Heavy Snow and Winter Weather in Nevada, February 20 to 21, 2010

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RENO NV
303 PM PST SAT FEB 20 2010

.SHORT TERM...
WAVE TRACKING SOUTH ALONG THE WEST SIDE OF A COLD TROUGH OVER THE
NORTHERN GREAT BASIN WILL REACH NORTHWEST NEVADA THIS EVENING AND
HELP PRODUCE MODERATE TO HEAVY SNOW SHOWERS SOUTH OF SUSANVILLE-
PYRAMID LAKE-LOVELOCK AND NORTH OF BRIDGEPORT-HAWTHORNE THIS
EVENING AND TONIGHT. SOUTH OF BRIDGEPORT-HAWTHORNE...GUSTY WEST
WINDS AND 700MB FLOW OF 30-40KTS SHOULD KEEP THE AREA SHADOWED
WITH ONLY LIGHT SHOWERS POSSIBLE TONIGHT.

SHOWERS STARTING TO INCREASE IN THE NORTHERN SIERRA AND RENO-
CARSON AREA. SNOW LEVELS ARE NEAR 5000 FEET AND SHOULD DROP TO
VALLEY FLOORS THIS EVENING WITH THE LOW SURFACE DEW POINTS AND
MODERATE TO HEAVY SHOWERS. DUE TO THE HIGHLY CONVECTIVE ENVIRONMENT
HEAVIER SHOWERS MAY PRODUCE SMALL HAIL OR SNOW PELLETS.

MODELS HAVE REMAINED CONSISTENT THE LAST 24-36 HOURS WITH
DEVELOPING A FRONT FROM PLUMAS COUNTY TO RENO-SPARKS TO CHURCHILL
COUNTY TONIGHT. ALONG THE FRONT AN IMPRESSIVE SNOW BAND AND/OR
TRAINING MODERATE TO HEAVY SNOW SHOWERS SHOULD DEVELOP DUE STRONG
LIFT IN THE DENDRITIC GROWTH ZONE...FRONTOGENESIS...AND GOOD
INSTABILITY FROM THE WAVE. SNOW FALL RATES OF 1-2 INCHES PER HOUR
ARE POSSIBLE WITH THE HEAVIER SHOWERS.

MAIN ISSUE WITH THE OPERATIONAL MODELS IS THEIR HIGH QPF...BOTH
GFS AND NAM PRODUCE WIDESPREAD AMOUNTS OVER 0.50 INCHES ALONG THE
FRONTAL BAND THROUGH SUNDAY MORNING. USING THE SREF AS A
COMPARISON...THE MAX QPF OUTPUT IS BELOW 0.50 INCHES...WITH A MEAN
IN THE 0.15-0.25 INCH RANGE. THINKING IS 0.15-0.25 INCHES OF
LIQUID IS REASONABLE...BUT A GOOD CHANCE FOR ISOLATED AMOUNTS UP
NEAR 0.50. THIS EQUATES TO UP TO 4 INCHES IS POSSIBLE FOR THE
SIERRA AND WESTERN NEVADA VALLEY FLOORS WITH ISOLATED AMOUNTS UP
TO 8 INCHES ESPECIALLY ON THE NORTHERN SIERRA PEAKS.

FOR SUNDAY STILL SOME QUESTION IN THE MODELS HOW QUICKLY THE
TROUGH/FRONT WILL PASS SOUTH AND EAST OF THE AREA AND IF
ADDITIONAL MOISTURE/ENERGY WILL UNDERCUT THE BLOCKING RIDGE ALONG
THE WEST COAST. 12Z GFS AND 18Z NAM...BRING ALMOST NO ADDITIONAL
MOISTURE AND ENERGY INTO THE AREA AND ALLOW THE TROUGH TO PASS
SOUTH OF THE AREA SUNDAY AFTERNOON. THIS COULD BRING A QUICK END
TO THE HEAVY SHOWERS SUNDAY MORNING...WITH ISOLATED TO SCATTERED
LIGHT SHOWERS INTO SUNDAY NIGHT.

