Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 011816
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
1216 PM MDT FRI MAY 1 2015

.SHORT TERM...TODAY THROUGH SUNDAY NIGHT

IMAGERY SHOWING NEAR ZONAL FLOW ALOFT OVR WY...WITH EXITING WEAK SW
TROF TO THE E AND ACROSS THE NRN AND CNTRL PLAINS...HIGH PRESSURE
RIDGE OVR THE SW CONUS. SFC HAS WEAK SFC HIGH PRESSURE ACROSS MUCH
OF THE WRN CONUS...LOWER PRESSURE UNDER THE HIGH IN THE DESERT
SW.

TODAY...DRYING AND WARMING WILL RULE BEHIND THURSDAY`S
SYSTEM...WITH MOST OF THE FA REMAINING DRY AND MOSTLY SUNNY UNDER
INCREASINGLY ZONAL FLOW ALOFT. VERY SLIGHT CHC THAT A WEAK STALLED
FRONT DRIFTING AROUND THE BORDER AREA WITH COLORADO THIS AFTERNOON
COULD TRIGGER A SHOWER OR TWO OVR THE SRN PORTION OF THE CWA.
SAT...A MODEST AREA OF MOISTURE ALONG WITH AN UPR LVL DISTURBANCE
ROTATING INTO THE REGION AROUND AN UPR LVL HIGH OVR THE SRN
CONUS...WILL INITIATE SOME SHOWERS AND ISOLATED THUNDER ACROSS
MAINLY THE SRN HALF OF THE FA LATER IN THE AFTERNOON. A SECONDARY
AREA OF SHOWERS WILL ALSO FORM 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.

SUNDAY...FRONT THRU THE FA E OF THE DIVIDE WITH SHALLOW UPSLOPE
UNDER WESTERLIES ALOFT. AN UPR DISTURBANCE WILL ROTATE INTO THE
REGION OUT OF THE SW CONUS FROM THE S DURING THE DAY...ALONG W/
INCREASED MID LVL MOISTURE. FRONT MAY BE LIFTED NWD DURING THE DAY
WITH SOME LOW P DEVELOPMENT OVR THE SW WITH BEST AREA OF PRECIP
FORCING OCCURRING BETWEEN ROCK SPRINGS...TO THE NE THRU MOST OF
NATRONA COUNTY. THIS WILL ALSO BE THE REGION WHERE BETTER...ALBEIT
WEAK...SHEAR WILL OCCUR. SO...CLOUDY SKIES WITH RAIN SHOWERS AND
ISOLATED THUNDER WILL RULE THE DAY ACROSS THE SW HALF OF THE
FA...WHERE HIGH LAPSE RATES BUT LOW BUOYANCY WILL BE FOUND. MORE
CLOUDY THAN SHOWERY TO THE NE...WHERE BOTH DECENT LAPSE RATES OR
BUOYANCY WILL BE HARDER TO FIND...AND MORE TERRAIN FORCED
PRECIP/CONVECTION IS EXPECTED.

.LONG TERM...MONDAY THROUGH FRIDAY

SYNOPSIS...A TROUGH OF LOW PRESSURE WILL DEVELOP OVER THE AREA
TUESDAY AND WEDNESDAY BRINGING INCREASING CHANCES OF SHOWERS AND
THUNDERSTORMS ALONG WITH COOLER TEMPERATURES. THIS COOL AND
UNSETTLED PATTERN MAY PERSIST THROUGH FRIDAY.

