FXUS65 KREV 242254
AFDREV

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RENO NV
254 PM PST THU DEC 24 2009

.SHORT TERM...

MORE OF AN INVERSION IN THE NV VALLEYS TODAY COMPARED TO
YESTERDAY. BASED ON AIRCRAFT SOUNDINGS AT RNO INVERSION IS NOT
THAT STRONG BUT IT'S STRONGER IN GREAT BASIN WITH READINGS STILL
IN THE 20S AT NFL. GFS/NAM CONTINUE TO INDICATE INCREASING TEMPS
750-800MB, WITH FRI HAVING THE STRONGEST INVERSIONS AND VALLEY-TO-
SIERRA TEMPERATURE DIFFERENCES. TNGT STILL APPEARS TO HAVE THE
BEST FZFG POTENTIAL IN THE VALLEYS. MODELS SHOW LIGHT WINDS AND
STRONG INVERSIONS. HOWEVER AS WAS THE CASE LAST NIGHT, MOISTURE
PROFILE/HYDROLAPSE IN LOWEST 500-1000 FT AGL IS QUESTIONABLE AS IS
THE INPUT FROM SNOW MELT. UPSTREAM CIRRUS MAY ALSO BE A LIMITING
FACTOR LATE TONIGHT. WILL CONTINUE AREAS OF FZFG TNGT/CHRISTMAS
MORNING BUT NOT MENTION DENSE FOG AT THIS TIME. BEST CHANCES FOR
DENSE FZFG ARE IN THE NFL/LOL SECTOR AND AROUND LAKES, ABOUT A 20%
CHANCE. AREAS AROUND MONO LAKE HAVE BEEN IN FOG MUCH OF TODAY
ALREADY. FZFG MAY RETURN FRI NGT BUT UPSTREAM CLOUDS COULD AGAIN
LIMIT EXTENT AND INTENSITY. SAT/SUN TEMPS ALOFT BEGIN TO COOL AND
WILL HELP LIMIT INVERSION STRENGTH AND HELP START MIXING OUT LOWER
VALLEYS. GENERAL CONFIDENCE IN FOG/INVERSION FORECAST IS HIGH (4
OUT OF 5).

DETERMINISTIC/ENSEMBLE MODELS CONTINUE TO PRODUCE A VARIETY OF
SOLNS REGARDING INCOMING TROF FOR SAT NGT INTO SUNDAY. THESE
SPLITTING TYPE SYSTEMS ARE NOTORIOUSLY HARD TO PREDICT AS IT IS.
EC, UKMET, AND A FEW SREF/GEFS MEMBERS BRING IN A STRONG LEAD
SHORT WAVE SAT NGT INTO SUN YIELDING A MODEST AMOUNT OF QPF ALONG
SIERRA, WHILE GFS AND MOST GEFS MEMBERS CONFINE TROF ENERGY
FURTHER WEST AND SLIDE SYSTEM DOWN ALONG COAST HITTING SRN CA
WITH MOST OF THE QPF SUN-MON. TELECONNECTIONS WITH NEGATIVE 500MB
HEIGHT ANOMALY NEAR ALEUTIAN ISLANDS WOULD TEND TO FAVOR MORE
PRECIP IN SRN CA, LESS IN OUR DISTRICT, HOWEVER THE SIGNAL IS
PRETTY WEAK. OVERALL WITH SUCH HIGH UNCERTAINTY AM INCLINED TO NOT
ADJUST PRECIP FORECAST ATTM, LEAVING LOW-END LIKELY IN SIERRA WITH
CHC/SLT CHC FOR WRN NV. SPILLOVER POTENTIAL INTO WRN NV, EVEN WITH
ECMWF, WOULD BE MARGINAL...CONFINED TO A QUICK HITTING UPPER FRONT
WITH AN OTHERWISE HOSTILE DYNAMICAL/STABILITY ENVIRONMENT FOR
SPILLOVER. ANY PRECIP, INCLUDING IN WRN NV, WOULD BE NEARLY ALL
SNOW BASED ON FCST SOUNDINGS AND SNOW LEVELS. FORECAST CONFIDENCE
FOR THIS WX SYSTEM HAS FALLEN COMPARED TO YESTERDAY, INTO THE LOW
CATEGORY (2 OUT OF 5).

