Area Forecast Discussion
Issued by NWS Phoenix, AZ

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000
FXUS65 KPSR 281610
AFDPSR

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PHOENIX AZ
910 AM MST SUN DEC 28 2014

.SYNOPSIS...
BELOW NORMAL TEMPERATURES ARE EXPECTED THROUGH THE REST OF THIS
WEEKEND AND EARLY NEXT WEEK WITH OCCASIONAL HIGH CLOUDS. ANOTHER
COLD AND POSSIBLY WET LOW PRESSURE SYSTEM WILL MOVE INTO THE REGION
BY MIDWEEK...WITH THE POTENTIAL FOR DESERT RAIN AND MOUNTAIN SNOW.
DRY BUT COOL WEATHER IS EXPECTED TO RETURN BY NEXT WEEKEND.

&&

.DISCUSSION...
MID AND HIGH LEVEL THIN CLOUDS ARE MAKING AN APPEARANCE ACROSS
SOUTHERN NEVADA AND WESTERN ARIZONA THIS MORNING...AS CAN BE SEEN ON
SOME VALLEY WEBCAMS JUST AFTER SUNRISE AND ON IR IMAGERY. CURRENT
TEMPERATURES ACROSS THE GREATER PHOENIX METRO SPAN UPPER 20S AND LOW
TO MID 30S...THE THIRD DAY IN A ROW OF COLD MORNING LOW
TEMPERATURES. LOCATIONS ALONG THE LOWER COLORADO RIVER FROM LAKE
HAVASU SOUTHWARD DIPPED INTO THE UPPER 20S THIS MORNING. WE SHOULD
REACH JUST ABOUT SEASONAL NORMAL TODAY AS WE START A GRADUAL WARM UP
THROUGH TUESDAY...BEFORE THE NEXT WEATHER SYSTEM MOVES THROUGH. AS
OF NOW...FORECAST IS IN GOOD SHAPE. MORE UPDATES LATER.

&&

.PREVIOUS DISCUSSION.../ISSUED 325 AM MST 28 DEC 2014/...

...THERE IS POTENTIAL FOR A MAJOR WINTER STORM IN ARIZONA
WEDNESDAY AND THURSDAY. PERIODS OF BRIEF HEAVY RAIN ON THE DESERTS
AND MOUNTAIN SNOW ARE POSSIBLE...

.TODAY THROUGH TUESDAY...
DRY WESTERLY FLOW ALOFT...WITH OCCASIONAL HIGH CLOUDS AND WARMER
TEMPERATURES ARE FORECAST THIS PERIOD.

TUESDAY NIGHT THROUGH THURSDAY...
MODELS CONTINUE TO BE VERY CONSISTENT WITH MOVING A VERY COLD
WESTERN CANADIAN TROF SOUTH TOWARD ARIZONA BY WEDNESDAY. INFACT
DETAILS OF THIS SYSTEM STILL INCLUDE IT PARTIALLY RETROGRADING
OFFSHORE SAN FRANCISCO AND LOS ANGELES...AND ENABLING IT TO PICK UP
COPIOUS AMOUNTS OF BOUNDARY LAYER FRONTAL BAND MOISTURE. THERE IS
POTENTIAL FOR WIDESPREAD RAIN AND SNOW AS THE MOIST FRONTAL BAND
GETS DIRECTED TOWARD PHOENIX BY WEDNESDAY EVENING.

BECAUSE THE MODELS ARE NOW VERY CONSISTENT WITH THE EVOLUTION OF
THIS COLD STORM TOWARD AZ...DETAILS ARE COMING INTO BETTER FOCUS
REGARDING THE AMOUNT OF RAIN...MOUNTAIN SNOW...AND SNOW LEVELS FOR
TUESDAY NIGHT THROUGH THURSDAY.  THE MOIST FRONTAL BAND IS FORECAST
TO MOVE WEST TO EAST...FROM SOUTHEAST CA TUESDAY NIGHT...INTO SOUTH
CENTRAL AZ BY WEDNESDAY EVENING.

AGAIN...AS THIS COLD CANADIAN SYSTEM MOVES PARTIALLY OFFSHORE...I.E
700 MB TEMPS OF MINUS 10 TO MINUS 12 C SWING OFFSHORE...LOW LEVEL
PRESSURE GRADIENTS INCREASE DRAMATICALLY TO THE POINT THAT NEAR GALE
FORCE SURFACE WINDS MECHANICALLY AND THERMODYNAMICALLY INJECT
COPIOUS AMOUNTS OF BOUNDARY LAYER MOISTURE INTO THE FRONTAL ZONE.
MOISTURE WILL CONTINUE TO BUILD ALONG THE FRONTAL ZONE AS IT MOVES
SOUTH TOWARD SAN DIEGO...AND WITH 500 MB HEIGHT FALLS ON THE ORDER
OF 120 METERS JUST OFF THE LOS ANGELES AND SAN DIEGO COAST TUESDAY
NIGHT AND WEDNESDAY MORNING...IMPLIED VERTICAL MOTION FIELDS SHOULD
INJECT MOISTURE HIGHER INTO THE MIDDLE ATMOSPHERE. THEREFORE ALL
INDICATIONS ARE THAT THE FRONTAL ZONE WILL BE JUICY...AND MIXED
TOGETHER WITH A NEAR ARCTIC TYPE AIRMASS...CLOUD PHYSICS PRECIP
GROWTH BECOMES VERY EFFICIENT RESULTING IN THE POTENTIAL FOR
WIDE-SPREAD RAIN AND MOUNTAIN SNOW.

