Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 220618
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
1118 PM MST SAT FEB 21 2015

.UPDATE...

06Z AVIATION DISCUSSION ADDED.


HAVE REMOVED THE NORTHERN PORTION OF THE WINTER HIGHLIGHTS /BIG
HORN BASIN...BIGHORN MTS AND JOHNSON COUNTY...AS WELL AS THE
HIGHLIGHTS OVER THE EXTREME WEST /TETON MTS & STAR VALLEY/ AS SNOW
HAS ENDED AND WINDS HAVE WEAKENED.

WSW IS ALREADY OUT. ALSO A PRELIMINARY PNS HAS BEEN ISSUED FOR
THIS STORM.


&&

.PREV DISCUSSION... /ISSUED 609 PM MST SAT FEB 21 2015/

UPDATE...

DECIDED TO EXPAND WINTER WEATHER ADVISORY TO INCLUDE MUCH OF
SOUTHWEST WYOMING...BASED ON CURRENT RADAR TRENDS...WEBCAMS...AND
OBSERVATIONS. ALSO...WYDOT JUST CLOSED INTERSTATE 80 BETWEEN
EVANSTON AND LITTLE AMERICA. LOOKS LIKE THE HEAVIEST SNOWFALL WILL
OCCUR THROUGH 9 PM...BUT WINDS WILL KEEP SIGNIFICANT BLOWING SNOW
AND PROBABLY HAZARDOUS TRAVEL ONGOING THROUGH THE NIGHT ESPECIALLY
OVER SWEETWATER COUNTY AS WELL AS SOUTH PASS ESPECIALLY EAST OF
ATLANTIC CITY...WHERE WHITEOUT CONDITIONS WILL BE POSSIBLE.

PREV DISCUSSION... /ISSUED 253 PM MST SAT FEB 21 2015/

SHORT TERM...TONIGHT THROUGH SUNDAY

SURFACE PRESSURE CHANGE ANALYSIS SHOWS A COUPLE OF SURGES IN THE
COLD HIGH PRESSURE...ONE COMING THROUGH NOW AND A SECOND COMING
OUT OF ALBERTA AND SASKATCHEWAN INTO NORTHERN MONTANA WITH THE
COLDEST AIR BEHIND THE CANADIAN SURGE.  MOST AREAS RECEIVING SNOW
WITH BRISK NORTH NORTHEAST WINDS OVER THE OPEN AREAS. SNOWFALL
REPORTS MAINLY IN THE 3 TO 5 INCH RANGE SO FAR AND SHOULD SEE A
GOOD PERIOD OF SNOW BEFORE THE PASSAGE OF THE SECOND PRESSURE SURGE.
THIS SHOULD BRING IN THE REAL COLD AIR AND END THE ACCUMULATING
SNOWFALL. LOW STRATUS AND FOG SHOULD BE COMMON OVER THE AREA AS
THE WIND DECREASES LATE TONIGHT AND IN THE EARLY MORNING WITH
STRONG INVERSIONS.  SHOULD SEE THE COLDEST AIR DOWN TO 15 BELOW IN
YELLOWSTONE NP. MANY AREAS WILL BE NEAR ZERO IN THE MORNING. HIGHS
WILL BE COLD WITH UPPER SINGLE DIGITS TO MID TEENS. WILL LEAVE
WARNINGS AND ADVISORIES AS IS FOR NOW WITH THE ABSAROKA MOUNTAINS
AND CODY FOOTHILLS EXPIRING AT 00Z AND THE REST AT 12Z.

LONG TERM...SUNDAY NIGHT THROUGH SATURDAY

A RELATIVELY QUIET BEGINNING OF THE FORECAST PERIOD WITH THE
DEPARTURE OF THE SYSTEM WHICH HAS BROUGHT SNOWFALL TO MUCH OF THE
FORECAST AREA. BEFORE ITS OFFICIAL DEPARTURE...A FEW LINGERING
SHOWERS WILL BE POSSIBLE ACROSS SOUTHERN WYOMING WITH PERIODS OF
GUSTY WIND. ANY PRECIPITATION IS NOT EXPECTED TO BE SIGNIFICANT
ACROSS THE AREA. TEMPERATURES WILL ALSO BE ABLE TO WARM ACROSS THE
AREA.

ONCE THIS SYSTEM RELEASES ITS INFLUENCE ON WESTERN AND CENTRAL
WYOMING...BUILDING NORTHERLY FLOW EXPECTED. HOWEVER...A DEVELOPING
AND STRENGTHENING AREA OF LOW PRESSURE OVER THE DESERT SOUTHWEST
WILL DISRUPT THE OVERALL FLOW PATTERN. THIS WILL LEAVE WIDESPREAD
LIGHT WIND ACROSS THE AREA TUESDAY AFTERNOON WITH PRECIPITATION
EXPECTED TO REMAIN OVER THE DESERT SOUTHWEST LEAVING WIDESPREAD DRY
CONDITIONS ACROSS THE FORECAST AREA.

