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000
FXUS65 KTWC 100410
AFDTWC

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TUCSON AZ
910 PM MST TUE FEB 9 2010

.SYNOPSIS...A PACIFIC LOW PRESSURE SYSTEM WILL BRING VALLEY RAIN AND
MOUNTAIN SNOW BY LATE WEDNESDAY INTO EARLY THURSDAY. THIS SYSTEM
WILL BE QUITE COLD WITH SIGNIFICANT SNOW ACCUMULATIONS POSSIBLE
ABOVE 4000 FEET OR EVEN LOWER. DRY AND WARMER CONDITIONS WILL BEGIN
FRIDAY AND CONTINUE INTO EARLY NEXT WEEK UNDER HIGH PRESSURE.

&&

.DISCUSSION...SATELLITE IMAGERY AND SURFACE OBSERVATIONS INDICATED
MOSTLY CLOUDY SKIES ACROSS COCHISE/SANTA CRUZ/SRN GRAHAM/GREENLEE
COUNTIES...AND MOSTLY CLEAR TO CLEAR SKIES ACROSS PIMA AND SE PINAL
COUNTIES. HOWEVER...CLOUDY SKIES WERE JUST WEST OF THIS FORECAST
AREA...AND CLOUD TOPS WERE COOLING DURING THE PAST 1-2 HOURS OVER
FAR SWRN AZ. PRECIP WAS ALSO ONGOING ACROSS SRN CALIFORNIA. STILL
QUITE DRY NEAR THE SURFACE WITH DEWPOINTS VALID 10/03Z RANGING FROM
THE MID 20S-LOWER 30S. 10/00Z KTWC SOUNDING DEPICTED A DRY SURFACE-
750 MB LAYER...A NEARLY SATURATED 650 MB LEVEL...THEN FAIRLY DRY
REGIME ABOVE 600 MB. SATELLITE IMAGERY AND 10/00Z PLOTS INDICATED
UPPER LOW CENTERED VICINITY LOS ANGELES WITH SHORTWAVE RIDGE EAST OF
THIS FORECAST AREA EXTENDING FROM NEW MEXICO TO WRN COLORADO.
MODERATE-STRONG SWLY FLOW PREVAILED OVER SRN AZ.

10/00Z GFS NOT YET AVAILABLE VIA AWIPS FOR THIS UPDATE. THUS 10/00Z
NAM PROGS UPPER LOW TO BE CENTERED JUST WEST OF SAN DIEGO 12Z WED...
THEN UPPER LOW TO MOVE TO NORTH-CENTRAL SONORA BY 12Z THUR. THIS
SOLUTION HAS TRENDED FARTHER SOUTH VERSUS PREVIOUS RUNS. SYSTEM TO
REMAIN PROGRESSIVE WITH TROUGH AXIS PROGGED TO BE WELL EAST OF SE AZ
BY LATE THUR AFTERNOON. SRN TRAJECTORY WOULD FAVOR THE MOST
SIGNIFICANT SNOWFALL ACCUMULATIONS TO BE ACROSS SANTA CRUZ AND
COCHISE COUNTIES AND PERHAPS KITT PEAK...AS WELL AS THE MOUNTAINS
VICINITY THE TUCSON METRO AREA...WITH LESSER AMOUNTS FURTHER NORTH.

HAVE OPTED TO MAINTAIN THE WINTER STORM WATCH WHICH IS IN EFFECT FOR
ALL ZONES EXCEPT FAR WRN PIMA COUNTY. THUS WILL DEFER TO MIDNIGHT
SHIFT AND AFTER FULL RECEIPT OF 10/00Z GFS/ECMWF SOLUTIONS. PRECIP-
FREE CONDITIONS TO OCCUR AREAWIDE STARTING THUR NIGHT THEN CONTINUE
INTO EARLY NEXT WEEK AS HIGH PRESSURE ALOFT PREVAILS OVER THE WRN
CONUS.

PLEASE REFER TO THE PREVIOUS DISCUSSION SECTION BELOW FOR ADDITIONAL
DETAIL.

&&

.PREV DISCUSSION...THE MODELS SHOW A BAND OF SHOWERS MAINLY ALONG
THE EASTERN PERIPHERY OF THE LOW EARLY WEDNESDAY ALONG THE COLORADO
RIVER BASIN. THE LOW IS PROGGED TO BE OVER SOUTHWESTERN ARIZONA OR
NORTHWESTERN SONORA BY MID TO LATE AFTERNOON ON WEDNESDAY. AS A
RESULT...EXPECT SHOWERS TO INCREASE FROM WEST TO EAST DURING THE
DAY. FOR NOW THE MODELS SHOW A BIT OF DISAGREEMENT WITH REGARD TO
QPF VALUES AND THUS SNOW AMOUNTS. HOWEVER...ALL OF THE MODELS ARE
SHOWING A STRONG JET AT AROUND 200-300MB...GENERALLY GREATER THAN
150 KNOTS DEVELOPING ACROSS THE SE CORNER OF ARIZONA...THUS SRN
PORTIONS OF ARIZONA WILL BE UNDER THE LEFT FRONT QUADRANT (LEFT EXIT
REGION) OF THE JET...SO DYNAMICS WITH THIS SYSTEM LOOK QUITE GOOD.
GFS MODEL CROSS-SECTIONS ARE SHOWING OMEGA VALUES OF 15 TO 20
MICROBARS PER SECOND BY LATE WEDNESDAY AFTERNOON OVER MAINLY CENTRAL
PORTIONS OF THE FORECAST AREA. THESE VALUES SHIFT EAST DURING THE
EVENING AND OVERNIGHT HOURS.

