Special Weather Statement, NWS Riverton, WY

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
203 PM MDT THU AUG 6 2009

.SHORT TERM...TONIGHT THROUGH SUNDAY
SUBTROPICAL PLUME DID COME NWD OVERNIGHT LAST NIGHT AS EXPECTED
WITH LATE NIGHT AND MORNING CONVECTION. AIRMASS IS QUITE
OVERTURNED NOW EAST OF THE DIVIDE WITH THREAT OF SEVERE WEATHER
DIMINISHED FOR NOW. THE JUICIEST AIR IS IN THE FAR NRN AND NERN
ZONES BUT WITH CLOUD COVER AND SHOWERS...TEMPS ARE DOWN AND THE
AVAILABLE CAPE IS CAPPED. BOTH OF THESE AREAS COULD FIRE AGAIN BUT
IT WILL TAKE SOME TIME AND PERHAPS THE ARRIVAL IN THE SW OF AN
APPROACHING SHORTWAVE. IN THE NEXT FEW HOURS...THE HOT SPOT SHOULD
BE THE WEST WHERE PLENTY OF SUNSHINE HAS OCCURRED ALL DAY WITH
INSTABILITY INCREASING BY THE HOUR. POTENTIAL FOR SVR STORMS
EXISTS THERE OVER THE NEXT COUPLE HOURS. OTHER AREAS COULD RECOVER
LATER BUT FOR SHORT TERM...THE FAR WEST WILL BE THE AREA TO WATCH.
SHORTWAVE ROTATING NEWD FROM BASE OF IMPRESSIVE UPPER LOW BY EARLY
EVENING SPREADING INCREASING QG FORCING INTO THE SWRN AND CENTRAL
AREAS THROUGH THE EVENING. ANOTHER ROUND OF EVEN STRONGER STORMS
MAY FORM AHEAD OF THIS DISTURBANCE BEGINNING IN THE SW IN A FEW
HOURS AND SPREADING NE INTO THE EVENING HOURS. LATE TONIGHT INTO
FRIDAY MORNING...THE COLD FRONT MOVES INTO THE WEST WITH A LINE OF
SHOWERS AND THUNDERSTORMS EXPECTED ALONG WITH RAPIDLY COOLING MID
LEVEL TEMPERATURES. INCREASING QG FORCING FRIDAY AFTERNOON AND
EVENING WILL BRING WIDESPREAD SHOWERS AND SOME THUNDERSTORMS TO
THE WEST WITH SCATTERED COVERAGE ACROSS THE NORTH AND MUCH LESS IN
OUR SERN ZONES. SNOW LEVELS COULD LOWER TO 8K FEET FRIDAY NIGHT
AND SATURDAY MORNING AND ONLY RISE TO 9K FEET OR SO SATURDAY
AFTERNOON. SNOW LEVELS COULD EVEN BE ARTIFICIALLY LOWER DURING
PERIODS OF HEAVIER CONVECTIVE SHOWERS. DEPENDING ON THE EXACT
EVOLUTION...WE COULD SEE SOME DECENT ACCUMULATION ABOVE 9000 TO
10000 FEET IN THE NW MTNS FRIDAY NIGHT AND SATURDAY. WITH BOTH THE
GFS AND THE NAM NOW FURTHER SOUTH WITH THE UPPER LOW SATURDAY...
THE CHANCE OF SHOWERS WILL BE HIGHER ACROSS OUR NRN AND EVEN
CENTRAL ZONES. TEMPERATURES WILL LIKELY BE QUITE COOL SATURDAY
WITH H7 TEMPS ONLY IN THE LOW TO MIDDLE SINGLE DIGITS. SYSTEM
MOVES EAST SATURDAY NIGHT WITH SUNDAY LOOKING LIKE A GENERALLY DRY
DAY WITH RECOVERING TEMPS. A FEW SHOWERS MAY DEVELOP OVER THE HIGH
COUNTRY BUT OTHERWISE SUNDAY LOOKS DRY.

