010
FXUS63 KLOT 181633
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
1133 AM CDT FRI APR 18 2025
KEY MESSAGES
- WINDY AND UNSEASONABLY WARM TODAY.
- SLIGHT CHANCE OF SHOWERS/STORMS LATE THIS AFTERNOON, BUT
BETTER CHANCES OF SCATTERED SHOWERS/STORMS THIS EVENING, SOME
POSSIBLY SEVERE.
- MUCH COOLER SATURDAY WITH SHOWER CHANCES LINGERING NEAR/SOUTH
OF HWY 24, ESPECIALLY EARLY.
- PERIODS OF SHOWERS AND POSSIBLY EMBEDDED THUNDERSTORMS ON
SUNDAY BECOMING MORE WIDESPREAD SUNDAY NIGHT. THERE REMAINS A
CONDITIONAL THREAT FOR STRONG TO SEVERE STORMS SUNDAY
AFTERNOON AND EVENING MAINLY FOR THE SOUTHERN PORTIONS OF OUR
AREA.
UPDATE
ISSUED AT 1133 AM CDT FRI APR 18 2025
MAIN FORECAST CHANGE:
- COOLER TEMPS ACROSS FAR NORTHERN ILLINOIS, ESPECIALLY THE LAKE
COUNTY IL SHORE
A COMBINATION OF A REMNANT OUTFLOW BOUNDARY DRIFTING SOUTHWARD
AND CLOUD COVER SLOWED THE WARMING TREND WITH NORTHWARD EXTENT
IN THE CWA. THE OUTFLOW BOUNDARY HAS SINCE WASHED OUT OVER FAR
NORTHERN COOK COUNTY, BUT EASTERLY WINDS ARE EXPECTED TO PERSIST
A WHILE LONGER FOR THE ILLINOIS NORTH SHORE AREA, ESPECIALLY FOR
LAKE COUNTY, WHICH MAY KEEP AN EASTERLY/ONSHORE COMPONENT UNTIL
THE 5-6PM TIMEFRAME. THESE LOCATIONS WILL STAY IN THE 50S PRIOR
TO THE SHIFT BACK TO SOUTHWESTERLY WINDS, IF IT HAPPENS. FOR
INLAND AREAS OF FAR NORTHERN ILLINOIS, PARTICULARLY FOR THE
IL/WI STATE LINE COUNTIES, THE SLOWED WARMING TREND THIS MORNING
AND MOSTLY CLOUDY SKIES SHOULD KEEP HIGHS IN THE 70S. FARTHER
SOUTH, HIGHS FROM AROUND 80F INTO THE LOWER 80S STILL APPEAR ON
TRACK DESPITE ONLY PARTIAL SUNSHINE.
TURNING BRIEFLY TO THE CONVECTIVE THREAT LATER TODAY INTO THIS
EVENING, THERE ARE NO BIG CHANGES IN THINKING FROM THE OVERNIGHT
FORECAST. THE KEY PIECE OF OBSERVATIONAL DATA IS THE EXCEPTIONALLY
STRONG EML AND ASSOCIATED CAPPING ON THE ILX AND DVN 12Z RAOB
SOUNDINGS. RECENT AIRCRAFT SOUNDINGS FROM MDW BACK UP THE
MORNING RAOBS. IT THUS REMAINS UNCLEAR IF AND WHEN CAPPING CAN
BE ERODED AHEAD OF THE APPROACHING COLD FRONT FOR THE CONDITIONAL
SEVERE THREAT, CONSIDERING THE MAGNITUDE OF THE CAPPING AND
RELATIVELY MODEST FORCING. WE'LL POST ANOTHER UPDATE BY THE
EARLY AFTERNOON WITH SOME OF OUR LATEST THINKING ON THE
THUNDERSTORM FORECAST LATER TODAY THROUGH THIS EVENING.
CASTRO
DISCUSSION
ISSUED AT 328 AM CDT FRI APR 18 2025
THROUGH SATURDAY:
THE PAIR OF SUPERCELLS THAT TRACKED ACROSS MUCH OF SOUTHERN IA
THURSDAY EVENING OUTRAN THE SURFACE BASED INSTABILITY AND
VAPORIZED BY THE TIME THEY REACHED THE MISSISSIPPI RIVER. WHILE
THOSE BOUNDARY LAYER BASED STORMS DISSIPATED, WATER VAPOR
IMAGERY THIS MORNING SHOWS A FAIRLY WELL DEFINED SHORTWAVE
MOVING EASTWARD INTO THE WESTERN GREAT LAKES, SO UNTIL THE WARM
FRONT AND THIS SHORT WAVE CLEAR THE AREA TOWARD SUNRISE, THERE
REMAINS CHANCE FOR SOME ISOLATED ELEVATED CONVECTION DEVELOPING
THROUGH AROUND SUNRISE NORTH OF I-80. UPSTREAM, A POORLY
ORGANIZED MCS HAS DEVELOPED OVER NORTHWESTERN IA WITH THIS
ACTIVITY EXPECTED TO REMAIN NORTH OF THE WISCONSIN BORDER THIS
MORNING.
