Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 272036
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
236 PM MDT MON JUL 27 2015

.SHORT TERM...TONIGHT THROUGH TUESDAY

A WEAK STATIONARY FRONTAL BOUNDARY APPEARS TO BE DRAPED EAST TO
WEST ALONG THE OWL CREEK MTNS AND ALONG THE JOHNSON/NATRONA COUNTY
LINE. NORTH OF THE BOUNDARY DEW POINTS ARE IN THE 40S AND 50S AND
SOUTH OF THE BOUNDARY THEY ARE ONLY IN THE 20S. MEANWHILE A STRONG
PACIFIC COLD FRONT IS PLOWING ITS WAY EAST ACROSS FAR WESTERN WY
AND WILL EVENTUALLY  THE AIRMASS ACROSS ALL OF WESTERN AND
CENTRAL WY THIS EVENING AS THE NORTHERN THIRD OF WYOMING
EXPERIENCES A SFC FRONTAL OCCLUSION. AS THIS PACIFIC FRONT
PROGRESSES EAST...THE POTENTIAL FOR STRONG THUNDERSTORMS EXISTS
ACROSS NORTHERN WY IN THE AREA OF HIGHER DEW POINTS AND RESULTANT
HIGHER CAPES. SPC HAS THIS AREA UNDER MARGINAL SO LARGE HAIL AND
DAMAGING WINDS ARE A POSSIBILITY IN THESE AREAS THROUGH AT LEAST
EARLY THIS EVENING. SO FAR...SMALL HAIL HAS BEEN REPORTED AT
MAMMOTH AND DUBOIS AND AS FAR SOUTH AS CASPER.

THE OTHER AREA OF CONCERN IS THE ON GOING RED FLAG WARNING FOR
PORTIONS OF THE CENTRAL...EASTERN AND SOUTHERN PORTIONS OF THE CWA
WHERE MINIMUM RH VALUES RANGING FROM THE UPPER SINGLE DIGITS TO
THE LOW TEENS HAVE BEEN COMMON ALONG WITH WINDS GUSTING AS HIGH
AS 35 KNOTS FROM THE SOUTHWEST AHEAD OF THE APPROACHING PACIFIC
COLD FRONT WITH EVEN STRONGER SHIFTING WINDS ANTICIPATED WITH THE
FROPA. WITH 3 HOURLY SFC PRESSURE RISES ANTICIPATED TO BE UP TO
12MB...MANY AREAS COULD SEE POST FRONTAL WIND GUSTS OF UP TO 50
MPH. IN ADDITION...A HAINES INDEX OF SIX IS IN PLACE IN THESE
AREAS...NOT TO MENTION THE SMALL POSSIBILITY OF A DRY LIGHTNING
STRIKE. WILL LET THE RED FLAG RIDE THROUGH 10 PM THIS EVENING FOR
FIRE WEATHER ZONES 279...280...281...283...285 AND 300.

THE AFOREMENTIONED PACIFIC COLD FRONT IS BEING SLUNG BY A VIGOROUS
EASTBOUND SHORTAVE FOR THIS TIME OF YEAR WHICH HAS BEEN TRACKING
EAST ACROSS MONTANA. THIS TROUGH WILL CLOSE OFF OVER EAST CENTRAL
MONTANA BY 06Z TONIGHT AND BECOME ALMOST VERTICALLY STACKED. THERE
IS PLENTY OF COLD AIR FOR THIS TIME OF YEAR WRAPPING AROUND THE
BACKSIDE OF THIS LOW WITH H7 TEMPS EXPECTED TO DROP BETWEEN -2 AND
-3C IN NW WYOMING LATER TONIGHT. IN FACT...TEMPS ARE ALREADY ONLY
IN THE 50S IN THE FAR WEST BEHIND THE FRONT. WITH THESE COLD H7
TEMPS...SNOW LEVELS SHOULD DROP TO BETWEEN 7K AND 7500 FEET IN
YELLOWSTONE PARK LATER TONIGHT THROUGH TUESDAY MORNING WHERE UP
TO A HALF INCH OF SNOW COULD FALL JUST BEFORE THE WX SYSTEM PULLS
OUT TO THE EAST AND THE RESULTANT AREA OF PRECIP SHRINKS. AREAS
LIKE TOGWOTEE PASS COULD SEE A PERIOD OF SNOWFALL AROUND MIDNIGHT
TONIGHT WITH GRANITE AND POWDER RIVER PASSES TO FOLLOW. THE
SOUTHERN WINDS COULD SEE A VERY BRIEF PERIOD OF LIGHT WET SNOW
ABOVE 8500 FEET IN THE WEE HOURS TUESDAY MORNING BEFORE SUNRISE.

FOR TUESDAY...A COOLER BUT PLEASANTLY MILD AND SUNNY DAY IS IN
STORE. WEST NORTHWEST WINDS WILL DIMINISH AS THE MONTANA WEATHER
SYSTEM CONTINUES TO PULL AWAY.

.LONG TERM...TUESDAY NIGHT THROUGH MONDAY

HIGH PRESSURE OVER THE AREA WILL KEEP WARM AND DRY CONDITIONS
THROUGH MUCH OF THE FORECAST PERIOD. MODELS IN GOOD AGREEMENT WITH
THIS QUIET SOLUTION FOR WESTERN AND CENTRAL WYOMING.

