Area Forecast Discussion
Issued by NWS Tucson, AZ

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FXUS65 KTWC 220301
AFDTWC

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TUCSON AZ
800 PM MST THU AUG 21 2014

.SYNOPSIS...AN UPPER LOW OVER SOUTHERN CALIFORNIA WILL WEAKEN AND
LIFT NORTHEAST ACROSS ARIZONA THROUGH EARLY FRIDAY.  THIS WILL
RESULT IN SCATTERED SHOWERS AND THUNDERSTORMS INTO TONIGHT BEFORE
ACTIVITY ENDS FROM WEST TO EAST LATE TONIGHT.  DRIER AIR BEHIND
THE LOW WILL LIMIT THUNDERSTORMS TO MAINLY EAST AND SOUTH OF TUCSON
FRIDAY AND THROUGH THE WEEKEND.  AFTER A COUPLE OF COOLER THAN
NORMAL DAYS HIGH TEMPERATURES WILL WARM BACK UP TO NORMAL OVER THE
WEEKEND.

&&

.DISCUSSION...IR SATELLITE IMAGERY CURRENTLY SHOWS A BLANKET OF
STRATUS REMAINING OVER THE EASTERN HALF OF THE FORECAST
AREA...WITH RADAR INDICATING SOME EMBEDDED NORTH SOUTH ORIENTED
SHOWER BANDS GRADUALLY PUSHING EAST. A CLOSED LOW IS CURRENTLY
SITTING ALONG THE SOUTHERN PORTION OF THE CALIFORNIA ARIZONA
BORDER...PROVIDING THE FORCING NEEDED FOR THE INCREASED SHOWER AND
THUNDERSTORM ACTIVITY SEEN OVER SOUTHWESTERN ARIZONA. MUCH OF THIS
ACTIVITY IS EXPECTED TO WEAKEN OVERNIGHT WITH THE LOSS OF DAYTIME
HEATING. MODELS PROG THE LOW PUSHING TO THE NORTHEAST
TONIGHT...BRINGING THE ASSOCIATED REMAINING SHOWER ACTIVITY ALONG WITH IT.
THE AMOUNT OF ACTIVITY THAT WILL BE SEEN OVER THE FORECAST AREA
OVERNIGHT HAS A BIT OF UNCERTAINTY WITH IT. MODEL SOLUTIONS WOULD
LEAD TO THE MAJORITY OF THE ACTIVITY STAYING NORTH OF THE FORECAST
AREA...WITH A SIGNIFICANTLY WEAKER TRAILING LINE OF MOSTLY SHOWERS
AND A FEW THUNDERSTORMS PUSHING THROUGH OVERNIGHT. AS SUCH THE
FORECAST CURRENTLY SEEMS ON TRACK...THOUGH A DELAY IN THE TURN COULD
LEAD TO INCREASED SHOWER ACTIVITY FURTHER SOUTH. OTHERWISE EXPECT
COOLER THAN NORMAL HIGH TEMPERATURES TOMORROW...WITH TUCSON METRO
EXPECTED TO HIT 91...WARMER THAN TODAYS HIGH OF 86...BUT STILL 7
DEGREES BELOW NORMAL.

FOR ADDITIONAL INFORMATION AND THE EXTENDED FORECAST SEE THE
PREVIOUS DISCUSSION BELOW.

&&

.AVIATION...VALID THRU 22/06Z.
A BROAD AREA OF BROKEN/OVERCAST MID AND HIGH LEVEL CLOUDS HAS
SETTLED OVER MUCH OF ARIZONA. ISOLATED TO SCATTERED SHOWERS CONTINUE
TO DEVELOP WITHIN THE BAND. THIS FEATURE WILL MOVE VERY LITTLE
THROUGH LATE EVENING...BEFORE SLIDING TO THE NORTH AND EAST
OVERNIGHT AND INTO EARLY FRIDAY. ISOLATED THUNDERSTORMS MAY DEVELOP
WITHIN THE EASTERN PERIPHERY OF THE CLOUD FEATURE...FAR WESTERN PIMA
COUNTY...AND IN AREAS TO THE EAST OF THE CLOUD BAND. VFR CONDITIONS
WILL BE THE GENERAL RULE...EXCEPT FOR BRIEF MVFR CONDITIONS IN AND
AROUND THUNDERSTORMS. AVIATION DISCUSSION NOT UPDATED FOR TAF
AMENDMENTS.

&&

.FIRE WEATHER...SCATTERED SHOWERS AND ISOLATED THUNDERSTORMS WILL
REMAIN IN THE OFFERING TONIGHT. THUNDERSTORM ACTIVITY WILL FAVOR FAR
WESTERN PIMA COUNTY...AND AREAS ALONG AND EAST OF A BROAD AREA OF
CLOUDINESS THAT HAS SETTLED OVER THE AREA. DRIER AIR WILL FILTER
INTO THE REGION AS LOW PRESSURE SLIDES TO THE NORTH FRIDAY INTO THE
WEEKEND. THIS WILL MAKE FOR LOWER RAIN/THUNDER CHANCES. AN INCREASE
OF STORMS COULD DEVELOP BY MID NEXT WEEK. OUTSIDE OF THUNDERSTORM
INFLUENCE 20 FT WINDS WILL BE DIURNALLY TERRAIN DRIVEN AND GENERALLY
LESS THAN 15 MPH.

