Cloud-related forecast discussions

SERIES OF POTENT SHORTWAVES TO CONTINUE TRACKING WELL TO THE N OF SRN CA IN WNWLY FLOW FM THE GULF OF AK...ACROSS THE PAC NW...INTO THE CENTRAL ROCKIES. AS EACH OF THESE SHORTWAVES PASSES BY TO THE NORTHEAST...HEIGHTS ALOFT WL LWR SLIGHTLY OVR SRN CA AND LOW LVL ONSHORE PRES GRADIENTS WL STRENGTHEN INCRG AFTERNOON AND EVENING WINDS IN THE DESERTS. IN BETWEEN SHORTWAVES...HEIGHTS ALOFT WL RECOVER OVR SRN CA AND LOW LVL GRADIENTS WL WEAKEN.

AS WITH LAST NIGHT...A WK EDDY CIRCLN IS IN PROGRESS ACROSS THE SRN CA CSTL WATERS AND SHOULD PERSIST THROUGH THE MORNING HOURS. ACARS SOUNDINGS FM BUR/ONT MON EVE SHOWED THE BASE OF THE MARINE INVERSION IN VLY AREAS OF 2-2.5 KFT MSL...SUFFICIENTLY DP FOR THE EXTENSIVE STRATUS FIELDOVER THE CSTL WATERS AND CSTL PLAIN TO SPREAD INTO MOST VLY AREAS BY MORNING. 10Z SATELLITE IMAGERY SHOWS THAT THE MARINE LYR HAS DEEPENED SUFFICIENTLY TO REACH THE LWR CSTL SLOPES THIS MORNING WITH THE STRATUS INTO NEWHALL...NEARLY TO BUO...ALMOST TO MWS...AND MOVG INTO THE CAJON PASS. A 09Z ACARS SOUNDING FM ONT SHOWED MARINE LYR MOISTURE TO AROUND 4K FT MSL WITH THE INVERSION BASE AROUND 5 KFT MSL. HAVE ADJUSTED ZONE WORDING TO INCLUDE PATCHY DRIZZLE THIS MORNING FOR MOST AREAS ON THE CSTL SIDE OF THE MTNS AND HAVE REDUCED CLEARING IN THE VALLEYS TODAY TO "BECOMING PRTLY CLDY IN THE AFTERNOON" DUE TO THE INCREASED DEPTH OF THE MARINE LYR.

MARTIN

UPR S/W TROF WITH CNTR NOW OVR S-SE BC STGR THAN ANTICIPATED. WNDS ALF ARE STRONGER AND THIS IN TURN HAS AFFECTED THE SFC ONSHR AND W TO E GRADIENTS WHICH ARE ALSO STGR THAN FCST FM YDAS PRGS. THE END RESULT HAS BEEN A STRETCHING OF THE MARINE LYR DUE TO MORE UVV BRINGG IN A CLOUD DECK AROUND 3 THSD FEET THICK. ACARS SOUNDINGS FM AROUND SEATAC AS WELL AS EARLIER PIREPS INDC TOP OF MARINE INVSN AND CLOUD TOPS ARND 4 THSD FT. THIS LYR THICK ENOUGH TO GENERATE A LTL DRZL CST AND CNTRL PGTSND. VSB STLT PICS SHOW SC DECK SOLID UP TO CASCD CREST. SOME CLRG OFF UPSTREAM OF N CST. QUESTION IS WL WEE SEE ANY SUNSHINE THIS AFTN...ANSWER IS NOT VERY MUCH IN THE INTERIOR BUT SOME ALONG NW CST AND IN STRT AND SAN JUANS. WL INDC A FEW SUNNY BREAKS FOR INTERIOR IN ZFP. PROGS SHOW FLAT UPR RDG FOR SUN FLWD BY APRCHG UPSTRM S/W TROF AS WINDS ALF BACK SUN AFTN. NEW ETA BRINGS 90 PCNT RH TO CST BY SUN SFTN. SFC GRADS REMAIN ONSHR THRU PD WITH VARING STRENGTH...PROBABLY WONT SEE MUCH SUN W SIDE. TIMING OF POPS FOR SUN AFTN ON CURRENT ZFP LOOKS GOOD. PROGS FOR MON STILL SHOW DIGG UPR LO OVR APCHG WA WHICH WL DOMINATE WX THRU 3 TO 5 DAY PD.

