Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 272043
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
243 PM MDT FRI MAR 27 2015

.SHORT TERM...TONIGHT THROUGH SATURDAY NIGHT

THE HIGH PRESSURE RIDGE THAT IS PRODUCING THE MILD WEATHER AND
RELATIVELY LIGHT WINDS WILL BEGIN TO FLATTEN TONIGHT AS THE
VIGOROUS TROUGH NOW OFF THE PACIFIC COAST AT 130W APPROACHES THE
ROCKIES AND INCREASES THE PRESSURE GRADIENT. AS THE TROUGH NEARS
THE ROCKIES A SMALL BUT VIGOROUS CLOSED LOW WILL FORM AND MOVE
EAST ALONG THE MONTANA WYOMING BORDER SATURDAY. AS IT DOES...THE
GRADIENT WILL BECOME EVEN TIGHTER OVER THE CWA. THIS WILL RESULT
IN INCREASING WESTERLY WINDS ACROSS THE AREA SATURDAY. THESE WINDS
WILL COMBINE WITH AN RH IN THE LOW TEENS WHICH HAS PROMPTED THE
ISSUANCE OF RED FLAG WARNINGS ACROSS THE WIND RIVER BASIN...THE
LOWER ELEVATIONS OF NATRONA COUNTY AND JOHNSON COUNTY VALID FROM
18Z SATURDAY THROUGH 03Z SUNDAY. MOST OF THE MOISTURE WITH THIS
TROUGH WILL REMAIN WELL TO THE NORTH OF WY OVER MONTANA CLOSER TO
THE LOW CENTER BUT THERE WILL BE JUST ENOUGH MOISTURE FOR SPOTTY
LIGHT SHOWERS IN THE NW MTNS SATURDAY AFTERNOON. 13MB 3 HOURLY SFC
PRESSURE RISES ARE EXPECTED BEHIND THE ASSOCIATED COLD FRONT OVER
NATRONA/JOHNSON COUNTIES WHICH CORRELATE WELL WITH THE 34 KNOT
SUSTAINED WIND ON THE BYG NAM MOS GUIDANCE AT BYG FOR 00Z SUNDAY.
ISSUED A HIGH WIND WATCH FOR BOTH ZONES 10 AND 11 FOR THE LOWER
ELEVATIONS OF JOHNSON COUNTY VALID FROM 22Z SUNDAY THROUGH 06Z
SUNDAY. WE WILL HAVE AN OVERLAP OF A RED FLAG WARNING AND HIGH
WIND WATCH FOR JOHNSON COUNTY FROM 00Z THROUGH 03Z SATURDAY
EVENING. SOME HIGH TEMPERATURE RECORDS SHOULD BE SHATTERED
SATURDAY AHEAD OF THE FRONT. WE DECIDED TO HANDLE THE BORDERLINE
COMMONPLACE HIGH WINDS EXPECTED OVER THE ABSAROKA MTNS AND CODY
FOOTHILLS SATURDAY WITH A SPECIAL WEATHER STATEMENT AS WELL FOR
THE BIG HORN BASIN...EVEN THOUGH THE WINDS IN THE BASIN WILL BE A
BIT LIGHTER WITH A MENTION OF THE POTENTIAL DANGER ASCT WITH ANY
AGRICULTURAL BURNING GIVEN THE EXPECTED FIRE WEATHER
CONDITIONS...AND IT BEING THE WEEKEND.

