Area Forecast Discussion
Issued by NWS Chicago, IL

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000 FXUS63 KLOT 022024 AFDLOT AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL 324 PM CDT TUE SEP 2 2014 .DISCUSSION... 324 PM CDT COLD FRONT WHICH MOVED THROUGH THE FORECAST AREA LAST NIGHT HAS SETTLED INTO THE OHIO RIVER VALLEY THIS AFTERNOON...WITH WEAK HIGH PRESSURE CENTER DEVELOPING EASTWARD FROM THE LOWER MISSOURI VALLEY. MODEST WEST-SOUTHWEST LOW LEVEL WINDS WERE NOTED ACROSS THE CWA... SEVERAL WEAK CONVERGENT ZONES INCLUDING AN OUTFLOW BOUNDARY FROM MORNING THUNDERSTORMS OVER NORTHWEST IL...THE LAKE BREEZE BOUNDARY ALONG THE ILLINOIS SHORE AND EXTENDING EAST ACROSS NORTHERN INDIANA...AND A SUBTLE TROUGH AXIS/MOISTURE GRADIENT ACROSS OUR SOUTHEASTERN COUNTIES. VISIBLE SATELLITE IMAGERY HAS SHOWN CU/TCU DEVELOPMENT ALONG THESE BOUNDARIES AT TIMES WITH A FEW ISOLATED SHOWERS HAVING FORMED ALONG THE BOUNDARIES ACROSS FAR EAST CENTRAL IL AND NORTHWEST INDIANA. ANOTHER WEAK CONVERGENCE AXIS WAS FROM NEAR MILWAUKEE BACK INTO NORTHWEST IL...ALSO WITH SOME CU DEVELOPMENT ALONG IT. WHILE AN ISOLATED SHOWER WILL BE POSSIBLE ALONG ANY OF THESE BOUNDARIES DURING THE REMAINDER OF THE AFTERNOON HOURS...THE HIGHER SURFACE DEW POINT AND MLCAPE AXIS OVER THE FAR SOUTHEASTERN CWA WILL HAVE THE BEST CHANCE (AND ONLY ISOLATED COVERAGE AT THAT) OF ANY MORE SIGNIFICANT SHOWERS OR ISOLATED THUNDER. LOSS OF DIURNAL INSTABILITY SHOULD END ISOLATED PRECIP THREAT BY EARLY EVENING. WEAK SURFACE PRESSURE GRADIENT NORTH OF SURFACE HIGH PRESSURE CENTER WILL ALLOW FOR LIGHT WINDS OVERNIGHT... AND WITH MAINLY CLEAR SKIES SHOULD SEE SOME PATCHY FOG DEVELOP ESPECIALLY ACROSS AREAS SOUTH OF CHICAGO WHERE DEW POINT TEMPS WILL BE A LITTLE HIGHER AND WHERE GROUND LIKELY REMAINS MOIST FROM RECENT HEAVY RAINS. ATTENTION THEN TURNS TOWARD THE WEST WEDNESDAY...AS SURFACE LOW PRESSURE DEVELOPMENT TAKES PLACE ACROSS THE HIGH PLAINS IN RESPONSE TO STRONG MID-LEVEL SHORT WAVE CROSSING THE NORTHERN ROCKIES. GUIDANCE HAS TRENDED DEEPER WITH THE MID-LEVEL WAVE AND THE RESULTING SURFACE LOW...WITH SOUTHERLY RETURN FLOW DEVELOPING AND STRENGTHENING ACROSS THE MISSISSIPPI VALLEY AND WESTERN LAKES REGION BY LATER WEDNESDAY AND WEDNESDAY NIGHT. THIS WILL RESULT IN THE COLD FRONT TO OUR SOUTH RETURNING NORTH AS A WARM FRONT WEDNESDAY NIGHT...AND