AS THE AREA OF LOW PRESSURE DISINTEGRATES TUESDAY
EVENING...NORTHWEST FLOW WILL BE ABLE TO RETURN TO THE AREA.
MOISTURE IS EMBEDDED IN THIS FLOW...RESULTING IN SOME APPROACHING
SHOWERS. NORTHERN WYOMING WILL SEE THE FIRST OF THESE
SHOWERS...WHOSE COVERAGE WILL INCREASE THROUGH WEDNESDAY.

BY WEDNESDAY AFTERNOON...MORE WIDESPREAD PRECIPITATION AND COOLING
TEMPERATURES APPEAR FAVORABLE ACROSS THE FORECAST AREA...AS MOST
MODELS REMAIN IN FAIR AGREEMENT WITH THE ARRIVAL OF THIS SYSTEM.
SHOWERS WILL REMAIN POSSIBLE THROUGH THURSDAY EVENING WHEN A DRIER
NORTH FLOW STARTS TO RETURN TO THE AREA.

AS THE FLOW SHIFTS A BIT MORE NORTHWESTERLY FRIDAY
AFTERNOON...ANOTHER PUSH OF MOISTURE WILL MOVE INTO WESTERN WYOMING.
MODELS BEGIN TO HAVE MORE DISAGREEMENT WITH THE APPROACH OF THIS
LATE SYSTEM...WITH THE GFS INDICATING SOME WIDESPREAD SHOWER
ACTIVITY...AND THE ECMWF PAINTS A LESS DIRE PICTURE WITH ONLY
SCATTERED SHOWERS OVER THE AREAS WEST OF THE CONTINENTAL DIVIDE.
THESE DISAGREEMENTS CONTINUE THROUGH SATURDAY NIGHT.

AVIATION.../06Z ISSUANCE/

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

ONGOING AREAS OF LIGHT SNOW/FLURRIES WITH MVFR CEILINGS OR VSBY
RESTRICTIONS WILL IMPROVE FROM NORTH TO SOUTH LATE TONIGHT INTO
SUNDAY MORNING. CONFIDENCE IS HIGHER FOR THIS TO OCCUR FOR
THE BIG HORN BASIN AND JOHNSON COUNTY...THAN ACROSS NATRONA COUNTY
AND THE WIND RIVER BASIN. HOWEVER WILL CONTINUE TO LIFT EVEN THOSE
TERMINALS TO VFR DURING SUNDAY MORNING...AND LATER SHIFTS CAN AMEND
TO MVFR CEILINGS IF NECESSARY. WINDS WILL GENERALLY BE RELATIVELY
LIGHT NORTHERLY TODAY. DESPITE MAINLY CLEAR SKIES AND LIGHT WINDS
TONIGHT...FOG CHANCES ARE LIMITED DUE TO THE DRY AIR MASS IN PLACE

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

ONGOING AREAS OF LIGHT SNOW/FLURRIES WITH MVFR CEILINGS OR VSBY
RESTRICTIONS OVER THE UPPER GREEN RIVER BASIN AND SWEETWATER COUNTY
WILL IMPROVE IN MOST AREAS LATE TONIGHT INTO SUNDAY MORNING.
HOWEVER...VERY WINDY WESTERLY WINDS OVER SWEETWATER COUNTY WILL KEEP
SOME BLSN IN THAT AREA...BUT NOT ENOUGH TO WARRANT A RESTRICTION AT
RKS TERMINAL AFTER 12Z. RKS HAS THE BEST CHANCE OF KEEPING MVFR OR
LOW VFR CEILINGS THROUGH MUCH OF THE DAY...BUT HAVE DECIDED TO KEEP
LOW VFR CEILINGS FOR MUCH OF THE DAY SUNDAY. VFR CONDITIONS ARE
EXPECTED OVER JACKSON HOLE WITH A STRATOCU DECK BETWEEN 030-050 KFT
AGL AT TIMES...WITH A NORTHERLY WIND.

FIRE WEATHER...

MUCH OF THE AREA WILL HAVE SNOW TONIGHT TAPERING OFF BY MORNING
WITH SNOW COVERING FUELS. TEMPERATURES WILL BE VERY COLD WITH HIGH
RELATIVE HUMIDITY. AREAS OF BRISK NORTH NORTHEAST WINDS WILL
GRADUALLY DECREASE BY MORNING. MIXING HEIGHTS WILL BE VERY LOW
WITH STRONG INVERSIONS SO SMOKE DISPERSAL WILL BE POOR.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
WINTER WEATHER ADVISORY UNTIL 5 AM MST SUNDAY
WYZ014-016-017-019-020-022-025>030.

WINTER STORM WARNING UNTIL 5 AM MST SUNDAY WYZ015-018.

&&

$$

UPDATE...WM
SHORT TERM...C.BAKER
LONG TERM...BRANHAM
AVIATION...WM
FIRE WEATHER...C.BAKER









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