FXUS65 KREV 111134
AFDREV

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RENO NV
330 AM PST FRI DEC 11 2009

.SHORT TERM...
THE FIRST IMPULSE IN THE ONE-TWO-THREE PUNCH IS MOVING OVERHEAD
THIS MORNING AND OVERRUNNING A VERY COLD LOW LEVEL AIRMASS IN PLACE
OVER THE GREAT BASIN. THE STABLE LOW LEVEL AIRMASS AND COPIOUS
MOISTURE IN THE ATMOSPHERE IS FIGHTING OFF OUR USUAL TENDENCY TO
DOWNSLOPE IN THE LEE OF THE SIERRA. THIS IS ALLOWING SNOW TO
SPILLOVER EASILY THIS MORNING. LIGHT SNOW IS EXPECTED TO CONTINUE
UNTIL THIS FIRST IMPULSE MOVES THROUGH LATE THIS MORNING. 

THE SECOND IMPULSE IN THE TRAIN OF THREE IS EXPECTED IN LATE THIS
EVENING INTO EARLY SATURDAY MORNING. ONCE AGAIN...A FEW INCHES OF
SNOWFALL IS EXPECTED IN THE SIERRA WITH UP TO A COUPLE INCHES OUT
INTO WESTERN NEVADA. 

THE THIRD OF THE THREE PUNCHES...WHICH IS DUE IN SATURDAY AFTERNOON
AND EARLY EVENING...IS CLEARLY THE MOST SIGNIFICANT. IT IS ALREADY
LOOKING IMPRESSIVE IN THE WATER VAPOR SATELLITE AT ABOUT 42N/155W
WITH A JET STREAK OF 125-140 KTS ON ITS SOUTHERN SIDE. THE SYSTEM
WILL WORK ITS WAY TO OFF THE NORTHERN CALIFORNIA COAST BY SATURDAY
AFTERNOON AS AN ASSOCIATED 130-KT JET STREAK NEARS THE CENTRAL
SIERRA. INTENSE FORCING WILL SET UP A PERIOD OF MODERATE TO HEAVY
PRECIPITATION LATE SATURDAY AFTERNOON AND EVENING FOR THE NORTHERN
SIERRA AND WESTERN NEVADA BEFORE THE HEAVIER FRONTAL PRECIP SAGS
SOUTH INTO MONO AND MINERAL COUNTIES OVERNIGHT INTO SUNDAY MORNING.

INTENSE LIFT AND HEAVY PRECIPITATION SHOULD HOLD IN A LOW LEVEL
ISOTHERMAL LAYER TO KEEP PRECIPITATION ALL SNOW FOR MOST AREAS.
HOWEVER...BASED ON THE 00Z GFS THERE IS A WINDOW OF OPPORTUNITY
FOR SNOW LEVELS TO RISE OFF THE LOWER VALLEY FLOORS SOUTH AND EAST
OF RENO (MAINLY NEAR AND SOUTH OF HIGHWAY 50) SATURDAY AFTERNOON
BEFORE HEAVIER PRECIP ARRIVES. FIGHTING THIS WILL BE LOW LEVEL
SOUTHEAST OR EVEN EAST FLOW AS WINDS BACK AHEAD OF THE 997-ISH MB
LOW OFF THE NORTHERN CALIFORNIA COAST. NAM BUFR SOUNDINGS INDICATE
A LOW LEVEL ISOTHERMAL LAYER OUT IN LOVELOCK WHICH IS INDICATIVE
OF A NON-MIXED OUT PROFILE AND ALL SNOW. WITH THE POSSIBILITY OF A
BRIEF RISE OF SNOW LEVELS ABOVE THE VALLEY FLOORS...I HAVE INTRODUCED
THE CHANCE FOR MIXED PRECIP SATURDAY AFTERNOON AND EVENING OUT IN
THE LOWEST ELEVATION AREAS OF THE BASIN EAST OF RENO. 

AS A FINAL NOTE...THE 06Z GFS HAS COME IN SLIGHTLY FURTHER SOUTH
WITH THE JET STREAK SATURDAY AFTERNOON AND EVENING. IF THE TREND
HOLDS...IT LOOKS LESS LIKELY THAT WE WILL MIX OUT SOME AND GO TO
RAIN IN THE LOWEST VALLEYS SOUTH AND EAST OF RENO. SNYDER

.LONG TERM...SUNDAY THROUGH THURSDAY...
SUNDAY FORECAST BEGINS WITH UPPER TROF SLOWLY PULLING TO THE EAST 
AWAY FROM THE FORECAST. MODELS APPEAR TO HAVE SLOWED THE EASTWARD 
PROGRESSION OF THIS FEATURE AND AS LONG AS IT LINGERS OVER THE RGN 
THERE WILL BE A CONTINUED CHANCE FOR PCPN. CHARACTER SHOULD CHANGE TO 
SHOWERS UNDER THE COLD POOL ALOFT WITH BEST CHANCES OVER THE SIERRA. 
OPTED TO BUMP UP POPS FOR THE SIERRA JUST A BIT SUNDAY. 
ACCUMULATIONS FOR THE HIGHER ELEVATIONS SHOULD CONTINUE INTO SUNDAY 
MRNG AND THIS COULD AFFECT TIMING OF THE WARNINGS THAT WILL BE IN 
PLACE OVER THE SIERRA THEN. 

