Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 261604
AFDPSR

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PHOENIX AZ
905 AM MST FRI DEC 26 2014

.SYNOPSIS...
EXPECT CLEAR SKIES AND COLD TEMPERATURES TONIGHT AND THROUGH THE
WEEKEND. ANOTHER LOW PRESSURE SYSTEM IS EXPECTED TO MOVE INTO THE
REGION NEXT WEDNESDAY AND THURSDAY...WITH A SLIGHT CHANCE OF SHOWERS.

&&

.DISCUSSION...
CLEAR SKIES AND CHILLY TEMPERATURES WILL GREET YOU THIS MORNING
ACROSS THE GREATER METRO PHOENIX AREA. SKY HARBOR OBSERVED A LOW OF
38F DEGREES THIS MORNING...REGISTERING AS THE COLDEST LOW
TEMPERATURE OBSERVED OF 2014 SO FAR...WITH THE SECOND COLDEST LOW
TEMPERATURE READING ON JANUARY 6 OF 39F DEGREES. MANY LOCATIONS
ACROSS THE VALLEY SAW TEMPERATURES IN THE MID TO LOW 30S AND EVEN A
FEW OUTLYING AREAS IN THE FAR SOUTHEAST CHANDLER...GILBERT...AND
QUEEN CREEK SAW UPPER 20S THIS MORNING.

FOR THE REMAINDER OF TODAY...EXPECTING CLEAR SKIES AND MOSTLY LIGHT
WINDS...OUTSIDE OF SOME BREEZES ACROSS EASTERN RIVERSIDE COUNTY IN
CALIFORNIA...WITH TEMPERATURES EXPECTED TO BE AROUND 5-10 DEGREES
BELOW SEASONAL NORMAL. FORECAST IS IN GOOD SHAPE FOR THE TIME BEING.

&&

.PREVIOUS DISCUSSION...
TODAY THROUGH SATURDAY...
A LARGE COLD LOW PRESSURE SYSTEM CONTINUED TO SETTLE OVER REGION
THIS MORNING. THE COLD FRONT ASSOCIATED WITH THIS SYSTEM WAS LOCATED
OVER FAR SOUTHEAST AZ AT 2AM WHERE IT WAS RAINING OFF AND ON. THE
COLD FRONT WILL MOVE INTO NEW MEXICO LATER THIS MORNING LEAVING A
BEHIND A CLEAR COLD DRY AIRMASS AND MUCH COLDER LOW TEMPERATURES
SATURDAY MORNING. DESERT LOW TEMPERATURES FROM THE MID 20S TO MID
30S ARE EXPECTED SATURDAY MORNING FROM SOUTHEAST CA TO SOUTH CENTRAL
AZ...SLIGHTLY WARMER ALONG THE COLORADO RIVER. THE FREEZE WARNING
ISSUED EARLIER WILL CONTINUE FOR MUCH OF THE SOUTH CENTRAL AZ
INCLUDING THE SUBURBS AROUND THE GREATER PHOENIX AREA.

SUNDAY AND MONDAY...
DRY AND WARMER WESTERLY FLOW ALOFT WILL MOVE INTO THE REGION REPLETE
WITH HIGH CLOUDS...ESPECIALLY SUNDAY. TEMPERATURES HOWEVER WILL
REMAIN BELOW NORMAL.

TUESDAY THROUGH THURSDAY...
THIS IS THE MOST EXCITING PERIOD AS WEATHER FORECASTS GET TOUGHER
AND MORE CHALLENGING. ANOMALOUS RIDGE AMPLIFICATION OVER ALASKA AND
FAR WESTERN CANADA PORTEND A VERY COLD CANADIAN AIRMASS TO DEEPEN
SOUTH/SOUTHWEST DOWN THE CALIFORNIA COAST. THIS VERY COLD TROF WILL
EVENTUALLY MOVE INTO SOUTHERN CA...THEN INTO AZ BY THE MIDDLE OF
NEXT WEEK. WHAT COMPLICATES THIS FORECAST TODAY ARE THAT MODEL
TIMING DIFFERENCES ARE EXTREME. FOR EXAMPLE THE GFS MOVES THE TROF
INTO SOUTHERN CA/AZ DURING THE TUE/WED TIME FRAME...WHEREAS THE
EUROPEAN IS 24 HOURS LATER.

