564  
FXUS63 KLOT 291954  
AFDLOT  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL  
254 PM CDT WED MAR 29 2017  
   
SHORT TERM
 
 
A CLOSED OFF UPPER LOW CONTINUES TO SPIN NORTHEASTWARD ACROSS  
SOUTH-CENTRAL KANSAS THIS AFTERNOON WITH AN ASSOCIATED SURFACE LOW  
FOLLOWING SUIT. AN INITIAL WAVE OF FORCING IS MOVING NORTHEASTWARD  
ACROSS THE FORECAST AREA WITH BRINGING AREAS OF LIGHT RAIN WITH  
IT. A PERSISTENT FEED OF LOWER LEVEL DRY AIR HAS BEEN HELPING TO  
ERODE THIS BAND OF LIGHT RAIN, BUT SOME COVERAGE HAS BEEN  
MAINTAINED. EXPECT THIS TO CONTINUE ACROSS THE CENTRAL AND EASTERN  
PART OF THE FORECAST AREA OVER THE NEXT COUPLE HOURS WITH SOME  
AREAS SEEING A LITTLE LIGHT RAIN OR EVEN JUST SPRINKLES WHERE  
SOMETHING DOES FALL.  
 
RADAR MOSAIC ALSO SHOWS ANOTHER AREA OF ASCENT, ORIENTED MORE  
WEST TO EAST, IN CONJUNCTION WITH A CORRIDOR OF MID LEVEL WARM  
ADVECTION. SCATTERED TO NUMEROUS SHOWERS HAVE DEVELOPED WITH THIS  
WITH THE BEST COVERAGE ACROSS SOUTHEASTERN IOWA AND NORTHEAST  
MISSOURI AT 240 PM CDT. THIS FORCING WILL CROSS THE AREA A LITTLE  
LATER THIS AFTERNOON AND VERY EARLY THIS EVENING BRINGING ANOTHER  
WINDOW FOR SCATTERED SHOWER ACTIVITY. IT APPEARS COVERAGE WILL  
WANE FOR A SHORT TIME AFTER THIS ACTIVITY LIFTS NORTH BUT THE  
PRIMARY AREA OF FORCING WILL ARRIVE IN THE WESTERN FORECAST AREA  
LATER THIS EVENING AND SPREAD EAST TONIGHT. EXPECT THAT THIS WILL  
BRING A MUCH MORE WIDESPREAD AREA OF SHOWERS/RAIN WITH IT.  
ELEVATED INSTABILITY SPREADS NORTHWARD OVERNIGHT AND THUNDER IS  
POSSIBLE JUST ABOUT ANYWHERE, THOUGH AREAS ALONG AND SOUTH OF  
INTERSTATE 80 HAVE THE BEST CHANCE. EAST TO NORTHEAST WINDS WILL  
INCREASE THIS EVENING AND GUSTS MAY PUSH 30 MPH THROUGH THE  
OVERNIGHT HOURS.  
 

 
   
LONG TERM
 
 
222 PM CDT  
 
THURSDAY THROUGH WEDNESDAY...  
 
MAIN CONCERNS WILL FOR THE LONG TERM FORECAST PERIOD WILL CENTER  
AROUND PCPN/TS CHANCES ASSOCIATED WITH 2 SRN STREAM UPPER CLOSED  
LOWS LIFTING OUT OF THE SWRN CONUS.  
 