CONDITIONS BEGIN TO CHANGE SUNDAY MORNING AS A WEAK DISTURBANCE
BEGINS TO MOVE TOWARDS THE STATE. THIS SYSTEM WILL GENERATE SOME
SHOWER AND THUNDERSTORM ACTIVITY SUNDAY AFTERNOON AND
EVENING...ESPECIALLY OVER NORTHERN AND WESTERN WYOMING. MODELS ARE
NOT IN GOOD AGREEMENT WITH THE PROGRESSION OF THIS SHOWER
ACTIVITY... BUT WILL LEAVE FORECAST TRENDING TOWARDS THE SOLUTION
FOR LOW COVERAGE OF SHOWERS AND THUNDERSTORMS. THE ECMWF INDICATES A
BETTER PUSH OF MOISTURE AND INSTABILITY ON MONDAY WITH EXPECTED
ZONAL FLOW. ISOLATED SHOWERS AND THUNDERSTORMS WILL BE POSSIBLE
ACROSS MUCH OF THE FORECAST AREA UNDER THIS SOLUTION. THE GFS IS NOT
AS AGGRESSIVE...WITH WEAKER MOISTURE AND INSTABILITY. UNDER ITS
SCENARIO...SHOWER COVERAGE WOULD CONTINUE TO BE FAIRLY LIMITED TO
THE NORTHERN AND WESTERN MOUNTAINS.

REGARDLESS...EXPECT GENERALLY WARMING CONDITIONS THROUGH MUCH OF THE
FORECAST PERIOD...WITH SLIGHTLY COOLER TEMPERATURES POSSIBLE SUNDAY
AND MONDAY.

&&

.AVIATION.../00Z ISSUANCE/

A STRONG PACIFIC COLD FRONT WILL CONTINUE TO PUSH INTO WEST AND
CENTRAL WYOMING THROUGH AREAS EAST OF THE DIVIDE LATER THIS
AFTERNOON AND EVENING. ISOLATED TO SCATTERED -TSRA WILL DEVELOP
ALONG THE FRONT MAINLY VCNTY AND NORTH OF A KJAC- KWRL-KBYG LINE
IN THE LATE AFTERNOON AND EVENING. SFC SW WINDS AT 20-35KTS WILL
PRECEDE THE FRONT WITH THE STRONGER WINDS FROM VCNTY KRKS-KCPR AND
ALONG THE EAST SLOPES FROM VCNTY KTHP-KLND. LLWS IN ADDITION TO
CONVECTIVE WIND SHEAR MAY BE ENCOUNTERED BEHIND THE COLD FRONT
VCNTY KCOD- KBYG THROUGH THE REMAINDER OF THE AFTERNOON WITH A
PREVAILING NORTHERLY SFC WIND AND SW WINDS 20-30KTS AT RIDGETOP
LEVEL UNTIL THE STRONG PACIFIC COLD FRONT ARRIVES FROM THE WEST.
SFC WEST WINDS AT 20-30KTS WILL FOLLOW THE COLD FRONT ACROSS
CENTRAL AND SOUTHERN WYOMING. WINDS WILL GRADUALLY DIMINISH
OVERNIGHT. THE NORTHERN ABSAROKA AND BIGHORN RANGES WILL LIKELY
REMAIN FQTLY OBSCURED WITH SCT -SHRA/-SHSN MONDAY NIGHT INTO
TUESDAY MORNING.

&&

.FIRE WEATHER...

...RED FLAG WARNING FOR ZONES 280 281 283 285 AND 300 THROUGH 10PM
TONIGHT...

JUST ABOUT EVERY ELEMENT WORTHY OF A RED FLAG WARNING WILL BE
PRESENT THROUGH THIS EVENING FROM WIDESPREAD STRONG WIND OVER 35
MPH ACROSS THE AREA WITH GUSTS AROUND 50 MPH POSSIBLE IN THE MORE
EXPOSED AND WIND PRONE AREAS...TO LOW HUMIDITY...TO DRY
THUNDERSTORMS AND A DRY COLD FRONT. AREAS EXPECTED TO BE MOST
AFFECTED WILL BE CASPER DISPATCH...SOUTHERN CODY DISPATCH...AND
RAWLINS DISPATCH AS THEY WILL HAVE THE MOST TIME AHEAD OF THE COLD
FRONT IN THE HOT AND DRY SOUTHWEST FLOW. MOST OF THE WET
THUNDERSTORM ACTIVITY WILL BE IN TETON DISPATCH WITH SEVERE
THUNDERSTORMS POSSIBLE IN NORTHERN CASPER AND CODY DISPATCH WITH
HAZARDS THAT RANGE FROM LARGE HAIL TO A TORNADO...WITH DAMAGING
DOWNBURST WINDS THE MOST LIKELY HAZARD THROUGH EARLY THIS EVENING.
IT WILL BE QUITE A BIT CALMER AND COOLER BEHIND THE FRONT FOR
TUESDAY BUT IT WILL REMAIN DRY...ESPECIALLY ACROSS THE SOUTHERN
THIRD OF THE STATE...WINDS WILL CALMER BUT A FEW GUSTS TO 25 MPH
CANNOT BE RULED OUT.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
RED FLAG WARNING UNTIL 10 PM MDT THIS EVENING FOR WYZ279>281-283-
285-300.

&&

$$

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


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