Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 270914
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
314 AM MDT SAT JUN 27 2015

.SHORT TERM...TODAY THROUGH MONDAY NIGHT

HIGH PRESSURE OVER THE AREA HAS SPARSE CLOUD COVER OVER THE AREA
WITH WIDESPREAD DRY CONDITIONS. THESE CONDITIONS WILL PERSIST
THROUGH THE MORNING...WITH A SURGE OF MOISTURE POSSIBLE THIS
AFTERNOON. SHOWERS AND THUNDERSTORMS MAY BE ASSOCIATED WITH THIS
MOISTURE...MAINLY IMPACT THE CENTRAL MOUNTAIN RANGES.

THIS PATTERN WILL THEN BECOME THE NORM FOR THE NEXT COUPLE OF
DAYS...WITH HIGH PRESSURE DRIVEN DRY CONDITIONS OVERNIGHT AND WEAK
MOUNTAIN SHOWER ACTIVITY IN THE AFTERNOON. TEMPERATURES WILL
CONTINUE TO WARM THROUGH THE FORECAST PERIOD...WITH HIGHS REACHING
INTO THE UPPER 90S BY SUNDAY AND MONDAY. THERE IS THE POSSIBLE FOR
SOME LOCATIONS TO PASS THE CENTURY MARK ON MONDAY ACROSS THE BIGHORN
BASIN. THUNDERSTORMS MAY CONTAIN GUSTY WIND AS WELL AS SMALL HAIL.

.LONG TERM...TUESDAY THROUGH SATURDAY

SYNOPSIS...A STRONG UPPER LEVEL HIGH PRESSURE SYSTEM OVER THE GREAT
BASIN WILL CONTINUE TO BRING HOT TEMPERATURES ACROSS THE AREA ON
TUESDAY.  THIS HIGH PRESSURE CENTER WILL DRIFT NW INTO N. CALIFORNIA
AND NEVADA ON WEDNESDAY RESULTING IN A NW FLOW ACROSS THE AREA AND A
COOLING TREND THROUGH THE 4TH OF JULY.  ISOLATED TO WIDELY SCATTERED
THUNDERSTORMS WILL BE MAINLY FOUND ALONG AND NORTH OF A CASPER TO
JACKSON LINE THROUGH THE PERIOD.

DISCUSSION...THE MAIN FORECAST CHALLENGE THROUGH THIS PERIOD
CONTINUES TO BE THE ADVERTISED NW DRIFT OF HOT UPPER HIGH OVER THE
GREAT BASIN AND DEGREE OF AMPLIFICATION OF DOWNSTREAM TROUGH LATTER
IN THE WEEK EAST OF THE ROCKIES.  MODELS HAVE SHOWN POOR RUN-TO-RUN
CONSISTENCY IN DISLODGING AND PROGRESSION OF UPPER LOW...CURRENTLY
OVER THE GULF OF ALASKA...OVER THE WESTERN U.S. RIDGE. THE GFS IS
ADVERTISING A SHARPER TROUGH CARVED OUT BY THIS SYSTEM OVER THE
NORTHERN PLAINS FRIDAY AND SATURDAY WHILE THE ECMWF MAINTAINS
HIGHER HEIGHTS ACROSS THE ROCKIES AND HIGH PLAINS. WHILE THE
MODELS CONTINUE FLIP-FLOPPING...WE WILL ALSO BE FLIPPING THE
CALENDAR TO JULY WHICH INCREASINGLY FAVORS THE MORE EXPANSIVE HOT
UPPER HIGH OVER THE GREAT BASIN/INTERMOUNTAIN WEST PER THE LATEST
ECMWF (AND PREVIOUS GFS RUNS) THROUGH THE 4TH.

THUS...THOUGH AN OVERALL COOLING TREND IS EXPECTED WEDNESDAY THROUGH
FRIDAY...FORECAST LEANS AWAY FROM THE MORE SIGNIFICANT CANADIAN AIR
INTRUSION ADVERTISED BY THE GFS BY THE 4TH.  THERE IS MODERATE
CONFIDENCE IN A COUPLE OF `RIDGE RIDERS` THAT WILL TRAVEL AROUND THE
NORTHERN PERIPHERY OF THE UPPER HIGH ACROSS THE NORTHERN ROCKIES
INTO THE NORTHERN PLAINS TUESDAY AND WEDNESDAY...BRINGING CHANCES OF
THUNDERSTORMS MAINLY ACROSS THE NORTHERN MTNS...BIGHORN
BASIN/JOHNSON COUNTY.

ECMWF SHOWS A DRIER AND WARMER WNW FLOW ACROSS THE AREA FRIDAY
THROUGH THE 4TH WHILE GFS SHOWS A MORE ACTIVE NW FLOW. FORECAST
KEEPS ISOLATED ACTIVITY AROUND PERIPHERY A MORE DOMINANT UPPER
HIGH...ACROSS THE NORTH AND EAST THURSDAY THROUGH THE FOURTH.

&&

.AVIATION.../12Z ISSUANCE/

ISOLATED AFTERNOON AND EVENING (21Z-03Z) THUNDERSTORMS COULD DEVELOP
OVER THE MOUNTAINS AND PUSH SOUTHEAST INTO ADJACENT FOOTHILLS BEFORE
QUICKLY DISSIPATING. THE BEST SHOT FOR THIS ISOLATED CONVECTION
APPEARS TO BE OVER THE SOUTHERN ABSAROKA AND SOUTHERN WIND RIVER
MTS. THE MAIN HAZARD FROM THIS ACTIVITY WILL BE STRONG OUTFLOW
WINDS. THE LIMITED COVERAGE OF THUNDERSTORMS PRECLUDE ANY TS MENTION
AT ANY TERMINAL FOR NOW. TERMINALS ARE EXPECTED TO BE VFR THROUGH
THE PERIOD...WITH MAINLY LIGHT WINDS. THE EXCEPTION WOULD BE NEAR
ANY THUNDERSTORMS...WITH THE MAIN HAZARD BEING STRONG OUTFLOW
WINDS.

&&

.FIRE WEATHER...

HIGH PRESSURE WILL KEEP DRY CONDITIONS OVER THE FORECAST AREA
THROUGH THE MORNING HOURS. A SURGE OF MOISTURE WILL INTRODUCE
ISOLATED SHOWERS AND THUNDERSTORMS THIS AFTERNOON. DRY OVERNIGHT
PERIODS WITH AFTERNOON SHOWERS AND THUNDERSTORMS WILL BE THE PATTERN
FOR THE NEXT COUPLE DAYS UNDER THE INFLUENCE OF THE AREA OF HIGH
PRESSURE. TEMPERATURES WILL WARM UNDER THE PRESENCE OF THE
RIDGE...WITH SOME AREAS APPROACHING 100 DEGREES BY SUNDAY AND
MONDAY. DAYTIME HUMIDITY WILL DECREASE AS WELL...THOUGH WIND IS
GENERALLY EXPECTED TO REMAIN LOWER THAN CRITICAL THRESHOLDS.

&&

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

$$

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



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