Area Forecast Discussion
Issued by NWS Chicago, IL

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000 FXUS63 KLOT 012205 AFDLOT AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL 505 PM CDT WED APR 1 2015 .SHORT TERM... 322 PM CDT THROUGH FRIDAY NIGHT... MAIN FORECAST CONCERNS ARE WITH SHOWER/THUNDERSTORM POTENTIAL LATE TONIGHT INTO THURSDAY MORNING...AND THEN WITH A SLIGHT POSSIBILITY FOR A PERIOD OF WINTRY WEATHER FRIDAY. GENERALLY QUIET CONDITIONS ONGOING THIS AFTERNOON...WITH ABOVE NORMAL TEMPS AND SUNNY SKIES IN PLACE. SURFACE PRESSURE FALLS WITH A TIGHTENING SURFACE GRADIENT IS ALLOWING FOR INCREASING SOUTHERLY FLOW THIS AFTERNOON AND WITH WARM FRONT LIFTING NORTH THROUGH THE CWA...TEMPS HAVE RISEN TO THE LOW TO MID 70S. COOLER CONDITIONS ARE STILL IN PLACE ACROSS FAR NORTHEAST ILLINOIS AT THIS TIME BUT STILL EXPECT A PERIOD OF WARMER CONDITIONS IN THE 60S TO FINALLY REACH THIS LOCATION LATE THIS AFTERNOON AND EARLY EVENING...AS WINDS TURN MORE SOUTHERLY AND USHER THE INLAND WARM AIR. DRY CONDITIONS WITH MAINLY CLEAR SKIES WILL BE IN PLACE THROUGH MUCH OF THE EVENING WITH EVEN A PERIOD OF STRENGTHENING WINDS EARLY...AS SYSTEM TO THE WEST APPROACHES. DONT ANTICIPATE PRECIP TO REACH THE CWA UNTIL AFTER THE MIDNIGHT TIME FRAME AS FOCUS FOR DEVELOPMENT WILL REMAIN WELL TO THE WEST AND SOUTHWEST DURING THIS TIME. INITIAL WAVE OF PRECIP WILL ACTUALLY STEM FROM NORTHWARD LIFTING SHORTWAVE ENERGY CURRENTLY OVER THE SOUTHERN PLAINS. AS THIS ENERGY LIFTS NORTH TOWARDS THE CWA...A VEERING AND STRENGTHENING LLJ WILL BE FOCUSED TOWARDS NORTHERN ILLINOIS. BOTH OF THESE FEATURES WILL AID IN THE DEVELOPMENT OF SHOWERS ACROSS NORTHERN ILLINOIS TOWARDS THE 7-9Z TIME FRAME. HAVE MAINTAINED MENTION OF THUNDER TONIGHT AS GUIDANCE IS INDICATING THAT THE INSTABILITY AXIS WEST OF THE CWA WILL SHIFT TOWARDS THE CWA LATE TONIGHT. HOWEVER...AS IT SHIFTS EAST...IT DOES APPEAR AS IT WILL BE SHUNTED MORE TOWARDS THE SOUTH. EVEN IF THIS WERE TO OCCUR STILL EXPECT AT LEAST WEAK INSTABILITY TO IN PLACE...ALONG WITH POTENTIAL FOR STEEPER MID LEVEL LAPSE RATES. SO THIS SHOULD ALL COME TOGETHER FOR THE POTENTIAL OF RUMBLES OF THUNDER LATE TONIGHT INTO EARLY THURSDAY MORNING. COVERAGE OF SHOWERS WITH EMBEDDED THUNDERSTORMS SHOULD INCREASE THROUGH MID TO LATE MORNING THURSDAY AS ANOTHER WAVE OF ENERGY SHIFTS EAST ACROSS THE UPPER MISSISSIPPI VALLEY...AND WITH SURFACE TROUGH/FRONTAL BOUNDARY APPROACH THE CWA. AS SURFACE DEWPOINTS INCREASE OUT OF THIS FRONT...DO FEEL THAT THUNDERSTORMS WILL STILL BE POSSIBLE ALONG THE FRONT. HOWEVER...ONCE AGAIN...INSTABILITY REMAINS TO BE WEAK AT BEST AND SKINNY CAPE AS WELL. NOT OVERLY CONCERNED WITH WIDESPREAD STRONG TO SEVERE THUNDERSTORMS THURSDAY MORNING ACROSS THE CWA WITH THIS WEAK CAPE IN PLACE. GUIDANCE DOES INDICATE THAT MOISTURE AND INSTABILITY WILL CONTINUE TO TRY AND INCREASE THROUGH MID AFTERNOON