Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 181736
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
1136 AM MDT MON AUG 18 2014

.SHORT TERM...TODAY THROUGH WEDNESDAY NIGHT

WV IMAGERY SHOWING SLOW TRANSITION TO MODEST NW FLOW ACROSS
WY...WITH RIDGING WEST AND EXPANDING TROF WEST OF THAT. SRN PORTION
OF ANOTHER WEAK UPR WAVE...ASSOCIATED WITH DEEP UPR TROF OVR S CNTRL
CANADA...WASHING OVR THE FA THIS EVENING WITH A FEW LIGHT
SHRA/ISOLATED TS AROUND. UPR HIGH OVR FOUR CORNERS. CURRENT SFC
PATTERN CONTINUES DIFFUSE HIGH PRESSURE FIELD OVR THE FA W/ SFC LOWS
TO THE SE AND SW OF WY.

TODAY...TRANSITION TO UPR DIRTY RIDGING WITH WY UNDER INCREASINGLY
NW FLOW WITH SOME MODEST MID LVL MOISTURE RETURNING FROM THE EPAC
AND OVR THE CWA...INCREASING PWS BY SOME 50 PERCENT OVR THOSE ON
SUNDAY. ISOLATED TO SCATTERED PRECIP CHCS PRIMARILY OVER THE
MOUNTAINS AND HILLS BY THE AFTERNOON. WITH LOW PRESSURE BUILDING
INTO S/SW WY...LL FLOW WILL BE UPSLOPE AND OUT OF THE N/NE ACROSS
MUCH OF THE AREA E OF THE DIVIDE...MORE WRLY W OF THE DIVIDE...
WHILE THE UPR FLOW REMAINS OUT OF THE W TO NW AND INCREASING IN
SPEED. DEEP SHEAR VALUES WILL THEREFORE INCREASE TO ABOVE 30-35 KTS
IN AREAS ACROSS THE ZONES E OF THE DIVIDE. HOWEVER DEEP MOISTURE
WILL NOT BE PRESENT YET...HOLDING OFF UNTIL TUESDAY...KEEPING MOST
SOUNDINGS W/ DRY LOWER LVLS. SO WHILE SOME STORMS WILL TRY TO WEAKLY
ORGANIZE...MOST LIKELY THE MAIN RESULT WILL BE IN PRODUCING A FEW
STRONGER WIND GUSTS...WITH SOME SMALL HAIL IN STORMS MAINLY OVR THE
NRN MOUNTAINS.

TUESDAY...UPR RIDGING AND NW FLOW OVR THE FA WILL GET BUSTED THRU
THE DAY WITH THE ARRIVAL OF A RELATIVELY STRONG UPR WAVE BEING
SQUEEZED OUT THRU THE INTERACTION OF THE CONUS LOW P TROF AND THE
UPR HIGH OVR THE SRN ROCKIES. THE SFC WILL RESPOND W AND
DEVELOPING/EVOLVING SFC LOW ENTERING THE STATE FROM THE S/SE WITH A
SFC BOUNDARY/TROF EXTENDING FROM S CNTRL TO N CNTRL WY. A DEEPER LEE
SIDE LOW WILL BE SPINNING UP IN E CNTRL CO. THE POSITIONING OF ALL
THESE UPR AND LWR PRESSURE FIELDS WILL HELP (RE)DIRECT MONSOONAL
MOISTURE BACK INTO THE STATE. ALL BECOMES MAXIMIZED BY LATE
AFTERNOON AND THRU THE OVERNIGHT PERIOD WITH PRECIP CHCS INCREASING
ACCORDINGLY. A FEW STRONGER STORMS MAY BE FOUND TUE AFTERNOON OVR
THE ERN COUNTIES THAT CAN TAKE ADVANTAGE OF MAX WARMING AHEAD OF THE
INCREASING CLOUDS...E TO SE FLOW INTO THE AREA AND REASONABLE DEEP
SHEAR. A FEW OF THESE STORMS COULD PRODUCE STRONG WINDS AND MARGINAL
SVR HAIL EARLY...BECOMING HEAVY RAIN PRODUCERS LATER IN THE EVENING.
ELSEWHERE...THRU THE EVENING AND OVERNIGHT...ATMOSPHERE BECOMES
INCREASINGLY TROPICAL WITH PERIODS OF HEAVY RAIN AND MORE ISOLATED
THUNDER...ESPECIALLY ACROSS THE WRN TO CNTRL ZONES. A FLASH FLOOD
WATCH MAY NEED TO BE ISSUED FOR PORTIONS OF THE FA...PARTICULARLY
FOR FAVORED TERRAIN LOCATIONS IN THE WRN MOUNTAINS...FOR TUE THRU
WED MORNING PERIOD...AS RAINFALL TOTALS MAY FALL INTO INTO THE 1 -3
IN CATEGORY...IT JUST DEPENDS ON HOW FAST IT FALLS. WILL CONTINUE TO
MONITOR FOR THIS POSSIBILITY.

