Area Forecast Discussion
Issued by NWS Chicago, IL

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000 FXUS63 KLOT 230041 AFDLOT AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL 641 PM CST SAT NOV 22 2014 .UPDATE... 631 PM CST MADE ADJUSTMENTS TO THE POP FORECAST FOR TONIGHT AS RADAR IS RATHER QUIET RIGHT NOW. THE WAVE AND ASSOCIATED RAIN SHOWERS OVER SOUTHWESTERN AND CENTRAL IL LOOKS LIKE THEY WILL BE THE BEST CHANCE FOR RAIN THROUGH TONIGHT. THEREFORE KEPT HIGHER POPS SOUTH OF I-80. CEILING HEIGHTS KEEP RISING OVER NORTHERN IL WHICH LEADS TO ME TO BELIEVE DRIER AIR WILL LIMIT RAINFALL. LEFT CHANCE POPS IN ACROSS THE CWA LATE THIS EVENING AS I DO NOT HAVE ENOUGH CONFIDENCE TO GO DRY. DEWPOINT DEPRESSIONS ARE IN THE SINGLE DIGITS ACROSS THE BOARD SO STILL THINKING AREAS OF FOG WILL PERSIST OVERNIGHT. NOT EXPECTING DENSE FOG AT THIS POINT IN TIME. LOOKING TOWARD TOMORROW...TIMING OF PRECIP AND FOG LOOK GOOD SO DID NOT MAKE ANY CHANGES. ALSO ADJUSTED TEMPERATURES A BIT HIGHER TONIGHT AND STILL MAY NOT HAVE GONE HIGH ENOUGH. WITH THE THICK CLOUD COVER AND SOUTHERLY WINDS...NOT EXPECTING TEMPS TO DROP VERY MUCH OVERNIGHT. JEE && .SHORT TERM... 258 PM CST FOR THE REMAINDER OF THIS AFTERNOON AND THROUGH TONIGHT...AN INFLUX OF WARM...MOIST AIR WILL CONTINUE TO STREAM INTO THE UPPER MIDWEST. LATEST SFC ANALYSIS SHOWS DEWPOINTS IN THE 50S OVER MISSOURI WHILE DEWPOINTS ACROSS NRN IL. NWRN IN HAVE CLIMBED INTO THE MIDDLE TO UPPER 40S. TEMPERATURES HAVE ALSO BEEN ON THE RISE THROUGH THE DAY WITH TEMPERATURES AT OR IN EXCESS OF 50F AS FAR NORTH AS MIDWAY AIRPORT AND AURORA AS OF 230PM CST. TEMPERATURES ARE EXPECTED TO CONTINUE TO RISE INTO THE EARLY EVENING UNDER THE PERSISTENT WARM...MOIST ADVECTION UNDER A STRONG SWLY LOW LEVEL JET WITH 850MB WINDS UP TO 45KT...TEMPERATURES WILL LIKELY HOVER AROUND THE 50 DEGREE MARK THROUGH THE NIGHT. ADDITIONAL LIGHT RAIN OR DRIZZLE WILL PERSIST THROUGH THE EVENING AND OVERNIGHT...WITH AREAS OF FOG POSSIBLE. KREIN && .LONG TERM... 258 PM CST MORE SIGNIFICANT RAINFALL IS SETTING UP FOR SUNDAY AND INTO SUNDAY NIGHT AS A STRONG SRN STREAM SHORTWAVE LIFTING OUT OF THE SOUTH HIGH PLAINS PHASES WITH ANOTHER SHORTWAVE LIFTING FROM THE SOUTHWEST GULF COAST REGION. THESE SYSTEM WILL RESULT IN A DEEPENING SFC LOW OF 984MB NEAR ST LOUIS BY SUNDAY EVENING. THE MODELS CONTINUE TO ADVERTISE PWAT VALUES AROUND 1.2 INCHES...WHICH IS AT THE 99TH PERCENTILE FOR PWATS FOR THE