FXUS65 KREV 232305
AFDREV

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RENO NV
305 PM PST WED DEC 23 2009

.SHORT TERM...

PRIMARY CONCERN OVER THE NEXT FEW DAYS WILL BE VALLEY INVERSIONS
AND ASSOC FOG CHANCES FOLLOWED BY POTENTIAL FOR WINTER WX SYSTEM
SAT NGT INTO SUN.

GFS/NAM TIME HEIGHT SECTIONS FOR RNO/LOL SHOW WARMING TEMPS IN
THE 700-800MB LAYERS ASSOC WITH RIDGING ALOFT, PEAKING FRI WHEN
INVERSIONS WILL BE STRONGEST. HAVE CONTINUED KEEPING NV VALLEYS
COOL COMPARED TO SURROUNDING HIGHER ELEVATIONS, ESPECIALLY FRIDAY,
GOING CLOSER TO RAW NAM 2M TEMPS VERSUS MOS GUIDANCE. MODIFIED
RICHARDSON NUMBERS ARE FAVORABLE FOR FOG IN VALLEYS, INCLUDING
RENO, WITH LIGHT PBL WINDS AND STRONG INVERSIONS AT NIGHT. HOWEVER
THE MOISTURE PROFILES IN THE LOWEST 500-1000 FT AGL AND INPUTS
FROM MELTING SNOW ARE MORE QUESTIONABLE. A LOT OF DRY AIR JUST OFF
THE SFC. ALSO, FOR TNGT UPSTREAM CIRRUS MAY OVERSPREAD THE REGION
BASED ON 300MB RH FROM GFS/EC, WHICH WOULD LIMIT RADIATIONAL FOG
COVERAGE. OVERALL WILL CONTINUE AREAS OF FZFG IN THE VALLEYS THRU
FRI NIGHT. FEEL THAT CHRISTMAS EVE NIGHT INTO CHRISTMAS MORNING
WILL BE THE BEST TIME FOR WIDESPREAD DENSE FZFG GIVEN OPTIMAL
COMBINATION OF LOW LEVEL MOISTURE, LIGHT WINDS, VERY STRONG
INVERSION, AND CLEAR SKIES ALOFT. ADVISORIES MAY BE NEEDED DURING
THIS TIMEFRAME. HIGHEST COVERAGE BOTH TNGT AND TMW NGT WOULD BE
OVER GREAT BASIN ZONES, AND LIKE LAST WEEK MAY CONTINUE WELL INTO
THE DAYTIME HOURS. CONFIDENCE LEVELS FOR THE INVERSION/FOG
FORECAST ARE MODERATE-HIGH (3.5 OUT OF 5).

BOTH GFS/ECMWF DEPICT MODEST UPPER TROF SWINGING INTO NRN CA AND
NV SAT NGT INTO SUN. THIS BRINGS A SLUG OF PRECIP INTO THE SIERRA
IN BOTH MODELS...HOWEVER GEFS ENSEMBLE DATA ARE QUITE DRY WITH A
LOT OF SPREAD/UNCERTAINTY IN POP FIELDS. BOTTOM LINE...HAVE
INCREASED POPS SLIGHTLY, PRIMARILY IN THE SIERRA, FOR SAT NGT.
SNOW LEVEL COULD BE QUITE LOW BASED ON 700MB TEMPS NEAR -7 TO -9C
AND ANY REMAINING VALLEY INVERSIONS. SPILLOVER INTO WRN NV
QUESTIONABLE, THOUGH GFS DOES BRING ABOUT 3" OF SNOW INTO RNO BY
12Z/SUN ASSOC WITH 400MB FRONTOGENETICAL BAND. CONFIDENCE LEVELS
REGARDING THIS SYSTEM THROUGH 12Z/SUN ARE MODERATE (3 OUT OF 5).

CS

.LONG TERM...SUNDAY THROUGH THURSDAY...

