FXUS66 KSGX 170955
AFDSAN

EXTREME SOUTHWESTERN CALIFORNIA AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SAN DIEGO CA
330 AM PDT FRI MAY 17 2002

.SYNOPSIS...HIGH PRESSURE WILL CONTINUE TO BUILD ALOFT THROUGH 
SATURDAY. THIS WILL WEAKEN THE ONSHORE FLOW AND LOWER THE MARINE 
LAYER. ON SUNDAY THE MARINE LAYER WILL BEGIN TO DEEPEN AS AN UPPER 
LEVEL TROUGH APPROACHES THE CALIFORNIA COAST. AS THE TROUGH MOVES 
INLAND MONDAY AND TUESDAY THERE IS A CHANCE OF SHOWERS OVER 
SOUTHWEST CALIFORNIA. 

.DISCUSSION...SUBSIDENCE FROM RIDGING ALOFT APPARENT IN WATER VAPOR 
IMAGERY AND WARMER TEMPS OVER HIGHER ELEVATIONS THURSDAY...AS WELL 
AS SOMEWHAT SHALLOWER MARINE LAYER. LOOKING AT THESE SHORT TERM 
TRENDS AND MODEL GUIDANCE...DROPPED MENTION OF DRIZZLE THROUGH  
SATURDAY. SATURDAY NIGHT AVN MODEL SHOWS INCREASING HIGH LEVEL 
RELATIVE HUMIDITY AS SW FLOW INCREASES AND ENTRAINS SUBTROPICAL 
MOISTURE AHEAD OF APPROACHING TROUGH. LEFT IN MENTION OF DRIZZLE ON 
SUNDAY FROM NET UPWARD VERTICAL VELOCITY/REDEEPENING OF MARINE LAYER 
AS TROUGH NEARS. ECMWF HAS COME AROUND TO THE MRF TIMING AND ON 
DEVELOPMENT OF THE TROUGH AND ITS MOVEMENT INLAND THROUGH TUESDAY 
BUT NOT AS DEEP. HOWEVER AVN CROSS SECTIONS SHOWS SURFACE TO 600 MB 
LAYER NEAR SATURATION MONDAY AND UP TO 700 MB ON TUESDAY...THUS  
MENTION OF SHOWERS STILL JUSTIFIED. ALL MODELS SHOW INITIAL 
ORIENTATION OF TROUGH ON MONDAY WITH RESPECT TO FORECAST AREA 
NEGATIVE TILT AS IT BEGINS TO MOVE INLAND. WILL HOLD OFF FOR ANOTHER 
MODEL RUN TO EVALUATE COLD CORE TEMPS BEFORE INCLUDING MENTION OF 
SNOW LEVELS AND THREAT OF TSTMS. MEDIUM RANGE MODELS GO OUT OF PHASE 
AFTER TUESDAY WITH MRF MORE PROGRESSIVE WHILE ECMWF HANGS BACK THE 
TROUGH OVER SE CALIFORNIA THROUGH THURSDAY. WILL LEAVE IN IMPROVING 
TREND FOR WED/THU FORECAST AS INHERITED DUE TO UNCERTAINITY. 

.SAN...NONE.

BALFOUR