Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 261909 AAA
AFDRIW

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE RIVERTON WY
109 PM MDT TUE MAY 26 2015

.UPDATE...

A FLASH FLOOD WATCH HAS BEEN ISSUED FOR THE BIGHORNS...OWL
CREEKS...JOHNSON...NATRONA...HOT SPRINGS AND WASHAKIE COUNTIES
THROUGH MIDNIGHT. SLOW MOVING SHOWERS AND THUNDERSTORMS ARE
FORMING AND MOVING ACROSS SATURATED SOILS AND ARE PRODUCING
RAINFALL RATES IN EXCESS OF 2 INCHES PER HOUR IN SOME CASES.
SHOWER AND THUNDERSTORM ACTIVITY IS EXPECTED TO DECREASE AFTER
MIDNIGHT.

.SHORT TERM...TODAY THROUGH THURSDAY NIGHT

WV IMAGERY SHOWING...SHOWING GENERAL LW TROF ACROSS WRN 2/3S OF THE
CONUS WITH EMBEDDED SW TROF OVR THE NRN...CNTRL...AND INTO THE SRN
PLAINS. UPSTREAM...ANOTHER SW TROF WITH CLOSED LOW RESIDES DOWN THRU
THE WRN CSTL STATES WITH CENTER OF CIRCULATION OVR WA. IN BETWEEN
THESE TWO...WEAK SW RIDGING OVR WY. SFC HAS A WEAK BROAD AREA OF
HIGH P ACROSS THE INTERMOUNTAIN WEST...WITH A WEAK TROF/DEVELOPING
WEAK FRONT FROM CNTRL MT...SW TO SERN OR.

SHOWERS MOSTLY ENDING THIS MORNING EXCEPT FOR SMALL CHANCES REMAINING
PRIMARILY OVR THE MOUNTAINS AND NW WY AS A SMALL UPR WAVE BRUSHES
THE REGION. OTHERWISE...INFLUENCES FROM THE UPSTREAM TROF/CLOSED LOW
SYSTEM PUSHES THE WEAK RIDGE EAST...WHILE DIVERGENT UPR SWRLY FLOW
BEGINS TO OVERTAKE THE FA FROM THE WEST WHILE A MODEST EMBEDDED SW
MOVES THRU THIS FLOW ACROSS THE WRN FA IN THE AFTERNOON...CATCHING
UP WITH THIS MORNINGS UPR WAVE...AND HELPING TO INITIATE SHOWERS AND
THUNDERSTORMS...ESPECIALLY W OF THE DIVIDE...ESPECIALLY ACROSS SW
WY. INCREASINGLY DEEP AND UNIDIRECTIONAL FLOW OUT OF THE SOUTHWEST
WILL LIKELY CREATE A MESO-LOW AND CONVERGENCE ZONE FROM THE SOUTH
PASS AREA...EASTERN FREMONT COUNTY...AND INTO AT LEAST WRN NATRONA
COUNTY. GENERALLY...MORE ISOLATED STORMS EXPECTED E OF THE DIVIDE.
HOWEVER...ANY STORMS NEAR/OVR THIS CONVERGENCE ZONE MAY BE SOMEWHAT
STRONGER THAN MOST BY EVENING...OFFERING UP HALF INCH TO THREE
QUARTER INCH HAIL IF/WHEN THEY FIRE. CONVECTION WILL DECREASE IN
COVERAGE OVERNIGHT...WITH A FEW BEING MOST PERSISTENT RIGHT UNDER
THE UPR WAVE AS IT MOVES QUICKLY OFF TO THE NE OVERNIGHT.

WED...UPR TROF CONTINUES TO MOVE TOWARD THE FA...WITH WEAK UP SW
RIDGE MOVING OVR WY ON THE TAILS OF THE OVERNIGHT SW TROF...AND SFC
LEE TROFFING OVR ERN WY. SOME DRY ENTRAINMENT STILL EXPECTED ACROSS
MUCH OF THE SW TO CNTRL FA...WITH ADEQUATE CAPE...UPR WIND SPEED AND
DEEP SHEAR BREAKING OUT ISOLATED TO SCATTERED TSRA ELSEWHERE. DEEP
SHEAR WILL BEGIN TO FAVOR SOME STORM ORGANIZATION...WITH ML CAPE
RISING ABOVE 500 J/JG FOR MOST AREAS...AND SB CAPE ECLIPSING 1200
J/KG IN SOME AREAS E OF THE DIVIDE. STORMS WILL BE ABLE TO ROTATE
SUSTAIN THEMSELVES BETTER WITH A SOME PRODUCING SMALL TO NEAR SEVERE
HAIL...ESPECIALLY FROM ERN FREMONT COUNTY...EWD AND INTO NATRONA AND
JOHNSON COUNTIES ALONG/NEAR/EAST OF ANOTHER DEVELOPING CONVERGENCE
ZONE. STORMS WILL ALSO MOVE AT A FASTER CLIP...20 TO 30 MPH AND
HOPEFULLY WITH SOMEWHAT LESS POTENTIAL OF
FLOODING.