.LONG TERM...SUNDAY NIGHT THROUGH THURSDAY
PATTERN CHANGE CONTINUES TO BE THE EMPHASIS OF THE EXTENDED
FORECAST AS FLOW OVER THE WESTERN US AND EASTERN PACIFIC BECOMES
ZONAL WITH GULF OF ALASKA LOW EXECUTING A SERIES OF FAST MOVING
SHORT WAVES IN THE FLOW. THIS WILL KEEP A THREAT OF AT LEAST
ISOLATED CONVECTION INTO THE WESTERN MOUNTAINS WITH MODERATE
TEMPERATURES AND BREEZY CONDITIONS ACROSS THE SOUTH AND ALONG THE
LEE SLOPES OF THE MOUNTAINS. EXTENDED MODELS ARE IN BETTER
AGREEMENT TONIGHT WHICH ADDS CONFIDENCE TO THE EXTENDED FORECAST.
IF THE SOUTHWEST MONSOON MOISTURE GETS ENTRAINED IN THE ZONAL FLOW
QUICKER...THE THREAT OF PRECIPITATION WILL INCREASE SIGNIFICANTLY.
AT THIS TIME THIS WOULD OCCUR ON THURSDAY AT THE EARLIEST.
TROPICAL STORM ACTIVITY IN THE EASTERN PACIFIC AS WELL AS CONVECTION
OVER THE DESERT SOUTHWEST COULD SIGNIFICANTLY ALTER THE
AVAILABILITY OF MOISTURE AS WELL.

&&

.AVIATION...
AS A STRONG UPPER LEVEL WEATHER DISTURBANCE TRACKS NORTHEAST THROUGH
A VERY UNSTABLE AIRMASS ACROSS WYOMING AHEAD OF AN APPROACHING LOW
PRESSURE CENTER IN NORTHERN CALIFORNIA... THERE IS THE POTENTIAL FOR
STRONG THUNDERSTORMS TO DEVELOP ACROSS MUCH OF THE AREA THIS
EVENING...ESPECIALLY ACROSS WESTERN...NORTHERN AND SOUTHERN WYOMING.
MANY OF THESE STORMS WILL LIKELY CONTAIN HAIL...LOCALLY HEAVY
RAINFALL AND GUSTY 40 TO 50 MPH WINDS. KRIW AND KLND ARE THE LEAST
LIKELY SITES TO EXPERIENCE THESE CONDITIONS...HOWEVER ALL THE ALL
TAF SITES HAVE THE POTENTIAL TO EXPERIENCE THESE STRONGER STORMS.
THESE STORMS WILL LIKELY PRODUCE WIDELY SCATTERED IFR CONDITIONS AND
SCATTERED MVFR CONDITIONS AS WELL AS MOUNTAIN OBSCURATIONS. THESE
SHOWERS WILL LINGER PAST MIDNIGHT BUT GRADUALLY DECREASE IN
INTENSITY AFTER MIDNIGHT AS THEY SLOWLY EXIT TO THE EAST. AS THE
TROUGH OF LOW PRESSURE ASSOCIATED WITH THE APPROACHING
AFOREMENTIONED LOW PRESSURE CENTER APPROACHES WYOMING...NUMEROUS
SHOWERS AND THUNDERSTORMS WILL OCCUR ACROSS WESTERN AND NORTHERN
WYOMING FRIDAY ALONG WITH ISOLATED TO WIDELY SCATTERED COVERAGE
ELSEWHERE. ANY SHOWER ACTIVITY AFTER MIDNIGHT TONIGHT WILL LIKELY
CREATE ISOLATED IFR CONDITIONS AND WIDELY SCATTERED MVFR CONDITIONS
ALONG WITH SOME MOUNTAIN OBSCURATIONS.

&&

.FIRE WEATHER...
A MAJOR UPPER LEVEL LOW WILL MOVE SLOWLY ACROSS THE NORTHERN GREAT
BASIN THROUGH FRIDAY AND ACROSS NORTHERN WYOMING ON SATURDAY.
SCATTERED TO NUMEROUS SHOWERS AND THUNDERSTORMS WILL COVER THE
AREA THROUGH THIS EVENING AND ACROSS MUCH OF THE WEST AND NORTH
THROUGH SATURDAY. TEMPERATURES WILL CONTINUE TO COOL EACH DAY WITH
HIGHS ONLY IN THE 60S AND 70S EAST OF THE DIVIDE BY SATURDAY AND
50S TO MID 60S OUT WEST. THE HIGH COUNTRY WILL BE IN THE 40S AND
50S WITH SOME SNOW IN THE NORTHERN MOUNTAINS ABOVE 8500 TO 9000
FEET AND ABOVE 10000 FEET FURTHER SOUTH. THE POTENTIAL FOR STRONG
TO SEVERE THUNDERSTORMS EXISTS THROUGH THIS EVENING WITH POTENTIALLY
DAMAGING WIND...LARGE HAIL AND WIND GUSTS OVER 60 MPH.

&&

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

&&

SHORT TERM...SKRBAC
LONG TERM...C.BAKER
AVIATION...LIPSON
FIRE WEATHER...SKRBAC

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