Area Forecast Discussion
Issued by NWS Reno, NV

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FXUS65 KREV 180620
AFDREV

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE RENO NV
1020 PM PST WED DEC 17 2014

.UPDATE...
ALREADY WEAK UPPER FORCING WAS WANING AND PUSHING THROUGH WESTERN
NV THIS EVENING AS BAND OF RAIN AND SNOW LAGGED BEHIND OVER CA.
MOST MODELS HAVE BACKED WAY OFF ON RAIN AND SNOW FOR AREAS EAST OF
THE SIERRA AS A RESULT OF THE LATEST TRENDS. THE BAND OF RAIN AND
SNOW, IF IT CAN HOLD TOGETHER, MAY REACH THE CREST WITH SOME LIGHT
SNOW POSSIBLE IN THE TAHOE BASIN AND ALONG HIGHWAY 89 NORTH OF
TRUCKEE, BUT SNOWFALL AMOUNTS ARE GOING TO BE LIGHT. WE UPDATED TO
SIGNIFICANTLY LOWER POPS ACROSS WESTERN NV AND EVEN PARTS OF THE
EASTERN SIERRA. SNOWFALL AMOUNTS UP TO 2 INCHES ARE POSSIBLE WEST
OF HIGHWAY 89 WITH AN INCH OR LESS AT KTVL/KTRK OVERNIGHT. WHILE
AMOUNTS WILL BE LIGHT, TRAVEL MAY BECOME SLICK OVERNIGHT INTO
THURSDAY MORNING FOR AREA PASSES, ESPECIALLY DONNER AND ECHO
SUMMITS, AND ALONG HIGHWAY 89. A FEW FLURRIES MAY VISIT THE
HIGHWAY 395 CORRIDOR. HOHMANN

&&

.SYNOPSIS...
HIGH PRESSURE WILL BUILD OVER THE REGION THURSDAY BEFORE ANOTHER
WEAK STORM SYSTEM BRINGS ANOTHER CHANCE OF RAIN AND SNOW FRIDAY
THROUGH SATURDAY. HIGH PRESSURE WILL THEN BUILD SUNDAY THROUGH THE
FIRST PART OF NEXT WEEK.

&&

.PREVIOUS DISCUSSION... /ISSUED 722 PM PST WED DEC 17 2014/

SYNOPSIS...

A WEAK AREA OF LOW PRESSURE MOVES ACROSS NORTHERN CALIFORNIA AND
WESTERN NEVADA TONIGHT AND EARLY THURSDAY. THIS LOW PRESSURE
SHOULD BRING ANOTHER ROUND OF LIGHT SNOW TO AREAS NORTH OF HIGHWAY
50 AND ALONG THE SIERRA CREST IN MONO COUNTY. ANOTHER SYSTEM WILL
IMPACT THE SAME AREAS FRIDAY WITH ANOTHER ROUND OF LIGHT SNOW. SLICK
AND SNOW COVERED ROADS ARE LIKELY FOR THE SIERRA PASSES THROUGH
FRIDAY. A FEW SLICK SPOTS ARE POSSIBLE IN THE LOWER VALLEYS AS WELL
THAT COULD IMPACT THE MORNING COMMUTE THURSDAY.

SHORT TERM...
NEXT WAVE IS MOVING TOWARD THE COAST NOW AND SHOULD IMPACT OUR
FORECAST AREA TONIGHT. THIS DOES NOT APPEAR TO BE AN OVERLY STRONG
WAVE...BUT THERE IS ENOUGH MOISTURE WITH IT TO PRODUCE LIGHT PCPN.
FORECAST MODELS DO NOT SHOW AN OVERWHELMING AMOUNT OF FORCING WITH
THIS SYSTEM. THERE IS A WARM AIR ADVECTION SIGNATURE WITH THE SYSTEM
THAT...COUPLED WITH FAIRLY DEEP MOISTURE...SHOULD PUSH PCPN OVER THE
SIERRA INTO WESTERN NEVADA OVERNIGHT.

THE BEST CHANCE FOR PCPN IN THE EVENING SHOULD BE IN NORTHEAST CA
FROM ABOUT THE TAHOE BASIN NORTHWARD. THEN THE PCPN CHANCES SPREAD
EAST AND SOUTH THROUGH THE OVERNIGHT HOURS. WITH WARM AIR ADVECTION
SYSTEMS LIKE THIS THE AMOUNT OF PCPN DOES NOT TEND TO BE VERY HIGH
OR DRIVEN MUCH BY TERRAIN INFLUENCES. THUS...IT IS NOT OUT OF THE
QUESTION FOR AREAS EAST OF THE SIERRA TO GET ABOUT AS MUCH AS
POINTS IN THE SIERRA AND FAR NORTHEAST CA. SNOW LEVELS COULD BE
DOWN TO EVEN THE LOWEST VALLEY FLOORS IN THIS WARM AIR ADVECTION
AS WELL. FORECAST SOUNDINGS EAST OF THE SIERRA SHOW A DROP TO NEAR
4500 FEET FOR THE SNOW LEVELS LATE TONIGHT.

