Area Forecast Discussion
Issued by NWS Chicago, IL

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000 FXUS63 KLOT 291807 AFDLOT AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL 107 PM CDT FRI AUG 29 2014 .DISCUSSION... 326 AM CDT THE MAIN FORECAST CONCERNS WILL CENTER AROUND CONVECTIVE CHANCES AND TIMING...AS WELL AS TEMPERATURES TODAY...AND THROUGH THE HOLIDAY WEEKEND. CURRENTLY...THE REMNANT MCV FROM YESTERDAY EVENINGS SHOWERS AND STORMS CONTINUES TO SHIFT ACROSS MY SOUTHERN COUNTIES EARLY THIS MORNING. HOWEVER...ONLY A FEW LIGHT SHOWERS/SPRINKLES REMAIN ACROSS THIS AREA...WITH DRIER AND FAIRLY STABLE CONDITIONS IN PLACE TO THE EAST. ANOTHER AREA OF CONVECTION IS ONGOING TO OUR WEST ACROSS IOWA AND INTO SOUTHERN MN. THIS ACTIVITY IS OCCURRING WITHIN A REGION OF GOOD LOWER LEVEL WARM AIR ADVECTION/ISENTROPIC ASCENT ON THE DOWNSTREAM PERIPHERY OF A REMNANT MID/UPPER LEVEL TROUGH OVER THE PLAINS STATES. DOWN STREAM UPPER RIDGING OVER THE GREAT LAKES REGION IS LEADING TO A STEERING FLOW TO THE NORTHEAST. THEREFORE...MOST IF NOT ALL OF THIS CONVECTIVE ACTIVITY WILL SHIFT INTO SOUTHERN WISCONSIN AND PERHAPS FAR NORTHWESTERN ILLINOIS THIS MORNING. IN SPITE OF THIS...I HAVE LEFT SLIGHT CHANCE POPS IN THIS MORNING AS IT IS POSSIBLE THAT SOME ADDITIONAL CELLS COULD DEVELOP IN ASSOCIATION WITH AN EASTWARD MOVING OUTFLOW BOUNDARY...AS EVIDENT OFF OF THE DMX RADAR MOVING TOWARDS EASTERN IOWA. THIS ACTIVITY...IF IT OCCURS IN ANY OF NORTHERN ILLINOIS EARLY THIS MORNING...WOULD LIKELY BE ISOLATED TO WIDELY SCATTERED...WITH THE MAIN FOCUS REMAINING NORTH. A SURFACE WARM FRONT IS EXPECTED TO LIFT NORTHWARD OVER THE AREA INTO THIS AFTERNOON...WITH A WARMER AIRMASS EXPECTED IN ITS WAKE. IN SPITE OF THIS...HOWEVER...HIGH TEMPERATURES ACROSS THE AREA WILL BE TRICKY AS IT APPEARS THAT SOME SCATTERED THUNDERSTORMS COULD DEVELOP ACROSS PORTIONS OF THE AREA THIS AFTERNOON...AND HAMPER THE MAGNITUDE OF THE WARMTH ACROSS SOME AREAS. MODEST HEIGHT FALLS ALOFT ARE FORECAST ACROSS NORTHERN ILLINOIS IN ASSOCIATION WITH THE AFOREMENTIONED UPPER TROUGH SHIFTING EASTWARD OVER THE MID MISSOURI VALLEY THIS AFTERNOON. IN SPITE OF THE FACT THAT THE BEST LARGE SCALE FORCING FOR ASCENT WITH THIS DISTURBANCE SHOULD REMAIN WEST OF THE AREA THIS AFTERNOON...THE EXPECTED INCREASE IN LOW-LEVEL THETA E IS EXPECTED TO PRODUCE AN UNCAPPED ATMOSPHERE FEATURING MLCAPES OF AROUND 1500 J/KG BY THIS AFTERNOON. A MAJORITY OF THE GLOBAL MODELS...AS WELL AS THE HIRES CONVECTIVE ALLOWING MODELS FIRE UP ACTIVITY WITHIN THIS REGIME THIS AFTERNOON. IT APPEARS WE WILL HAVE TO WATCH MORNING CLOUD COVER...HOWEVER...AS THIS COULD INHIBIT THE DEGREE OF WARMING A BIT THROUGH THE MORNING...AND POSSIBLY LEAD TO WEAKER INSTABILITY THAN WHAT THE MODELS ARE CURRENTLY ADVERTISING. IF THE CONVECTION DOES DEVELOP AS EXPECTED...MARGINAL DEEP LAYER SHEAR OF 25 TO 30 KT COULD ALLOW FOR A FEW STRONGER STORMS CAPABLE OF SOME ISOLATED HIGH WIND GUSTS. SOME TORRENTIAL DOWNPOURS WOULD