ART

WEAK OFFSHORE GRADIENTS HAVE HELPED CLEAR OUT STRATUS/FOG ALONG THE COAST AND SHRINK THE MARINE LAYER. LATEST ACARS SOUNDINGS INDICATE MARINE LAYER LESS THAN 1000 FEET. 12Z MODELS IN GENERAL AGREEMENT WITH INCREASING H5 HEIGHTS NEXT 24 HOURS. AT SURFACE...GRADIENTS SHOULD CONTINUE WEAK OFFSHORE THROUGH FRIDAY MORNING. NET RESULT WILL BE VERY SHALLOW MARINE LAYER OVERNIGHT...WITH STRATUS/FOG ONLY ACROSS THE COASTAL ZONES. FOR FRIDAT NIGHT/SATURDAY...MODELS INDICATE H5 HEIGHTS REMAIN STAGNANT WHILE SURFACE FLOW TURNS WEAK ONSHORE. THEREFORE...WILL EXPECT A LITTLE BETTER MARINE INFLUENCE WITH SOME PATCHY STRATUS/FOG IN VALLEYS FRIDAY NIGHT. AS FOR MOUNTAINS/DESERTS...WITH WEAK WESTERLY FLOW ALOFT...SKIES SHOULD REMAIN MOSTLY SUNNY THROUGH SATURDAY.

THOMPSON

MAIN CHG TO PREV FCST IS TO KEEP CLOUDS IN LONGER AND SUBSEQUENTLY LWR FCST MAX TEMPS A BIT.SATLT LOOPS SHW THAT LO CLD TRENDS CONT TO FAVOR INCRSG AREAL COVG WITH SC DECK NOW SPRDG SWWD ACROSS JUST ABT ALL OF CENTRAL AND PART OF SRN IL. CROSS SECTIONS AND ACARS DATA SHOW TOP OF INVERSION NR 9K FEET. DESPITE THIS...THIN BAND OF OVERLYING HI CLDNS OVR SRN WI HAS CAUSED SC DECK TO BREAK UP THERE. GUID SUG THAT INVERSION WL BE LOERG DURG TDY SO WL MENTION PTLY CLDY OR PARTIAL CLRG FOR THIS AFTN BUT WITH SUCH LGT LO LVL FLOW OVR NRN 1/2 STATE EXPCT THIS WL BE A SLO PROCESS.

NOT MUCH CHG TO 2ND AND 3RD PDS OF FCST. ALL MODLES CONT TO SHW SLO NEWD MOVNT OF SWRN U.S. SYS SO WL CONT WITH INCRSG CLDNS INTO CENTRAL IL ON SAT. SOME HI CLDS SHUD ALSO SPREAD INTO N AS WELL BUT EXPECT CHARACTER OF DAY TO BE PARTLY SUNNY.

MERZLOCK

SATELLITE IMAGERY LIMITED DUE TO GOES-9 TRANSMISSION PROBLEMS AT WALLOPS ISLAND...DON/T KNOW WHEN THIS WL IMPROVE.

WK EDDY PERSISTS ALG S CST AS INDICATED IN MARINE OBSERVATIONS AND IN LIMITED SATELITTE IMAGERY. THIS IS ALSO SUPPORTED BY THE RUC FORECASTS. 00Z SOUNDINGS SHOWED A MARINE LYR DEPTH OF 1500 FT AT VBG AND 3000 FT AT NKX. 07Z ACARS SOUNDING FROM ONT SHOWED A MARINE LYR DEPTH OF 4000 FT...THIS MESHES WELL WITH THE LOW CLOUDS EVIDENT ALG CSTL SLOPES IN THE AVBL SAT PICS. LTL CHG IN THIS PTN EXPCTD FOR THE NEXT 48 HOURS.

HOWEVER...UNLIKE LAST WEEK...COMBINATION OF COOLING/WK LIFT ASSOCD WITH THE UPR LOW TO THE N IS PROBABLY NOT STRONG ENOUGH TO ERODE THE PREEXISTING MARINE LYR IN NRN SECTIONS. HENCE...MORE EXTENSIVE NIGHT/MORNING LOW CLOUDS AND PATCHY FOG SHOULD CONTINUE IN CSTL/VLY AREAS THRU MON. DEPTH OF THE MARINE LYR SUGGESTS A SLOWER BREAKUP OF THE LOW CLOUDS IN THE VLYS AS OCCURED SAT. HAVE ADJUSTED AFTERNOON WORDING FOR THE VLYS TO "BECOMING PARTLY CLOUDY" INSTEAD OF "MOSTLY CLEAR".

MARTIN

FCST CONCERNS THIS PKG INCLUDE ANY PCPN CHCS NRN IL LATE SAT...AND RETURN TO COLDER TEMPS WITH STG CAA SAT NGT AND SUN.

18Z SFC ANALYSIS PLACES 1032MB HIGH OVR CNTRL WI. SFC TEMPS A FEW DEGS WARMER THAN LAKE MICHIGAN WATER TEMPS...WO WEAK LAND BREEZES HAVE OCCURED IN OTHWRS WEAK SYNOPTIC WINDS. VIS SAT PICS SHOWS STRATOCU OFF OF LM THINNING...WITH ORD ACARS SOUNDINGS SHOWING 900-925MB TEMPS HAVING WARMED FROM -5.3 TO -2.5 BTWN 12Z AND 18Z.

