Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 191729 AAA
AFDRIW

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE RIVERTON WY
1129 AM MDT TUE AUG 19 2014

.SHORT TERM...TODAY THROUGH THURSDAY NIGHT

WV IMAGERY SHOWING WEAK/FLAT RIDGING WITH A SLIGHT TILT TOWARD NW
FLOW. UPSTREAM...A SMALL BUT VIGOROUS SW IS BEING KICKED OUT AHEAD
OF THE MAIN CLOSED TROF OVR THE W COAST...IS NOW LOCATED FROM ERN NV
THRU CNTRL UT AND WILL IMPACT THE FA THRU THE DAY TODAY. FURTHER
UPSTREAM AND NW OF THE W COAST TROF...SW TROF JUST NOW MOVING INTO W
COAST OF BC WILL EFFECT THE REGION THRU THE MID/END OF WEEK PERIOD.
AT THE SFC...GENERAL LOW P ACROSS THE GREAT BASIN WITH SLIGHTLY
HIGHER P OVR WY. WEAK SFC BOUNDARY STRETCHING ACROSS THE SRN PORTION
OF WY FROM W TO E.

TODAY...FLAT UPR RIDGING TO SLIGHT NW FLOW OVR THE FA WILL BREAK
DOWN THRU THE DAY WITH THE ARRIVAL OF A RELATIVELY STRONG UPR WAVE
SHOOTING OUT AHEAD OF OUT THE WRN CONUS LOW P TROF. THE SFC WILL
RESPOND BY DEVELOPING/EVOLVING SFC LOW ENTERING THE STATE FROM THE S
WITH SFC BOUNDARY/TROF EXTENDING FROM S CNTRL TO NERN WY.
ADDITIONALLY...MONSOONAL MOISTURE IS HEADED BACK INTO THE STATE AND
WILL COMBINE WITH MOISTURE ASSOCIATED WITH THE SHORTWAVE COMING FROM
THE EPAC...GIVING A DEEP TROPICAL MOISTURE PROFILE TO MUCH OF REGION
THRU TONIGHT. PWS IN THE DESERT CONUS SW ARE INCREASING AND RANGING
FROM 1 TO 2 INCHES. INCREASING CHANCES FOR SCATTERED TO NUMEROUS
+SHRA/+TSRA WITH GUSTY WINDS...SMALL HAIL AND PERIODS OF VERY HEAVY
RAIN BY MID TO LATE AFTERNOON ACROSS THE SW/WRN ZONES...ISOLATED
EVERYWHERE ELSE. THRU THE EVENING...TRANSITIONING TO MORE OF A
STRATIFORM RAIN W/ ISOLATED EMBEDDED SHRA/TSRA REGIME. THERE WILL BE
SOME FLASH FLOOD POTENTIAL...ESPECIALLY EARLY WITH HEAVIER STORMS
AND OVR HIGHER TERRAIN OVERNIGHT WHERE TERRAIN FORCED RAINFALL WITH
CONVECTIVE ELEMENTS COINCIDE...ESPECIALLY ON S TO SW FACING SLOPES
OVERNIGHT AND THRU WED MORNING. AN SPS WILL BE SENT WITH THIS
ISSUANCE EXPLAINING THE SITUATION. SINCE THIS EVENT WILL BE
PARTIALLY CONVECTIVE...THEN STRATIFORM LATER TONIGHT...HESITATE IN
ISSUING FFW FOR ALL OF THE WRN MOUNTAINS AT THIS TIME AND ARE
CHOOSING TO WAIT AND SEE HOW ALL THIS EVOLVES. HOWEVER...LATER
DURING THIS FCST PERIOD...A FFW MAY BE NECESSARY ACROSS SOME OF THE
WRN MOUNTAIN ZONES AS THE GROUND SATURATES AND PRECIPITATION KEEPS
FALLING THROUGH AT LEAST THURSDAY.

WEDNESDAY AND THURSDAY...MOISTURE ENTRAINMENT FROM THE EPAC WILL
CONTINUE INTO THE WRN FA ALONG AND AHEAD OF MOBILE NRN BRANCH OF THE
W COAST TROF. MORE MODEST MONSOONAL MOISTURE WILL CONTINUE TO RETURN
INTO THE SRN/SERN ZONES. REGARDLESS OF MOISTURE SOURCE...PERIODS OF
RAINFALL...STRATIFORM AND EMBEDDED CONVECTIVE DURING THE DAYTIME
HRS...WILL CONTINUE THRU THURSDAY BEFORE FINALLY BEGINNING TO WANE
OVERNIGHT THURSDAY. DAYTIME NEARLY MOIST ADIABATIC LAPSE RATES WILL
RANGE FROM 7.5 TO 8.5 C/KM WITH CAPE PEAKING FROM 600 TO 1000 J/KG
DEPENDING ON WARMING WITH LITTLE INHIBITION. PERIODS OF HEAVY
RAINFALL AND SMALL HAIL CAN BE EXPECTED WITH ANY OF THIS EMBEDDED
ACTIVITY. E OF THE DIVIDE...CONVECTION WILL BE SOMEWHAT MORE
ISOLATED IN NATURE WITH LESS MOISTURE TO WORK WITH AND GUSTY WINDS
STILL A POSSIBILITY.

