Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 192021
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
221 PM MDT TUE AUG 19 2014

NEAR TERM...A MONSOONAL PUSH WAS BEGINNING TO MOVE INTO SOUTHWESTERN
PORTIONS OF THE FORECAST AREA AS OF 2PM...IT IS EXPECTED TO CONTINUE
ITS TREK NORTH AND EAST OVERSPREADING THE ENTIRE FORECAST AREA
THIS EVENING. DEW POINTS IN THE 50S AND EVEN 60 DEGREES WILL
ACCOMPANY THIS PUSH ALONG WITH PRECIPITABLE WATER VALUES WELL
ABOVE NORMAL...APPROACHING ONE INCH BY THIS EVENING WHICH PUTS US
IN THE 99TH PERCENTILE FOR THIS TIME OF YEAR.

AT THE SAME TIME...A SHORT WAVE THAT MOVED THROUGH UTAH OVERNIGHT
INTO THIS MORNING WILL MAKE ITS WAY INTO THE REGION...SUPPLYING
BROAD SCALE LIFT AND ENCOURAGING WIDESPREAD SHOWERS WITH SOME
EMBEDDED THUNDERSTORMS. WE ARE EXPECTING THE BULK OF THIS ENERGY TO
AFFECT THE SOUTHERN COUNTIES EARLY THIS AFTERNOON AND THEN
TRANSITION INTO CENTRAL AND EASTERN WYOMING LATER THIS EVENING. ALSO
AHEAD OF THIS FEATURE...THERE WILL BE SOME AREAS THAT WILL
DESTABILIZE IF SKIES ARE CLEAR ENOUGH...THIS IS PROBABLY WHERE THE
STRONGER STORMS WILL BE EARLY IN THE PERIOD. DEW POINTS ARE STILL
LOW ENOUGH TO ENCOURAGE SOME WIND GUSTS IN EXCESS OF 45 MPH AND
UPDRAFTS COULD BE STRONG ENOUGH TO SUPPORT SOME PENNY SIZED HAIL.
VILS WOULD HAVE TO EXCEED 65 IN ORDER TO GET LARGER HAIL...WHILE
THAT IS POSSIBLE IT IS DOUBTFUL THAT WE WILL BE ABLE TO MUSTER
ENOUGH CAPE...ESPECIALLY SINCE SHEAR WILL NOT BE STRONG ENOUGH TO
MAINTAIN MUCH OF AN UPDRAFT WITH ANY OF THESE STORMS.

.SHORT TERM...TONIGHT THROUGH FRIDAY

TONIGHT WE WILL TRANSITION TO MORE OF A WIDESPREAD SHOWER/ISOLATED
THUNDERSTORM WITH HEAVY RAIN SCENARIO AS THE RICH MOISTURE IS
STIRRED UP BY SOME PVA AND DECENT QG FORCING THROUGH ABOUT 06Z.
THERE WILL BE SOME FLASH FLOOD POTENTIAL...ESPECIALLY EARLY WITH
HEAVIER STORMS AND OVR HIGHER TERRAIN OVERNIGHT WHERE TERRAIN FORCED
RAINFALL WITH CONVECTIVE ELEMENTS COINCIDE...ESPECIALLY ON S TO SW
FACING SLOPES OVERNIGHT AND THRU WED MORNING. LIKED THE OVERNIGHT
SHIFT THINKING WITH AVOIDING A FLASH FLOOD WATCH...SINCE THIS EVENT
WILL BE PARTIALLY CONVECTIVE...THEN STRATIFORM LATER
TONIGHT...CHOOSING TO WAIT AND SEE HOW ALL THIS EVOLVES.
HOWEVER...LATER DURING THIS FCST PERIOD...A FFW MAY BE NECESSARY
ACROSS SOME OF THE WRN MOUNTAIN ZONES AS THE GROUND SATURATES AND
PRECIPITATION KEEPS FALLING THROUGH AT LEAST THURSDAY.

