Forecast Discussions mentioning ACARS, received at FSL on 09/20/99

...SYNOPSIS... AN UPPER LEVEL LOW OFF THE COAST WILL MOVE SLOWLY SOUTHWEST THROUGH TUESDAY AS HIGH PRESSURE BUILDS OVER SOUTHERN CALIFORNIA. THIS WILL LOWER THE MARINE LAYER WITH EARLIER BURNOFF OF THE COASTAL LOW CLOUDS AND FOG ON MONDAY AND LIMITING THE LOW CLOUDS AND FOG TO THE IMMEDIATE COAST ON TUESDAY. HIGH LEVEL MOISTURE FROM TROPICAL STORM HILARY WILL BEGIN TO MOVE OVER THE AREA MONDAY AFTERNOON AND INCREASE THROUGH TUESDAY. THIS WILL BRING A SLIGHT CHANCE OF THUNDERSTORMS TO THE MOUNTAINS AND DESERTS ON TUESDAY. AUTMN BEGINS ON THURSDAY...SEPTEMBER 23...AT 431 AM PDT.

...DISCUSSION... THE STRATUS AND FOG THAT COVERED THE AREAS WEST OF THE MOUNTAINS THIS MORNING HAS CLEARED OUT TO WELL OFFSHORE AT THIS TIME. VISIBLE SATELLITE LOOP INDICATES THE STRATUS IS BEING PULLED AWAY FROM THE COAST IN RESPONSE TO THE EDDY THAT IS SHOWING UP SOUTHWST OF THE CHANNEL ISLANDS. THE EDDY IS LIKELY A SURFACE REFLECTION OF THE WEAK 850 MB LOW THAT IS OFF THE SOUTHERN CALIFORNIA COAST. THE WIND NEAR POINT CONCEPTION IS NOT SUFFICIENT TO MAINTAIN AN EDDY...OTHERWISE. THE MARINE INVERSION IS NOT AS STRONG AS ON SATURDAY. A RECENT ACARS SOUNDING OUT OF ONTARIO SHOWED ABOUT A 5C DIFFERENCE IN TEMPERATURE BETWEEN THE BASE AND THE TOP OF THE INVERSION...MUCH LOWER THAN THE 9C DIFFERENCE ON SATURDAY.

THE 12Z MODELS ARE SIMILAR WITH THE RETROGRESSION OF THE CUT OFF LOW CURRENTLY OFF THE CENTRAL CALIFORNIA COAST. AS THE LOW RETROGRADES...HIGH PRESSURE BUILDS IN FROM THE SOUTHEAST INTO SOUTHERN CALIFORNIA THROUGH THE NEXT 48 HOURS. ALL THREE MODELS TAP INTO SPIN OFF MOISTURE FROM TROPICAL STORM HILARY AND BRING THIS MOISTURE NORTH INTO THE AREA BEGINNING MONDAY. THE ETA AND AVN BRING THE MOISTURE ACROSS ALMOST ALL OF SOUTHERN CALIFORNIA WHILE THE NGM KEEPS THE MOISTURE EAST OF THE CWFA AT LEAST THROUGH 12Z TUESDAY. THE AVN 850 MB COMPUTED LIFTED INDEX GRAPHICS ACTUALLY BRING 0 TO MINUS 2 LIFTED INDEX TO THE COASTAL AREAS ON TUESDAY. THE ETA IS NOT AS BULLISH AS THE AVN MODEL WITH THE MOISTURE...AT LEAST THROUGH 12Z TUESDAY...AND HAS KEEPS THE INSTABILITY EAST OF THE CWFA...MAINLY OVER THE SOUTHEAST DESERTS AND ARIZONA. FOR NOW...WILL LEAN TOWARDS A COMPROMISE BETWEEN THE ETA AND AVN SOLUTIONS AND INTRODUCE A SLIGHT CHANCE OF THUNDERSTORMS TO THE MOUNTAINS AND DESERTS FOR TUESDAY. IF THE AVN MODEL IS CORRECT...THE AREAS WEST OF THE MOUNTAINS COULD ALSO BE VULNERABLE. WILL LEAVE THAT TO LATER SHIFTS WITH UPDATED MODEL INPUT TO DECIDE THAT. WITH THE MARINE LAYER SUPPRESSED AND SIGNIFICANT THICKNESS/HEIGHT RISES THROUGH TUESDAY...EXPECT TUESDAY TO BE MUCH WARMER WEST OF THE MOUNTAINS. MID AND HIGH CLOUDS COULD OFFSET THIS WARMING...HOWEVER.

