FXUS65 KREV 251641 AAA
AFDREV

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE RENO NV
841 AM PST FRI DEC 25 2009

.UPDATE...
AREAS OF FZFG CONTINUE IN THE GREAT BASIN NEAR NFL/LOL AND AROUND
MONO LAKE PER OBS AND WEBCAMS. HAVE EXTENDED MENTION OF FZFG INTO
THE THE EARLY AFTN. NAM BUFR SOUNDINGS INDICATE FG BREAKING UP AT
LOL BTWN 19-22Z.

EARLY MORNING REV SOUNDING SHOWED MODERATE EASTERLY FLOW ~700MB
WITH 20KTS. REMOTE OBS ALSO SHOWING GUSTS BETWEEN 30-50 MPH ON
SIERRA RIDGES. THIS IS ASSOC WITH WEAK SHORTWAVE PASSING SOUTHWARD
THRU NV/UT TODAY. HAVE INCREASED RIDGE WINDS IN THE SIERRA TO
~35KTS TODAY AND THIS EVE.

OTHERWISE NO CHANGES TO ONGOING FORECAST. UPDATED GRIDS AND TEXT
OUT BY 9 AM PST.

CS

&&

.PREVIOUS DISCUSSION... /ISSUED 235 AM PST FRI DEC 25 2009/ 

.SHORT TERM...
LIGHT WINDS AND CLEAR SKIES HAVE ALLOWED US TO COOL OFF CONSIDERABLY 
AND CREATE A FEW AREAS OF FREEZING FOG AROUND THE REGION AGAIN THIS 
MORNING...MAINLY CENTERED AROUND PYRAMID LAKE...MONO LAKE...AND 
PORTIONS OF THE HUMBOLDT AND CARSON SINKS. NOT REALLY EXPECTING ANY 
DENSE FREEZING FOG THE RENO-SPARKS AREA THIS MORNING...ALTHOUGH SOME 
PATCHY AREAS OF FOG ARE NOT OUT OF THE QUESTION IN THE COLDER 
SURROUNDING VALLEYS. THERE REMAINS A LIGHT AND VERY DRY NORTHEAST 
WIND AT RIDGETOP THAT IS KEEPING THE BOUNDARY LAYER FROM MOISTENING 
UP AND GETTING FOG...WHICH IS GOOD. AS LONG AS WE CONTINUE TO SEE 
THIS DRY FLOW...WE CANNOT EXPECT TO SEE ANY WIDESPREAD FREEZING 
FOG...EXCEPT FOR SOME ISOLATED AREAS. FOR FRIDAY NIGHT...THERE AGAIN 
REMAINS A CHANCE FOR SOME PATCHY FREEZING FOG IN THE GREAT BASIN 
ZONES...BUT THE EXTENT WILL BE LIMITED DUE TO SOME MID AND HIGH 
CLOUDS MOVING IN TONIGHT...WHICH WILL HELP TO INSULATE.

STRONG TEMPERATURE INVERSIONS WILL CONTINUE THROUGH SATURDAY...WITH 
THE POTENTIAL OF CREATING SOME HAZE AND SMOG PROBLEMS AROUND THE 
RENO AREA AS THE SMOG GETS TRAPPED DOWN IN THE VALLEY UNDER THE 
INVERSION. 

A LITTLE BETTER AGREEMENT IN THE MODEL OVERALL FOR THE SHORT TERM 
PAST SATURDAY. THE RIDGE OF HIGH PRESSURE IS EXPECTED TO GET PINCHED 
OFF OVER WESTERN CANADA ON SATURDAY...ALLOWING FOR A PAIR OF 
SHORTWAVES TO MOVE INTO THE SOUTHWESTERN CONUS. LOOKS LIKE A PRETTY 
GOOD CHANCE AT A SPLITTING TROUGH EVEN BEFORE IT EVEN MAKES IT 
ONSHORE. THIS WILL BRING THE MAJORITY OF THE ENERGY AND MOISTURE 
INTO SOUTHERN CA SATURDAY NIGHT THROUGH SUNDAY. LOWERED POPS 
SLIGHTLY...BUT KEPT MOST AREAS IN THE CHANCE TO SLIGHT CHANCE 
CATEGORY. SNOW LEVELS LOOK TO BE TRICKY...RIGHT AROUND VALLEY FLOORS 
AT 4500 FEET. ANY PRECIP THAT WE DO RECEIVE LOOKS TO BE 
MINIMAL...REMAINING WELL BELOW ADVISORY CRITERIA IN ALL ELEVATIONS. 
THE WETTEST MODEL REMAINS THE 00Z ECMWF...WHICH IS NOTORIOUS FOR 
OVERDOING PRECIP AMOUNTS. LOTS OF QUESTIONS STILL REMAIN WITH THIS 
STORM...BUT LOOKS LIKE THE BEST THIS THE STORM WILL DO FOR THE AREA 
IS SCOUR OUT SOME OF THE HAZE AND SMOG FOR AT LEAST A COUPLE DAYS.  
HOON

.LONG TERM...MONDAY THROUGH THURSDAY...

