FXUS65 KVEF 051231
AFDVEF

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LAS VEGAS NV
430 AM PST SAT DEC 5 2009

.SYNOPSIS...HIGH PRESSURE WILL FINALLY BREAK DOWN TODAY AND GIVE WAY 
TO A COLD LOW PRESSURE SYSTEM DROPPING OUT OF CANADA INTO THE GREAT 
BASIN. THIS WILL BE THE BEGINNING OF A SIGNIFICANT CHANGE TO THE 
WEATHER PATTERN WHICH WILL ALLOW A SERIES OF STRONG PACIFIC STORMS 
TO BRING VALLEY RAIN AND MOUNTAIN SNOW TO THE REGION THIS COMING 
WEEK. THE FIRST SYSTEM WILL IMPACT THE ENTIRE REGION MONDAY AND 
MONDAY NIGHT.
&&

.SHORT TERM...IR IMAGERY SHOWED DENSE CIRRUS COVERING MUCH OF THE 
REGION THIS MORNING AND THAT HAS MODERATED TEMPERATURES SOMEWHAT. 
HOWEVER...NEAR OR BELOW FREEZING TEMPERATURES ARE OCCURRING IN THE 
FREEZE WARNING AREA SO THE WARNING WILL REMAIN IN EFFECT UNTIL 9 AM. 
THE NEXT PROBLEM WILL BE INCREASING WINDS THIS AFTERNOON OVER 
NORTHERN SAN BERNARDINO COUNTY...MAINLY WEST OF BAKER. ALSO...
THE LATEST NMM/ARW CROSS-SECTIONS INDICATE THE POTENTIAL FOR 
MODERATE TO POSSIBLY STRONG DOWNSLOPE WINDS ON THE EASTERN SIERRA 
SLOPES AND OWENS VALLEY THIS AFTERNOON. A WIND ADVISORY IS IN EFFECT 
FOR BOTH AREAS. THESE WINDS WILL OCCUR PRECEDING THE COLD FRONT 
PUSHING DOWN INTO THE SOUTHERN GREAT BASING THIS AFTERNOON AND 
EVENING. THIS WILL ALSO BRING BRIEF RAIN AND SNOW SHOWERS OVER 
LINCOLN COUNTY THIS EVENING AS THE FRONT SWEEPS ACROSS. SUNDAY WILL 
SEE A BRIEF REPRIEVE BEFORE THE NEXT AND MOST SIGNIFICANT WEATHER 
PROBLEM DEVELOPS MAINLY MONDAY AND MONDAY NIGHT. A SECOND LOW 
RETROGRADING AS IT DROPS OUT OF WESTERN CANADA IS FORECAST TO CLOSE 
OFF OVER NORTHERN CALIFORNIA SUNDAY. THE RESULTING WEST TO 
SOUTHWEST FLOW WILL TAP SUBTROPICAL MOISTURE OVER THE EASTERN 
PACIFIC AND PULL IT INLAND MONDAY AND MONDAY EVENING FOR WHAT 
APPEARS TO BE A WIDESPREAD PRECIPITATION EVENT...WITH RAPIDLY 
LOWERING SNOW LEVELS MONDAY NIGHT. IT LOOKS LIKE SNOW LEVELS WILL 
GENERALLY BE ABOVE 4000 FEET MONDAY AFTERNOON AND EVENING WHEN THE 
BEST COMBINATION OF MOISTURE AND UPWARD FORCING OCCUR. THE COLD CORE 
THEN SWINGS DOWN THROUGH SOUTHERN NEVADA MONDAY EVENING ON THE HEELS 
OF THE DEEPER MOISTURE. HOWEVER...THIS COULD PRODUCE A BRIEF PERIOD 
OF SNOW FOR THE WESTERN FOOTHILLS OF THE LAS VEGAS VALLEY LATE 
MONDAY EVENING. IT LOOKS FAVORABLE FOR SIGNIFICANT ACCUMULATIONS IN 
THE SOUTHERN SIERRA AND ALSO THE SPRING MOUNTAINS/SHEEP RANGE.
&&

.LONG TERM...A VERY ACTIVE PATTERN WILL REMAIN IN PLACE ACROSS THE 
SOUTHWESTERN UNITED STATES THROUGH THE EXTENDED. GIVEN THIS IT WOULD 
NOT BE SURPRISING IF SOME AREAS SAW MORE PRECIPITATION NEXT WEEK 
THAN IN THE LAST 11 MONTHS COMBINED. 