REGION. THIS PHASING OF THE TWO SRN STREAM SYSTEMS COMBINED WITH THE UNUSUALLY HIGH PWAT AIRMASS WILL COMBINE TO FOCUS STEADY RAINFALL ACROSS THE REGION THROUGH THE DAY ON SUNDAY AND INTO SUNDAY EVENING...WITH A CORRIDOR OF THE HEAVIEST RAIN OVER THE NWRN PORTIONS OF THE CWA...GENERALLY NORTH OF A LINE FROM PERU TO MCHENRY...WHERE UPWARDS OF A HALF INCH PER 3 HOUR PERIOD IS POSSIBLE. LATER SUNDAY NIGHT...THE PHASED SRN STREAM SYSTEM WILL BEGIN TO PHASE WITH A NRN STREAM SHORTWAVE...WITH LOW PRESSURE RAPIDLY DEEPENING AS IT LIFTS THROUGH THE UPPER GREAT LAKES. WHILE THE GUIDANCE IS RELATIVELY CONSISTENT ON THE TRACK AND TIMING OF THE DEEPENING LOW...THE GFS IS A MUCH STRONGER OUTLIER IN THE GUIDANCE ENVELOPE...DEEPENING THE CENTRAL PRESSURE TO ARND 966MB BY MIDDAY MONDAY...WHILE MUCH OF THE OTHER GUIDANCE ONLY DEEPENS THE LOW TO ARND 980MB. WHILE THE GFS SOLUTION MAY BE TOO DEEP...LIKELY DRAWING COLDER AIR INTO THE SYSTEM FASTER THAN THE OTHER MODELS...OR TAPPING MORE NRN STREAM ENERGY...THERE WILL STILL LIKELY BE A DEEP LOW DEVELOPING OVER THE UPPER GREAT LAKES SUNDAY NIGHT INTO MONDAY...WHOSE CIRCULATION PATTERN WILL COVER A LARGE EXPANSE FROM THE EAST-CENTRAL PLAINS TO NERN QUEBEC. A STRONG WLY- NWLY GRADIENT WILL SET UP ON THE SWRN PERIPHERY OF THE SYSTEM...LEADING TO STRONG...GUSTY WINDS ACROSS THE CWA FROM EARLY MONDAY MORNING THROUGH LATE MONDAY EVENING. THE STRONGEST WINDS SHOULD OCCUR FROM LATE MORNING THROUGH LATE EVENING...WITH SUSTAINED WINDS OF 25-30 MPH AND PEAK GUSTS OF 35-40 MPH. HOWEVER...IF THE SYSTEM EVOLVES A LITTLE DEEPER THAT THE CONSENSUS GUIDANCE AND DEEPENS CLOSER TO THE OUTLIER GFS SOLUTION...WINDS ACROSS THE AREA COULD REACH WIND ADVISORY LEVELS OF SUSTAINED WINDS OF 30 MPH OR HIGHER OR GUSTS OF 45 MPH OR GREATER. ALSO...AS THE SFC LOW BEGINS TO LIFT OUT TO THE NORTHEAST MONDAY AFTERNOON...MUCH COOLER AIR WILL BEGIN TO FILTER ACROSS THE REGION. AS THE PATTERN TRANSITIONS FROM ONE OF STRONG WARM ADVECTION SUNDAY NIGHT TO STRONG COLD ADVECTION BY MONDAY MORNING...FOLLOWING THE PASSAGE OF A STRONG ASSOCIATED COLD FRONT...LINGERING PCPN WILL BEGIN TO TRANSITION FROM RAIN TO SNOW. SINCE THE COLD AIR SHOULD EXTEND THROUGH THE ENTIRE ATMOSPHERE...THERE SHOULD BE A MORE STRAIGHTFORWARD TRANSITION FROM RAIN TO SNOW WITH LITTLE CHANCE FOR ANY FREEZING RAIN OR SLEET. THERE COLD BE SOME ISSUES WITH WATER FLASH FREEZING ON ELEVATED SURFACES AS SUB-FREEZING AIR QUICKLY FILTERS ACROSS THE REGION