BRINGING VERY WARM/HOT AND HUMID CONDITIONS BACK FOR THURSDAY AND FRIDAY. THUNDERSTORM POTENTIAL INCREASES WEDNESDAY NIGHT AND EARLY THURSDAY AS THE WARM FRONT PUSHES NORTH...BEFORE CAPPING INVERSION DEVELOPS IN 900-700 MB LAYER WITHIN WARM SECTOR THURSDAY. HOT AND HUMID CONDITIONS EXPECTED THURSDAY...WITH 850 MB TEMPS IN EXCESS OF +20 C AND 950 MB TEMPS APPROACHING +28 C. PROVIDED THAT ANY MORNING CLOUDS/PRECIP WITH EARLY MORNING WARM FRONTAL PASSAGE SCATTER...AND NO SIGNIFICANT CONVECTIVE OUTFLOW REMAINS...AFTERNOON HIGHS AROUND 90/LOWER 90S LOOK REASONABLE. COMBINED WITH DEW POINT TEMPS CLIMBING INTO THE 70S... HEAT INDEX VALUES OF 95-105 WOULD MAKE FOR UNCOMFORTABLE CONDITIONS. DEEP SURFACE LOW CONTINUES TO DEEPEN AS IT MOVES EAST ACROSS ONTARIO THURSDAY NIGHT...WITH A TRAILING COLD FRONT PUSHING INTO NORTHERN ILLINOIS PRE-DAWN FRIDAY. SHOWER/THUNDERSTORM POTENTIAL INCREASES EARLY FRIDAY WITH ARRIVAL OF FRONT...AND CONTINUES THROUGH FRIDAY NIGHT AS FRONT SAGS SOUTH INTO DOWNSTATE IL/IN. WITH FRONT BISECTING CWA DURING THE DAY...A FAIRLY LARGE TEMP SPREAD IS ANTICIPATED WITH UPPER 70S/LOW 80S ALONG THE WISCONSIN BORDER...AND UPPER 80S/NEAR 90 AGAIN ACROSS THE SOUTHERN COUNTIES OF THE FORECAST AREA WITH MOST OF THE CLOUD COVER AND PRECIP EXPECTED ALONG/NORTH OF THE ADVANCING FRONT AS CAPPING REMAINS IN PLACE SOUTH OF THE BOUNDARY. BROAD SURFACE HIGH PRESSURE THEN SPREADS ACROSS THE UPPER MIDWEST AND GREAT LAKES REGION SATURDAY AND SUNDAY...WITH DRY AND MUCH COOLER CONDITIONS WITH EASTERLY LOW LEVEL FLOW AND LAKE BREEZES. NEXT CHANCE OF ORGANIZED PRECIP APPEARS TO ARRIVE AT THE END OF THE FORECAST PERIOD ON TUESDAY. RATZER && .AVIATION... //ORD AND MDW CONCERNS...UPDATED 20Z... * NONE. KREIN //DISCUSSION...UPDATED 18Z... THE SHOWERS AND THUNDERSTORMS ASSOCIATED WITH A OLD MID LEVEL IMPULSE HAVE DISSIPATED AND NO PCPN IS NOW EXPECTED INVOF THE TERMINALS. LATEST RADAR IMAGERY SHOWS AN OUTFLOW BOUNDARY EXTENDING FROM KARR TO KVYS AND TRACKING TO THE SOUTHEAST. SOME NWLY WIND GUSTS FOLLOW THE PASSAGE OF THE OUTFLOW BOUNDARY...BUT DO NOT EXPECT MUCH...IF ANY...IMPACT AT THE TERMINALS. ALSO...A LAKE BREEZE BOUNDARY IS FORMING ALONG THE LAKE FRONT. WITH SFC WINDS APPROACHING 10KT AND WINDS JUST OFF THE DECK IN EXCESS OF 10KT...DO NOT EXPECT THE LAKE BREEZE BOUNDARY TO PUSH INLAND MUCH AND SHOULD REMAIN EAST