SLOWER OR FASTER MOVEMENT OF THE TROF INTO SOUTHERN CA/AZ WILL
DEPEND ON THE ORIENTATION AND AMPLITUDE OF THE UPSTREAM ALASKA/
WESTERN CANADIAN RIDGE POSITION...AND THIS FAR OUT IT LEADS TO MUCH
UNCERTAINTY.

HOWEVER...WHAT MAKES THIS SYSTEM EXCITING IS THE POTENTIAL FOR
SNOW...MAYBE LOTS OF IT OVER OUR JOSHUA TREE NATIONAL PARK ZONE 30.
NOVEMBER 20-21 2004 IS A CASE IN POINT. A SIMILAR COLD DRY TROF
MOVED OUT OF WESTERN CANADA AND RETROGRADED INTO SOUTHERN CA. WITH
AN INLAND TRACK FROM CANADA...THIS SYSTEM WAS INITIALLY BONE DRY.
HOWEVER AS IT PARTIALLY SWUNG OUT INTO THE PACIFIC...STRONG LOW
LEVEL PRESSURE GRADIENTS CREATED GUSTY OCEAN WINDS...INJECTING A
PLUME OF BOUNDARY LAYER MOISTURE UPSLOPE INTO COLDER TEMPS ALOFT AND
SNOW. 12 INCHES OF VERY DRY SNOW FELL IN JOSHUA TREE NATIONAL PARK
IN A BUSTED FORECAST. MODELS COULD NOT ACCOUNT FOR THE SUDDEN
MECHANICAL INJECTION OF LOW LEVEL MOISTURE INTO THE BOUNDARY LAYER.
WE WILL KEEP THIS IDEA IN MIND AS WE PROGRESS TOWARD NEXT WEEK.
OTHERWISE A BROAD BRUSH FORECAST FOR A SLIGHT CHANCE OF SHOWERS WED
AND THU IS ALL WE CAN OFFER FOR NOW.

FRIDAY...
CLEAR AND DRY WITH BELOW NORMAL TEMPERATURES IN THE AFTERMATH OF THE
ABOVE MENTIONED TROF.

&&

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

UPPER LEVEL TROUGH LINGERS OVER REGION WITH DRY NORTH-NORTHWESTERLY
FLOW ALOFT. EXPECT MOSTLY CLEAR SKIES AND LIGHT SURFACE WINDS
GENERALLY 10KT OR LESS AT ALL TAF SITES THROUGH AT LEAST 12Z
SATURDAY MORNING.

AVIATION DISCUSSION NOT UPDATED FOR AMENDED TAFS.

&&

.FIRE WEATHER...
SUNDAY THROUGH THURSDAY...
DRY AND ABNORMALLY COOL WEATHER WILL PREVAIL SUNDAY AND MONDAY...
WITH GENERALLY LIGHT WINDS AND PERIODS OF HIGH CLOUDS. BY TUESDAY...
ANOTHER STRONG LOW PRESSURE SYSTEM QUICKLY DROPS SOUTH OUT OF CANADA
AND CARVES OUT A LARGE COLD TROUGH OVER THE WESTERN UNITED STATES...
INCREASING THE POTENTIAL FOR STRONGER WINDS ACROSS SOUTHEAST
CALIFORNIA AND THE LOWER COLORADO RIVER VALLEY...BUT ALSO THE
CHANCES FOR MOUNTAIN SNOW AND VALLEY RAIN SHOWERS AFFECTING THE
DISTRICTS THROUGH THE MIDDLE OF NEXT WEEK.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...

AZ...FREEZE WARNING FROM MIDNIGHT TONIGHT TO 8 AM MST SATURDAY
     AZZ021-022-028.

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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