FXUS65 KREV 050041
AFDREV

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RENO NV
441 PM PST FRI DEC 4 2009

.SHORT TERM...
VALLEY INVERSIONS DID NOT WEAKEN ENOUGH TO ALLOW TEMPERATURES TO
CLIMB MUCH TODAY. HIGHS ONLY MANAGED TO REACH INTO THE LOWER 40S
IN MOST WESTERN NV VALLEYS TODAY WHILE TEMPS IN THE TAHOE BASIN
HIT 50 DEGREES. THE UPPER RIDGE HAS RETROGRADED TO NEAR 135W THIS
AFTERNOON AS A STRONG SHORT WAVE DROPS SOUTHWARD OVER WA AND
OREGON...DRIVEN BY A 120 KNOT JETSTREAM ALONG THE PAC NW COAST. 

A STRONG COLD FRONT ASSOCIATED WITH THIS SYSTEM WILL PUSH ACROSS
THE CWA ON SATURDAY. THE MAIN EFFECT WILL BE COLDER AIR THAT
SPREADS OVER THE AREA BEHIND FRONT. TEMPERATURES ARE EXPECTED TO
FALL THROUGH THE AFTERNOON...WITH MOST VALLEY LOCATIONS FROM I-80
SOUTH HITTING THEIR HIGHS IN THE LATE MORNING. GUSTY WINDS WILL
DEVELOP AHEAD OF THE FRONT AND CONTINUE AFTER FRONTAL PASSAGE
BEFORE DIMINISHING AFTER DARK SATURDAY EVENING. THE WINDS SHOULD
BE SUFFICIENT FOR ROUGH CONDITIONS ON LAKE TAHOE AND PYRAMID LAKES
SO LAKE WIND ADVISORIES HAVE BEEN ISSUED 9A-6P SATURDAY.

THE FRONT IS FAST MOVING AND WITHOUT A SIGNIFICANT AMOUNT OF
MOISTURE HOWEVER STRONG SHORT-LIVED FRONTOGENESIS IN WESTERN NV
SHOULD PRODUCE SCATTERED SNOW SHOWERS AND COULD BRING LIGHT
ACCUMULATIONS DURING THE HEAVIEST SHOWERS.

A MORE SIGNIFICANT SYSTEM AFFECTS THE CWA SUNDAY NIGHT THROUGH
MONDAY AS A SECOND SHORT WAVE MOVES SOUTH ALONG THE PAC NW COAST
INTO NORTHERN CA SUNDAY NIGHT. VERY COLD AIR WILL BE IN PLACE ON
MONDAY...H7 TEMPERATURES -15C TO -20C...AND SNOW WILL BE AT THE
LOWEST VALLEY FLOORS. THIS SYSTEM HAS A BIT OF AN OVER WATER
TRAJECTORY AND BRINGS MUCH HIGHER MOISTURE CONTENT INTO THE
REGION.

BECAUSE OF THE COLD AIR...SNOW RATIOS WILL BE HIGH...PERHAPS 20
TO 1 OR HIGHER. ACCUMULATIONS IN WESTERN NV VALLEYS ARE EXPECTED
AND THE SNOW MAY AFFECT THE MORNING COMMUTE ON MONDAY. CURRENT
THINKING IS FOR UP TO 18 INCHES IN THE HIGHER ELEVATIONS OF THE
SIERRA BY LATE MONDAY...WITH POTENTIAL A COUPLE OF INCHES IN
WESTERN NV VALLEYS. SKIES WILL BEGIN TO CLEAR MONDAY NIGHT AND
TEMPERATURES WILL DROP SIGNIFICANTLY...DEPENDENT ON HOW QUICKLY
CLEARING OCCURS. LOWS IN WESTERN NV VALLEYS WILL DIP INTO SINGLE
DIGITS AND BELOW ZERO TEMPS WILL OCCUR IN MANY MOUNTAIN VALLEYS.
RC

.LONG TERM...TUESDAY THROUGH FRIDAY...
MODELS HAVE COME INTO MUCH BETTER AGREEMENT OVER THE LAST FEW RUNS 
WITH A COMMON SOLUTION THAT IS A COMPROMISE BETWEEN THE PLETHORA OF 
PREVIOUS SOLUTIONS...SO CONFIDENCE IS INCREASING IN THE ENTIRE 
FORECAST.