SO...WHAT DOES THIS MEAN FOR OVERNIGHT? HIGH POPS AND LOW QPF WITH
MAYBE UP TO AN INCH OF SNOW FOR AREAS OF WESTERN NEVADA. IN THE
TAHOE BASIN...NORTHEAST CA AND THE HIGHER ELEVATIONS OF MONO
COUNTY...SNOW AMOUNTS SHOULD BE AN INCH OR TWO EXCEPT IN THE
HIGHEST ELEVATIONS WHERE UP TO 4 INCHES IS POSSIBLE. SO...WE WILL
NOT PLAN ANY ADVISORIES AT THIS TIME...BUT BE ADVISED THAT EVEN AN
INCH OF SNOW COULD RESULT IN SLICK ROADS FOR THE MORNING COMMUTE.
BE PREPARED FOR A FEW TRAVEL DELAYS FOR THE THURSDAY MORNING
COMMUTE FOR EASTERN CA AND FAR WESTERN NV.

THIS WAVE MOVES EAST THURSDAY AFTERNOON WITH A FEW LINGERING
SHOWERS DURING THE DAY. A BRIEF PERIOD OF RIDGING DEVELOPS LATE
THURSDAY EVENING...BUT THIS IS QUICKLY REPLACED BY ANOTHER WAVE
MOVING ACROSS THE NORTHERN FORECAST AREA FRIDAY. 20

FRIDAY AND SATURDAY...
ANOTHER WAVE MOVES ACROSS NORTHERN CA/NV FRIDAY WITH A WEAK COLD
FRONT PUSHING ACROSS THE REGION LATE FRIDAY MORNING. THE FRONT
FALLS APART AS IT CROSSES THE NORTHERN SIERRA WITH THE WAVE
WEAKENING CONSIDERABLY EAST OF THE CREST. THEREFORE, DO NOT EXPECT
SIGNIFICANT RAIN/SNOW TO OCCUR. BEST CHANCES FOR PRECIPITATION
WILL BE ALONG THE SIERRA FROM TAHOE NORTH INTO NORTHEAST CA
EXTENDING INTO NORTHERN WASHOE COUNTY. INSTABILITY ACROSS NORTHERN
NV MAY LEAD TO PRECIPITATION DEVELOPING IN THE CENTRAL BASIN AND
RANGE AS WELL. MOST FORECAST GUIDANCE LARGELY SHADOWS OUT THE
SIERRA FRONT, BUT WITH WESTERLY FLOW THINK THAT A FEW SPOTS MAY
END UP SEEING SHOWERS SO HAVE KEPT IN LOW END CHANCES. THE SYSTEM
QUICKLY EXITS EASTWARD, BUT UPSLOPE FLOW MAY KEEP SHOWERS
LINGERING IN THE SIERRA. SNOW LEVELS WILL MAINLY BE 4500-5500 FEET
NORTH OF HIGHWAY 50 WITH 5500-6500 FEET SOUTH OF THERE WITH SLICK
ROADS AND TRAVEL IMPACTS POSSIBLE.

A DIRTY RIDGE BEGINS TO BUILD INTO THE WEST SATURDAY INTO SUNDAY
WITH WARM AIR ADVECTION PRECIPITATION DEVELOPING ACROSS NORTHERN
CA/NV. THE JET IS DRIVEN INTO THE PACIFIC NORTHWEST OVER THE TOP
OF THE RIDGE, BUT THERE IS SOME DISAGREEMENT JUST HOW FAR NORTH THE
JET CORE WILL BE. THIS WILL IMPACT HOW FAR SOUTH WE MAY SEE
RAIN/SNOW DEVELOP LATE SATURDAY INTO SUNDAY. THE NORTHERN END OF
THE RANGE IS NORTH OF A PORTOLA-LOVELOCK LINE WITH THE SOUTHERN
END OF THE SPECTRUM BEING DOWN TO MAMMOTH AND THE WHITE MOUNTAINS.
FOR NOW HAVE MAINTAINED SLIGHT CHANCE POPS AREA WIDE WITH CHANCE
POPS ACROSS FAR NORTHERN CA/NV TO ACCOUNT FOR THE UNCERTAINTY.
SNOW LEVELS WILL BE RISING IN THIS PATTERN TO 6000-7000 FEET. DJ

LONG TERM...SUNDAY THROUGH WEDNESDAY/CHRISTMAS EVE...
SUNDAY THROUGH TUESDAY, EASTERN CALIFORNIA AND WESTERN NEVADA WILL
REMAIN UNDER AN UPPER RIDGE WHICH WILL STEER ANY DISTURBANCES INTO
THE PACIFIC NORTHWEST AND NORTHERN ROCKIES. THE RIDGE WILL BE RATHER
"DIRTY" WITH LOTS OF MID AND/OR HIGH CLOUDS, ESPECIALLY NORTH OF
INTERSTATE 80. TEMPERATURES SHOULD REMAIN MILD AT 5-10 DEGREES ABOVE
AVERAGE.