DISCUSSION...TWO PACIFIC SYSTEMS WILL BE THE MAIN WEATHER MAKERS
DURING THIS PERIOD.  THE FIRST IS A SUB-TROPICAL LOW NEAR 30N/135W
EARLY THIS MORNING AND THE SECOND IS A NORTHERN STREAM LOW MOVING
ALONG 50N...JUST SOUTH OF THE ALEUTIAN CHAIN.  BY MONDAY
MORNING...THE NORTHERN STREAM SYSTEM WILL PROGRESS TO NEAR VANCOUVER
ISLAND...KICKING THE SOUTHERN SYSTEM INTO SOUTHERN CALIFORNIA.
THIS LATTER SOUTHERN STREAM SYSTEM IS THEN FORECAST TO TRACK ENE
ACROSS THE SOUTHERN ROCKIES TUESDAY...LIFTING INTO THE CENTRAL HIGH
PLAINS ON WEDNESDAY.  THERE IS GOOD OVERALL AGREEMENT AMONG
MEDIUM-RANGE GUIDANCE WITH THIS DEPICTION.  THE MAIN PROBLEM
CONTINUES TO BE WHERE THE NORTHERN STREAM KICKER WILL GO.  30/12Z
ECMWF AND GFS WERE IN BETTER AGREEMENT DIGGING AT LEAST A GOOD CHUNK
OF THIS ENERGY SOUTH ACROSS THE WESTERN U.S. AS IT MAKES LANDFALL
TUESDAY.  ECMWF...AFTER HINTING AT THIS IDEA 12 HOURS AGO...HAS
REVERTED BACK TO A MORE PROGRESSIVE TROUGH PUSHING ACROSS THE
CANADIAN ROCKIES WEDNESDAY.  LATEST GFS/GEFS MEAN SHOW A CLOSED LOW
NEAR RENO BY 00Z THURSDAY...THE CANADIAN HAS THE CLOSED LOW A BIT
FURTHER NORTH.  FORECAST SIDES WITH THE GFS/CANADIAN RUNS THAT SHOW
BETTER RUN-TO-RUN CONSISTENCY.

SHOWERS AND THUNDERSTORMS SHOULD BEGIN TO SPREAD UP FROM THE SOUTH
ON MONDAY WITH SOME HINT OF SOME ENERGY FRAGMENTING FROM BAJA LOW
INTO THE AREA...COMBINING WITH LOW LEVEL ESE FLOW EAST OF THE
DIVIDE.  THE BEST CHANCE OF STORMS WILL BE ACROSS THE SOUTH AND EAST
WITH A FEW STRONG STORMS POSSIBLE MAINLY OVER NATRONA COUNTY.

MORE ABUNDANT MOISTURE WILL ADVECT INTO THE AREA TUESDAY AND
WEDNESDAY AS MAIN BAJA LOW LIFTS SOUTH AND EAST OF THE AREA.  MORE
CLOUD COVER...SHOWER AND THUNDERSTORM ACTIVITY WILL RESULT IN COOLER
TEMPERATURES BY WEDNESDAY.

IN THE THURSDAY TIMEFRAME...SOMEWHAT OF A BREAK IN PRECIPITATION IS
SHOWN BETWEEN THE BAJA LOW LIFTING OUT OF THE AREA AND THE UPPER LOW
DIGGING INTO THE INTERMOUNTAIN WEST...BUT CONFIDENCE IS LOW IN THESE
DETAILS.  THE BIGGER PICTURE OF A CLOSED LOW WOBBLING ACROSS THE
GREAT BASIN LATE IN THE WEEK WARRANTS KEEPING CHANCES OF
PRECIPITATION IN THROUGH THE END OF THE WEEK AND CONTINUING AN
OVERALL COOLING TREND.

.AVIATION.../18Z ISSUANCE/

VFR CONDITIONS WILL OCCUR THROUGH 18Z SATURDAY. ISOLATED SHOWERS OR STORMS
WILL DEVELOP IN FAR SOUTHERN WY AND ALONG THE UTAH/COLORADO
BORDERS FROM 21Z THROUGH 18Z SATURDAY. THIS ACTIVITY IS EXPECTED
TO STAY SOUTH OF THE KRKS AIRPORT.

.FIRE WEATHER...

FIRE DANGER BELOW CRITICAL TODAY AND FRIDAY WITH
WARM AND DRY CONDITIONS AND MOSTLY SUNNY SKIES. MINIMUM RH PERCENT
VALUES WILL DROP INTO THE TEENS ACROSS THE LOWER ELEVATIONS TO 20S
IN THE MOUNTAINS. TODAY...HOWEVER...WINDS WILL BE LIGHT AT LESS THAN
15 MPH FOR MOST LOCATIONS. SATURDAY WILL SEE SOME INCREASE IN WINDS
ACROSS THE SOUTHERN AND WESTERN DISPATCH ZONES...BUT ALSO MORE
CLOUD...HUMIDITY...AND PRECIPITATION CHANCES ACROSS THESE SAME
AREAS. SUNDAY CAN BE CHARACTERIZED AS COOLER AND MORE HUMID AREA
WIDE...WITH SOME CHANCE FOR PRECIPITATION ACROSS THE ENTIRE FORECAST
AREA.

&&

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

$$

SHORT TERM...BRAUN
LONG TERM...AEM
AVIATION...C.BAKER
FIRE WEATHER...BRAUN





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