BY LATE TUESDAY MORNING THE AFOREMENTIONED SYSTEM WILL BEGIN TO HEAD 
EASTWARD AND NORTHWEST FLOW ALOFT SHOULD DRY OUT THE AREA BY THE 
AFTERNOON. I LOWERED POPS IN THE WESTERN HALF OF THE AREA FOR THEN 
WITH A SHARP INCREASE AS ONE HEADS EASTWARD. THE MAIN ISSUE WILL BE 
THE LOW SNOW LEVELS WHICH COMBINED WITH LINGERING MOISTURE COULD 
ALLOW ANY PRECIP LEFT TO MIX WITH OR FALL AS SNOW BEFORE ENDING 
ESPECIALLY ABOVE 3500 FEET. BOTH SURFACE PRESSURE GRADIENTS AND 
UPPER SUPPORT LACK ON THE BACK SIDE OF THIS SYSTEM BY TUESDAY 
AFTERNOON TO SUPPORT ANY NOTABLE GUSTINESS TO WINDS. HIGH TEMPS WILL 
BE ABOUT 12 DEGREES BELOW NORMAL FOR THIS TIME OF YEAR OR SIMPLY 
STATED - BRRR!

TUESDAY NIGHT WILL BE COLD BUT SOME MODERATION WILL TAKE PLACE ON 
WEDNESDAY AS A SHORTWAVE RIDGE BUILDS OVER THE AREA. HOWEVER WARMING 
WILL BE HAMPERED BY WEAK MIXING AND CLOUDS ADVANCING IN AHEAD OF THE 
NEXT SYSTEM. THE ECMWF/GFS AND GEM ALL DIFFER ON THE START TIME OF 
THIS EVENT WITH PRECIPITATION POTENTIALLY MOVING INTO THE AREA AS 
EARLY AS WEDNESDAY MORNING. FOR NOW A BLEND OF THE START TIMES WAS 
USED BUT GIVEN THE CONSIDERABLE MODEL AND ENSEMBLE SUPPORT FOR A 
SYSTEM TO MOVE THROUGH IN THE WEDNESDAY NIGHT INTO THURSDAY 
AFTERNOON TIME FRAME POPS WERE BUMPED UP BIG TIME THEN. CROSS 
SECTIONS SHOW DECENT SPILLOVER OVER THE SIERRA SO DO NOT EXPECT RAIN 
SHADOWING TO PLAY A BIG ROLE GIVEN THE TRAJECTORY OF THE SYSTEM. 
THIS SHOULD BE AN AREA-WIDE EVENT THOUGH A LITTLE TOO EARLY TO 
SPECULATE ON JUST HOW MUCH PRECIP FALLS. SNOW LEVELS WILL RISE WITH 
WARM ADVECTION TO AROUND 6000 FEET WEDNESDAY NIGHT AND THEN TO 
AROUND 6000 FEET IN LINCOLN COUNTY AND 7000 FEET ELSEWHERE BY 
THURSDAY AFTERNOON BEFORE FALLING THURSDAY NIGHT TO AROUND 5500 FEET 
AS THE SYSTEM MOVES INLAND.

