Flash Flood and Hail in Arizona, August 20, 2015

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TUCSON AZ
943 AM MST THU AUG 20 2015

.SYNOPSIS...EXPECT ISOLATED TO SCATTERED THUNDERSTORMS MAINLY FROM
TUCSON EAST AND SOUTH THROUGH TONIGHT. INCREASING CHANCES FOR
SHOWERS AND THUNDERSTORMS WILL THEN BEGIN FRIDAY WITH THE BEST
CHANCE OF WIDESPREAD RAINFALL EARLY NEXT WEEK. THE INCREASED
MOISTURE WILL ALSO KEEP DAYTIME TEMPERATURES CLOSE TO SEASONAL
AVERAGES FRIDAY INTO EARLY NEXT WEEK.

&&

.DISCUSSION...SATELLITE IMAGERY WAS SHOWING SOME CLOUDS ALREADY
STARTING TO BUILD UP ACROSS FAR SOUTHEAST ARIZONA AND IN THE WHITE
MOUNTAINS THIS MORNING. MOISTURE WILL CONTINUE TO INCREASE TODAY AND
MORE SO THIS WEEKEND. THAT SAID...CHANCES OF RAIN WILL BE GOING UP
GRADUALLY STARTING TODAY. CURRENT FORECAST APPEARED TO HANDLE THE
TRENDS WELL...SO NO UPDATES NECESSARY THIS MORNING. PLEASE REFER TO
THE ADDITIONAL SECTIONS FOR MORE DETAILS.

&&

.AVIATION...VALID THRU 21/12Z.
FEW-SCT CLOUDS ABV 12K FT. AFT 20/21Z SCT TO LOCALLY BKN 8-10K FT
AND SCT-BKN LYRS AOA 12K FT. CHANCE FOR -SHRA/-TSRA FROM KTUS SOUTH
AND EAST...WITH AN EMPHASIS ON THE SOUTHERN TERMINALS KOLS AND KDUG.
BRIEF MVFR CONDITIONS AND GUSTY WINDS MAY BE POSSIBLE NEAR STRONGER
TSRA. OTHERWISE...WINDS WILL GENERALLY BE LIGHT AND TERRAIN DRIVEN
UNDER 10 KTS EXCEPT WLY 6-13KTS 20/21Z THRU 21/03Z. AVIATION
DISCUSSION NOT UPDATED FOR TAF AMENDMENTS.

&&

.FIRE WEATHER...MOISTURE WILL START TO INCREASE TODAY WITH ISOLATED
SHOWERS AND THUNDERSTORMS POSSIBLE...MAINLY SOUTHERN AREAS. THEN
THE MOISTURE AND THUNDERSTORMS CHANCES WILL INCREASE AND SPREAD
THROUGHOUT SOUTHEAST ARIZONA THROUGH THE WEEKEND INTO EARLY NEXT
WEEK. WINDS WILL BE TYPICAL FOR MID AUGUST AND GENERALLY LESS THAN
15 MPH THEN THE WIND WILL BECOME EASTERLY MONDAY AND TUESDAY...BUT
AGAIN GENERALLY REMAINING 15 MPH OR LESS.

&&

.PREV DISCUSSION...BASED ON VARIOUS NWP SOLUTIONS...THERE WERE NO
SIGNIFICANT ADJUSTMENTS TO THE INHERITED GRIDDED DATA
POP/WEATHER/TEMP FIELDS FOR THIS FORECAST ISSUANCE. HIGH PRESSURE
ALOFT CENTERED NEAR THE NRN BAJA CALIFORNIA COAST WILL WEAKEN INTO
THIS WEEKEND. THE DRY NWLY FLOW GENERALLY IN THE 700-300MB LAYER
WILL TRANSITION TO LIGHT WLY FLOW FRI AND CONTINUE SAT.

FOR TODAY AND TONIGHT...SEVERAL HRRR SOLUTIONS SUGGEST AN EARLY
START TO SHOWER/TSTM ACTIVITY...PERHAPS BY 18Z-19Z SOUTH-TO-
SOUTHEAST OF TUCSON AND ESPECIALLY FAVORING SANTA CRUZ/SWRN COCHISE
COUNTIES BY MID-AFTERNOON. MEANWHILE...THERE SHOULD BE ENOUGH
MOISTURE TO JUSTIFY ISOLATED SHOWERS/TSTMS THIS AFTERNOON INTO
TONIGHT FROM TUCSON EAST TO THE NEW MEXICO BORDER. PRECIP-FREE
CONDITIONS WILL CONTINUE ACROSS WRN PIMA COUNTY.

