Area Forecast Discussion
Issued by NWS Tucson, AZ

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FXUS65 KTWC 042148
AFDTWC

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TUCSON AZ
247 PM MST MON MAY 4 2015

.SYNOPSIS...THERE IS A CHANCE OF SHOWERS AND THUNDERSTORMS MAINLY
NORTH OF TUCSON INTO TONIGHT. A DECREASING CHANCE OF SHOWERS AND
THUNDERSTORMS WILL PERSIST TUESDAY...ESPECIALLY NORTHEAST OF TUCSON.
DRY CONDITIONS WILL THEN PREVAIL WEDNESDAY INTO THE UPCOMING
WEEKEND. EXPECT BREEZY TO PERHAPS WINDY CONDITIONS TO OCCUR THURSDAY
AND ESPECIALLY FRIDAY. DAYTIME TEMPERATURES WILL BE BELOW NORMAL
THIS WEEK.

&&

.DISCUSSION...THE UPPER PATTERN IS DEFINED BY AN APPROACHING UPPER
LOW POSITIONED OVER SOUTHERN CALIFORNIA/SOUTHWEST ARIZONA...ANOTHER
LOW IN THE GULF OF ALASKA AND A BROAD RIDGE OF HIGH PRESSURE SPREAD
OUT OVER THE EASTERN STATES. THE MAIN FORECAST CHALLENGE THIS
AFTERNOON WILL BE HANDLING THE LOCATION AND TIMING OF SHOWERS/
THUNDERSTORMS ACROSS THE FORECAST AREA AS THE UPPER LOW MOVES THRU
ARIZONA OVER THE NEXT 24 HOURS.

THE LATEST WATER VAPOR IMAGERY INDICATED THAT THE UPPER LOW HAS
SHIFTED INTO SOUTHERN CALIFORNIA/SOUTHWEST ARIZONA AND CONTINUES TO
BECOME NEGATIVELY TILTED AS IT MOVES NORTHEAST INTO ARIZONA. MODELS
ARE ALL IN GOOD AGREEMENT THAT THE BEST DEFORMATION/DIFLUENCE ALOFT
WILL OCCUR ACROSS THE NORTHERN HALF OF ARIZONA THIS AFTERNOON...
WITH THE SOUTHERN PERIPHERY OF THIS FIELD EXTENDING ACROSS THE
NORTHERN AND NORTHWEST PORTIONS OF TUCSON CWA /NORTH OF TUCSON/.

THE 12Z KTWC UPPER AIR SOUNDING INDICATED A MODERATELY UNSTABLE AND
MOIST ATMOSPHERE THIS MORNING...ESPECIALLY IN THE LOWER LEVELS WITH
A PRECIPITABLE WATER VALUE OF 0.89 INCHES. THIS VALUE IS QUITE HIGH
FOR THIS TIME OF YEAR...RESIDING SOMEWHERE NEAR THE 99TH PERCENTILE
OF VALUES. IN ANY EVENT...THE COMBINATION OF THE LARGE SCALE
FORCING...MOIST ATMOSPHERE AND INCREASING INSTABILITY WITH THE
APPROACHING UPPER LOW...THE NORTHERN HALF OF THE FORECAST AREA WILL
BE THE FAVORED LOCATION FOR CONVECTION THIS AFTERNOON AND TONIGHT.
THE LATEST KEMX RADAR IMAGERY SUPPORTS THIS...DEPICTING STORMS NORTH
OF PIMA AND COCHSIE COUNTIES. THESE STORMS WILL BE CAPABLE OF
PRODUCING LOCALIZED HEAVY RAINFALL...BRIEF THUNDERSTORM OUTFLOW
GUSTS THAT MAY LEAD TO LOCALIZED BLOWING DUST AND SMALL TO MEDIUM-
SIZED HAIL.

MEDIUM RANGE MODELS ARE ALL IN GOOD AGREEMENT ON THE TIMING OF THIS
UPPER LOW EJECTING OUT OF ARIZONA BY TUESDAY AFTERNOON. ADDITIONAL
STORM DEVELOPMENT WILL POSSIBLE ON THE SOUTHERN PERIPHERY OF THIS
FEATURE EARLY TUESDAY AS IT EJECTS...BUT THE INTENSITY WILL
SIGNIFICANTLY WANE THROUGH THE DAY.

OTHERWISE...THE UPPER LOW CURRENTLY IN THE GULF OF ALASKA WILL THEN
FILL INTO THE MEAN TROUGH ACROSS THE WESTERN STATES THE SECOND HALF
OF THE WORK WEEK. RIGHT NOW...BOTH THE GFS AND ECMWF ARE INDICATING
THAT THIS SYSTEM WILL DIVE SOUTH ALONG THE WEST COAST...WITH THE
BEST CHANCE FOR PRECIPITATION LIMITED TO THE GREAT BASIN. THIS PATH
WOULD LEAVE SOUTHEAST ARIZONA HIGH AND DRY...WITH BREEZY TO LOCALLY
WINDY CONDITIONS AND THE POSSIBILITY FOR CRITICAL FIRE WEATHER
CONDITIONS THE SECOND HALF OF THE WORK WEEK...ESPECIALLY FRIDAY.

&&

.AVIATION...VALID THRU 05/00Z. EXPECT ISOLATED -SHRA/-TSRA THROUGH
04/05Z MAINLY N AND E OF A KCGZ-KFHU LINE WITH BASES 9-12 KFT AGL.
THEREAFTER...EXPECT LOWERING CLOUD BASES TO 4-7 KFT AGL AND SLIGHTLY
INCREASED COVERAGE OF -SHRA/-TSRA N AND E OF A KGXF-KTUS-KDUG LINE.
BRIEF ERRATIC WIND GUSTS TO 45 KTS WILL BE POSSIBLE NEAR ANY
CONVECTION ALONG WITH POCKETS OF BLOWING DUST...MAINLY THIS
AFTERNOON. SURFACE WINDS WILL BE SWLY/WLY AT 10-18 KTS WITH GUSTS OF
20-25 KTS THIS AFTERNOON...DIMINISHING SHORTLY AFTER SUNSET.
AVIATION DISCUSSION NOT UPDATED FOR TAF AMENDMENTS.

&&

.FIRE WEATHER...AN APPROACHING STORM SYSTEM WILL BRING AT LEAST A
SLIGHT CHANCE OF PRECIPITATION TO THE REGION THROUGH TUESDAY. A FEW
SHOWERS AND THUNDERSTORMS ARE POSSIBLE THIS AFTERNOON...MAINLY NORTH
AND EAST OF TUCSON. PRECIPITATION COVERAGE IS EXPECTED TO INCREASE
SOMEWHAT LATE TONIGHT AND TOMORROW WITH THE BULK OF THE ACTIVITY
AGAIN PRIMARILY REMAINING NORTH AND EAST OF TUCSON. BRIEF AND
ERRATIC WIND GUSTS UP TO 40-50 MPH WILL BE POSSIBLE NEAR SHOWERS AND
STORMS...PARTICULARLY DURING THE AFTERNOON HOURS.

SUBSTANTIALLY DRIER CONDITIONS ARE ANTICIPATED FROM WEDNESDAY
THROUGH THE WEEKEND ALONG WITH BREEZY TO LOCALLY WINDY CONDITIONS.
THE COMBINATION OF LOW RELATIVE HUMIDITY VALUES AND STRONG WINDS MAY
PRODUCE AREAS OF CRITICAL FIRE WEATHER CONDITIONS ON THURSDAY AND
FRIDAY.

&&

.TWC WATCHES/WARNINGS/ADVISORIES...NONE.

&&

$$

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