Forecast Discussions mentioning ACARS, received at FSL on 09/21/99

CORRECTION TO SYNOPSIS

...SYNOPSIS... MOISTURE FROM WEAKENING TROPICAL STORM HILARY HAS BEGUN TO SPREAD OVER SOUTHERN CALIFORNIA. THIS MOISTURE WILL BRING SHOWERS AND THUNDERSTORMS TO THE AREA TUESDAY AFTERNOON THROUGH WEDNESDAY.

...DISCUSSION... CLEAR SKIES COVER THE CWFA THIS AFTERNOON EXTENDING OFFSHORE TO WEST OF SAN CLEMENTE ISLAND. BASED ON RECENT ACARS DATA IS APPEARS THE MARINE LAYER HAS LOWERED SIGNIFICANTLY TODAY. PRESSURE GRADIENTS ARE WEAK ONSHORE TO THE SOUTHERN DESERTS BUT WEAK OFFSHORE FROM THE COAST TO THE GREAT BASIN AND ARE TRENDING OFFSHORE IN BOTH DIRECTIONS.

SOME CU IS BEGINNING TO SHOW UP OVER THE MOUNTAINS OF NORTHERN BAJA AND APPEARS TO BE THE FIRST WAVE OF SPIN OFF MOISTURE FROM TROPICAL STORM HILARY. 12Z MODELS ARE IN GENERAL AGREEMENT INCREASING THE MOISTURE TONIGHT BUT THE MORE SIGNIFICANT MOISTURE WILL MOVE IN TUESDAY AFTERNOON THROUGH WEDNESDAY. A DEVELOPING OFFSHORE FLOW TONIGHT WILL COMBINE WITH INCREASED SUBSIDENCE TO PUSH WHAT IS LEFT OF ANY MARINE LAYER TO THE COAST ON TUESDAY AND BRING SIGNIFICANT WARMING MOST AREAS ON TUESDAY. THIS OFFSHORE FLOW WILL LIKELY PREVENT MUCH PRECIPITATION FROM THE TROPICAL MOISTURE FROM REACHING THE GROUND AT LEAST THROUGH TUESDAY EVENING. STILL...WITH THE SIGNIFICANT WARMING EXPECTED ON TUESDAY ALONG WITH THE INCREASE IN THE TROPICAL MOISTURE...EXPECT SOME THUNDERSTORM ACTIVITY OVER THE MOUNTAINS TUESDAY AFTERNOON. 00Z MRF TRACKS THE SURFACE LOW ASSOCIATED WITH HILARY NORTH NORTHWEST INTO THE SOUTHERN CALIFORNIA COASTAL WATERS BY WEDNESDAY MORNING WHILE THE 12Z AVN TAKES THE LOW NORTH ACROSS NORTHERN BAJA INTO THE GULF OF CALIFORNIA...SIMILAR TO THE TRACK OF HURRICANE NORA BACK IN 1997. EITHER WAY...THE MAIN IMPACT WILL BE THE MOUNTAINS AND DESERTS OF SOUTHERN CALIFORNIA...ESPECIALLY OF SAN DIEGO COUNTY AND THE COACHELLA VALLEY. THE WESTERLY TRACK WOULD ALSO IMPACT THE COASTAL AREAS WHILE THE MORE EASTWARD TRACK OF THE AVN WOULD KEEP MOST OF THE ACTIVITY AWAY FROM THE COAST. FOR NOW...WILL GO WITH LIKELY CATEGORY POPS FOR THE MOUNTAINS FOR TUESDAY NIGHT AND WEDNESDAY AND INCREASE THE DESERTS TO THE LIKELY CATEGORY FOR WEDNESDAY. ELSEWHERE...WILL KEEP POPS IN THE CHANCE CATEGORY FOR TUESDAY NIGHT AND WEDNESDAY. WILL LEAN TOWARDS THE NCEP QPF AMOUNTS...REFERENCE NFDQPFPFD.

