Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 020906
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
306 AM MDT THU OCT 2 2014

.SHORT TERM...TODAY THROUGH SATURDAY NIGHT

LW TROF ACROSS THE CONUS...BOOKENDED BY RIDGES OVR EACH OCEAN.
TROF AXIS HAS NOW PASSED THRU THE FA AND IS CURRENTLY ALIGNED NEAR
ALONG THE ERN WY BORDER...PUTTING THE CWA UNDER NW FLOW. UPSTREAM
IN THE UPR LVLS...NEXT SW/JET STREAK MOVING THRU THE MAIN FLOW IS
CURRENTLY LOCATED OVR SWRN CANADA...AND WILL ARRIVE LATER TODAY.
AT THE SFC...HIGH PRESSURE INCREASING ACROSS THE STATE WITH A
DEVELOPING FRONT WELL TO THE NORTH OVR SRN CA. THIS FEATURE WILL
BE PUSHED SWD BY THE ADVANCE OF THE UPR LVL SW.

TODAY/TONIGHT...LIGHT MIXING WINDS NEAR SFC ALONG WITH INCREASING
DRY DENSE AIR ALOFT OVERRUNNING A MODESTLY MOIST BOUNDARY LAYER WILL
POSSIBLY MAKE FOR AREAS OF PATCHY FOG TO BEGIN THE DAY.
OTHERWISE...FLAT TO MODEST RIDGING WILL QUICKLY GIVE WAY TO THE UPR
SW MENTIONED ABOVE AS IT SWINGS THRU THE BACKSIDE OF THE MAIN
TROF...DRIVING THE AFOREMENTIONED DEVELOPING COLD FRONT DOWN THRU
THE FA EAST OF THE DIVIDE IN A CLIPPER-LIKE FASHION IN THE AFTERNOON
AND EVENING. SPEED AND MODEST MOISTURE WILL BE THE LIMITING FACTORS
WITH THIS FEATURE. WHILE PRECIP CHANCES WILL DEFINITELY INCREASE E
OF THE DIVIDE...MAINLY ACROSS THE NRN MOUNTAINS...BIGHORN
BASIN...AND JOHNSON AND NATRONA COUNTIES...QPF LL REMAIN ON THE LOW
SIDE OF THINGS. CHANCES FOR LIGHT SNOW WILL BE POSSIBLE ACROSS THE
MOUNTAINS ONCE AGAIN ABOVE STARTING OUT JUST ABOVE 7000 FEET...BUT
LOWERING RATHER QUICKLY BEHIND THE FRONT TO BASIN LEVEL DURING THE
EVENING PERIOD. MEASURABLE AMOUNTS OF SNOW ATTM LOOK TO REMAIN
MOSTLY IN THE MOUNTAINS WITH AN INCH OR TWO AT MOST FROM THIS QUICK
MOVING SYSTEM. THE LOWER LEVELS OF JOHNSON...NATRONA...BIG HORN AND
PERHAPS EVEN FREMONT COUNTIES WILL PERHAPS BE PRIVY TO TRACE TO A
FEW TENTHS OF AN INCH OF SNOW. THIS SYSTEM WILL QUICKLY MOVE OUT OF
THE AREA OVERNIGHT...LEAVING MAINLY CLEAR COLD AND DRY SKIES BY FRI
MORNING. HAVE LOWERED MIN TEMPS TO FIT THIS PROFILE UNDER H7 TEMPS
RANGING FROM -7 TO -9 DEG C. OTHERWISE/ELSEWHERE...BREEZY TO LOW END
WINDY CONDITIONS WILL BE THE STORY OF THE DAY ACROSS THE FA...AS A
MODESTLY STRENGTHENING SFC P GRAD COMBINES WILL GREATLY INCREASING
UPR LVL FLOW UNDER MODEST INSTABILITY THROUGH THE DAY TO GIVE WEST
WINDS CAPABLE OF GUSTS OF 30 TO 40 MPH BY AFTERNOON.
ADDITIONALLY...WINDS WILL ALSO INCREASE AND BECOME GUSTY NORTHERLY
BEHIND THE FRONT LATE IN THE AFTERNOON AND INTO THE EVENING AS A
STRONG E/W ORIENTED P GRAD IS INTRODUCED TO THE FA...TRAVELING FROM
N TO S.

FRI/SAT...STRONG SFC HIGH P IN PLACE STILL UNDER RELATIVELY STRONG
NW FLOW TO BEGIN THE DAY FRIDAY...WITH MOSTLY SUNNY/DRY AND WARMING
CONDITIONS CWA WIDE. WINDS WILL ACCORDINGLY DROP FROM WHAT THEY WERE
THE PREVIOUS DAY AS THE SFC P GRAD WEAKENS UNDER THE HIGH PRESSURE
IN ADDITION TO WEAKENING WINDS ALOFT. SAT...HIGH PRESSURE BEGINS TO
WEAKEN AS LL WARMING CONTINUES AND LOWER PRESSURE IS INTRODUCED.
SFC TO 700 MB LAPSE RATES WILL BECOME UNSTABLE ENOUGH TO BRING DOWN
WINDS FROM ALOFT THIS DAY...RESULTING IN MODERATELY GUSTY W WINDS
MAINLY W OF THE DIVIDE AND OVR THE HIGHER MOUNTAINS...ALTHOUGH NOT
NEARLY AS STRONG AS THOSE OF TODAY. EARLY IN THE DAY ON SAT...A
SLIGHT INCREASE IN MAINLY MID LVL MOISTURE MAY INTRODUCE AN ISOLATED
SHOWER OR TWO OVR THE NRN HIGH MOUNTAINS...BUT NOTHING WORTH
MEASURING AT THIS POINT.

