Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 030604
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
1105 PM MST TUE FEB 2 2016

.SHORT TERM...TONIGHT THROUGH WEDNESDAY

THE CLOSED LOW THAT BROUGHT SIGNIFICANT AMOUNTS OF SNOW MAINLY TO
AREAS EAST OF THE DIVIDE...WITH THE HEAVIEST AMOUNTS AROUND THE
CASPER AREA...HAS RAPIDLY SCOOTED ON OUT TO SOUTHEAST NEBRASKA.
THE WESTERN EDGE OF THE ASCT STRATIFORM CLOUDINESS IS STILL
EXITING THE SOUTHEAST HALF OF THE CWA. LINGERING LIGHT SNOW
CONTINUES IN CPR AND RKS BUT SHOULD BE OVER WITHIN A COUPLE OF
HOURS. SMALL CONVECTIVE CELLS ARE ALREADY SEEN FORMING WITHIN THE
CLEARING SKIES BEHIND AND TO THE NORTHWEST OF THIS MASS OF
CLOUDINESS. THIS IS IN RESPONSE TO MINIMAL SFC HEATING BEHIND
THESE CLOUDS BUT MAINLY DUE TO COOLING ALOFT AHEAD OF THE NEXT
WEAK SHORTWAVE RIDING SOUTHEAST ALONG THE BACKSIDE OF THE MAIN
TROUGH. STEEP LAPSE RATES WILL BE ASCT WITH THIS FEATURE. THIS IS
SETTING UP TO BE A POTENTIALLY CLASSIC CASE FOR CONVECTIVE SNOW
BANDING TONIGHT WITH THE H7 LOW OVER THE CENTRAL PLAINS TONIGHT
ALONG WITH NW FLOW AT THE MID LEVELS AND AN UNSTABLE ATMOSPHERE.
HAVE ALREADY SENT AN SPS CONCERNING THE POTENTIAL FOR LOCALIZED
ACCUMULATING AREAS OF SNOW FOR CENTRAL WY...MAINLY EAST OF THE
DIVIDE...HITTING THE TYPICAL AREAS LIKE THE LEE OF THE OWL
CREEK/BRIDGER MTNS WITH THESE SNOW BANDS EXPANDING SOUTHEAST TO
POTENTIALLY AFFECT THE NORTHERN AND EASTERN WIND RIVER BASIN AS
WELL AS NATRONA COUNTY INCLUDING CASPER. ONCE THESE CELLS
FORM...THEY COULD REMAIN STATIONARY FOR A FEW HOURS UNTIL THE
SHORTWAVE CONTINUES TO EXIT TO THE EAST. THE SPS WAS WRITTEN WITH
AN ELEMENT OF UNCERTAINTY FOR TIMING AND AMNTS. SOME SNOW SHOWERS
WILL ALSO OCCUR IN WESTERN WY TONIGHT WITH THIS FEATURE. THEN ON
WEDNESDAY...THERE WILL BE A BRIEF BREAK IN THE SNOW SHOWER
ACTIVITY IN THE FAR WEST BEFORE WARM AIR ADVECTION/STRATIFORM
LIGHT SNOW TAKES OVER OUT THERE DURING THE DAY. SOUTHWEST WINDS
WILL INCREASE WED AROUND THE CASPER AREA INCREASING THE CHANCE
FOR BLOWING SNOW IN THAT AREA AS 10 TO 20 INCHES OF SNOWFALL WAS
COMMON IN THOSE AREAS DURING THE LAST 24 HOURS WITH PLENTY TO BLOW
AROUND. OTHERWISE...TEMPS WILL REMAIN SEASONABLY COLD WED ALONG
WITH DRY CONDITIONS EXPECTED WED. WEST WINDS WILL LIKELY BEGIN TO
INCREASE IN THE ABSAROKA MTNS AND CODY FOOTHILLS LATE.

