Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 210950
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
249 AM MST WED JAN 21 2015

.SHORT TERM...TODAY THROUGH FRIDAY NIGHT

A FEW SNOW SHOWERS ARE SHOWING UP ON RADAR THIS MORNING DUE TO A
PASSING SHORT WAVE. IT APPEARS TO BE PRETTY MOISTURE STARVED SO NOT
MUCH WILL COME OF IT BEFORE ITS INFLUENCE EXITS THE REGION LATER THIS
MORNING. OTHERWISE...EXPECT CLEARING AND COLDER AIR MOVING IN AT THE
MID LEVELS...WITH H7 TEMPS DROPPING TO NEARLY -17 C BY NOON. THIS
WILL BE SHORT LIVED HOWEVER AS A FOLD OVER RIDGE BRINGS IN SOME MORE
MILD TEMPERATURES ALOFT. DUE TO THE LACK OF RECENT SNOW COVER...THE
BASIN INVERSIONS WILL BE EASIER TO BREAK MOVING FORWARD. THE WAA
WILL ALSO ENCOURAGE SOME MORE FOG FORMATION OVERNIGHT UNDERNEATH
THESE INVERSIONS.

A LEE SIDE TROUGH FORMS ON THURSDAY...EVENTUALLY TURNING THE WIND
MACHINE BACK ON FROM EASTERN SWEETWATER COUNTY TO KAYCEE AND HELPING
THE MILD TEMPERATURES MIX TO THE SURFACE.

ON FRIDAY...ANOTHER QUICK SHORT WAVE WILL ZIP THROUGH THE AREA. IT
WILL NOT BE ACCOMPANIED BY MUCH MOISTURE OR DEEP COLD AIR BUT THERE
COULD BE A FEW RESULTING SNOW SHOWERS ACROSS THE NORTH AND IN THE
MOUNTAINS. A REINFORCING WAVE COULD BE NIPPING AT THE HEELS OF THE
FIRST WAVE...WHICH COULD KEEP SOME ISOLATED SHOWER ACTIVITY GOING
INTO FRIDAY NIGHT ACROSS YELLOWSTONE AND THE ABSAROKAS.

.LONG TERM...SATURDAY THROUGH WEDNESDAY

SYNOPSIS...NORTHWEST FLOW WILL BRING SOME VALLEY RAIN AND
MOUNTAIN SNOW SHOWERS ACROSS THE NORTH ON SATURDAY ALONG WITH THE
POTENTIAL FOR STRONG WINDS MAINLY ACROSS EASTERN JOHNSON COUNTY. A
RIDGE OF HIGH PRESSURE WILL BUILD IN TO THE AREA SUNDAY THROUGH
TUESDAY FOR GENERALLY DRY CONDITIONS WITH ABOVE NORMAL
TEMPERATURES...THOUGH INVERSIONS WILL HOLD SOME AREAS
BACK...ESPECIALLY OVER THE FAR WEST VALLEYS. CHANCES OF RAIN OR
SNOW SNOW SHOWERS RETURN ACROSS THE WEST ON WEDNESDAY.

