Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 092047
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
247 PM MDT WED APR 9 2014

.SHORT TERM...TONIGHT THROUGH SATURDAY

COLD FRONT IS EDGING INTO EXTREME SRN MT AND ABOUT TO ENTER FAR
NRN WYOMING ATTM. STABLE TRAPPED MOUNTAIN WAVE SIGNATURE UP NORTH
IS TRANSITIONING TO A MORE CONVECTIVE LOOK AS THE UNFAVORABLE JET
POSITION PULLS OFF A LITTLE TO THE EAST AND AT LEAST PUTS THE AREA
IN A NEUTRAL SUPPORT AREA. MINIMAL CAPE IN THE FORECAST
SOUNDING...100-200 J/KG/LI -1 TO -1.5 MAX...BUT ENOUGH TO POTENTIAL
SUPPORT A COUPLE THUNDERSTORMS WITH GUSTY WIND POTENTIAL OF 40 TO
50 MPH...MAINLY NORTH AND EAST INTO THE EVENING. FRONT WASHES OUT
IN NATRONA AND SRN FREMONT LATER TONIGHT WHICH COULD LEAVE BEHIND
A MOIST ENOUGH BOUNDARY LAYER FOR LOW CLOUDS AND OR FOG. ADDED IN
MORE CLOUDS AND PATCHY FOG FOR THIS AREA TOWARDS MORNING.
GENERALLY DRY THURSDAY THROUGH FRIDAY. WEAK DISTURBANCE TOMORROW
MAY PRODUCE AN ISOLATED HIGHER TERRAIN SHOWER/SPRINKLE UP NORTH
BUT PROBABLY NOT MUCH MORE. MID LEVEL TEMPERATURES COOL A LITTLE
BEHIND OUR SYSTEM TONIGHT SO THURSDAY WILL NOT BE AS WARM AS TODAY
BUT STILL MILD MOST AREAS AND THEN A REBOUND FRIDAY CLOSE TO
TODAY`S READINGS. ALL ATTENTION THEN TURNS TO OUR UPCOMING MAJOR
CHANGE TO COLD AND SNOW FOR THIS WEEKEND...AT LEAST FOR A BIG
CHUNK OF THE AREA. FOR SATURDAY...IT LOOKS LIKE THE COLD FRONT
WILL MOVE INTO THE NORTHERN BORDER AREA IN THE EARLY MORNING HOURS
AND COULD MAKE IT INTO CENTRAL WYO BY SATURDAY MORNING. THE TROUGH
DYNAMICS AND MOST OF THE FORCING LOOKS TO HOLD OFF UNTIL LATER
AFTERNOON OR JUST BEYOND THIS PERIOD SO KEEPING 20-30 POPS IN
CENTRAL WYOMING THROUGH 00Z. MAIN ACTION IN CENTRAL WYO SHOULD GET
STARTED JUST BEYOND THIS PERIOD. TEMPERATURES WILL BE TRICKY
ESPECIALLY IF THE SHALLOW FRONT DOES MAKE IT INTO THE WIND RIVER
BASIN AND NATRONA COUNTY SATURDAY MORNING OR TOWARDS MIDDAY. STILL
MILD SOUTH AND SOUTHWEST AHEAD OF THIS FRONT. EVEN TEMPERATURES IN
THE CODY FOOTHILLS ARE QUITE TRICKY AS EARLY FRONTAL PASSAGE WITH
COLD AIR RUSHING IN ALL DAY WITH FORCING ARRIVE LATE IN THE DAY
COULD HELP DEVELOPING RAIN CHANGE TO SNOW NEAR THE END OF THIS
PERIOD POTENTIALLY. WE CAN ADJUST THIS AND GO COLDER WITH TIME
DURING THE DAY TIME IF THIS POSSIBILITY SEEMS MORE CERTAIN. RIGHT
NOW GOING LOWER TO MID 40S FOR HIGHS SATURDAY WITH RAIN BUT IT`S
NOT IMPOSSIBLE THAT IT COULD CHANGE TO SNOW THAT FAR NORTH. EITHER
WAY...DRAMATIC CHANGE BACK TO WINTER CONDITIONS ARE LOOKING MORE
AND MORE CERTAIN FOR THE FAR NORTH AND AREAS EAST OF THE DIVIDE
BEGINNING SATURDAY AND CONTINUING INTO THE MEDIUM RANGE PERIOD. AN
SPS WILL BE ISSUED FOR THE HIGHEST IMPACTED AREAS...MAINLY CENTRAL
WYO.