WEDNESDAY...POST FRONTAL W/ THE UPR WAVE THAT CAUSED THE PREVIOUS
WET WEATHER ON THE WAY OUT AND INTO THE NRN PLAINS.
HOWEVER...MOISTURE...BOTH RESIDUAL AND MONSOONAL...WILL CONTINUE
ACROSS THE STATE W/ ANOTHER BUT WEAKER UPR LVL SW SWINGING OUT
ACROSS THE FA LATE IN THE DAY FROM W TO E...SETTING THE STAGE FOR
ANOTHER ROUND OF SHOWERS AND ISO THUNDER...PARTICULARLY OVR THE
W/NW/AND N FA. COLDER TEMPS BY WED NIGHT MAY ALLOW FOR SOME LIGHT SNOW
SHOWERS OVR THE HIGHER MOUNTAINS IN YELLOWSTONE ABOVE 11K FT...WITH
LITTLE TO NO ACCUMULATION.

.LONG TERM...THURSDAY THROUGH MONDAY

MEDIUM RANGE PERIOD WILL BE DOMINATED BY AN EXPECTED TROUGH THAT
COVERS WRN CANADA DOWN INTO THE NRN AND AT TIMES CENTRAL ROCKIES.
THE TIMING AND DETAILS VARY GREATLY BETWEEN MODELS AND MODEL RUNS.
THE BIGGER PICTURE IS FAIRLY CLOSE BUT THE TIMING OF THE
INDIVIDUAL SHORTWAVES THAT DROP INTO THE MEAN TROUGH VARY GREATLY
WHICH HAS A SIGNIFICANT IMPACT ON THE WEATHER ACROSS OUR AREA FROM
DAY TO DAY.

ON THURSDAY...ALL THE MODELS HAVE A SHORTWAVE IMPACTING AT LEAST
THE WRN AND NRN SECTIONS. THE GFS HAS SOME ENERGY MOVING UP AROUND
THE MONSOONAL RIDGE WHICH WOULD BRING A CHANCE OF SHOWERS/TSTMS IN
AT LEAST THE SERN ZONES IF THAT COMES TO FRUITION. HIT THE NRN AND
WRN SECTIONS THE HARDEST FOR NOW WITH MORE ISOLD COVERAGE ELSEWHERE.
ON FRIDAY...FRONTAL BOUNDARY FROM THE THURSDAY SYSTEM IS EXPECTED
TO HANG ACROSS THE NORTH WITH MORE POTENTIAL ENERGY SWINGING
ACROSS FROM THE WEST. LEAVING THE HIGHER SCT POPS ACROSS THE NORTH
AND WEST WITH MORE ISOLATED TO NIL COVERAGE THE FURTHER SOUTH YOU GO.
ON SATURDAY...THE GFS KEEPS THE FRONTAL BOUNDARY ACROSS THE FAR N
AND NERN ZONES WITH ONLY WEAK ENERGY SKIMMING MAINLY THE NORTH.
THE GEM AND ECMWF HOWEVER HAVE THE MAIN TROUGH SWINGING ACROSS
BEFORE RELOADING AGAIN FOR EARLY NEXT WEEK. THESE SOLUTIONS WOULD
BRING A MUCH HIGHER CHANCE OF PCPN AND CONSIDERABLY COOLER TEMPERATURES.
STICKING CLOSE TO A MODEL BLEND FOR NOW WITH SUCH VARYING SOLUTIONS.
THE UNCERTAINTY CONTINUES INTO EARLY NEXT WEEK WITH THE GFS
SWINGING THE WHOLE TROUGH THROUGH WITH A GOOD CHANCE OF SHOWERS
AND THUNDERSTORMS SUNDAY FOLLOWED BY SHARPLY COOLER/COLDER
UNSETTLED WX ON MONDAY. THE GEM HAS A NW FLOW WITH NO SIGNIFICANT
DISTURBANCES WHILE THE ECMWF DIGS SIGNIFICANT ENERGY INTO THE
PACNW AGAIN SUNDAY BEFORE STALLING IT OUT OVER ID ON MONDAY.
CONFIDENCE LOW THIS FAR OUT BUT WENT WITH MAINLY WRN AND NRN POPS
SUNDAY AND THEN ALL AREAS MONDAY WITH TEMPERATURES BEING A GENERAL
MODEL BLEND. WILL LOOK AT THE TRENDS AGAIN TONIGHT AND SEE IF WE
CAN DETERMINE THE CORRECT SOLUTION TO LEAN MORE TOWARDS.