THU...MAIN...BUT WEAKENING...UPR TROF MOVES INTO/ACROSS WY...WITH
UPR WINDS DECREASING IN SPEED...LOSING SOME OF THE BETTER MID/UPR
FORCING AND SHEAR PROFILES FROM THE DAY BEFORE ALONG WITH MORE DRY
AIR ENTRAINMENT. STILL...MODEST MOISTURE AND WARMTH WILL HELP TO
PRODUCE RELATIVELY GOOD CAPE...WITH DECENT MID/UPR VORTICITY
ROTATING WITHIN THE BASE OF THE UPR TROF OVR THE SWRN/SRN...TO CNTRL
AND THEN ONTO THE ERN CWA. THE SET UP WILL PRODUCE SCATTERED
SHRA/TSRA WEST AND EAST...WITH THE STRONGER MORE ROBUST STORMS OF
THE DAY OVR NATRONA AND JOHNSON COUNTIES...ESPECIALLY IN THE AREA OF
MAXIMUM LL CONVERGENCE FROM NRN TO ERN NATRONA COUNTY.
OVERNIGHT...PRECIP WILL LOSE A GOOD PORTION OF THE CONVECTIVE
COMPONENT AND BE DRIVEN BY THE DYNAMICS LEFT WITH THE UPR
TROF...WITH AREAL COVERAGE DECREASING RATHER QUICKLY AS SFC TO MID
LVL HIGH PRESSURE BUILDS IN ACROSS THE FA.

.LONG TERM...FRIDAY THROUGH TUESDAY

UPPER LOW CURRENTLY OVER FAR ERN WA/NRN ID WILL EVENTUALLY GET
KICKED SE AS CURRENT UPPER LOW NEAR THE SRN AK COAST MOVES INTO
WRN B.C. AT THE START OF THIS PERIOD. BOTH THE GFS AND ECMWF HAVE
A WEAKLY CLOSED OFF LOW OVER SWRN WY AT 12Z FRIDAY WITH SURFACE
HIGH AND ASSOCIATED COLD FRONT PUSHING SOUTH...EAST OF THE DIVIDE
..AS A SHORTWAVE SKIMS THE FAR NRN HIGH PLAINS. THE COMBINATION
OF THE UPSLOPE EAST OF THE DIVIDE AND MID LEVEL LOW SHOULD PROVIDE
A RATHER WET PERIOD THROUGH THE DAY FRIDAY...ESPECIALLY ALONG AND
EAST OF THE DIVIDE. GRADUAL IMPROVEMENT LATER FRIDAY NIGHT INTO
SATURDAY MORNING AS CIRCULATION WEAKENS AND MOST LIKELY MOVES OFF
TO THE EAST OR SE. RIDGE IS THEN EXPECTED TO BUILD OVER THE AREA
THROUGH THE WEEKEND WITH CONSIDERABLY WARMER TEMPERATURES. THE GFS
HAS MORE MOISTURE AND WEAK RIPPLES UNDERNEATH THE BUILDING HIGH
AND THUS MUCH MORE AFTERNOON AND EVENING CONVECTION. MOST OF THE
CONVECTION SHOULD BE OVER THE MOUNTAINS AND FORECAST INDICATES
THIS FOR NOW. EARLY NEXT WEEK...AN UPPER LOW CURRENTLY NEAR
40N/150W IS KICKED EWD BY A PIECE OF ENERGY THAT COMES OFF OF ERN
ASIA AND MOVES ACROSS THE PACIFIC. THIS UPPER LOW IS FORECAST TO
MOVE INTO THE NRN CA/OREGON AREA EARLY MONDAY AND THEN SLOW DOWN
AS IT MOVES INTO THE FAR NRN ROCKIES BEFORE STALLING OUT INTO
MIDWEEK. POTENTIAL FOR INCREASING THUNDERSTORM CHANCES EXIST IN
OUR NWRN CORNER WITH POTENTIAL COLD FRONT LYING IN THE AREA
COMBINED WITH AFTERNOON HEATING AND JET STREAK EARLY NEXT WEEK.
SOME STRONG STORMS COULD CERTAINLY DEVELOP IN THIS AREA IN THIS
FAVORED PATTERN SO WITH TIME WE`LL JUST HAVE TO SEE HOW THE DETAILS
WORK OUT. FOR NOW...ANOTHER UNSETTLED PERIOD TO START THE MEDIUM
RANGE PERIOD...FOLLOWED BY A WARMER WEEKEND WITH MOST
SHOWERS/TSTMS IN THE MTNS FOLLOWED BY A POTENTIAL INCREASE IN
TSTMS IN THE W/NW SECTIONS MONDAY INTO TUESDAY WITH APPROACHING
COLD FRONT AND UPPER TROUGH. CONTINUED WARM EAST OF THE DIVIDE
WITH SOME COOLING IN THE FAR WEST/NW.

.AVIATION.../18Z ISSUANCE/

SCT SHRA/ISOLD TSRA FROM SW INTO CENTRAL WYOMING TUESDAY AFTN/EVE
WITH MOST THUNDERSTORM ACTIVITY ACROSS THE SOUTHERN HALF OF
WYOMING. SHOWERS WILL LINGER ACROSS CENTRAL WYOMING PAST 06Z
TUESDAY WHILE THE WEST WILL PARTIALLY CLEAR WITH PATCHY FOG
REDEVELOPING EARLY TUESDAY MORNING.

.FIRE WEATHER...

FIRE DANGER LOW AND WILL REMAIN LOW THROUGH MOST OF
THE WORK WEEK. SEASONALLY COOL TO WARM TEMPERATURES WILL COMBINE
WITH RELATIVELY MOIST ATMOSPHERIC CONDITIONS TO PRODUCE DECENT
CHANCES FOR SHOWERS AND THUNDERSTORMS EACH AFTERNOON AND EVENING.
LIGHTNING AND STORM SPECIFIC GUSTY WINDS WILL BE THE MAIN THREAT.
GENERALLY TOO...SMOKE DISPERSAL EACH DAY...EXCEPT FOR SHORT PERIODS
IN THE VERY EARLY MORNING HOURS...WILL BE GOOD TO EXCELLENT.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
FLASH FLOOD WATCH UNTIL MIDNIGHT MDT TONIGHT
WYZ005>011-019-020-022.

&&

$$

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






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