FXUS65 KVEF 220454
AFDVEF

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LAS VEGAS NV
854 PM PST MON DEC 21 2009

.SYNOPSIS...A LOW PRESSURE SYSTEM OVER WESTERN WASHINGTON STATE WILL 
QUICKLY DROP SOUTHEAST OVERNIGHT AND TUESDAY INTO THE GREAT BASIN. 
THE SYSTEM WILL QUICKLY MOVE ACROSS THE AREA AND BRING A CHANCE FOR 
VALLEY RAIN AND MOUNTAIN SNOWS ALONG WITH STRONG WINDS AND COOLER 
TEMPERATURES. LOCALLY WINDY CONDITIONS WILL PERSIST INTO WEDNESDAY 
BEFORE TRANQUIL CONDITIONS PREVAIL THURSDAY AND CHRISTMAS.
&&

.UPDATE...TROUGH COMPLEX ALONG THE WEST COAST TO RAPIDLY SHIFT 
SOUTHEAST DURING THE NEXT 24-36 HOURS WITH THE LOW CENTERED TO THE 
EAST OF THE CWA BY 18Z WEDNESDAY. FROM THE PREVIOUS DISCUSSION... 
WINDS WILL CONTINUE TO BE THE MAIN CONCERN AS THIS SYSTEM CROSSES 
OVER THE AREA...HOWEVER A BRIEF PERIOD OF HEAVY SNOW IS POSSIBLE 
MAINLY OVER THE NORTHERN PORTION OF LINCOLN COUNTY FROM LATE TONIGHT 
THROUGH EARLY TUESDAY AFTERNOON. HAVE UPDATED GRIDS/ZONES EARLIER TO 
ADD A WINTER WEATHER ADVISORY FOR THIS AREA FOR TUESDAY. CURRENT 
HIGH WIND WATCH FOR THE SPRING MOUNTAINS FOR TONIGHT WILL REMAIN IN 
EFFECT AS DOWNSLOPE WINDS STILL POSSIBLE...WHILE WIND ADVISORIES FOR 
TUESDAY LOOK GOOD. WILL NEED TO CLOSELY MONITOR THE ADVISORIES 
HOWEVER AS LATEST NAM/GFS GUIDANCE POINT TO ISOLATED GUSTS REACHING 
HIGH WIND CRITERIA TUESDAY AFTERNOON ACROSS WESTERN SAN BERNARDINO 
COUNTY AND ALSO ESMERALDA/CENTRAL NYE COUNTIES. OVERALL TREND OF THE 
SHORT TERM IS ON TRACK.  DO NOT PLAN ANY FURTHER UPDATES TONIGHT.
&&

.PREV DISCUSSION...352 PM PST MON SHORT TERM...MODELS IN GOOD 
AGREEMENT IN THE SHORT TERM WITH THE LOW PROGGED OVER GREAT BASIN 
TUESDAY AFTERNOON. MAIN CONCERN WILL BE THE WINDS...BOTH PRE-FRONTAL 
WEST WINDS IN PRIMARILY WESTERN SAN BERNARDINO COUNTY AND THE POST 
FRONTAL WINDS IN NYE AND ESMERALDA COUNTIES. MODEL OUTPUT HAS LOW 
LEVEL WINDS IN THE 40 KNOT RANGE IN THESE AREAS AND HAVE GONE AHEAD 
AND ISSUED WIND ADVISORIES FOR THOSE AREAS FOR TONIGHT AND TUESDAY 
RESPECTIVELY. HAVE LEFT THE HIGH WIND WATCH IN EFFECT FOR THE SPRING 
MOUNTAINS OF CLARK COUNTY AS THERE IS STILL THE POTENTIAL FOR DOWN 
SLOPE WINDS THERE TONIGHT BUT IT IS NOT CLEAR CUT YET ON LOCAL MODEL 
RUNS AND WILL LET SWING SHIFT LOOK AT LATER GUIDANCE. IT IS LIKELY A 
WIND ADVISORY WILL BE ISSUED LATER FOR THE SPRING MOUNTAINS.