BY LATE MONDAY AFTERNOON. THE STRONG COLD ADVECTION WILL PERSIST THROUGH THE NIGHT WITH TEMPERATURES DROPPING TO THE LOWER TO MIDDLE 20S BY DAYBREAK TUESDAY MORNING. THE PERSISTENT COLD ADVECTION WILL ALLOW FOR LITTLE DIURNAL WARMING WITH HIGHS ON TUESDAY EXPECTED TO BE IN THE UPPER 20S TO ARND 30F OVER THE FAR SERN PORTIONS OF THE CWA. THERE BE SOME MODERATION TO TEMPERATURES ON WEDNESDAY AS WEAK SFC RIDGING CROSSES THE AREA ALLOWING TEMPERATURES TO RISE INTO THE MIDDLE 30S. WEDNESDAY SHOULD BE THE WARMEST OF THE EXTENDED PERIOD. A SERIES OF STRONG NORTHERN STREAM SHORTWAVES IS EXPECTED TO CARVE OUT A BROAD LONGWAVE TROUGH OVER ERN CONUS FROM THE MISSISSIPPI VALLEY TO THE EAST COAST. THE UPPER LEVEL PATTERN SHOULD DECREASE IN AMPLITUDE THROUGH THE EXTENDED PERIOD...BUT THE LOCAL AREA IS EXPECTED TO REMAIN UNDER GENERAL NWLY FLOW ALOFT...KEEPING A PERSISTENT FETCH OF COLD AIR ACROSS THE AREA. BY LATE NEXT WEEK...HIGH TEMPERATURES WILL STRUGGLE INTO THE LOWER TO MIDDLE 20S...WHILE OVERNIGHT LOWS DROP INTO THE LOWER TO MIDDLE TEENS...WITH THE POSSIBILITY OF THE NWRN PORTIONS OF THE CWA...INCLUDING THE ROCKFORD AREA DROPPING INTO THE SINGLE DIGITS FRIDAY AND SATURDAY MORNING. THE SERIES OF SHORTWAVES RIPPLING THROUGH THE NORTHWEST FLOW SHOULD BRING PERIODIC CHANCES FOR SOME LIGHT SNOW...BUT CONFIDENCE IS LOW ON THE EXACT TIMING. CONFIDENCE IS A LITTLE HIGHER THAT ANY SNOW THAT DOES OCCUR SHOULD BE LIGHT AS THERE WILL BE LITTLE ENVIRONMENTAL MOISTURE OR SFC FORCING. ANY SNOW GENERATION WILL HAVE TO RELY ON MID-LEVEL FORCING AND MEAGER MOISTURE CONTENT. KREIN && .AVIATION... //ORD AND MDW CONCERNS...UPDATED 00Z... * VFR CIGS FALLING TO MVFR THIS EVENING...POTENTIALLY IFR LATE TONIGHT. IFR POSSIBLE SUNDAY MORNING AND LIKELY BY LATE AFTERNOON. * MVFR VSBY EXPECTED TONIGHT WITH IFR POSSIBLE. * ISOLATED-SCATTERED SHOWER CHANCES INCREASE OVERNIGHT...ESPECIALLY JUST SOUTH OF THE TERMINALS. BETTER CHANCES LATE SUNDAY MORNING THROUGH THE AFTERNOON. STEADIEST RAIN BY EVENING. * SOUTH-SOUTHWEST WINDS BECOMING GUSTY SUNDAY MORNING. SHOULD TURN SOUTHEAST SUNDAY AFTERNOON THEN BACK SOUTH SUNDAY EVENING. * LOW LEVEL WIND SHEAR POSSIBLE TONIGHT WITH 45 KT WINDS AT 2000 FT AGL. MDB //DISCUSSION...UPDATED 00Z... CIG/VSBY TRENDS THROUGH SUNDAY MORNING ARE THE INITIAL CHALLENGE...FOLLOWED BY WIND TRENDS SUNDAY AFTERNOON AND TIMING/COVERAGE OF RAIN THROUGH SUNDAY EVENING. THE AREA REMAINS BETWEEN HIGH PRESSURE WELL TO THE EAST