796
FXUS63 KLOT 101207
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
707 AM CDT MON AUG 10 2026
KEY MESSAGES
- ADDITIONAL PERIODS OF STORMS ARE EXPECTED AT TIMES THROUGH
MID-WEEK. SOME OF THESE STORMS COULD BE SEVERE AND LEAD TO
ADDITIONAL FLOODING.
DISCUSSION
ISSUED AT 300 AM CDT MON AUG 10 2026
FORECAST REMAINS COMPLEX THROUGH MID-WEEK, AS A NEARLY
STATIONARY FRONTAL BOUNDARY LINGERS ACROSS THE FORECAST AREA
BENEATH AN ACTIVE WEST-NORTHWEST FLOW PATTERN ALOFT. PERIODIC
SHOWER AND THUNDERSTORM CHANCES WILL PERSIST WITH SOMEWHAT LOW
CONFIDENCE IN TIMING AND COVERAGE, ULTIMATELY MODULATED BY
INDIVIDUAL SHORT WAVES AND DAILY DIURNAL HEATING AND INSTABILITY
TRENDS. EPISODES OF SEVERE WEATHER AND LOCALLY HEAVY RAINFALL
REMAIN POSSIBLE.
IN THE NEAR-TERM, LATEST SURFACE ANALYSIS DEPICTS A NEARLY
STATIONARY FRONTAL BOUNDARY STRETCHING FROM AN AREA OF LOW
PRESSURE OVER WESTERN KS NORTHEAST INTO WESTERN IA, THEN EAST
ALONG ROUGHLY THE I-80 CORRIDOR ACROSS NORTHERN IL/NORTHWEST IN.
A VERY WARM/HUMID AIRMASS IS IN PLACE ALONG/SOUTH OF THE
BOUNDARY, CHARACTERIZED BY TEMPS IN THE UPPER 70/AROUND 80 AND
DEW POINTS IN THE MID-70S AT THIS LATE NIGHT HOUR. ALOFT,
SEASONABLY FAST WEST-NORTHWEST FLOW WAS IN PLACE ACROSS THE
REGION, SUPPLYING A TRACK FOR A SERIES OF MID-LEVEL SHORT WAVE
DISTURBANCES OF VARYING AMPLITUDE. ONE SUCH WAVE IS CURRENTLY
TRACKING FROM IA INTO NORTHERN IL, SUPPORTING A SOUTHWESTERLY 35
KT LOW-LEVEL JET ATOP THE LOW-LEVEL FRONTAL ZONE INTO NORTHEAST
IL/NORTHWEST IN. SEVERAL CLUSTERS OF STRONG THUNDERSTORMS HAD
DEVELOPED IN THE ENHANCED WARM/MOIST ADVECTION REGIME ASSOCIATED
WITH THIS JET LAST EVENING, WITH A SMALL MCS REMAINING ACROSS
OUR SOUTHERN TIER OF CWA COUNTIES AS OF 2 AM ON THE NOSE OF THE
JET IN THE VICINITY OF THE SURFACE FRONTAL BOUNDARY. GIVEN
ML/MUCAPE VALUES IN THE 3000-3500 J/KG RANGE, 35-40 KTS OF
EFFECTIVE SHEAR AND PRECIPITABLE WATER VALUES NEAR 2 INCHES,
SEVERE WEATHER AND LOCALLY HEAVY RAINFALL REMAIN A CONCERN WITH
THESE STORMS FOR THE NEXT HOUR OR SO, BEFORE THEY EVENTUALLY
SLIDE SOUTHEAST OF THE AREA TOWARD/JUST AFTER DAYBREAK AS THE
LOW-LEVEL JET GRADUALLY VEERS MORE WESTERLY AND WEAKENS WITH THE
PASSAGE OF THE MID-LEVEL WAVE.
