Severe Thunderstorm Warning 50, NWS Phoenix, AZ

SOUTHWEST ARIZONA/SOUTHEAST CALIFORNIA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PHOENIX AZ
245 PM MST FRI AUG 27 1999

.SYNOPSIS...
CONSIDERABLE MONSOON MOISTURE HAS SPREAD INTO THE AREA FROM THE
SOUTH...AND IN ADDITION THE ATMOSPHERE HAS BECOME QUITE UNSTABLE.
THIS WILL GIVE AN INCREASED THREAT FOR MAINLY AFTERNOON AND EARLY
NIGHTTIME SHOWERS AND THUNDERSTORMS OVER THE FORECAST AREA THROUGH
EARLY NEXT WEEK. THE POTENTIAL FOR VERY HEAVY RAIN WILL EXIST THROUGH
THE WEEKEND. A LOW PRESSURE SYSTEM APPROACHING THE WEST COAST EARLY
NEXT WEEK HAS THE POTENTIAL TO GRADUALLY DRY OUT THE FAR WESTERN
PORTIONS OF THE FORECAST AREA. THIS WILL LIMIT THUNDERSTORMS TO
SOUTHCENTRAL ARIZONA BY WEDNESDAY.

.DISCUSSION...
EVERYTHING STILL LOOKING ON TRACK FOR AN ACTIVE NIGHT TONIGHT...WITH
SCATTERED TO NMRS SHOWERS AND THUNDERSTORMS SPREADING GRADUALLY WWD
ACRS THE SRN AZ DESERTS INTO SWRN AZ. ATMOSPHERE OVER SRN AZ VERY
MOIST AND UNSTBL...PER HIGH CAPE VALUES ON PHX AND TUS RAOBS...AS
WELL AS VERY NEG LIFTED INDICES. GOES PWAT IMAGERY STILL SHOWS ENTIRE
CWA WITH PWAT VALUES 1.5 INCHES OR GREATER. SFC DEWPOINTS FM SERN CA
THRU PHX VERY HIGH...TYPICALLY UPPER 60S TO NEAR 80. VIS SAT LOOPS
AND 88D IMAGERY SHOW STORMS FIRING NICELY ACRS SERN AZ...ECNTRL
MTNS...AND ALONG THE MOG RIM. STEERING CURRENTS SHOULD BRING THESE
STORMS GRADUALLY WWD...PLUS OUTFLOWS FROM THESE STORMS COULD WELL END
UP MERGING IN THE PHX METRO AREA LATER THIS AFTN OR EARLY EVE WHICH
COULD THEN GENERATE VERY INTENSE...SEVERE...AND HEAVY RAINING STORMS
GIVEN THE EXPLOSIVE NATURE OF THE AMS ATTM. STORM DEVELOPMENT WILL BE
AIDED BY DYNAMICS OVER THE ERN PTNS OF THE FORECAST AREA...AS THERE
WILL BE A DEFORMATION ZONE OVERHEAD AS WELL AS H3 DIFLUENCE SHOWING
UP IN THE MODEL FORECASTS OF THE STREAMLINES. SHOULD LEAD TO TALL AND
HIGHLY REFLECTIVE STORMS. WILL GO WITH ETA SOLN...THO NO MODEL IS
PERFECT. MOST OF THE FORECAST AREA STILL UNDER THE GUN FOR SCT TSTMS
THRU SUN...AS HIGH H7 THETA-E AND 850 DEWPOINTS WILL REMAIN IN PLACE.
LESS DYNAMICS...AS THE DEFORMATION ZONE WILL WEAKEN...AND THE
INVERTED TROF TO THE S OF THE STATE SHOULD MOVE OFF TO THE W.
HOWEVER...UPPER HI FORECAST TO DRIFT INTO THE VICINITY OF FAR SRN NV
BY SAT NIGHT WHICH WOULD GIVE NICE NLY OR NELY STEERING CURRENT TO
SCNTRL AZ...THIS WOULD BRING RIM STORMS DOWN INTO THE DESERTS.

FOR TONIGHT WILL GO WITH SCT TO NUMEROUS SHOWERS AND TSTMS MOST
PLACES...CONTG THRU THE MORNING HOURS...WITH ISOLD STORMS ACRS SERN
CA. FAR WRN AREA STORMS HAVE BETTER POTENTIAL FOR STRONG WINDS AS MID
LVL DRYING IS TAKING PLACE OVER A RICH BOUNDARY LAYER...WITH STORMS
ACRS THE ERN CWA LIABLE TO GIVE HEAVY RAIN AND SOME GUSTY WINDS. WILL
KEEP CHANCE POPS MOST PLACES THRU SUN...WITH SLIGHT CHANCE FOR THE
FAR WRN ZONES...SUCH AS 21 AND 26. PER DRYING AND STABLIZING
ADVERTIZED OVER FAR SERN CA BY 36 HRS...WILL DROP MENTION OF STORMS
FOR LATER SAT NIGHT THRU SUN FOR THOSE ZONES. IN THE XTDD...MRF STILL
ADVERTISING A TROF TO APCH THE WEST COAST AND SUPPLY A DRYING SWLY
FLOW...BUT I WILL BELIEVE IT WHEN I SEE IT. WILL KEEP STANDARD BROAD
BRUSH FORECAST FOR ALL BUT FAR WRN AZ THRU WED. WILL MAKE A NOD TO
POTENTIAL DRYING BY TAKING WEATHER OUT OF THE XTDD FOR THE YUMA ZONE
ON TUE AND WED AND JUST GOING PARTLY CLOUDY TO FAIR. FEEL THERE COULD
INDEED BY A SLIGHT CHANCE THOSE DAYS BUT THAT CANNOT BE MENTIONED IN
THE XTDD FORECAST PER CURRENT GUIDELINES. 11