FXUS65 KREV 151458
AFDREV

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RENO NV
658 AM PST TUE DEC 15 2009

.UPDATE...
FOG IS THINNING NOW AS DRIER AIR BEGINS TO MIX DOWN THE MID LVLS.
ALSO NOTED WINDS BEGINNING TO INCREASE SLIGHTLY OFF THE VALLEYS
FLOORS. THIS WILL AID THE MIXING THROUGH THE MRNG. SOME PATCHES OF
FREEZING FOG WILL REMAIN FOR AN HOUR OR TWO THIS MRNG...BUT NOT
WIDESPREAD ENOUGH TO WARRANT KEEPING ADVISORY IN EFFECT SO WILL
CANCEL THE ADVISORY BUT CONTINUE TO MENTION SOME PATCHY FREEZING
FOG THROUGH ABOUT 9 AM. MLF

&&

.PREVIOUS DISCUSSION... /ISSUED 611 AM PST TUE DEC 15 2009/ 

UPDATE...
WEBCAMS IN THE VICINITY OF SUSANVILLE AND AIRPORT OBSERVATION AT
SUSANVILLE INDICATES VARYING OF DEGREES OF FREEZING FOG REMAIN
OVER THAT AREA THIS MORNING. MEANWHILE OBSERVATIONS FROM LOVELOCK
AND FALLON SHOW NO FOG. IN THE RENO/SPARKS AREA FOG HAS INCREASED
IN THE LAST 2 HOURS. AIRPORT HAS SHOWN SOME INDICATIONS OF
INCREASING VISIBILITY RECENTLY BUT WEBCAMS DO NOT SHOW MUCH
CHANGE IN TOWN. SUSPECT THAT FREEZING FOG EXTENDS NORTH OF SPARKS
TOWARD PYRAMID LAKE AS WELL THIS MORNING. AWOS AT MINDEN AIRPORT
SOUTH OF CARSON CITY DOES NOT INDICATE ANY FOG SO IT IS LIKELY
THAT WORST CONDITIONS ARE FROM RENO NORTH. WILL EXTEND THE
FREEZING FOG ADVISORY FOR THE RENO/SPARKS AREA AND LASSEN COUNTY
UNTIL 9 AM THIS MORNING AND ALLOW THE ADVISORY FOR LOVELOCK/FALLON
AREAS TO EXPIRE. SLIDE MTN OBSERVATION THIS MORNING HAS INDICATED
STRONGER WINDS THAN YESTERDAY AND SOME DRYING OF THE AIRMASS
ALOFT. MORNING SOUNDING AT KREV SHOWED A VERY SHALLOW
INVERSION...BUT THIS IS MAINLY ABOVE THE CITY. THE DRY AIR AT
SLIDE MTN SHOULD BEGIN MIXING DOWN...ERODING THE FOG FROM THE TOP
DOWN. EVEN THOUGH ADVISORY HAS BEEN EXTENDED..IT MAY NEED TO BE
CANCELED BEFORE 9 AM IF CONDITONS IMPROVE QUICKLY. ALSO ADDED
PATCHY FOG TO MONO COUNTY IN VICINITY OF MONO LAKE. MLF

PREVIOUS DISCUSSION... /ISSUED 403 AM PST TUE DEC 15 2009/ 

SHORT TERM...
FOG HAS BEEN LIFTING IN W-CNTRL NV SINCE 10Z...AND RECENT OBS AT
KRNO-KSVE ALSO INDICATED IMPROVING VSBY TO 1 MILE. VSBY HAS
IMPROVED AROUND WFO RENO AND ON WEB CAMERAS AROUND SUSANVILLE.
INCREASING RIDGE WINDS AND LOWERING HUMIDITY AT SLIDE MTN APPEARS
TO BE HELPING TO DECREASE THE FOG COVERAGE AND IF THIS TREND
CONTINUES THE FZFG ADVISORY SHOULD EXPIRE AS SCHEDULED AT 6 AM.

