Area Forecast Discussion
Issued by NWS Riverton, WY

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757
FXUS65 KRIW 040953
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
353 AM MDT WED MAY 4 2016

.SHORT TERM...WEDNESDAY THROUGH FRIDAY NIGHT

WITH THE UPCOMING ACTIVE WEATHER...WE WILL SPEND VERY LITTLE TIME ON
THE WEATHER THROUGH THURSDAY MORNING. WITH RIDGING STILL IN
CONTROL...IT CONTINUES TO LOOK VERY NICE WITH A GOOD DEAL OF
SUNSHINE AND WARM TEMPERATURES. ACROSS THE MOUNTAINS...ALTHOUGH WE
CAN NOT RULE OUT A SHOWER...THE CHANCE IS SO SMALL WE LEFT THE POPS
SILENT.

THINGS WILL BECOME A BIT MORE ACTIVE THURSDAY AFTERNOON AS AN UPPER
LEVEL LOW MOVING INTO CALIFORNIA THROWS SOME MID LEVEL MOISTURE INTO
THE AREA AND IT INTERACTS WITH SOME JET ENERGY TO PRODUCE A FEW
SHOWERS AND THUNDERSTORMS ACROSS AREAS WEST OF THE DIVIDE. AT THIS
TIME...COVERAGE DOES NOT LOOK THAT GREAT AND CONVECTION SHOULD BE
RESTRICTED TO WEST OF THE DIVIDE THROUGH THURSDAY. MEANWHILE...AREAS
EAST OF THE DIVIDE WILL HAVE ONE MORE DRY AND QUITE WARM DAY.
GUIDANCE NUMBERS CONTINUE TO CLIMB WITH THE OUTSIDE CHANCE THAT SOME
OF THE WARMER SPOTS LIKE GREYBULL COULD GET CLOSE TO 90.
SOUTHWESTERLY BREEZES WILL DEVELOP AHEAD OF THE FRONT...BUT NOTHING
UNUSUAL FOR WYOMING.

THINGS DEFINITELY GET MORE ACTIVE FOR FRIDAY. THE REGION WILL BE IN
A SQUEEZE PLAY AS A COLD FRONT DROPS SOUTHWARD OUT OF MONTANA AND
INTERACTS WITH THE MORE UNSTABLE AIR ACROSS THE AREA. MODELS ARE
SHOWING A DECENT AMOUNTS OF DIRECTION SHEAR IN THE AFTERNOON WITH
THE INSTABILITY AND SOME DECENT JET SUPPORT. THIS COULD LEAD TO SOME
STRONG AND POSSIBLY SEVERE STORMS IF A SHORTWAVE CAN CROSS AT THE
RIGHT TIME AND BRING A BIT OF EXTRA LIFT. THE STORM PREDICTION
CENTER HAS SOUTHWESTERN WYOMING UNDER A MARGINAL DAY 3 RISK.
CONTINUITY HAD THIS COVERED FAIRLY WELL SO WE LEFT IT ALONE.

THINGS THEN TRANSLATE TO A MORE STRATIFORM PRECIPITATION AT NIGHT AS
THE HEATING OF THE DAY AND INSTABILITY IS LOST. THE MODELS CONTINUE
TO BE RATHER ROBUST IN AMOUNTS OF RAIN WITH OVER A HALF AN INCH A
GOOD POSSIBLY OVERNIGHT IN AREAS FAVORED BY LOW LEVEL EASTERLY
FLOW. THROUGH FRIDAY NIGHT...THIS LOOKS TO BE MAINLY A RAIN EVENT
WITH 700 MILLIBAR TEMPERATURES EXPECTED TO REMAIN AT PLUS 2 OR
ABOVE. THIS WOULD PUT SNOW LEVELS AROUND 9500 OR ABOVE. THIS COULD
INCREASE FLOOD POTENTIAL IN SOME AREAS IF RAIN FALLS ON THE SNOWPACK
IN A PLACE LIKE THE WIND RIVER RANGE. STAY TUNED.

.LONG TERM...SATURDAY THROUGH WEDNESDAY

SYNOPSIS...AN UPPER LEVEL LOW PRESSURE SYSTEM WILL SLOWLY LIFT
ACROSS THE GREAT BASIN AND CENTRAL ROCKIES OVER THE
WEEKEND...BRINGING CONTINUED WIDESPREAD RAIN AND MOUNTAIN SNOW
ACROSS MUCH OF THE AREA THROUGH SATURDAY NIGHT.  RAIN AND HIGHER
MOUNTAIN SNOW COULD BE QUITE ALONG THE EAST SLOPES OF THE DIVIDE AND
ADJACENT CENTRAL BASINS WITH SOME FLOODING POSSIBLE.  THE UPPER
LEVEL LOW PRESSURE SYSTEM WILL MOVE EAST INTO THE PLAINS ON MONDAY
WITH A COOL AND UNSETTLED NORTHWEST FLOW EXPECTED TO PREVAIL BEHIND
THIS SYSTEM THROUGH MID-WEEK.

DISCUSSION...MEDIUM-RANGE MODELS IN GOOD OVERALL AGREEMENT AT THE
OUTSET WITH REX BLOCK OVER THE INTERMOUNTAIN WEST...RIDGE OVER THE
NORTHERN ROCKIES AND CUTOFF NEAR LAS VEGAS 12Z SATURDAY. SPREAD
INCREASES LIFTING THE UPPER LOW NE INTO THE CENTRAL ROCKIES LATE IN
THE WEEKEND WITH THE ECMWF DISPLAYING THE NORTHERN MOST TRACK THIS
MORNING.  FORECAST CONTINUES TO BE WEIGHTED TOWARD THE MORE
CONSISTENT GFS/GEFS MEAN...TAKING UPPER LOW INTO THE FOUR CORNERS
REGION SUNDAY MORNING BEFORE JUMPING THE DIVIDE AND ENDING UP NEAR
GOODLAND KS ON MONDAY.  ALSO PREFER THE GFS EVOLUTION OF MORE WELL-
DEFINED HIGH AMPLITUDE RIDGE DEVELOPING ALONG 130W EARLY NEXT WEEK
WITH LOWERING HEIGHTS DOWNSTREAM ACROSS THE NORTHERN ROCKIES.

