Forecast Discussions mentioning ACARS, received at FSL on 05/15/99

...SYNOPSIS... A BROAD UPPER TROUGH WILL REMAIN ACROSS THE WEST THROUGH SUNDAY WITH A COASTAL EDDY EACH NIGHT AND MORNING FOR SOUTHERN CALIFORNIA. THIS WILL CONTINUE THE EXTENSIVE NIGHT AND MORNING LOW CLOUDS WEST OF THE MOUNTAINS WITH BREEZY CONDITIONS IN THE MOUNTAINS AND DESERTS.

...DISCUSSION... ONLY MINOR CHANGES IN THE PATTERN AFFECTING SRN CA THE NEXT TWO DAYS AS THE BROAD UPPER TROUGH CONTINUES OVR THE WRN U.S. WITH LOW LVL FLOW/PRESSURE GRADIENTS OVR SRN CA FAVORABLE FOR A CSTL EDDY NIGHT THRU MORNING HOURS THRU SUN MORNING. AS THE UPR TROUGH AXIS SHARPENS AND MOVES EAST ON SUN AND MON...SHORTWAVE RDGG ALF AND LOW LVL FLOW UNFAVORABLE FOR EDDY DEVELOPMENT SHOULD FINALLY RESULT IN A DECRG MARINE INFLUENCE W OF THE MTNS SUN NIGHT AND MONDAY.

ACARS SOUNDINGS THIS MORNING SHOW THE MARINE LYR DEEPENED SLIGHTLY IN THE PAST 24 HOURS TO AROUND 4500 FEET THIS MORNING...UP FM 4000 FT THU MORNING. PAST TWO RUNS OF THE COAMPS MDL (HTTP://205.67.220.5/TAMS/) HAVE SHOW THE EDDY CRCLN DRIFTING S AND DISSIPATING DURG THE DAY TODAY (NORMAL DIURNAL DECAY OF THE EDDY) WITH WK ONSHR W TO NW FLOW DEVELOPING THIS AFTERNOON AT LOW LVLS AS OPPOSED TO THE PERSISTENT S FLOW ALG THE CST YESTERDAY AFTERNOON. 03Z ETA 850 MB RH SHOWS A DECR IN 850 MB RH W OF THE MTNS TODAY. THIS...IN COMBINATION WITH THE DECAY OF THE EDDY...SHOULD ALLOW FOR FASTER...GREATER CLEARING W OF THE MTNS. HOWEVER...WITH MDT ONSHR LOW LVL FLOW CONTINUING AND WITH THE REDEVELOPMENT OF THE EDDY TONIGHT...LOW CLOUDS SHOULD SPREAD QUICKLY INLAND TO THE LOWER CSTL SLOPES THIS EVENING.

DEEP MARINE LYR...INCR IN ONSHR FLOW...AND LIFT PROVIDED BY A SHORTWAVE MOVG ACROSS SRN CA SAT NIGHT...COULD BE SUFFICIENT FOR MEASURABLE DRIZZLE OR LIGHT RAIN TO DEVELOP ALG AND W OF THE MTNS FM THE INLAND EMPIRE AND ORANGE COUNTY S. WL TAKE A CLOSER LOOK AT THIS FOR THE AFTERNOON ISSUANCE.

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MARTIN

MAIN CONCERNS THIS FCST PKG INCLUDE PSBL RETURN OF CLOUDS FROM SE TNGT...AND PCPN CHCS LATE SAT/EARLY SUN.

MEAN TROF OVR WRN US...WITH WEAKENING UPR RDG IN THE EAST. PESKY UPR LOW THAT AFFECTED IL ON WED DRIFTING SE OVR THE CAROLINAS. MODELS FAIRLY CONSISTENT IN CONTG TO WKN ERN RDG...AND EJECTING SEVERAL WEAK IMPLUSES OUT OF POS TILT WRN TROF INTO THE PLAINS NEXT SVRL DAYS.

VIS SAT IMGRY SHOWING SC/AC CLOUD DECK S OF UIN-SPI-SBN LINE AT 18Z. SOME CONCERN THAT THESE CLOUDS WOULD ADVECT BACK TO THE NW THIS AFTERNOON...INTO NRN IL. 1835Z ACARS SOUNDING OUT OF ORD CONFIRMS MODEL FCSTS OF FLOW BECOMING MORE SERLY FROM ABOUT H85 ON UP...BUT SUBSIDENCE ASSOCIATED WITH ADVANCING UPR RDG AXIS AND DRY NERLY SFC FLOW HELPING TO ERODE NRN EDGE OF CLOUD DECK. AS FLOW BECOMES MORE SRLY TONIGHT COULD SEE CLDS MOVE BACK INTO NRN IL. MODEL TIME SCTNS SHOW INCRS IN LOW LVL RH OVRNGT...SO SOME AMNT OF CLOUDS TNGT REASONABLE. MID/HIGH CLDS TO MOV INTO IL SAT...FROM CONVECTION OVR PLAINS TO OUR WEST LTR TONIGHT.

S/WV SEEN IN WV IMGRY LIFTING TO NORTHEAST OUT OF CO THIS AFTERNOON WL BRING BEST UPR DIVERGENCE AND 850-500MB Q VECTOR CONV TO PTNS OF CNTRL AND NRN PLNS TNGT. SCND S/WV FCST BY ETA TO LIFT OUT A BIT FARTHER TO EAST SATURDAY NIGHT...WITH DECENT H85 WAA AND 40KT LO LVL JET ORIENTED MORE TOWARD ERN IA/NWRN IL BY 12Z SUN. H85 WRMFNT THEN LIFTS TO THE NORTH ON SUNDAY...WITH BEST PCPN CHCS ENDING SUN AM. THUS WL CONT CHC POPS FOR SAT NGT INTO EARLY SUN.

