FXUS65 KREV 261059
AFDREV

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RENO NV
259 AM PST SAT DEC 26 2009

.SHORT TERM...

FREEZING FOG THIS MORNING NOT AS WIDESPREAD AS IT HAS BEEN THE
PAST FEW MORNINGS DUE TO A LACK OF LOW LEVEL MOISTURE. BEST
COVERAGE IS FROM SUSANVILLE EASTWARD INTO THE LOWER VALLEYS OF
NORTHERN WASHOE COUNTY WHERE THERE ARE AREAS WITH VISIBILITY BELOW
A QUARTER OF A MILE. PATCHY FREEZING FOG HAS BEEN REPORTED IN THE
LOW LYING...SNOW COVERED VALLEYS OF PERSHING AND CHURCHILL
COUNTIES.

CIRRUS SHIELD IS EVIDENT ON SATELLITE IMAGERY PUSHING INTO THE
WEST THIS MORNING AHEAD OF THE LOW SITTING OVER THE EASTERN
PACIFIC. THIS SYSTEM SHOULD HELP MIX OUT SOME OF THE VALLEY
INVERSIONS TODAY AS COOLER AIR MOVES IN ALOFT. WINDS ARE FAIRLY
LIGHT SURFACE AND ALOFT THOUGH...SO HAZE WILL LIKELY LINGER
THROUGH THE DAY.

UPPER TROUGH AND THE ASSOCIATED SHORTWAVE LOCATED NEAR 135W/35N AS
OF 10Z ARE SHOWING UP WELL ON WATER VAPOR...WITH UNSTABLE CU IN
THE COLD AIR TO THE NORTHWEST. DETERMINISTIC AND ENSEMBLE MODELS
ARE BRINGING THE BEST LIFT...INSTABILITY...AND DEEPEST MOISTURE
ACROSS THE SIERRA TONIGHT INTO SUNDAY MORNING. APPEARS THE BEST
CHANCES FOR PRECIP ALONG THE CREST EASTWARD OVER THE SIERRA WILL
BE FROM LAKE TAHOE NORTHWARD. WITH BETTER MODEL AGREEMENT...HAVE
RAISED POPS THROUGH THE SIERRA TO LIKELY. SNOW AMOUNTS WILL BE
LIGHT...MAYBE 2-5 INCHES IN THE SIERRA. LIMITED MOISTURE WILL BE
ABLE TO SPILL OVER INTO WESTERN NV VALLEYS...BUT ITS NOT OUT OF
THE QUESTION THAT WE COULD SEE A DUSTING UP TO A HALF INCH OF SNOW
THROUGH THE RENO/SPARKS/CARSON CITY METRO AREA EARLY SUNDAY
MORNING.

SYSTEM EXITS THE AREA MONDAY WITH THE MAJORITY OF THE ENERGY
DIVING SOUTH TOWARD BAJA CA WHERE THE GEFS HAS IMPRESSIVE HEIGHT
ANOMALIES. BROAD SCALE TROUGH REMAINS OVER THE AREA THROUGH
TUESDAY WITH NO CLEAR SIGNALS FOR PRECIP THROUGH THIS PERIOD UNTIL
A SHORTWAVE BEGINS TO PUSH INTO THE REGION DURING THE FIRST PART
OF THE LONG TERM FORECAST. DF
 
.LONG TERM...TUESDAY THROUGH FRIDAY...

LARGE TROUGH OVER THE WEST TUESDAY...WITH MINIMAL RAIN/SNOW CHANCES 
FOR THE SIERRA AND WESTERN NEVADA. MODELS DEVELOP RIDGING IN THE 
EASTERN PACIFIC OR ALONG THE WEST COAST. DIFFERENCES IN POSITION OF 
THE RIDGE BRINGS UNCERTAINTY TO TRACK OF SYSTEM THAT MAY AFFECT THE 
NORTHERN AREAS TUESDAY NIGHT THROUGH WEDNESDAY.