LINGERING MOISTURE AND ENERGY RIPPLING THROUGH THE FLOW MAY SET OFF 
SOME LIGHT PRECIPITATION FOR FRIDAY. BY SATURDAY BOTH THE ECMWF AND 
OPERATIONAL GFS BUILD ENOUGH OF A RIDGE INTO THE SOUTHWEST U.S. TO 
ALLOW US TO CATCH A BREAK AND DRY OUT AND WARM UP TO AROUND NORMAL. 
BY NEXT SUNDAY ANOTHER SYSTEM MAY HEAD IN FROM THE PACIFIC ACROSS 
CALIFORNIA. HOWEVER MODELS WERE ALL OVER THE PLACE WITH THIS. ONE 
INTERESTING THING WAS THE 00Z GFS ENSEMBLE MEAN FOR A TENTH OF AN 
INCH OF PRECIPITATION WAS AT LEAST 80 PERCENT OVER THE ENTIRE CWFA 
AT 192 HOURS...HOWEVER THE OPERATIONAL GFS AND ECMWF WERE NOT AS 
WET. FOR NOW JUST WENT WITH SLIGHT CHANCE TO GHOST POPS. WE CAN THEN 
ADJUST AS CONFIDENCE IN THE TRACK BECOMES BETTER. 
&&

.AVIATION...FOR MCCARRAN...WINDS WILL BE LIGHT THROUGH TONIGHT. 
DIRECTION WISE CARRIED VRB IN THE TAF THROUGH 05Z SUNDAY...HOWEVER 
COMPONENT MOST LIKELY SHOULD BE EAST TO SOUTHEAST THIS 
AFTERNOON...THEN GOING SOUTHWEST EARLY THIS EVENING WITH A NORTHWEST 
SHIFT SOMETIME AROUND 05Z SUNDAY. BKN-OVC AOA 25K FEET THIS 
MORNING...THEN FEW-SCT BETWEEN 8K-12K FEET TONIGHT. A STORM SYSTEM 
IS EXPECTED TO AFFECT THE AREA MONDAY AFTERNOON INTO TUESDAY MORNING 
WITH CEILINGS BELOW 5K FEET LIKELY AS WELL AS RAIN. THE 
PRECIPITATION MAY BRIEFLY MIX WITH SNOW ESPECIALLY TOWARDS THE WEST 
SIDE OF THE VALLEY BEFORE ENDING TUESDAY MORNING BUT NO ACCUMULATION 
IS EXPECTED IN THE VALLEY.  

FOR THE REST OF SOUTHERN NEVADA...NORTHWEST ARIZONA...AND SOUTHEAST 
CALIFORNIA...LIGHT WINDS TO START ACROSS THE AREA HOWEVER WINDS 
SHOULD INCREASE THIS AFTERNOON IN THE SIERRA...OWENS VALLEY AND 
AROUND KDAG AS A SYSTEM APPROACHES THE AREA WITH GUSTS OVER 25 KTS 
ESPECIALLY IN THE EASTERN SIERRA. THESE GUSTY WINDS WILL LESSEN BY 
LATER TONIGHT. OTHERWISE NO SIGNIFICANT WINDS ARE EXPECTED ELSEWHERE 
THROUGH TONIGHT. BKN-OVC CLOUDS AOA 25K FEET THIS MORNING...THEN 
FEW-SCT AROUND 8K-12K FEET THIS EVENING. A STORM SYSTEM WILL AFFECT 
THE AREA MONDAY AFTERNOON INTO TUESDAY MORNING WITH LOW 
CEILINGS...REDUCED VSBY AND PRECIPITATION. SNOW LEVELS ARE EXPECTED 
TO DROP TO AROUND 3500 FEET BY LATE MONDAY NIGHT. 
&&

.VEF WATCHES/WARNINGS/ADVISORIES...
NV...FREEZE WARNING UNTIL 9 AM PST THIS MORNING FOR NVZ016-020>022.
AZ...FREEZE WARNING UNTIL 10 AM MST THIS MORNING FOR AZZ002-036.
CA...FREEZE WARNING UNTIL 9 AM PST THIS MORNING FOR CAZ523>527.
     WIND ADVISORY FROM NOON TODAY TO 6 PM PST THIS EVENING FOR 
     CAZ519-520.
     WIND ADVISORY FROM NOON TODAY TO 10 PM PST THIS EVENING FOR 
     CAZ523-524.
&&

$$

ADAIR/STACHELSKI

HTTP://WEATHER.GOV/LASVEGAS