FXUS65 KREV 041047
AFDREV

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RENO NV
247 AM PST FRI DEC 4 2009

.SHORT TERM...
UPPER LEVEL RIDGE WILL BEGIN TO RETROGRADE TODAY AS SHORTWAVE
ENERGY RIDES OVER THE RIDGE AND SLIDES SOUTH. THIS WILL HELP THE
INVERSION BREAK TODAY WITH MAX TEMPS FORECAST TO BE 3-6 DEGREES
WARMER IN THE VALLEYS AND A DEGREE OR TWO WARMER IN THE HIGHER
ELEVATIONS.

ABOVE MENTIONED SHORTWAVE DIVES SOUTH OUT OF BRITISH COLUMBIA
SENDING A COLD FRONT THROUGH THE AREA SATURDAY. STRONG
FRONTOGENESIS ALONG WITH DECENT LIFT AND JET ENERGY SHOULD PRODUCE
SCATTERED SNOW SHOWERS ACROSS THE CWA. MODELS ARE NOT GENERATING
MUCH...IF ANYTHING...IN THE WAY OF QPF...BUT THERE ARE TOO MANY
DYNAMICS TO IGNORE. KEPT CHANCE POPS IN THE FORECAST AND QPF LIGHT
AS SIGNIFICANT MOISTURE IS THE ONE INGREDIENT THAT IS LACKING.

700 MB WINDS ARE ABOUT 30-35 KTS AHEAD OF THE FRONT...SO RIDGES
WILL LIKELY BE GUSTING UP TO 70 KTS SATURDAY MORNING. BEHIND THE
FRONT...A STRONG PRESSURE GRADIENT SETS UP...BUT THE UPPER LEVEL
WIND SUPPORT IS LACKING. SURFACE WINDS RELAX SUNDAY BEFORE
INCREASING AGAIN MONDAY MORNING AHEAD OF ANOTHER POTENT SHORTWAVE.

THIS IMPULSE WILL BE THE REINFORCING SHOT OF COLD AIR THAT WILL
PLUNGE TEMPERATURES DOWN IN THE EXTENDED PERIOD. IT WILL BE
PHASING UP WITH MOISTURE MOVING INTO THE WEST FROM THE PACIFIC
...THE PERFECT RECIPE FOR SNOW. THE GFS/ECMWF/GEFS MODELS HAVE ALL
BEGUN TO COME INTO AGREEMENT FOR PRECIP ACROSS MUCH OF THE CWA
MONDAY MORNING. THE QUESTION THOUGH REMAINS JUST HOW MUCH MOISTURE
CAN BE ENTRAINED INTO THIS SYSTEM BENEATH THE AMPLIFIED RIDGE IN
THE GULF OF ALASKA. THE EXACT AMOUNT OF MOISTURE WILL BE A HUGE
INFLUENCE WITH BUFKIT INDICATING SNOW RATIOS OF 25-30:1 DUE TO
THE EXTREMELY COLD AIR MASS IN PLACE. CURRENTLY AM THINKING 6-12
INCHES ACROSS THE HIGH SIERRA WITH 1-2 INCHES OF SNOW POSSIBLE BY
RUSH HOUR MONDAY MORNING THROUGH THE RENO/SPARKS/CARSON CITY AREA. 

KEEP UP WITH THE LATEST FORECASTS THROUGH THE WEEKEND AS THIS COULD BE
A SIGNIFICANT IMPACT ON TRAVEL MONDAY MORNING. DF

.LONG TERM...TUESDAY THROUGH THURSDAY...
MODELS CONVERGING AND IN MUCH BETTER AGREEMENT THIS MORNING FOR 
MON-TUE. MODELS THEN DIVERGE AND THERE IS QUITE A BIT OF UNCERTAINTY 
FOR WED-THU...BUT STILL SHOW A GENERALLY WET PATTERN. OUT 
THERE...GFS IS DRIER AND MORE SPLIT FLOW THAN EC WHICH IS WETTER AND 
MORE CONSOLIDATED. ATTM...LEANING MORE TOWARD GFS WHICH HAS BEEN 
LEADING THE WAY LAST TWO DAYS WITH MON-TUE AND THE LONGER BREAK IN 
PRECIP SEEMS A BIT MORE REASONABLE. IT ALSO HAS GOOD SUPPORT FROM 
ITS ENSEMBLE...BUT WILL BLEND IN EC ALSO DUE TO UNCERTAINTY. 

MON-TUE...COLD!!! EC/GFS/GEFS SHOW 530 DM UPPER LOW MOVG INTO NW NV 
BY AFTERNOON WITH SOME MOISTURE AND GOOD FRONTOGENESIS JUST TO ITS 
SOUTH. HAVE NOT DONE MUCH WITH POPS...BUT LOWERED ALL SNOW LEVELS TO 
MAKE SURE THEY ARE AT VALLEY FLOORS. GFS/EC SHOW H7 TEMPS OF -18C BY 
00Z...AND GEFS HAS NORMALIZED ANOMALIES OF -3 BY 00Z. ALL PRECIP 
WILL BE SNOW...AND JUST A QUESTION OF AMOUNTS. COULD BE UP TO 2 FEET 
IN SIERRA IF GFS IS RIGHT WITH VERY COLD TEMPS AND HIGH SNOW RATIOS. 
A FEW INCHES POSSIBLE ALSO IN WRN NV...BUT THESE AMOUNTS ARE VERY 
UNCERTAIN. 

MAIN MESSAGE MON-TUE WILL BE THE VERY COLD AIRMASS. HIGH TEMPS WILL 
STRUGGLE TO REACH 30 MOST AREAS MON-TUE WITH SIERRA VALLEYS LIKELY 
NOT MAKING IT OUT OF THE TEENS. OVERNIGHT LOWS MON NIGHT COULD BE 
EXTREMELY COLD IF SKIES CLEAR...WHICH NOW LOOKS POSSIBLE. TEMPS 
COULD BE BELOW ZERO NOT JUST FOR SIERRA VALLEYS...BUT ALSO WRN NV IF 
THERE IS SNOW COVER AND CLEAR SKIES. THIS WILL BE WATCHED CLOSELY.

WED-THU...FLOW BECOMES MORE WEST AND MOISTURE SHOULD BEGIN TO 
INCREASE. QUESTION IS HOW FAST AND HOW MUCH. MODELS ARE ALL DRIER 
THIS MORNING...BUT EC REMAINS WETTEST AND FASTEST. HAVE SHOWN A SLOW 
INCREASE IN POPS FROM TUE-NIGHT THRU THU. HAVE ALSO LOWERED SNOW 
LEVELS THROUGH WED NIGHT AS MODELS ARE COLDER...AND COLD AIR DAMMING 
IS A CONCERN WITH SUCH A COLD AIRMASS. SYSTEM COULD STILL BE 
SIGNIFICANT...BUT NOW LOOKS A BIT DRIER THAN 24 HRS AGO. STILL A FEW 
DAYS OUT SO THINGS CAN STILL CHANGE BACK. WALLMANN

&&

.AVIATION...VFR WITH LGT WINDS THRU FRI NIGHT. INCREASING WEST WINDS AHEAD OF 
COLD FRONT SAT WITH PK GUSTS 25-30 KTS AT AREA TERMINALS. WALLMANN

&&

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

$$

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