TEMP WISE...FAN/FWC NUMBERS VERY CLOSE...AND WL FOLLOW CLOSELY. MAY GO A BIT ABOVE GUIDANCE SUNDAY...AS PROGGED H85 TEMPS WOULD SUGGEST PSBLY BREAKING THE 80 DEG MARK. OF COURSE...CHI AREA COOLER DUE TO LK BREEZE. .CHI...NONE.

RATZER

...SYNOPSIS... A BROAD UPPER TROUGH OF LOW PRESSURE OVER THE WEST WILL BEGIN TO MOVE EAST ON SUNDAY WITH WEAK HIGH PRESSURE BUILDING INTO SOUTHERN CALIFORNIA FOR MONDAY. A COASTAL EDDY WILL ONCE AGAIN BRING LOW CLOUDS AND PATCHY FOG TO AREAS WEST OF THE MOUNTAINS TONIGHT THROUGH SUNDAY MORNING. FOR THE MOUNTAINS AND DESERTS... BREEZY TODAY...WITH A LITTLE WARMER TEMPERATURES FOR SUNDAY.

...DISCUSSION... COAMPS AND OFF TIME ETA RUNS (PROBABLY 00Z AND 12Z RUNS AS WELL...XCP OUTPUT FM THOSE RUNS IS ON A COARSE 80 KM GRID EFFECTIVELY MASKING SUBSYNOPTIC DETAIL) CONTINUE TO PERFORM WELL IN SHOWING THE DIURNAL TRENDS OF THE EDDY WITH THE ETA PERFORMING REASONABLY WELL IN SHOWING TRENDS WITH THE MARINE LYR DEPTH.

ACARS SOUNDINGS SHOW THE BASE OF THE MARINE INVERSION BETWEEN 4000 AND 4500 FEET...SIMILAR OF SLIGHTLY LWR THAN 24 HOURS AGO. HOWEVER...COOLING IN THE LYR FM 850 MB TO 700 MB FM THE INLAND EMPIRE AND ORANGE COUNTY N HAS WEAKENED THE INVERSION IN THOSE AREAS...WITH RESULTING PATCHIER STRATUS COVERAGE IN THOSE AREAS VERSUS THE PAST FEW NIGHTS. HOWEVER...A 15Z ACARS SOUNDING FM LAX SHOWED THAT THE MARINE INVERSION IS BEGINNING TO STRENGTHEN WITH STRATUS COVERAGE ACTUALLY GREATER NOW THAN OVERNIGHT.

CSTL LOW CLOUDS...COAMPS AGAIN SHOWS THE EDDY DECAYING TODAY...THEN REFORMING TONIGHT...A TYPICAL DIURNAL CYCLE FOR THE EDDY. HOWEVER...S WINDS ALG THE CST THIS MORNING ARE ACTUALLY STRONGER THAN 24 HOURS AGO AND HENCE DECAY OF EDDY AND ASSOCD CLEARING TODAY MAY BE SLOWER THAN SHOWN BY MDLS AND THAT INDICATED IN THE CURRENT PACKAGE. HAVE UPDATED TO INDICATE A SOMEWHAT SLOWER AND LESS EXTENSIVE CLEARING PATTERN FOR TODAY. 00Z ETA SHOWED A GRADUAL DECR IN MARINE LYR DEPTH...TO AROUND 3000 FEET...DURG THE FORECAST PD. HOWEVER...GIVEN THAT HEIGHTS ALF ARE FORECAST TO CHANGE LTL DURG THE NEXT 24 HOURS...WOULD NOT EXPECT MUCH CHANGE...OTHER THAN DIURNAL CHANGES...THRU SUN MORNING. ETA ALSO SHOWS THAT MOST 850 MB COOLING HAS ALREADY OCCURRED (ALSO SUPPORTED BY THE 15Z ACARS SOUNDING FM LAX)...SO COOLING SUFFICIENT TO BREAK THE MARINE INVERSION APPEARS UNLIKELY. HENCE...AREAS OF LOW CLOUDS AND FOG SHOULD ONCE AGAIN PUSH INLAND TONIGHT TO THE LOWER CSTL SLOPES. FOR AFTERNOON FORECAST ISSUANCE...WL EVALUATE INCOMING 12Z GUIDANCE FOR INFO ON EDDY PERSISTENCE FOR SUN AND RESULTING IMPACTS ON CLRG W OF MTNS FOR SUN.

TEMPERATURES...HEIGHTS/TEMPERATURES ALF ARE LWR/COOLER TODAY THAN FRI...WITH SOME INCR/WARMING SHOWN FOR SUN. FOR MTNS AND DESERTS... THIS SHOULD RESULT IN SOME COOLING TODAY VERSUS FRI WITH SOME WARMING FOR SUN. FOR AREAS W OF THE MTNS...THE MARINE INFLUENCE SHOULD MODULATE THESE TRENDS SOMEWHAT WITH SLIGHT COOLING TODAY...AND LIKELY A SLOWER WARMUP ON SUNDAY THAN SHOWN IN THE MOS GUIDANCE OR IN THE CURRENT FORECAST PACKAGE...ESPECIALLY IF THE EDDY THAT DEVELOPS TONIGHT PERSISTS WELL INTO SUN. WL NOT MAKE ANY CHANGES FOR THE LATTER PERIODS NOW...BUT WL TAKE A CLOSER LOOK FOR THE AFTERNOON ISSUANCE.

WINDS...STRONGEST GRADIENTS TO SUPPORT WINDS IN THE MTNS AND DESERTS ARE TODAY WITH WEAKENING OF THE GRADIENTS SHOWN FOR SUN. EVEN TODAY...HOWEVER...WINDS SHOULD REMAIN BLO ADV LVLS.

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MARTIN