ALSO BE A THREAT FROM THESE STORMS GIVEN THE JUICY AIRMASS FEATURING PWATS UP AROUND 1.75 INCHES. HOWEVER...EXTREME RAINFALL AMOUNTS APPEAR UNLIKELY GIVEN STORM MOTIONS SHOULD BE TO THE NORTHEAST AROUND 30 KT. SO THE STORMS SHOULD NOT BE OVER A GIVEN AREA TOO LONG. MUCH OF THIS ACTIVITY APPEARS TO BE HIGHLY DIURNAL IN NATURE...SO A RAPID DOWNWARD TREND IS EXPECTED AFTER SUNSET THIS EVENING. ANOTHER ROUND OF THUNDERSTORMS APPEARS LIKELY ON SATURDAY...ESPECIALLY DURING THE MORNING AS THE UPPER LEVEL TROUGH...AND AN ASSOCIATED SURFACE LOW AND COLD FRONT SHIFTS EASTWARD ACROSS ACROSS SOUTHERN WI AND NORTHERN IL. THIS ACTIVITY COULD LINGER INTO SATURDAY EVENING ACROSS MY FAR SOUTHEASTERN COUNTIES. HOWEVER...IT APPEARS THAT FOLLOWING THIS FRONTAL PASSAGE WE WILL BE GREETED WITH A NICE DRY SUNDAY ACROSS THE AREA. UNFORTUNATELY...LABOR DAY APPEARS THAT IT COULD BE ANOTHER ACTIVE WEATHER DAY ACROSS THE REGION. ANOTHER POTENT UPPER LEVEL DISTURBANCE AND SURFACE FRONT IS FORECAST TO SHIFT OVER THE AREA BY MONDAY. WE WILL ALSO HAVE TO WATCH FOR THE THREAT OF SEVERE THUNDERSTORMS ON MONDAY AND MONDAY NIGHT AS A 45 TO 50 KT MID LEVEL JET IS PROGGED TO DEVELOP ACROSS THE AREA. THIS COULD RESULT IN DEEP LAYER SHEAR STRONG ENOUGH TO PRODUCE MORE ORGANIZED STRONG TO SEVERE CONVECTION. VERY HEAVY RAINFALL WOULD AGAIN BE A CONCERN WITH THIS ACTIVITY...ESPECIALLY IF THE FRONTAL ZONE STALLS OUT OVER THE AREA MONDAY NIGHT. AT THIS TIME...HOWEVER...UNCERTAINTY IS HIGH ON WHERE THE SURFACE FRONT MAY STALL AND HENCE WHAT AREAS COULD BE FAVORED FOR EXTENDED PERIODS OF VERY HEAVY RAINFALL. THE ACTIVE WEATHER PATTERN COULD SHIFT SOUTH OF THE AREA FOR A DAY OR TWO INTO MID TO LATE WEEK BEFORE MORE PERIODS OF CONVECTION ARE POSSIBLE. OVERALL...IT APPEARS TEMPERATURES WILL REMAIN ON THE WARM SIDE OF AVERAGE...WITH THE POTENTIAL FOR SOME VERY WARM AND HUMID DAYS LATER NEXT WEEK. KJB && .AVIATION... //ORD AND MDW CONCERNS...UPDATED 18Z... * ISOLATED-SCATTERED SHRA/TSRA THIS AFTERNOON THRU EARLY EVENING. * SOUTH WINDS WITH GUSTS 15-20 KT THIS AFTERNOON. * ANOTHER PERIOD OF SHRA/TSRA LIKELY AFTER MIDNIGHT INTO SATURDAY MORNING. * SOUTHWEST WINDS GUSTING AROUND 20 KT SATURDAY. RATZER //DISCUSSION...UPDATED 18Z... FAIRLY LOW CONFIDENCE CONVECTIVE FORECAST THIS PERIOD...WITH TERMINALS WITHIN A BROAD AREA OF SOUTH WARM/MOIST ADVECTION DOWNSTREAM OF AN UPPER TROUGH OVER THE CENTRAL PLAINS BUT WITH OTHERWISE LITTLE ORGANIZED FOCUS FOR TSRA. SATELLITE EVIDENCE OF WEAK MID-LEVEL DISTURBANCE OVER NORTHEAST IL EARLY THIS AFTERNOON...WITH MORNING ELEVATED SHRA/TSRA DECAYING OVER NORTH CENTRAL IL. DIURNAL TSRA WERE FORMING IN LOW LEVEL INSTABILITY AXIS ALONG IL/IN BORDER REGION GENERALLY ALONG A DEC- GIJ-MKG LINE...AND THIS LOOKS TO REMAIN THE FOCUS REGION FOR HIGHEST COVERAGE TSRA THIS AFTERNOON. HIGH-RES GUIDANCE STILL DEVELOPS SOME ISOLATED SHRA/TSRA INTO THE TERMINAL REGION IN ASSOCIATION WITH AFOREMENTIONED MID-LEVEL DISTURBANCE AND NORTH OF SURFACE WARM FRONT WHICH