MEDIUM RANGE MODELS SUGGEST ONCE THE MOISTURE FROM HILARY IS OUT OF THE PICTURE LATER IN THE WEEK... SHOULD SEE TEMPERATURES NEAR OR PERHAPS ABOVE NORMAL BY NEXT WEEKEND WITH DRY WEATHER AS THE RIDGE BECOMES ESTABLISHED OVER THE WEST COAST.

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HORTON

SAT PIX SHOW LOW LVL CRCLN CNTR HAS CONTD TO DRIFT WWD AWAY FROM SRN CA CST DURG THE PAST 24 HRS...ALLOWING FOR BETTER THAN XPCTD CLRG ON SUN. WWD MIGRATION OF THE LOW LVL CRCLN...OFSHR PRES GRAD TRENDS AND WKNG INVERSION WORKED TOGETHER TO KEEP SKIES CLR ACRS MOST OF THE FCST AREA THRU THIS A.M.. LOW CLDS WERE BEGINNING TO BLD TOWARD THE CST OF L.A. AND VTU COUNTIES ATTM...AND SUSPECT PTCHY OR AREAS OF LOW CLDS IN THOSE AREAS THIS A.M.. ALSO...SOME LOW CLDS/FOG WL PROBABLY DVLP ACRS PORTIONS OF THE SBA CNTY CNTRL CST AS WELL. OTHERWISE...XPCT MOSTLY CLR SKIES ACRS THE FCST AREA TODAY. UPR RDG BEGINNING TO BLD NWWD INTO SRN CA WHICH HAS CAUSED MARINE INVERSION TO LOWER...WITH ACARS SOUNDINGS SHOWING INVERSION CURRENTLY 2000-2500 FT. PROFILERS SHOWS SEVERAL DEGS OF WARMING ABOVE 1500 FT. LWRG INVERSION...WKR ONSHR FLOW...PLENTY OF SUN AND INCREASES IN H85 TEMPS/THICKNESS SHOULD CAUSE TEMPS TO RISE AN AVG OF 4-8 DEGS IN MOST AREAS TODAY OVER SUN.

MDLS ARE IN VERY GOOD AGREEMENT WITH UPR PATTERN THRU 48 HRS...SHOWING UPR LOW CONTG TO SPIN SEVERAL HUNDRED MILES W OF PT CONCEPTION...WITH UPR HIGH OVER SWRN STATES CONTG TO BLD WWD INTO SRN CA. THIS WILL SET UP INCRGLY DEEP SLY FLOW THRU THE ATMOS...WHICH WL BEGIN TO TRANSPORT SUBTROPICAL MSTR...INCLUDING THAT ASSOCD WITH "HILLARY"...INTO SRN CA. WITH SUCH COLD WATER TEMPS OFF OUR CST...THERE IS ABSOLUTELY NO THREAT OF "HILLARY" MAINTAINING EVEN TROPICAL STORM STATUS EVEN IF ITS CRCLN WERE EVENTUALLY TO DRIFT ACRS THE FCST AREA. HWVR...IT SEEMS INEVITABLE THAT AT LEAST SOME OF ITS MID AND HIGH LVL MSTR WL BE STEERED INTO SRN CA...WITH ALL MODELS INDICATING ABUNDANT AND INCRGLY DEEP MSTR PUSHING TOWARD THE FCST AREA TUE. AVN AND NGM ARE SIMILAR IN TIMING WITH THEIR MSTR FIELDS...SHOW QUITE AN IMPRESSIVE SWATH OF HIGH RH VALUES FROM H85 THRU H5 SPRDG ACRS THE SERN HALF OF THE FCST AREA TUE. THE ETA IS A BIT SLOWER...ESPECIALLY WITH MSTR IN THE LOWEST LVLS. THE AVN BRINGS VERY HIGH PRECIPITABLE WATER VALUES TO XTRM SRN CA BY TUE AFTERNOON... WITH THE ETA A BIT SLOWER ON THESE AS WELL. WITH THE BULK OF THE MSTR STILL SO FAR S AND STILL ENTRAINED ARD THE CRCLN OF THE HURRICANE...BLV THE ETA MAY HAVE THE BEST HANDLE ON THE MSTR FIELD...AND TEND TO FAVOR THE SLOWER ETA SOLN.