DISCUSSION...STRONG 160 KNOT JET ACROSS THE CENTRAL PACIFIC WILL
MOVE UP AND OVER THE RIDGE ALONG THE WEST COAST OVER THE NEXT COUPLE
OF DAYS WITH LEAD JET STREAK DIGGING INTO THE NORTHERN PLAINS ON
SATURDAY.  ECMWF TAKES THIS ENERGY ON A FURTHER EASTWARD TRAJECTORY
THAN THE GFS...SO THE ECMWF SOLUTION WOULD RESULT IN LITTLE OR NO
PRECIPITATION ACROSS THE AREA SATURDAY AND WEAKER NW FLOW.  GFS
SHOWS H7 WIND SPEEDS OF 60-70 KTS ACROSS NE WYOMING SATURDAY WHICH
WOULD RESULT IN THE POSSIBILITY OF HIGH WINDS ALONG THE INTERSTATE
90 CORRIDOR.  FORECAST LEANS TOWARD SLOWER PROGRESSION OF LONGWAVE
PATTERN PER GFS.  STRONG NW FLOW SHOULD ALSO RESULT IN GOOD MIXING
FOR MOST AREAS EAST OF THE DIVIDE...SO BUMPED UP MAX TEMPERATURES
INTO THE 40S FOR MOST AREAS.   BOTH ECMWF AND GFS SHOW 585DM H5 HIGH
CENTERED OVER UTAH BY MONDAY WITH H7 TEMPS 5-6C OVER WYOMING.  THIS
WILL BRING A VERY WARM AIRMASS INTO THE MOUNTAINS AND ADJACENT
FOOTHILLS...BUT WINDS ALOFT WILL BECOME LIGHT SO STEEP INVERSIONS
WILL BE LIKELY OVER THE JACKSON AND STAR VALLEYS...AND TO A LESSER
EXTENT OVER THE CENTRAL BASINS.   GFS AND ECMWF ALSO IN GOOD
AGREEMENT ON LIFTING AN UPPER LOW NORTH OUT OF THE BAJA AREA ALONG
BACKSIDE OF UPPER RIDGE ACROSS THE INTERMOUNTAIN WEST TUESDAY AND
WEDNESDAY.  THIS WILL BRING CHANCES OF RAIN AND SNOW SHOWERS INTO
THE WEST BY WEDNESDAY...THOUGH PRECIPITATION AMOUNTS ARE EXPECTED TO
BE LIGHT AS THIS ENERGY FRAGMENTS AND DAMPENS OUT QUITE A BIT BY THE
TIME IT REACHES WYOMING.

&&

.AVIATION.../12Z ISSUANCE/

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

ISOLATED TO LOCALLY SCATTERED SNOW SHOWERS WILL CONTINUE THIS
MORNING WITH THE BEST COVERAGE OVER THE HIGHER TERRAIN. IFR AND MVFR
CONDITIONS WILL OCCUR IN SNOW SHOWERS. HOWEVER THE MAJORITY OF THE
LOWER ELEVATIONS WILL SEE CEILINGS BETWEEN 040-080 KFT AGL WITH
ONLY BRIEF PERIODS WHERE IFR/MVFR ARE POSSIBLE. HAVE KEPT ALL
TERMINALS VFR FOR NOW...WITH VCSH TO COVER THE CONVECTION COMING OFF
THE MOUNTAINS THIS MORNING. THE UPPER LEVEL SYSTEM RESPONSIBLE FOR
THIS CONVECTION WILL EXIT THE AREA MIDDAY WITH DECREASING CLOUDINESS
EXPECTED FROM NORTH TO SOUTH BETWEEN 17Z AND 23Z TODAY. CLEAR SKIES
AND LIGHT WINDS TONIGHT COULD ALLOW FOR PATCHY FOG TO DEVELOP IN
BASINS AND VALLEYS AFTER 06Z THURSDAY.

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

MODEL SOUNDINGS ARE HINTING AT A STRATOCU DECK /025-045 KFT AGL/
IMPACTING SWEETWATER COUNTY...INCLUDING THE KRKS TERMINAL...THIS
MORNING. CONFIDENCE IN THE EXTENT OF THIS CLOUD DECK IS LOW...AND
HAVE DECIDED TO KEEP THE KRKS CEILING IN THE VFR CATEGORY FOR NOW.
THIS DECK SHOULD SCATTER/LIFT DURING THE AFTERNOON. OTHERWISE...THE
MAIN CONCERN WILL BE SOME LOW CEILINGS AND FOG OVER THE FAR WESTERN
VALLEYS. HOWEVER THE EXPECTED EXTENT ONLY WARRANTS A VCFG FOR NOW AT
KJAC...WHILE A LIGHT NORTHERLY FLOW SHOULD KEEP KBPI AND KPNA FROM
FOG/LOW CLOUDS IMPACTING THOSE TERMINALS. ANOTHER A CHANCE FOR SOME
FOG WILL OCCUR TONIGHT.

&&

.FIRE WEATHER...

MIXING/INVERSION HEIGHTS WILL REMAIN ON THE LOW SIDE THROUGH AT
LEAST FRIDAY. SIGNIFICANT PRECIPITATION IS NOT EXPECTED ANYWHERE
OVER THE FORECAST AREA UNTIL POSSIBLY THIS WEEKEND ACROSS THE NORTH
AND NORTHEAST DISPATCH ZONES.

&&

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

$$

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







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