00Z GFS/ECMWF RUNS HAVE FLIPPED THEIR SOLUTIONS OVER 24 HOURS AGO. 
GFS IS NOW THE SOLUTION THAT BRINGS A COLD UNSTABLE SYSTEM LATE 
TUESDAY NIGHT AND WEDNESDAY TO AREAS NORTH OF US-50.  QPF 
AMOUNTS/SNOW RATIOS COULD LEAD TO NEAR ADVISORY ACCUMULATIONS ALONG 
AND NORTH OF I-80 IF THE SOLUTION VERIFIES.  ECMWF BUILDS THE RIDGE 
CLOSER TO THE COAST...LIKE YESTERDAYS 00Z GFS...KEEPING THE SYSTEM 
NORTH OF THE AREA WITH WARM AIR ADVECTION PRECIP WEDNESDAY.

THE MAIN CAUSE FOR THE MODEL DIFFERENCES IS THE PROGRESSIVE SPLIT 
FLOW IN THE PACIFIC.  MODELS CANT DECIDE IF THE ENERGY ASSOCIATED 
WITH JET BETWEEN 30-40N WILL PHASE WITH THE MAIN UPPER LOW NEAR THE 
ALEUTIANS.  AMPLITUDE OF THE PATTERN IS DETERMINED IF THE ENERGY 
PHASES WITH THE MAIN LOW...ALLOWING A LONG WAVE TROUGH TO PUSH FROM 
150W ON MONDAY TO 140W ON TUESDAY.  WHEN THE HIGHER AMPLITUDE FLOW 
APPEARS IN THE MODELS AND ENSEMBLES...THE UNSTABLE SYSTEM PASSES 
OVER THE NORTHERN AREAS. THE LESS AMPLIFIED PATTERN YIELDS THE 
LIGHTER WARM AIR ADVECTION PRECIPITATION.   

DID NOT CHANCE THE FORECAST MUCH BASED ON THE DIFFERENT ENVIRONMENTS 
AND MECHANISMS THAT COULD BRING RAIN/SNOW TO THE AREA TUESDAY NIGHT 
AND WEDNESDAY.

AFTER WEDNESDAY MODELS HAVE SETTLED ON A FLAT OR DIRTY RIDGE OVER 
THE SIERRA WITH MAIN STORM TRACK NORTH OF THE AREA.  MODELS BRING 
LIGHT WARM AIR ADVECTION PRECIP WITH SYSTEM BRUSHING THE FAR 
NORTHERN AREAS.  LEFT THE SLIGHT CHANCE WORDING FOR RAIN/SNOW AS 
THERMAL PROFILES UNCERTAIN TO DETERMINE SNOW LEVELS/PRECIP TYPE AND 
STRENGTH OF VALLEY INVERSIONS. BRONG

&&

.AVIATION...FOG HAS DEVELOPED IN THE BASINS FROM SUSANVILLE...TO
PYRAMID LAKE AND INTO LOVELOCK AND FALLON AREA. NORTHEAST FLOW
OVER THE SIERRA HAS RELAXED...ALLOWING FOG TO FORM AT TRUCKEE.

MAIN CONCERN IS WHEN FOG WILL BURN OFF WITH CLOUDS APPROACHING FROM 
THE WEST AHEAD OF SYSTEM OFF THE CALIFORNIA COAST.  FOG AT TRUCKEE 
SHOULD DISSIPATE BY 18Z WITH INCREASED SOUTHWEST FLOW OVER THE 
SIERRA.  MODELS HINTING THAT FOG OR LOW STRATUS DECK WITHING 1000 
FEET OF THE SURFACE WILL REMAIN IN PLACE ALL DAY NEAR FALLON AND 
LOVELOCK AS LOW LEVEL FLOW IS VERY LIGHT.
  
SYSTEM WILL MOVE INTO THE SIERRA 00-06Z SUNDAY WITH CHANCE FOR 
MVFR/IFR CONDITIONS DUE TO SNOW.  SNOW MAY REACH WESTERN NEVADA 
04Z-12Z SUNDAY WITH POTENTIAL FOR MVFR CONDITIONS AT KRNO. BRONG

&&

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

$$

HTTP://WEATHER.GOV/RENO