.LONG TERM...WEDNESDAY NIGHT THROUGH TUESDAY

PERIOD STARTS OFF WITH POTENTIAL FOR HIGH WINDS ALONG THE CODY
FOOTHILLS DUE TO DECENT GAP FLOW SIGNATURE WITH 10-15C H7 GRADIENT.
WHAT`S EVEN MORE POTENTIALLY SIGNIFICANT IS THE MOUNTAIN WAVE
SIGNATURE. WRFNMM AND WRFARW ARE SHOWING A NEAR OR SLIGHTLY ABOVE
STRONG MTN TOP (H7) INVERSION WITH ABOUT 30-35KTS OF FLOW INTO THE
BARRIER. SOME GUIDANCE IS A LITTLE LIGHTER WHICH IS POTENTIALLY
BOTHERSOME. WE USUALLY NEED AT LEAST 30KTS INTO THE BARRIER TO GET
DECENT MTN WAVE. FROUDE CALCULATIONS ARE NEAR ONE OR A LITTLE UNDER
DEPENDING ON THE BARRIER HEIGHT INPUT AND WIND SPEED. CROSS SECTIONS
ARE ALSO SHOWING POTENTIAL WAVE BREAKING ON THE LEE OF THE
ABSAROKAS. WITH THE PEAK IN THE TRADITIONAL LATE NIGHT HOURS. GAP
FLOW HIGH WIND COULD OCCUR EARLIER. WILL ISSUE A HIGH WIND WATCH FOR
THE CODY FOOTHILLS FROM 00Z-18Z THU FOR THIS DOUBLE (GAP AND MTN
WAVE) HIGH WIND POTENTIAL. SPLITTING TROUGH IS STILL APPROACHING ON
THURSDAY WITH SOME LIGHT UPGLIDE SNOW POTENTIAL IN THE WEST AHEAD OF
IT LATE WED NGT AND THUR MORNING BEFORE MORE SHOWERY SNOW ARRIVES
DURING THE AFTERNOON AND EVENING. STILL SOME SIGNIFICANT DIFFERENCES
ON THE DETAILS SO STICKING WITH IDEA OF SOME SNOW MOVING ACROSS THE
I80 CORRIDOR AS EARLY AS THURSDAY AFTERNOON AND MORE OF CENTRAL
WYOMING THURSDAY NIGHT...ESP NORTH AND EAST. RIDGING THEN MOVES IN
FRIDAY BUT FLATTENS QUICKLY FRIDAY NIGHT LEADING TO RAPIDLY
INCREASING WINDS IN THE NRN MTNS WITH MORE HIGH WIND POTENTIAL ALONG
THE CODY FOOTHILLS FRIDAY NIGHT INTO SATURDAY. BEEFED UP WIND THEN
QUITE A BIT. BEYOND THAT...MODELS ARE MORE IN AGREEMENT WITH A SHORT
LIVED COLD BLAST LATE SAT INTO SAT NIGHT WITH RAPID MID LEVEL
WARMING BY SUNDAY ALREADY CONTINUING INTO EARLY NEXT WEEK AS RIDGE
STRENGTHENS NEAR OR SLIGHTLY INLAND ALONG THE WEST COAST. GENERALLY
LOOKS DRY EARLY NEXT WEEK. THE GFS AND GEM HAVE QUITE A BIT OF PCPN
WITH A WEAK SHORTWAVE IN AN UPGLIDE PATTERN ALONG THE BIGHORNS AND
NERN ZONES MON/MON NGT WHILE THE ECMWF IS DRY. WILL KEEP OUR CHANCE
POPS UP THERE AND SEE HOW THIS EVOLVES AS WE GET A LITTLE CLOSER

&&

.AVIATION.../06Z ISSUANCE/

ISOLATED SNOW SHOWERS ASSOCIATED WITH A SHORTWAVE DIVING SOUTHEAST
ACROSS THE COWBOY STATE...WILL COME TO AN END BY AROUND 15Z. BRIEF
IFR/MVFR CONDITIONS ARE POSSIBLE FROM THIS SHOWER ACTIVITY. THIS
WOULD PRIMARILY IMPACT TERMINALS EAST OF THE DIVIDE AND WILL HAVE
ONLY VCSH IN A FEW EASTERN TERMINALS DUE TO THE ISOLATED NATURE OF
THE SNOW SHOWERS. THERE COULD ALSO BE PATCHY MORNING FOG IN A FEW
PLACES BOTH WEST AND EAST...BUT CLOUD COVER AND MARGINAL BOUNDARY
LAYER MOISTURE SHOULD LIMIT COVERAGE AND WILL KEEP ALL MENTION OUT
OF AREA TERMINALS FOR NOW.  IN THE AFTERNOON EXPECT ONLY SOME
MID/HIGH LEVEL CLOUDINESS AND DRY CONDITIONS. THE MAIN EXCEPTION
WILL BE SOME LOW CEILINGS OVER THE WESTERN MOUNTAINS WITH SOME LIGHT
SNOW OR FLURRIES...BUT THIS ACTIVITY IS NOT EXPECTED TO IMPACT
WESTERN TERMINALS. BREEZY WEST TO SOUTHWEST ARE EXPECTED IN KRKS-
KCPR CORRIDOR.  A FAST MOVING STORM SYSTEM WILL APPROACH THE AREA
TONIGHT. THE MAIN IMPACT WILL BE INCREASING MOUNTAIN TOP WINDS AND
BREEZY TO WINDY CONDITIONS...AND/OR SOME LLWS ON THE LEE SIDE OF
MOUNTAINS RANGES. AFTER 06Z...COULD SEE SOME SNOW SHOWERS AND MVFR
CEILINGS DEVELOP OVER THE FAR WEST...BUT MUCH IF NOT ALL OF THE
ACTIVITY SHOULD HOLD OFF UNTIL AFTER 12Z SATURDAY.

&&

.FIRE WEATHER...

THERE WILL BE SOME INVERSIONS RETURNING TO VALLEYS AND BASINS
AFTER TONIGHT...MAINLY IN SOUTHWEST WYOMING CLOSER TO WHERE THE
SURFACE HIGH PRESSURE CENTER SETS UP. ACROSS THE REST OF WESTERN
AND CENTRAL WYOMING...SMOKE DISPERSAL WILL IMPROVE WITH A
DESTABILIZING AIRMASS AS AN UPPER LEVEL DISTURBANCE TRACKS
SOUTHEAST ACROSS THE AREA TONIGHT THAT WILL PRODUCE SCATTERED SNOW
SHOWERS. THERE WILL BE A FEW ADDITIONAL SHOTS AT SOME MORE SNOW
AS THE WEEK MOVES ALONG BUT NOTHING SIGNIFICANT ON THE HORIZON. A
STRONG RIDGE COULD BE BUILDING IN NEXT WEEK...WITH DRIER AND
CALMER WEATHER POSSIBLE...IN ADDITION TO SOME STRONG INVERSIONS IN
ANY SNOW COVERED VALLEYS OR BASINS.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
HIGH WIND WATCH FROM WEDNESDAY AFTERNOON THROUGH THURSDAY
MORNING FOR WYZ003.

&&

$$

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



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