.LONG TERM...SATURDAY NIGHT THROUGH WEDNESDAY
BY 00Z SUNDAY THE ANTICIPATED WEEKEND SNOW EVENT SHOULD BE WELL
UNDERWAY WITH ISENTROPIC LIFT STREAMING NORTH OF THE SOUTHBOUND COLD
FRONT AND RIGHT ENTRANCE REGION DYNAMICS FROM THE 90 KNOT JET
SEEDING THE LOWER DECK OF LOW LEVEL UPSLOPE STRATOCU BENEATH
RESULTING IN ENHANCED STRATIFORM SNOWFALL ACROSS MUCH OF THE AREA
EAST OF THE DIVIDE SATURDAY NIGHT AND INTO SUNDAY
MORNING...ESPECIALLY IN UPSLOPE FAVORED AREAS. THE GFS AND THE EURO
HAVE PRETTY MUCH COME INTO AGREEMENT AS FAR AS TAKING THE BULK OF
THE ENERGY/VORT MAX WITH THIS SHORTWAVE SOUTH SOUTHEAST ALONG THE
IDAHO/WY BORDER...A GOOD SET UP FOR LANDER TO RECEIVE SUBSTANTIAL
SNOWFALL...IF NO WHERE ELSE...AS FAR AS THE LOWER ELEVATIONS ARE
CONCERNED. FOR NOW...HAVE PAINTED MAINLY SNOW ADVISORY AMOUNTS FOR
THE MOUNTAINS EAST OF THE DIVIDE AS WELL AS THE LOWER ELEVATIONS
ADJACENT TO THE MOUNTAINS ARCHING FROM LANDER TO CASPER. THIS SEEMS
REASONABLE IF YOU CONSIDER WE WILL HAVE ABOUT 12 HOURS TO WORK
WITH...NAMELY FROM 06Z TO 18Z SUNDAY...TO ACCUMULATE A HALF INCH OF
SNOW AN HOUR ONCE H7 TEMPS COOL ENOUGH FOR MOST OF THE PRECIP TO
FALL AS SNOW. H7 TEMPS WILL DROP TO BETWEEN -8C TO -9C BY 06Z SUNDAY
IN NORTHERN AND CENTRAL WY EAST OF THE DIVIDE AND TO -14C IN JOHNSON
CO THERE BY 18Z SUNDAY WITH -12C IN NORTHWEST WY ACDG TO THE ECMWF
WHICH IS ACTUALLY NOT AS COLD AS PREVIOUS RUNS BUT COLD ENOUGH FOR
ALL SNOW. THE SNOWFALL WILL DECREASE SUNDAY AFTERNOON AND EVENING.
BY LATE SUNDAY NIGHT...PATCHY FOG WILL LIKELY FORM IN THE CENTRAL
BASINS AFTER SKIES CLEAR AND WARM AIR ALOFT STREAMS IN STRENGTHENING
THE NOCTURNAL INVERSIONS. THE FOG SHOULD LAST THROUGH ABOUT MID
MORNING MONDAY. OTHERWISE EXPECT WARMER AND DRIER CONDITIONS MONDAY.
ON TUESDAY THE NEXT UPSTREAM TROUGH WILL DELIVER MORE PRECIP TO
WESTERN WY WITH THE ASSOCIATED COLD FRONT SPREADING THE PRECIP TO
AREAS EAST OF THE DIVIDE TUESDAY NIGHT AND WEDNESDAY WITH ISENTROPIC
LIFT GETTING UNDERWAY NORTH OF THIS NEXT FRONT INCREASING THE
CHANCES OF PRECIP.

&&

.AVIATION.../00Z ISSUANCE/
WEST OF THE DIVIDE...KJAC/KPNA/KBPI/KRKS ROUTES

VFR CONDITIONS WILL OCCUR AT THE TERMINAL SITES THROUGH TONIGHT.
GUSTY WIND WILL OCCUR AT MOST SITES UNTIL 02Z. SCATTERED RAIN
SHOWERS WITH HIGH ELEVATION SNOW SHOWERS WILL OCCUR THROUGH 05Z AS A
COLD FRONT MOVES THROUGH THE REGION THIS EVENING PUSHING THE SHOWERS
SOUTH FROM MT TO JACKSON. A FEW THUNDERSTORMS MAY ALSO OCCUR. LOCAL
MVFR CONDITIONS ARE POSSIBLE IN THE NORTHWEST WY MOUNTAINS THROUGH
05Z.

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

VFR CONDITIONS WILL OCCUR AT THE TERMINAL SITES THROUGH TONIGHT. A
GUSTY WIND WILL OCCUR AT MOST TERMINAL SITES THROUGH 01Z. A COLD
FRONT WILL PUSH INTO NORTHERN WY BY 22Z WITH A WIND SHIFT TO THE
NORTH. THIS FRONT WILL PUSH SOUTH INTO CENTRAL WY AFTER 01Z WITH A
NORTH WIND DEVELOPING. SCATTERED RAIN SHOWERS WILL OCCUR OVER THE
NORTH AFTER 22Z AND CONTINUE THROUGH 06Z. A FEW THUNDERSTORMS MAY
ALSO OCCUR OVER THE NORTH LATE IN THE AFTERNOON AND EVENING. A FEW
RAIN SHOWERS MAY OCCUR AROUND KCPR AFTER 03Z UNTIL 08Z THURSDAY.

&&

.FIRE WEATHER...
A COLD FRONT WILL CONTINUE TO SLIDE SOUTH FROM FAR NORTHERN
WYOMING INTO CENTRAL WYOMING THROUGH THE EVENING HOURS WITH A
SHARP WIND SHIFT FROM A GENERALLY WESTERLY DIRECTION TO A
NORTHERLY DIRECTION WITH STRONG WIND GUSTS CONTINUING INTO THE
EVENING. THE FRONT WILL BE ACCOMPANIED BY SHOWERS AND ISOLATED
THUNDERSTORMS WITH WIND GUSTS OVER 40 MPH POSSIBLE NEAR ANY
SHOWERS. LOW RH`S AHEAD OF THE COLD FRONT WILL KEEP ELEVATED FIRE
DANGER INTO THE EVENING FROM CENTRAL INTO SOUTHWESTERN WYOMING
WHERE RELATIVE HUMIDITIES WILL BE IN THE TEENS TO LOWER 20S WITH
WEST WINDS OF 25 TO 40 MPH. MANY AREAS WILL BECOME WINDY AGAIN
THURSDAY WITH A GENERAL WEST TO NORTHWEST DIRECTION.

&&

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

&&

$$

SHORT TERM...SKRBAC
LONG TERM...LIPSON
AVIATION...LIPSON
FIRE WEATHER...SKRBAC








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