OF ORD/MDW. UNDER GENERALLY CLEAR SKIES...EXPECT THE SFC LAYER TO DECOUPLE FROM WINDS ALOFT AND WINDS SHOULD BECM LGT/VRBL OVERNIGHT. THERE IS A CHANCE FOR SOME PATCHY GROUND FOG OVERNIGHT TONIGHT. THERE IS A CHANCE IT COULD IMPACT DPA/RFD...BUT IS UNLIKELY AT ORD/MDW/GYY. WILL LEAVE MENTION OUT OF THE TAFS FOR NOW SINCE IT SHOULD BE CONFINED TO SHELTERED...LOW LYING AREAS. KREIN //ORD AND MDW CONFIDENCE...UPDATED 20Z... * MEDIUM TO HIGH CONFIDENCE IN ORD/MDW REMAINING DRY THIS AFTERNOON. * HIGH CONFIDENCE IN ALL OTHER FORECAST ELEMENTS. KREIN //OUTLOOK FOR ORD/MDW FOR 00Z THURSDAY-12Z TUESDAY...UPDATED 12Z... WEDNESDAY NIGHT AND THURSDAY...PREDOMINANTLY VFR...THOUGH THERE IS A SLIGHT CHANCE OF A THUNDERSTORM. FRIDAY...PRETTY GOOD CHANCE OF A PERIOD OR TWO OF TSRA. SATURDAY THROUGH MONDAY...VFR. NORTHEAST WINDS LIKELY. KJB && .MARINE... 251 PM CDT MAIN FORECAST CONCERN IS WITH STRONG SOUTHERLY WINDS AHEAD OF DEEPENING LOW PRESSURE MOVING FROM THE NORTHERN PLAINS ON THURSDAY TO JAMES BAY BY FRIDAY MORNING. TIGHTENING PRESSURE GRADIENT AND PRESSURE FALLS WILL RESULT IN STRENGTHENING SOUTHERLY FLOW DURING THE DAY WEDNESDAY AND ESPECIALLY WEDNESDAY NIGHT. SUSTAINED WINDS WILL PEAK ON THURSDAY IN THE LOWER 20 KT RANGE...BUT HAVE LOWER CONFIDENCE IN GUST SPEEDS DUE WARM AIR MASS OVERSPREADING THE REGION. WILL CONTINUE TO MENTION 30 KT GUSTS IN THE GLF. ITS POSSIBLE THAT NEAR SHORE WATERS COULD SEE MORE FREQUENT GUSTS UP TO 30 KT IN STRONG OFFSHORE FLOW...WITH FORECAST SOUNDINGS INDICATING THAT POTENTIAL. SMALL CRAFT ADVISORY WINDS ARE LIKELY THURSDAY MORNING INTO THURSDAY EVENING FOR THE ENTIRE NEARSHORE. THEN A STRONG COLD FRONT TRAILING FROM THE LOW PRESSURE AREA WILL SWEEP DOWN THE LAKE THURSDAY NIGHT THROUGH FRIDAY AFTERNOON. A PERIOD OF DECENT NORTHERLY WINDS COULD SET UP ESPECIALLY ON THE SOUTHERN HALF OF THE LAKE...BUT HIGH PRESSURE WILL QUICKLY SPREAD EASTWARD AND REDUCE SPEEDS. THEREFORE...IT IS UNCERTAIN WHETHER DURATION OF NORTHERLY WINDS WILL BE SUFFICIENT TO RESULT IN WAVES BUILDING TO SMALL CRAFT ADVISORY CRITERIA. FOR NOW DO HAVE WAVES ABOVE CRITERIA IN GRIDS. WINDS WILL BECOME RELATIVELY LIGHT OVER THE WEEKEND WITH EXPANSIVE HIGH PRESSURE SLOWLY SLIDING EASTWARD...THOUGH NORTHERLY. RC && .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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