FOR MONDAY THROUGH WEDNESDAY MORNING...MOISTURE WITHIN THE TROUGH
WILL KEEP SNOW SHOWERS LINGERING ACROSS THE SIERRA MAINLY SOUTH OF
US-50 MONDAY MORNING...AS WELL AS THE REST OF MONO AND PORTIONS OF
MINERAL COUNTY. TEMPERATURES WILL BE 5 TO 10 DEGREES BELOW NORMAL
IN THE WAKE OF THE TROUGH. AS THIS WAVE EXITS...A SHORTWAVE RIDGE
BUILDS INTO THE WEST MONDAY NIGHT INTO TUESDAY FOR A PERIOD OF
DRYING...THOUGH TEMPS WILL REMAIN ON THE COOL SIDE. THE DRYING
WILL BE SHORT LIVED AS ANOTHER MOIST WAVE IS PROGGED TO PUSH INTO
THE WEST TUESDAY NIGHT INTO WEDNESDAY. BRONG/DF

.LONG TERM...WEDNESDAY THROUGH SATURDAY...
PATTERN CONTINUES TO BE PROGRESSIVE THROUGH THIS PERIOD WITH ANOTHER
SYSTEM AFFECTING THE WEST COAST AND WESTERN U.S. LATE THURSDAY
THROUGH LATE FRIDAY OR SO. OVERALL TREND OF THE MODELS ARE IN DECENT
AGREEMENT BUT THERE ARE THE USUAL SYSTEM TIMING AND AMPLITUDE
DIFFERENCES.

WEDNESDAY THE UPPER FLOW IS ZONAL ACROSS THE EASTERN PACIFIC INTO
THE WEST COAST UNDER A DEEP UPPER LOW IN THE GULF OF ALASKA. THERE
IS ENOUGH OF A MOISTURE TAP TO KEEP LIGHT OROGRAPHIC SNOW GOING IN
THE SIERRA BUT WOULD NOT EXPECT MUCH SPILLOVER EXCEPT FOR SOME LIGHT
SHOWERS IN THE FOOTHILLS.

FOR THURSDAY ANOTHER UPPER LOW SWINGS TO NEAR 45N/140W WITH THE
TROUGH AXIS APPROACHING 130W. THIS PUMPS A RIDGE UP ACROSS THE
WESTERN U.S SO FOR OUR AREA WOULD EXPECT JUST SOME LINGERING SHOWERS
IN THE SIERRA. BY THURSDAY NIGHT THE TROUGH MOVES TO THE WEST COAST
AND THE LATEST EC/GFS RUNS BRING A GOOD SURGE OF MOISTURE INTO THE
SIERRA WITH SOME SPILLOVER POSSIBLE INTO WESTERN NV VALLEYS. THE GFS
IS MORE STRONGLY NEGATIVELY TILTED AND BEGINS TO SHOW SIGNS OF A
SPLIT WHICH HAS BEEN TYPICAL WITH SYSTEMS MOVING INTO THE WEST COAST
FOR THE PAST SEVERAL WEEKS.

FRIDAY AND FRIDAY NIGHT THE UPPER LOW IS STILL PARKED OFF OF THE
PACNW COAST BUT MODELS SHOW A DEFINITIVE SPLIT WITH THE BEST ENERGY
AND DYNAMICS SOUTH OF ROUGHLY 35N INTO SOUTHERN CA. THE GFS IS
FASTER IN MOVING THE TROUGH EAST BUT THERE IS CURRENTLY LITTLE
ENSEMBLE SUPPORT FOR THIS SOLUTION. AT ANY RATE EXPECT SNOW LEVELS
TO DROP ON FRIDAY WITH SNOW LIKELY FRIDAY AND FRIDAY NIGHT AND JUST
LINGERING SIERRA SNOW SHOWERS SATURDAY.

