Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 210535
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
1135 PM MDT WED MAY 20 2015

.SHORT TERM...TONIGHT THROUGH THURSDAY

COMPLEX...GENERALLY WEAK...FLOW PATTERN OVER THE AREA TODAY WITH
WEAKENING DEFORMATION ZONE HANGING OVER THE CENTRAL ZONES BEHIND OUR
SYSTEM FROM YESTERDAY. ANOTHER CIRCULATION OVER SERN ID IS KEEPING A
LARGE SWATH OF LOW CLOUDS WEST OF THE DIVIDE. SOME SUNSHINE ALONG
THE FAR SRN PORTION OF OUR CWFA IS ALLOWING FOR CONVECTION TO FORM
WITH WHICH WILL HAVE TO CONTEND WITH OVER PORTIONS OF OUR S AND W
INTO THE EVENING. NEXT 24 HOURS ARE ACTUALLY A CHALLENGING FORECAST
WHEN LOOKING AT THE DETAILS. POTENTIAL WEAK SHORTWAVE WHICH MAY BE
OVER CENTRAL NV AND FAR WRN UT IS FORECAST TO MOVE UP INTO OUR SWRN
ZONES THIS EVENING WITH A SWATH OF LIGHT PCPN BEFORE DYING OUT OVER
THE WEST OVERNIGHT. THE ERN FRINGE OF THIS SYSTEM MAY INTERACT WITH
OUR LEFTOVER DEFORMATION ZONE AS IT LIFTS ACROSS CENTRAL WYO
THURSDAY MORNING. MAY JUST BE A BAND OF MID TO SC TYPE CLOUDS WITH A
FEW HIGHER TERRAIN SHOWERS BUT SOMETHING TO WATCH. WOULDN`T RULE OUT
A FEW SPRINKLES ACROSS THE LOWER ELEVATIONS OF CENTRAL WYO IN THE
MORNING. OTHERWISE...WE WAIT FOR OUR MORE ORGANIZED SHORTWAVE THAT
MOVES INTO SWRN WYO NEAR THE END OF THIS PERIOD. SOME TIMING
DIFFERENCES BUT POTENTIAL FOR A MID LEVEL CIRCULATION TO ENHANCE
PCPN IN OUR SRN AND SWRN ZONES IS THERE AT THE END OF THIS PERIOD
AND ESPECIALLY INTO MY PARTNER`S PERIOD THURSDAY NIGHT. WILL
INCREASE THE SHOWER/ISOLD TSTM POTENTIAL IN THIS AREAS RIGHT UP TO
THE WIND RIVERS/GREEN MTNS ZONE AND POINTS SOUTH AND WEST FOR
THURSDAY AFTERNOON. THIS SYSTEM IS VERY VISIBLE THIS AFTERNOON AS IT
HEADS INTO SRN NV. JUST THE BEGINNINGS OF ANOTHER MAJOR CHANGE TO
UNSETTLED WEATHER THAT MY LONGER TERM PARTNER GETS TO DEAL WITH.