ON SATURDAY AND SUNDAY...AREA OF SIGNIFICANT WARM CONVEYOR
BELT/UPSLOPE PRECIPITATION IS EXPECTED TO DEVELOP ON THE NE QUADRANT
OF THE SW LOW FROM SOUTHEAST INTO CENTRAL WYOMING. PRECIPITATION
RATES ARE EXPECTED TO BE 0.50-1 INCH/6HR OVER AT LEAST A 24 HOUR
PERIOD ALONG THIS SWATH ARCING ROUGHLY FROM A CHEYENNE TO CASPER
(OR ALONG I25) LINE AND WESTWARD INTO THE EAST SLOPES OF THE WIND
RIVER AND SOUTHERN ABSAROKA RANGES. MAIN CONCERN CONTINUES TO BE
FLOODING POTENTIAL...WITH SNOW LEVELS EXPECTED TO BE BETWEEN
8.5-9KFT THROUGH SATURDAY EVENING BEFORE LOWERING SUNDAY MORNING
TO NEAR 8KFT. THE LITTLE WIND RIVER IS FORECAST BY THE MBRFC TO
CREST OVER A FOOT ABOVE FLOOD STAGE NEAR RIVERTON ON SUNDAY IF
THESE TEMPERATURES AND PRECIPITATION AMOUNTS PLAY OUT. 10 TO 20
INCHES OF SNOW IS FORECAST ABOVE 9000 FEET ACROSS THE WIND RIVER
AND SOUTHERN ABSAROKA RANGE FROM FRIDAY NIGHT THROUGH SUNDAY
MORNING.

MONDAY THROUGH WEDNESDAY...THERE IS DECREASING CONFIDENCE IN
PRECIPITATION PLACEMENT AND TIMING DURING THIS PERIOD.  WITH
PREFERRED GFS SCENARIO BETWEEN EXITING UPPER LOW AND NORTHERN STREAM
KICKER DIVING ACROSS THE NORTHERN ROCKIES...KEPT HIGHER POPS ACROSS
THE SOUTH AND WEST ON MONDAY GIVING EXITING UPPER LOW MORE
INFLUENCE...AND THEN TURNING ATTENTION TO THE NORTHERN STREAM KICKER
ON TUESDAY AND WEDNESDAY THAT WILL SPREAD SOME VALLEY RAIN AND
MOUNTAIN SNOW SHOWERS ACROSS MAINLY THE NORTH HALF OF THE CWA.

&&

.AVIATION.../12Z ISSUANCE/

WEST OF THE CONTINENTAL DIVIDE...KJAC/KRKS/KPNA/KBPI AIRPORTS

VFR CONDITIONS WILL CONTINUE AT ALL TERMINALS THROUGH 12Z/THURSDAY
WITH UPPER RIDGE AXIS OVER THE FORECAST AREA BY THAT TIME. WIND
SPEEDS WILL GENERALLY BE 6-9KTS AFTER ABOUT 20Z/WEDNESDAY...WITH
SLIGHTLY HIGHER SPEEDS OF 10-12KTS AT KRKS. SOME WEAK CONVECTION IS
POSSIBLE OVER THE HIGHER TERRAIN FROM 21Z/WEDNESDAY TO 03Z/THURSDAY.

EAST OF THE CONTINENTAL DIVIDE...KCPR/KCOD/KRIW/KWRL/KLND AIRPORTS

VFR CONDITIONS WILL CONTINUE AT ALL TERMINALS THROUGH 12Z/THURSDAY
WITH UPPER RIDGE AXIS OVER THE FORECAST AREA BY THAT TIME. WIND
SPEEDS WILL BE LESS THAN 10KTS AT ALL SITES WITH ONE EXCEPTION. THAT
WILL BE KCPR WHERE SOUTHWEST WIND AROUND 12KTS WILL INCREASE AROUND
15Z/WEDNESDAY AND CONTINUE UNTIL AROUND 03Z/THURSDAY.

&&

.FIRE WEATHER...

MAINLY DRY AND WARM CONDITIONS WILL CONTINUE THROUGH THURSDAY
MORNING FOR ALL AREAS. RELATIVE HUMIDITY WILL FALL INTO THE TEENS
FOR MANY LOWER ELEVATION LOCATIONS...BUT WINDS SHOULD REMAIN LIGHT
TO MODERATE. THE CHANCE OF SHOWERS WILL INCREASE IN THE WEST
THURSDAY AFTERNOON WITH A CHANCE OF THUNDERSTORMS FOR ALL AREAS ON
FRIDAY. A WETTING RAIN IS A GOOD POSSIBILITY FOR MANY AREAS FROM
FRIDAY NIGHT INTO THE WEEKEND. EXPECT MAINLY FAIR MIXING CONDITIONS
AND SMOKE DISPERSAL TODAY WITH FAIR TO GOOD SMOKE DISPERSAL
THURSDAY.
&&

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

$$

SHORT TERM...HATTINGS
LONG TERM...AEM
AVIATION...MCDONALD
FIRE WEATHER...HATTINGS



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