ONCE THESE STORMS EXIT THE AREA, THERE SHOULD BE A BIT OF A
LULL IN CONVECTION IN THE WAKE OF THE DEPARTING SHORT WAVE AND
DURING OUR DIURNAL INSTABILITY MINIMUM. CAN'T COMPLETELY RULE
OUT SOME ISOLATED ELEVATED SHOWER OR THUNDERSTORM DEVELOPMENT
HOWEVER, DUE TO THE PRESENCE OF THE FRONT/BAROCLINIC BOUNDARY
AND WEAKER BUT STILL PERSISTENT WARM/MOIST ADVECTION. CHANCES
SHOULD INCREASE AGAIN AT LEAST A BIT TOWARD MIDDAY AND INTO THIS
AFTERNOON HOWEVER, AS ANOTHER WEAK WAVE OR MCV (CURRENTLY
APPROACHING THE OMAHA VICINITY) TRACKS EAST DURING A MORE
FAVORABLE DIURNAL TIME. CAM GUIDANCE LOOSELY DEPICTS THIS,
THOUGH DIFFERENCES IN COVERAGE AND TIMING DETAILS ARE EVIDENT.
ADDITIONAL DEVELOPMENT ALSO APPEARS POSSIBLE LATER THIS EVENING,
AS THE NOCTURNAL LOW-LEVEL JET MODESTLY INCREASES ATOP THE
FRONTAL ZONE. THERE DOES APPEAR TO BE A SOMEWHAT CONSISTENT
SIGNAL HOWEVER, AMONGST BOTH CAM AND GLOBAL GUIDANCE, FOR AN MCS
TO DEVELOP OVER NE/SD LATER TONIGHT WHICH WOULD APPROACH THE
FORECAST AREA AROUND/AFTER DAYBREAK TUESDAY. THIS SUPPORTS THE
HIGHEST POPS (60-70%) DURING TUESDAY, AND HAVE LIMITED OTHER
MENTIONS TO JUST CHANCE (20-40%) LATER TODAY AND TONIGHT. GIVEN
OTHERWISE SIMILAR INSTABILITY/SHEAR AND MOISTURE PARAMETERS, WE
REMAIN IN SPC/WPC SLIGHT RISKS FOR SEVERE WEATHER AND EXCESSIVE
RAINFALL DURING THESE PERIODS.
NO SIGNIFICANT CHANGE IN THE PATTERN IS EVIDENT INTO MID-WEEK
(WEDNESDAY, POSSIBLY THURSDAY), THOUGH CONFIDENCE GRADUALLY
DECREASES BASED ON IMPACTS OF ANY PRECEDING CONVECTIVE/MCS
OUTFLOW IMPACTS. GLOBAL ENSEMBLES CONTINUE TO SHOW A SIGNAL FOR
OUR FRONTAL BOUNDARY TO SHIFT A LITTLE FARTHER SOUTH THURSDAY
NIGHT AND FRIDAY, WHICH MAY DECREASE OUR SHOWER/THUNDERSTORM
POTENTIAL BRIEFLY LATE IN THE WEEK, THOUGH THIS SIGNAL IS WEAKER
THAN EARLIER RUNS AND THERE ARE INDICATIONS THAT THE FRONT MAY
LIFT BACK INTO THE AREA NEXT WEEKEND. THUS, IT APPEARS THAT OUR
SEASONABLY WARM/HUMID AND PERIODICALLY STORMY PATTERN MAY
PERSIST FOR MUCH OF THE FORECAST PERIOD.
RATZER
AVIATION /12Z TAFS THROUGH 18Z TUESDAY/
ISSUED AT 707 AM CDT MON AUG 10 2026
KEY MESSAGES:
- MVFR CEILINGS (INITIALLY IFR AT RFD) LIKELY TO PERSIST
THROUGH THIS MORNING.
- THUNDERSTORMS ARE LIKELY TO AFFECT THE TERMINALS LATE THIS
AFTERNOON INTO TONIGHT. SOME POTENTIAL EXISTS FOR A PROLONGED
PERIOD OF TSRA IMPACTS AT/NEAR THE TERMINALS DURING THIS TIME
FRAME.
- ANOTHER ROUND OF THUNDERSTORMS IS LIKELY TOWARDS THE END OF
THE CURRENT 30-HOUR TAF PERIOD FOR ORD AND MDW.
MVFR STRATUS HAS DEVELOPED ACROSS NORTHERN ILLINOIS AND
NORTHWEST INDIANA AND IS LIKELY TO PERSIST AT THE TERMINALS
THROUGH MOST, IF NOT ALL, OF THIS MORNING. ADDITIONALLY, IFR
CEILINGS AND MVFR VISIBILITIES ARE BEING OBSERVED AT RFD AT
PRESS TIME, BUT THESE CONDITIONS ARE EXPECTED TO IMPROVE BY MID-
MORNING.
