595
FXUS63 KLOT 202339
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
639 PM CDT MON APR 20 2026
KEY MESSAGES
- NEXT CHANCE OF THUNDERSTORMS AS EARLY AS TUESDAY AFTERNOON,
BUT MAINLY TUESDAY EVENING NEAR/SOUTH OF I-88.
- WARMING TREND THROUGH THURSDAY (EXCEPT COOLER NEAR LAKE
MICHIGAN ON WEDNESDAY).
- MORE WIDESPREAD SHOWERS AND THUNDERSTORMS WITH ANOTHER COLD
FRONT THURSDAY NIGHT INTO FRIDAY.
DISCUSSION
ISSUED AT 235 PM CDT MON APR 20 2026
EARLY AFTERNOON SURFACE ANALYSIS DEPICTS HIGH PRESSURE DRIFTING
OFF TO THE EAST OF THE FORECAST AREA, CENTERED NEAR LAKE ERIE
AS OF 18Z. FARTHER TO THE WEST, THE SURFACE PRESSURE GRADIENT
WAS TIGHTENING FROM THE PLAINS INTO THE UPPER MIDWEST/NORTHERN
LAKES, WITH SOUTHERLY WINDS EXPECTED TO GRADUALLY INCREASE
TONIGHT AS A LOW-LEVEL JET DEVELOPS ACROSS THE REGION. OUR
TYPICALLY WARMER URBAN CORE OF THE CHICAGO METRO WILL LIKELY SEE
AT LEAST OCCASIONAL GUSTS IN THE 20-25 MPH DEVELOP OVERNIGHT,
WITH BREEZY SOUTHWEST FLOW AREA-WIDE AFTER SUNRISE TUESDAY. BY
TUESDAY AFTERNOON GUSTS OF 30-35 MPH APPEAR LIKELY ESPECIALLY
SOUTH OF I-80. WITHIN THE BROADER REGION OF SOUTHWEST FLOW,
ANOTHER EML PLUME IS FORECAST TO SPREAD ATOP A RELATIVELY DRY
BOUNDARY LAYER DURING THE DAY TUESDAY, WITH FAIRLY WARM TEMPS OF
+10/12C NOTED AT THE BASE OF THE PLUME AROUND 800-850 MB. THIS
LOOKS TO PROVIDE A CAP WITH RESPECT TO SURFACE-BASED CONVECTION
THROUGH MUCH OF THE MORNING/MIDDAY HOURS. FORECAST SOUNDINGS DO
INDICATE SOME COOLING OF THE COLUMN AHEAD OF A LOW-AMPLITUDE
SHORT WAVE DURING THE AFTERNOON AND EVENING HOWEVER, WHICH ALONG
WITH THE NORTHEASTWARD ADVECTION OF A NARROW AXIS OF LOW-LEVEL
MOISTURE MAY BE SUFFICIENT TO ERODE THE CAP. VARIOUS CAM
GUIDANCE (4KM NAM, RAP, HRRR) DOES DEVELOP ISOLATED TO SCATTERED
SHOWERS AND THUNDERSTORMS ROUGHLY ALONG AND SOUTH OF I-88 BY
LATE AFTERNOON. GREATER COVERAGE AND CONFIDENCE HOWEVER REMAINS
ALONG/SOUTH OF I-80 TOWARD/DURING THE EVENING HOURS AS THE
SURFACE COLD FRONT PUSHES SOUTHEAST. DESPITE THE RELATIVELY
LIMITED MOISTURE RETURN, WEST-NORTHWEST 40-45 KT WINDS ALOFT
WOULD SUPPORT SOME MODEST DEEP-LAYER SHEAR, WHICH MAY PROVIDE
ENOUGH UPDRAFT LONGEVITY TO PRODUCE A SUB-SEVERE SMALL HAIL AND
GUSTY WIND THREAT. THE ORIENTATION OF THE MID-LEVEL FLOW MAY
ALSO SUPPORT SLOW STORM MOTIONS/BACK BUILDING FOR A VERY
LOCALIZED TRAINING/HEAVY RAINFALL/FLASH FLOOD POTENTIAL AS THE
FRONT CONTINUES TO SAG SOUTH AND SOUTHEAST TUESDAY NIGHT.