FOR TODAY AND TONIGHT...PACIFIC MOISTURE BAND CONTINUES TO BE 
ORIENTED OVER NWRN CA-SWRN OREGON WITH LATEST GUIDANCE INDICATING
SLOWER PROGRESS TOWARD NERN CA BY LATE THIS AFTERNOON...WITH ONLY
LIGHT PRECIP AMOUNTS AFFECTING NWRN LASSEN-SURPRISE VLY AND PARTS
OF NWRN NV BY 00Z. ELSEWHERE THE PRECIP POTENTIAL FOR TODAY IS
FINISHED. SATELLITE-BASED PW VALUES FOR THIS MOISTURE OFF THE
COAST HAS DECREASED BELOW 1.5 INCHES WHICH INDICATES A WEAKER
SUBTROPICAL MOISTURE FETCH FOR THIS SYSTEM AND MINIMAL RUNOFF
CONCERNS FOR THE REGION. RIDGE WINDS TODAY WILL BE STRONG WITH
GUSTS OF 70 MPH OR MORE...BUT THESE WINDS WILL STRUGGLE TO MIX TO
LOWER ELEVATIONS.

BY TONIGHT...PRECIP IS STILL LIKELY TO SPREAD INTO NERN CA-NWRN NV
THIS EVENING AND POSSIBLY REACH THE I-80 CORRIDOR NORTH OF TAHOE
BY LATE TONIGHT...BUT QPF VALUES CONTINUE TO DECREASE WITH MAX
VALUES IN SWRN LASSEN COUNTY AROUND 0.50 INCH WITH MUCH LOWER
VALUES AWAY FROM THIS AREA. ISOTHERMAL MODEL SOUNDING PROFILES
BELOW 700 MB ARE LIKELY TO KEEP PRECIP TONIGHT ACROSS NERN CA-FAR
NWRN NV AS SNOW...AND A FEW INCHES COULD ACCUMULATE NEAR THE HWY
36-44 CORRIDORS WEST OF SUSANVILLE. HOWEVER...THE POTENTIAL FOR
LOW-END ADVISORY CRITERIA SNOWFALL WOULD COVER A SMALL AND
SPARSELY POPULATED PORTION OF THE LASSEN ZONE SO NO ADVISORY IS
PLANNED WITH THIS FORECAST ISSUANCE.

BY WEDNESDAY...THE MOISTURE BAND IS EXPECTED TO FINALLY REACH THE
TAHOE BASIN BUT WITH WEAKENING DYNAMICS AND THINNING MOISTURE
PLUME THE AMOUNT OF PRECIP WILL BE LIGHT. THE WEAKER VALLEY
INVERSION WILL ALLOW FOR TEMPS TO RISE ABOVE FREEZING IF PRECIP
CARRIES INTO WRN NV...LIMITING THE POTENTIAL FOR ANY FREEZING OR
FROZEN PRECIP BELOW THE POSTED SNOW LEVEL NEAR 6000 FEET. THERE IS
UNCERTAINTY TO WHETHER RAIN WOULD SPREAD EAST INTO WRN NV AS THE
GFS DOES GENERATE OVER 0.10 INCH OF QPF ACROSS RENO AND
LOVELOCK...BUT THE LATEST NAM AND ECMWF INDICATE LITTLE OR NO
MEASURABLE PRECIP CARRYING ACROSS THESE AREAS. WILL KEEP CHANCE
POPS FOR RENO AND PARTS OF W-CNTRL NV FOR NOW.

THE MOISTURE PLUME IS EXPECTED TO SHUT DOWN QUICKLY BY WED
EVENING WITH LITTLE OR NO ADDITIONAL PRECIP IN PARTS OF THE LASSEN
AND TAHOE ZONES. MJD

LONG TERM...THURSDAY THROUGH MONDAY...
MEDIUM RANGE MODELS IN DECENT AGREEMENT EARLY IN THE EXTENDED PERIOD 
THIS FORECAST BUT BY SUNDAY...DAY 6...MODELS START TO DIVERGE QUITE 
A BIT. CONFIDENCE REMAINS MUCH HIGHER IN THE DAY 3 THROUGH 5 PERIOD 
AS RIDGE DEVELOPS OVER THE RGN. PRIMARY DIFFERENCES IN THE MODELS 
EARLY ARE THAT OPERATIONAL GFS IS MUCH DIRTIER WITH THIS RIDGE THAN 
EITHER OPERATIONAL ECMWF AND CANADIAN AND MOST GEFS ENSEMBLE 
MEMBERS. 