WITH DVLPG SLY FLOW ALF AND INCRSG HIGH LVL MSTR...BLV MARINE LAYER CLOUDS WL BE PTCHY AT BEST TONIGHT AND TUE A.M....AND CONFINED TO THE CST. EVEN WITH SLOWER ETA SOLN...EXPECT HIGH AND MID LVL CLOUDINESS TO BEGIN TO INCRS ACRS SRN SNXS BY TUE A.M....OVERSPREADING THE FCST AREA DURG THE DAY. IN SPITE OF INCRSD CLD CVR...RISES IN THICKNESSES AND H85 TEMPS ALG WITH DECRSD ONSHR LOW LVL FLOW AND SUBTROPICAL AIR MASS SHOULD ALLOW FOR SEVERAL DEGS MORE WARMING ON TUE. LATER SHIFTS MAY EVEN NEED TO FURTHER BUMP UP TEMPS FOR CSTL SXNS FOR TUE. OF COURSE... IF THE CLOUD COVER IS MORE SOLID THAN CURRENTLY XPCTD...WARMING WL BE NEGATED.

WITH THE CAUSE OF ANY SHOWER/TSTM ACTIVITY EXPECTED TO BE MORE DRIVE BY SYNOPTIC PATTERN THAN TOPOGRAPHY...FEEL THAT CSTL AND VLY LOCATIONS WL HAVE NEARLY AS GOOD A CHANCE OF PRECIPITATION AS MTNS AND DSRTS. THEREFORE...HAVE ADDED SLGT CHC SHWRS/TSTSM TO CSTL AND VLY SXNS OF L.A. COUNTY AND VLYS OF VTU COUNTY FOR LATE IN THE DAY TUE. LOOKS LIKE THE BULK OF THE MSTR ALG WITH BEST THREAT OF SHOWER/TSTM ACTIVITY WL OCCUR ACRS THE FCST AREA LATE TUESDAY NIGHT AND WED. THEN...AXIS OF BEST MSTR SHIFTS TO THE N AND E OF THE AREA BY THU...THERE MAY BE ENOUGH RESIDUAL MSTR FOR AT LEAST A SLGT CHC OF SHWRS/TSTMS INTO THU. AT THIS TIME... IT IS TOO EARLY TO FCST PRECIP AMTS WITH ANY DEGREE OF CONFIDENCE... BUT THERE IS A FAIR CHANCE THAT MUCH OF THE FCST AREA WL RECEIVE AT LEAST MEASURABLE PRECIP BETWEEN TUE AFTERNOON AND THU MORNING. AND...ALTHOUGH A SMALL SHIFT IN THE AXIS OF BEST MSTR WOULD LEAVE THE FCST AREA DRY...THE POTENTIAL ALSO EXISTS FOR WDSPRD PRECIP AMOUNTS OF UP TO ONE HALF WITH LOCALLY HEAVIER TOTALS.

LATER THU AND FRI...UPR RIDGE BLDG INTO NRN CA WL BEGIN TO DRY THING OUT...WITH TEMPS LIKELY TO BE SOMEWHAT ABOVE NORMAL FOR A CHANGE. MODELS CONT TO HINT AT SOME WK OFSHR FLOW FRI AND SAT...WHICH IF IT WERE TO OCCUR...WOULD CAUSE SOME ADDITIONAL WARMING IN CSTL SXNS.

LAX 002. BRUNO

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