084 FXUS65 KPSR 130444 AFDPSR AZZ020>028-CAZ030>033-131045- SOUTHWEST ARIZONA/SOUTHEAST CALIFORNIA FORECAST DISCUSSION NATIONAL WEATHER SERVICE PHOENIX AZ 945 PM MST WED FEB 12 2003 .SYNOPSIS... ABUNDANT MOISTURE ASSOCIATED WITH AN UPPER LEVEL LOW PRESSURE SYSTEM WILL SPREAD SHOWERS ACROSS SOUTHWEST ARIZONA AND SOUTHEAST CALIFORNIA TONIGHT AND THURSDAY. LOCALLY HEAVY RAINFALL CAN BE EXPECTED DURING THIS TIME. LINGERING SHOWERS WILL CONTINUE FRIDAY BEFORE HIGH PRESSURE MOVES IN FOR THE WEEKEND AND GIVES DRIER AND WARMER WEATHER INTO THE EARLY PART OF NEXT WEEK. .DISCUSSION... UPPER CLOSED LOW THAT WAS OFF THE SRN CA COAST HAS OPENED UP...AND IS IN THE SLOW PROCESS OF MOVING EAST AND INLAND OVER THE DESERT SW. SLY FLOW AHEAD OF THE LOW HAS CREATED A SUBSTANTIAL TROPICAL TAP AND IS INGESTING COPIOUS AMOUNTS OF MOISTURE INTO SERN CA AND AZ. GOES PWAT IMAGERY SHOWS BETWEEN 1.5 AND 2 INCHES OF WATER JUST OFF CNTRL BAJA...AND LOCAL RAOBS ARE NEARLY SATURATED BELOW 500MB...WITH PWAT AT OR OVER 1 INCH. SCT TO NUMEROUS SHOWERS HAVE DEVELOPED CWA WIDE...BUT ARE FOCUSED EAST OF PHX AS OF 9 PM...AND FOR THE MOST PART THEY HAVE BEEN LIGHT THIS EVE. HOWEVER THERE ARE POCKETS OF MODERATE SHOWERS EMBEDDED IN THE OVERALL PRECIP SHIELD...AND RAINFALL IS EXPECTED TO CONTINUE TO INCREASE IN COVERAGE AND INTENSITY AS THE NIGHT WEARS ON. SURFACE DEWPOINTS HAVE RISEN DRAMATICALLY AS THE SHOWERS MOISTEN THE LOWER AMS...WITH RISES OF 20 TO OVER 30 DEGREES IN THE PAST 24 HOURS. VALUES OVER THE DESERTS NOW ARE IN THE 50S TO LOW 60S. A LOOK AT IR IMAGERY SHOWS A SERIES OF RIPPLES MOVING UP FROM THE SOUTH...EACH OF THESE WAVES WILL ADD ENHANCED UVV TO THE ATMOSPHERE AS THEY PASS THRU THE AREA OVER THE NEXT 24-48 HOURS...AND WITH THE AIRMASS SO WET THEY WILL HELP SPUR LOCALLY HEAVY SHOWERS. LATEST ETA CONTINUES THE IDEA OF SLOWLY MOVING THE TROF EAST DURING THE NEXT 2 DAYS...WITH A SERIES OF VORT MAXES/SHORT WAVES MOVING THRU THE TROF AND ACROSS THE CWA. LATEST ETA AGAIN OVERDOING THE FORCING/UVV AND QPF VALUES...IF IT WAS CORRECT BY 06Z MUCH OF THE WRN CWA SHOULD HAVE PICKED UP OVER ONE HALF INCH OF RAIN....WHEREAS AS OF 9 PM MOST PLACES HAVE PICKED UP LESS THAN 2 TENTHS. ETA SUGGESTS COPIOUS RAINFALL WILL OCCUR OVER SERN CA AND SWRN AZ TONIGHT...WIHT UVVS OF OVER 10 MICROBARS/SEC ...THEN THE DYNAMICS WILL SHIFT EAST AND MAINLY AFFECT THE AZ PORTION OF THE CWA THU INTO FRI. LATEST FOUS GUIDANCE FROM THE ETA GIVES AROUND 2.5 INCHES OF QPF FOR PHOENIX OVER THE NEXT 48 HOURS. THIS LIKELY OVERDONE...BUT EVEN IF HALF OF THIS OCCURS IT WILL BE A SIGNIFICANT EVENT FOR OUR AREA. IN ANY CASE...FOR THE REST OF TONIGHT RAIN WILL BE ON THE INCREASE EVERYWHERE...WITH THE HEAVIEST AMOUNTS LIKELY TO FALL WELL AFTER MIDNIGHT. HAVE REMOVED THUNDERSTORMS FROM THE FORECAST TONIGHT AS AMS TOO STABLE...AND NO LIGHTNING HAS BEEN SEEN SO FAR ON THIS SHIFT. FOR THE MOST PART THE PRECIP HAS BEEN STRATIFORM...WITH NOT MUCH CONVECTION NOTED. AS THE TROF PUSHES FURTHER EAST THU AND SOME COLDER AIR ALOFT MOVES IN FROM THE WEST...AIRMASS SHOULD START TO DESTABILIZE...AND THERE WILL BE AN INCREASING THREAT FOR STORMS. THIS IS ALREADY IN THE FORECAST. BY THU AFTN MOST OF THE HEAVY PRECIP SHOULD BE CONFINED TO AZ...WITH LESSER AMOUNTS IN SERN CA...AND THE OVERALL FOCUS AREA WILL LIKELY SHIFT TO MARICOPA COUNTY EAST BY FRI...WITH DECREASING THREATS FOR SERN CA AND THE FAR WESTERN DESERTS AS RH STARTS TO DECREASE AND UVVS LESSEN. AGAIN THIS TREND COVERED NICELY IN THE CURRENT PACKAGE. HAVE MADE SOME MINOR TWEAKS TO THE FORECAST WORDING FOR TONIGHT AND THURSDAY...MAINLY REMOVING THUNDERSTORMS...OTHERWISE NO BIG CHANGES NEEDED. WITH AIRMASS SO WET AND STRONG FORCING STILL EXPECTED OVER AZ LATER TONIGHT THRU THU...COULD SEE SOME FLOODING PROBLEMS AND THUS WILL LET THE FLASH FLOOD WATCH CONTINUE IN EFFECT. 11 .PSR...FLASH FLOOD WATCH FOR SOUTH-CENTRAL AND SOUTHWEST ARIZONA...ZONES 21 THROUGH 28...FROM MIDNIGHT THROUGH 5 PM THURSDAY. $$