Freeze Warning 3, NWS Las Vegas, NV

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LAS VEGAS NV
403 AM PST THU DEC 3 2009

.SYNOPSIS...HIGH PRESSURE WILL KEEP DRY WEATHER IN PLACE...BUT
LOCALLY GUSTY NORTH WINDS WILL BRING SUBFREEZING TEMPERATURES TO
MUCH OF THE MOJAVE DESERT TONIGHT AND FRIDAY NIGHT. THE HIGH
PRESSURE RIDGE WILL BREAK DOWN GOING INTO THE WEEKEND...LEADING TO A
SIGNIFICANT PATTERN CHANGE WITH A SERIES OF STRONG PACIFIC STORMS
IMPACTING THE REGION LATE IN THE WEEKEND THROUGH NEXT WEEK.
&&

.UPDATE...AFTER CHECKING OUT A FEW MORE PIECES OF ENSEMBLE GUIDANCE
AND LOW TEMPS SO FAR THIS SEASON...HOISTED FREEZE WARNINGS FOR
NORTHERN SAN BERNARDINO COUNTY AND THE DESERTS OF CLARK COUNTY
TONIGHT...AND EXPANDED THE WARNING TO INCLUDE THE REST OF SAN
BERNARDINO COUNTY AND THE COLORADO RIVER VALLEY FRIDAY NIGHT.
WARNINGS ARE NOT NEEDED FOR PAHRUMP AND KINGMAN AREAS SINCE THEY
HAVE ALREADY HAD FREEZES.
&&

.PREV DISCUSSION...
313 AM PST THU DEC 3 2009

.SHORT TERM...SKIES WERE CLEAR AREAWIDE AND WINDS WERE GENERALLY
LIGHT AT 215 AM. MAIN CONCERN FOR THIS PACKAGE IS THE POSSIBILITY OF
FREEZING TEMPERATURES IN LAS VEGAS AND OTHER DESERT AREAS TONIGHT
AND FRIDAY NIGHT. LOCAL HIGH RESOLUTION MODELS SUGGEST WINDS WILL
STAY UP A BIT TONIGHT...WHICH WOULD BE A LIMITING FACTOR...BUT
FRIDAY NIGHT LOOKS COLDER THAN IT DID YESTERDAY. HIGH LEVEL MOISTURE
PROGS SUGGEST A THINNER CLOUD LAYER FRIDAY NIGHT THAN PREVIOUS RUNS
SHOWED...AND LOW LEVEL TEMPERATURE AND DEWPOINT FORECASTS KEEP
GETTING COLDER AND DRIER WITH EACH SUCCESSIVE RUN. HAVE NOT MADE A
FINAL CALL YET...BUT AM LEANING TOWARD ISSUING A FREEZE WARNING FOR
BOTH NIGHTS BEFORE THE END OF MY SHIFT. AREAS PROTECTED FROM NORTH
WINDS WOULD BE MOST AT RISK TONIGHT...AND A MORE WIDESPREAD FREEZE
WOULD BE LIKELY FRIDAY NIGHT. OTHER CONCERNS ARE NORTH WIND DOWN THE
COLORADO RIVER VALLEY TODAY...WHICH SHOULD STAY JUST BELOW LAKE WIND
ADVISORY AND RED FLAG WARNING CRITERIA IN MOST AREAS...AND POTENTIAL
FOR SNOW IN THE PANHANDLE OF LINCOLN COUNTY SATURDAY NIGHT. LOOKS
LIKE THAT SHOULD STAY BELOW ADVISORY CRITERIA AS WELL.