THE OPERATIONAL GFS AND ECMWF ARE IN BETTER AGREEMENT TODAY IN 
SHOWING A TROUGH MOVING IN FROM THE NORTHWEST AND EFFECTIVELY 
UNDERCUTTING THE RIDGE OVER THE WESTERN UNITED STATES ON SUNDAY. THE 
ECMWF IS STILL A BIT MORE PROGRESSIVE REGARDING PRECIP...BUT THE GFS 
HAS BEGUN TO FOLLOW SUIT BY INTRODUCING SOME QPF OVER THE SIERRA AND 
WESTERN NEVADA SUNDAY MORNING. THIS IN TURN AS INCREASED CONFIDENCE 
IN A WETTER SOLUTION AND HAVE INCREASED POPS ALONG THE SIERRA AND IN 
THE FOOTHILLS. A MODERATELY DEFINED BAND OF INSTABILITY NEEDED FOR 
PRECIP IN THE EASTERN PARTS OF THE CWA WILL COME THROUGH SUNDAY 
MORNING BUT WILL MOST LIKELY BE SHORT LIVED...ONLY LASTING THROUGH 
SUNDAY AFTERNOON. SNOW LEVELS FOR THIS STORM WILL BEGIN AROUND 5000 
FEET AT THE START BUT WILL DROP TO VALLEY FLOORS AS COLDER AIR IS 
RUSHED IN WITH THE LEADING EDGE OF THE TROUGH. BOTH OPERATIONAL GFS 
AND ECMWF CLOSE THE LOW OVER THE GREAT BASIN AND SLOWLY BRING IT 
EAST...KEEPING CHANCES OF PRECIPITATION ACROSS NEVADA THROUGH 
MONDAY. HOWEVER...ENSEMBLE MEMBERS FOR BOTH MODELS INDICATE RATHER 
LOW CONFIDENCE IN HOW LONG IT WILL TAKE FOR THE LOW TO MOVE OUT OF 
THE REGION SO HAVE KEPT SLIGHT CHANCES THROUGH THE PERIOD. 

A WEAK...DIRTY RIDGE WILL BUILD IN WITH ITS AXIS ALONG THE
WESTERN SEABOARD BEGINNING TUESDAY MORNING. THERE WILL BE LITTLE
REPRIEVE BY WAY OF CLOUD COVER AND PRECIPITATION HOWEVER AS IT
LOOKS TO BE A ONE-TWO PUNCH SET UP FOR THE NORTHERN GREAT BASIN.
ANOTHER TROUGH IS EXPECTED TO FORM IN THE GULF OF ALASKA AND PUSH
A SHORTWAVE IN OVER THE NORTHWESTERN US BY THURSDAY...KEEPING A
CHANCE OF SNOW IN THE FORECAST WITH BELOW NORMAL AFTERNOON HIGH
TEMPERATURES THROUGH THE END OF THE PERIOD. HOWEVER...THE SEVERITY
OF THIS NEXT DISTURBANCE IS STILL UNKNOWN AT THIS TIME. -LABELLE-

.AVIATION...

FOR KRNO-KTVL...VFR CONDITIONS THROUGH 08Z THEN VICINITY PATCHY
FZFG DEVELOPING IN LIGHT WIND/CLEAR SKY CONDITIONS. VISIBILITY
COULD DROP TO 2 MILES AT TIMES. HOWEVER...ANY POTENTIAL FOG
CONDITIONS WILL DISSIPATE BY 12Z BASED ON NAM FORECAST SOUNDINGS.

FOR KTRK-KLOL...VFR CONDITIONS THROUGH 08Z THEN FZFG DEVELOPING
IN LIGHT WIND/CLEAR SKY CONDITIONS. PRONE VALLEYS COULD EXPERIENCE
LESS THAN 1 MI VISIBILITY THROUGH 15Z. -LABELLE-

&&

.REV WATCHES/WARNINGS/ADVISORIES...
NV...NONE.
CA...NONE.
&&

$$

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