Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 271756
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
1156 AM MDT THU AUG 27 2015

.SHORT TERM...TODAY THROUGH SATURDAY NIGHT

WV IMAGERY SHOWING LARGE BROAD RIDGE OVR THE CNTRL
CONUS....BOOKENDED BY AMPLIFIED TROFFING. BUSTING THRU THE RIDGE TOP
OVR WY AND INTO THE HIGH/CNTRL PLAINS...IS A STRONG SW/JET
STREAK...LEAVING SHRA IN ITS WAKE ACROSS THE 30 PERCENT OF THE FA
TONIGHT/THIS MORNING...WITH STRONG TO SEVERE STORMS RIDING THE NOSE
OF THE DISTURBANCE INTO SD AND NE. A SMALL EMBEDDED SW DISTURBANCE
IS PRESENT OVR NV THIS MORNING...ARRIVING LATER THIS AFTERNOON OVR
WY. THE SFC HAS DEVELOPING LEE CYCLONE ACROSS ERN WY AND INTO THE
HIGH/CNTRL PLAINS...HIGH P WRN WY AND GREAT BASIN...WITH STATIONARY
FRONT IN BETWEEN.

THIS MORNING THRU TONIGHT...UPR WAVE AND LEE LOW WILL MOVE EWD INTO
THE PLAINS...EVENTUALLY TAKING SOME PRECIP WITH IT. HOWEVER...A BACK
DOOR FRONT SHOULD REMAIN...STRETCHING FROM THE FRONT RANGE OF CO
INTO SRN WY...CONTINUING ENOUGH LL FORCING TO KEEP SCATTERED AREAS
OF SHRA/ISOLATED TSRA LINGERING...WHILE WRAP AROUND MOISTURE
INCREASES CHANCES ACROSS THE NERN ZONES. PATCHY FOG  WILL BE
POSSIBLE FOR MANY LOCATIONS WEST OF THE DIVIDE...WITH LOCALIZED
AREAS LIKE THE STAR/JACKSON VALLEYS SEEING ISOLATED PATCHY DENSE
FOG. BY THIS AFTERNOON...BACKSIDE OF THE UPR WAVE WILL FINALLY BEGIN
TO CLEAR THE AREA...DRAGGING THE SFC FRONT WITH IT. AS THE UPR WAVE
MOVES OUT...COOL UPR LVLS WILL INCREASE INSTABILITY ACROSS THE
WEST/NORTHWEST...WITH DIURNAL PARTIALLY TERRAIN FORCED SHRA/TSRA
INCREASING ONCE AGAIN. THESE STORMS WILL BE ISOLATED TO
SCATTERED...SOUTH TO NORTH...AND A BIT STRONGER AS INDIVIDUAL STORMS
THAN WEDNESDAY...WITH GUSTY WINDS AND SMALL POSSIBLE IN ADDITION TO
BRIEF HEAVY RAIN.

FRI...UPR RIDGE BUILDS BACK OVR THE FA WITH SOME WEAK WARMING
BEGINNING AGAIN. LITTLE IF ANY PRECIP IS EXPECTED ACROSS THE
FA...WITH ONLY AN ISOLATED STORM POSSIBLE ACROSS THE HIGHER PEAKS OF
CNTRL/NRN WY. DRYING...BUT GENERALLY LIGHT WINDS ACROSS MOST OF THE
REGION.

