FXUS65 KREV 262234
AFDREV

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RENO NV
234 PM PST SAT DEC 26 2009

.SHORT TERM...
HIGH AMPLITUDE RIDGE OVER THE WEST COAST HAS BECOME A CLOSED HIGH
CIRCULATION OVER CANADA WHILE DISORGANIZED SHORTWAVE ENERGY
UNDERCUTTING THE RIDGE BEGINS MOVING INTO THE WEST COAST. MID AND
HIGH CLOUDS AHEAD OF THE SHEARING TROUGH ARE SPREADING OVER CA/NV
WITH RADAR RETURNS STARTING TO SHOW UP NEAR THE CENTRAL SIERRA.
JUDGING BY THE STRENGTH AND SPEED OF THE RETURNS THIS IS MOST
LIKELY JUST VIRGA AS THE LOWER LEVELS SLOWLY MOISTEN UP. MIXING
BELOW MOUNTAIN TOP LEVEL HAS BEEN VERY POOR TODAY WITH STRONG
INVERSIONS PERSISTING IN VALLEY LOCATIONS BRINGING COLD AND HAZY
CONDITIONS.

MODELS AGREE ON THE OVERALL PICTURE REGARDING THE INCOMING
SHORTWAVE ENERGY BUT THEY DIFFER ON THE DETAILS SUCH AS TIMING AND
TOTAL QPF. THE GFS IS MORE AGGRESSIVE WITH QPF 00-12Z TONIGHT WITH
THE NAM KEEPING A DRY FORECAST THROUGH 12Z. IT'S REALLY TOUGH TO
PICK A MORE LIKELY SCENARIO WITH NO DISTINCT FORCING MECHANISMS
VISIBLE ON WV IMAGERY. FEELING THE GFS IS TOO AGGRESSIVE AND MOST
OF THE INITIAL QPF WILL EVAPORATE BEFORE IT REACHES THE GROUND...
HOWEVER CANNOT IGNORE THE HIGHER QPF OF THE GFS SO KEPT WITH
DECENT POPS TONIGHT. NEARLY ISOTHERMAL LAPSE RATES NEAR THE
SURFACE WILL KEEP SNOW LEVELS AT MOST VALLEY FLOORS.

SNOW CHANCES CONTINUE THROUGH THE DAY SUNDAY HOWEVER AMOUNTS WILL
BE VERY LIGHT WITH NO SIGNIFICANT FORCING MECHANISM...JUST
DISORGANIZED VORT ENERGY PASSING OVERHEAD. THERE IS A SHALLOW
LAYER OF INSTABILITY ASSOCIATED WITH THE COOLING ALOFT WHICH
SHOULD BRIEFLY ENHANCE SNOWFALL RATES. TOTAL SNOWFALL FROM TONIGHT
THROUGH SUNDAY IS NOT EXPECTED TO BE MORE THAN A DUSTING IN THE
LEE OF THE SIERRA WITH UP TO 3 INCHES POSSIBLE IN THE SIERRA.

A SECONDARY SHEARED APART TROUGH AXIS MOVES ACROSS CA/NV MONDAY
AND MONDAY NIGHT WHICH MAY HAVE ENOUGH UPPER FORCING FOR AT LEAST
SOME SHOWERS IN THE SIERRA. SHORTWAVE RIDGING OCCURS BEHIND THE
EXITING TROUGH AXIS TUESDAY BEFORE A FAST MOVING SHORTWAVE
MOVES INTO THE PACNW TUESDAY NIGHT. MODELS HAVE BEEN AT ODDS
REGARDING THE TRACK OF THIS FEATURE AS WELL WHICH IS DISCUSSED
BELOW. JORDAN

.LONG TERM...WEDNESDAY THROUGH SATURDAY...

