Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 190552
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
1152 PM MDT MON AUG 18 2014

.SHORT TERM...TONIGHT THROUGH TUESDAY (ISSUED AT 317 PM MDT)

SHOWERS AND THUNDERSTORMS CONTINUE TO DEVELOP ACROSS THE FORECAST
AREA THIS AFTERNOON. COVERAGE WILL INCREASE THROUGH THE LATE
AFTERNOON...WITH ACTIVITY WANING WITH THE LOSS OF DAYLIGHT. GUSTY
WIND IS THE PRIMARY THREAT WITH ACTIVITY TODAY. A FEW SHOWERS MAY
LINGER INTO THE OVERNIGHT HOURS...BUT WIDESPREAD DRY CONDITIONS ARE
FAVORABLE.

AN AREA OF LOW PRESSURE OFF THE CALIFORNIA COAST WILL PUSH ITS WAY
INLAND. AS THE SYSTEM MOVES FURTHER EAST...IT WILL PUSH A SURGE OF
MOISTURE ACROSS THE FORECAST AREA. ENHANCED INSTABILITY IS ALSO
ANTICIPATED...WITH SHOWERS AND THUNDERSTORMS EXPECTED ACROSS MUCH OF
THE AREA. THE WESTERN THIRD OF THE FORECAST AREA LOOKS MOST
FAVORABLE FOR DEVELOPMENT. PWATS ACROSS THE AREA DO LOOK TO INCREASE
DURING THE AFTERNOON...SO PERIODS OF HEAVY RAIN ARE NOT OUT OF THE
QUESTION WITH ACTIVITY TUESDAY. GUSTY WIND LOOKS FAVORABLE AS
WELL...AS IT USUALLY DOES ACROSS WYOMING. SMALL HAIL AND CLOUD TO
GROUND LIGHTNING WILL ALSO BE POTENTIAL THREATS WITH ANY STORM
ACTIVITY.

.LONG TERM...TUESDAY NIGHT THROUGH MONDAY

A SHORT WAVE AHEAD OF A TROUGH DIGGING ACROSS THE WEST COAST WILL
CONTINUE TO MOVE NORTHEAST ACROSS THE FORECAST AREA. BROAD SCALE
LIFT WITH THE WAVE IN ADDITION TO A SMALL BUT WELL PLACED SPEED MAX
AND FAVORABLE QG FORCING WILL HAVE PLENTY OF FUEL TO WORK WITH AS A
RICH PLUME OF MONSOONAL MOISTURE ARRIVES WITH THE WAVE. HOWEVER...
INSTABILITY MAY BE LIMITED SOMEWHAT BY CLOUD COVER EARLIER IN THE
DAY BUT WITH ALL OF THE DYNAMIC SUPPORT THERE WILL BE AT LEAST
SCATTERED SHOWERS AND STORMS. THERE COULD PERHAPS BE A FEW STRONGER
STORMS ALONG THE I-25 CORRIDOR WHERE THERE WILL BE A BETTER CHANCE
OF CLEAR TO PARTLY CLOUDY SKIES EARLIER IN THE DAY ALLOWING FOR
STEEPER LAPSE RATES TO DEVELOP. PWAT VALUES WILL BE CLOSE TO THE
99TH PERCENTILE BY THE EVENING ALONG AND EAST OF THE DIVIDE...SO IF
SOME OF THE DEEPER INSTABILITY IS REALIZED THEN SOME OF THESE STORMS
COULD PRODUCE TORRENTIAL DOWNPOURS AND INCREASE FLASH FLOOD HAZARD.

THERE WILL BE A BIT OF A BREAK ON WEDNESDAY...ESPECIALLY IN THE
UPPER LEVEL SUPPORT DEPARTMENT. THERE WILL HOWEVER BE SOME
INSTABILITY TO BE REALIZED IF THERE ARE SOME BREAKS IN THE CLOUDS
DURING THE DAY...AND THERE ARE EXPECTED TO BE. SO THERE WILL BE
MAINLY AIRMASS TYPE THUNDERSTORMS WITH SOME WEAK SHEAR AND MODERATE
STORM MOVEMENT...10 TO 20KTS...SO ONLY MINOR IMPACTS EXPECTED ON
WEDNESDAY.

