Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 040557
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
1155 PM MDT THU SEP 3 2015

UPDATE...ADDED 06Z AVIATION DISCUSSION.

.SHORT TERM...TONIGHT THROUGH FRIDAY

WV IMAGERY SHOWING BROAD RIDGING PATTERN ACROSS MOST OF THE CONUS
EXCEPT FOR THE WCST WHERE A DEEP TROF EXISTS. WY CURRENTLY UNDER SW
FLOW. EMBEDDED JET STEAK ON ERN PORTION OF NRN BRANCH JET CURRENTLY
MOVING OVR/OUT OF NWRN WY WITH A FEW SHOWERS PRODUCED ON ITS NOSE IN
SRN MT. SRN SRN BRANCH OF JET MOVING THIS WAY OUT OF THE DESERT SW
W/NOSE JUST COMING INTO SRN CO...W/ SHRA/TSRA JUST OUT IN FRONT OVR
CNTRL CO. SFC HAS HIGH P SOUTH OVR CO...LOW P NE OVR ND WITH
ATTENDING MINOR COLD FRONT DRAPED TO THE SW AND CURRENTLY OVR/NEAR
WY/MT BORDER...TRYING TO SAG SWD.

WARM AND DRY TODAY...BUT NOT AS DRY AS WEDNESDAY...OR AS WINDY FOR
MOST LOCATIONS EXCEPT THE FAR WEST...WHERE S TO SWRLY SFC WINDS ARE
GUSTING 20 TO 30 MPH. A FEW ISOLATED/WIDELY SCATTERED SHRA/TSRA
AROUND THE NWRN FA THIS EVENING...FORCED MAINLY DUE TO PROXIMITY TO
NRN JET AND WEAK SFC FRONT DEVELOPING SFC LOW FROM THE NRN BIG HORN
BASIN WWD AND OVR YELLOWSTONE NW. JET/DISTURBANCE AND MONSOONAL
MOISTURE NEAR S/SERN ZONES PROVIDING FOR OTHER ISOLATED ACTIVITY
THIS EVENING.

FRI...UPR DISTURBANCES ALONG WITH COMBINING/DEVELOPING SFC LOW MOVE
OFF TO THE N AND E...LEAVING LITTLE BUT A NEARLY STATIONARY FRONT
ACROSS CNTRL WY IN THEIR WAKE. THE AIRMASS THAT WILL REPLACE TODAY`S
...WHILE RELATIVELY DRY...WILL NOT BE AS DRY AS THE FIRST HALF OF
THIS WEEK...AND WILL BE JUST ENOUGH COOLER THAT RH VALUES WILL
REMAIN ABOVE CRITICAL FOR FIRE WX PURPOSES. THAT SAID...WITH THE
STATIONARY FRONT STRETCHED FROM E TO W ACROSS CNTRL WY AND A
RELATIVELY STRONG LOCAL HIGH P MASS OVR ONCE AGAIN OVR CNTRL
CO...PRESSURE DIFFERENCES BETWEEN THE TWO ALONG WITH SLIGHTLY
INCREASED FLOW ALOFT FROM THE APPROACH OF THE SRN BRANCH OF THE JET
WILL MAKE FOR A BREEZY TO WINDY DAY ACROSS THE SRN FORECAST ZONES.
WERE IT NOT FOR MODEST AMOUNTS OF MOISTURE THAT KEEP ADVECTING INTO
WY FROM THE S...IT WOULD BE ANOTHER RED FLAG DAY. ONE SMALL AREA TO
WATCH MAY ACROSS FIRE WX ZONE 277 IN THE EXTREME SWRN CORNER OF WY
WHERE DOWNSLOPING WINDS OFF THE UINTA MOUNTAINS MAY CREATE LOCAL RED
FLAG CONDITIONS. ADDITIONALLY...AN ISOLATED THUNDERSTORM OR TWO MAY
ALSO BE POSSIBLE IN THE AREA LATE IN THE DAY/EVENING...WITH
OCCASIONAL LIGHTNING A POSSIBLITY. ELSEWHERE...ISO SHRA/TSRA WILL BE
POSSIBLE ACROSS THE HIGHER TERRAIN OF THE NRN ABSAROKAS AND POSSIBLY
BIG HORN MOUNTAINS AS TERRAIN FORCED ELEVATED CONVERGENCE AND A WEAK
UPR DISTURBANCE COMBINE TO ERODE THE CAP NEAR EVENING.