PRECIPITATION CHANCES ARE GREATEST OVER MOSTLY HIGHER TERRAIN 
BEGINNING TONIGHT STARTING FROM THE NORTHWEST TO THE SOUTHEAST AS 
TUESDAY PROGRESSES. MOISTURE IS EXPECTED TO BE MOSTLY ON THE EAST 
SIDE OF THE SYSTEM AND LIMITED. DYNAMIC LIFT WILL BE STRONG ENOUGH 
TO OVER COME THE LIMITED AMOUNT THOUGH. IT IS NOT OUT OF THE 
QUESTION TO SEE BRIEF FLURRIES IN THE HIGHER ELEVATIONS OF THE LAS 
VEGAS VALLEY AS THE SURFACE FRONT COMES THROUGH ON TUESDAY. 

QUICK CLEARING FROM WEST TO EAST WILL OCCUR WEDNESDAY ALONG WITH 
BREEZY NORTHWEST TO NORTH WINDS...ESPECIALLY IN NORTH-SOUTH 
ORIENTATED VALLEYS SUCH AS THE COLORADO RIVER VALLEY AS HIGH 
PRESSURE BUILDS IN ALONG WITH TIGHT PRESSURE GRADIENTS ACROSS 
SOUTHERN NEVADA.

.LONG TERM...CYCLONIC FLOW WILL STILL BE PRESENT ACROSS THE AREA ON 
CHRISTMAS EVE SO IN THE AIR THERE SHOULD BE A FEELING OF CHRISTMAS 
IN THE FORM OF A WINTRY CHILL. NORTH-SOUTH SURFACE PRESSURE 
GRADIENTS WILL BEGIN TO WEAKEN BUT NOT ENOUGH TO TOTALLY RID THE 
COLORADO RIVER VALLEY OF A GUSTY NORTH BREEZE AT 15-20 MPH. DRY 
NORTHERLY FLOW WILL KEEP JUST ABOUT ANY SIGNIFICANT CLOUDS OUT AND 
ALLOW SANTA AND COMPANY A MAINLY CLEAR SKY TO FLY IN ACROSS THE 
AREA. MODEL GUIDANCE HAS TRENDED DOWN WITH NUMBERS FOR CHRISTMAS DAY 
AND HAVE FOLLOWED IN THAT DIRECTION. ALTHOUGH CHRISTMAS DAY WILL 
DAWN CHILLY HIGHS WILL WARM A LITTLE OVER THURSDAY BUT NOT ALL THAT 
MUCH AS MIXING REMAINS WEAK.

AN UPPER RIDGE IS FORECAST TO BUILD INTO CALIFORNIA AND NEVADA 
CHRISTMAS DAY INTO SATURDAY WHICH WILL ALLOW FOR A LITTLE WARMING AS 
THE RIDGE REMAINS OVER THE SOUTHWEST U.S.. LIMITING WARMING ANY 
FURTHER THOUGH WILL BE A STEADY INCREASE IN HIGH CLOUDS THAT ADVANCE 
IN AHEAD OF THE NEXT PACIFIC SYSTEM. MIXING WILL BE WEAKER SATURDAY 
THEN INCREASE SUNDAY AS THE RIDGE DEPARTS...HOWEVER BY THEN CLOUDS 
WILL BE THICKER WHICH WILL OFFSET ANY SHOT A BIGGER JUMP UP IN 
TEMPS. OVERALL TEMPS WILL BE A HAIR BELOW NORMAL.

PRECIP-WISE 2009 HAS BEEN LIKE A LUMP OF COAL AND WE'LL HAVE A FEW 
CHANCES BEFORE THE YEAR ENDS TO TRY AND CATCH SOMETHING. ONE OF OUR 
FINAL OPPORTUNITIES WILL BE A SYSTEM THAT THE MODELS AGREE NOW WILL 
MAINLY PASS ALONG THE SOCAL COAST EARLY NEXT WEEK. IF ENOUGH 
MOISTURE CAN SPREAD FAR ENOUGH NORTH WE MAY GET SOMETHING OUT OF 
THIS BUT THE BIG QUESTION IS HOW MUCH. FOR NOW HAVE ONLY GONE WITH 
LOW END POPS AS CLOUDS LOOK TO BE A BETTER GUARANTEE. THE GFS 
ENSEMBLE MEAN SUPPORTS ROUGHLY 20%-30% POPS ACROSS THE HIGHER 
TERRAIN IN THE WESTERN CWFA FROM SUNDAY AFTERNOON THROUGH MONDAY 
AFTERNOON. BEYOND THAT THERE MAY BE ANOTHER SYSTEM IN THE WORKS FOR 
TUESDAY AND HAVE KEPT IN LOW END POPS THEN AS WELL UNTIL THE MODELS 
COME INTO BETTER AGREEMENT. THIS SYSTEM MAY FOLLOW THE TRACK OF THE 
EARLY WEEK SYSTEM SO WILL HAVE TO WATCH IT AS IT WILL BE GETTING 
INTO THE NEW YEAR'S EVE TIME FRAME. 
&&