SAT...WRN CONUS UPR TROF STARTS TO IMPINGE ON AND INFLUENCE THE
CNTRL CONUS RIDGE POSITION...WITH INCREASING UPR LVL SWRLY FLOW
OVERTAKING MOST OF THE CWA...AND ALLOWING FOR FAST MOVING
DISTURBANCES TO RACE THRU. AT THE SAME TIME A SFC LOW P SYSTEM OVR
MT WITH A LEADING TROF WITH DEEPEN OVR ERN WY THRU THE DAY...WHILE
THE FRONT DEVELOPS FURTHER TO THE NW OF THE FA. LOW P WILL ALSO
DEVELOP OVR THE ERN GREAT BASIN. THE COMPLICATED SFC PATTERN WILL
INCREASE THE P GRAD LOCALLY IN ADDITION TO THE FASTER FLOW ALOFT.
SFC WINDS WILL INCREASE ACCORDINGLY OUT OF THE S TO SW INT THE TEENS
WITH AREAS ACROSS THE W...NW...AND SW MOST LIKELY TO SEE GUSTS TO 25
MPH OR HIGHER...ESPECIALLY OVR THE YELLOWSTONE AREA. HOTTEST TEMPS
AND DRIEST LOCATIONS WILL BE EAST OF THE DIVIDE WITH MIN RH VALUES
DROPPING INTO THE MID TEENS ACROSS THE LOW ELEVATIONS...MID 20S IN
THE MOUNTAINS. W OF THE DIVIDE WILL BE PRIVY TO A BIT MORE MOISTURE
AND CLOUD...WITH MIN RH VALUES RANGING FROM THE MID 20S TO NEAR 40
FOR MOST LOCATIONS. FROM A FIRE WX PERSPECTIVE...THERE ARE REALLY TWO
AREAS OF CONCERN THAT MAY WARRANT MARGINAL RED FLAG WATCH/WARNING
CONDITIONS. ONE IS YELLOWSTONE AND THE NORTHERN ABSAROKAS...WHERE
ALTHOUGH MIN RH VALUES WILL NOT BE CRITICAL...WINDS WILL BLOW 20 TO
25 MPH WITH GUSTS TO 35 MPH POSSIBLE ALONG WITH THE POTENTIAL FOR
ISOLATED THUNDERSTORMS BY AFTERNOON. THE OTHER WILL BE ACROSS THE
WIND RIVER AND BIG HORN BASINS IN ADDITION TO NATRONA AND JOHNSON
COUNTIES. HERE...MIN RH VALUES WILL FALL INTO THE MIDDLE TEENS WITH
WINDS GUSTING 20 TO 25 MPH AT TIMES...ESPECIALLY OVER THE ERN FCST
ZONES. WILL NEED ANOTHER DAY TO FIGURE OUT JUST WHAT LOCATIONS TO
INCLUDE/LEAVE OUT.

.LONG TERM...SUNDAY THROUGH THURSDAY

THE 00Z MODELS SHOW THE FORECAST AREA IN GENERALLY A SOUTHWEST FLOW
THROUGH THE EXTENDED...AHEAD OF A LONGWAVE TROUGH OVER THE WEST
COAST...WITH A COUPLE OF SHORTWAVE TROUGHS AND ASSOCIATED COLD
FRONTS IMPACTING THE FORECAST AREA. ALTHOUGH THE FORECAST AREA COULD
STILL SEE SOME ELEVATED TO POSSIBLY EXTREME FIRE WX
CONDITIONS...MODELS HAVE TRENDED WITH SLIGHTLY COOLER/MORE MOIST
ENVIRONMENT IN THE EXTENDED DECREASING CHANCES FOR EXTREME FIRE WX
PARAMETERS.

A COLD FRONT SHOULD BE STALLED OUT ACROSS NORTHERN WYOMING/SOUTHERN
MONTANA SUNDAY MORNING. A SIGNIFICANT SHORTWAVE TROUGH PUSHING
ACROSS THE EXTREME NORTHERN ROCKIES/SW CANADA SUNDAY/SUNDAY NIGHT
SHOULD REINFORCE/PUSH THIS FRONT FARTHER SOUTH ACROSS THE FORECAST
AREA LATE SUNDAY/SUNDAY NIGHT. 00Z MODELS ARE NOW PROGGING MORE
AVAILABLE MOISTURE FOR THIS FRONT...AND HAVE EXPANDED POPS TO THE
ENTIRE AREA SUNDAY AFTERNOON AND EVENING. THE SURFACE FRONT WILL
PROBABLY WASH OUT MONDAY...BUT WEAK DISTURBANCES IN THE SOUTHWEST
FLOW COMBINED WITH DAYTIME HEATING COULD RESULT IN ISOLATED
MOUNTAINS THUNDERSTORMS POSSIBLY SPILLING INTO THE ADJACENT
FOOTHILLS AT TIMES. MODELS ARE SHOWING A STRONGER TROUGH PUSHING
ACROSS THE PACIFIC NORTHWEST AND SW CANADA WEDNESDAY AND THURSDAY
WHICH COULD SEND ANOTHER COLD FRONT INTO THE AREA. HOWEVER ONCE
AGAIN THE MAIN IMPACTS SHOULD REMAIN WELL NORTHWEST/NORTH OF THE
AREA.