THE MODELS ARE COMING INTO BETTER ALIGNMENT WITH THE TIMING AND  
STRENGTH OF THE FIRST IN THE SERIES OF SRN STREAM CLOSED LOWS, BY  
THURSDAY MORNING, THE UPPER LOW CENTER SHOULD LIFT INTO THE SRN  
PLAINS WITH THE ASSOCIATED SFC LOW IN NCNTRL OK AND A WARM FRONT  
EXTENDING EAST INTO THE OHIO VALLEY. STRONG HIGH PRESSURE OVER ERN  
CANADA WILL KEEP GENERALLY EAST TO NORTHEAST FLOW OVER THE REGION.  
THE SFC PRESSURE GRADIENT SHOULD STRENGTHEN THROUGH THE DAY  
THURSDAY, WITH WIND GUSTS ARND 30 MPH DEVELOPING. AS THE SYSTEM  
CONTINUES TO LIFT TO THE NORTHEAST, PCPN WILL CONTINUE THROUGH THE  
DAY. LATEST GUIDANCE SUGGESTS THAT THE INSTABILITY AXIS SHOULD BE  
SUPPRESSED SOUTH IN THE MORNING, WITH LINGERING TS CHANCES GENERALLY  
SOUTH OF I-80 IN THE MORNING AND GRADUALLY SPREADING NORTH TO THE  
WISCONSIN BORDER THROUGH THE DAY. WITH DEEP LAYER MOISTURE ALSO  
INCREASING THROUGH THE DAY, WITH PWATS IN EXCESS OF 1 INCH OVER THE  
AREA, PERIODS OF MODERATE TO HEAVY RAINFALL WILL BE POSSIBLE. SOME  
AREA RIVERS ALREADY RUNNING HIGH FROM PREVIOUS RAINFALL, ADDITIONAL  
RISES WILL BE LIKELY, AND SOME RIVER FLOODING WILL BE POSSIBLE. THE  
TEMPERATURE FORECAST FOR THURSDAY ALSO PRESENTS SOME CHALLENGES AS  
IT WILL BE HIGHLY DEPENDENT ON HOW FAR NORTH THE WARM FRONT WILL  
LIFT. AT THIS TIME, EXPECT THAT THE WARM FRONT WILL GET HUNG UP  
NEAR THE SRN TIP OF LAKE MICHIGAN. THE NRN PORTIONS OF THE CWA  
SHOULD BE IN THE COOL ELY FLOW, NORTH OF THE WARM FRONT AND TEMPS  
SHOULD TOP OUT IN THE LOWER TO MIDDLE 40S. SOUTH OF THE FRONT,  
TEMPS SHOULD BE ABLE TO REACH INTO THE LOWER 60S. LATEST GUIDANCE  
WOULD SUGGEST THAT THE FRONT WILL LAY OUT NEAR THE I-80 CORRIDOR,  
BUT THERE COULD BE SOME BUST POTENTIAL TO THE TEMPERATURE FORECAST  
IF THE FRONT CAN SURGE A LITTLE FARTHER NORTH OR IS SUPPRESSED MORE  
TO THE SOUTH. EXPECT RAIN TO CONTINUE THROUGH THURSDAY NIGHT AND  
POSSIBLY LINGER INTO FRIDAY MORNING AS THE MAIN SFC LOW LIFTS NEWD  
THROUGH NRN IL/IN. ANY LINGERING RAIN SHOULD END BY FRIDAY  
AFTERNOON. AS THE SFC LOW PASSES NORTHEAST OF THE CWA, WINDS WILL  
SHIFT FROM GENERALLY ELY TO MORE NORTHERLY, ALLOWING FOR COOLER  
CONDITIONS TO OVERSPREAD THE ENTIRE CWA, WITH HIGHS FRIDAY ONLY IN  
THE LOWER TO MIDDLE 40S. THE FAR SERN PORTIONS OF THE CWA SHOULD  
SEE TEMPS PEG 50 F. SHOULD THE LOW TAKE A SLIGHTLY SLOWER TRACK,  
THE WARMER AIR COULD LINGER OVER THE ERN PORTIONS OF THE CWA A  
LITTLE LONGER.  
 