RISING HEIGHTS/SUBSIDENCE THEN DEVELOPS WEDNESDAY, WITH SURFACE
HIGH PRESSURE DRIFTING ACROSS THE GREAT LAKES REGION SUPPORTING
A PERIOD OF DRY, QUIET WEATHER MID-WEEK. AFTER TEMPERATURES IN
THE 70S ON TUESDAY (RIGHT UP TO THE LAKE MICHIGAN SHORE GIVEN
THE BREEZY SOUTHWEST WINDS), WEDNESDAY LOOKS EQUALLY MILD IN
MANY AREAS, THOUGH ONSHORE WINDS NEAR THE LAKE WILL LIKELY HOLD
TEMPS IN THE 50/60S CLOSER TO THE SHORE. BREEZY SOUTHWEST LOW-
LEVEL FLOW THEN DEVELOPS AGAIN THURSDAY, WARMING TEMPERATURES
INTO THE LOWER 80S IN MOST AREAS EVEN NEAR THE LAKE.
GUIDANCE IS IN GOOD AGREEMENT IN DEVELOPING A DEEP LONG-WAVE
TROUGH FROM THE WESTERN CONUS EASTWARD LATER IN THE WEEK. THIS
LOOKS TO SPREAD MORE WIDESPREAD SHOWER AND THUNDERSTORM CHANCES
BACK INTO THE AREA THURSDAY NIGHT INTO FRIDAY, AHEAD OF A COLD
FRONT CURRENTLY DEPICTED TO CROSS THE AREA FRIDAY
AFTERNOON/EVENING. MODELS CONTINUE TO INDICATE STRONGER
INSTABILITY AND GREATER SEVERE THREAT WOULD RESIDE UPSTREAM (MO-
CENTRAL/SOUTHERN IL) FROM THE WFO LOT CWA AHEAD OF THE COLD
FRONT, BUT WE'LL HAVE TO MONITOR DETAILS IN FURTHER TRENDS AS
THIS END OF THE WEEK PERIOD APPROACHES.
RATZER
AVIATION /00Z TAFS THROUGH 06Z WEDNESDAY/
ISSUED AT 639 PM CDT MON APR 20 2026
FORECAST CONCERNS INCLUDE...
CHANCE OF THUNDERSTORMS TUESDAY AFTERNOON.
SOUTHEAST WINDS THIS EVENING SHIFTING SOUTHWEST OVERNIGHT.
LOW LEVEL WIND SHEAR OVERNIGHT.
GUSTY SOUTHWEST WINDS TUESDAY.
A LAKE BREEZE HAS MOVED INLAND AND CLOSE TO BOTH ORD/MDW.
CONFIDENCE IS LOW FOR THIS BOUNDARY MOVING THROUGH EITHER
TERMINAL BUT PREVAILING WINDS WILL BE FAVORING SOUTHEAST THIS
EVENING FOR THE ENTIRE AREA. THERE COULD BE A FEW HOURS OF
EAST/SOUTHEAST WINDS AT ORD/MDW. DIRECTIONS WILL TURN BACK MORE
SOUTHERLY LATE THIS EVENING AND THEN SOUTHWEST OVERNIGHT. GUSTS
ARE EXPECTED TO SLOWLY INCREASE OVERNIGHT FOR ORD/MDW, POSSIBLY
INTO THE LOWER 20KT RANGE. ELSEWHERE GUSTS WILL LIKELY NOT
DEVELOP UNTIL AFTER DAYBREAK AND AS LOW LEVEL WINDS STEADILY
INCREASE, LOW LEVEL WIND SHEAR IS EXPECTED. IF GUSTS DO NOT
MATERIALIZE AT ORD/MDW, LOW LEVEL WIND SHEAR WILL ALSO BE
NEEDED. GUSTS INTO THE LOWER/MID 20KT RANGE WILL CONTINUE INTO
TUESDAY AFTERNOON.
WHILE THERE REMAINS QUITE A BIT OF UNCERTAINTY, THE MOST RECENT
GUIDANCE IS SHOWING A BROKEN LINE OF THUNDERSTORMS DEVELOPING
RIGHT OVER THE CHICAGO TERMINALS DURING THE MID/LATE AFTERNOON
TUESDAY. ITS POSSIBLE THIS ACTIVITY DEVELOPS JUST SOUTHEAST OF
THE TERMINALS BUT GIVEN THE POTENTIAL, PROB THUNDER MENTION
SEEMS REASONABLE WITH THIS FORECAST. CURRENT MODEL TRENDS WOULD
SUGGEST ANYTHING THAT DEVELOPS WOULD BE SOUTHEAST OF THE
TERMINALS BY/AFTER 00Z ALONG WITH A POSSIBLE WIND SHIFT TO
NORTHEAST EARLY TUESDAY EVENING. CMS
LOT WATCHES/WARNINGS/ADVISORIES
IL...NONE.
IN...NONE.
LM...NONE.
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