WARMING ON TUESDAY COMBINED WITH INCREASED HUMIDITY AND CLOUD COVER FOR TUESDAY NIGHT SHOULD KEEP MINIMUM TEMPERATURES WEDNESDAY MORNING WELL ABOVE NORMAL.

MRF CONTINUES TO INDICATE THE CUT OFF LOW OFF THE CENTRAL COAST WILL RETROGRADE THROUGH THE WEEK AS THE SUBTROPICAL HIGH BUILDS NORTHWEST AND EVENTUALLY MERGES WITH THE HIGH OVER THE GULF OF ALASKA BY FRIDAY. THIS WILL BRING A NORTHEAST TO EAST FLOW ACROSS SOUTHERN CALIFORIA FRIDAY AND SATURDAY. SURFACE PRESSURE GRADIENTS ALSO GO BRIEFLY OFFSHORE FROM THE GREAT BASIN TO THE INLAND EMPIRE BUT REMAIN ONSHORE OVER COASTAL SAN DIEGO COUNTY. HOWEVER...SIGNIFICANT INCREASE IN THICKNESS BY SATURDAY SHOULD BRING WARMING MOST AREAS.

SAN 0234

.SAN...NONE.

HORTON

...SYNOPSIS... MOISTURE FROM WEAKENING TROPICAL STORM HILARY...WHICH IS OFF THE WEST COAST OF THE SOUTHERN TIP OF BAJA THIS AFTERNOON...WILL BEGIN TO SPREAD OVER SOUTHERN CALIFORNIA TONIGHT. THIS MOISTURE WILL BRING SOME SHOWERS AND THUNDERSTORMS TO THE AREA TUESDAY AFTERNOON THROUGH WEDNESDAY. RAINFALL AMOUNTS UP TO AN INCH IS POSSIBLE IN THE MOUNTAINS OF SAN DIEGO COUNTY AND SOUTHERN RIVERSIDE COUNTY WITH BETWEEN A QUARTER OF AND INCH AND ONE INCH POSSIBLE ELSEWHERE BY WEDNESDAY MORNING.

...DISCUSSION... CLEAR SKIES COVER THE CWFA THIS AFTERNOON EXTENDING OFFSHORE TO WEST OF SAN CLEMENTE ISLAND. BASED ON RECENT ACARS DATA IS APPEARS THE MARINE LAYER HAS LOWERED SIGNIFICANTLY TODAY. PRESSURE GRADIENTS ARE WEAK ONSHORE TO THE SOUTHERN DESERTS BUT WEAK OFFSHORE FROM THE COAST TO THE GREAT BASIN AND ARE TRENDING OFFSHORE IN BOTH DIRECTIONS.

