FXUS65 KREV 270522 AAA
AFDREV

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE RENO NV
922 PM PST THU NOV 26 2009

UPDATED AVIATION SECTION

.SHORT TERM...
MODELS STARTING COME INTO BETTER AGREEMENT REGARDING THE EVOLUTION
OF THE NEXT TROF MOVING INTO THE WEST COAST LATE TONIGHT. BOTH
GFS AND ECMWF CONTINUE TREND OF DEVELOPING A CLOSED LOW IN THE
BASE OF THE TROF BY EARLY FRIDAY OVER CENTRAL CA THEN DROPPING
THIS LOW INTO SOUTHERN CA BY SATURDAY. EARLIER THE NAM HAD BEEN
HOLDING THIS LOW JUST OFF SHORE WHICH WOULD RESULT IN MUCH LESS
PCPN FOR THE AREA. HOWEVER...THE 26/18 UTC NAM NOW HAS SHIFTED
THIS LOW BACK INLAND AND ITS TRACK IS NOW CLOSER TO THAT OF THE
GFS/ECMWF. THIS LEADS TO A BIT MORE CONFIDENCE IN A WETTER
SOLUTION FOR THE EASTERN SLOPES OF THE SIERRA.

THERE APPEARS TO BE TWO PERIODS OF HEAVIER PCPN FRIDAY. THE
INITIAL ONE WILL BE WITH THE SFC FRONT AS IT DIVES SOUTH LOWERING
SNOW LEVELS AS IT GOES. AHEAD OF THIS FRONT WINDS WILL BRIEFLY
INCREASE BUT NOT SIGNIFICANTLY IN THE VALLEY LOCATIONS. RIDGE
GUSTS INCREASE EARLY FRIDAY IN RESPONSE TO APPROACHING LOW AND
INCREASING UPPER LVL WINDS ASCD WITH JET.

AFTER THE INITIAL PERIOD OF PCPN ANOTHER PERIOD WILL DEVELOP WITH
THE UPPER LVL DEFORMATION AXIS...AND AS LOW/MID LVL FLOW SHIFTS TO
A MORE NORTHERLY/NORTHEASTERLY DIRECTION BY LATE AFTERNOON UPSLOPE
FLOW WILL BECOME ENHANCED ON THE EAST SLOPES OF THE SIERRA. THIS
SHOULD LEAD TO A PERIOD OF HEAVIER PCPN INTO THE EVENING. THIS
PROLONGED PERIOD OF ENHANCED PCPN SHOULD BE ENOUGH TO PRODUCE
SEVERAL INCHES OF SNOW...FIRST IN WESTERN LASSEN COUNTY THEN
SPREADING SOUTH INTO THE TAHOE BASIN. SNOW LEVELS DROP DURING THE
AFTERNOON EAST OF THE SIERRA AND ANY PCPN THERE SHOULD SWITCH TO
ALL SNOW BY THE EVENING. ACCUMULATIONS OF 2 TO 4 INCHES ARE
POSSIBLE IN WESTERN NV BY LATE FRIDAY NIGHT/EARLY SATURDAY. FOR
NOW WILL ISSUE ADVISORIES FOR SNOW IN LASSEN COUNTY AND THE TAHOE
BASIN BUT WILL HOLD OFF ON ISSUING ANYTHING FOR THE FAR WESTERN
VALLEYS OF NV. TIMING THE HEAVIEST SNOW STARTS TO GET A BIT MORE
TRICKY BEYOND EARLY FRIDAY EVENING. LOCATION OF HEAVIEST BAND IS
IN QUESTION AS WELL...BASED ON HOW FAR EAST OR WEST THE LOW
DEVELOPS.

SNOW SHOULD TAPER OFF ACROSS THE FAR NRN CWA IN THE EVENING THEN
INCREASE ACROSS PORTIONS OF MONO COUNTY AND EAST INTO
MINERAL/LYON COUNTY. ACCUMULATIONS COULD REACH ADVISORY CRITERIA
THIS AREA BY LATE FRIDAY NIGHT/EARLY SATURDAY AS WELL. BUT FOR NOW
WILL HOLD OFF ON ISSUING ANYTHING THERE. 

SNOW SHOWERS SHOULD LINGER INTO SATURDAY AFTN/EVNG OVER THE HIGH
SIERRA WHERE ACCUMULATIONS OF MORE THAN A FOOT ARE POSSIBLE BEFORE
THE SNOW SHOWERS END BY LATE SATURDAY NIGHT. INCREASING EASTERLY FLOW
BEHIND THE SFC COLD FRONT AND THE UPPER LEVEL LOW WILL USHER
COLDER AIR INTO THE RGN BY LATE SAT NIGHT WITH HIGHS SUNDAY
STRUGGLING TO REACH THE MID 40S IN THE WESTERN NV VALLEYS AND THE
MID 30S IN THE SIERRA. MLF

