Area Forecast Discussion
Issued by NWS Tucson, AZ

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FXUS65 KTWC 241614
AFDTWC

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TUCSON AZ
910 AM MST WED DEC 24 2014

.SYNOPSIS...THERE IS A WIND ADVISORY IN EFFECT FROM 10 AM UNTIL 9 PM
CHRISTMAS DAY FOR MUCH OF COCHISE...EASTERN SANTA CRUZ...AND EXTREME
SOUTHEASTERN PIMA COUNTIES. COOL AND DRY CONDITIONS TODAY WILL GIVE
WAY TO A STORM SYSTEM DIGGING INTO THE DESERT SOUTHWEST LATE THIS
WEEK. CHRISTMAS DAY WILL SEE BREEZY TO WINDY CONDITIONS AND A CHANCE
OF VALLEY RAIN AND MOUNTAIN SNOW THURSDAY NIGHT.  THIS SHOULD BE A
QUICK SHOT OF PRECIPITATION...ENDING IN THE EASTERN AREAS FRIDAY
MORNING. COOL AND DRY WEATHER WILL THEN RETURN THIS WEEKEND.

&&

.DISCUSSION...VERY COOL MORNING IN SOUTHEASTERN AZ WITH KTUS SEEING
ITS FIRST 32 DEG MORNING SINCE JAN 6. THIS IS THE THIRD LATEST
FREEZE ON RECORD. WV IMAGERY AND 24/12Z UPPER-AIR PLOTS SHOW A
LONGWAVE TROUGH WITH NEUTRAL TILT POSITIONED OVER THE CENTER OF THE
CONUS FOLLOWED BY A RIDGE OVER THE WRN STATES. OUR NEXT WEATHER
MAKER IS IN THE FORM OF A PACIFIC TROUGH WHICH IS LOCATED JUST WEST
OF THE 130W BENCHMARK AND MOVING TOWARD THE PACIFIC NW STATES. THIS
TROUGH WILL QUICKLY DIVE SOUTHEAST THROUGH THE GREAT BASIN MAKING
FOR AN INTERESTING WEATHER SCENARIO ON CHRISTMAS DAY. IR SATELLITE
IMAGERY CURRENTLY INDICATING CLEAR TO MOSTLY CLEAR SKIES OVER MUCH
OF SOUTHERN AZ...WITH HIGH CLOUDS BEGINNING TO STREAM INTO NRN AZ.

THE CHALLENGE OF THE DAY WILL BE THE TIMING OF THE NEXT SYSTEM
PROGGED TO IMPACT THIS AREA CHRISTMAS AFTERNOON INTO CHRISTMAS
NIGHT. 24/12Z GFS SEEMS TO BE SLIGHTLY FASTER WITH THIS DISTURBANCE
VERSUS 24 HRS AGO. THAT SAID...WILL PROBABLY NEED TO INCREASE POPS
TO AT LEAST A SLIGHT CHANCE FOR THE 25/18Z-26/00Z PERIOD...
ESPECIALLY FOR AREAS MAINLY NORTH AND WEST OF TUCSON. STILL
EXPECTING THE HIGHEST QPF AMOUNTS / ALBEIT NOT ALL TOO IMPRESSIVE /
TO BE IN THE HIGHER ELEVATIONS AND MAINLY SE OF A LINE EXTENDING
FROM ABOUT SAFFORD THROUGH TUCSON AND SELLS. WIND WILL ALSO BE AN
ISSUE TOMORROW WITH PORTIONS OF SOUTHEASTERN AREAS IN A WIND
ADVISORY FROM 10 AM UNTIL 9 PM. MORE INFORMATION ON WIND SPEEDS AND
RAIN/SNOW AMOUNTS IS IN THE PREVIOUS DISCUSSION BELOW. FORECAST FOR
TODAY IS ON TRACK AND NO UPDATES ARE PLANNED.

&&

.AVIATION...VALID THRU 25/18Z.
CLEAR SKIES THIS MORNING...WITH CLOUD COVER INCREASING AOA 20K FT
AGL THIS AFTERNOON AND TONIGHT. WINDS THIS MORNING WILL INCREASE OUT
OF THE EAST OR SOUTHEAST AT 10 TO 15 KTS...MAINLY IN AND AROUND THE
KTUS TERMINAL...BEFORE RELAXING AFTER ABOUT 24/18Z. CEILINGS WILL
THEN DECREASE TO AROUND 10 TO 15K FT AGL OVERNIGHT AND INTO THURSDAY
MORNING. GUSTY SOUTHERLY OR SOUTHWESTERLY WINDS WILL BEGIN TO
INCREASE AFT 25/12Z. AVIATION DISCUSSION NOT UPDATED FOR TAF
AMENDMENTS.

&&

.FIRE WEATHER...DRY CONDITIONS INTO CHRISTMAS MORNING. A LOW
PRESSURE SYSTEM MOVING ACROSS THE FOUR CORNERS REGION WILL BRING A
SUBSTANTIAL INCREASE IN SLY/SWLY WINDS THURSDAY AFTERNOON WITH THE
HIGHEST WIND SPEEDS DEVELOPING OVER AREAS TO THE SOUTH OF TUCSON AND
EAST OF THE I-19 CORRIDOR. GUSTS MAY APPROACH 55 MPH AT THE HIGHEST
ELEVATIONS AND 35 TO 45 MPH ACROSS VALLEY LOCATIONS. THIS SYSTEM
WILL ADDITIONALLY BRING A CHANCE OF VALLEY RAIN AND MOUNTAIN SNOW
THURSDAY NIGHT INTO EARLY FRIDAY MORNING. DRY CONDITIONS WILL THEN
PREVAIL FRIDAY AFTERNOON THROUGH NEXT TUESDAY.

