Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 181811 AAA
AFDRIW

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE RIVERTON WY
1211 PM MDT SAT APR 18 2015

.SHORT TERM...SATURDAY THROUGH MONDAY NIGHT

THE WEATHER THIS MORNING CONTINUES TO BE INFLUENCED BY THAT PAIN IN
THE NECK UPPER LEVEL LOW...NOW SPINNING OVER SOUTHERN COLORADO. IN
AREAS WHERE THE RAIN FELL AT A DECENT RATE...WE HAVE SOME PATCHY FOG
AS WELL. THE LOCAL RADAR IS LARGELY QUIET...BUT THE CHEYENNE RADAR
SHOWS ANOTHER ROUND OF PRECIPITATION MOVING INTO SOUTHEASTERN
WYOMING. THE QUESTION...ONCE AGAIN...LOOKS TO BE HOW FAR NORTH AND
WEST THE PRECIPITATION WILL EXTEND AND HOW FAR HEAVY WILL THE
PRECIPITATION WILL BE. THE MODELS ONCE AGAIN HAVE SOME DIFFERENCES
IN THE DETAILS. SO...FOR THE MOST PART WE LEFT THINGS ALONE. THIS
MEANS THE BEST CHANCE WILL BE ACROSS AREAS FROM SOUTH OF A LINE FROM
LANDER TO AROUND SOUTHERN JOHNSON COUNTY. HOWEVER...THERE IS A
DIFFERENCE IN THE FORECAST TODAY.

THIS IS IN THE FORM OF A COLD FRONT AND TROUGH MOVING IN FROM THE
NORTHWEST...A SO CALLED KICKER. NO...NOT IN THE FORM OF RICH KARLIS
OR JASON ELAM...BUT A SYSTEM THAT WILL FINALLY GET THE LOW MOVING
AWAY TO THE EAST. THERE IS A WRINKLE THOUGH. THERE ALWAYS IS. THE
MODELS ARE SHOWING A BAND OF ENHANCED PRECIPITATION DEVELOPING ALONG
SOME CONVERGENCE NEAR THE EASTERN PORTION OF THE COUNTY WARNING AREA.
AT THIS POINT WE HAVE THE HIGHER POPS ACROSS THE EAST FOR THE
AFTERNOON. HOWEVER...WITH THE FRONT MOVING IN FROM THE NORTH
EVERYONE WILL HAVE A CHANCE OF SHOWERS AND EVEN A FEW THUNDERSTORMS.
WITH THE COLD AIR ALOFT THERE COULD BE SOME SMALL HAIL OR GRAUPEL
WITH ANY OF THE STORMS. IN ADDITION...THERE WILL BE SOME DECENT COLD
ADVECTION BEHIND THE FRONT SO PLACES PRONE TO STRONG NORTHERLY WINDS
IN THIS SITUATION...LIKE THE NORTHERN BIG HORN BASIN...JOHNSON
COUNTY AND NEAR THE OWL CREEK RANGE WILL SEE GUSTY WINDS THIS
AFTERNOON. NOT HIGH WINDS...BUT GUSTS TO 45 OR 50 MPH ARE POSSIBLE.
LIKE YESTERDAY...WITH THE BULK OF THE PRECIPITATION EXPECTED DURING
THE DAYLIGHT HOURS...EVEN WHERE SNOW DOES FALL IT WILL NOT STICK ON
THE ROADS WITH THE HIGH SUN ANGLE. THE LOW WILL FINALLY GET KICKED
TO THE EAST.

