Area Forecast Discussion
Issued by NWS Riverton, WY

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729
FXUS65 KRIW 022018
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
218 PM MDT MON MAY 2 2016

.SHORT TERM...TONIGHT THROUGH THURSDAY

RIDGE SETTLES IN ACROSS THE AREA THROUGH THE PERIOD WITH DRY AND
WARMER CONDITIONS DOMINATING. A WEAK SYSTEM PASSING OFF TO THE
EAST WILL BRING SOME HIGH CLOUDS AND A LIGHT TO MODEST NORTH WIND
INTO THIS EVENING BUT IT WILL REMAIN DRY. OTHERWISE...AS THE
AIRMASS WARMS UP...WE SHOULD GRADUALLY SEE MORE AND MORE CUMULUS
BUILDING OVER THE MOUNTAINS DURING THE AFTERNOON AND I WOULDN`T
BE SURPRISED TO SEE AN ISOLD SHOWER OVER THE HIGHER PEAKS OF THE
ABSAROKAS TUESDAY AND OR WEDNESDAY AFTERNOON. OTHERWISE...99
PERCENT OF THE AREA WILL SEE DRY AND WARMER CONDITIONS WITH
LIMITED CLOUD COVER EXCEPT FOR A FEW MORE CUMULUS FLOATING OFF THE
MOUNTAINS LATE IN THE AFTERNOON AND EARLY EVENING. ON
THURSDAY...WE START TO SEE THE INITIAL INFLUENCE OF OUR LARGE
WESTERN U.S. TROUGH THAT WILL HAVE A HUGE IMPACT FOR US IN THE
MEDIUM RANGE PERIOD. SOME MID LEVEL MOISTURE AND WARM AFTN TEMPS
WITH WEAK UPPER FORCING WILL LIKELY RESULT IN AT LEAST ISOLD TO
WIDELY SCT SHOWERS AND TSTMS THU AFTN ALONG AND WEST OF THE
DIVIDE. THE EAST WILL REMAIN WARM AND DRY EXCEPT AN OUTSIDE SHOT
OF AN ISOLD SHOWER/TSTM FORMING OVER THE HIGHER TERRAIN OF SRN
SWEETWATER.

.LONG TERM...THURSDAY NIGHT THROUGH MONDAY

SYNOPSIS...NOT MUCH CHANGE IN THINKING COMPARED TO PREVIOUS
FORECAST DISCUSSION. THE UPPER LEVEL LOW PRESSURE SYSTEM WILL SPIN
SLOWLY ACROSS THE GREAT BASIN FRIDAY AND ACROSS THE CENTRAL AND
NORTHERN ROCKIES THROUGH THE WEEKEND. SHOWERS AND THUNDERSTORMS WILL
INCREASE ACROSS THE AREA FRIDAY WITH A FEW STRONG/SEVERE STORMS
POSSIBLE DURING THE AFTERNOON. MORE WIDESPREAD RAIN AND MOUNTAIN
SNOW IS THEN EXPECTED SATURDAY ACROSS CENTRAL WYOMING WITH
PRECIPITATION DECREASING IN INTENSITY AND COVERAGE FROM THE NORTH
AND WEST LATE SUNDAY AND MONDAY.

DISCUSSION...GFS AND ECMWF REMAIN IN OVERALL AGREEMENT BRINGING
CLOSED/CUTOFF LOW INTO THE FOUR CORNERS REGION OVER THE WEEKEND WITH
THE MOST SIGNIFICANT SPREAD IN THE NORTHERN STREAM. GFS CONTINUES
TO SHOW MIDWEEK OMEGA BLOCK OVER THE U.S. MORPHING INTO A REX
BLOCK OVER THE WESTERN U.S. BY SATURDAY WITH UPPER HIGH OVER THE
NORTHERN ROCKIES AND SW LOW IN THE FOUR CORNERS REGION. FORECAST
CONTINUES TO FAVOR THE GFS THAT HAS SHOWN MUCH BETTER RUN-TO-RUN
CONSISTENCY.