VERY COLD CONDITIONS REMAIN OVER THE REGION TUESDAY WITH H7 
TEMPERATURE ANOMALY OF -2.35 CENTERED ALONG THE SIERRA AND WRN 
NEVADA. THE REAL COLD CONDITIONS WILL BE SHORT-LIVED AS SHORTWAVE 
RIDGING BUILDS OVER THE WEST COAST LATE TUESDAY BRINGING WARMING 
ALOFT. MEANWHILE THE PACIFIC JET STREAK CONTINUES TO UNDERCUT THE 
HIGH CENTER OVER THE GULF OF ALASKA AND THE MUCH ADVERTISED 
SUBTROPICAL MOISTURE TAP FINALLY APPROACHES THE WEST COAST. MID AND 
HIGH CLOUDS AHEAD OF THE MOISTURE SHOULD LIMIT MUCH OF THE SOLAR 
HEATING TUESDAY KEEPING THE VALLEY TEMPERATURES MUCH BELOW 
NORMAL...EVEN THOUGH THE HIGH SIERRA WILL SLOWLY WARM.

THERE IS STILL ROUGHLY A 12 HOUR DISCREPANCY REGARDING WHEN THE 
MOISTURE MOVES ONSHORE AND THE PRECIP BEGINS LATE TUESDAY INTO 
WEDNESDAY...HOWEVER MODEL SOLUTIONS ARE VERY SIMILAR ONCE THE WET 
PATTERN STARTS. OVERRUNNING PRECIP INITIALLY MOVES INTO THE SIERRA 
WITH A DECENT AMOUNT OF SPILLOVER INTO THE LEESIDE BY LATE 
WEDNESDAY. RIDGE LEVEL WINDS NEVER GO MUCH ABOVE 30 KTS AHEAD OF THE 
PRECIP BAND WHICH WILL PREVENT MIXING AND KEEP COLD TEMPS TRAPPED IN 
THE VALLEYS. EVEN THOUGH H7 TEMPS WARM TO ROUGHLY -3C WEDNESDAY 
EVENING...EXPECTING AN ISOTHERMAL TEMP PROFILE WHICH WOULD BRING ALL 
SNOW TO MOST ELEVATIONS. A STEADY STREAM OF UPPER LEVEL IMPULSES 
COMBINED WITH MOIST OROGRAPHIC FLOW WILL KEEP THE PRECIP GOING INTO 
THURSDAY EVENING. SNOW LEVELS WILL GRADUALLY RISE THROUGH THURSDAY 
AS VERTICAL MIXING SLOWLY SCOURS OUT THE LOW LEVEL COLD AIR. SNOW 
LEVELS SHOULD NEVER RISE MUCH ABOVE 6000 FEET WITH H7 TEMPS COOLING 
TO -6C BY THURSDAY AFTERNOON WHEN THE LOW LEVELS ARE FINALLY MIXED.

MODEL QPF HAS LOWERED QUITE A BIT FROM THE SOLUTIONS A FEW DAYS 
AGO...BUT THIS DOES STILL APPEAR TO BE A MODEST WINTER STORM FOR THE 
REGION. QPF VALUES ALONG THE SIERRA CREST HAVE BEEN FAIRLY STEADY AT 
3-4 INCHES DURING THIS LATE WEEK STORM WITH AROUND AN INCH POSSIBLE 
IN EXTREME WRN NEVADA. JORDAN

&&

.AVIATION...
WEST FLOW ALOFT IS STARTING TO INCREASE AHEAD OF AN APPROACHING COLD 
FRONT. WINDS OVER THE SIERRA CREST ARE ALREADY GUSTING TO 40 MPH AND 
LENTICULAR CLOUDS ARE PRESENT ALONG THE LEE OF THE SIERRA. COLD 
FRONT DROPS SOUTH INTO NRN NV/CA AFTER 06Z AND CROSSING THE TAF 
SITES AROUND 18Z SATURDAY. SURFACE WINDS AHEAD OF THE FRONT DON'T 
APPEAR TO BE OVERLY STRONG...HOWEVER POST-FRONTAL NORTH WINDS 
INCREASE SATURDAY AFTERNOON WITH GUSTS 25-30 MPH. THE FRONT WILL 
ALSO TRIGGER A LINE OF LOWER CLOUDS HOWEVER VFR CONDITIONS ARE 
EXPECTED. JORDAN
&&

.REV WATCHES/WARNINGS/ADVISORIES...
NV...LAKE WIND ADVISORY FROM 8 AM TO 6 PM PST SATURDAY FOR NVZ002-004.

CA...LAKE WIND ADVISORY FROM 8 AM TO 6 PM PST SATURDAY FOR CAZ072.

&&

$$

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