.LONG TERM...SUNDAY THROUGH THURSDAY

SYNOPSIS...NORTHWEST FLOW ALOFT WILL PREVAIL SUNDAY AND MONDAY
BETWEEN AN UPPER RIDGE ALONG THE WEST COAST AND UPPER TROUGH ACROSS
THE EASTERN U.S.   THIS FLOW WILL BECOME MORE ZONAL OR WESTERLY
TUESDAY THROUGH THURSDAY WITH POSSIBLY SOME RAIN AND SNOW SHOWERS
PUSHING INTO THE NORTHERN MOUNTAINS WEDNESDAY AND THURSDAY.

DISCUSSION...A CLIPPER IN NW FLOW WILL DIVE SOUTHEAST ACROSS THE
NORTHERN HIGH PLAINS SUNDAY AROUND BACKSIDE OF UPPER LOW OVER
ONTARIO THAT IS EXPECTED TO WOBBLE SLIGHTLY WESTWARD.  ECMWF
GENERATES SOME PRECIPITATION ACROSS EASTERN WYOMING...IN THE VCNTY
OF THE BIGHORNS AND ALONG AND EAST OF I-25...WHILE THE GFS KEEPS
THINGS DRY.   BOTH MODELS SHOW SOME CONTINUITY WITH THEIR RESPECTIVE
QPF FIELDS.  WE CONTINUED THE IDEA OF A DRY FORECAST ON SUNDAY WITH
OUR NEIGHBORS TO THE NORTH AND EAST ALSO KEEPING ANY CHANCES OF
PRECIPITATION WELL AWAY FROM OUR BORDERS FOR NOW.  TEMPERATURES WERE
COOLED A FEW DEGREES ACROSS EASTERN ZONES ON THE EDGE OF A DEEPER
COLDER AIRMASS SPILLING INTO THE NORTHERN PLAINS.

FLOW BACKS MONDAY WITH WEAK WARM ADVECTION RESULTING IN TEMPERATURES
A FEW DEGREES WARMER.

TRANSITION TO A MORE ZONAL FLOW BEGINS ON TUESDAY AS UPPER TROUGH
LIFTS OUT OF THE EASTERN U.S. AND OMEGA BLOCK BREAKS DOWN OVER
ALASKA/BERING SEA.  ECMWF AND GFS IN BETTER AGREEMENT KEEPING MEAN
RIDGE JUST OF THE WEST COAST WEDNESDAY AND THURSDAY WITH ELONGATED
W-E TROUGH DOWNSTREAM ACROSS CANADA.  THUS...ANY FRONTAL INTRUSIONS
FROM THE NORTH LATE IN THE WEEK WILL LIKELY BE SHALLOW AND MAINLY
DRY...COOLING TEMPERATURES BY ONLY A FEW DEGREES.  SLIGHT CHANCE
POPS WERE KEPT IN THE NORTHERN MOUNTAINS WEDNESDAY AND THURSDAY AS
WESTERLY FLOW MOISTENS UP A BIT.


&&

.AVIATION.../12Z ISSUANCE/

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

PATCHY LOW LYING FOG IS POSSIBLE IN THE CENTRAL BASINS UNTIL 15Z.
A NEW COLD FRONT WILL MOVE INTO NORTHERN WY AROUND 21Z AND MOVE
QUICKLY SOUTH INTO CENTRAL WY AFTER 01Z. RAIN SHOWERS WILL TURN TO
SNOW SHOWERS OVER NORTHERN WY BY 00Z WITH MVFR TO IFR CONDITIONS
AND MOUNTAIN OBSCURATION. THE RAIN AND SNOW SHOWERS WILL MOVE INTO
CENTRAL WY BY 03Z AND CONTINUE UNTIL 09Z WITH MVFR TO IFR CONDITIONS
AND MOUNTAIN OBSCURATION. CONDITIONS OVER THE NORTH WILL IMPROVE
AFTER 06Z FRIDAY. PLEASE SEE INDIVIDUAL TAF SITES FOR MORE DETAILS.

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

PATCHY LOW FOG AND LOW CLOUDS WILL OCCUR UNTIL 16Z WITH LOCAL MVFR
TO IFR CONDITIONS. VFR CONDITIONS WILL OCCUR OVER THE REGION THROUGH
TONIGHT. ISOLATED RAIN OR SNOW SHOWERS WILL OCCUR IN THE NW WY
MOUNTAINS LATE IN THE AFTERNOON INTO THE EVENING...ENDING BY 06Z
FRIDAY. EXPECT GUSTY WIND AT THE TAF SITES BY 18Z UNTIL 01Z. KRKS
WILL SEE A WIND SHIFT AFTER 06Z AS THE COLD FRONT BRUSHES BY THAT
AREA.

&&

.FIRE WEATHER...

FIRE DANGER WILL REMAIN LOW ACROSS THE FORECAST AREA THROUGH THE
REST OF THE WEEK. HOWEVER...BREEZY TO WINDY CONDITIONS WILL DEVELOP
TODAY AS SURFACE LOW PRESSURE IS REPLACED BY HIGH PRESSURE AND UPPER
LEVEL WINDS BEGIN TO INCREASE IN RESPONSE TO A STRENGTHENING UPPER
LEVEL JET STREAK. ADDITIONALLY...ANOTHER CHANCE FOR LIGHT
PRECIPITATION...BOTH LIQUID AND FROZEN...WILL PRESENT ITSELF LATER
THIS AFTERNOON AND INTO EVENING...MAINLY EAST OF THE DIVIDE AS
ANOTHER COLD FRONT PUSHES SOUTH ACROSS THE FORECAST AREA.

&&

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

$$

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







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