Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 252010
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
210 PM MDT SAT APR 25 2015

.SHORT TERM...TONIGHT THROUGH SUNDAY

IMAGERY SHOWING RIDGE E OVR THE CNTRL AND NRN PLAINS...DEEP TROF...
WITH CLOSED LOW CENTER OVR WA...AND AXIS ORIENTED FROM WA DOWN THRU
SRN CALIFORNIA. EJECTING SW CURRENTLY MOVING N AND E ACROSS ERN
WY...WITH THUNDERSTORMS FIRING. NEXT WAVE UPSTREAM LOCATED OVR NV
AND ASSOCIATED WITH MUCH BETTER MOISTURE...WILL ARRIVE LATER THIS
EVENING AND OVERNIGHT. SFC SHOWS LOW PRESSURE FROM ERN MT SWD INTO
CNTRL WY AND THEN INTO ERN CO. WARM FRONT ERN WY WHERE CURRENT
THUNDERSTORM ACTIVITY IS TAKING PLACE...COLD FRONT S INTO CNTRL
WY...TRAILING OFF INTO NERN CO THEN E INTO KA.

2 TO 4 INCHES OF SNOWFALL OCCURRED OVERNIGHT ACROSS THE HIGHER
MOUNTAINS OF THE WEST...INCLUDING TETON PASS...WITH MORE SNOW LIKELY
LATER TONIGHT. OTHERWISE...MODEST INSTABILITY WILL KEEP ISOLATED
WIDELY SCATTERED SHOWERS AND MAYBE AN THUNDERSTORM OR TWO AROUND
THROUGH THE AFTERNOON. HOWEVER...THE MAIN SHOW BEGINS TONIGHT AS THE
NEXT STRONGER WAVE...COPIOUS MOISTURE...MODEST JET...AND UPR
DIFLUENCE ARRIVE WHILE A COLD FRONT PUSHES S ACROSS THE FA THIS
EVENING AND OVERNIGHT. WHILE SNOW LEVELS WILL START OUT ABOVE 10000
FEET EARLY...THEY WILL DROP TO 7000 - 7500 FEET OVERNIGHT AS THE
MAIN TROF CONTINUES ITS EWD PUSH. GOOD MOIST UPGLIDE OVR THE SFC AND
MID LVL FRONT WILL ALLOW MOUNTAIN QPF VALUES TO APPROACH 1 INCH OR
MORE FROM THE SRN ABSAROKA AND TETONS...S OVR THE WYOMING...WIND
RIVER AND SALT MOUNTAINS. H7 TEMPS...ALTHOUGH TRICKY TO PIN POINT AS
THE MAIN UPR TROF BECOMES NEGATIVELY TILTED...LOOKS TO DROP TO
BETWEEN -3 AND -5 DEG C ACROSS THE WEST AT LEAST THROUGH THE MORNING
HRS. AS BEST MOISTURE AND LOWEST TEMPS ALIGN OVERNIGHT TONIGHT AND
THRU SUNDAY MORNING...WW ADVISORY CONDITIONS WILL CERTAINLY BE
POSSIBLE EVEN IF SNOW IS VERY WET OR MIXED WITH RAIN FOR A FEW HRS.
HAVE ADDED A FEW ADDITIONAL HIGHLIGHTS TO THE CURRENT
ADVISORY...INCLUDING MOUNTAIN ZONES 2 12 AND 24...AND LOWER
ELEVATION ZONES 16 AND 25 INCLUDING BONDURANT AND DUBOIS...WITH MUCH
OF THIS AREA GETTING ADVISORY LEVEL SNOW IN RATHER SHORT ORDER
BETWEEN MIDNIGHT TONIGHT AND NOON SUNDAY. HAVE CHOSEN TO KEEP THE
BIGHORN MOUNTAINS AND CASPER MOUNTAIN OUT OF THE HIGHLIGHTS FOR NOW
AS UPPER LEVELS ARE INDICATED TO REMAIN ON THE WARM SIDE AT GREATER
THAN -1.5 AT 700 MB FOR MOST OF THE EVENT. WORTHY WATCHING
HOWEVER.