.LONG TERM...THURSDAY NIGHT THROUGH WEDNESDAY

THE BLOCKING HIGH OVER WESTERN CANADA WILL CONTINUE TO BE A PLAYER IN
OUR WEATHER PATTERN OVER THE NEXT SEVERAL DAYS. THE PACIFIC LOW
PRESSURE CENTERS WILL ENCOUNTER THE HIGH ON THEIR EAST BOUND JOURNEY
AND WILL SPLIT WITH THE SOUTHERN PORTION OF THE SPLIT ENERGY
AFFECTING OUR WEATHER. THE NEXT SUCH LOW WILL HAVE BEEN DIVERTED
SOUTH TO OFF THE SOUTHERN CALIFORNIA COAST BY 00Z FRIDAY. BY
THURSDAY NIGHT OUR CWA WILL BE UNDERNEATH THE SOUTHERLY DIFLUENT
FLOW AHEAD OF THIS CLOSED LOW. WITH THE INCREASED INSTABILITY AND
DIFLUENT FLOW...SCATTERED THUNDERSTORMS WILL BE A GOOD BET EVERYWHERE
THURSDAY EVENING. THE MOST NUMEROUS CONVECTIVE CELLS WILL TAKE
PLACE IN SOUTHWEST WY WHERE A WEAK MID LEVEL CIRCULATION IS PROGGED
TO BE LOCATED AS IT DRIFTS IN FROM THE SOUTHWEST. THEN ON FRIDAY WE CAN
EXPECT MORE WIDESPREAD CONVECTION EXCEPT WILL BE MORE WIDELY
SCATTERED IN BIG HORN AND JOHNSON COUNTIES AS THE GFS TAKES THE
CALIFORNIA UPPER CLOSED LOW INTO THE VEGAS AREA AND THE EURO FARTHER
SOUTH ACROSS THE ARIZONA/CA BORDER WITH A BAGGY TROUGH EXTENDING
NORTHEAST INTO WYOMING FROM THAT MAIN LOW. THEN ON SATURDAY AND
SATURDAY NIGHT...KEEPING WITH TRADITION AND CLIMATOLOGY FOR COOL WET
MEMORIAL DAY WEEKENDS FOR THIS PART OF THE WORLD...A COOLISH WET
SATURDAY AND SATURDAY NIGHT IS IN STORE AS THE REMNANTS OF THIS
CLOSED LOW AGAIN TRACKS DIRECTLY OVER WY. H7 TEMPS ARE PROGGED TO
DROP TO BETWEEN ZERO AND -1 BY 12Z SUNDAY AS COOLER AIR WRAPS AROUND
THE BACKSIDE OF THE LOW. WHEN YOU FIGURE IN ADIABATIC COOLING FROM
SFC UPSLOPE FLOW EAST OF THE DIVIDE AROUND THE BACKSIDE OF THE LOW
COMBINED WITH SAID H7 TEMPS...SNOW LEVELS COULD ONCE AGAIN EASILY
DROP TO BETWEEN 7500 AND 8000 FEET IN THE BIG HORN MTNS AND EAST
SLOPE OF THE WIND RIVER RANGE BY SUNDAY MORNING. FOR NOW HAVE
INDICATED LIGHT SNOW AMOUNTS WITHOUT ANY OTHER STRONG DYNAMICS TO
SPEAK ASCT WITH THIS LOW PRESSURE CENTER BY 12Z SUNDAY WITH THE BULK
OF THE PRECIP LIKELY OCCURRING EARLIER SATURDAY NIGHT BEFORE MAXIMUM
COOLING. HOWEVER...WILL MONITOR THIS EVENT THROUGH THE REST OF THIS WEEK.
THIS LOW WILL GET SHEARED APART BEFORE A SHORT LIVED TRANSITORY
RIDGE BUILDS IN FROM THE WEST AND A SHORT PERIOD OF DRYING TAKES
PLACE LATER SUNDAY AND INTO SUNDAY NIGHT. THIS IS THE POINT WHERE
THE GFS AND THE EURO BEGIN TO DIVERGE WITH EACH OTHER AND WITH TIME.
THE EURO HAS BEEN CONSISTENTLY SLOWER WITH THE NEXT PACIFIC LOW
COMING OUT OF THE PAC NW AND SHEARING IT APART IN MT BY NEXT
WEDNESDAY. THE EURO ALSO WANTS TO BREAK DOWN THE WESTERN CANADIAN
RIDGE. THE GFS IS FASTER AND HOLDS THIS NEXT PAC NW LOW TOGETHER
WHEN IT BRINGS IT ACROSS WY IN ONE PIECE BY TUESDAY EVEN THOUGH THE
PREVIOUS RUN SHEARED IT APART AND THE EURO WAS THE MODEL THAT
SHEARED THIS LOW APART IN THE PREVIOUS RUN. THE GFS KEEPS THE
WESTERN CANADIAN HIGH IN PLACE. IT SEEMS AS IF THE EURO MAY NOT KNOW
WHAT TO DO WITH THE BLOCKING HIGH. WITH THIS MODEL
UNCERTAINTY...WILL LEAVE A  CONTINUED CHANCE OF AFTERNOON AND
EVENING SHOWERS AND THUNDERSTORMS THROUGH EARLY NEXT WEAK WITH THE
ASSUMPTION THAT THE HIGH WILL REMAIN WHERE IT IS WITH MORE
SOUTHWESTERLY DIFLUENT FLOW AHEAD OF THIS NEXT PAC NW LOW.

TEMPERATURES SHOULD BEGIN TO WARM UP THROUGH EARLY NEXT WEEK AFTER
THE CLOSED LOW WEAKENS ON SUNDAY.

