Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 010852
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
252 AM MDT WED OCT 1 2014

.SHORT TERM...TODAY THROUGH FRIDAY NIGHT

LARGE LW TROF ACROSS THE CONUS W OF THE MISSISSIPPI...WITH TWO
EMBEDDED SW TROFS ROTATING THRU THE FLOW...ONE TO THE E AND OVR THE
NRN PLAINS...THE OTHER JUST NOW KNOCKING ON THE DOORSTEP OF THE WRN
FA. FURTHER UPSTREAM...SW RIDGING COMING INTO THE PAC NW WITH SW
TROF ON ITS HEALS COMING OUT OF SRN AK. SFC SHOWS
DEVELOPED/ASSOCIATED CYCLONE OVR THE NRN PLAINS W/ TRAILING FRONT
FROM ERN MT SWD AND THRU CNTRL WY...THEN OFF TO THE SW AND INTO THE
GREAT BASIN...WHILE ANOTHER SFC TROF/DEVELOPING FRONT JUST
APPROACHING THE NW CORNER OF THE STATE.

THIS MORNING/TODAY THRU TONIGHT...SOME MID/UPR LIFT INCREASES AS SW
NEAR WRN CORRIDOR CONTINUES TO TRANSLATE THRU THE LARGE-SCALE FLOW
AND OVR THE WRN HALF OF THE CWA BY MORNING...WITH COLDER AND
MODESTLY DIFLUENT UPR LVLS. ATTM...WV IMAGERY SHOWS TWO PLUMES OF
MOISTURE ASSOCIATED WITH APPROACHING SW TROF...ONE RIDING INTO THE
NW CORNER OF WY WITH THE TROF/FRONT...THE OTHER TO THE S AND RIDING
THRU RATHER FLAT FLOW OUT OF THE SERN ID/NRN UT...WITH DRIER AIR IN
BETWEEN THE TWO. PRECIPITATION...BOTH LIQUID AND FROZEN WILL
CONTINUE ACROSS THE NRN CWA THROUGH THE DAY BEHIND A FRONT...WHILE
THE CNTRL CWA INCLUDING THE WIND RIVER BASIN AND WIND RIVER
MOUNTAINS WILL HAVE TO WAIT A BIT LONGER FOR THE MAIN UPR TROF TO
PUSH FURTHER ACROSS AND FORCE THE FRONT FURTHER SOUTH THROUGH THE
LATE MORNING BEFORE PRECIP THERE GETS GOING. H7 TEMPS WILL CONTINUE
TO FALL FROM AROUND -1 DEG C TO NEAR -4 THRU THE MORNING HRS. WHILE
COLD AIR WILL BE ADVECTING INTO THE FA FROM THE N...CLOUD COVER WILL
KEEP SOME WARMER AIR TRAPPED AT LOWER ELEVATIONS AND WILL HAVE TO
MIX OUT THOROUGHLY...WHICH MAY TAKE A WHILE.
ADDITIONALLY...EVAPORATIVE COOLING FROM ALOFT MAY BE LIMITED AS
CEILINGS CONTINUE TO FALL THIS MORNING...CLOSING THE DRY/MOIST AIR GAP
QUICKLY. SNOW TOTALS FROM EARLY THIS MORNING THROUGH THE EVENING
PERIOD WILL VARY FROM AS MUCH AS 6 INCHES OVER PORTIONS OF THE
ABSAROKAS AND BIGHORN MOUNTAINS...5 INCHES IN THE TETONS...AND UP TO
4 INCHES IN THE WIND RIVERS. RAIN SHOWERS MAY MIX WITH SNOW OR
BECOME ALL SNOW FOR A BRIEF PERIOD AT SOME LOWER ELEVATIONS IN THE
WESTERN BIGHORN BASIN THIS MORNING...WITH MOSTLY AN INCH OR LESS
POSSIBLE AT SOME LOCATIONS...BUT WITH ISOLATED AREAS...SUCH AS
AROUND MEETEETSE RECEIVING 1 TO 2 INCHES OF SNOWFALL...IF THE
CHANGEOVER OCCURS.  TONIGHT...UPR TROF AXIS PUSHES ACROSS THE
FA...QUICKLY CLEARING SKIES AND COOLING THINGS OFF FROM W TO E BY
MIDNIGHT OR SHORTLY THEREAFTER. PATCHY FOG MAY BECOME AN ISSUE
OVERNIGHT AND INTO THURSDAY MORNING WEST OF THE DIVIDE AND OVER
PARTS OF THE BIGHORN BASIN.

