Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 171824
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
1224 PM MDT FRI APR 17 2015

.SHORT TERM...FRIDAY THROUGH SUNDAY NIGHT

ANOTHER DAY HAS ARRIVED AND IT IS ANOTHER DAY IN DEALING WITH ONE OF
THE MOST FICKLE FEATURES IN WEATHER FORECASTING...THAT UPPER LEVEL
LOW THAT IS NOW SPINNING OVER SOUTHERN COLORADO. ANOTHER SPOKE OF
ENERGY HAS ROTATED INTO SOUTHEASTERN WYOMING BUT FOR NOW IS STAYING
OUT OF OUR COUNTY WARNING AREA. SEEING THE CHEYENNE RADAR...IT
ALMOST MAKES ME THINK I AM BACK IN MY ALMA MATER STATE OF NORTH
CAROLINA RATHER THAN WYOMING AS IT ALMOST LOOKS LIKE A SPIRAL BAND
FROM A LAND FALLING HURRICANE. SO FAR THE MOVEMENT HAS BEEN MORE
WEST THAN NORTH...SO THE BEST CHANCE OF SEEING EFFECTS FROM IT
WOULD BE ACROSS EASTERN SWEETWATER COUNTY...SO THE HIGHEST POPS
WERE PUT THERE. LIKE YESTERDAY...THE MILLION DOLLAR QUESTION IS
HOW FAR NORTH AND WEST WILL THE PRECIPITATION GET. THE GFS BRINGS
IT FURTHER NORTH TO AROUND KAYCEE TO RIVERTON LINE WHILE THE NAM
IS SLIGHTLY FURTHER SOUTH. CONTINUITY HANDLED THIS FAIRLY WELL SO
WE MADE FEW CHANGES...BUT THE EXACT NORTHERN EDGE IS IMPOSSIBLE TO
PREDICT AT THIS TIME. THE AIR IS STILL FAIRLY WARM WITH 700
MILLIBAR TEMPERATURES AROUND MINUS 2 SO SNOW LEVELS WOULD LIKELY
BE MAINLY ABOVE 7500 FEET. AND EVEN HERE...SINCE MOST OF ANY SNOW
WILL FALL DURING THE DAY IT WOULD MELT ON ROADS WITH THE STRONG
APRIL SUN...SO WE WILL NOT ISSUE ANY HIGHLIGHTS AT THIS TIME.
MEANWHILE...IT WILL BE A TALE OF TWO AREAS WITH THE NORTHWEST HALF
OF THE AREA HAVING A FAIRLY NICE DAY WITH SOME SUNSHINE AND NEAR
NORMAL TEMPERATURES. THE MODELS HAVE THE SAME DISAGREEMENT WITH
THE PRECIPITATION PATTERN TONIGHT BUT BOTH THE NAM AND GFS KEEP
THE LIONS SHARE OF THE QPF EAST OF THE RIVERTON COUNTY WARNING
AREA.

FINALLY ON SATURDAY...A COLD FRONT AND KICKER TROUGH WILL MOVE IN
FROM THE NORTHWEST AND FINALLY GIVE THE UPPER LEVEL LOW A BOOT TO
THE NORTH AND EAST. JUST ABOUT EVERYONE WILL HAVE A CHANCE FOR
SHOWERS ONCE AGAIN AS THE FRONT DROPS SOUTHWARD. THE MODELS ARE ALSO
SHOWING A HEAVIER BAND OF PRECIPITATION WITH SOME CONVERGENCE AS THE
FRONT DROPS SOUTHWARD. AGAIN...BOTH THE MODELS KEEP THIS FEATURE
JUST EAST OF OUR AREA BUT WE DID ADJUST POPS UPWARD IN EASTERN
NATRONA AND JOHNSON COUNTY IN CASE OF A SHIFT TO THE EAST.

