Area Forecast Discussion
Issued by NWS San Diego, CA

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000
FXUS66 KSGX 280456
AFDSGX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SAN DIEGO CA
900 PM PST SAT DEC 27 2014

.SYNOPSIS...
COLD WEATHER WILL CONTINUE TONIGHT WITH FROST IN MANY VALLEY AND
LOWER DESERT LOCATIONS. DAYTIME TEMPERATURES WILL CONTINUE A LITTLE
BELOW NORMAL THROUGH MONDAY...THOUGH SUNDAY NIGHT WILL BE MILDER DUE
MOSTLY TO INCREASING CLOUDS. A STORM SYSTEM MOVING SOUTH ALL THE WAY
FROM CANADA WILL BRING PRECIPITATION MAINLY LATE TUESDAY THROUGH
WEDNESDAY NIGHT TO THE REGION...WITH A FEW LINGERING SHOWERS
THURSDAY. SNOW LEVELS WILL BE VERY LOW...POSSIBLY 2000 TO 2500 FEET
ABOVE SEA LEVEL AT TIMES...AND 6 INCHES OF SNOW...POSSIBLY
MORE...COULD FALL IN THE MOUNTAINS. A THUNDERSTORM IS EVEN POSSIBLE
LATE TUESDAY NIGHT OR WEDNESDAY DUE TO INSTABILITY WITH THE COLD AIR
ALOFT. FAIR WEATHER WILL RETURN BY FRIDAY UNDER NORTHWEST FLOW
ALOFT...WITH TEMPERATURES REMAINING A LITTLE BELOW SEASONAL NORMALS.

&&

.DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE...
SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO
COUNTIES...

A FEW HIGH CLOUDS WERE MOVING THROUGH...WITH OTHERWISE CLEAR
CONDITIONS AND LIGHT WINDS OCCURRING. IT WAS ALREADY QUITE COLD AT 8
PM IN SOME LOW LYING VALLEY LOCATIONS...SUCH AS 38 AT RIVERSIDE
MARCH AND 36 AT THERMAL. WITH LESS WIND...THE NEAR OR SLIGHTLY BELOW
FREEZING TEMPS WILL BE MORE WIDESPREAD TONIGHT...AND THE HIGH CLOUDS
WILL ONLY VERY SLIGHTLY MODIFY THEM. FROST ADVISORIES ARE IN EFFECT
FOR THE VALLEYS AND LOWER DESERTS FOR TONIGHT. THE HIGH DESERTS HAVE
ALREADY HAD A COUPLE LIGHT FREEZES THIS SEASON...SO NO ADVISORY IS
IN EFFECT THERE. AN INCREASE IN HIGH CLOUDS AND A SOMEWHAT WARMER
PROFILE WILL RESULT IN SUNDAY NIGHT BEING MILDER...MOSTLY IN THE MID
30S TO MID 40S AT LOWER ELEVATIONS.