LARGE TROUGH OVER THE WEST EARLY IN THE WEEK...WITH MINIMAL 
RAIN/SNOW CHANCES FOR THE SIERRA AND WESTERN NEVADA.  MODELS DEVELOP 
RIDGING ALONG THE WEST COAST WITH LARGE AMPLITUDE DIFFERENCES FOR 
WEDNESDAY AND THURSDAY.

THE MAIN CAUSE FOR THE MODEL DIFFERENCES IS THE POTENTIAL FOR A 
SPLIT FLOW IN THE PACIFIC.  GFS HAS A MORE ACTIVE SOUTHERN BRANCH OF 
THE JET BETWEEN 30N AND 40N...PUSHING ENERGY UNDERNEATH THE MAIN 
UPPER LOW OVER THE ALEUTIANS.  THIS YIELDS A LESS AMPLIFIED PATTERN 
IN THE PACIFIC BETWEEN 150W AND THE WEST COAST.  THE ECMWF KEEPS THE 
SOUTHERN BRANCH LESS PROGRESSIVE AND ALLOWS THE ENERGY TO PHASE INTO 
ALEUTIAN LOW FOR A LONG WAVE TROUGH AND A MUCH MORE AMPLIFIED RIDGE 
NEAR 130W.

BOTH MODELS BRING PRECIPITATION INTO THE NORTHERN SIERRA AND 
NORTHWEST NEVADA...BUT IN VERY DIFFERENT ENVIRONMENTS.  GFS DEVELOPS 
WARM AIR ADVECTION PRECIP AS IT BUILDS ITS FLAT RIDGE OVER THE WEST 
COAST. DEPENDING ON SURFACE INVERSIONS THE GFS SOLUTION COULD BE A 
MIXED BAG OF HIGH ELEVATION SNOW...RAIN IN THE 5000-7000 ELEVATION 
RANGE...AND RAIN/SLEET/FREEZING RAIN IN THE LOWER VALLEYS THAT MAY 
OR MAY NOT MIX OUT WITH THE SYSTEM. ECMWF HAS A MUCH COLDER 
NORTHWEST FLOW WITH A QUICK MOVING AND POTENTIALLY VERY UNSTABLE 
SNOW STORM PASSING OVER NORTHWEST NEVADA WEDNESDAY.  TRACK OF THE 
ECMWF SYSTEM IS ALSO HIGHLY VARIABLE DUE TO POSITION OF THE JET THAT 
WILL DRIVE THE SYSTEM ONSHORE.  KEPT RAIN/SNOW CHANCES AT 30 PERCENT 
FOR AREAS NORTH OF US-50.

ENSEMBLES HAVE MORE SUPPORT FOR THE ECMWF KEEPING THE RIDGE OFFSHORE 
WEDNESDAY...THEN AMPLIFYING THE RIDGE FOR THURSDAY WITH THE 
CONSOLIDATED TROUGH NEAR 140W.  THUS WILL LEAN MORE TOWARDS THE 
ECMWF...BUT KEEP A BIT OF THE GFS BLEND...SINCE THERE ARE A FEW 
ENSEMBLE MEMBERS THAT SUPPORT IT.  BRONG  

&&

.AVIATION...

RIDGE BUILDING OVERHEAD WILL KEEP VALLEY INVERSIONS IN PLACE THIS 
MORNING AND AGAIN TONIGHT.

FOG HAS DEVELOPED IN ONLY IN A FEW LOCATIONS EARLY THIS MORNING 
BASED ON WEB CAMS AND SURFACE OBSERVATIONS.  AREAS WITH FOG INCLUDE 
PORTIONS OF THE HUMBOLDT AND CARSON SINKS...AND IN THE MONO LAKE 
BASIN.  VISIBILITY AT LOVELOCK HAS BEEN HIGHLY VARIABLE THE LAST 
COUPLE HOURS...RANGING FROM 8 MILES DOWN TO 1/4 MILE.  EXPECT THAT 
TO SETTLE TOWARDS 1/2 TO 2 MILE RANGE 10Z-18Z.

FOG POTENTIAL AT THE SIERRA TERMINALS IS VERY LOW DUE TO THE DRY 
NORTHEAST FLOW OVER THE SIERRA.  FOR RNO...FOG POTENTIAL VERY LOW 
WITH LACK OF SNOW COVER BUT REDUCED VISIBILITY TO 4 MILES IS 
POSSIBLE AS HAZE COLLECTS IN THE INVERSION LAYER THE NEXT COUPLE 
DAYS.  

FOG SHOULD RETURN TO LOVELOCK TONIGHT...WITH TRK...TVL AND RNO FOG 
FREE FRIDAY NIGHT/SATURDAY MORNING.  BRONG
  

&&

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

$$

HTTP://WEATHER.GOV/RENO