WEDNESDAY AND THURSDAY...ABUNDANT MOISTURE REMAINS IN THE REGION BUT
THE UPPER LEVEL FORCING LEAVES FOR WEDNESDAY...EXPECT MORE OF AN
AIRMASS/DIURNAL THUNDERSTORM SETUP. THERE HAVE BEEN SOME MODEL
UNCERTAINTIES AS TO WHAT WILL ARRIVE ON THURSDAY...BUT THERE MAY BE
SOME MORE BROAD LIFT ARRIVING WITH ANOTHER SHORT WAVE QUICKLY
FOLLOWED BY A LONG WAVE TROUGH ACROSS THE GREAT BASIN TAKING ON A
NEGATIVE TILT AND MOVING OUR DIRECTION...IF THIS SHOULD HAPPEN THEN
SOME MORE ORGANIZED THUNDERSTORM ACTIVITY COULD VISIT THE FORECAST
AREA BUT HAVE KEPT A HEALTHY DOSE OF UNCERTAINTY IN THE GRIDS AT
THIS TIME. IN ANY EVENT...IT IS LIKELY THAT MUCH OF THE FORECAST
AREA WILL CONTINUE TO SEE BOUTS OF HEAVY RAIN THROUGH THURSDAY AND
PERHAPS EVEN FRIDAY...ESPECIALLY ACROSS THE WESTERN
MOUNTAINS.

.LONG TERM...FRIDAY NIGHT THROUGH TUESDAY

MEDIUM-RANGE MODEL SOLUTIONS ARE IN FAIR AGREEMENT TODAY. BROAD
TROUGH ACROSS THE NORTHWEST U.S. TO BEGIN THE FORECAST PERIOD WITH
HIGH PRESSURE OVER THE SOUTHERN U.S. HELPING TO SLOW THE EASTWARD
PROGRESSION OF THE TROUGH. MODELS HAVE SLOWED SOME FROM MONDAY RUNS
REGARDING THE UPCOMING WEEKEND WEATHER SYSTEM. OVERALL...COOLER
PATTERN TO BE IN PLACE ACROSS THE ROCKIES THROUGH THE PERIOD.

LONG WAVE TROUGH WILL BE POSITIONED OVER THE INTERMOUNTAIN WEST AND
PACIFIC NORTHWEST TO BEGIN THE PERIOD. WEAK SHORTWAVE ENERGY IS
LIKELY TO BE EJECTED EASTWARD ACROSS WYOMING FRIDAY NIGHT AND FAVOR
PRECIPITATION IN THE NORTHWEST AND NORTHERN MOUNTAINS. HAVE NUDGED
PRECIP CHANCES UPWARD IN THESE TWO LOCATIONS. THIS SCENARIO IS
LIKELY TO PLAY OUT THROUGH SATURDAY AS THE TROUGH NEARS WYOMING AND
SOUTHWEST FLOW ALOFT REMAINS OVER THE FORECAST AREA. SOME ADDITIONAL
SUB-TROPICAL MOISTURE WILL BE DRAWN NORTH INTO WYOMING SATURDAY
AFTERNOON AND EVENING. HAVE INCREASED PRECIP CHANCES EAST OF THE
DIVIDE TO ACCOUNT FOR THIS. IF MODELS CONTINUE TO COME MORE IN-LINE
IT IS LIKELY THAT PRECIP CHANCES WOULD NEED TO BE BUMPED UP AND
EXPANDED WEST SATURDAY AFTERNOON AND EVENING. MID-LEVEL CIRCULATION
WILL MOVE ALONG OUR NORTHERN BORDER SATURDAY NIGHT KEEPING SHOWERS
GOING ALL NIGHT ACROSS THE NORTH. THIS CIRCULATION WILL SWING TO OUR
EAST BY MIDDAY SUNDAY SUBSEQUENTLY DRAWING COOLER AIR SOUTH ACROSS
WYOMING IN CYCLONIC FLOW. FAVORABLE WEST-NORTHWEST FLOW SHOULD KEEP
THE BIGHORNS AND PORTIONS OF THE NORTHWEST MOUNTAINS WET SUNDAY
AFTERNOON/NIGHT. ELSEWHERE...THE SOUTHWEST LOOKS TO BE THE DRIEST
AREA SUNDAY AND SUNDAY NIGHT AS THE UPPER FLOW BACKS TO THE WEST AND
DRIER AIR PUSHES IN FROM THE WEST. ZONAL FLOW FOR MONDAY WILL LEAD
TO PRIMARILY DRY CONDITIONS. THE EXCEPTION WILL BE THE FAR NORTH
WHERE ENOUGH MOISTURE WILL LINGER TO AT LEAST GENERATE
OROGRAPHICALLY-DRIVEN CONVECTION. THE NEXT PIECE OF ENERGY WILL DIG
SOUTHEAST ALONG THE FRONT SIDE OF AN EASTERN PACIFIC RIDGE MONDAY.
THIS WILL REINFORCE THE TROUGH FOR TUESDAY AND BRING ANOTHER SHOT OF
COOLER AIR AND INCREASE PRECIP CHANCES ACROSS THE WEST.