&&

.AVIATION.../18Z ISSUANCE/

A SURGE OF MOISTURE ACROSS WESTERN AND CENTRAL WYOMING THIS
AFTERNOON WILL BE ABLE TO INITIATE SOME SHOWER AND THUNDERSTORM.
ACTIVITY IS ANTICIPATED ACROSS MUCH OF THE FORECAST AREA...WITH
VCSH/VCTS INCLUDED AT MOST TERMINALS THROUGH 03Z. THE MOUNTAINS
LOOK TO BE FAIRLY SUSCEPTIBLE TO DEVELOPMENT THIS AFTERNOON...WITH
STORMS CREATING SOME MOUNTAIN OBSCURATION THROUGH THE EARLY
EVENING. TERMINAL WISE...GUSTY WIND LOOKS TO BE THE PRIMARY THREAT
WITH SHOWER ACTIVITY TODAY. SHOWERS WILL WANE WITH LOSS OF
DAYLIGHT...AND WIDESPREAD VFR CONDITIONS ARE EXPECTED BY 06Z-08Z
THIS EVENING. ANOTHER SURGE OF MOISTURE WILL BEGIN TO MOVE OVER
THE AREA AFTER 12Z TUESDAY...WITH INCREASING AND LOWERING CLOUD
COVER ANTICIPATED.

&&

.FIRE WEATHER...

TODAY WILL SEE IMPROVING FIRE WEATHER CONDITIONS REGION WIDE AS SOME
MOISTURE BEGINS TO HEAD BACK INTO THE FORECAST AREA FROM THE EPAC.
WHILE THE SOUTHERN ZONES WILL STILL REMAIN ON THE DRY SIDE...WINDS
WILL NOT BE AS ROBUST AND GENERALLY WILL REMAIN AT LESS THAN 15
MPH OVER THE DRY AREAS. HOWEVER...THERE WILL BE A SLIGHT CHANCE
FOR AN ISOLATED LATE DAY THUNDERSTORM ACROSS THE SOUTHERN ZONES
WITH SOME DRY LIGHTNING POTENTIAL...SO LOCALIZED/ISOLATED ENHANCED
FIRE DANGER DURING THIS SHORT PERIOD. TUESDAY...MONSOONAL MOISTURE
INFILTRATES THE STATE WITH INCREASING RAINFALL CHANCES STATEWIDE
BY EVENING AS A VIGOROUS UPPER LEVEL DISTURBANCE HELPS IN
TRANSFORMING THE NEW AVAILABLE MOISTURE INTO PRECIPITATION THAT
WILL REACH THE GROUND. WEDNESDAY WILL KEEP COOLER MOIST CONDITIONS
AROUND...ALONG WITH LOW FIRE DANGER.

&&

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

$$

SHORT TERM...BRAUN
LONG TERM...SKRBAC
AVIATION...BRANHAM
FIRE WEATHER...BRAUN





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