PRIMARILY ACROSS THE FAR SOUTHEAST CWA. AS THE FRONT FINALLY PUSHES THROUGH THIS AREA DURING THIS TIME...CANNOT RULE OUT AN ISOLATED STRONGER STORM. HOWEVER...THIS BETTER DEVELOPMENT WILL LIKELY END UP OCCURRING JUST SOUTH OF THE CWA. GUIDANCE VARIES TO THE SOUTHWARD PUSH OF THIS PRECIP THURSDAY NIGHT AND HAVE MADE LITTLE CHANGES TO THE GOING FORECAST FOR THIS PERIOD...WITH HIGH CHANCE POPS FOR ONLY RAIN ACROSS THE SOUTHERN CWA. ACTIVE MID LEVEL FLOW WILL USHER IN ADDITIONAL SHORTWAVE ENERGY OUT AHEAD OF DIGGING LONG WAVE TROUGH ON FRIDAY. THIS SHOULD ALLOW PRECIP TO BLOSSOM OVER THE SOUTHERN HALF OF THE CWA THROUGHOUT THE DAY. HAVE MAINTAINED RAIN DURING THIS TIME BUT HAVE SOME CONCERNS THAT SOME SNOW FLAKES COULD BE OBSERVED AS WELL. COLD AIR FILTERING ON THE BACKSIDE OF THIS SYSTEM ON FRIDAY WILL ALLOW THE COLUMN TO COOL WITH CRYSTALS PRESENT BY THE AFTERNOON. BOUNDARY LAYER TEMPS APPEAR TO BE THE LIMITING FACTOR BUT IF ENOUGH FORCING AND COOLING WERE TO OCCUR...A FEW SNOWFLAKES COULD BE POSSIBLE LATE IN THE DAY FRIDAY. RODRIGUEZ && .LONG TERM... 322 PM CDT SUNDAY THROUGH WEDNESDAY... UNSETTLED WEATHER TO START THE NEW WEEK WITH SERIES OF WAVES TO MOVE ALONG A WEST-EAST ORIENTED FRONTAL BOUNDARY TO BE IN THE VICINITY THROUGH THE PERIOD. INITIALLY SPRAWLING HIGH PRESSURE ACROSS THE OHIO VALLEY AND CONTINUED RISING HEIGHTS ALOFT AS RIDGING BUILDS EAST OUT OF THE PLAINS STATES WILL MAINTAIN A MILDER DAY SUNDAY. AS A FRONTAL BOUNDARY SAGS SOUTH ACROSS WISCONSIN SUNDAY AND LOW PRESSURE APPROACHES THE AREA FROM THE WEST... EXPECT AN AREA OF AT LEAST SCATTERED RAIN TO BREAK OUT ACROSS AREAS MAINLY NORTH OF I-80. LIMITED MOISTURE MAY CURTAIL PRECIP COVERAGE UNTIL BETTER FORCING ALONG WITH INCREASE GULF MOISTURE ARRIVES MONDAY. BEST PERIOD FOR RAIN AND EVEN THUNDERSTORMS AT THIS POINT APPEARS TO COME MONDAY NIGHT AND TUESDAY AS MODELS CONTINUE TO DEVELOP STRONG LOW PRESSURE ACROSS THE CENTRAL PLAINS MONDAY ASSOCIATED WITH A SHARP UPPER LEVEL TROUGH. THIS LOW THEN RIDES EAST ALONG AFOREMENTIONED FRONTAL BOUNDARY NEAR THE IL-WI BORDER EARLY TUESDAY AND DRAGS A TRAILING COLD FRONT ACROSS THE AREA LATER TUESDAY. WITH SECONDARY UPPER TROUGH COMBINED WITH COLD AIR ALOFT DROPPING SOUTH INTO THE AREA WEDNESDAY BEHIND COLD FRONT... WILL MAINTAIN SHOWER CHANCES THROUGH EVENING. ED F && .AVIATION... //ORD AND MDW CONCERNS...UPDATED 22Z... * GUSTY SOUTH WINDS THROUGH MID EVENING. * SMALL CHANCE FOR TSRA LATE TONIGHT INTO THURSDAY MORNING. * LLWS POTENTIAL TONIGHT. KJB/BMD //DISCUSSION...UPDATED 18Z... AN AREA OF LOW PRESSURE MOVING EASTWARD ACROSS SOUTHERN CANADA WILL CONTINUE TO DEEPEN AS IT MOVES OVER SOUTHWESTERN ONTARIO THIS EVENING. OVERALL THIS WILL CONTINUE TO LEAD TO AN INCREASE IN SOUTHERLY WINDS ACROSS THE AREA THROUGH THIS EVENING. HOWEVER...IN SPITE OF THIS SURFACE WIND INCREASE...IT APPEARS THAT A LOW LEVEL JET FEATURING 50+ KT