&&

.PREV DISCUSSION...MODELS INDICATE THAT THE AFOREMENTIONED PACIFIC
TROUGH WILL MOVE INTO NORTHERN CALIFORNIA AND NEVADA LATE THIS
AFTERNOON AND INTO NORTHWEST ARIZONA BY EARLY THURSDAY MORNING. BY
LATE THURSDAY AFTERNOON THE CENTER OF CIRCULATION WITH THE LOW WILL
BE OVER EASTERN UTAH WITH THE ASSOCIATED TROUGH AXIS EXTENDING INTO
WESTERN ARIZONA. BY EARLY FRIDAY MORNING THE LOW SHOULD BE OVER THE
GENERAL VICINITY OF THE FOUR CORNERS REGION WITH A VORT MAX SWINGING
THROUGH THE BASE OF THE TROUGH ACROSS SOUTHERN ARIZONA.

AT THIS TIME THE MODELS ARE INDICATING THAT PRIME TIME FOR
PRECIPITATION WITH THE SYSTEM WILL OCCUR CHRISTMAS EVENING AND
OVERNIGHT INTO EARLY FRIDAY MORNING. MODELS HAVE SHOWN SIGNIFICANT
CHANGES IN THE PAST 12 TO 24 HOURS WITH THE NAM SHOWING MOS POP FOR
THURSDAY NIGHT FOR TUCSON GOING FROM SINGLE DIGIT POPS TO AROUND 50
PERCENT. IN ADDITION...THE ECMWF FOR TUCSON WENT FROM 63 PERCENT
FROM THE 23/00Z RUN TO 77 PERCENT FOR THE 24/00Z RUN. FOR AREAS TO
THE EAST NAM MOS POP NUMBERS FOR DOUGLAS WENT FROM 28 PERCENT TO 69
PERCENT FOR 12 HOUR POP. THE ECMWF POP FOR DOUGLAS INCREASED TO
AROUND 80 PERCENT FOR THURSDAY NIGHT. MEANWHILE...THE GFS IS THE
OUTLIER WITH THIS SYSTEM AND ONLY SHOWS SLIGHT TO LOW END CHANCE
CATEGORY POPS FOR THIS EVENT. ALL THAT SAID...I LEANED TOWARD THE
WETTER NAM/EURO SOLUTIONS AND INCREASE THE POP FOR THURSDAY NIGHT.
THE CAVEAT IS THAT I MAY NOT HAVE INCREASED IT ENOUGH. I KEPT POPS
HIGH ENOUGH TO REFLECT THE RECENT MODEL CHANGES...BUT NOT HIGH
ENOUGH TO BE OUT OF CONSISTENCY WITH SURROUNDING OFFICES.

STRONG WINDS WILL ALSO ACCOMPANY THIS SYSTEM ON THURSDAY...SO A WIND
ADVISORY WAS ISSUED FOR MAINLY EASTERN PORTIONS OF SANTA CRUZ COUNTY
AND MUCH OF COCHISE COUNTY FOR THURSDAY. SEE DETAILS BELOW IN THE
WATCH/WARNING/ADVISORY SECTION.

THIS SYSTEM WILL ALSO BRING SIGNIFICANTLY COOLER AIR TO THE REGION
FOR FRIDAY THROUGH THE WEEKEND WITH HIGH TEMPERATURES DROPPING
AROUND 10 TO 15 DEGREES FOR FRIDAY AND SATURDAY...AND 5 TO 10 DEGS
ON SUNDAY COMPARED TO WHAT IS EXPECTED ON CHRISTMAS DAY.

QPF VALUES WITH THIS SYSTEM ARE NOT EXPECTED TO VERY MUCH...WITH
VALUES RANGING FROM AROUND A TENTH TO A THIRD OF AN INCH. SNOW
AMOUNTS WITH THIS SYSTEM THEREFORE EXPECTED TO BE ON THE LIGHT
SIDE...WITH AMOUNTS GENERALLY RANGING FROM AROUND 1 TO 4 INCHES FOR
THE HIGHEST MOUNTAIN PEAKS.

ONCE THE SYSTEM EXITS THE REGION LATE SATURDAY/EARLY SUNDAY...DRY
NORTHWEST FLOW WILL DEVELOP BEHIND THE SYSTEM...WITH ANOTHER SYSTEM
APPROACHING FOR THE EARLY TO MIDDLE PART OF NEXT WEEK. HOWEVER...
SIGNIFICANT DIFFERENCES AMONG THE MODELS WITH THIS STORM...SO FOR
NOW I ONLY SHOW SINGLE DIGIT POPS FOR THAT PERIOD OF TIME.

&&

.TWC WATCHES/WARNINGS/ADVISORIES...WIND ADVISORY FROM 10 AM TO 9 PM
      MST THURSDAY FOR AZZ503-507-508-512-513.

&&

$$

VISIT US ON FACEBOOK...TWITTER...YOUTUBE...AND AT WEATHER.GOV/TUCSON

DISCUSSION...FRENCH
PREV DISCUSSION...MOLLERE
AVIATION/FIRE WEATHER...CARLAW/MEADOWS






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