SOME CU IS BEGINNING TO SHOW UP OVER THE MOUNTAINS OF NORTHERN BAJA AND APPEARS TO BE THE FIRST WAVE OF SPIN OFF MOISTURE FROM TROPICAL STORM HILARY. 12Z MODELS ARE IN GENERAL AGREEMENT INCREASING THE MOISTURE TONIGHT BUT THE MORE SIGNIFICANT MOISTURE WILL MOVE IN TUESDAY AFTERNOON THROUGH WEDNESDAY. A DEVELOPING OFFSHORE FLOW TONIGHT WILL COMBINE WITH INCREASED SUBSIDENCE TO PUSH WHAT IS LEFT OF ANY MARINE LAYER TO THE COAST ON TUESDAY AND BRING SIGNIFICANT WARMING MOST AREAS ON TUESDAY. THIS OFFSHORE FLOW WILL LIKELY PREVENT MUCH PRECIPITATION FROM THE TROPICAL MOISTURE FROM REACHING THE GROUND AT LEAST THROUGH TUESDAY EVENING. STILL...WITH THE SIGNIFICANT WARMING EXPECTED ON TUESDAY ALONG WITH THE INCREASE IN THE TROPICAL MOISTURE...EXPECT SOME THUNDERSTORM ACTIVITY OVER THE MOUNTAINS TUESDAY AFTERNOON. 00Z MRF TRACKS THE SURFACE LOW ASSOCIATED WITH HILARY NORTH NORTHWEST INTO THE SOUTHERN CALIFORNIA COASTAL WATERS BY WEDNESDAY MORNING WHILE THE 12Z AVN TAKES THE LOW NORTH ACROSS NORTHERN BAJA INTO THE GULF OF CALIFORNIA...SIMILAR TO THE TRACK OF HURRICANE NORA BACK IN 1997. EITHER WAY...THE MAIN IMPACT WILL BE THE MOUNTAINS AND DESERTS OF SOUTHERN CALIFORNIA...ESPECIALLY OF SAN DIEGO COUNTY AND THE COACHELLA VALLEY. THE WESTERLY TRACK WOULD ALSO IMPACT THE COASTAL AREAS WHILE THE MORE EASTWARD TRACK OF THE AVN WOULD KEEP MOST OF THE ACTIVITY AWAY FROM THE COAST. FOR NOW...WILL GO WITH LIKELY CATEGORY POPS FOR THE MOUNTAINS FOR TUESDAY NIGHT AND WEDNESDAY AND INCREASE THE DESERTS TO THE LIKELY CATEGORY FOR WEDNESDAY. ELSEWHERE...WILL KEEP POPS IN THE CHANCE CATEGORY FOR TUESDAY NIGHT AND WEDNESDAY. WILL LEAN TOWARDS THE NCEP QPF AMOUNTS...REFERENCE NFDQPFPFD.

WARMING ON TUESDAY COMBINED WITH INCREASED HUMIDITY AND CLOUD COVER FOR TUESDAY NIGHT SHOULD KEEP MINIMUM TEMPERATURES WEDNESDAY MORNING WELL ABOVE NORMAL.

MRF CONTINUES TO INDICATE THE CUT OFF LOW OFF THE CENTRAL COAST WILL RETROGRADE THROUGH THE WEEK AS THE SUBTROPICAL HIGH BUILDS NORTHWEST AND EVENTUALLY MERGES WITH THE HIGH OVER THE GULF OF ALASKA BY FRIDAY. THIS WILL BRING A NORTHEAST TO EAST FLOW ACROSS SOUTHERN CALIFORIA FRIDAY AND SATURDAY. SURFACE PRESSURE GRADIENTS ALSO GO BRIEFLY OFFSHORE FROM THE GREAT BASIN TO THE INLAND EMPIRE BUT REMAIN ONSHORE OVER COASTAL SAN DIEGO COUNTY. HOWEVER...SIGNIFICANT INCREASE IN THICKNESS BY SATURDAY SHOULD BRING WARMING MOST AREAS.

SAN 0234

.SAN...NONE.

HORTON

MAIN FORECAST CONCERN CONTINUES TO BE LAKE EFFECT CLOUDS/SHOWERS FOR CHICAGO AND NW IND...DIURNAL CAA STRATOCU AND EFFECT ON TEMPS ELSEWHERE. 04Z ACARS SOUNDING OUT OF ORD SHOWS H85 TEMP AROUND 2.8 DEG C. SOUTHERN LAKE MICHIGAN DATA BUOY SHOWS WATER TEMP OF 19 DEG C. H85-LAKE DELTA T OF 16 DEG C RESULTING IN RAIN SHOWERS ALONG THE ILLINOIS SHORE OF THE LAKE. KLOT WSR88-D SHOWING A FEW 45 TO 50 DBZ RETURNS IN SOME OF THESE SHOWERS VCNTY OF WAUKEGAN.