ONE SIDE NOTE...ANY T STORMS THAT DEVELOP WILL LIKELY HAVE A LOW WET
BULB ZERO SO MOST THUNDERSTORMS THAT DEVELOP THIS WEEK WILL HAVE THE
POTENTIAL TO PRODUCE SMALL HAIL AS IS COMMON FOR THIS TIME OF
YEAR.

&&

.AVIATION.../06Z ISSUANCE/

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

CEILINGS SHOULD STAY VFR AT ALL TERMINALS OVERNIGHT AND THURSDAY.
COULD BE LOCAL MVFR AT KCPR LATE TONIGHT WHERE EASTERLY LOW LEVEL
FLOW MAY ADVECT LOWER CLOUD DECK FROM VICINITY OF KDGW WHERE RECENT
MVFR NOTED. CLOUD COVER WILL DETERMINE WHETHER OR NOT KRIW OR KLND
GET VCFG OR FG. IF THERE IS SUFFICIENT CLEARING THEN BEST CHANCE FOR
FOG WOULD BE 11Z-15Z/THU. MOUNTAIN TOP OBSCURATIONS WILL PERSIST
THROUGH MID-MORNING THURSDAY...PARTICULARLY ALONG THE WIND RIVER
MOUNTAINS.  SHORTWAVE AND ASSOCIATED MID-CLOUDS AND INSTABILITY WILL
LIFT NORTH OF THE FORECAST AREA BY 15Z/THU WITH MORE SUNSHINE
EXPECTED THAN PREVIOUS DAYS. NEXT SHORTWAVE WILL APPROACH SOUTHWEST
WYOMING THURSDAY AFTERNOON...BUT BEST INSTABILITY APPEARS TO BE
ALONG AND TO THE SOUTHWEST OF THE CONTINENTAL DIVIDE. ANY ISOLATED
THUNDERSTORMS WILL MAINLY BE CONFINED TO THE HIGHER TERRAIN AFTER
21Z/THU. ANY DEEP CONVECTION WILL WANE WITH THE SETTING SUN.

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

SHORTWAVE LIFTING NORTH ALONG THE WESTERN BORDER CONTINUES TO
GENERATE WEAK SHOWERS FROM KJAC SOUTH TO KEMM LATE WEDNESDAY
EVENING. SATELLITE TRENDS INDICATE WARMING CLOUD TOPS AND EXPECT
MUCH OF THIS ACTIVITY TO WANE BY 09Z/THU. CEILINGS WILL REMAIN VFR
THROUGH THE FORECAST PERIOD AT ALL TERMINALS. MOUNTAIN TOP
OBSCURATIONS WILL BE WIDESPREAD UNTIL 12Z/-15Z/THU AS SHORTWAVE
FINALLY LIFTS NORTH AND CLEARS THE REGION. NEXT SHORTWAVE WILL
ROTATE NORTH THURSDAY AFTERNOON. SHOWERS WILL BE MOST NUMEROUS OVER
THE HIGHER TERRAIN AFTER 20Z/THU. INSTABILITY WILL BE SUFFICIENT...
ESPECIALLY AFTER 21Z/THU TO GENERATE ISOLATED TO WIDELY SCATTERED
THUNDERSTORMS. SHOWERS AND STORMS WILL CONTINUE UNTIL ABOUT 04Z/FRI
BEFORE DECREASING IN COVERAGE AND INTENSITY.

&&

.FIRE WEATHER...


RELATIVELY LOW FIRE DANGER CONTINUES WITH GENERALLY MOIST
FUELS...HIGHER RH`S AND NO SIGNIFICANT WIND THROUGH THURSDAY. ISOLATED
THUNDERSTORMS ARE EXPECTED THOUGH ACROSS THE SOUTH AND WEST THROUGH
EARLY THIS EVENING AND IN MANY AREAS THURSDAY AFTERNOON. DEEPER
MOISTURE WILL MOVE TOWARD THE AREA FOR FRIDAY AND BRING ANOTHER
CHANCE OF A WETTING RAIN FROM FRIDAY INTO FRIDAY NIGHT WITH HIGH
ELEVATION SNOW POSSIBLE AS WELL.

&&

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

&&

$$

SHORT TERM...PS
LONG TERM...LIPSON
AVIATION...CNJ
FIRE WEATHER...PS





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