Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 211753
AFDRIW

AREA FORECAST DISCUSSION...UPDATE
NATIONAL WEATHER SERVICE RIVERTON WY
1154 AM MDT THU MAY 21 2015

.SHORT TERM...THURSDAY THROUGH SATURDAY NIGHT

THE ACTIVE WEATHER PATTERN LOOKS TO CONTINUE INTO THE FIRST PART OF
THE HOLIDAY WEEKEND. AS OF RIGHT...WE STILL GAVE A FEW LIGHT SHOWERS
MAINLY WEST OF THE DIVIDE BUT NOTHING OF CONSEQUENCE. THIS LOOKS TO
BE ON OF THE QUIETEST TIMES OF THE SHORT TERM FORECAST PERIOD.
THINGS WILL BECOME MORE ACTIVE THIS AFTERNOON AS ANOTHER SHORTWAVE
APPROACHES THE AREA FROM THE SOUTH AND WEST AND BRINGS ANOTHER ROUND
OF SHOWERS AND THUNDERSTORMS. MUCH LIKE YESTERDAY...THE VAST
MAJORITY OF THESE LOOK TO STAY WEST OF THE DIVIDE. ALSO...WITH MORE
SOUTHERLY FLOW AND MORE SUNSHINE THAN YESTERDAY...TEMPERATURES
SHOULD WARM UP CLOSER TO NORMAL LEVELS...BUT STILL RATHER CHILLY FOR
THIS TIME OF YEAR. A FEW OF THESE STORMS COULD SNEAK EAST OF THE
DIVIDE...BUT MOST AREAS EAST OF THE DIVIDE SHOULD REMAIN MAINLY DRY
MOST OF THE TIME. MOST SHOWERS SHOULD WANE AFTER MIDNIGHT ALTHOUGH A
FEW COULD LINGER AS A DEEPER PUSH OF MOISTURE MOVES TOWARDS THE
AREA.

THE AFOREMENTIONED PUSH OF MOISTURE WILL BEGIN TO PUSH NORTHWARD
ACROSS THE AREA IN THE MORNING...BUT THE BEST CHANCE OF RAIN WILL BE
DURING THE AFTERNOON. AS OF THIS TIME...THE BEST CHANCE OF SHOWERS
WILL BE ACROSS CENTRAL PORTIONS OF THE AREA WITH LESSER CHANCES
FURTHER NORTH. AS FOR THE CHANCE OF ANY STRONGER STORMS...THE BEST
CHANCE WOULD BE FURTHER NORTH WHERE THE START OF THE RAIN WOULD BE
LATER AND THERE COULD BE SOME MORNING SUNSHINE THAT COULD
DESTABILIZE THINGS SOMEWHAT. BY LATER FRIDAY NIGHT...AN INTERESTING
SCENARIO AS BOTH THE NAM AND THE GFS SHOW A DRY SLOT TRYING TO WORK
INTO THE AREA...ALTHOUGH THE NAM IS MORE PRONOUNCED WITH IT THAT THE
GFS IS.