COOLER AIR THEN MOVES IN BEHIND THE FRONT FOR SATURDAY NIGHT AND
SUNDAY. THE MODELS ONCE AGAIN ARE IN SOMEWHAT OF A DISAGREEMENT. THE
GFS KEEPS SOME ENERGY OVER THE AREA WITH SHOWERS CONTINUING ACROSS
MUCH OF THE AREA...ESPECIALLY ACROSS THE SOUTHWEST. MEANWHILE...THE
NAM IS DRIER WITH KICKING THE ENERGY FURTHER TO THE EAST AND HAVING
ONLY SPOTTY MOUNTAIN SHOWERS. THIS FAR OUT...WE TWEAKED POPS AND QPF
DOWN JUST A BIT BUT WILL NOT MAKE SWEEPING CHANGES UNTIL THE MODELS
COME INTO BETTER AGREEMENT.

.LONG TERM...MONDAY THROUGH FRIDAY NIGHT

A NORTHWEST FLOW PATTERN REMAINS IN THE FORECAST TO BEGIN THE WORK
WEEK NEXT WEEK. THE UPPER LEVEL LOW THAT WE`VE BEEN TRACKING IN
PREVIOUS DISCUSSIONS...WILL PROGRESS EASTWARD OVER THE GREAT LAKES
THROUGH WEDNESDAY...PER THE 00Z GFS/ECMWF RUNS. THIS WILL ALLOW A
SHORT WAVE RIDGE TO MOVE OVER THE NORTHERN AND CENTRAL ROCKIES
THROUGH 00Z THURSDAY.

THE GFS CONTINUES TO BE THE MORE PROGRESSIVE MODEL DIGGING A LONG
WAVE TROUGH OVER THE WESTERN CONUS...WHILE THE ECMWF RESOLVES A CUT
OFF LOW DIGGING SOUTHWARD ALONG THE CALIFORNIA COAST WEDNESDAY AND
THURSDAY. IT THEN REINFORCES THIS LOW WITH A DIGGING TROUGH FROM THE
GULF OF ALASKA AND BEGINS TO MERGE THESE TWO FEATURES OVER THE GREAT
BASIN BY THE END OF THE FORECAST PERIOD. AS A RESULT...THE GFS
CONTINUES TO SPEW SHORT WAVES FROM THE SOUTHWEST OVER THE FORECAST
AREA. KEPT A SLIGHT CHANCE TO CHANCE OF MAINLY RAIN SHOWERS FROM
WEDNESDAY AFTERNOON ONWARD. A RAIN/SNOW MIX TO SNOW SHOWERS WILL BE
POSSIBLE DURING THE OVERNIGHT HOURS.

.AVIATION.../18Z ISSUANCE/

AN UPPER LEVEL LOW PRESSURE SYSTEM WILL DRIFT ACROSS SOUTHERN
COLORADO THROUGH FRIDAY. AREAS OF MVFR CEILINGS AND VISIBILITY REDUCED
IN -SHRASN...WITH LOCAL IFR CEILINGS AND VISIBILITY REDUCED IN
-SHRASN BR WILL CONTINUE TO ROTATE AROUND THE NORTHERN END OF
THIS SYSTEM MAINLY VICINITY AND SOUTH OF A KCPR- KLND-KRKS LINE
THROUGH THE PERIOD. TERMINALS NORTH OF THIS LINE WILL SEE
PREVAILING VFR CONDITIONS WITH MAINLY SKC ACROSS THE NORTHERN
THIRD OF WYOMING.

.FIRE WEATHER...

UNSETTLED WEATHER WILL CONTINUE MAINLY SOUTH OF A RIVERTON TO NORTH
OF CASPER LINE...AS AN UPPER LEVEL LOW KEEPS CLOUDS AS WELL AS
SOME RAIN AND SNOW SHOWERS OVER THE AREA THROUGH TODAY. FURTHER
NORTH AND WEST...EXPECT MORE SUNSHINE AND WARMER TEMPERATURES
ALONG WITH LOWER HUMIDITY...BUT REMAINING ABOVE CRITICAL LEVELS. A
COLD FRONT WILL DROP SOUTHWARD ACROSS THE AREA SATURDAY WITH A
CHANCE OF SHOWERS FOR MOST AREAS FOLLOWED BY COOL DOWN.

&&

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

$$

SHORT TERM...HATTINGS
LONG TERM...LAVOIE
AVIATION...C.BAKER
FIRE WEATHER...HATTINGS






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