CS

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

WITH MORE CONSENSUS OF MAIN UPPER LOW CLOSING OFF WELL TO THE
SOUTH OF THE REGION NEAR THE CA-MEXICO BORDER EARLY NEXT
WEEK...PRECIP POTENTIAL IS VIRTUALLY FINISHED FOR MON-MON NIGHT. A
FEW SHOWERS HAVE BEEN LEFT IN THE SIERRA SOUTH OF TAHOE FOR MONDAY
IN CASE THE SYSTEM TRACKS A BIT FARTHER NORTH...OTHERWISE DRY BUT
MOSTLY CLOUDY WEATHER AND LIGHT WINDS ARE EXPECTED ACROSS THE
REGION.

A MORE ACTIVE PACIFIC JET IS THEN EXPECTED TO CLOSE OUT THE YEAR
WITH THE FIRST SYSTEM TRACKING TO THE NORTH AND SPREADING MAINLY
LIGHT PRECIP ACROSS THE REGION WITH BEST PROBABILITY FOR NERN CA-
NWRN NV NORTH OF I-80 ON TUES NIGHT THEN TAPERING OFF BY EARLY
WED. ALTHOUGH THE 12Z GFS WASHES THIS SYSTEM OUT AS IT MOVES
INLAND...THERE IS MODERATE AGREEMENT AMONG THE GFS ENSEMBLE
MEMBERS AND SUPPORT FROM THE ECMWF AND CANADIAN RUNS TO ADD A
LITTLE MORE CONFIDENCE AND BUMP UP POPS TO CHANCE WORDING FOR ERN
CA FROM TAHOE NWD. COLD AIR MASS IS INDICATED TO PREVAIL WITH THIS
SHORTWAVE RESULTING IN MOSTLY SNOW EVEN DOWN TO LOWER ELEVATIONS.

SHORTWAVE RIDGE THEN MOVES INTO ERN CA-WRN NV ON WEDNESDAY THEN
PUSHES EAST BY THURSDAY AS A STRONGER TROF APPROACHES THE WEST
COAST. PRELIMINARY INDICATIONS ARE FOR THIS TROF TO SPLIT WITH THE
SRN PORTION REACHING THE SIERRA BY LATE THURSDAY. SNOW LEVELS MAY
HAVE A CHANCE TO RISE TO NEAR 6000 FT WITH INCREASING SW FLOW
ALOFT BEFORE THE TROF AND COLD FRONT PUSHES ACROSS THE REGION NEXT
THURS NIGHT. CURRENTLY THE POPS WERE KEPT AS SLGT CHC BUT ENSEMBLE
DATA IS IN FAIRLY GOOD AGREEMENT FOR A SYSTEM SEVEN DAYS AWAY.
SINCE THE PROJECTED PASSAGE OF COLD FRONT COINCIDES WITH NEW YEARS
EVE...THIS SYSTEM WILL NEED TO BE WATCHED DUE TO POTENTIAL IMPACTS
ON HOLIDAY ACTIVITIES AND ROAD CONDITIONS AS 2010 BEGINS.

THE QUICK PROGRESSION OF EACH SHORTWAVE NEXT WEEK WILL NOT ALLOW
FOR MUCH SURFACE WARMING BETWEEN SYSTEMS...RESULTING IN CONTINUED
BELOW NORMAL TEMPS OVER MOST VALLEY AREAS THRU NEXT WEEK. THE ONLY
POSSIBLE OPPORTUNITY FOR WARMING IS IF THE APPROACHING TROF ON
THURSDAY SLOWS DOWN ENOUGH TO BRING A LONGER PERIOD OF SW FLOW TO
THE REGION. MJD

&&

.AVIATION...

DRY WEATHER WITH LIGHT WINDS WILL PREVAIL OVER THE REGION THRU
THURSDAY. A DAY OF INVERSION CONDITIONS WITH FURTHER STRENGTHENING
TONIGHT MAY PRODUCE MORE AREAS OF FZFG COMPARED TO LAST
NIGHT...ALTHOUGH CONTINUED COLD TEMPS AND A NE RIDGE WIND
COMPONENT WOULD KEEP THE POTENTIAL FOR WIDESPREAD DENSE FZFG ON
THE LOW SIDE. DEW POINT SPREADS ARE ALSO SOMEWHAT HIGH AROUND 8
DEGREES C AS OF MID AFTERNOON. THE W-CNTRL NV VALLEYS INCLUDING
KLOL APPEAR TO HAVE THE BEST FOG PROBABILITY EARLY CHRISTMAS
MORNING...ALTHOUGH CONFIDENCE IS NOT HIGH ENOUGH TO INSERT IFR
CIGS OR VSBY AS A PREVAILING CONDITION. OTHERWISE THE CIGS/VSBY
SHOULD REMAIN VFR OR OCNL MVFR FOR THE REMAINING TAF SITES THRU
CHRISTMAS DAY. MJD

&&

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

$$

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