.LONG TERM...SUNDAY STARTS THE EXTENDED AND THE PATTERN IS POISED TO
BRING A BIG CHANGE FOR OUR AREA. THE UPPER LOW OVER THE PACIFIC
NORTHWEST IS PROGGED TO MOVE SOUTH AND THEN PIVOT ITS BASE EAST
ACROSS OUR CWA LATE MONDAY AND EARLY TUESDAY. MODELS DIFFER IN DEPTH
AND SPEED OF THE TROUGH WITH THE GFS DEEPER THAN THE ECMWF AND THE
GEM DEEPEST YET. ALL AGREE ON A DECENT AMOUNT OF MOISTURE BEING
BROUGHT IN AHEAD OF THE TROUGH. DID RAISE THE POPS FOR THE ENTIRE
AREA MONDAY AND TUESDAY AS CONFIDENCE IS HIGHER IN THEIR BEING
SUFFICIENT MOISTURE AND DYNAMICS. SOME UNCERTAINTY ON SNOW AMOUNTS
AND LEVELS ESPECIALLY FOR THE SIERRA HOWEVER THERE IS THE POTENTIAL
FOR ACCUMULATIONS MONDAY AND TUESDAY THAT MAY WARRANT A WATCH AS THE
EVENT GETS CLOSER. ANOTHER CONCERN WAS FOR TEMPERATURES AS THE
TROUGH PASSAGE IS FORECAST TO BRING A GOOD AIR PUSH. DID LOWER
TEMPERATURES TUESDAY AND WEDNESDAY. LOOKING BEYOND DAY 7 ANOTHER
MOIST SYSTEM IS PROGGED TO MOVE ONTO THE WEST COAST FOR ANOTHER
CHANCE FOR ACTIVE WEATHER AT WEEKS END.

.AVIATION...FOR MCCARRAN...LIGHT WINDS WILL CONTINUE THROUGH THE
MORNING BEFORE THE SWITCH TO NORTHEAST WINDS LATER. FOR NOW EXPECT
WIND SPEEDS TO BE IN THE 8-12 KNOT RANGE BUT MAY BE STRONGER DUE TO
TIGHTENING PRESSURE GRADIENTS. IN THE LATE AFTERNOON DIRECTIONS ARE
EXPECTED TO SWITCH OVER TO THE NORTH FOR THE REST OF THURSDAY. SKIES
ARE EXPECTED TO REMAIN SPARSE.

FOR THE REST OF SOUTHERN NEVADA...NORTHWEST ARIZONA...AND SOUTHEAST
CALIFORNIA...LIGHT DIURNAL WINDS EXPECTED FOR THE REST OF THIS
MORNING. IN THE AFTERNOON EXPECT INCREASED WIND SPEED FROM THE NORTH
WITH THE STRONGEST WINDS BEING GUSTY TO AROUND 25 KNOTS IN THE
COLORADO RIVER VALLEY. SKIES WILL BE ESSENTIALLY CLEAR THROUGH
FRIDAY MORNING.

.FIRE WEATHER...LOCALLY GUSTY NORTH WINDS...ESPECIALLY DOWN THE
COLORADO RIVER VALLEY...WILL USHER IN A VERY DRY AIRMASS TODAY.
MINIMUM RELATIVE HUMIDITY VALUES WILL DROP INTO THE LOW TEENS AND
SINGLE DIGITS TODAY AND FRIDAY. HOWEVER...WIND GUSTS ARE EXPECTED
TO GENERALLY REMAIN BELOW 35 MPH.
&&

.VEF WATCHES/WARNINGS/ADVISORIES...
NV...FREEZE WARNING FROM MIDNIGHT TONIGHT TO 8 AM PST FRIDAY FOR
     NVZ016-020-022.
     FREEZE WARNING FROM 10 PM FRIDAY TO 9 AM PST SATURDAY FOR
     NVZ016-020>022.
AZ...FREEZE WARNING FROM 11 PM FRIDAY TO 10 AM MST SATURDAY FOR
     AZZ002-036.
CA...FREEZE WARNING FROM MIDNIGHT TONIGHT TO 8 AM PST FRIDAY FOR
     CAZ523-524.
     FREEZE WARNING FROM 10 PM FRIDAY TO 9 AM PST SATURDAY FOR
     CAZ523>527.
&&

MORGAN/JACQUES

HTTP://WEATHER.GOV/LASVEGAS