&&

.AVIATION.../00Z ISSUANCE/

A SURGE OF MOISTURE MOVING INTO SOUTHERN WY ALREADY VCNTY KRKS WILL
CONTINUE TO MOVE NORTH THROUGH 06Z. AHEAD OF THE MAIN PUSH EXPECT
ANY AREAS WITH PARTLY CLOUDY OR CLEAR SKIES TO SEE -SHRA AND -TSRA
TO DEVELOP 18-21Z OVER THE MOUNTAINS FIRST AND SPREAD INTO THE
FOOTHILLS SHORTLY THEREAFTER...MAINLY FROM A KEVW TO VCNTY KAFO IN
THE EARLY AFTERNOON PUSHING INTO NW WYOMING VCNTY AND NORTH OF KJAC
IN THE LATE AFTERNOON AND EVENING. THUNDERSTORM COVERAGE WILL BE
MORE ISOLATED FURTHER EAST. EXPECT OCCNL MVFR CONDITIONS...MOUNTAIN
OBSCURATIONS AND TURBULENCE VCNTY TSRA.

THE SYSTEM WILL SPREAD SCT-NMRS SHRA S-N ACROSS WEST AND CENTRAL
WYOMING AFTER 00Z WITH A FEW EMBEDDED THUNDERSTORMS LINGERING. CIGS
WILL LOWER MAINLY INTO THE FL050-100 RANGE WITH ALONG WITH HIGHER
MTN OBSCURATIONS AND POSSIBLY LCL MVFR CONDITIONS IN RA AND BR.
GRADUAL IMPROVEMENT IS EXPECTED FROM SW-NE AFTER 10Z AS THE SYSTEM
LIFTS INTO NE WYOMING.

&&

.FIRE WEATHER...

HEIGHTS AND TEMPERATURES ARE SHOWING A LOWERING AND COOLING TREND
OVER THE NEXT WEEK. ANOTHER SURGE OF MONSOONAL MOISTURE WILL ARRIVE
TODAY AND WILL BRING SHOWERS AND WET THUNDERSTORMS THROUGH AT LEAST
THURSDAY. THERE COULD BE A COLD FRONT ARRIVING THIS WEEKEND THAT
WILL DROP TEMPERATURES FURTHER...TO WELL BELOW NORMAL FOR THIS TIME
OF YEAR...ALONG WITH BRING ANOTHER DOSE OF WETTING RAINS TO THE
REGION. RELATIVE HUMIDITY WILL REMAIN ON THE HIGH SIDE AND THERE ARE
NO WIDESPREAD HIGH WIND EVENTS ON THE HORIZON EITHER.

FORECAST CONFIDENCE IS PRETTY LOW ONCE WE GET INTO NEXT WEEK. THERE ARE
SOME INDICATIONS THAT THERE COULD BE ANOTHER RIDGE BUILDING IN WHILE
OTHER MODELS ARE POINTING TO AN EXACTLY OPPOSITE SOLUTION. THAT
SAID...WHILE THERE COULD BE SOME IGNITIONS WITH THE LIGHTNING OVER
THE NEXT SEVERAL DAYS...THERE WILL BE PLENTY OF RAIN TO ACCOMPANY
THE STORMS...ESPECIALLY ONCE WE GET PAST THIS AFTERNOON WITH MORE
MUD SLIDES...FLASH FLOODING...AND DIFFICULT TRAVEL ALONG ANY BACK
COUNTRY OR UNPAVED ROADS.

&&

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

$$

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




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