20/00Z GFS/ECMWF/CMC SUGGEST THAT PRECIP COVERAGE SHOULD INCREASE
STARTING FRI FOLLOWED BY SCATTERED TO PERHAPS NUMEROUS SHOWERS/
TSTMS...ESPECIALLY SUN INTO MON NIGHT OR SO. THIS SCENARIO SEEMS
REASONABLE GIVEN THAT THE MID AND UPPER FLOW IS PROGGED TO BECOME
ELY/SELY STARTING SUN AND CONTINUING EARLY NEXT WEEK. HAVE NOTED
SOME DISCREPANCIES BETWEEN THE GFS/ECMWF SOLUTIONS FOR EARLY NEXT
WEEK. IN GENERAL...THE 20/00Z GFS TRENDED SOMEWHAT DRIER VERSUS
PREVIOUS GFS SOLUTIONS FOR QPF/S SUN-MON...AND THE 20/00Z ECMWF WAS
MARKEDLY MORE ROBUST WITH QPF/S FOR MON-TUE VERSUS THE 20/00Z GFS.

BASED ON THESE DIFFERENCES...OPTED TO MAKE ONLY VERY MINOR
MODIFICATIONS TO THE GRIDDED DATA POPS SUN-TUE. AT THIS TIME...
APPEARS THAT THE BEST COVERAGE OF SHOWERS/TSTMS SHOULD OCCUR ROUGHLY
SUN AFTERNOON INTO MON NIGHT...OR PERHAPS EARLY TUE. THEREAFTER...
THE GFS TRANSITIONED THE UPPER HIGH CENTER FROM NEAR THE FOUR
CORNERS REGION INTO SERN NEW MEXICO/WEST TEXAS NEXT WED. SWLY FLOW
ALOFT ASSOCIATED WITH THIS SCENARIO WOULD CONFINE SHOWERS/TSTMS
MAINLY TO ERN SECTIONS BY NEXT WED. HOWEVER...THE ECMWF MAINTAINED
THE UPPER HIGH TO BE CENTERED NEAR THE FOUR CORNERS. SLY/SELY FLOW
ALOFT ASSOCIATED WITH THIS SCENARIO WOULD KEEP PRECIP CHANCES HIGHER
FURTHER WWD VERSUS THE GFS SOLUTION. THE UPSHOT...ISOLATED TO
SCATTERED SHOWERS/TSTMS AREA-WIDE NEXT WED...WITH HIGHER POPS ACROSS
GREENLEE/GRAHAM/COCHISE COUNTIES.

HIGH TEMPS TODAY WILL BE QUITE SIMILAR TO TEMPS ACHIEVED WED... AND
WILL CONTINUE TO AVERAGE ABOUT 2-5 DEGS F ABOVE NORMAL. SOME MINOR
COOLING WILL THEN OCCUR FRI FOLLOWED BY NEAR NORMAL DAYTIME TEMPS
FOR MUCH OF THE AREA SAT-TUE. HIGH TEMPS SHOULD TREND UPWARD AND
GENERALLY A FEW DEGS ABOVE NORMAL NEXT WED.

&&

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

&&

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SPECIAL WEATHER STATEMENT
NATIONAL WEATHER SERVICE TUCSON AZ
1243 PM MST THU AUG 20 2015

AZZ502>504-515-202015-
UPPER SANTA CRUZ RIVER AND ALTAR VALLEYS INCLUDING NOGALES AZ-
TUCSON METRO AREA INCLUDING TUCSON/GREEN VALLEY/MARANA/VAIL AZ-
TOHONO O'ODHAM NATION INCLUDING SELLS AZ-
BABOQUIVARI MOUNTAINS INCLUDING KITT PEAK AZ-
1243 PM MST THU AUG 20 2015

...SIGNIFICANT WEATHER ADVISORY FOR CENTRAL PIMA COUNTY UNTIL 115 PM
MST...

AT 1242 PM MST...DOPPLER RADAR WAS TRACKING A STRONG THUNDERSTORM 10
MILES SOUTHWEST OF THREE POINTS...MOVING NORTHEAST AT 10 MPH.

REDUCED VISIBILITIES DUE TO BLOWING DUST AND WIND GUSTS UP TO 50 MPH
WILL BE POSSIBLE WITH THIS STORM.

LOCATIONS IMPACTED INCLUDE...
THREE POINTS.

LAT...LON 3186 11147 3198 11159 3220 11135 3200 11121
TIME...MOT...LOC 1942Z 215DEG 7KT 3200 11140