THE MAIN CHALLENGE FOR THIS TIME FRAME WILL BE WEDNESDAYS WEATHER 
ACROSS WESTERN NEVADA. MODELS STILL BACK AND FORTH BETWEEN A FLATTER 
OVERRUNING PATTERN FOR WEDNESDAY AS ADVERTISED BY THE LATEST EC RUNS 
VERSUS THE POSSIBILITY OF A COLDER MORE UNSTABLE INSIDE SLIDER 
SYSTEM PER THE LATEST GFS. WHICH SCENARIO PANS OUT DEPENDS UPON THE 
STRENGTH OF THE SHORTWAVE MOVING INTO THE PACNW TUESDAY NIGHT INTO 
EARLY WEDNESDAY AND AMPLITUDE OF RIDGING IN ITS WAKE.

THE GFS SCENARIO WHICH WAS SIMILAR TO YESTERDAYS EC WOULD POINT TO 
THE POSSIBILITY OF UP TO A FEW INCHES OF SNOW IN THE WESTERN NV 
VALLEYS WEDNESDAY MORNING MAINLY NORTH OF US-50. 700MB WINDS OF 
30-35KT TUESDAY NIGHT INTO EARLY WED WOULD ALLOW VALLEYS TO MIX AND 
SCOUR OUT FOG/HAZE THAT HAS PREVAILED FOR THE LAST SEVERAL DAYS. THE 
MILDER OVERRUNNING PATTERN PER THE EC WOULD PUSH PRECIPITATION 
ACROSS THE AREA MAINLY NORTH OF INTERSTATE 80. 

IT STILL LOOKS LIKE THERE WILL BE SOME RIDGING ALONG THE WEST COAST 
THURSDAY WITH SHOWERS POSSIBLE NORTH IF THE FLATTER EC VERIFIES.  
FRIDAY MODELS CONTINUE TO BRING A SHORTWAVE ACROSS THE PACNW 
FLATTENING THE RIDGE AND BRINGING ANOTHER CHANCE OF PRECIPITATION 
NORTH OF INTERSTATE 80 FOR NEW YEARS DAY INTO SATURDAY. 

WILL INCREASE POPS FOR WED NORTH OF HWY 50 AND RETAIN THE CHANCE 
POPS THAT WE HAVE...MAINLY NORTH OF I-80...FOR THURSDAY THROUGH 
SATURDAY. SNOW LEVELS AND PRECIPITATION TYPE THROUGH THE PERIOD WILL 
DEPEND UPON STRENGTH AND PERSISTENCE OF VALLEY INVERSIONS. 

CURRENT FORECAST KEEPS TEMPERATURES BELOW NORMAL FOR HIGHS WED-THUR 
AND THEN CLOSE TO NORMALS INTO THE NEW YEAR. JAH  

&&

.AVIATION...

LIGHT SNOW SHOWERS WILL MOVE INTO THE SIERRA BY 03Z SUNDAY BECOMING 
MORE WIDESPREAD WITH CONTINUOUS LIGHT SNOW AFTER 06Z. THERE WILL BE 
LIGHT SNOW ACCUMULATIONS 1 TO 3 INCHES AT KTRK AND KTVL LATE 
TONIGHT INTO SUNDAY. EXPECT MOUNTAIN OBSCURATION WITH GENERALLY MVFR 
CONDITIONS AT KTRK AND KTVL IN LIGHT SNOW AND FOG. THERE COULD BE 
SOME PERIODS OF IFR CONDITIONS MAINLY AFTER 12Z. 

FOR WESTERN NEVADA...
KRNO AND KLOL...THERE WILL BE SOME LIGHT SNOW IN THE AREA GENERALLY 
BETWEEN 06Z-18Z BUT LITTLE IF ANY ACCUMULATION IS EXPECTED. VFR 
CONDITIONS ARE FORECAST ALTHOUGH AFTER 12Z THERE COULD BE PERIODS OF 
MVFR CONDITIONS IN LIGHT SNOW/FOG. JAH

&&

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

$$

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