FXUS65 KREV 032346
AFDREV

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RENO NV
346 PM PST THU DEC 3 2009

.SHORT TERM...
UPPER RIDGE ALONG THE WEST COAST WILL RETROGRADE OVER THE NEXT 
COUPLE OF DAYS AS SHORT WAVE ENERGY RIDES OVER THE RIDGE AND SLIDES 
SOUTH. A VIGOROUS SHORT WAVE IN THE EASTERN GULF OF ALASKA WILL MOVE 
INTO WESTERN CANADA TONIGHT THEN DROP SOUTH THROUGH WA AND OREGON 
FRIDAY AND FRIDAY NIGHT. THIS SHORT WAVE WILL DRIVE A STRONG COLD 
FRONT ACROSS WESTERN NV ON SATURDAY BRINGING BRISK WINDS...A FEW 
SNOW SHOWERS...AND FALLING AFTERNOON TEMPERATURES. TEMPS ON
SATURDAY WILL STRUGGLE TO GET INTO THE 40S IN WESTERN NV...WITH
SIMILAR HIGHS IN MANY SIERRA VALLEYS.

MODELS SHOW MOISTURE WITH THIS SYSTEM TO BE MEAGER...NOT TOO 
SURPRISING GIVEN ITS OVERLAND TRAJECTORY. THE GFS AND NAM GENERATE 
LITTLE IF ANY QPF ON SATURDAY WHILE THE ECMWF SHOWS A COUPLE
HUNDREDTHS OVER EXTREME WESTERN NV BY LATE SAT AFTN. TIME-HEIGHT
CROSS SECTIONS SHOW STRONG FRONTOGENESIS OVER WESTERN NV AROUND
MID DAY SAT SO EVEN WITH THE LOW MODEL RH/QPF NUMBERS WILL HAVE
SLIGHT CHANCE POPS IN MOST AREAS WITH UP TO A HALF INCH OF SNOW
WITH THE FRONTAL BAND. NORTH WINDS WILL REMAIN BRISK INTO THE
EVENING BEHIND THE COLD FRONT. DECREASING WINDS AND CLOUDS ARE
EXPECTED SATURDAY NIGHT.

FOR SUNDAY THROUGH MONDAY NIGHT...THE TROUGH CARVED OUT BY
SATURDAY'S SYSTEM WILL BEGIN RELOADING WITH ANOTHER IMPULSE
DIGGING DOWN THE PACIFIC NORTHWEST COAST. THIS SYSTEM IS PROVING
TO VERY TRICKY AS MOISTURE DIFFERENCES FROM THE ECMWF TO THE GFS
ARE GOING TO MAKE A WORLD OF DIFFERENCE ON HOW MUCH SNOW WE CAN
RING OUT OF THE ATMOSPHERE LATE SUNDAY NIGHT AND MONDAY.

BASED ON THE GENERAL HEIGHT PATTERN CONSENSUS OF THE VERY CONSISTENT 
ECMWF FOR SUNDAY NIGHT AND MONDAY...I AM LEANING TOWARDS THE WETTER 
ECMWF (MAYBE 50% TOWARDS). IN ADDITION...THE NAM HAS COME TOWARDS 
THE ECMWF THROUGH SUNDAY AFTERNOON. THE DETAILS OF IMPORTANCE LIE IN 
HOW MUCH MOISTURE WE CAN ENTRAIN BENEATH THE HUGE RIDGE OVER THE 
GULF OF ALASKA. THE ECMWF ENTRAINS A GOOD AMOUNT INTO CENTRAL 
CALIFORNIA BY MONDAY MORNING WHILE THE GFS ENTRAINS GOOD MOISTURE 
BUT SENDS IT INTO SOUTH-CENTRAL CALIFORNIA. WHERE THE MOISTURE 
ENTRAINS IS BASED ON THE ORIENTATION AND WESTWARD DIGGING OF THE 
SECONDARY LOW. THE GFS HAS A MORE NORTHWEST TO SOUTHEAST ORIENTATION 
AND FASTER DIGGING MOVEMENT TO THE LOW...WHILE THE ECMWF AND NAM  
WHICH ALSO HAVE DECENT SUPPORT FROM THE SREF SPAGHETTI PLOTS 
INDICATE A SLOWER DIGGING AND MORE WEST TO EAST ORIENTATION. THE 
EC/NAM SOLUTION WOULD ALLOW MORE MOISTURE TO BE BROUGHT INTO PLAY 
FOR THE REGION ON MONDAY.   