DETERMINISTIC MODELS AND GEFS SUPPORT LIKELY OR CATEGORICAL POPS IN
THE SIERRA WITH BEST CHANCES OF SPILLOVER THURSDAY NIGHT THROUGH
FRIDAY NIGHT SO INCREASED POPS THOSE PERIODS. GEFS ALSO POINTS TO
LINGERING SIERRA SNOW SHOWERS WEDNESDAY INTO WED NIGHT BEFORE THE
SHORTWAVE RIDGE BUILDS IN BRIEFLY THURSDAY. FOR SATURDAY AND
SATURDAY NIGHT KEPT CURRENT MENTION OF CHANCE POPS WITH NO CHANGE TO
PREVIOUS FORECAST DUE TO INCREASED UNCERTAINTY. EXPECT TEMPERATURES
TO BE ABOVE NORMAL FOR WED AND THUR AND BELOW NORMAL FRIDAY INTO THE
WEEKEND AS UPPER TROUGH MOVES THROUGH REGION. JAH

&&

.AVIATION...
UNSTABLE AIR MASS THIS AFTERNOON AND OVERNIGHT AS EVIDENCED BY
MODERATE CU ACROSS THE AREA. SHOWERS ARE CURRENTLY SPREADING INTO
THE SIERRA AND SPILLING OVER INTO THE FOOTHILLS. AN EAST-WEST
ORIENTED UPPER TROUGH WILL SWING ACROSS NORTHERN NEVADA
TONIGHT...INTENSIFYING A COLD FRONT THAT WILL BE POSITIONED FROM
ABOUT PORTOLA-KRNO-KNFL BY 12Z. THIS FRONT WILL FOCUS A RATHER
NARROW NW-SE ORIENTED BAND OF MODERATE TO HEAVY SNOW FROM ABOUT
05Z-13Z ACROSS PARTS OF WESTERN NEVADA AND NORTHEAST CA...AND IS
EXPECTED TO AFFECT KRNO AND KTRK. EXPECT ANY GIVEN LOCATION TO GET A
PERIOD OF MAYBE 3-4 HOURS OF HEAVY SNOW.

KRNO...A PERIOD OF MODERATE TO HEAVY SNOW IS LIKELY TONIGHT
BEGINNING AS EARLY AS 04Z AND TAPERING OFF BY 14Z. THE BEST WINDOW
OF HEAVY SNOW LOOKS TO BE AROUND 07Z-12Z. ACCUMULATION OF 2 INCHES
IS LIKELY WITH UP TO 4 INCHES POSSIBLE. IF BAND OF SNOW BECOMES
CONVECTIVE THEN THERE COULD BE A BRIEF PERIOD OF HEAVY SNOW WITH
RATES UP TO AN INCH PER HOUR ALONG WITH SOME SNOW PELLETS. MVFR
CONDITIONS WITH PERIODS OF IFR CONDITIONS ARE EXPECTED WITH EVEN
BRIEF LIFR CONDITIONS POSSIBLE.

KTRK...SNOW TONIGHT FROM 03Z-15Z TOMORROW. MVFR CONDITIONS WILL
LOWER TO IFR WITH POSSIBLE LIFR CONDITIONS IN SNOW BY 06Z WITH
CONDITIONS IMPROVING AFTER 15Z. EXPECT 4 INCHES ACCUMULATION WITH UP
TO 6 INCHES POSSIBLE.

KTVL...A PERIOD OF MVFR CONDITIONS IN LIGHT SNOW AS HEAVIER SNOW
SHOULD STAY NORTH OF TERMINAL. KTVL COULD STILL PICK UP AN INCH OR 2
OF SNOW MAINLY 03Z-12Z.  JAH

&&

.REV WATCHES/WARNINGS/ADVISORIES...
NV...WINTER WEATHER ADVISORY FROM 10 PM THIS EVENING TO 10 AM PST
     SUNDAY NVZ003 AND EASTERN NVZ004.

CA...WINTER WEATHER ADVISORY FROM 10 PM THIS EVENING TO 10 AM PST
     SUNDAY FOR SOUTHERN CAZ071.

&&

HTTP://WEATHER.GOV/RENO