IS ACROSS DOWNSTATE IL AT THIS TIME... THOUGH COVERAGE IS EXPECTED TO REMAIN LOW. A GREATER CHANCE OF SHRA/TSRA APPEARS TO BE LATE TONIGHT AND EARLY SATURDAY AS THE MAIN UPPER TROUGH APPROACHES. WEAK COLD FRONT APPROACHES FROM THE WEST SATURDAY MIDDAY AND AFTERNOON...WITH THUNDERSTORMS EXPECTED TO DEVELOP ALONG THE FRONT BY EARLY AFTERNOON. CURRENT SUITE OF HIGH-RES GUIDANCE SUGGESTS TSRA POTENTIAL MAY BE GREATER JUST EAST/SOUTH OF CHICAGO TERMINALS SATURDAY AFTERNOON. OUTSIDE OF ANY PRECIP VIS REDUCTION EARLY SATURDAY...SOME MVFR HAZE POSSIBLE OVERNIGHT IN MOIST WARM SECTOR AIR. WINDS...GENERALLY SOUTH 10-12 KT WITH GUSTS TO 20 KT OR SO POSSIBLE THIS AFTERNOON. WIND MAY BACK A BIT TO THE SOUTH- SOUTHEAST LATE THIS AFTERNOON/EARLY EVENING AS SURFACE WARM FRONT APPROACHES...BEFORE SHIFTING SOUTHWEST OVERNIGHT AND SUNDAY AHEAD OF THE COLD FRONT. DEEPER MIXING SUNDAY SUGGESTS GUSTS AGAIN IN THE 20 KT RANGE OR A LITTLE HIGHER DURING THE LATE MORNING AND AFTERNOON. RATZER //ORD AND MDW CONFIDENCE...UPDATED 18Z... * MEDIUM IN SHRA/TSRA BEING LOW IN COVERAGE IN IMMEDIATE TERMINAL AREA THIS AFTERNOON...GREATER COVERAGE EAST/SOUTH OF CHICAGO. * LOW IN TSRA TIMING/COVERAGE LATER TONIGHT/SATURDAY. * MEDIUM-HIGH IN WIND TRENDS THROUGH PERIOD. RATZER //OUTLOOK FOR ORD/MDW FOR 00Z SUNDAY-12Z FRIDAY...UPDATED 12Z... * SATURDAY NIGHT THROUGH SUNDAY NIGHT...VFR. WX NIL. * MONDAY...CHANCE OF TSRA. * TUESDAY...VFR. WX NIL. * WEDNESDAY THROUGH THURSDAY NIGHT...SLIGHT CHANCE OF TSRA KREIN && .MARINE... 315 AM CDT HIGH PRESSURE OVER THE EASTERN GREAT LAKES WILL MOVE EAST OFF THE NEW ENGLAND COAST FRIDAY EVENING...WHILE A TROUGH OF LOW PRESSURE DEVELOPS ACROSS THE CENTRAL PLAINS AND UPPER MIDWEST. THIS WILL SET UP A PERIOD OF SOUTHERLY WINDS AND AS THE TROUGH MOVES EAST...THE GRADIENT OVER THE LAKE WILL STRENGTHEN AND WINDS SHOULD INCREASE TO THE 20-25 KT RANGE. A DEEPENING LOW CENTER WITHIN THE TROUGH WILL MOVE TO WISCONSIN SATURDAY AND ACROSS NORTHERN LAKE MICHIGAN SATURDAY EVENING. AS THE LOW TRACKS TO THE NORTHEAST...AN ASSOCIATED COLD FRONT WILL MOVE DOWN THE LAKE SATURDAY AFTERNOON AND EVENING...QUICKLY SHIFTING WINDS TO NWLY FOLLOWING THE FROPA. THE LATEST MODEL GUIDANCE SUGGESTS THAT THE FRONT WILL WASH OUT LATE SATURDAY NIGHT BEFORE REACHING THE SOUTH END OF THE LAKE AND WINDS THERE SHOULD NEVER TRULY SHIFT NWLY...BUT RATHER BECOMING LIGHT AND VARIABLE LATE SATURDAY NIGHT THROUGH SUNDAY MORNING. A DEEPER LOW IS EXPECTED TO DEVELOP OVER THE DAKOTAS AND LIFT THROUGH WESTERN ONTARIO SUNDAY NIGHT AND MONDAY BRINGING A RETURN TO BRISK SOUTH TO SOUTHWEST WINDS TO THE LAKE FOR MONDAY AND TUESDAY. KREIN && .LOT WATCHES/WARNINGS/ADVISORIES... IL...NONE. IN...NONE. LM...NONE. && $$ VISIT US AT HTTP://WEATHER.GOV/CHICAGO (ALL LOWERCASE) FOLLOW US ON FACEBOOK...TWITTER...AND YOUTUBE AT: WWW.FACEBOOK.COM/US.NATIONALWEATHERSERVICE.CHICAGO.GOV WWW.TWITTER.COM/NWSCHICAGO WWW.YOUTUBE.COM/NWSCHICAGO

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