442
FXUS63 KIWX 180522
AFDIWX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE NORTHERN INDIANA
122 AM EDT MON MAY 18 2026
KEY MESSAGES
- SEVERE THUNDERSTORMS MONDAY AFTERNOON AND EVENING. HEAVY RAIN,
DAMAGING WIND GUSTS, AND HAIL ARE THE THE PRIMARY HAZARDS.
- ADDITIONAL SEVERE STORMS ARE ANTICIPATED ON TUESDAY.
- REMAINING WARM AND HUMID THROUGH TUESDAY. COOLER AND DRY BY
WEDNESDAY WITH HIGHS ONLY IN THE 60S.
DISCUSSION
ISSUED AT 211 PM EDT SUN MAY 17 2026
THE PRIMARY SEVERE WEATHER CONCERN TOMORROW EVOLVES AROUND WHAT IS
EXPECTED TO BE AN UPSTREAM, MATURE, MCS EARLY MONDAY MORNING.
MCS MAINTENANCE IS DEPENDENT, IN PART, ON MID-LEVEL LAPSE
RATES, INSTABILITY, AND BULK SHEAR. INSTABILITY APPEARS TO BE A
NON- ISSUE ACROSS NORTHERN IL AND EVEN NORTHWEST IN ON EITHER
SIDE OF SUNRISE MONDAY, WITH MUCAPE VALUES OF 1,000-1,500 J/KG.
LAPSE RATES NEAR 7C/KM ARE ALSO FAVORABLE, BUT SHEAR MAGNITUDE
OF ONLY 25 KNOTS IMMEDIATELY AHEAD OF THE LINE IS MARGINAL AT
BEST, IN CONTRAST TO INCREASED SHEAR VALUES BEHIND THE LINE
(WHICH COULD BE A RESULT OF THE MODEL-DERIVED CONVECTION). GIVEN
THE AFORMENTIONED NUANCES, AND UPPER-LEVEL FORCING LIFTING
NORTH OF OUR AREA, AN INCOMING MCS FROM ILLINOIS IN THE MORNING
WILL BE IN A WEAKENING PHASE AS IT APPROACHES NORTHWEST INDIANA
JUST AFTER SUNRISE.
THE ASSOCIATED OUTFLOW BOUNDARY FROM THIS MCS IS LIKELY THE CATALYST
FOR AFTERNOON SEVERE THUNDERSTORMS (BEST WINDOW OF OPPORTUNITY
APPEARS TO BE 1P TO 10P EDT). FORECAST SOUNDINGS AHEAD OF THE LINE
SHOW BOUNTIFUL SFC AND MU CAPE IN EXCESS OF 2,000 J/KG WHILE BULK
SHEAR VALUES IS MARGINAL NEAR 25 KNOTS. STEEP LOW-LEVEL LAPSE RATES
SUGGEST DAMAGING WINDS ARE THE PRIMARY HAZARD, AS DOES THE
ANTICIPATED LINEAR STORM MODE. A SOUTHWESTERLY LOW-LEVEL SHEAR
VECTOR INDICATES A LOW TORNADO PROBABILITY FOR THE NORTHERN EDGE OF
ANY MCS THAT DEVELOPS. CANNOT RULE OUT HAIL FROM ANY DISCRETE STORMS
THAT DEVELOP AHEAD OF THE LINE. A FINAL PLAUSIBLE OUTCOME FOR MONDAY
IS THAT THE WEAKENING MCS MOVES FROM WEST TO EAST MONDAY MORNING
THROUGH MIDDAY, RESULTING IN ABUNDANT CLOUD COVER AND STEADY RAIN
FOR A TIME WHICH ULTIMATELY SQUASHES THE SEVERE WEATHER RISK. GIVEN
THE PRECEDING INSTABILITY (AS NOTED ABOVE) THIS SEEMS UNLIKELY.
SIMILARLY, SEVERE WEATHER APPEARS INCREASINGLY LIKELY ON TUESDAY AS
THE SLOW-MOVING COLD FRONT ULTIMATELY RESPONSIBLE FOR THIS ACTIVE
WEATHER PATTERN MOVES THROUGH. INSTABILITY CONTINUES TO BE AMPLE
WHILE 0-6KM SHEAR IMPROVES TOWARD 35 KNOTS. WITH THE COLD FRONT IN
PLAY, TORNADOES ARE POSSIBLE AS WELL. FINE DETAILS REMAIN TO BE
SEEN, BUT EARLY INDICATIONS SUGGEST A LINE OF THUNDERSTORMS DEVELOPS
TO OUR WEST AND MOVES THROUGH TUESDAY AFTERNOON AND EVENING.
NOTABLY COOLER STARTING WEDNESDAY WITH HIGHS ONLY IN THE 60S. HIGH
PRESSURE PROMOTES DRY WEATHER THROUGH AT LEAST FRIDAY, PERHAPS INTO
SATURDAY, DEPENDING ON THE TIMING OF THE NEXT DISTURBANCE LIFTING IN
FROM THE SOUTHERN PLAINS.
AVIATION /06Z TAFS THROUGH 06Z TUESDAY/
ISSUED AT 117 AM EDT MON MAY 18 2026
VFR CONDITIONS WILL CONTINUE THROUGH MIDDAY WITH THE ONLY
CONCERN BEING SOME MARGINAL LLWS AS THE LOW LEVEL FLOW BEGINS TO
RAMP UP SOMEWHAT. A DECAYING MCS OUT OF IOWA, MISSOURI, AND
ILLINOIS WILL BE THE FOCUS FOR POTENTIAL STRONG TO SEVERE STORMS
TODAY. THIS COMPLEX IS EXPECTED TO WEAKEN OVER ILLINOIS THIS
MORNING AND THEN REINTENSIFY ACROSS NORTHERN INDIANA AROUND
18Z. BREEZY SOUTHERLY WIND WILL GUST AS HIGH AS 30 KTS TODAY,
EVEN OUTSIDE OF ANY SHOWERS OR STORMS. THE STRONGEST STORMS MAY
END UP EAST OF IN-15 TOWARDS KFWA, BUT HAVE MAINTAINED A PROB30
AT KSBN FOR 17-20Z AND A FEW HOURS LATER AT KFWA FROM 19-22Z.
MVFR CEILINGS AND VISIBILITIES POSSIBLE WITHIN ANY STORMS. THE
MAIN THREATS TODAY WILL BE HEAVY RAIN AND DAMAGING WIND GUSTS UP
TO 50KTS.
IWX WATCHES/WARNINGS/ADVISORIES
IN...NONE.
OH...NONE.
MI...NONE.
MARINE...NONE.
DISCUSSION...BROWN
AVIATION...JOHNSON
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