.AVIATION...FOR MCCARRAN...SOUTH TO SOUTHWEST WINDS WILL DEVELOP 
THIS EVENING AND INCREASE OVERNIGHT AND ESPECIALLY AFTER 17Z TUESDAY 
WHEN SUSTAINED WINDS OF 15-20 KTS ARE EXPECTED WITH GUSTS TO AROUND 
30 KTS. WINDS WILL BECOME WESTERLY BETWEEN 20Z AND 22Z TUESDAY 
BEFORE BECOMING NORTHWEST BETWEEN 23Z TUESDAY AND 00Z WEDNESDAY. BKN 
CLOUDS AOA 16K FEET TONIGHT...THEN CLOUD BASES WILL LOWER TOMORROW 
DROPPING DOWN TO AT LEAST 5K FEET AROUND 17Z TUESDAY THROUGH 03Z 
WEDNESDAY AND A RAIN SHOWER COULD AFFECT THE AIRPORT COMPLEX THEN. 
SNOW LEVELS ARE FORECAST TO DROP TO AROUND 3500 FEET BY 00Z 
WEDNESDAY AND LOCAL MOUNTAINS WILL LIKELY BE OBSCURED.  

FOR THE REST OF SOUTHERN NEVADA...NORTHWEST ARIZONA...AND SOUTHEAST 
CALIFORNIA...WINDS WILL BEGIN TO INCREASE TONIGHT MAINLY AFTER 10Z 
AND NOTICEABLY INCREASE AFTER 16Z TUESDAY AS A COLD FRONT MOVES 
THROUGH THE AREA. GUSTS OF 25-45 KTS ARE LIKELY IN THE VALLEYS AND 
LOWER DESERT ELEVATIONS THEN AND MECHANICAL TURBULENCE WILL BE 
LIKELY. VSBY COULD DROP AS LOW AS 2SM-5SM IN AREAS OF BLDU...WILL 
CARY A TEMPO IN KDAG FOR THIS GIVEN THEY ARE ONE OF THE MORE PRONE 
SITES AND ALSO WILL SEE AT LEAST 35 KT GUSTS. OTHERWISE CLOUD BASES 
WILL LOWER TO 7K-12K FEET TONIGHT AND THEN DOWN TO AT LEAST 5K-10K 
FEET TUESDAY AFTERNOON. SCATTERED SHRA AND SHSN ARE EXPECTED 
ESPECIALLY IN THE HIGHER TERRAIN OF THE NORTH. MOUNTAIN OBSCURATIONS 
WILL BE LIKELY. GIVEN THE TRAJECTORY OF THE STORM WILL ONLY CARY A 
VCTY SHOWER AT KBIH...BUT IF PRECIP FALLS IT SHOULD COME DOWN AS 
SNOW WHICH WOULD RESULT IN MUCH LOWER VSBY HERE THEN CURRENTLY 
FORECAST. PRECIP SHOULD BE GONE FROM KDRA AND KVGT BY THE TIME SNOW 
LEVELS LOWER ENOUGH SO WILL KEEP PRECIP ALL RAIN AT THESE SITES. 
&&

.VEF WATCHES/WARNINGS/ADVISORIES...
NV...WIND ADVISORY FROM 10 AM TUESDAY TO MIDNIGHT PST TUESDAY NIGHT 
     FOR NVZ014-017.
     WINTER WEATHER ADVISORY FROM 10 PM THIS EVENING TO 4 PM PST 
     TUESDAY FOR NVZ015.
     HIGH WIND WATCH FROM MIDNIGHT PST TONIGHT THROUGH TUESDAY 
     MORNING FOR NVZ019.
AZ...NONE.
CA...WIND ADVISORY FROM 10 AM TUESDAY TO MIDNIGHT PST TUESDAY NIGHT 
     FOR CAZ519>522.
     WIND ADVISORY FROM 4 AM TUESDAY TO MIDNIGHT PST TUESDAY NIGHT 
     FOR CAZ523>526.
&&

$$

JENSEN/JACQUES/STACHELSKI

HTTP://WEATHER.GOV/LASVEGAS