SHOWERS AND THUNDERSTORMS ARE EXPECTED THROUGH
MUCH OF THE WEEK.  SNOW LEVELS WILL DROP TO NEAR 9000 FEET ACROSS THE
NORTHERN MOUNTAINS BEHIND A COLD FRONT TUESDAY NIGHT AND WEDNESDAY
WITH A FEW INCHES OF SNOW POSSIBLE OVER THE BIG HORN...ABSAROKA AND
WIND RIVER RANGES.

.SPOTTER INFORMATION STATEMENT...

SPOTTERS ARE ASKED TO REPORT HAIL AND WIND GUSTS IN EXCESS OF 40 MPH
FROM THUNDERSTORMS TODAY AND TONIGHT. THANK YOU.

TEAM RIVERTON

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