THU...QUICK MODEST RIDGING GIVES WAY TO YET ANOTHER AND FINAL UPR SW
TROF THAT WILL SWING THRU THE BACKSIDE OF THE MAIN TROF...DRIVING
ANOTHER COLD FRONT DOWN THRU THE FA EAST OF THE DIVIDE.
HOWEVER...MOISTURE WILL BE AT A PREMIUM WITH THIS FEATURE. WHILE
PRECIP CHANCES WILL INCREASE E OF THE DIVIDE...MAINLY ACROSS THE NRN
MOUNTAINS AND BIGHORN BASIN...QPF WILL REMAIN QUITE LOW. CHANCES FOR
LIGHT SNOW WILL BE POSSIBLE ACROSS THE MOUNTAINS ONCE AGAIN ABOVE
7000 FEET...BUT MEASURABLE AMOUNTS ATTM LOOK TO REMAIN MOSTLY BE AN
INCH. THIS SW TROF WILL QUICKLY MOVE OUT OF THE AREA THROUGH THE
EVENING...WITH ONLY CLEAR COLD AND DRY SKIES LEFT BY FRI
MORNING.

FRI...LARGE MAIN UPR TROF PROGRESSES EWD INTO THE CNTRL CONUS...WITH
RIDGING W SLOWLY INCHES ITS WAY TOWARDS WY...LEAVING THE FA UNDER NW
FLOW ALOFT AND STRONG HIGH PRESSURE AT THE SFC. FRI WILL BE
CHARACTERIZED BY MOSTLY CLEAR AND SUNNY SKIES...AND WARMER AFTER A
RATHER COLD START.

.LONG TERM...SATURDAY THROUGH WEDNESDAY

SYNOPSIS...UPPER RIDGE OF HIGH PRESSURE ALONG THE WEST COAST WILL
BRING A DRY AND MILD NORTHWEST FLOW ACROSS THE AREA THROUGH TUESDAY.

DISCUSSION...THIS PERIOD BEGINS WITH AN UPPER RIDGE ALONG THE WEST
COAST AND DOWNSTREAM LONGWAVE TROUGH ACROSS THE EASTERN U.S.  THIS
PATTERN IS EXPECTED TO SLOWLY DEAMPLIFY EARLY NEXT WEEK AS UPSTREAM
OMEGA BLOCK BUILDS ALONG THE ALEUTIAN CHAIN WITH UPPER LOW OVER THE
CENTRAL PACIFIC (NEAR 43N/165W EARLY THIS MORNING) BECOMING THE
EASTERN ANCHOR OF THIS BLOCK NEAR JUNEAU AK.  GFS AND ECMWF
DETERMINISTIC RUNS BOTH SHOW INCREASING SPREAD FROM THEIR RESPECTIVE
ENSEMBLE MEANS ON HOW THIS BLOCK WILL EVOLVE.  FORECAST PREFERS THE
LEAST AMOUNT OF TRANSITION OFFERED BY THE GEFS MEAN THIS
MORNING...HOLDING ON TO FLATTER WESTERN RIDGE...EASTERN TROUGH
PATTERN THROUGH MIDWEEK AND KEEPING POLAR FRONT NORTH OF THE AREA.
EXPECT TEMPERATURES TO BE 5-8F DEGREES ABOVE NORMAL THROUGH THE
PERIOD WITH A SLIGHT WARMING TREND MONDAY AND TUESDAY.  SLIGHT
CHANCE POPS WERE INTRODUCED TO THE NORTHERN MOUNTAINS ON WEDNESDAY
AS FLOW BECOMES MORE WESTERLY.