I KEPT THE INHERITED QPF/SNOW AMOUNT FORECAST FROM THE NIGHT SHIFT. I
ALSO INHERITED A WINTER STORM WATCH FOR ALL OF THE HIGHER ELEVATIONS
OF SOUTHEAST ARIZONA FOR WEDNESDAY AFTERNOON THROUGH LATE THURSDAY
MORNING. WITH SOME CONTINUED UNCERTAINTY WITH REGARD TO THE
TIMING...POSITION AND STRENGTH OF THIS SYSTEM I DECIDED TO HOLD OFF
ON ANY UPGRADES TO THE WATCH...SINCE TRYING TO FINE TUNE AREAS THAT
MAY NEED A WARNING ARE STILL A BIT DIFFICULT AND ALL DEPEND ON THE
TRACK OF THE STORM AS WELL AS HOW MUCH OF THE TROPICAL MOISTURE THE
LOW IS ABLE TO TAP INTO AND PULL NORTHWARD INTO ARIZONA.

AS THE LOW MOVES OVER SRN ARIZONA AND NORTHERN SONORA THE MODELS
PROG THIS SYSTEM TO BE COLD WITH 500MB TEMPS OF AROUND -24 TO -26
DEGS CELSIUS. THEREFORE...I INCLUDED A MENTION OF A SLIGHT CHANCE OF
THUNDERSTORMS TO ACCOMPANY THE SHOWERS...MAINLY FOR WEDNESDAY
AFTERNOON AND THROUGHOUT THE OVERNIGHT HOURS. SNOW LEVELS ARE
EXPECTED TO BE RATHER LOW WITH SNOW LEVELS GENERALLY AROUND
3500-4500 FEET DURING THE OVERNIGHT HOURS WEDNESDAY NIGHT INTO
THURSDAY. THIS COULD ALSO CHANGE DEPENDING ON WHAT SUBSEQUENT MODEL
RUNS HAVE TO SHOW WITH REGARD TO SNOW LEVELS.

THE LOW SHOULD QUICKLY KICK TO THE EAST THURSDAY WITH PRECIPITATION
CHANCES DECREASING CONSIDERABLY DURING THE DAY. FOR THE REMAINDER OF
FORECAST PERIOD EXPECTING RIDGING TO GENERALLY INFLUENCE THE WEATHER
ACROSS THE DESERT SOUTHWEST WITH SOME SYSTEMS DROPPING OVER THE TOP
OF THE RIDGE AND PERHAPS CLIPPING THE EASTERN SECTIONS OF THE
FORECAST AREA FROM TIME TO TIME. HOWEVER...FOR NOW I KEPT A DRY
FORECAST GOING.

&&

.AVIATION...THRU 16Z WEDNESDAY CLOUD DECKS LOWERING TO 6-9K FT AGL.
AFTER 16Z WEDNESDAY AND CONTINUING INTO WEDNESDAY NIGHT CLOUD DECKS
GENERALLY 5-8K FT AGL...ALONG WITH AREAS OF MVFR AND LOCAL IFR
CONDITIONS DUE TO VALLEY SHRA AND HIGHER TERRAIN SHSN ESPECIALLY
KTUS VICINITY EWD/SWD. SURFACE WINDS THRU 16Z WEDNESDAY SELY/SLY
GENERALLY LESS THAN 10 KTS. SURFACE WINDS DURING THE 16Z-23Z PERIOD
SWLY-WLY 10-20 KTS WITH GUSTS TO AROUND 25 KTS. AVIATION DISCUSSION
NOT UPDATED FOR TAF AMENDMENTS.

&&

.FIRE WEATHER...NO FIRE WEATHER CONCERNS AT THIS TIME.

&&

.TWC WATCHES/WARNINGS/ADVISORIES...WINTER STORM WATCH IN EFFECT
     FROM WEDNESDAY AFTERNOON THROUGH THURSDAY MORNING
     AZZ019-029-030-032>035 ABOVE 4000 FEET.

$$

BF
PREV DISCUSSION...MOLLERE

WEATHER.GOV/TUCSON