Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 041700
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
1100 AM MDT FRI SEP 4 2015

.SHORT TERM...TODAY THROUGH SUNDAY NIGHT

SHOWERS AND THUNDERSTORMS CONTINUE TO MOVE THROUGH THE AREA THIS
MORNING...WITH SOME STORMS PRODUCING BRIEF PERIODS OF SMALL HAIL.
SHOWER COVERAGE IS STILL EXPECTED TO WEAKEN THROUGH THE EARLY
MORNING HOURS...WITH ISOLATED ACTIVITY MAINLY LINGERING OVER THE
NORTHWESTERN MOUNTAINS.

A SIMILAR SITUATION IS EXPECTED TO SET UP TODAY...WITH ONE LAST MILD
DAY. SHOWERS AND THUNDERSTORMS EXPECTED TO INCREASE IN COVERAGE OVER
THE NORTHWEST MOUNTAINS DURING THE LATE AFTERNOON HOURS. ACTIVITY
WILL SPREAD TO INCLUDE THE LOWER ELEVATIONS AND AREAS EAST OF THE
CONTINENTAL DIVIDE. HOWEVER...THE BEST LOCATIONS FOR DEVELOPMENT
WILL REMAIN OVER THE WESTERN AND NORTHERN MOUNTAINS. SHOWERS AND
THUNDERSTORMS WILL LINGER THROUGH THE EVENING HOURS...WITH COVERAGE
LESSENING AFTER MIDNIGHT.

SHOWER AND THUNDERSTORM COVERAGE WILL NOT DISAPPEAR COMPLETELY
TONIGHT AS A COLD FRONT ADVANCES TOWARDS WESTERN WYOMING. AHEAD OF
THE FRONT...THE PRESSURE GRADIENT WILL INCREASE ACROSS THE AREA.
THIS GRADIENT WILL RESULT IN GUSTY SURFACE WINDS FOR SATURDAY
AFTERNOON...ESPECIALLY OVER THE WIND CORRIDOR BETWEEN ROCK SPRINGS
AND CASPER.

THE COLD FRONT WILL BEGIN TO MOVE INTO WESTERN WYOMING SATURDAY
EVENING...BRINGING WITH IT A TASTE OF FALL. AS THE SYSTEM PROGRESSES
ACROSS THE STATE...EXPECT COOLING TEMPERATURES...GUSTY WINDS WITH A
WIND SHIFT...AND AN INCREASE IN SHOWERS AND THUNDERSTORMS. THESE
CONDITIONS WILL LAST INTO THE EVENING HOURS...WHEN TEMPERATURES WILL
COOL ENOUGH TO PRODUCE SOME SNOWFALL OVER THE WESTERN THIRD OF THE
STATE.

THE AREA OF LOW PRESSURE INTRODUCING THE COLD FRONT TO THE STATE
WILL LIFT NORTH AND MOVE QUICKLY OUTSIDE OF THE THE STATE BY SUNDAY
AFTERNOON. AS THIS SYSTEM EXITS...EXPECT A QUICK RETURN TO DRY
CONDITIONS ACROSS THE FORECAST AREA. AFTERNOON HUMIDITY MAY DIP
INTO THE LOW TEENS SUNDAY AFTERNOON FOR PORTIONS OF NATRONA
COUNTY...WHERE GUSTY WIND MAY ALSO BE PRESENT. THIS MAY RESULT IN
ELEVATED FIRE BEHAVIOR FOR SUNDAY AFTERNOON.

.LONG TERM...MONDAY THROUGH FRIDAY

THE GFS AND GEM CONTINUE TO SHOW A SHORTWAVE TROUGH PUSHING EAST
SOUTHEAST ACROSS THE FORECAST AREA IN THE TUESDAY/TUESDAY NIGHT TIME
FRAME...WHILE THE 00Z ECMWF IS MUCH WEAKER WITH THIS SYSTEM. ALSO
MODELS HINT AT A WEAKER WAVE IN THE FLOW ON MONDAY. AT THE
SURFACE...MODELS SHOW A WEAK FRONTAL BOUNDARY OVER NORTHERN WYOMING
MONDAY...SLIDING SOUTH ACROSS THE AREA MONDAY NIGHT/TUESDAY. DUE TO
MODEL DIFFERENCES WILL CONTINUE ONLY SLIGHT POPS OVER THE
NORTH/NORTHWEST MOUNTAINS MONDAY/TUESDAY WITH AN EMPHASIS ON
TUESDAY/TUESDAY NIGHT. IF THE GFS AND GEM CONTINUE THEIR
TREND...POPS WILL NEED TO BE EXPANDED ESPECIALLY EAST OF THE DIVIDE
TUESDAY.