&&

.AVIATION.../18Z ISSUANCE/

SHOWERS AND THUNDERSTORMS WILL BE POSSIBLE ACROSS MOST TERMINALS
WEST OF THE CONTINENTAL DIVIDE THIS AFTERNOON. SHOWERS ARE MOST
FAVORABLE TO DEVELOP OVER THE MOUNTAIN AREAS...CREATING A MORE
HAZARDOUS SCENARIO FOR ANY PILOTS ATTEMPTING TO USE MOUNTAIN
AVIATION PASSES. SMALL HAIL AND ERRATIC WIND WILL BE POSSIBLE WITH
THUNDERSTORM DEVELOPMENT THIS AFTERNOON. THOUGH DEVELOPMENT LOOKS
BEST FOR AREAS WEST OF THE DIVIDE...SOME ACTIVITY WILL ALSO BE
POSSIBLE EAST OF THE CONTINENTAL DIVIDE...WITH LESS COVERAGE
EXPECTED. THE EXCEPTION MAY BE KCPR...WHICH CONTINUES TO BE
MONITORED. THUNDERSTORM ACTIVITY WILL WANE BY MIDNIGHT...THOUGH
ISOLATED SHOWERS WILL CONTINUE THROUGH AROUND 09Z. THE RETURN OF
HIGH PRESSURE WILL THEN BRING A PERIOD OF GENERALLY DRY
CONDITIONS THROUGH 18Z FRIDAY.


&&

.FIRE WEATHER...

FIRE DANGER DECREASES TODAY WITH MINIMUM RH VALUES RANGING FROM THE
20S TO NEAR 40 PERCENT FOR MOST LOCATIONS AND WINDS GENERALLY OUT OF
THE NORTH AT LESS THAN 15 MPH. ISOLATED TO WIDELY SCATTERED
THUNDERSTORMS WILL BE POSSIBLE TOO...MAINLY WEST OF THE DIVIDE.
LIGHTNING COVERAGE WILL HAVE TO BE MONITORED THIS AFTERNOON FOR THE
POSSIBLITY OF A LIGHTNING START WITH THE THUNDERSTORM ACTIVITY.
FRIDAY AND SATURDAY...WARMING AND DRYING CONDITIONS RETURN TO THE
REGION...WITH FIRE DANGER ON THE INCREASE. ALTHOUGH MINIMUM RH
VALUES WILL APPROACH THE MIDDLE TEENS (PERCENT) ON FRIDAY...WINDS
WILL REMAIN RELATIVELY LIGHT AROUND THE AREA KEEPING FIRE DANGER
BELOW CRITICAL. SATURDAY...HOWEVER...WILL SEE FIRE DANGER INCREASING
TO AT LEAST THE ELEVATED CATEGORY...IF NOT HIGHER FOR SOME
LOCATIONS...AS A WARM/HOT DRY ATMOSPHERE BUILDS INTO THE REGION AS
WINDS INCREASE FROM THE SOUTHWEST. A COMBINATION OF DECREASING
MOISTURE VALUES AND INCREASING GUSTY WINDS FOR SOME LOCATIONS EAST
OF THE DIVIDE WILL APPROACH CRITICAL VALUES LATER IN THE AFTERNOON.
ADDITIONALLY...WINDS GUSTING TO 35 MPH AT TIMES ALONG WITH CHANCES
FOR ISOLATED THUNDERSTORMS OVER THE NORTHWEST FORECAST AREA SATURDAY
AFTERNOON WILL ALSO ENHANCE FIRE DANGER EVEN THOUGH MOISTURE VALUES
WILL NOT BE AS LOW AS THEY WILL BE EAST OF THE DIVIDE.


&&

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

$$

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



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