Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 031909
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
108 PM MDT SUN MAY 3 2015

.SHORT TERM...TODAY THROUGH TUESDAY NIGHT

THE SHOWERS ASCT WITH A WEAK SOUTHERN STREAM VORT LOBE THAT WAS OVER
THE GREAT BASIN EARLIER THIS EVENING WILL ONLY PRODUCE SOME BRIEF
SPRINKLES OR LIGHT SHOWERS IN SOUTHWEST WY THIS MORNING. IN THE
MEANTIME...A COOL FRONT EJECTING FROM A NORTHERN STREAM SHORTWAVE
WITH THE ASCT VORT MAX TRACKING EAST ALONG THE MONTANA CANADIAN
BORDER WILL BE DROPPING DOWN ACROSS NORTHERN WYOMING AND APPEARS
THAT IS HAS REACHED AS FAR SOUTH AS WRL. THIS FRONT WILL GET LODGED
UP AGAINST THE CONTINENTAL DIVIDE TODAY THUS KEEPING THE BEST
INSTABILITY AND INVERTED V SOUNDINGS ALONG AND WEST OF THE DIVIDE AS
SOMEWHAT MORE STABLE AIR DRIFTS IN BEHIND THE FRONT EAST OF THE
DIVIDE TODAY. HAVE MENTION OF SCATTERED THUNDERSTORMS WITH SMALL HAIL
AND GUSTY WINDS WEST OF THE DIVIDE AND A FEW SHOWERS WITH ISOLD
THUNDER WITH GUSTY WINDS EAST. MONDAY WILL FEATURE MORE WIDESPREAD
THUNDERSTORM ACTIVITY WITH LIFTED INDICES DOWN TO MINUS 3 IN SOME
AREAS. WRF QPF INDICATES RELATIVELY STRONG AND MORE FOCUSED
CONVECTIVE INDUCED PRECIP ALONG AND WEST OF THE DIVIDE MONDAY
COMPARED TO THE MORE WIDESPREAD LIGHTER SHOWERS ASCT WITH PROBABLE
VIRGA SHOWERS THAT IT APPEARS TO BE INDICATING FOR THIS AFTERNOON.
TUESDAY SHOULD BE EVEN MORE ACTIVE AS FAR AS CONVECTION GOES GIVEN
THE ANTICIPATED UPSTREAM STRONG CLOSED LOW MAKING LANDFALL ONTO THE
PAC NW/BRITISH COLUMBIA COAST ALONG WITH AN ARM OF LOWER PRESSURE
EXTENDING EAST SOUTHEAST INTO WY. IN FACT THE GFS INDICATES AN H7
CIRCULATION OVER PATHFINDER RESERVOIR TUESDAY WITH THE EURO SHOWING
A WEAKER LOW BUT NO DISCERNIBLE H7 CIRCULATION OVER JOHNSON COUNTY
TUE. NEGATIVE LIFTED INDICES AND WEAK CAPES COUPLED WITH THE H7
CIRCULATION SHOULD RESULT IN QUITE AN ACTIVE CONVECTIVE DAY TUE/TUE
EVE. TEMPERATURES WILL NOT FLUCTUATE MUCH THROUGH TUE.

.LONG TERM...WEDNESDAY THROUGH SUNDAY

SYNOPSIS...A TROUGH OF LOW PRESSURE OVER THE WESTERN U.S. WILL BRING
COOLER CONDITIONS WITH CHANCES OF SHOWERS AND THUNDERSTORMS
WEDNESDAY THROUGH SATURDAY.  THE SYSTEM AND WILL LIFT INTO THE HIGH
PLAINS ON SUNDAY WITH DECREASING CHANCES OF SHOWERS.

DISCUSSION...MEDIUM RANGE MODELS HAVE SHOWN GOOD OVERALL AGREEMENT
AND RUN-TO-RUN CONSISTENCY OVER THE LAST 36 HOURS IN THE DEVELOPMENT
OF TROUGH ACROSS THE WESTERN U.S. WEDNESDAY THROUGH FRIDAY.   THE
TWO WEATHER MAKERS DURING THIS PERIOD ARE A SUB-TROPICAL UPPER LOW
NEAR 30N/140W AND A NORTHERN STREAM UPPER LOW NEAR 50N/150W. THE
SUB-TROPICAL LOW WILL GET KICKED INTO THE SW U.S. ON MONDAY AND
LIFT NNE ACROSS THE CENTRAL HIGH PLAINS INTO STRONG DOWNSTREAM
RIDGE ON WEDNESDAY. THIS SYSTEM WILL BRING INCREASING SHOWERS AND
THUNDERSTORMS UP FROM THE SOUTH ON WEDNESDAY...HOWEVER THE
HEAVIEST AND MOST WIDESPREAD PRECIPITATION IS EXPECTED EAST OF THE
AREA...ALONG AND EAST OF THE STORM TRACK.