&&

.PREV DISCUSSION...A SOUTH TO NORTH BLANKET OF MID/HIGH LEVEL CLOUDS
REMAINS ACROSS THE CENTRAL PORTION OF THE AREA WHICH IS LIMITING THE
DESTABILIZATION OF THE AIR MASS AS EXPECTED WITH JUST A FEW LIGHT
SHOWERS OVER THIS REGION AT THIS TIME.  THIS AREA WILL VERY SLOWLY
SHIFT EAST AND CONTINUE TO POP OUT LIGHT SHOWERS AND POSSIBLY AN
ISOLATED THUNDERSTORM.  RATHER COOL TEMPERATURES UNDER THIS BAND
FOR AUGUST.

THE MORE INTERESTING AREAS INTO THE EVENING WILL BE THE MOSTLY CLEAR
SECTIONS TO THE EAST AND WEST.  THE EASTERN AREAS ARE POPPING OFF
THUNDERSTORMS AS INSTABILITY HAS INCREASED.  WILL NEED TO WATCH
THOSE FOR GUSTY WINDS.  THEY WILL BE MOVING TO THE NORTH-NORTHEAST
AROUND 20 MPH SO NOT EXPECTING FLOODING ISSUES UNLESS WE GET
REPETITIVE TRAINING ECHOES.  FAR WESTERN PIMA COUNTY IS A POTENTIAL
HOT SPOT AS ENOUGH SUNSHINE HAS BROUGHT THE NEEDED INSTABILITY AND
THIS REGION IS IN A MORE DYNAMICALLY INFLUENCED REGION.  POTENTIAL
EXISTS FOR STRONG STORM DEVELOPMENT THIS AFTERNOON INTO THE EARLY
EVENING.  WILL NEED TO WATCH AND SEE HOW THINGS UNFOLD OVER THE NEXT
COUPLE OF HOURS.

BY MIDNIGHT...THE CLOSED LOW WILL BE OVER SOUTHWEST AZ AND BEGINNING
TO MOVE NORTHEAST AS IT SLOWLY OPENS UP AND WEAKENS.  BY DAWN IT
SHOULD BE SQUARELY OVER CENTRAL AZ AND SLIDING INTO COLORADO AND NEW
MEXICO BY SUNSET.  TOTAL ATMOSPHERIC MOISTURE WILL BEGIN TO DECREASE
AS THE SYSTEM MOVES EAST...HOWEVER THERE SHOULD BE ENOUGH MOISTURE
AND LIFT FOR SCATTERED SHOWERS AND ISOLATED THUNDERSTORMS TO
CONTINUE TO BE A POSSIBILITY OVERNIGHT.  I TWEAKED POPS UP A BIT
WITH THIS THOUGHT IN MIND.  WITH THE TROUGH AXIS STILL MOVING ACROSS
THE EASTERN PART OF THE FORECAST AREA FRIDAY...WOULD EXPECT SOME
CONVECTION TO DEVELOP DURING THE DAY WITH HEATING FROM ROUGHLY
TUCSON EASTWARD.  TWEAKED POPS UPWARD A BIT THIS PERIOD AS WELL
ACROSS EASTERN AREAS.  WITH THE TROUGH OVERHEAD...SOME CLOUDS AND
SHOWERS IT WILL BE ANOTHER COOLER THAN NORMAL DAY ACROSS THE AREA.
FRIDAY NIGHT THE TROUGH FINALLY MOVES OFF TO THE EAST...TAKING
WHATEVER CONVECTION IS LEFT WITH IT.

FOR SATURDAY AND SUNDAY...THE AREA IS LEFT UNDER WEAK WESTERLY FLOW
AS A DEEP LOW PASSES BY WELL TO THE NORTH.  THIS WILL RESULT IN
LOWER PW VALUES OVERALL WITH THE HIGHEST LEVELS ALONG THE INTNL
BORDER...A LOW GRADE MONSOON SETUP WITH ISOLATED TO SCATTERED
CONVECTION MAINLY EAST AND SOUTH OF TUCSON BOTH AFTERNOONS AND
EVENINGS AND LITTLE IF ANYTHING DURING THE LATE NIGHT AND EARLY
MORNING HOURS.  WITH LESS CONVECTION AND CLOUD COVER AFTERNOON HIGH
TEMPERATURES WILL REBOUND TO MORE NORMAL LEVELS FOR THE LATTER PART
OF AUGUST.

DIFFERENCES REMAIN IN THE LONGER RANGE GUIDANCE BUT STARTING TO COME
AROUND TO A MORE SIMILAR SOLUTION FOR THE FIRST HALF OF NEXT WEEK.
IT LOOKS AS THOUGH THE NEXT TROPICAL SYSTEM WILL SEND A PULSE OF
MOISTURE NORTH INTO AZ EARLY IN THE WEEK.  TIMING AND DEGREE OF
MOISTURE INFLUX REMAINS IN QUESTION BUT I WENT AHEAD AN TWEAKED POPS
UPWARD SOME FOR MONDAY INTO TUESDAY WITH THE GREATER LIKELIHOOD OF
IT HAPPENING.  CERNIGLIA

&&

$$

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