ORIGINALLY WE HAD SLIGHT CHANCE TO LOW CHANCE POPS IN THE FAR NRN 
PORTIONS OF THE CWA STARTING FRIDAY NIGHT INTO SATURDAY NIGHT DUE TO 
PRESENCE OF WEAK SHORT WAVES RIDING OVER THE RIDGE. WITH MOST 
ENSEMBLE MEMBERS SUPPORTING A DRIER SOLUTION HAVE OPTED TO LOWER THE 
POPS TO NO MORE THAN SLIGHT CHANCE. TEMPS ALSO A BIT OF PROBLEM AS 
EVEN THOUGH RIDGE BUILDS OVER THE AREA DO NOT SEE ANY SIGNIFICANT 
LOW LVL FLOW STRONG ENOUGH TO SCOUR OUT VALLEY INVERSIONS 
COMPLETELY. INVERSIONS ARE LIKELY STRENGTHENED BY LOW LVL ERLY FLOW. 
SO WILL KEEP TEMPS JUST AOB GUIDANCE VALUES...NEAR SEASONAL AVERAGES.

MODEL DIFFERENCES BECOME MUCH MORE NOTICEABLE STARTING SUNDAY AS 
OPERATIONAL GFS WANTS TO BEGIN BREAKING DOWN THE RIDGE WHILE ECMWF 
HOLDS IT IN PLACE A BIT LONGER. ENSEMBLE MEMBERS ALSO STARTING TO 
SHOW QUITE A BIT OF SPREAD BY SUNDAY IN THEIR SOLUTIONS. THUS WILL 
MAKE ONLY MINOR TWEAKS TO POPS FOR BOTH SUNDAY AND MONDAY AND RAISE 
SNOW LEVELS JUST A BIT. WILL KEEP MOST POPS IN THE NW 1/3 OF THE CWA 
FROM THE TAHOE BASIN NORTH. GFS CONTINUES TO BE MORE AGGRESSIVE AND 
COLDER WITH LOW DROPPING DOWN THE COAST ON MONDAY THAN THE ECMWF. 
AGAIN ENSEMBLE MEMBERS SHOW QUITE A BIT OF SPREAD SO CONFIDENCE IN 
THIS TIME FRAME IS ALMOST NON EXISTENT. MLF 

AVIATION...
FOG STARTING TO LIFT A BIT TO THE EAST IN VICINITY KLOL THIS MRNG. 
EARLIER FOG LIFTED AT KTRK. WINDS ALOFT ARE STARTING TO INCREASE AND 
THESE HIGHER WINDS AND SOME DRYING ALOFT IN THE SIERRA IS 
RESPONSIBLE FOR BREAKING UP FOG AND LOW CLOUDS AT KTRK. THINK AS 
DRIER AIR TRIES TO WORK ITS WAY DOWN TODAY THAT FOG WILL BEGIN TO 
MIX OUT FOR MOST OF THE RGN. KTVL/KTRK SHUD REMAIN VFR MOST OF THE 
NEXT 24 HOURS. CLOUDS SHUD THICKEN IN THE SIERRA LATER TODAY 
WITH LIGHT RAIN/SNOW DEVELOPING WELL NORTH OF THE TAHOE BASIN. THIS 
WILL DROP SOUTH OVERNIGHT AND IMPACT KTRK/KTVL AFTER 16/06 UTC.

FOR KLOL AND KRNO FOG SHUD MIX OUT TODAY...BECOMING AT LEAST MVFR BY 
AFTERNOON. KLOL MAY MIX OUT SOONER. OVERNIGHT CLOUDS BEGIN TO 
INCREASE BUT PCPN SHUD STAY TO THE NORTH AND WEST OF THESE TAF SITES.

KRNO SHUD GO ABOVE 1 MILE VSBY BY 15/17 UTC TODAY WITH CIG RISING 
ABOVE 1000 FT BY 17/20 UTC. MLF

&&

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

$$

HTTP://WEATHER.GOV/RENO