000
FXUS65 KRIW 240925
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
325 AM MDT FRI MAY 24 2013

.SHORT TERM...TODAY THROUGH SUNDAY NIGHT

SOUTH-SOUTHWESTERLY FLOW AHEAD OF AN UPPER LEVEL TROUGH BROUGHT
STRONG THUNDERSTORMS THROUGH THE NORTHERN BIGHORN BASIN INTO THE
WEE MORNING HOURS LAST NIGHT...THIS TROUGH WILL CONTINUE ITS TREK
NORTHEASTWARD INTO MONTANA TODAY AS VERY DRY SOUTHWEST FLOW FILLS
IN BEHIND IT. MUCH OF THE AREA WILL HAVE MINIMUM RELATIVE
HUMIDITIES BELOW 20 PERCENT TODAY WITH MUCH OF CENTRAL AND
SOUTHERN WYOMING BOTTOMING OUT BELOW 10 PERCENT THIS AFTERNOON.
700MB FLOW REMAINS BREEZY WITH 20 TO 25 KNOTS EXPECTED ACROSS THE
STATE. THESE WINDS WILL EASILY MIX DOWN THIS AFTERNOON WITH MIXING
AS HIGH AS 500MB POSSIBLE WHERE WINDS WILL BE BETWEEN 30 AND 40
KNOTS...THESE WINDS COULD MIX DOWN OCCASIONALLY IN SWEETWATER
COUNTY...SOUTHEAST FREMONT COUNTY AND NATRONA COUNTIES. THE VERY
LOW HUMIDITY AND GUSTY WINDS RAISE FIRE WEATHER CONCERNS...BUT NO
HIGHLIGHTS ARE ANTICIPATED DUE TO THE LACK OF CRITICALLY DRY
FUELS. THE WARM...DRY...AND GUSTY SOUTHWEST FLOW IS EXPECTED TO
REMAIN ACROSS MUCH OF THE AREA FOR THE REMAINDER OF THE SHORT
TERM.

OTHERWISE...THE CHANCE FOR THUNDERSTORMS IS VERY LOW TODAY BUT A
BIT HIGHER ON SATURDAY ACROSS THE NORTHERN MOUNTAINS...SUNDAY
LOOKS MORE ACTIVE AS A THETA E RIDGE...SOME MID LEVEL MOISTURE...A
SIGNIFICANT AMOUNT OF CAPE AND LIFTED INDICIES BELOW -3 POSSIBLY
MOVING INTO JOHNSON COUNTY ON SUNDAY AFTERNOON...THERE IS ALSO A
DECENT AMOUNT OF 0-6KM SHEAR THAT COULD ORGANIZE THE STORMS LONG
ENOUGH FOR THEM TO BECOME SEVERE...WITH STRONG OUTFLOW
WINDS...HAIL LARGER THAN THE SIZE OF QUARTERS...AND A WEAK TORNADO
NOT OUT OF THE QUESTION. THUNDERSTORMS ARE ALSO POSSIBLE ACROSS
THE WESTERN AND CENTRAL PORTIONS OF WYOMING...BUT THEY ARE NOT
EXPECTED TO BE AS ORGANIZED AS THE STORMS ALONG THE I25 CORRIDOR
NORTH OF CASPER...AND POSSIBLY IN THE BIGHORN BASIN AGAIN. IN
ADDITION...THERE MAY NOT BE ENOUGH MOISTURE TO WORK WITH SOUTH OF
THE OWL CREEKS. THERE IS STILL A LOT OF ROOM FOR VARIATION THIS
FAR OUT...BUT THIS SITUATION WILL BE WATCHED CLOSELY GIVEN THE
AMOUNT OF INSTABILITY AND SHEAR ANTICIPATED.

