290
FXUS65 KTWC 042102
AFDTWC
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TUCSON AZ
202 PM MST SAT JUL 4 2009
SYNOPSIS
DRIER AIR WILL BEGIN TO PENETRATE SOUTHERN ARIZONA FROM
THE NORTHWEST DURING THE LATTER HALF OF THE HOLIDAY WEEKEND BUT A
FEW SHOWERS AND THUNDERSTORMS WILL STILL DEVELOP EACH DAY INTO EARLY
NEXT WEEK...ESPECIALLY SOUTH AND EAST.
DISCUSSION
THUNDERSTORMS HAVE FORMED ALONG THE DEFORMATION ZONE
AND EDGE OF UPPER LEVEL MOISTURE AXIS EXTENDING FROM NORTHEAST
SONORA INTO SOUTHWEST NEW MEXICO. OTHERWISE...CONVECTION THUS FAR
LIMITED TO THE HIGHER TERRAIN IN RIM COUNTRY. THUS PREVIOUS
THINKING OF BEST COVERAGE THIS EVENING ACROSS ERN PORTION OF CWFA
STILL HOLDS ALTHOUGH POPS MAY NEED TO BE A SHADE LOWER THAN
PREVIOUSLY ANTICIPATED. EVEN SO...ANY STORM COULD PRODUCE LOCALLY
HEAVY RAIN TO DUE VERY SLOW MOVEMENT AND HIGH ATMOSPHERIC MOISTURE
CONTENT.
UPPER TROUGH APPROACHING THE PACIFIC NW COAST WILL TEMPORARILY
SUPPRESS UPPER RIDGE SOUTHWARD ENOUGH TO BRING A DRY NW FLOW ALOFT
OVER SOUTHERN AZ. THIS WILL LIMIT BUT NOT ELIMINATE THUNDERSTORM
COVERAGE SUNDAY AND MONDAY. THUS HAVE TRENDED POPS DOWNWARD. BY MID
TO LATE WEEK THE ELONGATED HIGH WILL BECOME RECENTERED AND BUILD
OVER NM WHICH WILL INDUCE A SOUTHERLY FLOW AND PERHAPS HERALD A
RETURN TO BETTER MOISTURE OVER SOUTHERN AZ. MODELS ONLY POINT TO A
GRADUAL SEEPAGE OF MOISTURE NWD BUT WOULD NOT BE SURPRISED TO SEE AT
LEAST ONE DECENT SURGE. THEREFORE HAVE NOT LOWERED POPS
SIGNIFICANTLY YET. THE OTHER SIDE OF THE COIN IS THAT TEMPERATURES
ALOFT ARE FORECAST TO RISE SIGNIFICANTLY AS THE HIGH BUILDS...500 MB
HEIGHTS AS HIGH AS 6000 METERS ON THE ECMWF AND 500 MB TEMPERATURES
AS WARM AS -2C ALONG THE INTERNATIONAL BORDER ON THE GFS BY FRIDAY.
THIS WOULD CERTAINLY SUPPRESS CONVECTION DESPITE THE INCREASED
MOISTURE. NOT CONVINCED THE HIGH AND THE TEMPERATURES WILL BUILD
THIS QUICKLY AS THIS IS A SIGNIFICANT DEPARTURE FROM PREVIOUS MODEL
RUNS. THEREFORE WILL AWAIT ANOTHER RUN OR TWO BEFORE RAISING
TEMPERATURES INTO HEAT ADVISORY TERRITORY...KD
AVIATION
LOOKING AT LESS AREAL COVERAGE THIS EVENING WITH CHANCE
OF STORMS IMPACTING THE TERMINAL SITES LESS THAN FRIDAY. STILL
LOOKING AT A 30% CHANCE OF STORMS IMPACTING KDUG/KOLS SITES WITH A
20% CHANCE AT KTUS. CIGS WITH STORMS DOWN TO 10KFT AGL WITH WIND
GUSTS UP TO 40 KTS. DEBRIS CLOUDS AROUND 12-15K AGL OVERNIGHT INTO
SUNDAY MORNING WITH TYPICAL DIURNAL WIND FLOW. AVIATION DISCUSSION
NOT UPDATED FOR TAF AMENDMENTS.
FIRE WEATHER
A MOIST ATMOSPHERE ACROSS SOUTHEAST ARIZONA WILL
RESULT IN ISOLATED TO SCATTERED THUNDERSTORMS INTO SUNDAY. A TROUGH
MOVING TOWARDS THE WEST COAST WILL SHIFT A DRIER WESTERLY FLOW INTO
ARIZONA NEXT WEEK...ALTHOUGH ENOUGH MOISTURE SHOULD REMAIN IN PLACE
TO MINIMIZE ANY FIRE WEATHER RISKS AND CONCERNS.
TWC WATCHES/WARNINGS/ADVISORIES
NONE.
KD/JRG
WEATHER.GOV/TUCSON
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