AND THIS LEADS US TO THE QUESTION OF SATURDAY. IF THE MODELS ARE TO
BE BELIEVED...MUCH OF SATURDAY MORNING COULD BE DRY EAST OF THE
DIVIDE...ESPECIALLY THE FURTHER EAST YOU GO. THERE COULD EVEN BE
SOME CLEARING. AND THIS LEADS TO THE POSSIBILITY OF MORE CONVECTIVE
PRECIPITATION RATHER THAN STRATIFORM. AGAIN...IT DEPENDS OF THE
MODEL. THE GFS IS COOLER BUT STILL HAS NEGATIVE LIFTED INDICES AND
UP TO 300 J/KG OF CAPE IN HE AFTERNOON. MEANWHILE...THE
NAM..ALTHOUGH IT TENDS TO OVERDO INSTABILITY A BIT...IT IS SHOWING
AROUND 1000 J/KG OF CAPE OVER PORTIONS OF THE AREA ALONG WITH LIFTED
INDICES DROPPING TO AS LOW AS -5. THE NEW STORM PREDICTION CENTER DAY
3 OUTLOOK BRINGS A MARGINAL AREA INTO SOUTHEASTERN WYOMING AS
WELL. THERE IS ALSO A DECENT JET NEAR THE AREA AND WITH THE LOW
NEAR BY THERE WILL BE SOME DIRECTIONAL SHEAR AROUND. AGAIN...THE
BIG X FACTOR WILL BE HOW MUCH SUNSHINE AND DESTABILIZATION THERE
IS. SOMETHING TO WATCH. BY SATURDAY NIGHT...AGAIN THE QUESTION IS
WHETHER OR NOT THE S WORD WILL ENTER THE FORECAST AGAIN. THIS
SYSTEM STILL LOOKS WARMER THAN THE LAST ONE. THE GFS IS THE
COLDEST WITH -1 CELSIUS AT 700 MILLIBARS WHICH WOULD BRING SNOW
LEVELS DOWN TO AROUND 8000 FEET. THE NAM IS WARMER WITH LEVELS
CLOSER TO 9000 FEET. THE GFS WAS CLOSER LAST TIME THOUGH.
CONTINUITY HAD THIS REPRESENTED FAIRLY WELL SO WE MADE FEW
CHANGES.

.LONG TERM...SUNDAY THROUGH THURSDAY

SUNDAY WILL SEE THE WEATHER SYSTEM MOVE SLOWLY INTO EASTERN WY BY
00Z MONDAY. RAIN WILL BE MORE LIKELY IN THE EASTERN ZONES SUNDAY
WITH SCATTERED RAIN SHOWERS ELSEWHERE. THE WEATHER SYSTEM MOVES OUT
SUNDAY NIGHT WITH SHOWER ACTIVITY DECREASING. MONDAY LOOKS DRIER
CENTRAL AND EAST WITH A WEST FLOW ALOFT. SHOULD SEE SLIGHT CHANCE
FOR SHOWERS AND STORMS OVER MUCH OF THE REGION MON AFTERNOON AND
EVENING. FLOW ALOFT TURNS TO THE WEST SOUTHWEST FOR TUESDAY. DO NOT
SEE A WEATHER DISTURBANCE IN THE FLOW ON TUE BUT THERE IS ENOUGH
MOISTURE AND INSTABILITY FOR CONVECTION TO FIRE UP IN THE WEST AND
SOUTHWEST. ONLY ISOLATED ACTIVITY FOR THE REST OF THE AREA. WEDNESDAY
AND THURSDAY WILL SEE A SOUTHWEST FLOW ALOFT PER THE GFS MODEL
WITH A DECENT WEATHER DISTURBANCE ON THURS PM AND NIGHT. THEN A
POTENTIAL WEATHER DISTURBANCE AGAIN NEXT THURSDAY. THE ECMWF MODEL
IS A TAD DIFFERENT NEXT WED AND THURS WITH LESS POTENT WEATHER
SYSTEMS IN THE FLOW. WILL KEEP THE SCATTERED SHOWERS AND ISOLATED
STORMS IN THE FORECAST BOTH DAYS. AS FOR TEMPS...A COOL DAY ON
SUNDAY THEN WARMING UP A FEW DEGREES FOR MON AND TUE. THEN MAYBE A
LITTLE COOLER WED AND NEXT THURS DUE TO MORE CLOUDS AND MORE
COVERAGE OF SHOWERS/STORMS.


