Area Forecast Discussion
Issued by NWS Tucson, AZ

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FXUS65 KTWC 281545
AFDTWC

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TUCSON AZ
844 AM MST SUN DEC 28 2014

.SYNOPSIS...EXPECT A SLOW WARMING TREND TO START THE WEEK. A STRONG
AND COLD STORM SYSTEM WILL THEN BRING A GOOD CHANCE OF VALLEY RAIN
AND MOUNTAIN SNOW SHOWERS WEDNESDAY NIGHT INTO THURSDAY...WITH SNOW
POTENTIALLY REACHING THE VALLEY FLOORS EAST OF TUCSON THURSDAY
EVENING.

&&

.DISCUSSION...AFTER ANOTHER COLD START THIS MORNING...TEMPERATURES
WILL CONTINUE TO REBOUND NICELY TODAY UNDER DRY NORTHWESTERLY FLOW
ALOFT. 12Z KTWC SOUNDING SHOWS TEMPS ABOUT 5C TO 15C WARMER THAN 24
HOURS AGO ABOVE 800 MB WITH A FEW DEGREES OF WARMING BELOW 800 MB.
THERE WILL NOT BE DEEP ENOUGH SUSTAINED MIXING TODAY TO TAP INTO THE
MUCH WARMER AIR IN THE VALLEYS WITH TEMPERATURES ABOUT 5 DEGREES
WARMER THAN YESTERDAY. SOME MAINLY THIN CIRROFORM CLOUDINESS WILL
MOVE INTO THE AREA THIS AFTERNOON.

GENERALLY BENIGN WEATHER WITH A WARMING TREND MONDAY AND TUESDAY
UNDER WESTERLY FLOW ALOFT AS THE LONGWAVE PATTERN IN THE WESTERN US
UNDERGOES CONSIDERABLE CHANGES. AN INITIALLY OPEN BROAD UPPER LEVEL
TROUGH MONDAY OVER THE NORTHERN ROCKIES AND GREAT BASIN WILL
TRANSLATE INTO AN UPPER LEVEL LOW AS A STRONG DISTURBANCE DIGS DOWN
THE BACKSIDE OF THE TROUGH/EASTERN SIDE OF THE AMPLIFIED RIDGE
ACROSS THE EASTERN PACIFIC. THIS UPPER LOW WILL DIG AND BRIEFLY
RETROGRADE ACROSS CALIFORNIA TUESDAY INTO WEDNESDAY BEFORE VERY
SLOWLY MOVING EAST ACROSS ARIZONA THURSDAY INTO EARLY FRIDAY. THIS
SETUP WILL ALLOW CONSIDERABLE BACKING OF THE FLOW TO RESULT IN AMPLE
PACIFIC AND GULF OF CALIFORNIA MOISTURE TO BE PICKED UP AND
TRANSPORTED INTO SOUTHEAST ARIZONA ALONG WITH THE COLD FRONT. WE ARE
STILL WORKING OUT DETAILS WITH TIMING...QPF AND SNOW LEVELS WHICH
WILL BE QUITE LOW. MORE DETAILS IN THE AFTERNOON DISCUSSION AFTER
FULL REVIEW OF THE 12Z GUIDANCE.


&&

.AVIATION...VALID THRU 29/18Z.
SKC WITH CIRRUS CLOUDS INCREASING FROM THE NW...BECOMING BROKEN IN
COVERAGE 28/18Z THRU 29/06Z...CLEARING THEREAFTER. NORMAL DIURNAL
WIND TRENDS GENERALLY LESS THAN 10 KTS EXPECTED THROUGH THE PERIOD.
AVIATION DISCUSSION NOT UPDATED FOR TAF AMENDMENTS.

&&

.FIRE WEATHER...SEASONABLY COOL AND DRY CONDITIONS WITH GENERALLY
LIGHT TERRAIN DRIVEN WINDS THROUGH TUESDAY.  THEN A DEEP LOW
PRESSURE SYSTEM WILL BRING A GOOD CHANCE OF VALLEY RAIN AND MOUNTAIN
SNOW BY LATE WEDNESDAY INTO THURSDAY.  LOWER SNOW LEVELS ARE
POSSIBLE WITH THIS STORM MEANING A GOOD CHANCE OF SNOW TO THE VALLEY
FLOORS EAST AND SOUTH OF TUCSON.  BREEZY TO WINDY CONDITIONS ARE
EXPECTED AHEAD OF THIS SYSTEM...DEVELOPING BY LATE TUESDAY AND
STRENGTHENING WEDNESDAY.

&&

.TWC WATCHES/WARNINGS/ADVISORIES...
 HARD FREEZE WARNING UNTIL 9 AM MST FOR AZZ504-506.
 FREEZE WARNING UNTIL 9 AM MST SUNDAY FOR AZZ502-505.

&&

$$

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