AS TROF PUSHES TO THE EAST SUNDAY NIGHT AND MONDAY A RIDGE WILL TRY 
TO BUILD INTO THE RGN. MODELS DIFFER SLIGHTLY WITH PROGRESSION AND 
AMPLITUDE OF THIS FEATURE. GFS REMAINS A BIT MORE PROGRESSIVE AND 
FLATTER WITH THE RIDGE WHILE ECMWF IS MORE AMPLIFIED. GEFS ENSEMBLE 
MEMBERS SHOW A RANGE OF SOLUTIONS AS WELL BUT MORE TEND TO FAVOR THE 
OPERATIONAL GFS. THIS WOULD SUPPORT PERIODS OF SLIGHT CHANCE TO LOW 
CHANCE POPS THROUGH MUCH OF THE EXTENDED FORECAST FOR NORTHERN AREAS 
OF THE CWA AS WEAK IMPULSES MOVE OVER THE RIDGE. THESE WEAK IMPULSES 
BECOME SOMEWHAT SELF SUSTAINING AS WELL IN THAT THEY HELP TO FLATTEN 
THE RIDGE EVEN MORE AND ALLOW THE NEXT IMPULSE TO COME OVER THE 
RIDGE. WHILE I AM NOT COMPLETELY ENTHUSIASTIC ABOUT THIS SOLUTION I 
ALSO CAN FIND LITTLE TO ARGUE AGAINST IT...EXCEPT FOR THE 
OPERATIONAL ECMWF. THUS WILL MAINTAIN BASICALLY WHAT WAS ALREADY IN THE 
FORECAST FOR THE BULK OF THE EXTENDED PERIOD.

ONLY SIGNIFICANT CHANGE WAS TO TREND TEMPS DOWN THROUGH THE 
EXTENDED. ALTHOUGH A RIDGE TRIES TO BUILD...LOW LVL COLD AIR SHOULD 
REMAIN TRAPPED IN MOST VALLEYS WITH LITTLE IN THE WAY MIXING TO MOVE 
THIS COLD AIR OUT. ALSO OPTED TO LOWER SNOW LEVELS THROUGH MUCH OF 
THE EXTENDED. ALTHOUGH MODELS INDICATE WARMING TEMPS ALOFT THINK IF 
PCPN DOES DEVELOP WITH LOW LVL COLD AIR TRAPPED IN THE VALLEYS AND 
RELATIVELY DRY AIR BELOW THE INITIAL LEVEL OF PCPN DEVELOPMENT...
EVAPORATIVE COOLING WOULD FORCE SNOW LEVELS DOWN. THE AMOUNT OF LOW 
LVL COLD AIR TRAPPED IN THE VALLEYS WOULD ALSO RESULT IN PERIODS OF 
HAZE/FREEZING FOG IN MANY VALLEYS LOCATIONS...ESPECIALLY FOR MONDAY 
THROUGH WEDNESDAY. MLF
&&

.AVIATION...
FIRST BAND OF LIGHT SNOW IS LIFTING NORTH ACROSS WRN NV AND THE 
CENTRAL/NRN SIERRA NOW. VSBYS HAVE DROPPED TO 1-3 MILES IN THIS 
BAND. MORE SNOW SHOULD DEVELOP THROUGH THE MRNG AND LIFT NORTH WITH 
PERIODS OF IFR-MVFR CONDITIONS AT KTVL/KTRK/KRNO THROUGH ABOUT 19Z 
THIS MORNING. KLOL NOT LIKELY TO GO DOWN AS FAR OR LAST AS LONG. 
THERE SHOULD BE A BIT OF A BREAK EAST OF THE SIERRA THIS AFTERNOON 
AND EVENING.

MEANWHILE ANOTHER SYSTEM BEGINS TO IMPACT THE SIERRA LATE THIS 
EVENING AND OVERNIGHT WITH ADDITIONAL SNOW DEVELOPING AFTER 05Z. 
THIS SNOW SHOULD BECOME WIDESPREAD AND HEAVY WITH IFR CONDITIONS 
LIKELY AFTER ABOUT 10Z TONIGHT. THIS SNOW WILL CONTINUE THROUGH MUCH 
OF SATURDAY INTO SATURDAY NIGHT IN THE SIERRA WITH CONDITIONS 
REMAINING MVFR OR LOWER. 

EAST OF THE SIERRA THE BREAK CONTINUES UNTIL LATE THIS EVENING. 
AFTER MIDNIGHT TONIGHT LIGHT SNOW SHOULD DEVELOP BUT WILL BECOME 
SOMEWHAT INTERMITTENT WITH PERIODS OF MVFR CONDITIONS. SNOW WILL 
INCREASE LATE IN THE DAY SATURDAY AND CONTINUE INTO SATURDAY NIGHT 
WITH MORE WIDESPREAD MVFR-IFR CONDITIONS AFTER MIDNIGHT SATURDAY 
NIGHT. MLF
&&

.REV WATCHES/WARNINGS/ADVISORIES...
NV...WINTER WEATHER ADVISORY UNTIL 10 AM PST THIS MORNING FOR 
     NVZ001>003.

     WINTER STORM WARNING FROM 10 PM THIS EVENING TO 4 AM PST SUNDAY 
     FOR NVZ002.

     WINTER STORM WATCH FROM SATURDAY AFTERNOON THROUGH LATE SATURDAY 
     NIGHT FOR NVZ001-003>005.

CA...WINTER WEATHER ADVISORY UNTIL 10 AM PST THIS MORNING FOR 
     CAZ071>073.

     WINTER STORM WARNING FROM 10 PM THIS EVENING TO 4 AM PST SUNDAY 
     FOR CAZ071-072.

     WINTER STORM WATCH FROM SATURDAY AFTERNOON THROUGH LATE SATURDAY 
     NIGHT FOR CAZ070.

     WINTER STORM WARNING FROM 10 AM SATURDAY TO 10 AM PST SUNDAY FOR 
     CAZ073.

&&

$$

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