&&

.AVIATION.../18Z ISSUANCE/

EAST OF THE DIVIDE...KCOD/KCPR/KLND/KRIW/KWRL ROUTES

MVFR CIGS WILL PERSIST UNTIL ABOUT 19Z OVER THE NORTH CENTRAL AND
NORTHEAST WIND RIVER BASIN BEFORE THIS AREA OF CLOUDS SHRINKS FROM
THE EDGES. OTHERWISE VFR CEILINGS WILL PREVAIL THROUGH THE FORECAST
PERIOD. OVERALL EXPECT LESS CLOUD COVER FOR A CHUNK OF TODAY WHICH
WILL AID MIXING AND LIFT CEILINGS. THE NEXT IN A SERIES OF
SHORTWAVES WILL APPROACH SOUTHWEST WYOMING THIS AFTERNOON. FAVORABLE
INSTABILITY AND DYNAMICS WILL BE TO THE SOUTHWEST OVER SOUTHWEST
WYOMING. ISOLATED THUNDERSTORMS WILL MAINLY BE CONFINED TO THE
HIGHER TERRAIN AFTER 22Z/THU AND IS EXPECTED TO DISSIPATE WITH THE
SETTING SUN. HAVE LINGERED VCSH INTO THE 06Z-08Z/FRI TIME FRAME
ACROSS CENTRAL WYOMING...WITH LITTLE IF ANY SHOWER ACTIVITY
IMPACTING KWRL OR KCOD.

WEST OF THE DIVIDE...KBPI/KJAC/KPNA/KRKS ROUTES

MVFR CIGS WILL PERSIST UNTIL ABOUT 19Z ACROSS THE WESTERN PORTION OF
THE GREEN RIVER BASIN BEFORE THIS AREA OF CLOUDS SHRINKS FROM THE
EDGES. A SMALL PATCH OF LINGERING IFR CIGS WILL VANISH OVER
SWEETWATER COUNTY JUST IN TIME FOR A HIGHER DECK OF VFR CLOUDINESS
MOVING IN FROM THE SOUTH WHICH SHOULD DELAY ANY CONVECTIVE ACTIVITY
THERE UNTIL MAINLY AFTER 22Z THIS AFTERNOON. THE NEXT IN A SERIES OF
SHORTWAVES WILL ROTATE NORTH THIS AFTERNOON. SHOWERS WILL BE MOST
NUMEROUS OVER THE HIGHER TERRAIN...WITH DEVELOPMENT OCCURRING
BETWEEN 22Z-23Z TODAY. LIFTED INDICES AND CAPE ARE SUFFICIENT TO
GENERATE WIDELY SCATTERED THUNDERSTORMS AFTER 22Z THIS AFTERNOON.
SIMILAR TO LAST NIGHT...THE SHOWERS AND STORMS WILL CONTINUE UNTIL
ABOUT 04Z/FRI WITH SOME SHOWERS LINGERING UNTIL WELL AFTER 06Z/FRI.
CLOUD HEIGHTS OVERNIGHT TONIGHT INTO FRIDAY WILL BE LOWER...ALBEIT
VFR/MVFR...AS A RESULT OF THE INCREASED LOW-LEVEL MOISTURE ACROSS
SOUTHWEST WYOMING.

&&

.FIRE WEATHER...

FIRE DANGER RELATIVELY LOW AS SEASONAL TO COOL AND RELATIVELY MOIST
CONDITIONS PERSIST. A WEAK UPPER LEVEL DISTURBANCE WILL BRING
ISOLATED CONVECTION MAINLY WEST OF THE DIVIDE TODAY. DEEPER MOISTURE
WILL MOVE TOWARD THE AREA FOR FRIDAY AND BRING ANOTHER CHANCE OF A
WETTING RAIN FROM FRIDAY INTO FRIDAY NIGHT. THERE WILL BE A CHANCE
OF SHOWERS AND THUNDERSTORMS THAT WILL PERSIST INTO SATURDAY AS
WELL.

&&

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

&&

$$

SHORT TERM...HATTINGS
LONG TERM...AR
AVIATION...CNJ/LIPSON
FIRE WEATHER...HATTINGS










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