.LONG TERM...SUNDAY NIGHT THROUGH SATURDAY

SUNDAY NIGHT THROUGH SATURDAY...

BY SUNDAY EVENING...THE UPPER CIRCULATION WILL PROPAGATE RAPIDLY
SOUTH TO NEW MEXICO. DURING THIS TIME FRAME WE WILL STILL BE
EXPERIENCING SOME WRAP ISENTROPIC LIFT BEFORE MIDNIGHT JUST BEFORE
MOST OF THE ENERGY SHIFTS SOUTH. SWEETWATER COUNTY WILL LIKELY BE
THE LAST PLACE TO SEE SIGNIFICANT PRECIP BEFORE TAPERING OFF AS
ANOTHER FRONTAL SURGE MOVES SOUTH. THERE WILL BE SOME LINGERING CAPE IN
SWEETWATER EARLY SUNDAY EVENING AS WELL. POST FRONTAL 3 HOURLY
SURFACE PRESSURE RISES OF 7MB WILL INITIATE INCREASING NORTH WINDS
OVER MUCH OF THE AREA EAST OF THE DIVIDE ALONG WITH SWEETWATER
COUNTY. H7 TEMPS WILL BE AROUND -4C SO THE SNOW LEVEL SHOULD HOVER
AROUND 6500 OR 7K SUNDAY NIGHT. WILL LET THE WINTER WEATHER ADVISORY
RIDE THROUGH MIDNIGHT SUNDAY NIGHT FOR THE WIND RVR MTNS. WIND AND
SNOW WILL LIKELY MAKE FOR ADVERSE TRAVEL OVER SOUTH PASS. PRECIP WILL
END FROM NORTHWEST TO SOUTHEAST SUNDAY NIGHT WITH SOME LINGERING
PRECIP IN SOUTHERN NATRONA AND EASTERN SWEETWATER COUNTY THROUGH
ABOUT 15Z MONDAY. WILL NOT ADD ANY OTHER MTN ZONES TO THE WINTER
WEATHER ADVISORY AS THE BULK OF THE SNOWFALL WILL BE MOVING SOUTH OF
THE OTHER MTN RANGES DURING THE COURSE OF THE NIGHT.

ON MONDAY...A FLATTENING POSITIVELY TILTED RIDGE WILL BUILD IN FROM
THE WEST ENDING THE INTERRUPTION TO THE MILD SPRING WEATHER. THE
WARMING TREND WILL CONTINUE THROUGH TUESDAY WITH HIGHS IN THE 70S IN
MANY AREAS BY TUESDAY EAST OF THE DVD AND 60S WEST. THEN BY
WEDNESDAY...THE NEXT UPSTREAM TROUGH WILL APPROACH THE PACIFIC
COAST. AS THE TROUGH APPROACHES...THE SOUTHWEST FLOW AHEAD OF IT
WILL BEGIN TO DRAW IN LIMITED MOISTURE TO NW WY. WITH THE SCANT
MOISTURE IN PLACE COUPLED WITH INCREASING INSTABILITY ALONG WITH THE
INFLUENCE OF BEING UNDERNEATH THE RIGHT REAR QUAD OF THE NEXT JET
STREAK...WILL KEEP ISOLD THUNDERSTORMS IN NW WY FOR WED AFTERNOON. ON
THURSDAY WE WILL SEE AN INCREASING CHANCE OF THUNDERSTORMS IN
WESTERN WY AS THE TROUGH MARCHES ACROSS THE PACIFIC NW TOWARD IDAHO
AND MONTANA ALONG WITH WIDESPREAD NEGATIVED LIFTED INDICES. BY
FRIDAY MORNING...THE CHANCES OF THUNDERSTORMS WILL SPREAD TO AREAS
EAST OF THE DIVIDE AND WILL INCREASE WEST AS THE TROUPHPA TAKES
PLACE ALONG WITH A SFC LOW TRACKING EAST ACROSS MT WHICH WILL SLING
A WELL DEFINED PACIFIC SFC COLD FRONT ACROSS THE CWA AS WE SIT
UNDERNEATH THE LEFT REAR QUAD OF ANOTHER JET STREAK.