WHILE THE ACARS SOUNDINGS SHOW SFC-7KFT WNDS OUT OF DUE NORTH...MODELS ARE ALL FCSTG A SHIFT TO THE NE THROUGH THE BOUNDARY LAYER TODAY. WITH H85 CAA CONTG...EXPECT LAKE EFFECT CLOUDS AND SCT RAIN SHOWERS TO CONT TO AFFECT AREAS NEAR THE LAKE. THIS SHOULD COME TO AN END THIS EVENING HOWEVER...AS SFC RDG AXIS DROPS TO OUR SOUTH...AND WK WAA RETURNS WITH SFC WINDS BECMG WRLY. AWAY FROM THE LAKE...COLD POOL OVR AREA TODAY SHOULD ALLOW BKN DECK OF SC TO FORM...BUT WITHOUT PCPN. FAN AND FWC GUIDANCE GIVING MAX TEMPS IN THE 55 TO 60 RANGE TODAY...AND THIS LOOKS RSNBL GIVEN CLDS. WARMER TEMPS WED WITH WAA BRINGING H85 TEMPS BACK ABOVE 10 DEG C BY 00Z THU.

WITH SFC RDG PASSING OVR TNGT...CLR SKIES AND LGT WNDS WL SET UP GOOD RADIATIVE CONDITIONS. TEMPS IN NRN IL WL PROBABLY DIP INTO THE UPPER 30S OUT OF METRO CHI...GIVEN MODEL DEWPOINT FCSTS. SOME PATCHY FROST POSSIBLE IN SOME SUBURBAN/RURAL AREAS...BUT FEEL IT WILL NOT BE EXTENSIVE ENOUGH TO WARRANT AN ADVISORY.

.CHI...NONE.

RATZER

NO 12Z MODEL DATA YET THIS MORNING DUE TO DATA INGEST PROBLEMS AT NCEP...BUT LOOKS LIKE THE LAST SUNNY AND WARM DAY THIS MONTH (THIS YEAR TOO?) WILL BE WEDNESDAY. MEAN UPR RDG WL HOLD NR 115W THRU WED. MDLS STILL CONSISTENT IN PUSHING A 160KT JET EWD ACRS N PAC AND SLAMMING A S/W TROF ACRS B.C. THU AND KNOCKING THE RDG DOWN...KABLAM! STEEERIKE! MEDIUM RANGE MDLS ARE CONSISTENT IN RETROGRADING THE RDG BACK OUT TO 145W THIS WEEKEND. BAD NEWS FOR SUMMER IS MDLS CONTINUE RETROGRADING RDG BACK OUT TO 160W EARLY NXT WEEK WITH A BIG COLD TROF SETTING UP OVR THE PACNW...WHICH HAS BEEN THE MDL SOLN THE PAST FEW DAYS. THUS MDLS HAVE BEEN VERY CONSISTENT PAST FEW DAYS AND ACCEPTED.

SHORT TERM PROBLEM REMAINS MAX TEMPS AND MORNING ST/FOG COVERAGE. 850MB TEMPS PER ACARS DATA ARE ABT 1C WARMER THAN MDLS. WL NUDGE TEMPS BACK UP TO NR 80 TODAY PGTSND. SFC THERMAL TROF SHFTS E TNGT SO WED SHOULD BE SLGTLY COOLER...BUT WL LIKELY NUDGE WED MAXES UP A DEGREE TOO. VERY LITTLE ST/FG THIS MORNING SO EASY CALL FOR MORE COVERAGE WED...XPCT MAIN INTRUSION THRU CHEHALIS GAP GIVEN SHIFT TO SW GRAD.

EXTENDED...XPCT MARGINAL SW MARINE PUSH WED NIGHT FOLLOWED BY CDFNT EARLY THU MORNING. AVN IS HAIR FASTER THAN PVS RUN. DEFINITE CD ADVECTION AT 850 MB WITH TEMPS CRASHING DOWN TO 7.6C THU AFTN. WL NEED TO LOWER MAXES THU ON AFTN PKG. REMAINDER OF FCST STILL ON TRACK PER PVS FCSTS. KAM SEA 000 OLM 000 UIL 001

.KATX...VCP32/CSR32...OPERATIONAL.

.SEW...NONE.