Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 300919
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
319 AM MDT THU APR 30 2015

.SHORT TERM...TODAY THROUGH SATURDAY NIGHT

IMAGERY SHOWING STRONG AMPLIFIED UPR LVL RIDGING FROM SRN AZ UP THRU
ND AND WELL INTO NCNTRL CANADA. SMALL WAVE MOVING THRU THE NRN/NWRN
FA INTO CNTRL MT CURRENTLY...WITH -SHRA AND VIRGA MOVING ACROSS THIS
REGION THIS MORNING. UPSTREAM...EXTENDING FROM SERN BC THRU NRN ID AND INTO
NRN CALIFORNIA...THE NEXT SW TROF...THAT WILL AFFECT THE FA
TODAY...IS MOVING QUICKLY EWD. SFC HAS WEAK LOW PRESSURE STARTING TO
INFILTRATE THE CWA WITH FRONT THRU SWRN MT (AND BILLINGS)...THEN OFF
TO THE SW AND OVR ERN ID.

TODAY...THE SW TROF...MENTIONED ABOVE...MOVES ACROSS THE FA...ALONG
WITH A LOW TO MODEST INCREASE IN MID LVL MOISTURE...PUSHING A FRONT
DOWN ACROSS THE FA THIS MORNING WITH GUSTY NORTH WINDS. INCREASING
INSTABILITY WILL KICK OFF A FEW (MAINLY) MOUNTAIN SHOWERS WITH
ISOLATED THUNDER OCCURRING ALONG AND AHEAD THE FRONT. WHERE THUNDER
CAN/WILL OCCUR...THE ATMOSPHERE WILL HAVE STRONG LAPSE RATES BUT
VERY LITTLE BUOYANCY AOB 150 J/KG FOR MOST LOCATIONS WITHIN THE CWA.
SHORT-LIVED MODERATELY STRONG LOW LEVEL SHEAR AT/NEAR THE FRONT WILL
OFFER A CHANCE FOR SHORT DURATION STRONGER STORMS ABLE TO PUSH OUT
SOME MODERATELY STRONG WIND GUSTS TO PERHAPS 50 MPH...WITH A
SECONDARY THREAT OF SMALL HAIL...PARTICULARLY ACROSS ZONES 19 AND
30.

FRIDAY THRU SAT NIGHT...DRYING AND WARMING WILL RULE BEHIND TODAY`S
SYSTEM...WITH 99 PERCENT OF THE FA BEING CLOUD FREE UNDER
INCREASINGLY ZONAL FLOW ALOFT. VERY SLIGHT CHC THAT A WEAK STALLED FRONT
DRIFTING AROUND THE BORDER AREA WITH COLORADO ON FRIDAY AFTERNOON
COULD TRIGGER A SHOWER OR TWO OVR EXTREME SRN SWEETWATER COUNTY.
SAT...A WEAK AREA OF MOISTURE AND UPR LVL DISTURBANCE ROTATES INTO
THE REGION AROUND AN UPR LVL HIGH OVR THE SRN CONUS...INITIATING
MORE SHOWERS AND ISOLATED THUNDER ACROSS MAINLY THE SRN HALF OF THE
FA...WITH A SECONDARY AREA OF SHOWERS PUSHING OVR THE NRN FA SAT
NIGHT AS A MODEST CLIPPER TYPE FRONT MOVES THRU THE NRN AND NERN
ZONES. AGAIN...LITTLE MOISTURE AND LOW INSTABILITY WITH PRECLUDE
MUCH IN THE WAY OF SIGNIFICANT PRECIP OF ANY KIND.

.LONG TERM...SUNDAY THROUGH THURSDAY

SYNOPSIS...CHANCES OF SHOWERS AND THUNDERSTORMS IN THE FORECAST FOR
EACH DAY DURING THE EXTENDED PERIOD WITH A GRADUAL COOLING TREND
OVERALL.

DISCUSSION...A WEAK COLD FRONT WILL PUSH SOUTH ACROSS NORTH AND
CENTRAL WYOMING SATURDAY NIGHT/SUNDAY MORNING WITH SOME RIPPLES IN
WESTERLY FLOW INTERACTING WITH THE FRONT SUNDAY AFTERNOON.  SHOWERS
AND THUNDERSTORMS ARE EXPECTED TO BLOSSOM ACROSS THE SOUTH AND WEST
AND MOVE INTO CENTRAL AREAS THROUGH THE AFTERNOON.

