341  
FXUS63 KLOT 200720  
AFDLOT  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL  
220 AM CDT WED MAR 20 2019  
   
UPDATE
 
 
930 PM CDT  
 
NO MAJOR CHANGES TO THE FORECAST MESSAGE OF A WET WEDNESDAY  
MORNING COMMUTE, THE DAY OF THE VERNAL EQUINOX AT 4:58PM CDT. MADE  
TWEAKS TO HOURLY POP TRENDS AND ADDED HOURLY RESOLUTION THROUGH  
MID DAY TOMORROW IN LINE WITH RADAR AND CAM GUIDANCE TRENDS.  
EXTROPLATING OFF THE CURRENT POSITION OF THE AREA OF RAIN OVER  
CENTRAL IA UP TO ABOUT I-80 LENDS CREDENCE TO THE GRADUAL  
NORTHWARD SHIFT OF THE RAIN THAT HAS OCCURRED WITH THIS SYSTEM. IT  
NOW LOOKS LIKE MOST IF NOT ALL LOCATIONS IN THE CWA IN NORTHERN IL  
WILL RECEIVE MEASURABLE RAIN.  
 
STRONG MOISTURE TRANSPORT IN 925-850 MB LAYER FROM 25-35 KT LLJ  
WILL TRANSLATE THE 0.6"+ PWAT AXIS SAMPLED BY 00Z TOP RAOB  
EASTWARD OVER OUR AREA, SUPPORTING STEADY TO TEMPORARY MODERATE  
RAINFALL RATES. MAIN FACTOR KEEPING RAIN AMOUNTS IN CHECK WILL BE  
THE FAST MOVING NATURE OF THE FORCING FROM THE SHORT-WAVE AND  
ISENTROPIC ASCENT, WITH MAXIMUM AMOUNTS FROM NEAR/SOUTH OF I-88  
DOWN TO AROUND KANKAKEE RIVER UP TO ABOUT 1/4 INCH, WITH LIGHTER  
AMOUNTS NORTH TO AROUND 0.1" IN STATE LINE COUNTIES. OTHER ITEM OF  
NOTE IS THE RATHER BREEZY SOUTH-SOUTHWEST WINDS THAT WILL  
ACCOMPANY THE TAIL END OF THE MORNING RAIN AND PERSIST INTO THE  
EARLY AFTERNOON, ESPECIALLY SOUTHEAST OF I-55. SURFACE LOW WILL  
OPEN UP BUT STILL LOOKING AT STOUT PRESSURE FALLS MID MORNING  
THROUGH EARLY AFTERNOON FOR GUSTS UP TO 30 MPH IN FAVORED  
CORRIDOR PER FORECAST SOUNDINGS.  
 
FINALLY, TOMORROW AFTERNOON INTO THE EARLY EVENING, AS ANOTHER  
STOUT SHORT WAVE DIVES SOUTHEAST WITH COLD FRONT CROSSING THE  
REGION, AT LEAST WIDELY SCATTERED SHOWERS APPEAR PROBABLE. DID NOT  
CHANGE POPS FOR MID TO LATE PM HOURS, BUT DID CHANGE SHOWERS TO  
COVERAGE WORDING GIVEN GOOD CONFIDENCE IN THEM OCCURRING.  
 
CASTRO  
 

 
   
SHORT TERM
 
 
1232 PM CDT  
 
THROUGH WEDNESDAY...  
 
SEASONAL CONDITIONS REMAIN IN PLACE AFTERNOON WITH PLENTIFUL  
SUNSHINE, SOME SCATTERED CUMULUS, AND READINGS IN THE UPPER 40S  
TO NEAR 50. WATER VAPOR IMAGERY DEPICTS AN INCREASING CLOUD  
SHIELD OUT AHEAD OF THE UPPER LOW CURRENTLY ACROSS THE CENTRAL  
PLAINS. WARM ADVECTION AHEAD OF THE COMPACT UPPER LOW WILL DRIVE  
AN AREA OF RAIN ACROSS THE AREA LATE TONIGHT INTO WEDNESDAY.  
 
MORNING RAOBS INDICATE AN AXIS OF PWAT VALUES IN EXCESS OF 0.6",  
NOT A DIRECT GULF OF MEXICO FEED BUT AN AXIS OF INCREASED  
MOISTURE. SOUTHWEST FLOW AHEAD OF THE LOW WILL INDUCE ISENTROPIC  
ASCENT AND EVENTUALLY LEAD TO LOWER LEVEL SATURATION. FOR TONIGHT  
THE ASCENT IS MORE MODEST, BUT WILL INCREASE OVERNIGHT INTO MORE  
SO WEDNESDAY MORNING. ENSEMBLE PLUMES SUGGEST MOST AREAS WILL SEE  
UNDER 0.25" OF QPF, BUT THE LIFT AND MOISTURE ARE SUPPORTIVE OF  
HIGHER TOTALS MAINLY ALONG AND SOUTH OF I-80 AND INTO NORTHWEST  
INDIANA.  
 