MODELS HAVE BEEN PRETTY CONSISTENT WITH BRINGING A BROAD CANADIAN
TROUGH SOUTH INTO THE NORTHERN AND CENTRAL UNITED STATES
SUNDAY/MONDAY AND COVERING MOST OF THE US BY TUESDAY. AN ENE-WSW
ORIENTED TROUGH AXIS WILL END FROM THE HUDSON BAY TO LAKE SUPERIOR
TO WYOMING TO CENTRAL CALIFORNIA TUESDAY...A SOMEWHAT ODD
CONFIGURATION...WITH A LOW CLOSING OFF OVER THE SAN JOAQUIN VALLEY.
THE CLOSED LOW WILL TRACK SOUTHEAST AND MOVE INTO INLAND PARTS OF
SOUTHERN CALIFORNIA BY WEDNESDAY AND BECOME DISCONNECTED FROM THE
BROAD TROUGH WHICH WILL SHIFT TO THE EASTERN US. AHEAD OF THE
LOW...ONSHORE FLOW WILL DEVELOP AND BECOME RATHER STRONG WITH A
RAPID INCREASE IN LOW-LEVEL MOISTURE TUESDAY. SHOWERS WILL BEGIN
SOMETIME TUESDAY...MOST LIKELY MIDDAY AND GRADUALLY INCREASE IN THE
AFTERNOON. STRONG GUSTY WESTERLY WINDS WILL OCCUR IN THE MOUNTAINS
DUE TO THE TIGHTENING PRESSURE/TEMP GRADIENTS AND SOME MOUNTAIN WAVE
POTENTIAL. THE COLD AIR WILL ARRIVE LATE TUESDAY...WITH 850 MB TEMPS
FALLING TO -1 TO -4 DEG C TUESDAY NIGHT...AND THAT COMBINED WITH THE
SHOWERY NATURE OF THE PRECIP RESULTING IN SOME EVAPORATIVE COOLING
COULD RESULT IN SOME OCCASIONAL SNOW LEVELS BELOW 3000 FEET
MSL...LOCALLY AROUND 2000 FEET MSL IN SAN BERNARDINO COUNTY WHERE
THE COLDEST AIR WILL BE. THE HIGH DESERTS OF SAN BERNARDINO COUNTY
SHOULD RECEIVE SOME SNOW...ESPECIALLY AS LOW-LEVEL WINDS TURN
NORTH/NORTHEAST BEHIND THE LOW FOR UPSLOPING THERE. WHILE MOISTURE
WILL NOT BE GREAT...ONLY EXTENDING TO 700 MB AT MOST...THE
INSTABILITY WILL HELP BRING LOCALLY HEAVY SHOWERS AND POSSIBLY EVEN
A FEW THUNDERSTORMS TUESDAY NIGHT/WED AS 500 MB TEMPS FALL BELOW -25
DEG C. PRECIP AMOUNTS OVERALL SHOULD BE 1/4 TO 1/2 INCH AT LOWER
ELEVATIONS AND A LITTLE OVER 1/2 INCH IN THE MOUNTAINS...WHICH WILL
TRANSLATE TO 6 INCHES OR MORE OF SNOW. THE COLDER AIRMASS ASSOCIATED
WITH THE CLOSED LOW WILL MEAN THAT SNOW-TO-LIQUID RATIOS WILL BE
HIGHER THAN NORMAL...BETWEEN 10-1 AND 12-1. WHILE THE UPPER LOW WILL
REMAIN JUST TO THE EAST WED NIGHT/THU...NORTH FLOW ALOFT ON THE BACK
SIDE OF THE LOW WILL BRING IN SOMEWHAT DRIER AIR AND REDUCE THE
CHANCE OF PRECIPITATION THEN. SOME LOCAL GUSTY NORTH WINDS COULD
OCCUR THROUGH THE CAJON PASS AND OTHER N-S ORIENTED PASSES/CANYONS
WED NIGHT/THU...BUT AT THIS TIME THE WINDS DO NOT LOOK TOO STRONG AS
COLD AIR ADVECTION WILL BE LIMITED DUE TO THE LOCATION OF THE UPPER
LOW...AND THERE WILL BE TOO MUCH INSTABILITY FOR A GOOD MOUNTAIN
WAVE. TEMPS WILL BE WELL BELOW NORMAL WED/THU WITH MOSTLY 50S AT
LOWER ELVEATIONS...WITH THU NIGHT BEING A POTENTIAL FROST NIGHT IN
THE VALLEYS/LOWER DESERTS IF CLOUD COVER AND WIND SPEEDS ASSOCIATED
WITH THE UPPER LOW ARE BOTH DECREASED.

FAIR WEATHER SHOULD RETURN FRIDAY AS A RIDGE WILL DEVELOP OFF THE
COAST AND WITH THE UPPER LOW HAVING EXITED TO THE EAST...WITH SLOW
RECOVERIES IN THE TEMPS...POSSIBLY REACHING NEAR NORMAL BY AROUND
NEXT SUNDAY.

&&

.AVIATION...
280400Z...INCREASING HIGH CLOUDS TONIGHT BECOMING BKN-OVC200 SUNDAY
WITH LIGHT WINDS AND UNRESTRICTED VIS THROUGH 290000Z.

&&

.MARINE...
800 PM...GENERALLY LIGHT WINDS SUNDAY AND MONDAY. A WINTER STORM
WILL BRING STRONGER ONSHORE WINDS TUESDAY AFTERNOON THROUGH
WEDNESDAY ALONG WITH SCATTERED SHOWERS.

&&

.SKYWARN...
SKYWARN ACTIVATION IS NOT REQUESTED. HOWEVER WEATHER SPOTTERS ARE
ENCOURAGED TO REPORT SIGNIFICANT WEATHER CONDITIONS.

&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...FROST ADVISORY FROM MIDNIGHT TONIGHT TO 8 AM PST SUNDAY FOR THE
     SAN BERNARDINO AND RIVERSIDE COUNTY VALLEYS-THE INLAND
     EMPIRE...SAN DIEGO COUNTY VALLEYS...SAN DIEGO COUNTY
     DESERTS...COACHELLA VALLEY.

PZ...NONE.

&&

$$

PUBLIC...MAXWELL
AVIATION/MARINE...MOEDE





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