THE LONG WAVE DIGGING ACROSS THE WEST COULD TAKE ON A NEGATIVE TILT
AND BEGIN TO MOVE THROUGH ON THURSDAY. THERE IS STILL SOME
SIGNIFICANT MODEL AGREEMENT AT THIS TIME BUT THERE IS POTENTIAL FOR
MORE ORGANIZED THUNDERSTORM ACTIVITY ON THURSDAY IF THE GFS SOLUTION
COMES TO FRUITION.

MEDIUM RANGE PERIOD BEGINS WITH GLOBAL MODELS IN FAIRLY GOOD
AGREEMENT WITH SYNOPTIC FEATURES. TROUGH TAKES SHAPE ACROSS THE
PACIFIC NORTHWEST AND THE NORTHERN ROCKIES BY FRIDAY WHILE RIDGE
SETS-UP ALONG THE GULF COAST STRETCHING NORTH INTO THE SOUTHERN
PLAINS. ANOTHER FEATURE WILL BE AN UPPER LOW TO OUR SOUTHWEST TO
BEGIN THE PERIOD. GFS IS FARTHER NORTH AT 00Z/FRI THAN THE ECMWF AND
IS FASTER IN SHEARING THIS LOW NORTHEAST ACROSS THE CENTRAL ROCKIES.
ECMWF LAGS THIS FEATURE BY ABOUT 12-18 HOURS WHICH WOULD CERTAINLY
IMPACT THE WEATHER PICTURE ACROSS SOUTHERN AND PERHAPS CENTRAL
WYOMING. THE 12Z RUN OF THE ECMWF HAS TRENDED TO AT LEAST BRINGING
THIS FEATURE NORTHEAST AS OPPOSED TO THE EARLIER SOLUTION OF HOLDING
BACK NEAR SOUTHERN CA. TIMING OF THIS FEATURE COULD HAVE IMPACTS BUT
IT APPEARS THIS SHORTWAVE WILL WEAKEN AND SHEAR APART AS IT TRACKS
NORTHEAST AND MAY HAVE MORE IMPACT ON SOUTHEAST WYOMING.

WE THEN TURN OUR ATTENTION TO SHORTWAVE POISED TO OUR WEST BY
00Z/SATURDAY. MID-LEVEL CIRCULATION LOOKS TO TRACK ACROSS THE
NORTHERN PORTION OF THE FORECAST AREA. BOTH THE GFS AND ECMWF BRING
THIS FEATURE ACROSS THE STATE FROM WEST-TO-EAST ON SATURDAY.
AGAIN...TIMING WILL PLAY A BIG ROLE IN THE COVERAGE AND INTENSITY OF
CONVECTION SATURDAY AFTERNOON. QUICKER GFS SOLUTION WOULD FAVOR BEST
CONVECTION ACROSS OUR EASTERN ZONES AND THE BIGHORNS...WHILE SLOWER
ECMWF WOULD PRODUCE MORE WIDESPREAD CONVECTION. FOR NOW HAVE
CONTINUED WITH MODEL BLEND AND HAVE HIT THE NORTH AND EASTERN ZONES
HARDEST. PASSAGE OF THIS FEATURE WILL USHER A COLD FRONT SOUTH
ACROSS CENTRAL WYOMING EAST OF THE CONTINENTAL DIVIDE. THERE SHOULD
BE SOME GOOD CLEARING BEHIND THIS WAVE. HAVE COOLED SUNDAY MORNING
LOWS A DEGREE OR TWO BUT THEY MAY NEED TO BE DROPPED FURTHER PENDING
FUTURE MODEL RUNS. ZONAL FLOW ANTICIPATED FOR SUNDAY AND HAVE
TRENDED MORE TOWARD CLIMATOLOGICAL PRECIP CHANCES BUT HAVE CONTINUED
TO FAVOR THE FAR NORTH FOR BEST CHANCES.

