Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 211747
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
1047 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.../18Z ISSUANCE/

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

UPPER LEVEL DISTURBANCE WILL BRING LINGERING ISOLATED TO LOCALLY
SCATTERED SNOW SHOWERS THIS MORNING...INTO THE EARLY AFTERNOON
HOURS...WITH THE BEST COVERAGE OVER THE HIGHER TERRAIN. BRIEF IFR
AND MVFR CONDITIONS WILL OCCUR IN THESE SNOW SHOWERS...AND THE
MAJORITY OF THE LOWER ELEVATIONS WILL SEE CEILINGS BETWEEN 030-080
KFT AGL. GIVEN THE ISOLATED NATURE AND SHORT DURATION OF THESE SNOW
SHOWERS...PLACED VCSH IN A FEW TAFS FOR THE REMAINDER OF THE
MORNING...INTO THE EARLY AFTERNOON HOURS. THE UPPER LEVEL SYSTEM
RESPONSIBLE FOR THIS CONVECTION WILL EXIT THE AREA MIDDAY WITH
DECREASING CLOUDINESS EXPECTED FROM NORTH TO SOUTH BETWEEN 19Z AND
23Z TODAY. CLEAR SKIES AND LIGHT WINDS TONIGHT COULD ALLOW FOR
PATCHY FOG TO DEVELOP IN BASINS AND VALLEYS AFTER 06Z THURSDAY. ANY
PATCHY FOG THAT DEVELOPS WILL DIMINISH BY 15Z THURSDAY...GIVING MAY
TO FEW/SCT HIGH CLOUDS AND VFR CONDITIONS. GUSTY WINDS COULD ALSO
DEVELOP AFTER 09Z THURSDAY...MAINLY NEAR KCPR.

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

A LOWERING BKN TO OVC STRATOCU DECK...MAY BRING MVFR TO BRIEF IFR
CEILINGS LATE THIS MORNING THROUGH THE EARLY AFTERNOON...MAINLY
FROM KPNA TO KRKS. THIS DECK SHOULD SCATTER/LIFT DURING THE MID TO
LATE AFTERNOON. OTHERWISE...THE MAIN CONCERN WILL BE SOME LOW
CEILINGS AND FOG OVER THE FAR WESTERN VALLEYS TONIGHT INTO EARLY
THURSDAY MORNING. HOWEVER THE EXPECTED EXTENT ONLY WARRANTS A VCFG
FOR NOW AT KJAC...WHILE A LIGHT VRB WINDS SHOULD KEEP KBPI AND KPNA
FROM FOG/LOW CLOUDS IMPACTING THOSE TERMINALS. ANY FOG THAT DEVELOPS
WILL LIFT BY 15Z THURSDAY...AND MAINLY VFR CONDITIONS ARE EXPECTED
THROUGH THE REST OF THE DAY.

&&

.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...ANGLIN
FIRE WEATHER...ALLEN











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