Winter Storm Watch 3, NWS Las Vegas, NV

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LAS VEGAS NV
820 PM PST SUN DEC 7 2014

.SYNOPSIS...A SYSTEM PASSING ACROSS CALIFORNIA AND NEVADA TONIGHT
WILL RESULT IN ABUNDANT HIGH LEVEL CLOUDS ACROSS THE AREA. LESS IN
THE WAY OF HIGH CLOUDS ARE EXPECTED FOR MONDAY IN THE WAKE OF THIS
SYSTEM. ANOTHER SYSTEM WILL APPROACH THE AREA ON THURSDAY AND MOVE
ACROSS THE AREA BETWEEN FRIDAY AND SATURDAY MORNING RESULTING IN
ANOTHER ROUND OF WIDESPREAD PRECIPITATION. THIS IS EXPECTED TO BE A
COOLER SYSTEM WITH SNOW LEVELS ARE EXPECTED TO BE AROUND 6000 FEET.
&&

.UPDATE...WIDESPREAD HIGH CLOUD COVER IS STREAMING OVERHEAD THIS
EVENING AS A SHORT WAVE PIVOTS INTO THE PACIFIC NORTHWEST. I UPDATED
THE SKY COVER GRIDS TO ACCOUNT FOR THIS AND BUMPED UP THE SKY
COVERAGE TO MOSTLY CLOUDY.  OTHERWISE THE FORECAST IS ON TRACK THIS
EVENING AND NO UPDATES ARE PLANNED.

&&

.PREVIOUS DISCUSSION....FROM 355 PM...

.SHORT TERM...TONIGHT THROUGH WEDNESDAY NIGHT.

A BAND OF HIGH LEVEL CLOUDS WILL TRANSITION EAST ACROSS THE AREA
TONIGHT AS A MID/UPPER LEVEL TROUGH AXIS LIFTS NORTHEAST ACROSS
CALIFORNIA AND NEVADA. HIGH CLOUDS WILL DECREASE IN COVERAGE
TOMORROW IN THE WAKE OF THIS FEATURE RESULTING IN MAINLY SUNNY
CONDITIONS. A STUBBORN INVERSION AROUND 900 MB WILL KEEP WINDS AT
THE SURFACE VERY LIGHT FOR MONDAY AND ALSO KEEP HIGH TEMPS CLOSE TO
WHAT WE SAW TODAY ACROSS THE AREA.

A SOUTHWEST TO WEST-SOUTHWEST FLOW ALOFT IS EXPECTED ACROSS THE AREA
FROM MONDAY NIGHT THROUGH WEDNESDAY NIGHT. THIS WILL KEEP US MILD,
EVEN FOR DECEMBER, WITH HIGH TEMPS AVERAGING 5-8 DEGREES ABOVE
NORMAL. THE INVERSION WILL CONTINUE TO HOLD FIRM BETWEEN 900 AND 950
MB RESULTING IN CONTINUED LIGHT WINDS IN THE LOWER ELEVATIONS. LOWS
TUESDAY MORNING AND WEDNESDAY MORNING MAY BE A TAD LOWER THAN WHAT
WE HAVE SEEN LATELY AS CLOUD COVER WILL BE LESS. HOWEVER, CIRRUS
WILL FILL BACK IN THROUGH THE DAY ON WEDNESDAY AS THE NEXT SYSTEM
STARTS TO APPROACH FROM THE PACIFIC WHICH SHOULD SHAVE A DEGREE OR
TWO AT LEAST OFF HIGH TEMPS FROM WHAT WE SEE ON TUESDAY.

.LONG TERM...THURSDAY THROUGH SATURDAY.

THE MAIN EVENT THIS WEEK WILL BE ANOTHER MID/UPPER LEVEL TROUGH AND
ASSOCIATED COLD FRONT PRESSING ON THROUGH FROM THE PACIFIC. THIS
SYSTEM WILL BE A COOLER SYSTEM THAN THE LAST ONE WE SAW AND AS OF
NOW DOES NOT APPEAR AS MOIST WITH MODEL PWATS FROM THE ECMWF AND GFS
ONLY FORECAST TO REACH BETWEEN A HALF AND THREE-QUARTERS OF AN INCH.

BASED ON THE 12Z MODELS, THERE IS STILL A SPREAD IN TIMING WITH THE
OPERATIONAL GFS FASTEST, THE OPERATIONAL ECMWF SLOWEST AND THE GEM
AND MANY OF THE GFS ENSEMBLES IN THE MIDDLE. I OPTED TO USE THE
LATER AS A BLEND WITH SOME TILT TOWARD THE ECMWF FOR TIMING. WE KEPT
IN THE POSSIBILITY OF PRECIPITATION ENTERING THE FAR WESTERN CWFA
THURSDAY AFTERNOON BASED ON THE OPERATIONAL GFS, HOWEVER, IF THE
SLOWER ECMWF IS CORRECT, EVEN THESE AREAS STAY DRY THEN. THE MAIN
WINDOW FOR PRECIPITATION IS EXPECTED TO BE BETWEEN FRIDAY MORNING
AND AFTERNOON IN THE WESTERN HALF OF THE CWFA AND BETWEEN FRIDAY
EVENING AND SATURDAY MORNING FOR LINCOLN COUNTY, LAS VEGAS AND THE
COLORADO RIVER VALLEY AND POINTS EAST.