THERE SHOULD BE A LULL IN THE PCPN FRIDAY NIGHT THROUGH SATURDAY  
NIGHT AS SHORTWAVE UPPER RIDGING BUILDS ACROSS THE MIDWEST AS THE  
FIRST UPPER LOW EXITS TO THE NORTHEAST AND THE NEXT SRN STREAM  
SYSTEM BEGINS TO LIFT OUT OF THE SWRN CONUS. WHILE CONFIDENCE IS  
RELATIVELY HIGH IN ANOTHER ROUND OF PCPN ASSOCIATED WITH THIS NEXT  
SYSTEM, TIMING OF ONSET IS STILL QUESTIONABLE AS THE LONGER RANGE  
MODELS DIVERGE SOMEWHAT ON THE SPEED WITH WHICH THE UPPER LOW LIFTS  
NEWD. GENERALLY FAVOR THE SLOWER ECMWF SOLUTION WITH THIS SYSTEM AS  
THE GFS HAS A BIAS OF LIFTING THESE TYPES OF SYSTEMS TOO FAST. SO,  
WILL CARRY CHANCE POPS SPREADING INTO NERN IL BY SUNDAY MORNING,  
THERE IS A STRONG CHANCE THAT THE ONSET OF THIS PCPN COULD BE  
DELAYED INTO SUNDAY AFTERNOON OR EVENING. THE GENERAL TREND WITH  
THIS SYSTEM SHOULD BE VERY SIMILAR TO THE FIRST SYSTEM, THOUGH THERE  
ARE INDICATIONS THAT IS SHOULD TAKE A MORE SOUTHERLY TRACK. THE GFS  
IS NOT ONLY TRENDING FASTER, BUT IS ALSO TRENDING FARTHER NORTH,  
THROUGH CENTRAL IL/IN, WHILE THE ECMWF IS TRENDING FARTHER SOUTH,  
THROUGH THE OHIO VALLEY. AGAIN, FAVOR THE MORE SOUTHERLY TRACK OF  
THE SFC LOW AS WELL, WHICH WOULD LIMIT PCPN POTENTIAL TO LIGHTER  
RAIN AND ANY ASSOCIATED TS POTENTIAL TO THE FAR SRN PORTIONS OF THE  
CWA, IF ANY TS CAN EVEN SPREAD THAT FAR NORTH.  
 

 
   
AVIATION
 
 
FOR THE 18Z TAFS...  
 
MAIN FORECAST CONCERNS ARE RAINFALL TIMING, HOW QUICKLY IFR  
ARRIVES OVERNIGHT OR EARLY THURSDAY MORNING AND HOW LOW CIGS/VSBY  
GO THURSDAY.  
 
LOW PRESSURE CENTERED OVER SOUTHEAST KANSAS WILL GRADUALLY MOVE  
NORTHEASTWARD INTO THURSDAY IN CONJUNCTION WITH AN UPPER LEVEL LOW  
OVER WESTERN OKLAHOMA. THE FIRST ROUND OF FORCING IS APPROACHING  
FROM THE WEST WITH A BAND OF RAIN ASSOCIATED WITH IT OVER WESTERN  
AND NORTH-CENTRAL ILLINOIS. PERSISTENT FLOW OF LOWER LEVEL DRY AIR  
WILL INHIBIT THE EASTWARD PUSH OF THIS RAIN SOMEWHAT INTO MID  
AFTERNOON BUT EXPECT LIGHT RAIN AT RFD SOON. COVERAGE MAY DECREASE  
SOMEWHAT ON THE LEADING EDGE INTO THE CHICAGO AREA SO WILL  
MAINTAIN VCSH MENTION THERE FOR NOW. EXPECT THE WINDOW OF  
POTENTIAL LIGHT RAIN TO BE FOCUSED IN THE LATE AFTERNOON AND VERY  
EARLY EVENING TIME FRAME BEFORE FORCING EASES UP FOR A SHORT TIME.  
WILL PROBABLY STILL SEE SPOTTY COVERAGE EARLY THIS EVENING.  
PRIMARY FORCING ARRIVES LATE THIS EVENING AND CONTINUES OVERNIGHT  
AND MOVES FROM WEST TO EAST. THIS WILL BRING GREATER COVERAGE OF  
RAIN ALONG WITH SOME EMBEDDED THUNDER OVERNIGHT. CIGS WILL LOWER  
OVERNIGHT BUT MAY FLUCTUATE BETWEEN MVFR/VFR BEFORE TRENDING TO  
IFR TOWARD MORNING. MAY NEED TO ADJUST THE TIMING OF IFR ONSET AS  
IT MAY OCCUR FASTER THAN CURRENTLY INDICATED.  
 