SUNDAY LOOKS TO BE A COOLER DAY BEHIND THE FRONT. ANOTHER SHORTWAVE
WILL MOVE IN BEHIND THE FRONT AND BRING ANOTHER ROUND OF UNSETTLED
WEATHER. MOISTURE WILL BE LESS THOUGH. HOWEVER...THIS MAY BRING MY
LEAST FAVORITE TYPE OF PRECIPITATION...CONVECTIVE SNOW BANDS OR IN
THIS CASE RAIN BANDS SINCE IT IS APRIL. THE PROBLEM IS THEY ARE
DIFFICULT TO PINPOINT AND THE MODELS USUALLY DO A TERRIBLE JOB IN IT
AS WELL. FOR NOW...CONTINUITY LOOKED REASONABLE SO WE LEFT THINGS
ALONE. ON MONDAY...THE MODELS LOOK TO DIVERGE A BIT WITH THE NAM
DRYING THINGS OUT WITH RIDGING STARTING TO BUILD. MEANWHILE...THE
GFS KEEPS SOME INSTABILITY WITH SHOWERS OVER THE AREA...MAINLY WEST
OF THE DIVIDE. AGAIN...WITH THE MODEL DIFLUENCE WILL HAVE AGAIN
DECIDED TO LEAVE CONTINUITY AS IS UNTIL THE SOLUTION BECOMES
CLEARER.

.LONG TERM...TUESDAY THROUGH SATURDAY

A CHAOTIC AND RATHER SLOW SPRING PATTERN WILL IMPACT THE REGION
FROM MIDDLE OF NEXT WEEK INTO THE WEEKEND. ON TUESDAY AND
WEDNESDAY...WEAK UPPER LEVEL RIDGING IS EXPECTED WITH A WEAK
BACKDOOR COLD FRONT PUSHING INTO THE AREA FROM THE EAST/NORTHEAST
ON WEDNESDAY. MOISTURE WILL BE LIMITED...WITH THE BEST SHOT AT
ANY CONVECTION...MAINLY IN THE AFTERNOON/EVENING OVER THE HIGHER
TERRAIN OF WESTERN WYOMING. ALTHOUGH CANNOT RULE OUT AN ISOLATED
THUNDERSTORM BOTH DAYS...NOT ENOUGH CONFIDENCE OR COVERAGE TO
INCLUDE IN FORECAST.

ON THURSDAY...A SHALLOW QUASI-STATIONARY SURFACE FRONT SHOULD BE SET
UP EAST OF THE DIVIDE. MODELS ARE HINTING AT A MODEST EAST TO
SOUTHEAST...HIGH PLAINS UPSLOPE...SURFACE FLOW INTO JOHNSON AND
NATRONA COUNTIES. IN THE UPPER LEVELS...HEIGHTS BEGIN TO FALL IN
RESPONSE TO AN UPPER LOW SLIDING SOUTHEAST ALONG THE BRITISH
COLUMBIA COAST...AND AN UPPER LOW OVER SOUTHERN CALIFORNIA
BEGINNING TO SHIFT INTO THE SOUTHERN ROCKIES. ALTHOUGH SYNOPTIC
FORCING APPEARS TO BE WEAK THURSDAY...SUFFICIENT MOISTURE AND
INSTABILITY ESPECIALLY WITH DAYTIME HEATING SHOULD GIVE THE ENTIRE
AREA A CHANCE OF CONVECTION. HAVE GONE SLIGHT VALLEY/CHANCE
MOUNTAIN POPS. ALSO MODELS ARE SHOWING DIRECTIONAL SHEAR OVER THE
FAR EAST...WHERE EAST TO SOUTHEAST LOW LEVEL FLOW VEERS TO THE
SOUTHWEST AND WEST PRETTY QUICKLY. THIS WILL AID IN STORM
DEVELOPMENT...BUT THUNDERSTORM STRENGTH SHOULD BE LIMITED BY
PROJECTED UPPER LEVEL FORCING AND OVERALL QUALITY OF MOISTURE.

ON FRIDAY...THE SOUTHERN UPPER LOW OPENS UP AND QUICKLY EJECTS INTO
THE SOUTH CENTRAL PLAINS...WHILE THE NORTHERN SYSTEM BEGINS TO DIG
ACROSS THE PACIFIC NORTHWEST. IN RESPONSE TO THE SOUTHERN STREAM
ENERGY...STRONG LEE CYCLOGENESIS IS EXPECTED ACROSS THE COLORADO
FRONT RANGE LATE THURSDAY NIGHT INTO FRIDAY MORNING. AS THIS OCCURS
THE QUASI-STATIONARY FRONT THAT PROBABLY SHIFTED EAST OF THE AREA
LATE THURSDAY AFTERNOON...EVENING COMES BACK DOWN AS A COLD FRONT
FRIDAY MORNING...TRANSITIONING INTO AN INVERTED SURFACE TROUGH
HUNG UP ALONG THE DIVIDE. THIS SURFACE BOUNDARY COMBINED WITH A
RELATIVELY MOIST ATMOSPHERE WILL CONTINUE TO GIVE THE ENTIRE AREA
A CHANCE OF MOUNTAIN SNOW/VALLEY RAIN SHOWERS. AN ISOLATED
THUNDERSTORM REMAINS POSSIBLE.