THE MAIN DIFFERENCE WITH THIS SYSTEM WILL BE THAT IT IS COOLER AND
THUS SNOW LEVELS WILL BE LOWER, DROPPING TO 5500-6000 FEET PER
PROGGED 700 MB TEMPS OFF THE GFS AND ECMWF OF -4C. EVAPORATIONAL
COOLING COULD ALSO DRIVE THEM SLIGHTLY LOWER AS WELL. SO, WE MAY
HAVE A SHOT TO SEE THE FIRST REAL ACCUMULATION OF SNOW OF THE SEASON
IN THE SPRING MOUNTAINS AS WELL AS AREAS IN OF LINCOLN COUNTY SUCH
AS HIGHWAY 93 NORTH OF PIOCHE BY FRIDAY NIGHT.

AHEAD OF THE SYSTEM ON THURSDAY, A 120 KT+ JETSTREAM AT 250 MB IS
SHOWN TO PASS OVER THE AREA. FORECAST SOUNDINGS KEEP AN INVERSION
AROUND OVER THE VALLEYS AROUND 900-950 MB, WHICH WILL INHIBIT THE
STRONGER WINDS ALOFT FROM MIXING DOWN. THE 9KM ARW DOES NOT SHOW A
WELL DEVELOPED CRITICAL LAYER OF ANY SORTS, BUT 500 MB HEIGHT PROGS
FROM THE MODELS SHOW A HYDRAULIC JUMP SIGNATURE TO THEM TO THE LEE
OF THE SOUTHERN SIERRA. WINDS TO 50 KTS ARE POSSIBLE DOWN TO AT
LEAST MID-SLOPE. WE MAY NEED SOME SORT OF WIND HEADLINES HERE FOR
THURSDAY AFTERNOON AND EVENING, HOWEVER, THE ARRIVAL OF
PRECIPITATION MAY CUT WINDS OFF EARLIER OR DELAY THEIR ONSET.

DEPENDING ON HOW SLOW THE SYSTEM IS TO EXIT, SHOWERS MAY LINGER INTO
SATURDAY ACROSS THE EASTERN HALF OF THE AREA. WE KEPT POPS IN TO
ACCOUNT FOR A SLOWER SOLUTION. OTHERWISE LOOK FOR A SLIGHT DROP IN
TEMPS, BUT REALLY TEMPS AT BEST MAY GET BACK CLOSER TO NORMAL AND
MAINLY FOR HIGHS. BREEZY TO LOCALLY WINDY CONDITIONS ARE POSSIBLE
SATURDAY INTO NEXT SUNDAY IN THE COLORADO RIVER VALLEY.

ONE SIDE NOTE IS THAT IT APPEARS LAS VEGAS HAS A LOCK ON BREAKING
THE RECORD FOR THE LEAST NUMBER OF DAYS IN A YEAR WITH A LOW
TEMPERATURE BELOW 40 DEGREES. THE CURRENT RECORD IS 28 DAYS SET IN
2010. SO FAR THIS YEAR THERE HAS ONLY BEEN 9 SUCH DAYS AND BASED ON
THE CURRENT FORECAST THE ODDS OF TYING OR EXCEEDING 28 DAYS THIS
YEAR ARE QUITE LOW.
&&

.AVIATION...FOR MCCARRAN...SCT-BKN CLOUDS AOA 25K FEET THROUGH
MONDAY EVENING. WINDS WILL BE 8 KTS OR LESS AND TREND TOWARD TYPICAL
DIURNAL DIRECTIONS.

FOR THE REST OF SOUTHERN NEVADA...NORTHWEST ARIZONA...AND SOUTHEAST
CALIFORNIA...SCT-BKN CLOUDS AOA 25K FEET THROUGH MONDAY EVENING.
WINDS WILL BE 10 KTS OR LESS AND TREND TOWARD TYPICAL DIURNAL
DIRECTIONS.
&&

.SPOTTER INFORMATION STATEMENT...SPOTTER ACTIVATION IS NOT EXPECTED
THROUGH WEDNESDAY NIGHT. SPOTTERS ARE ENCOURAGED TO REPORT ANY
SIGNIFICANT WEATHER CONDITIONS ACCORDING TO STANDARD OPERATING
PROCEDURES FROM THURSDAY THROUGH SATURDAY.
&&

UPDATE...OUTLER
PREVIOUS DISCUSSION...STACHELSKI

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