THE TWO MAIN CONCERNS DURING THIS FORECAST PERIOD WILL BE NUMBER
ONE...THE POTENTIAL FOR STRONG/SEVERE THUNDERSTORMS ACROSS CENTRAL
WYOMING ON FRIDAY AS COLD FRONT DROPS SOUTH TO NEAR A LANDER-CASPER
LINE WITH SE FLOW BRINGING HIGHER CAPES ALONG THE FRONT...AND
SOUTHERLY DIFLUENT FLOW ALOFT. SECOND...DESPITE DIFFERENCES IN THE
GFS AND ECMWF...BOTH MODELS HAVE CONSISTENTLY PAINTED A BAND OF
SIGNIFICANT PRECIPITATION ACROSS CENTRAL WYOMING FRIDAY NIGHT INTO
SATURDAY ON THE NE QUADRANT OF THE FOUR CORNERS LOW. GFS HAS COOLED
MID-LEVEL TEMPERATURES SIGNIFICANTLY OVER THE LAST 24 HOURS WHICH IS
A GOOD THING FOR HOLDING ONTO THE MOUNTAIN SNOWPACK AND ALLEVIATING
FLOODING CONCERNS WITH SNOW LEVELS FORECAST TO LOWER BELOW 9KFT
WHERE HEAVIER PRECIPITATION (>=1 INCH OF WATER) IS FORECAST. GFS QPF
TOTALS ARE SIGNIFICANT (2 INCHES PLUS) WHEN COMPARED TO THE STILL
ROBUST NUMBERS OFFERED BY THE ECMWF (AROUND 1.5 INCHES). FRIDAY
NIGHT AMOUNTS ACROSS THE UPSLOPE FAVORED AREAS OF CENTRAL WYOMING
RANGE FROM AROUND 1.5 TO 2.0 INCHES. GFS WANTS TO PRODUCE ANOTHER
ROUND OF SUBSTANTIAL PRECIP FROM AROUND MIDDAY SATURDAY THROUGH
SATURDAY AFTERNOON AS SURFACE LOW DEEPENS OVER NORTHEAST
COLORADO. THIS COULD POTENTIALLY GENERATE EAST TO NORTHEAST
UPSLOPE FLOW INTO NATRONA AND FREMONT COUNTIES. BOTTOM LINE...EVEN
IF THESE QPF NUMBERS ARE OVERDONE...WIDESPREAD ONE INCH PLUS
TOTALS LOOK TO BE A GOOD BET ACROSS FREMONT... NATRONA... AND
JOHNSON COUNTIES. ACCORDINGLY...HAVE LOWERED SATURDAY MAX TEMPS
SLIGHTLY FROM MODEL GUIDANCE ACROSS CENTRAL WYOMING.

FOUR CORNERS LOW IS EXPECTED TO GRADUALLY FILL AND LIFT ACROSS THE
CENTRAL ROCKIES INTO THE CENTRAL HIGH PLAINS SUNDAY AND MONDAY WITH
THE HEAVIER PRECIPITATION EXPECTED TO SHIFT OUT INTO THE PLAINS ON
SUNDAY. HOWEVER...THE PATTERN ACROSS MUCH OF THE FORECAST AREA
REMAINS CONVOLUTED THROUGH AT LEAST SUNDAY WITH LINGERING SHOWERS
ACROSS THE WESTERN MOUNTAINS. BELIEF HERE IS THAT PESKY NORTHWEST
FLOW ALOFT WILL ALLOW FOR AT LEAST CONTINUED LOW CHANCES OF PRECIP
THROUGH MONDAY.

&&

.AVIATION.../00Z ISSUANCE/

WEST OF THE CONTINENTAL DIVIDE...KJAC/KRKS/KPNA/KBPI AIRPORTS

VFR CONDITIONS TO OCCUR THROUGH TUESDAY.

EAST OF THE CONTINENTAL DIVIDE...KCPR/KCOD/KRIW/KWRL/KLND AIRPORTS

VFR CONDITIONS TO OCCUR THROUGH TUESDAY.

&&

.FIRE WEATHER...

A DRYING AND WARMING TREND WILL CONTINUE THROUGH MIDWEEK. BY
WEDNESDAY...RELATIVE HUMIDITY WILL FALL INTO THE TEENS ACROSS SOME
OF THE BASINS EAST OF THE DIVIDE. HOWEVER...WINDS ARE EXPECTED TO
REMAIN LIGHT TO MODERATE THROUGH WEDNESDAY NIGHT. OTHER THAN AN
ISOLATED AFTERNOON OR EVENING THUNDERSTORM OVER THE
MOUNTAINS...EXPECT DRY WEATHER THROUGH WEDNESDAY NIGHT. WITH LIGHT
WINDS...MIXING AND SMOKE DISPERSAL SHOULD REMAIN FAIR TO POOR.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...NONE.

&&

$$

SHORT TERM...PS
LONG TERM...AEM/CNJ
AVIATION...PS
FIRE WEATHER...PS



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