NEXT WEDNESDAY AND WEDNESDAY NIGHT (CHRISTMAS EVE), MODELS ARE
STARTING TO SHOW AN INTERESTING POSSIBILITY AS THE LARGE UPPER RIDGE
NEAR THE PACIFIC COAST BEGINS TO REBUILD/RETROGRADE INTO THE EASTERN
PACIFIC AND GULF OF ALASKA. THIS RETROGRESSION IS EXPECTED TO ALLOW
A SLIDER-TYPE SYSTEM TO DIVE DOWN INTO THE WESTERN CONUS BY LATE ON
CHRISTMAS EVE OR ON CHRISTMAS DAY/NEXT THURSDAY. ENSEMBLE SPREADS
ARE LARGE AT THIS TIME INDICATING VAST UNCERTAINTY, ALTHOUGH THE
UNCERTAINTY APPEARS TO BE RELATED TO THE STRENGTH AND HOW FAR SOUTH
THE SLIDER DIGS VERSUS WHETHER IT ACTUALLY EXISTS. THEREFORE, I
WOULD EXPECT SOME KIND OF SLIDER SYSTEM WITH INCREASING WINDS FOR
WEDNESDAY FOLLOWED BY COLDER TEMPERATURES FOR CHRISTMAS DAY. AS FAR
AS PRECIPITATION AND HOW COLD IT GETS, THAT REMAINS VERY UNCERTAIN
AT THIS TIME. STAY TUNED! SNYDER

AVIATION...
PLENTY OF MOISTURE AND A SLIGHTLY UNSTABLE ATMOSPHERE IS PRODUCING
WIDESPREAD CUMULUS ALONG WITH ISOLATED SHOWERS IN THE SIERRA AND OUT
INTO FAR WESTERN NEVADA THIS AFTERNOON. CLOUD DECKS ARE GENERALLY
VFR, ALTHOUGH AREAS OF TERRAIN OBSCUREMENT CAN BE FOUND IN EASTERN
CALIFORNIA/THE SIERRA AND IN SHOWERS ELSEWHERE.

TONIGHT (GENERALLY AFTER 03Z-06Z), YET ANOTHER WEAK UPPER
DISTURBANCE WILL MOVE OVER NORTHEAST CALIFORNIA AND WESTERN NEVADA.
THIS WILL BRING A ROUND OF SCATTERED (WESTERN NEVADA) TO NUMEROUS
(SIERRA) LIGHT SNOW SHOWERS TO THE REGION. THIS WILL BRING DOWN
CIGS/VIS TO IFR IN THE SIERRA WITH AREAS OF MVFR CIGS OUT INTO
WESTERN NEVADA. -SHSN SHOULD DISSIPATE QUICKLY BY AROUND 12Z/4AM
THURSDAY AS THE DISTURBANCE EXITS THE REGION.

AS FAR AS SNOWFALL ACCUMULATIONS ON UNTREATED PAVEMENTS/TARMACS,
SHOWERS SHOULD REMAIN LIGHT AND SPORADIC ENOUGH THAT WESTERN NEVADA
TERMINALS BELOW 5000 FEET REMAIN MAINLY WET, WITH A DUSTING POSSIBLE
ABOVE 5000 FEET (KRTS). IN THE NORTHERN SIERRA AND FOR NORTHEAST
CALIFORNIA (KSVE), SNOW COULD BE A BIT HEAVIER SO A LIGHT
ACCUMULATION (UP TO AN INCH) IS POSSIBLE ON UNTREATED PAVEMENT.

THURSDAY AND THURSDAY NIGHT, A RIDGE BUILDS OVERHEAD FOR DRIER
CONDITIONS BEFORE A COLD FRONT MOVES INTO EXTREME NORTHERN NEVADA
AND CALIFORNIA FRIDAY FOR ANOTHER SHOT AT MOUNTAIN SNOW AND VALLEY
RAIN. SNYDER

&&

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

$$

FOR MORE INFORMATION FROM THE NATIONAL WEATHER SERVICE VISIT...
HTTP://WEATHER.GOV/RENO (ALL LOWER CASE)







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