MEDIUM RANGE MODELS THEN SHOW A DRY AND MILD NORTHWEST FLOW
DEVELOPING WEDNESDAY THROUGH FRIDAY.

&&

.AVIATION.../18Z ISSUANCE/

VFR CONDITIONS WILL PREVAIL THROUGH THE FORECAST PERIOD.

SOUTHWEST FLOW ALOFT WILL REMAIN OVER THE REGION AHEAD OF A LARGE
UPPER TROUGH OVER THE NW U.S. GUSTY S-SW SFC WINDS 10-25KTS WILL
ONCE AGAIN PREVAIL OVER MUCH OF THE AREA THIS AFTERNOON. THEN...A
SUB-TROPICAL DISTURBANCE WILL LIFT ACROSS THE CENTRAL
ROCKIES...CLIPPING EASTERN WYOMING IN THE LATE AFTERNOON AND
EVENING...WITH ISOLATED THUNDERSTORMS MAINLY VCNTY AND EAST
INTERSTATE 25...KCPR-KBYG FROM 00Z-07Z SATURDAY.
ADDITIONALLY...ANOTHER UPPER LEVEL DISTURBANCE WILL RIDE OUT OF THE
BASE OF THE TROUGH...ALLOWING ISOLATED TO WIDELY SCATTERED
THUNDERSTORMS TO SPREAD ACROSS WESTERN WYOMING...VCNTY AND WEST OF
THE CONTINENTAL DIVIDE 22Z FRI-06Z SATURDAY WITH A FEW STORMS
PUSHING INTO THE CENTRAL BASINS...VCNTY KLND-KRIW-KCOD-KWRL 01Z-06Z.
THE MAIN HAZARD WITH THESE STORMS WILL BE ERRATIC OUTFLOW WIND GUSTS
35-50KTS. MOST OF THE STORM ACTIVITY SHOULD DISSIPATE 06Z-09Z
SATURDAY...WITH A FEW SHOWERS LINGERING OVER THE HIGH
WESTERN/NORTHWESTERN MOUNTAINS UNTIL AROUND SUNRISE.

&&

.FIRE WEATHER...

SHOWERS AND THUNDERSTORMS WILL BE FOUND THIS AFTERNOON...ESPECIALLY
OVER THE WESTERN MOUNTAINS AND VALLEYS. ACTIVITY WILL PERSIST INTO
THE OVERNIGHT HOURS. AN APPROACHING COLD FRONT WILL BRING A
TIGHTENED PRESSURE GRADIENT OVER THE AREA FOR SATURDAY
AFTERNOON...WITH GUSTY WIND EXPECTED. THESE WINDS WILL PRECEDE AN
APPROACHING COLD FRONT. THIS COLD FRONT WILL INTRODUCE COLDER
TEMPERATURES ACROSS THE AREA...WITH SHOWERS AND THUNDERSTORMS
EXPECTED. DURING THE OVERNIGHT HOURS...SOME SNOW IS ALSO POSSIBLE
OVER THE WESTERN AND NORTHERN MOUNTAINS. THIS SYSTEM WILL EXIT THE
FORECAST AREA BY SUNDAY AFTERNOON...AND WILL BE REPLACED BY DRIER
AND WARMER CONDITIONS...WITH SOME GUSTY WIND POSSIBLE. THIS MAY
BRING ELEVATED FIRE BEHAVIOR ACROSS NATRONA COUNTY. THIS SITUATION
WILL BE MONITORED.

&&

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

&&

$$

SHORT TERM...BRANHAM
LONG TERM...WM
AVIATION...JB
FIRE WEATHER...BRANHAM


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