THE GFS HAS BACKED OFF ON THE SPEED OF THE NEXT UPSTREAM TROUGH AND
IS MORE SIMILAR TO PREVIOUS RUNS OF THE ECMWF IN HOLDING THE TROUGH
BACK OVER THE PAC NW WITH THE GFS CLOSING THE ASCT LOW JUST WEST OF
OREGON. WITH MOISTURE AND INSTABILITY INCREASING AGAIN OUT AHEAD OF
THIS FEATURE AND WITH THE MODELS SEEMINGLY COMING INTO MORE OF AN
AGREEMENT WITH THIS NEXT FEATURE...WILL NOT TINKER WITH POPS MUCH
FOR SATURDAY. TEMPS WILL BEGIN TO COOL SLIGHTLY FRIDAY AND
SATURDAY.

&&

.AVIATION.../00Z ISSUANCE/

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

VFR TO MVFR (KCOD) THIS EVENING...WITH A RETURN TO MVFR CONDITIONS
FOR ALL TERMINAL LOCATIONS OVERNIGHT...LASTING THROUGH THE FORECAST
PERIOD. TERMINALS WILL SEE PRECIPITATION AND LOWER CIGS SPREADING
INTO THE REGION FROM THE SOUTH...WITH MVFR CONDITIONS IN LOW CIGS
AND VIS RESTRICTIONS DUE TO PRECIPITATION...EVEN RAIN MIXING WITH OR
TURNING TO ALL SNOW AFTER 06Z. MOUNTAIN OBSCURATIONS WILL OBVIOUSLY
BECOME PREVALENT DURING THE SAME PERIOD OF TIME. OCCASIONAL IFR
CONDITIONS EXPECTED MOST TERMINALS AFTER 09Z...ALSO LASTING THROUGH
THE END OF THE FORECAST.

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

NEAR MVFR TO MVFR IN LOW CIGS AS SHOWERS AND ISOLATED THUNDERSTORMS
THIS EVENING. THEN CIGS AND VIS WILL DROP...BECOMING PRIMARILY
MVFR...TO OCCASIONAL IFR...IN LOW CIGS AND VIS...AS RAIN AND/OR SNOW
SHOWERS BECOME INCREASINGLY LIKELY AFTER 07Z. THESE CONDITIONS WILL
PERSIST UNTIL THE END OF THE FORECAST PERIOD IN ONE RESPECT OR
ANOTHER.

&&

.FIRE WEATHER...

FIRE DANGER LOW TODAY...REMAINING LOW THROUGH THE
WEEKEND AS SEASONALLY COOL AND MUCH MORE MOIST WEATHER TAKES OVER.
WIDELY SCATTERED SHOWERS WITH AN ISOLATED THUNDERSTORM OR TWO WILL
OCCUR THIS AFTERNOON AND EVENING. MUCH BETTER CHANCES FOR A WETTING
RAIN OR SNOW OCCUR TONIGHT...LASTING THROUGH SUNDAY FOR MOST
AREAS...AS A LARGE VIGOROUS DISTURBANCE ROLLS OVER THE FORECAST
AREA. THEN...A WARMING AND DRYING TREND BEGINS ON MONDAY...LASTING
THROUGH MOST OF THE WORK WEEK.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
WINTER WEATHER ADVISORY FROM 9 PM THIS EVENING TO MIDNIGHT MDT
SUNDAY NIGHT WYZ002-012-014-015-024.

WINTER WEATHER ADVISORY FROM MIDNIGHT TONIGHT TO NOON MDT SUNDAY
WYZ016-025.

&&

$$

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






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