&&

.AVIATION.../12Z ISSUANCE/

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

A LARGE AREA OF LOW CEILINGS AND SCATTERED TO LOCALLY WIDESPREAD
PRECIPITATION IS EXPECTED ACROSS NORTHERN WYOMING
/KBYG...KCOD...KWRL/ IN WAKE OF A COLD FRONT THIS MORNING. THIS WILL
RESULT IN WIDESPREAD MVFR CEILINGS...AS WELL AS IFR REDUCTIONS
MAINLY DUE TO LOW VSBY IN PRECIP. ENOUGH COLD AIR IS EXPECTED THAT A
SNOW COULD OCCUR DOWN TO AROUND 5500 FT. FARTHER SOUTH ACROSS
CENTRAL WYOMING /KCPR...KLND...KRIW/ THERE IS LESS CERTAINTY OF
EXTENT/TIMING OF THE STRATUS DEVELOPING THIS MORNING AFTER FRONTAL
PASSAGE. EITHER WAY...THE BULK OF THE PRECIPITATION SHOULD BE NORTH
OF THOSE TERMINALS THIS MORNING. SCATTERED SHOWERS AND POSSIBLY AN
ISOLATED THUNDERSTORM WILL OCCUR IN THE AFTERNOON WITH PERSISTENT
LOW CEILINGS IN THE 020-040 KFT AGL. THE TROUGH EXITS THE AREA THIS
EVENING WITH IMPROVING CONDITIONS...BUT A MOIST NORTHWEST FLOW COULD
RESULT IN SHOWER ACTIVITY FORMING OVER THE MOUNTAINS AND PUSHING
SOUTHEAST INTO THE LOWER ELEVATIONS THROUGH ABOUT 06Z. MOUNTAINS
WILL BE OBSCURED THROUGH MUCH OF THE DAY...WITH IMPROVING CONDITIONS
LATE IN THE DAY INTO THE OVERNIGHT.

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

SHOWER ACTIVITY WILL BE MOST CONCENTRATED ACROSS THE FAR WEST THIS
MORNING IMPACTING KJAC. MVFR AND POSSIBLY IFR CONDITIONS WILL BE
POSSIBLE IN SNOW AT KJAC THIS MORNING. THIS SHOWER ACTIVITY IS THEN
EXPECTED TO OVERSPREAD THE REST OF THE WEST/SOUTHWEST WYOMING
BETWEEN 16Z AND 20Z...AS THE MAIN TROUGH BEGINS TO PUSH THROUGH.
THIS COULD RESULT IN PERIODS OF MVFR CONDITIONS...BUT NOT OF
CONFIDENCE TO PREVAIL MVFR CONDITIONS AT ANY TERMINAL. RAPIDLY
IMPROVING CONDITIONS ARE EXPECTED BETWEEN 22Z AND 02Z...WITH SKIES
TRYING TO CLEAR OUT.  SOME WARM AIR ADVECTION STRATOCU DECK IS THEN
EXPECTED TO DEVELOP OVER PORTIONS OF FAR WEST WYOMING AFTER 06Z.

&&

.FIRE WEATHER...

FIRE DANGER WILL REMAIN LOW ACROSS THE FORECAST AREA THROUGH THE
REST OF THE WEEK. SHOWERS WILL CONTINUE ACROSS WEST AND NORTH
THROUGH THE DAY...ENDING FROM WEST TO EAST LATER THIS AFTERNOON AND
THROUGH THIS EVENING. THE BEST CHANCE FOR WETTING RAINS (OR SNOW)
WILL OCCUR IN THE TETONS EARLY...THEN THE ABSAROKA AND BIGHORN
MOUNTAINS THROUGH THE REST OF THE DAY...WITH BEST CHANCES ACROSS THE
LOWER ELEVATIONS COMING IN THE CODY FOOTHILLS AND SOUTHERN BIGHORN
BASIN. ANOTHER CHANCE FOR LIGHT PRECIPITATION WILL PRESENT ITSELF BY
THURSDAY AFTERNOON OVER THE NORTHEASTERN DISPATCH ZONES...EXITING
QUICKLY BY EVENING.

&&

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

$$

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







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