AND DEVELOPING LOW PRESSURE TO THE SOUTHWEST ALLOWING A STEADY SOUTH-SOUTHWEST WIND TO CONTINUE. MVFR CIGS HAVE ACTUALLY IMPROVED TO VFR SOUTH OF A LINE FROM UGN TO KC75. UPSTREAM OBS SHOW A MIX OF VFR AND MVFR AND AM EXPECTING MVFR TO RETURN BUT IT MAY TAKE LONGER THAN THE 00Z TAF REFLECTS. BEYOND THAT HAVE TRENDED TOWARD BETTER CEILINGS THROUGH TOMORROW MORNING BY CARRYING BKN VS OVC IFR CIGS FROM LATE TONIGHT ONWARD. AM CONCERNED THAT IFR MAY EITHER BE BRIEF LATE TONIGHT OR NOT DEVELOP AT ALL SO WILL NEED TO WATCH THIS CLOSELY. HAVE HIGHER CONFIDENCE IN IFR DURING THE LATER AFTERNOON HOURS SUNDAY HOWEVER. HAVE THE SAME CONCERN FOR VSBY AS WELL AND HAVE TRENDED UP THROUGH THE EVENING. WITH NO REAL CHANGE IN AIRMASS AND LITTLE COOLING EXPECTED NOR ANY LOSS OF CLOUD COVER...SIGNIFICANT VSBY REDUCTION WILL BE DIFFICULT. 3-5SM VSBY MAY END UP PREVAILING SO MAY NEED TO MAKE FURTHER ADJUSTMENTS OVERNIGHT. IN TERMS OF SHOWERS...AN UPPER WAVE IS LIFTING NORTHEAST FROM MISSOURI THIS EVENING AND WITH LOW LEVEL WINDS INCREASING INTO TONIGHT COVERAGE MAY INCREASE TO THE NORTH OF THE CURRENT BAND. STILL THINKING GYY WILL HAVE THE BEST CHANCE OVERALL BUT MDW MAY BE SKIRTED IF MORE SOLID NORTHWARD DEVELOPMENT OCCURS. OTHERWISE THE INCREASING LOW LEVEL WINDS MAY RESULT IN SPOTTY NEW DEVELOPMENT ACROSS THE REST OF THE AREA SO OPTED FOR A VCSH MENTION AT ALL BUT RFD...THOUGH THE CHANCE AT RFD IS NOT ZERO. LOW PRESSURE WILL TAKE SHAPE TO THE SOUTHWEST TOMORROW AND MOVE NORTHEAST BRINGING A BAND OF WIDESPREAD RAIN WITH IT. EXPECT THIS TO MOVE INTO RFD DURING THE EARLY AFTERNOON AND POINTS EAST INTO THE EVENING. AM EXPECTING THAT THERE WILL BE MORE SCATTERED COVERAGE AHEAD OF IT FROM LATE MORNING ONWARD. CIGS/VSBY WILL FALL AS THE MORE WIDESPREAD RAIN MOVES IN WITH LIFR POSSIBLE. HAVE GOOD CONFIDENCE IN THE WIND FORECAST THROUGH THE MORNING THROUGH GUSTS COULD BE A LITTLE HIGHER DURING THE DAY. THE TRACK OF THE SURFACE LOW WILL DICTATE HOW WINDS EVOLVE INTO THE EVENING AND AT THIS POINT FEEL THAT THE LOW WILL MOVE OVER NORTH-CENTRAL ILLINOIS SUPPORTING A SHIFT TO SOUTHEAST WINDS DURING THE AFTERNOON...THEN BACK SOUTH DURING THE EVENING. THERE MAY BE AN OPPORTUNITY FOR MORE OF AN EAST COMPONENT SUNDAY AFTERNOON THAN CURRENTLY INDICATED. MDB //ORD AND MDW CONFIDENCE...UPDATED 00Z... * HIGH CONFIDENCE IN VFR CIGS FALLING BACK TO MVFR. MEDIUM CONFIDENCE IN TIMING. LOW-MEDIUM CONFIDENCE IN FALLING TO IFR BEFORE SUNDAY