.LONG TERM...FRIDAY NIGHT THROUGH THURSDAY

A CHANGE IN THE WEATHER IS ON-TAP FOR THE LABOR DAY WEEKEND. MODELS
IN GOOD AGREEMENT IN BRINGING A COLD FRONT ACROSS THE FORECAST AREA
SATURDAY NIGHT AS SHORTWAVE ENERGY EJECTS OUT OF IDAHO INTO MONTANA.
BEST CHANCE FOR PRECIPITATION WILL BE ACROSS NORTHWEST WYOMING...
INCLUDING YELLOWSTONE NATIONAL PARK. RIGHT NOW...ACCUMULATIONS OF AN
INCH OR SO LOOK POSSIBLE ACROSS THE POPULATED AREAS OF THE PARK. JET
ENERGY IN COMBINATION WITH THE COLD FRONT WILL CAUSE INCREASING
CONVECTION COVERAGE SATURDAY EVENING EAST OF THE CONTINENTAL DIVIDE
AND ACROSS SOUTHWEST WYOMING. WITH STRONG PRESSURE GRADIENT...
SUSPECT THAT GUSTY OUTFLOW WIND WOULD BE THE MAIN HAZARD...
PARTICULARLY ACROSS SOUTHWEST WYOMING. PRESSURE GRADIENT REMAINS
TIGHT ON SUNDAY AS SHORTWAVE ENERGY PROGRESSES ACROSS MONTANA. THIS
WILL LEAD TO BREEZY TO WINDY CONDITIONS SUNDAY...AS WEST-NORTHWEST
FLOW PREVAILS. WIND SPEEDS SUNDAY MAY NEED TO BE INCREASED IN AREAS
THAT FAVOR NORTHWEST FLOW. ANY PRECIPITATION IS LIKELY TO BE
CONFINED TO NORTHWEST WYOMING AND THE NORTHERN BIGHORN MOUNTAINS.
TEMPERATURES SUNDAY WILL BE AROUND 10 DEGREES BELOW NORMAL WITH 700
TEMPS ONLY CLIMBING TO AROUND 0 OR +2C IN THE NORTH AND 6C IN THE
SOUTH. DRIER ZONAL FLOW ANTICIPATED FOR LABOR DAY WITH HIGH
TEMPERATURES REBOUNDING 3-5F. TUESDAY COULD BE INTERESTING AS BOTH
THE ECMWF AND GFS DEPICT ANOTHER SHORTWAVE SWEEPING ACROSS THE
FORECAST AREA. COULD SEE THAT CHANCE OF PRECIPITATION COULD BE
ADJUSTED UPWARD...ESPECIALLY OVER NORTHWEST WYOMING TUESDAY...AS
DETAILS BECOME MORE FINE TUNED WITH LATER MODEL RUNS. THE REMAINDER
OF THE WEEK LOOKS TO BE DRY WITH WEST-NORTHWEST FLOW ALOFT
PREVAILING. TEMPERATURES LOOK TO BE STEADY AND A TAD BELOW NORMAL
DURING THESE LATER PERIODS OF THE FORECAST.

&&

.AVIATION.../06Z ISSUANCE/

A FEW SHOWERS AND THUNDERSTORMS WILL CONTINUE TO RACE NORTHEAST
ACROSS THE NW QUARTER OF WY THROUGH 08Z.  ERRATIC OUTFLOW WIND GUSTS
OF 30-45KTS WILL BE THE MAIN HAZARD.

ON FRIDAY...VFR CONDITIONS WILL PREVAIL.  A SOUTHERLY FLOW ALOFT
WILL REMAIN OVER THE REGION AHEAD OF A LARGE UPPER TROUGH OVER THE
NW U.S.  GUSTY S-SW SFC WINDS 10-20KTS WILL ONCE AGAIN PREVAIL OVER
MUCH OF THE AREA IN THE AFTERNOON. A SUB-TROPICAL DISTURBANCE WILL
LIFT ACROSS THE CENTRAL ROCKIES...CLIPPING EASTERN WYOMING IN THE
LATE AFTERNOON AND EVENING.  ISOLATED THUNDERSTORMS WILL BE MAINLY
VCNTY AND EAST INTERSTATE 25...KCPR-KBYG...WITH THIS SYSTEM FROM 00Z-
06Z SATURDAY.  ISOLATED TO WIDELY SCATTERED THUNDERSTORMS WILL ALSO
SPREAD INTO 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 IN THE EVENING HOURS.
THE MAIN HAZARD WITH THESE STORMS WILL BE ERRATIC OUTFLOW WIND GUSTS
35-45KTS.  MOST OF THE CONVECTION SHOULD DISSIPATE 06Z-08Z SATURDAY.

&&

.FIRE WEATHER...

FIRE DANGER ELEVATED TODAY ACROSS MOST OF THE FORECAST AREA FOR ONE
REASON OR ANOTHER. WEST OF THE DIVIDE IT IS FOR GUSTY SOUTHWEST
WINDS BUT WITH RH VALUES MOSTLY REMAINING AT OR ABOVE 20 PERCENT.
EAST OF THE DIVIDE IT IS DUE TO RH VALUES LOWERING INTO THE UPPER
TEEN TO AROUND 20 PERCENT...BUT WITH LESS WIND OVERALL.
OTHERWISE...MOISTURE AND PRECIPITATION CHANCES WILL BE ON A SLOW
INCREASE ACROSS THE FORECAST AREA...WITH BEST AREAS FOR SHOWERS AND
ISOLATED THUNDER ACROSS SOUTHEASTERN WYOMING AS SMALL AREA OF
MONSOONAL MOISTURE MOVES INTO THE REGION FROM CO. ANOTHER AREA THAT
MAY SEE A FEW ISOLATED SHOWERS/STORMS WILL BE FROM WEST CENTRAL WY
TO NW WYOMING. ONE AREA TO WATCH ON FRIDAY WILL BE ACROSS FIRE WX
ZONE 277 IN THE EXTREME SWRN CORNER OF WY WHERE DOWNSLOPING WINDS
OFF THE UINTA MOUNTAINS MAY CREATE LOCAL RED FLAG CONDITIONS.
ADDITIONALLY...AN ISOLATED THUNDERSTORM OR TWO MAY ALSO BE POSSIBLE
IN THE SAME AREA  BY FRIDAY EVENING...WITH OCCASIONAL LIGHTNING A
POSSIBLITY. IN GENERAL HOWEVER...ENOUGH MOISTURE WILL BE AROUND
ACROSS THE FORECAST AREA THROUGH THE WEEKEND TO KEEP MOST
FORECAST ZONES JUST BELOW CRITICAL LEVELS.

&&

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

$$

SHORT TERM...BRAUN
LONG TERM...CNJ
AVIATION...AEM
FIRE WEATHER...BRAUN


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