MODELS SEEM TO BE STRUGGLING BEYOND SUNDAY. THE 00Z RUN OF THE ECMWF
WAS ADVERTISING A CONTINUATION OF THE PACIFIC NORTHWEST/NORTHERN
ROCKIES TROUGH AND BRINGING A REINFORCING SHOT OF ENERGY INTO THE
TROUGH ON THE DOWNSTREAM SIDE OF THE PACIFIC RIDGE BY EARLY NEXT
WEEK. IT WOULD BE DIFFICULT TO FIND ANY TRACES OF THAT SOLUTION WITH
THE 12Z RUN WHICH HAS TRENDED TOWARD THE 12Z GFS RUN. BOTH NOW
INDICATING A BUILDING RIDGE ACROSS THE WESTERN US WITH TAIL END OF
THE TROUGH CROSSING THE FORECAST AREA MONDAY INTO TUESDAY. NEEDLESS
TO SAY...FOR NOW HAVE TRENDED AGAIN TOWARD CLIMATOLOGICAL PRECIP
CHANCES GIVEN SUCH UNCERTAINTY. ONE AREA OF AGREEMENT IS THAT
TEMPERATURES WILL BE BELOW NORMAL THROUGH THE MEDIUM RANGE PERIOD.

&&

.AVIATION.../06Z ISSUANCE/

A MOISTURE-LADEN UPPER LOW PRESSURE SYSTEM WILL LIFT NE ACROSS THE
GREAT BASIN TODAY AND ACROSS WYOMING TONIGHT.  AHEAD OF THIS SYSTEM
THE AIRMASS WILL DE-STABILIZE ACROSS EASTERN IDAHO AND FAR WEST
WYOMING WHERE THE BEST THUNDERSTORM COVERAGE IS EXPECTED THIS
AFTERNOON...MAINLY FROM A KEVW TO VCNTY KAFO IN THE EARLY AFTERNOON
PUSHING INTO NW WYOMING VCNTY AND NORTH OF KJAC IN THE LATE
AFTERNOON AND EVENING.  THUNDERSTORM COVERAGE WILL BE MORE ISOLATED
FURTHER EAST.

THE SYSTEM WILL SPREAD SCT-NMRS SHRA S-N ACROSS WEST AND CENTRAL
WYOMING TUESDAY EVENING WITH A FEW EMBEDDED THUNDERSTORMS
LINGERING.  CIGS WILL LOWER MAINLY INTO THE FL100-130 RANGE WITH
ALONG WITH HIGHER MTN OBSCURATIONS AND POSSIBLY LCL MVFR CONDITIONS
IN RA BR.  GRADUAL IMPROVEMENT IS EXPECTED FROM SW-NE 08Z-12Z AS THE
SYSTEM LIFTS INTO NE WYOMING.


&&

.FIRE WEATHER...

SHOWERS AND THUNDERSTORMS WILL BE FOUND THROUGH THE EVENING HOURS.
GUSTY WIND WILL BE THE PRIMARY THREAT. A CALM PERIOD IS EXPECTED
OVERNIGHT. AN AREA OF LOW PRESSURE WILL THEN PUSH A SURGE OF
MOISTURE OVER THE AREA TUESDAY...HELPING TO INITIATE WIDELY
SCATTERED TO SCATTERED SHOWER AND THUNDERSTORM ACTIVITY. MODELS
INDICATE THAT THE AMOUNT OF ATMOSPHERIC MOISTURE MAY PERMIT THE
DEVELOPMENT OF HEAVY RAINFALL. ADDITIONAL STANDARD THREATS OF GUSTY
WIND AND SMALL HAIL ALSO EXIST. PERIODS OF SHOWERS AND THUNDERSTORMS
WILL BE EXPECTED OVER THE NEXT FEW DAYS AS MOISTURE CONTINUES TO BE
PUSHED ACROSS THE FORECAST AREA.

&&

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

$$

SHORT TERM...BRANHAM
LONG TERM...ALLEN/JONES
AVIATION...AEM
FIRE WEATHER...BRANHAM







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