SOUTH-SOUTHWESTERLY WINDS AT AROUND 2,000 FEET AGL WILL DEVELOP THIS EVENING...LIKELY SETTING THE STAGE FOR LLWS OVERNIGHT. A COLD FRONT...ASSOCIATED WITH THE SOUTHERN CANADA LOW... WILL SHIFT ACROSS THE AREA BY EARLY THURSDAY AFTERNOON...WITH WEST- NORTHWESTERLY WINDS LIKELY IN ITS WAKE. THE OTHER CONCERN DURING THE PERIOD IS THE POSSIBILITY FOR A PERIOD OR TWO OF SHOWERS WITH SOME EMBEDDED THUNDERSTORMS...PARTICULARLY VERY LATE TONIGHT AND AGAIN BY LATE THURSDAY MORNING. UNCERTAINTY STILL EXISTS ON THE EXTENT OF THE THUNDER WITH THIS ACTIVITY...BUT IT DOES APPEAR THAT AN UPPER LEVEL DISTURBANCE...CURRENTLY MOVING NORTHEASTWARD OVER SOUTHWESTERN MISSOURI...WILL SHIFT OVER PORTIONS OF NORTHERN ILLINOIS BY DAYBREAK THURSDAY. THIS COULD RESULT IN SOME SCATTERED SHOWER AND THUNDERSTORM DEVELOPMENT OVER THE TERMINALS AFTER 09 UTC. FINALLY...ANOTHER AREA OF SHOWERS AND PERHAPS SOME ADDITIONAL STORMS LOOK TO MOVE OVER THE AREA BY LATE MORNING INTO THE EARLY AFTERNOON AHEAD OF THE APPROACHING COLD FRONT. KJB //ORD AND MDW CONFIDENCE...UPDATED 22Z... * MEDIUM-HIGH IN WIND TRENDS. * LOW-MEDIUM IN EXTENT OF TSRA LATE TONIGHT INTO THURSDAY MORNING. * HIGH IN REMAINING ELEMENTS. KJB/BMD //OUTLOOK FOR ORD/MDW FOR 00Z FRIDAY-12Z WEDNESDAY...UPDATED 12Z... THURSDAY NIGHT...VFR LIKELY. FRIDAY...CHANCE LIGHT RAIN AND MVFR. GUSTY NORTH TO NORTHEAST WINDS. SATURDAY...VFR. CHANCE -RA AT NIGHT. SUNDAY...CHANCE -RA. SOUTH WINDS SHIFTING TO EAST. MONDAY...CHANCE -SHRA AND IFR/MVFR. SLIGHT CHANCE TSRA AT NIGHT. TUESDAY...CHANCE TSRA AND MVFR. RC && .MARINE... 317 PM CDT TIGHTENING GRADIENT BEHIND DEPARTING HIGH OVER THE EASTERN GREAT LAKES AND AHEAD OF APPROACHING DEEP LOW PRESSURE OVER THE NORTHERN PLAINS/SOUTHERN MANITOBA WILL PRODUCE A PERIOD OF BRISK SOUTH WINDS ACROSS THE LAKE TONIGHT. GIVEN STABLE THERMAL PROFILE... EXPECT A MUCH LARGER THAN TYPICAL VARIATION BETWEEN THE 10 M WIND AND WINDS AT A 30 M PILOT HOUSE LEVEL. WINDS WILL THEN SLOWLY SUBSIDE AS SOMEWHAT MORE RELAXED GRADIENT IN VICINITY OF FRONTAL BOUNDARY MOVES EAST THURSDAY. NORTHWEST WINDS COMBINED WITH A MORE NEUTRAL TO SLIGHTLY UNSTABLE THERMAL PROFILE BEHIND A SECONDARY COLD FRONT THEN ADVECT ACROSS THE LAKE LATER THURSDAY NIGHT INTO FRIDAY. TIGHTENING GRADIENT COMBINED WITH COLD AIR ADVECTION MEANS THIS PERIOD WILL HAVE TO BE MONITORED CLOSELY FOR A PERIOD OF BRISK NORTHWEST FLOW BEFORE GRADIENT RELAXES AS HIGH PRESSURE APPROACHES FROM THE WEST. A SPRAWLING HIGH PRESSURE RIDGE INFLUENCES THE LAKE SATURDAY BEFORE LOW PRESSURE THEN APPROACHES FROM THE WEST LATER SUNDAY. ED F && .LOT WATCHES/WARNINGS/ADVISORIES... IL...NONE. IN...NONE. LM...SMALL CRAFT ADVISORY...INDIANA NEARSHORE WATERS UNTIL 1 PM THURSDAY. SMALL CRAFT ADVISORY...ILLINOIS NEARSHORE WATERS UNTIL 10 AM THURSDAY. && $$ 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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