Area Forecast Discussion
Issued by NWS Tucson, AZ

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FXUS65 KTWC 210406 CCA
AFDTWC

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TUCSON AZ
906 PM MST WED AUG 20 2014

.SYNOPSIS...AN UPPER LOW OVER SOUTHERN CALIFORNIA WILL MOVE ACROSS
NORTHERN ARIZONA LATE THURSDAY NIGHT AND EARLY FRIDAY WITH SCATTERED
SHOWERS AND THUNDERSTORMS EXPECTED THURSDAY AFTERNOON AND NIGHT.
DRIER AIR BEHIND THE LOW WILL LIMIT CONVECTION TO MAINLY EASTERN AND
SOUTHERN AREAS FRIDAY THROUGH THE WEEKEND.  AFTER A COUPLE OF COOLER
THAN NORMAL DAYS...HIGH TEMPERATURES WILL WARM BACK UP TO NORMAL
OVER THE WEEKEND.

&&

.DISCUSSION...IT WAS A RELATIVELY DOWN DAY FOR CONVECTION ACROSS
SOUTHEAST ARIZONA AS SHOWERS AND THUNDERSTORMS GENERALLY STRUGGLED
AWAY FROM HIGHER TERRAIN AREAS. THE ONE EXCEPTION WAS EARLIER THIS
EVENING ACROSS CENTRAL AND WESTERN PIMA COUNTY WHERE A FEW SPOTS HAD
ONE TO TWO INCHES OF RAINFALL. THESE SHOWERS AND STORMS HAVE JUST
ABOUT DISSIPATED THOUGH BASED ON LATEST RADAR IMAGERY.
MEANWHILE...MOSTLY CLOUDY SKIES PREVAIL FOR AREAS FROM TUCSON
WESTWARD...WITH MOSTLY CLEAR TO PARTLY CLOUDY SKIES EAST OF TUCSON.

THERE IS A CONSIDERABLE AMOUNT OF MID/UPPER LEVEL MOISTURE THAT IS
STREAMING TO THE NORTHEAST FROM THE BAJA REGION...MOST OF WHICH IS
ATTRIBUTED TO TROPICAL STORM LOWELL. THE OTHER MAJOR FEATURE IS AN
ANOMALOUS UPPER LEVEL LOW THAT IS ALONG THE SOUTHERN CALIFORNIA
COAST AT THIS TIME. THE BIG QUESTION HEADING INTO THURSDAY WILL BE
HOW MUCH CLOUD COVERAGE WILL LIMIT INSTABILITY AND THUS CONVECTION
AS THERE WILL CERTAINLY BE FAVORABLE DYNAMICS EAST OF THE UPPER LOW
TO WORK WITH DURING THE DAY THURSDAY. MADE SOME MINOR TWEAKS TO THE
GRIDS BY INCREASING POPS AND SKY COVERAGE ACROSS THE CENTRAL AND
WESTERN DESERTS THROUGH THURSDAY MORNING AS SOME SHOWERS ARE
POSSIBLE WITH ANY EMBEDDED DISTURBANCES IN THE MID/UPPER LEVEL
MOISTURE PLUME. SEE PREVIOUS FORECAST DISCUSSION BELOW FOR
ADDITIONAL INFORMATION.

&&

.AVIATION...VALID THRU 21/00Z.
BKN-OVC SKIES ABOVE 10K FT AGL MAINLY FROM KTUS WESTWARD INTO
THURSDAY MORNING WITH SCT-BKN CLOUDS EAST OF KTUS. CHC OF -SHRA
ACTIVITY WILL GENERALLY BE LOW DURING THE OVERNIGHT HOURS WITH THE
BEST CHANCES WEST OF KTUS. AFTER 22/18Z SCT SHRA/TSRA ARE EXPECTED
TO DEVELOP ACROSS SOUTHEAST ARIZONA...BUT CONFIDENCE TOO LOW ON
TIMING AND LOCATION TO INCLUDE IN ANY TAF SITES. WINDS WILL
GENERALLY REMAIN LIGHT AND LESS THAN 10 KTS...THOUGH THUNDERSTORMS
THURSDAY WOULD RESULT IN GUSTY AND ERRATIC WINDS.