ANOTHER ROUND OF THUNDERSTORMS IS EXPECTED TO DEVELOP IN THE
AREA LATE THIS AFTERNOON INTO THIS EVENING AS A CONVECTIVELY-
ENHANCED DISTURBANCE ORIGINATING FROM THE CENTRAL PLAINS MOVES
INTO THE AREA. CONFIDENCE IN THE TIMING AND LOCATION OF THIS
THUNDERSTORM THREAT HAS INCREASED SINCE THE LAST TAF ISSUANCE,
AND HAVE INTRODUCED VCTS MENTIONS AND TEMPO GROUPS FOR TSRA AT
THE CHICAGO METRO TERMINALS FOR THE LATE AFTERNOON THROUGH EARLY
EVENING TIME FRAME. MAINTAINED A PROB30 GROUP FOR THIS TIME
FRAME AT RFD AS IT APPEARS THAT THE MAIN SURFACE BOUNDARY THAT
CONVECTION WILL INITIALLY FOCUS ALONG SHOULD REMAIN SOUTH OF
RFD, THUS RESULTING IN LOWER CONFIDENCE IN STORMS OCCURRING NEAR
RFD DURING THIS TIME WINDOW.
ADDITIONAL THUNDERSTORMS MAY DEVELOP LATER THIS EVENING INTO THE
OVERNIGHT HOURS AS ANOTHER UPPER-LEVEL DISTURBANCE ENTERS THE
REGION AND A LOW-LEVEL JET PROVIDES ASCENT OVER THE TOP OF
OUTFLOW BOUNDARIES LEFT BEHIND BY THE EARLIER STORMS. EXACTLY
WHERE THESE BOUNDARIES AND STORMS WILL FOCUS REMAINS UNCERTAIN,
BUT HAVE THIS POTENTIAL HIGHLIGHTED IN PROB30 GROUPS FOR ALL OF
OUR TAF SITES. WHEREVER THESE STORMS DO DEVELOP, IT IS POSSIBLE
THAT THEY WILL TRAIN OVER THE SAME LOCATIONS AND RESULT IN
PROLONGED THUNDERSTORM IMPACTS FOR SEVERAL HOURS. ANY STORMS,
EITHER WITH THIS ROUND OR THE EARLIER ROUND, WILL HAVE THE
POTENTIAL TO PRODUCE STRONG DOWNBURST WINDS, ERRATIC WIND
SHIFTS, HAIL, AND SHARPLY REDUCED VISIBILITIES.
ONE ADDITIONAL ROUND OF STORMS IS LIKELY TO OCCUR SOMETIME
TOMORROW MORNING THROUGH THE EARLY-MID AFTERNOON, SO RIGHT
TOWARDS THE END OF THE CURRENT 30-HOUR TAF PERIOD FOR ORD AND
MDW. GIVEN THE EXISTING UNCERTAINTIES AT THIS TIME OVER TIMING
AND EXACTLY WHERE THESE STORMS WILL TRACK, OPTED FOR PROB30
GROUPS FOR TSRA AT THE VERY END OF THE ORD AND MDW TAFS, BUT
SOME REFINEMENT TO THIS WILL LIKELY BE NEEDED AS FORECAST AND
OBSERVATIONAL TRENDS BECOME CLEARER. IN THE WORST CASE, A MATURE
LINE OR BOW MAY PLOW THROUGH THE LOCAL AIRSPACE TOMORROW AND
PRODUCE SEVERE TO POTENTIALLY SIGNIFICANT SEVERE WIND GUSTS,
BUT IT IS NOT YET CLEAR WHETHER THAT WILL HAPPEN.
LASTLY, CONFIDENCE IN WIND DIRECTION TRENDS FOR TODAY IS FAIRLY
LOW, PARTLY DUE TO POTENTIAL THUNDERSTORM INFLUENCES AND PARTLY
BECAUSE SYNOPTIC FLOW WILL BE FAIRLY LIGHT. A WEAK LAKE BREEZE
DOES LOOK LIKE IT WILL RESULT IN EASTERLY WINDS BEING OBSERVED
AT ORD AND MDW AND NORTHEASTERLY WINDS AT GYY THIS AFTERNOON.
OGOREK
LOT WATCHES/WARNINGS/ADVISORIES
IL...NONE.
IN...NONE.
LM...NONE.
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