FXUS65 KREV 231129
AFDREV

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RENO NV
330 AM PST WED DEC 23 2009

.SHORT TERM...

VERY COLD TEMPERATURES ACROSS THE AREA THIS MORNING...WITH MANY OF 
THE COLDER VALLEYS ALREADY IN THE SINGLE DIGITS. EASTERLY GRADIENT 
WIND WILL CONTINUE THROUGH THIS MORNING...WITH STRONG NORTH TO EAST 
WINDS ON THE RIDGES. THESE STRONGER WIND ALOFT HAVE KEPT THE 
BOUNDARY LAYER MIXED AND FREEZING FOG FROM DEVELOPING UNDER THE 
CLEAR SKIES. THE DEPARTING UPPER LOW WILL PUSH EAST INTO THE CENTRAL 
CONUS TODAY...ALLOWING FOR HIGH PRESSURE TO BUILD INTO THE WEST 
COAST THROUGH CHRISTMAS.

AS THE RIDGE OF HIGH PRESSURE CONTINUES TO BUILD OVER THE NEXT FEW 
DAYS...AND WINDS ALOFT DECREASE SIGNIFICANTLY...WE WILL SEE A RETURN 
OF FREEZING FOG TO THE WESTERN GREAT BASIN. WARMER AIR WILL MOVE IN 
ALOFT...WHICH WILL STRENGTHEN THE TEMPERATURE INVERSIONS IN THE 
VALLEYS. CLEAR SKIES AT NIGHT WILL MAXIMIZE NOCTURNAL COOLING...WITH 
PATCHY FREEZING FOG DEVELOPING IN THE COLDEST VALLEYS FIRST...THEN 
SPREADING INTO THE REST OF THE BASIN VALLEYS OVER THE NEXT COUPLE 
NIGHTS. WITH THE RIDGE BUILDING DIRECTLY OVERHEAD...AND NOTHING 
REALLY ON THE HORIZON TO BREAK THESE INVERSIONS...COULD SEE MULTIPLE 
DAYS OF FREEZING FOG ACROSS THE REGION...MAKING FOR ICY SLICK 
MORNINGS AS THE FREEZING FOG PUTS DOWN A LIGHT COATING OF RIME ICE 
ON ROAD SURFACES. MAY NEED TO HAVE FREEZING FOG ADVISORIES IF FOG 
BECOMES MORE WIDESPREAD THAN PATCHY.  HOON

.LONG TERM...SATURDAY THROUGH TUESDAY...

THE UPPER RIDGE REMAINS STRONG ENOUGH ON SATURDAY TO KEEP PATCHY 
FREEZING FOG IN VALLEY AREAS THROUGH THE MORNING. THE RIDGE IS 
WEAKENED BY AN APPROACHING PACIFIC TROUGH THAT IS HANDLED 
DIFFERENTLY BY MEDIUM RANGE MODELS. THE GFS...CANADIAN...AND ECMWF 
ALL SHOW THE TROUGH SPLITTING OFF THE COAST DURING THE WEEKEND. THE 
ECMWF SHOWS MORE SOUTHERN STREAM ENERGY THAT UNDERCUTS THE RIDGE AND 
BRINGS PRECIP IN FASTER THAN THE GFS/CANADIAN MODELS. THE GFS 
DEVELOPS A CLOSED LOW OFF THE NORTHERN BAJA COAST SUNDAY NIGHT THEN 
TAKES THE LOW ACROSS BAJA MONDAY INTO TUESDAY WITH MINIMAL EFFECT ON 
WEATHER FOR NORTHERN CA AND NV. THE EC KEEPS A WEAK TROUGH OVER CA 
AND WESTERN NV DURING THAT TIME. GFS ENSEMBLE CHARTS SHOW LESS 
SPREAD IN THEIR SOLUTIONS THAN THE ECMWF FOR THE WEEKEND AND EARLY 
NEXT WEEK...PROVIDING SOME SUPPORT FOR LESS PRECIP DURING THE 
PERIOD. FOR NOW WILL CONTINUE WITH A FORECAST SHOWING SLIGHT CHANCES 
OF RAIN AND SNOW IN THE SIERRA AND PORTIONS OF NORTHWEST NV SUNDAY 
THROUGH TUESDAY.

HIGH TEMPERATURES WILL REMAIN BELOW NORMAL THROUGH THE PERIOD WITH 
HIGHS IN THE MID AND UPPER 30S WESTERN NV AND 30S IN THE MOUNTAINS. 
LOW TEMPS WILL STAY COLD AS WELL BUT CLOUD COVER MAY HELP KEEP THEM 
FROM DROPPING SIGNIFICANTLY. RC
&&

.AVIATION...

VFR CONDITIONS THROUGH THU 06Z THEN PATCHY FZFG DEVELOPING IN LIGHT 
WIND/CLEAR SKY CONDITIONS. RC
&&

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

$$

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