THE EXACT AMOUNT OF MOISTURE WILL BE HUGE GIVEN THE COLD AIRMASS 
ALLOWING FOR THE POSSIBILITY OF SNOW RATIOS OF 15-20/1 OR HIGHER. 
EVEN THE GFS INDICATE A TENTH TO TWO-TENTHS OF PRECIP MONDAY...WHICH 
COULD MEAN SEVERAL INCHES OF POWDERY SNOW. THE ECMWF HAS MORE LIKE 
1/2 INCH OF PRECIP WHICH WOULD MEAN MUCH MORE SNOWFALL. STAY TUNED 
GIVEN THAT THE SNOWFALL IS DUE IN MONDAY AND COULD AFFECT TRAVEL 
SIGNIFICANTLY. SNYDER/RC

.LONG TERM...TUESDAY THROUGH THURSDAY...

FOR TUESDAY...WE SHOULD SEE A BRIEF RESPITE IN BETWEEN STORM 
SYSTEMS...BUT LEFT A CHANCE OF PRECIP IN THE FORECAST TO ACCOUNT FOR 
ANY TIMING ERRORS. 

MEDIUM RANGE MODELS CONTINUE TO CONVERGE ON A WETTER AND MORE MILD 
SOLUTION FOR THE SECOND HALF OF NEXT WEEK. THERE IS UNCERTAINTY WITH 
ACTUAL RAIN/SNOW AMOUNTS...BUT CONFIDENCE IS HIGH THAT WEATHER WILL 
BECOME WET FOR THE WED-FRI PERIOD ACROSS CA/NV. THIS WETTER PATTERN 
WILL BE CAUSED BY A GULF OF ALASKA RIDGE THAT IS EXPECTED TO AMPLIFY 
AND PUSH NORTH INTO INLAND ALASKA. AS THIS UPPER RIDGE MOVES 
NORTHWARD...THIS WILL OPEN UP THE JET STREAM TO THE SOUTH...PUSHING 
A DEEP SUBTROPICAL MOISTURE TAP INTO CA/NV. 

THERE IS PLENTY OF UNCERTAINTY STILL WITH THIS SUBTROPICAL MOISTURE 
TAP...AND HOW MUCH WILL AFFECT THE REGION. IF THE EURO MODEL IS 
CORRECT...WE COULD SEE A HEAVY RAIN/SNOW EVENT THROUGH SEVERAL DAYS 
LATE NEXT WEEK. THE JET STREAM WOULD STRETCH BACK FAR ACROSS THE 
PAC...CREATING A FIRE HOSE EFFECT OF DEEP MOISTURE WITH HIGHER SNOW 
LEVELS AND HEAVY MOUNTAIN SNOW AND VALLEY RAINS. IF THE GFS IS 
CORRECT...THE RAIN/SNOW WILL BE MUCH LESS...AND SNOW LEVELS WILL BE 
LOWER. 12Z GFS ENS SHOW A MEAN VERY SIMILAR TO THE EC...WHICH WOULD 
INDICATE A THE POTENTIAL FOR HEAVY RAIN/SNOW. BOTH MODELS TEND TO 
UNDERESTIMATE SNOW LEVELS WHEN WE GET INTO A SUBTROPICAL WINTER 
PATTERN...WHICH COULD MEAN THAT SNOW LEVELS MAY BE EVEN HIGHER. IT 
IS STILL TOO EARLY TO SPECULATE ABOUT SPECIFIC RAIN/SNOW 
AMOUNTS...ALTHOUGH THERE IS STRONG POTENTIAL FOR HEAVY RAIN/SNOW 
THROUGH THE PERIOD. FORECAST CONFIDENCE OF OVERALL MILDER AND WETTER 
WEATHER REMAINS GOOD AND CONTINUES TO RISE.  HOON

&&

.AVIATION...
VFR WITH LGT WINDS THRU FRI. GEN NLY FLOW ALF THRU TNGT BECMG NWLY 
FRI AHD OF SYS DROPPING SWD THRU THE PACNW. HOON

&&

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

$$

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