NORTHERN STREAM UPPER LOW WILL DIG SOUTH ALONG THE WEST COAST
WEDNESDAY AND THURSDAY...ROUGHLY ALONG 120W.  THERE MAY BE SOMEWHAT
OF A BREAK IN PRECIPITATION WEDNESDAY NIGHT INTO THURSDAY BETWEEN
THE TWO SYSTEMS...THE FIRST UPPER LOW OUT OF THE SUB-TROPICS LIFTING
INTO THE NORTHERN PLAINS THURSDAY MORNING.  UPPER LOW ALONG THE WEST
COAST IS EXPECTED TO CUTOFF FROM THE FLOW THURSDAY AND THEN SLOWLY
WOBBLE EASTWARD ACROSS THE GREAT BASIN FRIDAY AND SATURDAY.
UPSTREAM PACIFIC TROUGH IS FORECAST TO MOVE INTO THE EASTERN
PACIFIC...ACROSS 140W AROUND 00Z SATURDAY...AND ACT AS THE KICKER
FOR THE GREAT BASIN LOW THAT IS EXPECTED TO ACCELERATE OUT OF
NORTHERN UTAH NE ACROSS WYOMING INTO THE HIGH PLAINS SATURDAY NIGHT
AND SUNDAY.  NUMEROUS SHOWERS AND A FEW THUNDERSTORMS ARE EXPECTED
ACROSS MUCH OF THE AREA THURSDAY AFTERNOON THROUGH SATURDAY OUT
AHEAD OF THE UPPER LOW...THE BEST CHANCE FOR HEAVIER PRECIPITATION
ON FRIDAY NIGHT AND SATURDAY FROM SOUTHEAST INTO CENTRAL WYOMING.
SNOW LEVELS ARE EXPECTED TO BE BETWEEN 7KFT AND 8KFT ACROSS MOST OF
THE AREA THURSDAY THROUGH SATURDAY.

&&

.AVIATION.../18Z ISSUANCE/

EAST OF THE DIVIDE...KCOD/KCPR/KLND/KRIW/KWRL ROUTES

SHOWERS AND THUNDERSTORMS CONTINUE TO MOVE OVER THE CONTINENTAL
DIVIDE THIS AFTERNOON...THOUGH GENERALLY REMAIN OVER THE HIGHER
ELEVATIONS. SHOWER AND THUNDERSTORM COVERAGE WILL INCREASE THROUGH
THE REMAINDER OF THE AFTERNOON...WITH PERIODS OF MVFR TO IFR
CONDITIONS POSSIBLE. GUSTY WIND AND SMALL HAIL MAY ACCOMPANY STORM
ACTIVITY...WITH ENHANCED TURBULENCE POSSIBLE OVER CENTRAL AND
NORTHERN MOUNTAIN PASSES. CONDITIONS WILL IMPROVE AFTER 04Z THIS
EVENING FOR THE AREA...WITH GENERALLY DRY CONDITIONS ANTICIPATED
THROUGH MONDAY. GUIDANCE DEMONSTRATES A BRIEF LOWERING OF CLOUD
COVER...POTENTIALLY SOME VALLEY FOG AFTER 12Z...BUT THIS IS EXPECTED
TO BE SHORT LIVED AND IS COVERED BY VCFG FOR A FEW HOURS IN THE
CURRENT TAF SET.

WEST OF THE DIVIDE...KBPI/KJAC/KPNA/KRKS ROUTES

SHOWERS AND THUNDERSTORM COVERAGE CONTINUES TO INCREASE THIS
AFTERNOON AND WILL REMAIN OVER THE AREA INTO THE EVENING HOURS.
STRONG WIND AND SMALL HAIL MAY ACCOMPANY SOME STORM ACTIVITY. STORM
ACTIVITY MAY BRING BRIEF PERIODS OF MVFR CONDITIONS TO AREA
AIRPORTS...ESPECIALLY KRKS...KBPI...AND KPNA. KJAC WILL SEE MORE
ACTIVITY OVER ITS SURROUNDING MOUNTAINS...POTENTIALLY RESULTING IN
INCREASED TURBULENCE OVER THESE AREA. CONDITIONS WILL IMPROVE
OVERNIGHT...WITH MOSTLY DRY CONDITIONS EXPECTED AFTER 06Z THIS
EVENING.

&&

.FIRE WEATHER...

WESTERLY ZONAL FLOW WILL BE OVER THE AREA THROUGH TODAY WITH WEAK
LOW PRESSURE TROUGHS IN THE NORTHERN BRANCH OF THE FLOW AND
SUBTROPICAL MOISTURE MOVING UP FROM THE GREAT BASIN IN THE
SOUTHERN BRANCH OF THE FLOW. DIURNALLY DRIVEN SCATTERED SHOWERS
WITH ISOLATED THUNDERSTORMS REFORMING LATE THIS MORNING AND
INCREASING THROUGH THE AFTERNOON WILL RESULT. CLOUD COVER AND
SCATTERED PRECIPITATION WILL HAVE A COOLING EFFECT TODAY. WITH
MODERATE FLOW ALOFT AND CONVECTIVE CELLS MOVING 25 TO 30 MPH...WIND
GUSTS TO 45 MPH WILL ACCOMPANY THE THUNDERSTORMS ALONG WITH SMALL
HAIL. THE COVERAGE OF SHOWERS AND THUNDERSTORMS IS EXPECTED TO
INCREASE THIS AFTERNOON. WITH STRONG CONVECTIVE MIXING AND MODERATE
FLOW ALOFT GOOD OR BETTER SMOKE DISPERSAL IS EXPECTED THIS AFTERNOON.
THESE MAINLY LATE DAY THUNDERSTORMS ARE EXPECTED TO PERSIST THROUGH
AT LEAST TUESDAY.

&&

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

$$

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










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