&&

.FIRE WEATHER...ISOLATED TO SCATTERED MAINLY AFTERNOON AND EVENING
THUNDERSTORMS ARE EXPECTED THROUGH EARLY NEXT WEEK. HOWEVER...THE
OVERALL TREND IN THUNDERSTORM ACTIVITY WILL BE DIMINISHED FROM
RECENT WEEKS. THE STRONGER THUNDERSTORMS WILL STILL PRODUCE
STRONG...GUSTY AND ERRATIC WINDS. OUTSIDE OF THE THUNDERSTORM
OUTFLOWS...20-FOOT WINDS WILL EXHIBIT NORMAL DIURNAL TRENDS AND WILL
BE MOSTLY LESS THAN 15 MPH. ALSO...RELATIVE HUMIDITY VALUES WILL
FALL A BIT LOWER THAN RECENT WEEKS DUE TO SOME DRYING IN THE
ATMOSPHERE...BUT WE WILL REMAIN ABOVE CRITIC RELATIVE HUMIDITY
THRESHOLDS THROUGH EARLY NEXT WEEK.

&&

.PREV DISCUSSION...SOMEWHAT INTERESTING WEATHER THE NEXT 36 HOURS AS
THE UPPER LOW MOVING OVER SOUTHERN CALIFORNIA OPENS UP AND MOVES
NORTHEAST ACROSS CENTRAL/NORTHERN ARIZONA LATER THURSDAY NIGHT INTO
FRIDAY MORNING.  WATER VAPOR LOOP SHOWING HIGH LEVEL MOISTURE FROM
TS LOWELL BEGINNING TO MOVE NORTH OVER SOUTHERN AZ WITH THE MID
LEVEL MOISTURE EXPECTED LATE TONIGHT INTO THURSDAY WHICH WILL
MOISTEN UP THE ATMOSPHERE SOME.  WHILE THE TIMING OF THE SYSTEM IS
NOT IDEAL (A BIT LATE) IT SHOULD BE GOOD ENOUGH TO HELP FIRE OFF
CONVECTION THURSDAY AFTERNOON INTO THURSDAY NIGHT WITH SOMEWHAT OF A
FOCUS OVER CENTRAL/WESTERN PIMA COUNTY.  VARIETY OF HI RES MODELS
INDICATE PLENTY OF MID AND HIGH LEVEL CLOUDS AROUND WITH SCATTERED
SHOWERS AND ISOLATED THUNDERSTORMS.  I HAVE TWEAKED POPS UP SOME
FROM WHAT WE HAD...ESPECIALLY WEST FOR THE AFTERNOON AND EVENING
HOURS.  AS A RESULT OF THE ANTICIPATED INCREASE IN CLOUDS AND
SHOWERS WE WILL HAVE ANOTHER COOLER THAN NORMAL DAY.

FOR FRIDAY...THE UPPER LOW...ACTUALLY AN OPEN TROUGH BY THIS POINT
WILL MOVE NE OF THE AREA WITH SOMEWHAT DRIER AIR MOVING OVER THE
REGION AT THE MID AND HIGH LEVELS WITH THE EXCEPTION OF ALONG THE
ITRNL BORDER WHICH WILL DIMINISH THE SHOWER AND THUNDERSTORM THREAT
AREAS FROM TUCSON NORTH AND WEST.  ANOTHER COOLER THAN NORMAL DAY
THANKS TO A BOTTOMING OUT OF HEIGHTS AND THICKNESS WITH THE PASSING
TROUGH.

FOR THE WEEKEND THROUGH MONDAY THERE WILL BE WESTERLY FLOW ACROSS
THE REGION LIMITING THE ATMOSPHERIC MOISTURE TO SLIGHTLY BELOW
NORMAL LEVELS.  STILL ENOUGH FOR ISOLATED TO SCATTERED CONVECTION
BUT PRIMARILY OVER SOUTH AND EAST OF TUCSON.  WITH FEWER CLOUDS AND
REBOUNDING THICKNESS VALUES HIGH TEMPERATURES ARE EXPECTED TO
REBOUND TO NORMAL LEVELS.

FOR THE MIDDLE OF NEXT WEEK SOME QUESTIONS ARISE DUE TO A POTENTIAL
TROPICAL SYSTEM OVER THE EASTERN PACIFIC.  THE QUESTION RELATES TO
HOW MUCH MOISTURE MAY BE TOSSED OUR WAY AND WHEN AS THE SYSTEM HEADS
OUT TO SEA.  IF LITTLE TO NONE OF THIS TROPICAL MOISTURE MAKES IT
THIS FAR NORTH WE WILL CONTINUE TO HAVE LESS THAN NORMAL CONVECTION
ACROSS THE AREA.  IF IT MANAGES TO MAKE IT HERE IT WILL BE MORE
ACTIVE.  WITHOUT A CLEAR SIGNAL AT THIS TIME I HAVE LEFT THE
FORECAST AS IS WHICH IS A LOW GRADE MONSOON FORECAST WITH NEAR
NORMAL TEMPERATURES.  CERNIGLIA

&&

$$

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