A GOOD CHANCE OF PRECIPITATION BEGINS TUESDAY EVENING OVER OUR
SOUTHEAST CA ZONES...ESPECIALLY JOSHUA TREE NATIONAL PARK ZONE 30.
SNOW LEVELS BY TUESDAY MIDNIGHT SHOULD FALL TO NEAR 4400 FT...THEN
FALL RAPIDLY AGAIN TO AROUND 1700 FEET BY MID MORNING WEDNESDAY.
PRECIP AMOUNTS ARE NORMALLY HARD TO FORECAST IN THIS AREA BECAUSE
LOW LEVEL UPSLOPE WINDS HAVE TO BE JUST RIGHT IN THE SHADOW OF THE
COASTAL RANGE...BUT FROM TUESDAY EVENING THROUGH WEDNESDAY MORNING
THERE COULD BE 0.62 INCHES OF WATER EQUIVALENT. PERHAPS 6 INCHES OF
SNOWFALL ABOVE 4500 FEET...AND 2-4 INCHES ABOVE 2500 FEET BY
WEDNESDAY AFTERNOON.

THE FRONTAL BAND THEN MOVES BODILY TOWARD SOUTHWEST AND SOUTH
CENTRAL AZ ON WEDNESDAY. PRECIPITATION SLOWLY INCREASES BUT WILL
LIKELY REACH A PEAK IN SOUTH CENTRAL AZ...INCLUDING PHOENIX AND
VICINITY LATE WEDNESDAY AFTERNOON AND WEDNESDAY NIGHT. UPSLOPE
PRECIP IN SOUTH CENTRAL AZ WILL BE PARTICULARLY EFFICIENT INTO
THURSDAY MORNING. SNOW LEVELS WILL BE ARTIFICIALLY HIGH IN SOUTH
CENTRAL AZ WEDNESDAY NIGHT/EARLY THURSDAY AS THE COLD FRONT STALLS
A BIT.  FORECAST SNOW LEVELS WILL LIKELY RANGE FROM BETWEEN 5000 FT
IN PHOENIX TO 6000 FT IN GLOBE WEDNESDAY AT MIDNIGHT...THEN FALL
ANOTHER 1000 FEET BY THURSDAY MORNING. STORM TOTAL PRECIP AMOUNTS
HAVE THE POTENTIAL TO RANGE FROM 0.60 IN PHOENIX...BETWEEN WEDNESDAY
AND THURSDAY AFTERNOONS...TO NEAR 1.50 INCHES IN THE UPSLOPE AREAS
JUST NORTH THROUGH EAST OF PHOENIX...INCLUDING THE TOWNS OF
GLOBE/MIAMI. AS SNOW LEVELS FALL LATE WEDNESDAY NIGHT INTO THURSDAY
THERE COULD BE HEAVY AMOUNTS IN THE HIGHER ELEVATIONS.

FRIDAY THROUGH SUNDAY...
CLEARING AND MUCH COLDER AS THE CANADIAN SYSTEM SLOWLY EXITS THE
STATE.


&&

.AVIATION...
SOUTH-CENTRAL ARIZONA INCLUDING KPHX...KIWA...KSDL...AND SOUTHEAST
CALIFORNIA/SOUTHWEST ARIZONA...INCLUDING KIPL AND KBLH...

INCREASING HIGH CLOUDS OVER THE REGION EXPECTED TODAY...CLEARING
THIS EVENING AFTER 00Z MONDAY. WINDS WILL REMAIN LIGHT...LESS THAN
10KT THROUGH 12Z MONDAY MORNING.

AVIATION DISCUSSION NOT UPDATED FOR AMENDED TAFS.

&&

.FIRE WEATHER...
TUESDAY THROUGH SATURDAY...
A STRONG LOW PRESSURE SYSTEM WILL QUICKLY DROP SOUTH OUT OF CANADA
AND CARVE OUT A LARGE COLD TROUGH OVER THE WESTERN UNITED STATES.
THIS STORM WILL BRING INCREASED CLOUD COVER...AND THE POTENTIAL FOR
PERIODS OF GUSTY WINDS ACROSS RIDGETOPS IN SOUTHEAST CALIFORNIA AND
SOUTH-CENTRAL ARIZONA THROUGH THURSDAY...BUT ALSO THE CHANCES FOR
MOUNTAIN SNOW AND VALLEY RAIN SHOWERS TO ALL DISTRICTS THROUGH
FRIDAY MORNING. SNOW LEVELS DROP TO AROUND 4500 FEET ACROSS SOUTH
CENTRAL ARIZONA BY LATE THURSDAY MORNING...FOLLOWED BY WIDESPREAD
SUB-FREEZING TEMPERATURES OVER THE LOWER DESERTS BOTH FRIDAY AND
SATURDAY MORNINGS. WARMER TEMPERATURES AND DRIER CONDITIONS ARE
EXPECTED NEXT WEEKEND.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...

AZ...NONE.

CA...NONE.

&&

$$

VISIT US ON FACEBOOK...TWITTER...AND AT WEATHER.GOV/PHOENIX

DISCUSSION...DEWEY
PREVIOUS DISCUSSION...VASQUEZ
AVIATION...MEYERS
FIRE WEATHER...MEYERS





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