THE STRONGER SHORTWAVE ENERGY WILL PASS TO OUR SOUTH, BUT BROAD  
HEIGHT FALLS AND THE MAIN TROUGH AXIS WILL STILL BE SHIFTING  
THROUGH WEDNESDAY AFTERNOON, THUS CLOUDY CONDITIONS WILL PERSIST  
WITH SOME LIGHT RAIN/SHOWERS STILL POSSIBLE. GIVEN THE EXTENSIVE  
CLOUD COVER, EXPECT HIGHS TO FALL SHY OF 50 IN MOST AREAS.  
 
KMD  
 

 
   
LONG TERM
 
 
220 AM CDT  
 
THURSDAY NIGHT THROUGH TUESDAY...  
 
THE MAIN FORECAST CONCERNS THROUGH THE PERIOD CONTINUE TO BE THE  
INCREASING POTENTIAL FOR A DECENT COLD FRONT TO SHIFT OVER THE  
AREA FRIDAY, FOLLOWED BY BRIEF WARMUP FOR THE WEEKEND, THEN THE  
POSSIBLE ARRIVAL OF ANOTHER STRONG COLD FRONT SUNDAY NIGHT.  
 
IT APPEARS THAT A WEATHER SYSTEM DROPPING SOUTHEASTWARD OVER THE  
EASTERN GREAT LAKES WILL PUSH A COLD FRONT DOWN LAKE MICHIGAN AND  
ACROSS OUR AREA EARLY FRIDAY. WITH THE PASSAGE OF THIS FRONT,  
WINDS ARE LIKELY TO SHIFT NORTHEASTERLY OFF THE LAKE SETTING UP A  
CHILLY AFTERNOON (TEMPS IN THE 30S) FOR AREAS AROUND THE LAKE.  
WHILE TEMPERATURES WILL BE A BIT WARMER INLAND, TEMPERATURES HERE  
WILL LIKELY REMAIN IN THE MID 40S.  
 
SURFACE HIGH PRESSURE SHOULD QUICKLY DROP SOUTHWARD OVER THE AREA  
FRIDAY NIGHT BEFORE SHIFTING TO OUR SOUTHEAST DURING THE DAY ON  
SATURDAY. AFTER A COLD START TO THE DAY (TEMPS IN THE MID TO UPPER  
20S) TEMPERATURES SHOULD WARM BACK TO NEAR 50 ON SATURDAY. THE  
ONLY EXCEPTION TO THIS WILL BE AREAS ALONG THE LAKESHORE, WHERE  
IT APPEARS PROBABLE THAT A LAKE BREEZE WILL DEVELOP SATURDAY  
AFTERNOON AND RESULT IN LOCALLY COOLER CONDITIONS.  
 
TEMPERATURES SHOULD INCH UP A BIT MORE FOR SUNDAY, BUT THERE WILL  
BE INCREASING CLOUD COVER AND CHANCE FOR SOME RAIN SHOWERS IN  
ASSOCIATION WITH AN APPROACHING WEATHER DISTURBANCE. WHILE THERE  
CONTINUES TO BE UNCERTAINTIES ON THE DETAILS OF THE FORECAST  
SUNDAY NIGHT INTO MONDAY, THERE CONTINUES TO BE SIGNS THAT ANOTHER  
STRONG COLD FRONT COULD DROP SOUTHWARD ACROSS THE AREA AS STRONG  
HIGH PRESSURE BUILDS SOUTHWARD OVER THE NORTHERN PLAINS. NOT ONLY  
WOULD THIS RESULT IN BELOW NORMAL TEMPERATURES FOR EARLY NEXT  
WEEK, BUT IT COULD ALSO RESULT IN A PERIOD OF WINTRY PRECIPITATION  
SUNDAY NIGHT INTO MONDAY. HOWEVER, CONFIDENCE ON THE WINTRY  
PRECIPITATION IS LOW AT THIS TIME.  
 
KJB  
 

 
   
AVIATION
 
 
FOR THE 06Z TAFS...  
 
RAIN WILL BE DEVELOPING OVER THE AREA TOWARDS DAYBREAK AS OUR NEXT  
WEATHER DISTURBANCE SHIFTS OVER THE AREA. EXPECT A PERIOD OF LIGHT  
TO MODERATE RAIN DURING THE EARLY MORNING, WHICH MAY PRODUCE A  
SHORT PERIOD OF IFR VISIBILITIES. LOW MVFR CIGS WILL ALSO DEVELOP  
OVER THE AREA FOR THE MORNING. WHILE CIGS MAY IMPROVE SOME IN THE  
AFTERNOON, IT APPEARS THAT ANOTHER BATCH OF RAIN SHOWERS COULD  
MOVE ACROSS THE TERMINALS LATE IN THE AFTERNOON AND EARLY EVENING  
AS A COLD FRONT MOVES OVER THE AREA. CIGS MAY THEN TREND BACK DOWN  
INTO THE MVFR RANGE LATER TONIGHT WITH WINDS SHIFTING TO THE NORTHWEST  
IN THE WAKE OF THE FRONT.  
 
KJB  
 

 
   
LOT WATCHES/WARNINGS/ADVISORIES
 
 
IL...NONE.  
IN...NONE.  
LM...NONE.  

 
 

 
 
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