SHOWERS WILL REMAIN POSSIBLE THROUGH THE DAY THURSDAY. TOUGH TO  
SAY WHEN THE BEST CHANCES WILL OCCUR AT THIS POINT. CIGS/VSBY WILL  
REMAIN LOW BUT DO NOT HAVE GREAT CONFIDENCE IN THE SPECIFICS WITH  
GUIDANCE SHOWING A WIDE RANGE OF POSSIBILITIES. HAVE TRENDED DOWN  
FROM THE PREVIOUS FORECAST FOR THURSDAY MORNING. WITH THE LOW  
TRACKING TO THE SOUTH OF THE TERMINALS EXPECT LOW CONDITIONS TO BE  
PERSISTENT THROUGH THE DAY BUT SHOWERS MAY PROVIDE INTERMITTENT  
IMPROVEMENT.  
 
FINALLY, EAST-NORTHEAST TO NORTHEAST WINDS WILL INCREASE INTO THE  
EVENING AS THE LOW APPROACHES. SPEEDS WILL EASE THURSDAY AFTERNOON  
AND TREND MORE NORTHEASTERLY.  
 
MDB  
 

 
   
MARINE
 
 
253 PM CDT  
 
NORTHEAST TO EAST FLOW WILL INCREASE THROUGH THE REMAINDER OF THE  
AFTERNOON AND THIS EVENING AS LOW PRESSURE LIFTS FROM THE SOUTHERN  
TO CENTRAL PLAINS. THE LOW IS EXPECTED TO TRACK EAST FROM MISSOURI  
TONIGHT TO SOUTHERN LOWER MICHIGAN BY FRIDAY MORNING. EAST WINDS  
ARE EXPECTED TO INCREASE TO 30KT TONIGHT INTO THURSDAY WITH  
OCCASIONAL GALE FORCE GUSTS TO 35 KT POSSIBLE, MAINLY ACROSS THE  
SOUTH HALF OF THE LAKE. THE LOW WILL CONTINUE EAST TO THE EASTERN  
GREAT LAKES FRIDAY NIGHT AND THEN MERGE WITH ANOTHER LOW OFF THE  
MID ATLANTIC COAST SATURDAY. WINDS OVER LAKE MICHIGAN WILL BACK TO  
THE NORTHERLY, AND REMAINING MODERATELY STRONG. THERE IS AN  
INCREASING CHANCE FOR MORE WIDESPREAD GALE FORCE WINDS FRIDAY INTO  
FRIDAY NIGHT IN THE NORTHERLY FLOW. THIS PATTERN WILL KEEP SMALL  
CRAFT ADVISORY CONDITIONS IN PLACE FOR THE ILLINOIS NEARSHORE  
WATERS INTO SATURDAY, WITH THE LIKELIHOOD THAT A SMALL CRAFT  
ADVISORY WILL BE NEEDED FOR THE INDIANA WATERS AS WELL WHEN WINDS  
TURN NORTHERLY ON FRIDAY. HIGH PRESSURE WILL BEGIN TO BUILD OVER  
THE REGION ON SATURDAY ALLOWING WINDS TO RELAX SOME OVER THE  
WEEKEND BEFORE ANOTHER LOW TRACKS THROUGH THE OHIO VALLEY EARLY  
NEXT WEEK.  
 

 
   
LOT WATCHES/WARNINGS/ADVISORIES
 
 
IL...NONE.  
IN...NONE.  
LM...SMALL CRAFT ADVISORY...IL NEARSHORE WATERS UNTIL 4 AM SATURDAY.  
 

 
 

 
 
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