FXUS65 KREV 241137
AFDREV
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RENO NV
330 AM PST THU DEC 24 2009
.SHORT TERM...
FREEZING FOG IS HAVING TROUBLE FORMING OVER THE GREAT BASIN THIS
MORNING...MOST LIKELY DUE TO SOME HIGH CIRRUS MOVING IN AND
NORTHEAST FLOW RIDGE TOP FLOW AT ABOUT 30 MPH. A FEW AREAS ARE
VISIBLE THAT HAVE FORMED SOME FREEZING FOG...NAMELY NEAR PYRAMID
LAKE AND MONO LAKE...BUT THAT'S ABOUT IT SO FAR. ANY OTHER FREEZING
FOG THAT DEVELOPS EARLY THIS MORNING WILL BE VERY LOCALIZED AND
PATCHY.
CHRISTMAS EVE NIGHT AND CHRISTMAS MORNING WE WILL HAVE A BETTER
CHANCE FOR FREEZING FOG AS THE RIDGE CONTINUES TO BUILD INTO THE
WEST COAST AND WINDS ALOFT WEAKEN. CLEAR SKIES AND STRONG
TEMPERATURE INVERSIONS WILL BE THE STRONGEST...SO STILL FEEL THAT
THE AMOUNT OF MOISTURE INPUT FROM MELTING SNOW WILL BE CRITICAL TO
HOW MUCH FOG FORMATION WE SEE.
THE NEXT ITEM OF CONCERN IN THE SHORT TERM IS A SPLITTING TROUGH
TRYING TO MOVE INTO NORTHERN CA/NV ON SATURDAY NIGHT INTO SUNDAY AS
THE RIDGE GETS CUTOFF TO THE NORTH. STILL SOME HIGH UNCERTAINTY WITH
THE TIMING AND AMOUNT OF MOISTURE. GFS/NAM SEEM TO BE TRENDING LATER
WITH THE BEST CHANCE OF MOISTURE...MORE SUNDAY INSTEAD OF SATURDAY
NIGHT...BUT GFS ENS AND ECMWF KEEP THE MOISTURE COMING IN SAT NIGHT
INSTEAD. CONTINUED TO INCREASE POPS...MAINLY IN THE SIERRA
ZONES...WITH SNOW LIKELY LATE SATURDAY NIGHT ALONG THE SIERRA CREST.
THIS IS EXPECTED TO BE A PRETTY WEAK SYSTEM...ALTHOUGH THERE REMAINS
SOME POTENTIAL FOR AN INCH OR TWO ON THE VALLEY FLOORS BY SUNDAY
MORNING IF EVERYTHING COMES TOGETHER. AT THE SAME TIME...THERE IS
ALSO A CHANCE WE MAY SEE NOTHING AT ALL. HOON
.LONG TERM...SUNDAY THROUGH THURSDAY...
FORECASTS FOR THIS PERIOD ARE BASED ON THE ECMWF AND GFS ENSEMBLE
MEAN CHARTS WHICH SHOW A SPLITTING TROUGH ALONG THE WEST COAST
SUNDAY WHICH WEAKENS AND MOVES INLAND MONDAY. IT IS DIFFICULT TO BE
TOO CONFIDENT IN DETAILS (SPECIFIC POPS AND QPF) WITH THIS SYSTEM AS
THE OPERATIONAL GFS SHOWS LITTLE RUN-TO-RUN CONSISTENCY AND THE
ENSEMBLE MEMBERS CONTINUE TO SHOW PRETTY GOOD SPREAD IN THEIR
SOLUTIONS.
THE OPERATIONAL GFS HAS TRENDED SLOWER BRINGING IN PRECIP OVER THE
PAST COUPLE OF RUNS. IT LOOKS MORE LIKE THE SOLUTION FROM YESTERDAY
WHICH KEPT THE LOW JUST OFF THE CA COAST...DROPPING IT SOUTHWARD AND
THEN ACROSS NORTHERN BAJA WITH SOME PRECIP IN THE SIERRA BUT NOT
MUCH MAKING ITS WAY INTO WESTERN NV. THE OPERATIONAL ECMWF ALSO
SHOWS A DECENT SPLIT WITH A VORTICITY MAX MOVING SOUTH THEN ACROSS
BAJA...BUT KEEPING ENOUGH ENERGY OVER CA AND WESTERN NV TO KEEP A
CHANCE OF RAIN/SNOW SUNDAY INTO MONDAY.
THE FORECAST WILL SHOW THE HIGHEST POPS ON SUNDAY AS WHATEVER ENERGY
IS AVAILABLE WITH THE TROUGH MOVES INTO THE SIERRA AND EXTREME
WESTERN NV. WILL KEEP QPF VALUES ON THE LIGHT SIDE AT THIS TIME FOR
THE SUNDAY/SUNDAY NIGHT PERIOD...WITH ONLY A FEW HUNDREDTHS IN
EXTREME WESTERN NV AND AROUND A QUARTER INCH IN THE HIGHER
ELEVATIONS OF THE SIERRA.
SNOW LEVELS WILL BE NEAR VALLEY FLOORS SATURDAY NIGHT AND SUNDAY...
HOWEVER FOR THIS WEEKEND SYSTEM THE CURRENT MODELS DO NOT SUPPORT
ACCUMULATIONS GREAT THAN AN INCH OR SO IN WESTERN NV VALLEYS AND 6
INCHES ABOVE 7000 FEET IN THE SIERRA.
WHATEVER IS LEFT OF THE TROUGH WEAKENS AND MOVES EAST OF THE CWA
MONDAY AND MONDAY NIGHT. THE GFS BUILDS A RIDGE ALONG THE WEST COAST
ON TUESDAY. THE ECMWF WAITS UNTIL WEDNESDAY TO AMPLIFY THE RIDGE...
WITH A SECONDARY SHORT WAVE MOVING ACROSS NORTHERN NV TUESDAY AND
TUESDAY NIGHT THAT COULD PRODUCE ADDITIONAL RAIN OR SNOW TUE INTO
WED. WILL CONTINUE A SLIGHT CHANCE BOTH DAYS AND ADD MORE DETAIL AS
NEEDED IN LATER FORECAST PACKAGES. RC
.AVIATION...
THE STRONG EAST FLOW OVER THE RIDGES THIS MORNING IS HELPING TO
LIMIT FREEZING FOG DEVELOPMENT AT TAF SITES. THE 12Z PACKAGE WILL
SHOW NO IMPACTS FROM FOG AT KRNO/KTVL WHILE KTRK AND KLOL MAY YET
BECOME IFR FOR A TIME NEAR SUNRISE THROUGH ABOUT 15Z.
KTRK SAW REDUCED VSBY FOR A TIME OVERNIGHT BEFORE CONDITIONS
IMPROVED TO VFR. WILL HAVE TO FOLLOW THAT SCENARIO IN THE KTRK TAF
WITH TEMPO VSBY TO 1/2SM FZFG TIL 15Z. SIMILAR CONDITIONS ARE
POSSIBLE AT KLOL ALTHOUGH DO NOT EXPECT VSBY TO DROP BELOW 1SM BR
THERE THROUGH 15Z. RC
&&
.REV WATCHES/WARNINGS/ADVISORIES...
NV...NONE.
CA...NONE.
&&
$$
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