BY SATURDAY...THE NORTHERN STREAM UPPER LOW COULD BE SHIFTING EAST
INTO THE INTERMOUNTAIN WEST/ROCKIES REGION...CONTINUING THE COOL AND
INCLEMENT WEATHER.

.AVIATION.../18Z ISSUANCE/

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

A COLD FRONT WILL PUSH SOUTH ACROSS THE MONTANA/WYOMING BORDER
EARLY IN THE PERIOD AND ACROSS THE AREA BY EARLY AFTERNOON. SCT
MVFR/IFR -SHRASN WILL FOLLOW THE FRONT WITH MORE NUMEROUS -SHSN
OVER THE MOUNTAINS WITH MOUNTAINS BECOMING FREQUENTLY OBSCURED
THROUGH THE AFTERNOON. THE FRONT WILL ALSO PRODUCE UPSLOPE IFR
RASN VICINITY KCPR THROUGH THE AFTERNOON INTO THE EVENING.
DRIER AIR WILL WRAP INTO NORTHEAST WYOMING TONIGHT...BUT AREAS OF
MVFR/IFR CEILINGS ARE EXPECTED TO BANK UP AGAINST THE EAST SLOPES
OF THE DIVIDE...VICINITY KCOD-KLND.

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

A COLD FRONT WILL PUSH SOUTH ALONG THE DIVIDE THIS AFTERNOON WITH
SCT- NUMEROUS SHSN RESULTING IN FREQUENT MOUNTAIN OBSCURATIONS
AFTER 20Z. THIS ACTIVITY IS EXPECTED TO BE MORE WIDELY SCT OVER
THE VALLEYS WITH POSSIBLE BRIEF MVFR/IFR CONDITIONS AT ALL
TERMINALS THROUGH THIS EVENING. MOST OF THIS ACTIVITY WILL SHIFT
ALONG AND EAST OF THE CONTINENTAL DIVIDE AFTER 06Z SUNDAY.

.FIRE WEATHER...

THE SOUTH AND EAST WILL BE COLD WITH SHOWERS OF RAIN AND SNOW.
NORTHEAST WINDS WILL SLOWLY DIMINISH THROUGH SATURDAY. WITH
WIDESPREAD LOW CLOUDS THE MIXING HEIGHTS WILL BE LOW WITH POOR SMOKE
DISPERSAL..ESPECIALLY IN AREAS OF PRECIPITATION.

A PACIFIC TROUGH WILL APPROACH THE WEST THIS AFTERNOON WITH
CONVECTION DEVELOPING OVER THE WEST AND NORTH MOUNTAINS.  CLOUDS
WILL INCREASE THROUGH THE MORNING WITH AREAS OF SHOWERS AND ISOLATED
THUNDERSTORMS DEVELOPING. TEMPERATURES WILL BE WARMER IN THE NORTH
AND WEST. MIXING HEIGHTS WILL INCREASE WITH THE CONVECTIVE LIFT
WITH GOOD SMOKE DISPERSAL OR BETTER EXPECTED IN THE AFTERNOON.
AREAS OF HEAVIER PRECIPITATION OVER THE HIGHER MOUNTAINS WILL LOCALLY
CAUSE LOWER MIXING HEIGHTS AND DEVELOPING POOR SMOKE DISPERSAL.

&&

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

$$

SHORT TERM...HATTINGS
LONG TERM...WM
AVIATION...ALLEN
FIRE WEATHER...HATTINGS






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