.LONG TERM...SUNDAY THROUGH FRIDAY

MEDIUM-RANGE FORECAST MODELS ARE IN FAIRLY GOOD AGREEMENT WITH THE
OVERALL PATTERN ANTICIPATED OVER THE PERIOD. SOME DIFFERENCES IN
TIMING AND STRENGTH LATER NEXT WEEK...BUT OVERALL TRENDS ARE SIMILAR.
THE UPPER LEVEL DISTURBANCE WILL SWEEP EAST OF WYOMING BY 12Z/SUNDAY
WITH WEAK RIDGING ALREADY BUILDING IN FROM THE SOUTHWEST. THE RIDGE
AXIS REMAINS WEST OF THE STATE THROUGH MONDAY WITH WEST-NORTHWEST
FLOW PREVAILING ACROSS THE FORECAST AREA. CONDITIONS WILL BE DRY
BOTH DAYS WITH LESS WIND. FAVORED NORTHWEST VALLEY AND BASIN
LOCATIONS COULD STILL SEE SOME GUSTY LATE DAY AND EARLY EVENING
NORTHWEST WIND. 700MB TEMPERATURES MAKE A SLIGHT REBOUND MONDAY AND
HAVE NUDGED MAX TEMPS UPWARDS. RIDGE AXIS TRAVERSES THE FORECAST
AREA TUESDAY AS UPSTREAM TROUGH PUSHES INTO THE PACIFIC NORTHWEST.
AS A RESULT...SOUTHWEST FLOW WILL INCREASE ALOFT AND AT THE SURFACE.
WARM AIR ADVECTION COUPLED WITH LIKELY INCREASING SOUTHWEST SURFACE
WIND WILL ALLOW FOR TEMPERATURES TO CLIMB TO 10 TO 15 DEGREES ABOVE
NORMAL ON TUESDAY. RELATIVE HUMIDITIES AND WIND MAY BE SUFFICIENT
TUESDAY FOR POSSIBLE FIRE HIGHLIGHTS IN ZONES WITH CRITICAL FUELS.
MOST RECENT MODEL RUNS HAVE BACKED OFF SLIGHTLY ON SWEEPING A COLD
FRONT EAST THROUGH THE FORECAST AREA LATE TUESDAY AFTERNOON AND
EARLY EVENING. FRONTAL PUSH NOW COMES ABOUT 6 HOURS LATER WHICH WILL
HELP WESTERN AREAS STAY WARM THROUGH TUESDAY AFTERNOON. THIS DELAY
AND THE SLOWING APPROACH OF THE INCOMING SYSTEM WILL ALSO PUSH BACK
PRECIP CHANCES IN WEST/NORTHWEST WYOMING UNTIL TUESDAY NIGHT. 700MB
TEMPERATURES WILL COOL SIGNIFICANTLY BEHIND THIS COLD FRONT...
FALLING FROM +2C TO +5C TUESDAY TO -6C TO -10C WEDNESDAY. THE
ASSOCIATED UPPER LOW WILL STAY WELL TO OUR NORTH OVER THE SOUTHERN
CANADIAN PRAIRIE PROVINCES...SO ENTRENCHED UPSLOPE IS NOT LIKELY IN
THE CENTRAL BASINS WEDNESDAY. THE NORTHWEST AND BIGHORN MOUNTAINS
WILL SEE THE BEST CHANCE FOR ACCUMULATING SNOW. UNSTABLE NORTHWEST
FLOW ALOFT AND FAVORABLE MID-LEVEL LAPSE RATES COULD LEAD TO SOME
RAIN/SNOW SHOWERS OFF THE SOUTHERN ENDS OF THE AREA MOUNTAIN RANGES.
THIS WILL LIKELY DEPEND UPON THE SOUTHWARD EXTENT OF THE MID-LEVEL
MOISTURE. GUSTY NORTHWEST WIND EXPECTED WEDNESDAY IN NORTHERN
JOHNSON COUNTY AND THE NORTHERN BIG HORN BASIN. THE OVERALL TREND IS
FOR THE COOLER TEMPERATURES AND UNSETTLED CONDITIONS TO REMAIN FOR
THURSDAY AND FRIDAY. BOTH THE GFS AND ECMWF INDICATE A BIT MORE
CYCLONIC FLOW ALONG WITH SUFFICIENT MOISTURE TO GENERATE A BETTER
CHANCE OF RAIN/SNOW THURSDAY WITH PERHAPS DRIER CONDITIONS FRIDAY IN
PREVAILING NORTHWEST FLOW ALOFT.

&&

.AVIATION.../00Z ISSUANCE/

VFR CONDITIONS WILL PREVAIL ACROSS THE AREA THROUGH 12Z SATURDAY.
WINDS WILL INCREASE ACROSS THE WIND CORRIDOR...VICINITY 50SM NE
KRKS-KCPR...AND ALONG THE LEE SLOPES OF THE DIVIDE AFTER 06Z
SATURDAY AHEAD OF AN APPROACHING COLD FRONT.  THE COLD FRONT WILL
PUSH EAST ACROSS THE AREA LATE SATURDAY MORNING AND AFTERNOON.
BEHIND THE FRONT...NW WINDS WILL INCREASE WITH STRONG WINDS POSSIBLE
ACROSS JOHNSON CO SATURDAY EVENING INCLUDING KBYG.

&&

.FIRE WEATHER...

...RED FLAG WARNING FOR ZONES 280...281 AND 283...

TONIGHT WILL BE MOSTLY CLEAR WITH INCREASING WEST WINDS. ON
SATURDAY...A COLD FRONT AND UPPER LEVEL DISTURBANCE WILL MOVE
ACROSS THE AREA. THE COMBINATION OF GUSTY WINDS AND LOW RELATIVE
HUMIDITY WILL BRING CRITICAL FIRE WEATHER TO LOWER ELEVATION
ZONES WITH CRITICAL FUELS INCLUDING THE WIND RIVER BASIN...NATRONA
COUNTY AND JOHNSON COUNTY. CONDITIONS SHOULD IMPROVE FOR SUNDAY WITH
COOLER TEMPERATURES AND LESS WIND.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
RED FLAG WARNING FROM NOON TO 9 PM MDT SATURDAY WYZ280-281-283.

HIGH WIND WATCH FROM SATURDAY AFTERNOON THROUGH SATURDAY EVENING
WYZ010-011.

&&

$$

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




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