NEXT WEATHER FEATURE TO AFFECT NRN IL IS S/W SEEN IN WV IMGRY OVER SASKATCHEWAN. MODELS FAIRLY SIMILAR IN MVG THIS S/W INTO THE GRTLKS REGION BY SAT EVE...AND HELPING CUTOFF LOW OVR TX/OK EWD...BUT DIFFERENCES IN TIMING AND DETAILS. ETA/NGM AND AVN ALL POINT TWDS DRY FRONTAL PASSAGE FOR NRN IL AS TROPOSPHERE VERY DRY AND GULF COMPLETELY CUT OFF. STRONG DYNAMICS INDICATED BY BOTH LOW LVL Q VECTOR CONVERGENCE AND MODLE OMEGA FIELDS...BUT TSECTIONS INDICATE THAT THERE JUST ISN/T MUCH MOISTURE TO WORK WITH. LOW LVL RH DOES INCR BEHIND FRONT...BUT UPR DYNAMICS SHIFTED E BY THEN. LIKELY JUST MOCLDY...WINDY AND COLDER BEHIND FRONT INTO SUNDAY...MAYBE A FEW FLURRIES.

RATZER

MAIN FCST PROBLEM FOR THIS EVE IS POTENTIAL LOW CLD DEVELOPMENT IN THE BASIN. PRES GRADS TRENDING STRONGLY OFFSHORE...AND RAWS OBS IN THE MTNS AND HIR VLYS REPORTING PRETTY GUSTY N-NW WINDS. ACARS SOUNDINGS FROM LAX AND BUR SHOW NRLY WINDS DEVELOPING AT ABT 2500 FT AND REACHING 40 TO 45 KTS AT 8000 FT. TEMP PROFILES INDICATE ISOTHERMAL CONDS TO 4000 FT...WHICH IS NOT CONDUCIVE TO WIDESPREAD LOW CLD DEVELOPMENT. IN SHORT...THINK THAT LOW CLOUDS WILL BE AT A MINIMUM TONIGHT AND UPDATED ZONES TO REFLECT THIS. STILLENUF LOW LVL MOISTURE FOR HAZE/FOG...BUT OTHERWISE JUST SOME HIGH CLOUDS. DIDN/T CHG TEMPS...THO IN HINDSIGHT WISH I HAD. LUKS LIKE TEMPS WILL BE 3 TO 5 DEGREES WARMER THAN ZONES INDICATE...AT LEAST IN THE VLYS AND DOWNTOWN.

00Z RUNS IN AND ALL ARE SOMEWHAT DEEPER THAN 12Z RUNS. ETA LUKS THE STRONGEST WITH A SIGNIFICANT VORT MAX ABT 300 MI W OF PT CONCEPTION SUN AFTN. NGM MUCH FURTHER N WITH VORT AND NOT QUITE A DEEP...THO MOISTURE PROFILES LUK SIMILAR WITH LEADING EDGE OF PCPN REACHING SLO COUNTY LATE IN THE DAY. ALL MODELS AGREE THAT THIS WILL BE A NEG TILT TROF...WHICH WILL ENHANCE CHANCE FOR PCPN FURTHER S. JET PROGS SUGGEST CONTD DIGGING OF THE SYS THRU SUN AFTN...SUPPORTING THE ETA/S THEORY. OVERALL...THINK CHANGES FOR PCPN IN LA BASIN ON MON ARE VERY GOOD...DESPITE LOW POPS FROM THE AVN/NGM. AMNTS STILL TOO EARLY TO SAY...BUT SLY FLOW AHD OF SYS WILL ENHANCE PRECIP OVER THE TRANSVERSE RANGES AND ADJ FOOTHILLS.

WOFFORD

NOT MUCH CHG TO GOING FCST. CURR GUID PKGS FAIRLY CONSISTENT IN HANDLING SRN PLAINS SYS AND WITH TIMING ON CDFNT DUE TO BE APCHG STA BY SAT NGT.

MOS TEMPS IN CLOSE AGRMT AS WELL AND WL MAKE LTL OR NO CHNGS TO PREV TEMP FCSTS.

WL KEEP SUNNY/MSTLY SUNNY GOING FOR TDY. SOME CI MAY DRIFT INTO PARTS OF CNTRL IL BUT STILL BE MOSTLY SUNNY. H5 RDG AXIS INDUCED AHD OF SRN PLAINS SYS FINALLY GETS INTO W CENTRL/SRN IL BY EARL SAT SO EXPECT INCRS IN HI CLDS CNTRL IL BY THEN. ONLY OTR FLY IN THE OINTMENT AS FAR AS CLDS ARE CONCERNED WUD BE IN NERN IL THIS MORNING. 11-3.9UM SAT LOOP SHOWS SC CONTG TO DVLP ACRS SRN LM. ACARS WND PROFILE FM RFD ABT 08Z SHWD NLY WND AOB 7K FT SO SOME OF THESE CLOUDS MAY DRIFT INTO EXTRM NERN IL/NWRN IN DURG THE NEXT FEW HOURS. FOR NOW WL KEEP OUT OF FCST AS TEMPS OVR LK MARGINAL FOR LK INDUCED CLOUDS.