ON MONDAY...BOTH GFS AND ECMWF REMAIN IN RELATIVELY GOOD AGREEMENT
MOVING NORTHERN STREAM UPPER LOW TO NEAR 50N/130W AND KICKING
SUB-TROPICAL UPPER LOW ACROSS THE NORTHERN BAJA PENINSULA.  GFS AND
ECMWF THEN QUICKLY DIVERGE IN THEIR SOLUTIONS TUESDAY THROUGH
THURSDAY WITH BOTH OF THESE SYSTEMS.  GFS DIGS THE NORTHERN STREAM
UPPER LOW SSE ACROSS THE NW U.S. TUESDAY AND WEDNESDAY WHILE THE
ECMWF KEEPS THE LOW MOVING EAST ALONG ROUGHLY 50N...ACROSS THE
NORTHERN ROCKIES ON WEDNESDAY.  THESE DIFFERENCES IN THE PATH OF
THIS `KICKER` LOW AFFECT THE PATH OF THE SUB-TROPICAL LOW LIFTING
OUT OF THE SW U.S.  GFS PULLS THIS LOW BACK FURTHER WEST THAN ECMWF
AS IT LIFTS ACROSS THE ROCKIES INTO THE HIGH PLAINS TUESDAY AND
WEDNESDAY.  GFS AND ECMWF ENSEMBLE MEANS GENERALLY REFLECT THEIR
RESPECTIVE DETERMINISTIC RUNS.  THE BEST SOLUTION MOST LIKELY LIES
SOMEWHERE IN BETWEEN...NOT AS MUCH DIGGING AS GFS...NOT AS
PROGRESSIVE AS ECMWF WITH NORTHERN STREAM LOW...AND BAJA LOW LIFTING
ACROSS THE CENTRAL HIGH PLAINS ON WEDNESDAY.  EITHER WAY...THE
RESULT WILL BE A ACTIVE PERIOD ACROSS MOST OF THE REGION WITH LOW
LEVEL E-SE UPSLOPE FLOW ACROSS CENTRAL AND EASTERN WYOMING MONDAY
AND TUESDAY...WITH SW FLOW ALOFT AND SOME ENERGY FRAGMENTING OFF
BAJA LOW ACROSS WYOMING.   MORE UNCERTAINTY ON THE
EXTENT...PLACEMENT AND TYPE OF PRECIPITATION WEDNESDAY AND
THURSDAY...WITH THE COMPROMISE APPROACH KEEPING HIGHER POPS ACROSS
THE NORTH AND EAST WITH A GRADUAL COOLING TREND OF A FEW DEGREES
EACH DAY.


&&

.AVIATION.../12Z ISSUANCE/

A COLD FRONT WILL CONTINUE TO SOUTHWARD TODAY INTO CENTRAL WY BY
15Z. A NORTH TO NORTHWEST WIND WILL OCCUR BEHIND THE FRONTAL
PASSAGE. ISOLATED SHOWERS WILL OCCUR OVER THE NORTH THIS
MORNING...ENDING IN THE AFTERNOON. LOCAL MVFR CONDITIONS MAY OCCUR
IN THE NORTH UNTIL 18Z. ISOLATED TO WIDELY SCATTERED SHOWERS WILL
SPREAD SOUTH INTO CENTRAL WY AFTER 18Z THROUGH 02Z FRIDAY. A
THUNDERSTORM OR TWO CANNOT BE RULED OUT. IN FAR SOUTHERN WY ISOLATED
SHOWERS AND STORMS ARE POSSIBLE...BUT NOT IMPACTING KRKS AIRPORT.
THE COLD FRONT PUSHES INTO SOUTHERN WY AFTER 01Z FRIDAY WITH THE
WIND TURNING TO THE EAST OR NORTHEAST. ANY LINGERING SHOWERS WILL
END BY 03Z FRIDAY IN CENTRAL AND SOUTHERN WY WITH VFR CONDITIONS
PREVAILING TONIGHT.

&&

.FIRE WEATHER...

MORE WIND AROUND TODAY...GUSTING 20 TO 30 MPH AT
TIMES...BUT ALSO COOLER WITH  TEMPERATURES RUNNING SOME 5 TO 20 DEG
BELOW THOSE OF WEDNESDAY. MORE CLOUD COVER ALSO HELP KEEP MINIMUM RH
VALUES ABOVE 20 PERCENT NEARLY EVERYWHERE. THERE WILL ALSO BE A
CHANCE  TODAY FOR MAINLY MOUNTAIN DERIVED/MAINTAINED ISOLATED TO
WIDELY SCATTERED SHOWERS. A FEW THUNDERSTORMS WILL MOST LIKELY BE
LOCATED ACROSS SOUTH CENTRAL WYOMING FROM THE GREEN AND RATTLESNAKE
MOUNTAINS SOUTHWARD. SATURDAY AFTERNOON ACROSS THE SOUTHERN FORECAST
AREA...AND AGAIN ON SUNDAY/SUNDAY NIGHT...WILL SEE THE NEXT BEST
MODEST CHANCES FOR CONVECTIVE PRECIPITATION AS ANOTHER COUPLE OF
WEAK DISTURBANCES ALONG WITH A FRONT ON SUNDAY...QUICKLY ROTATE
THROUGH THE FORECAST AREA.

&&

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

$$

SHORT TERM...BRAUN
LONG TERM...AEM
AVIATION...AR
FIRE WEATHER...BRAUN








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