AFTERNOON. IFR IS POSSIBLE LATE TONIGHT BUT MAY IMPROVE EARLY SUNDAY MORNING. * MEDIUM-HIGH CONFIDENCE IN MVFR VSBY TONIGHT. LOW-MEDIUM CONFIDENCE IN TIMING. * HIGH CONFIDENCE THAT COVERAGE OF SHOWERS WILL GENERALLY BE LOW THROUGH TONIGHT...WITH MOST ACTIVITY JUST TO THE SOUTH. HIGH CONFIDENCE THAT COVERAGE WILL INCREASE LATE SUNDAY MORNING AND ESPECIALLY LATE SUNDAY AFTERNOON. * HIGH CONFIDENCE IN WIND TRENDS THROUGH SUNDAY MORNING. MEDIUM CONFIDENCE SUNDAY AFTERNOON/EVENING. * MEDIUM CONFIDENCE IN LLWS POTENTIAL. MDB //OUTLOOK FOR ORD/MDW FOR 12Z MONDAY-12Z SATURDAY...UPDATED 00Z... MONDAY...CHANCE OF RAIN AND SNOW. STRONG WEST TO SOUTHWEST WINDS. TUESDAY...MVFR CIGS POSSIBLE. WESTERLY WINDS. WEDNESDAY...CHANCE OF SNOW AND MVFR. SOUTH TO SOUTHWEST WINDS. THURSDAY...SNOW SHOWERS AND MVFR POSSIBLE. NORTHWEST WINDS. FRIDAY...VFR PROBABLE. WEST TO SOUTHWEST WINDS. MDB && .MARINE... 231 PM...STRONG LOW PRESSURE WILL MOVE ALONG THE U.S. CANADIAN BORDER TONIGHT AND BEGIN TO WEAKEN AS IT REACHES WESTERN LAKE SUPERIOR SUNDAY AFTERNOON. ANOTHER LOW WILL DEVELOP OVER THE SOUTHERN PLAINS TONIGHT AND MOVE NORTH TO SOUTHERN LAKE MICHIGAN AS IT DEEPENS AND ABSORBS THE LOW OVER WESTERN LAKE SUPERIOR. THIS LOW WILL THEN DEEPEN FURTHER AS IT REACHES EASTERN LAKE SUPERIOR MONDAY MORNING. THERE STILL REMAINS SOME DIFFERENCES AMONG THE MODELS REGARDING HOW STRONG THIS LOW WILL BECOME. THE DEEPEST OF THE MODELS HAS A PRESSURE OF 28.5 INCHES MIDDAY MONDAY. CONFIDENCE IS INCREASING FOR MID/HIGH END GALES SO WILL BE GOING WITH A GALE WATCH FOR MONDAY AND MONDAY NIGHT. STORM FORCE GUSTS OR PREVAILING WINDS ALSO REMAIN POSSIBLE. WINDS STEADILY DIMINISH TUESDAY AS HIGH PRESSURE MOVES ACROSS THE SOUTHERN LAKES AND OHIO VALLEY. ANOTHER LOW IS EXPECTED TO MOVE ACROSS THE LAKES REGION WEDNESDAY INTO THURSDAY...BRINGING NORTHWEST WINDS TO 30KT ON THURSDAY. CMS && .LOT WATCHES/WARNINGS/ADVISORIES... IL...NONE. IN...NONE. LM...GALE WATCH...LMZ080-LMZ669-LMZ671-LMZ673-LMZ675-LMZ740-LMZ741- LMZ742-LMZ743-LMZ744-LMZ745-LMZ777-LMZ779-LMZ870-LMZ872- LMZ874-LMZ876-LMZ878...6 AM MONDAY TO 3 AM TUESDAY. GALE WATCH...LMZ261-LMZ362-LMZ364-LMZ366-LMZ563-LMZ565-LMZ567- LMZ868...NOON MONDAY TO 3 AM TUESDAY. && $$ VISIT US AT HTTP://WEATHER.GOV/CHICAGO (ALL LOWERCASE) FOLLOW US ON FACEBOOK...TWITTER...AND YOUTUBE AT: WWW.FACEBOOK.COM/NWSCHICAGO WWW.TWITTER.COM/NWSCHICAGO WWW.YOUTUBE.COM/NWSCHICAGO

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