.LONG TERM...MEMORIAL DAY THROUGH FRIDAY

MEDIUM RANGE GLOBAL MODELS CONTINUE TO ADVERTISE AN UPPER TROUGH
ACROSS THE WESTERN U.S. AND UPPER RIDGE OVER THE CENTRAL U.S. TO
BEGIN THIS PERIOD...WITH A SLOW PROGRESSION OF THIS OVERALL PATTERN
THROUGH THE WEEK.  A SIGNIFICANT PORTION OF THIS TROUGH ALONG THE
WEST COAST IS KICKED ACROSS THE ROCKIES ON MONDAY WITH THE TROUGH
RELOADING ALONG THE WEST COAST.  GFS CONTINUES TO BRING MORE OF THIS
ENERGY INTO DOWNSTREAM RIDGE WHILE ECMWF LIFTS MOST OF THE ENERGY
WEST OF THE DIVIDE OR INTO WESTERN MONTANA BY 18Z MONDAY.  FORECAST
REFLECTS THE ECMWF HERE...KEEPING BEST MOISTURE AND INSTABILITY
ACROSS NORTHERN WYOMING WHILE A DRIER S-SW FLOW PREVAILS ACROSS THE
SOUTH.

MODELS SHOW INCREASING SPREAD WITH THE HANDLING OF THIS NEXT TROUGH
TO TAKE SHAPE ALONG THE WEST COAST.  THE ECMWF CLOSES OFF AN UPPER
LOW OVER ARIZONA BY TUESDAY NIGHT WHILE GFS AND CANADIAN SHOW A
BROADER OPEN TROUGH PUSHING INTO THE GREAT BASIN.  ECMWF THE ODD MAN
OUT WITH CLOSED SW LOW...AND IS SUSPECT ESPECIALLY THIS LATE IN
THE SEASON. FORECAST LEANS TO THE GFS OVERALL MID TO LATE WEEK
WITH THIS FIRST TROUGH PUSHING ACROSS THE CENTRAL ROCKIES AND INTO
THE PLAINS ON WEDNESDAY WITH ADDITIONAL PACIFIC ENERGY DIGGING A
BROAD TROUGH ACROSS THE WEST AND HIGH PLAINS BY THURSDAY. MOIST
AND UNSTABLE SOUTHERLY FLOW WILL BRING CHANCES OF SHOWERS AND
THUNDERSTORMS ACROSS THE AREA TUESDAY AHEAD OF THE UPPER
TROUGH...THE BEST MOISTURE AND INSTABILITY REMAINING ACROSS THE
NORTH. COOLER AND UNSETTLED FOLLOW ON WEDNESDAY AND THURSDAY...THE
HEART OF THIS TROUGH EXPECTED TO PUSH INTO THE NORTHERN PLAINS BY
FRIDAY.

&&

.AVIATION.../12Z ISSUANCE/

VFR CONDITIONS WILL PREVAIL TODAY AND TONIGHT. ONLY ISOLATED SHOWERS
ARE POSSIBLE IN NW WY THROUGH 01Z SATURDAY.

&&

.FIRE WEATHER...

STRONG SOUTHWEST FLOW WILL DOMINATE MUCH OF THE STATE THROUGH THE
WEEKEND WITH MINIMUM RELATIVE HUMIDITIES EXPECTED TO DROP BELOW 10
PERCENT ACROSS SOUTHERN AND CENTRAL WYOMING. IN ADDITION...WINDS
WILL REMAIN GUSTY AT 15 TO 25 MPH SUSTAINED WITH GUSTS UP TO 45
MPH IN FAVORED AREAS POSSIBLE IN THE AFTERNOON. IN ADDITION...IT
WILL BE UNSTABLE ON SATURDAY AND SUNDAY AFTERNOON...WITH A HAINES
INDEX OF 6 EXPECTED ACROSS MUCH OF THE AREA ON SATURDAY AS IT WILL
BE TOO DRY FOR THUNDERSTORMS. THERE COULD BE STRONG STORMS ON
SUNDAY AFTERNOON ALONG THE I25 CORRIDOR...WITH LESS ORGANIZED AND
THEREFORE WEAKER STORMS EXPECTED ACROSS THE HIGHER ELEVATIONS OF THE
REST OF THE STATE...SOME OF THESE STORMS COULD SNEAK OFF INTO THE
LOWER ELEVATIONS AND SEND OUT STRONG OUTFLOW WINDS.

&&

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

$$

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