313
FXUS63 KIWX 172338
AFDIWX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE NORTHERN INDIANA
738 PM EDT SUN MAY 17 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 /00Z TAFS THROUGH 00Z TUESDAY/
ISSUED AT 734 PM EDT SUN MAY 17 2026
VFR CONDITIONS TO DOMINATE FOR THE FIRST 12 TO 15 HOURS OF THE
PERIOD WITH THE ONLY CONCERN BEING SOME MARGINAL LLWS AS THE LOW
LEVEL FLOW BEGINS TO RAMP UP SOMEWHAT. ATTENTION THEN SHIFTS TO
WHAT WILL LIKELY BE A WEAKENING COMPLEX OF STORMS MOVING OUT OF
IL AROUND/AFTER 15Z. CAMS STILL INDICATING PLENTY OF INSTABILITY
AHEAD OF THIS AREA TO ALLOW FOR THE POTENTIAL OF RE-
DEVELOPMENT/INTENSIFICATION OF STORMS AS THEY ENTER THE AREA.
STRONGEST STORMS MAY END UP MORE TOWARDS KFWA, BUT HAVE
CONTINUED WITH PROB30 AT KSBN AND A FEW HOURS LATER AT KFWA. THE
AREA SHOULD BE FAIRLY PROGRESSIVE WITH CLEARING SKIES BEHIND IT.
IWX WATCHES/WARNINGS/ADVISORIES
IN...NONE.
OH...NONE.
MI...NONE.
MARINE...NONE.
DISCUSSION...BROWN
AVIATION...FISHER
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