.LONG TERM...FRIDAY THROUGH TUESDAY
MEDIUM RANGE PERIOD LOOKS TO STILL BE DOMINATED BY A RIDGE ALONG
AND JUST OFF THE PACNW COAST NWD TO JUST OFFSHORE THE B.C. COAST
AND A RELOADING TROUGH OVER THE NRN ROCKIES/INTERMOUNTAIN WEST.
FORECAST LEANING MORE ON THE MORE CONSISTENT GFS. THE ECMWF HAS
HAD SOME WILD SWINGS SINCE LAST NIGHT RANGING FROM A TROUGH EARLY
NEXT WEEK JUST TO OUR WEST...THEN A BIG RIDGE AND NOW ANOTHER COLD
TROUGH SWINGING THROUGH AT THE END OF THE MEDIUM RANGE PERIOD. THE
GFS IS CHANGING WITH EACH RUN TOO BUT NOT WITH AS WILD OF SWINGS.
CONFIDENCE STILL LOW ON TIMING AND DETAILS FOR EARLY NEXT WEEK.
TROUGH HANGS JUST TO THE WEST FRIDAY AND THEN STARTS TO SWING EWD
ON SATURDAY. THE GFS KEEPS A FRONTAL BOUNDARY DRAPED ACROSS THE NE
HALF OF THE AREA WITH NE UPSLOPE AND HIGHER DEW POINTS AND THUS
HIGHER CAPES. KEEPING THE HIGHER CHANCE POPS BEHIND THE EXPECTED
FRONT IN THIS AREA ALONG WITH COOLER TEMPERATURES. MILDER WITH
LOWER TO NIL POPS FURTHER SW. BRIEF SHORTWAVE RIDGING POSSIBLE
SUNDAY BEFORE THE NEXT SHORTWAVE RELOADS/REDEVELOPS THE MEAN TROUGH
OVER THE INTERMOUNTAIN WEST. MAINLY ISOLD POPS IN THE NORTH AND
WEST. NEXT TROUGH EARLY NEXT WEEK COULD BE QUITE COOL WITH A GOOD
CHANCE OF SHOWERS AND SOME THUNDERSTORMS ACROSS AT LEAST THE NORTH
HALF. THE GFS IS NOT AS FAR SOUTH WITH THIS SYSTEM AS THE ECMWF
BUT BOTH WOULD INDICATE A GOOD CHANCE OF PCPN FOR AT LEAST THE
NORTH AND EVEN FURTHER SOUTH IF THE EURO IS RIGHT. FOR NOW GOING
WITH CHANCE MTN POPS AND ISOLD ELSEWHERE. ENOUGH COOL AIR WITH THE
FRIDAY/SATURDAY SYSTEM TO PRODUCE SOME HIGHER MOUNTAIN SNOW BUT
MAINLY ABOVE 10-11K. SYSTEM EARLY NEXT WEEK HAS THE POTENTIAL TO
BE COLDER YET WITH SNOW LEVEL DOWN TO 9K OR SO BY TUESDAY/TUESDAY
NIGHT BUT CONFIDENCE STILL LOW ON THE DETAILS THIS FAR OUT.

.AVIATION.../18Z ISSUANCE/

A SURGE OF MOISTURE MOVING INTO SOUTHERN WY ALREADY VCNTY KRKS
WILL CONTINUE TO MOVE NORTH THROUGH 06Z. AHEAD OF THE MAIN PUSH
EXPECT ANY AREAS WITH PARTLY CLOUDY OR CLEAR SKIES TO SEE
-SHRA AND -TSRA TO DEVELOP 18-21Z OVER THE MOUNTAINS FIRST AND
SPREAD INTO THE FOOTHILLS SHORTLY THEREAFTER...MAINLY FROM A KEVW
TO VCNTY KAFO IN THE EARLY AFTERNOON PUSHING INTO NW WYOMING VCNTY
AND NORTH OF KJAC IN THE LATE AFTERNOON AND EVENING. THUNDERSTORM
COVERAGE WILL BE MORE ISOLATED FURTHER EAST. EXPECT OCCNL MVFR
CONDITIONS...MOUNTAIN OBSCURATIONS AND TURBULENCE VCNTY TSRA.

THE SYSTEM WILL SPREAD SCT-NMRS SHRA S-N ACROSS WEST AND CENTRAL
WYOMING AFTER 00Z WITH A FEW EMBEDDED THUNDERSTORMS LINGERING. CIGS
WILL LOWER MAINLY INTO THE FL050-100 RANGE WITH ALONG WITH HIGHER
MTN OBSCURATIONS AND POSSIBLY LCL MVFR CONDITIONS IN RA AND BR.
GRADUAL IMPROVEMENT IS EXPECTED FROM SW-NE AFTER 10Z AS THE SYSTEM
LIFTS INTO NE WYOMING.

.FIRE WEATHER...

FIRE WEATHER CONCERNS WILL CONTINUE TO FALL TODAY...REMAINING LOW
ACROSS MOST OF THE FORECAST AREA THROUGH AT LEAST THURSDAY. THE
BIG HORN BASIN HOWEVER WILL REMAIN RELATIVELY DRY FOR MOST OF THE
DAY...BUT WINDS WILL REMAIN WELL UNDER 15 MPH FOR TH MOST PART.
THE REASON FOR THE LOW CONCERN IS THAT BOTH MONSOONAL AND PACIFIC
MOISTURE WILL INFILTRATE THE STATE WITH INCREASING RAINFALL
CHANCES STATEWIDE BY EVENING AS A VIGOROUS UPPER LEVEL DISTURBANCE
HELPS IN DEVELOPING SHOWERS AND THUNDERSTORMS FROM THIS MOISTURE.
THE OVERNIGHT PERIODS WILL GENERALLY SEE MORE STRATIFORM AND
LIGHTER RAINFALL WHILE DAYTIME PERIODS WILL BE CHARACTERIZED BY
BOTH STRATIFORM RAIN WITH EMBEDDED ISOLATED THUNDER. HEAVY RAIN
WILL BE POSSIBLE AT TIMES...ESPECIALLY DURING THE DAYS.

&&

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

$$

SHORT TERM...BRAUN
LONG TERM...SKRBAC
AVIATION...ALLEN
FIRE WEATHER...BRAUN




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