LONG TERM...MONDAY THROUGH THURSDAY...
THE GFS IS LOOKING LIKE THE OUTLIER WITH THE 00Z/12Z MODEL RUNS 
BEYOND MONDAY...SO I HAVE SIDED WITH THE ECMWF/CANADIAN FOR THIS 
PACKAGE. THE DIFFERENCE BETWEEN THE GFS AND ECMWF/CANADIAN LIES IN 
THEIR RESPECTIVE HANDLING OF ENERGY RIDING OVER A RIDGE AXIS SET TO 
DEVELOP ALONG THE BRITISH COLUMBIA COAST THIS WEEKEND. THE GFS KEEPS 
ALL THE ENERGY ALONG THE NORTHERN CONUS NEXT WEEK WHILE THE ECMWF 
AND CANADIAN DIG THE ENERGY INTO THE WEST-CENTRAL CONUS/NORTHERN 
ROCKIES. IF YOU ARE NOT CONVINCED YET...TELECONNECTIONS RELATED 
TO AN UPPER RIDGE NEAR THE BRITISH COLUMBIA COAST SUPPORT WELL BELOW 
NORMAL TEMPERATURES FOR OUR REGION IN THE 6-10 DAY PERIOD.

TO THE DETAILS...THE PREFERRED ECMWF AND CANADIAN WILL BRING A 
BACKDOOR COLD FRONT INTO THE WESTERN GREAT BASIN TUESDAY NIGHT AND 
WEDNESDAY MORNING WITH NORTHERLY FLOW ALOFT. THEREAFTER...THEY KEEP 
CHILLY NORTH FLOW OVER THE REGION AS THE PATTERN GRINDS TO A HALT 
WITH A CENTRAL CONUS TROUGH AND A NORTHEAST PACIFIC RIDGE. WITH THE 
EXPECTED PATTERN...I HAVE KNOCKED DOWN HIGHS SIGNIFICANTLY BEYOND 
TUESDAY (WE MAY MIX BEST TUESDAY AS THE FRONT APPROACHES) IN 
DEFERENCE TO THE ECMWF LOW LEVEL TEMPERATURE FIELDS. SNYDER

AVIATION...
VFR WITH NO CONCERNS THROUGH TONIGHT. FRIDAY...AN UPPER LOW ABOUT 
350SM WEST OF KACV AT 22Z THU WILL MOVE TO BTWN KUKI AND KSAC BY 00Z 
SAT. AN ASSOCIATED COLD FRONT WILL MV THRU KRNO/KTRK/KTVL IN THE 
21Z-00Z TIME FRAME FRIDAY...LOWERING CIGS/VSBYS SIGNIFICANTLY FOR A 
FEW HOURS. 

KTRK/KTVL WILL BE SNOW...WITH KTVL GOING BELOW VIS MINS QUICKLY 
CIRCA 00Z SAT (AS MINS ARE HIGH AT 5SM VIS). KTRK WILL FLIRT WITH 
THE 015 CIG/1.25 VIS APPCH MINS INTO EARLY EVENING IN -SN/SN BEFORE 
POSSIBLY RISING LATER AS MAIN SNOW AREA SHIFT SOUTH. 

FOR KRNO...CIGS COULD FALL BRIEFLY BELOW 020 CIRCA 22Z FRI TO 01Z 
SAT AS THE COLD FRONT MOVES THRU. HOWEVER...AT THIS TIME THE MAIN 
PREVAILING LOW VIS/CIG THREAT LOOKS TO BE 03Z-10Z SAT WHEN LOW LVL 
WNDS TURN N-NE AND UPSLOPE FLOW INTO THE SIERRA BEGINS...BRINGING 
THE THREAT OF AT LEAST TEMPO -SN/SN WITH CIGS/VSBYS BLO 015AGL AND 
1.5 SM. 

AS FAR AS RUNWAY SNOW AND ICE ISSUES FOR KRNO...POSSIBILITY EXISTS
FOR UP TO 2-3 INCHES ACCUMULATION ON RUNWAYS. ATTM...SNOW LOOKS TO
BECOME HEAVY AFTER 03-04Z...WHEN ACCUMULATING SNOW WILL BECOME
POSSIBLE. PROB OF SEEING 2 INCHES ON RUNWAYS ABOUT 60-70 PCT ATTM. 
SNYDER/WALLMANN

&&

.REV WATCHES/WARNINGS/ADVISORIES...
NV...WINTER WEATHER ADVISORY FROM NOON FRIDAY TO MIDNIGHT PST FRIDAY 
     NIGHT FOR NVZ002.

CA...WINTER WEATHER ADVISORY FROM 9 AM TO 9 PM PST FRIDAY FOR CAZ071.

     WINTER WEATHER ADVISORY FROM NOON FRIDAY TO MIDNIGHT PST FRIDAY 
     NIGHT FOR CAZ072.

&&

$$

HTTP://WEATHER.GOV/RENO