OTHERWISE, THE BIG STORY MUCH OF TODAY WILL BE THE WINDY AND
UNSEASONABLY WARM CONDITIONS. A LARGE AND STRONG ELEVATED MIXED
LAYER (EML) SAMPLED BY THE TOP/DDC EVENING SOUNDINGS WILL
OVERSPREAD THE AREA THIS AFTERNOON, WITH A VERY STRONG CAPPING
INVERSION AT THE BASE OF THE EML. GUIDANCE IS PRETTY AGGRESSIVE
WITH CLOUD COVER TODAY DEVELOPING AN EXTENSIVE STRATOCUMULUS
DECK BENEATH THE BASE OF THE EML. GIVEN SATELLITE TRENDS,
GUIDANCE SEEMS OVERLY AGGRESSIVE DEVELOPING THAT CLOUD COVER, SO
THINKING THERE SHOULD BE A GOOD DEAL OF SUNSHINE THIS MORNING
AND LIKELY INTO THE EARLY AFTERNOON. GIVEN THE STRENGTH OF THE
CAP, SUNSHINE AND SURFACE HEATING WILL DO LITTLE TO ERODE THE
CAP WITH CHANCE OF SURFACE BASED CONVECTION NEAR ZERO THROUGH
MID AFTERNOON.
WATER VAPOR IMAGERY EARLY THIS MORNING SHOWS A BLOSSOMING AREA
OF MID-HIGH LEVEL CLOUDINESS OVER THE SOUTHERN HIGH PLAINS,
LIKELY A RESPONSE TO INCREASING ASCENT IN ADVANCE OF JET STREAK
EMBEDDED WITHIN THE SUBTROPICAL JET. SREF GUIDANCE, WHICH TENDS
TO SMOOTH OUT THE NOISIER HIGH RES MODELS, SHOWS A VORT MAX
EMERGING OVER THE SOUTHERN PLAINS THIS MORNING ASSOCIATED WITH
THAT SUBTROPICAL JET, BRINGING THAT SHORTWAVE AND ASSOCIATED JET
STREAK NORTHEASTWARD INTO THE MID MISSISSIPPI VALLEY THIS
EVENING.
LARGE SCALE ASCENT ASSOCIATED WITH THE APPROACHING SHORTWAVE AND
JET STREAK COMBINED WITH MODEST FRONTAL FORCING SHOULD
EVENTUALLY WORK TO WEAKEN THE VERY STRONG CAP THIS EVENING.
QUESTION BECOMES, WILL THE FORCING BE ENOUGH WEAKEN THE CAP
SUFFICIENTLY FOR SFC BASED CONVECTION OR WILL CAP PREVAIL. MOST
GUIDANCE SUGGESTS THE CAP WILL BREAK, BUT THERE ARE SOME CAMS
THAT DON'T BREAK THE CAP AND KEEP INTENSE CONVECTION SOUTH OF
OUR CWA. PREPONDERANCE OF MODEL GUIDANCE WOULD SUGGEST
SUFFICIENT WEAKENING OF THE CAP FOR AT LEAST WIDELY SCATTERED
THUNDERSTORMS TO DEVELOP ALONG/AHEAD OF THE FRONT THIS EVENING.
HAVE MAINTAINED LIKELY POPS THIS EVENING FOR THIS SCENARIO,
HOWEVER THERE REMAINS A CHANCE THAT THE CAP COULD WIN OUT AND
THINGS REMAIN MOSTLY DRY. NOT UNCOMMON IN THESE SITUATIONS WITH
EXTREMELY STEEP MID LEVEL LAPSE RATES (NEARLY DRY ADIABATIC) FOR
A SUBTLE WAVE TO KICK OFF HIGH BASED ELEVATED WEAK CONVECTION,
SO DO HAVE SOME SLIGHT CHANCE POPS FOR THAT LATE THIS AFTERNOON.
ANY DEEP CONVECTION THAT CAN DEVELOP THIS EVENING WILL BE DOING
SO IN AN OTHERWISE QUITE FAVORABLE ENVIRONMENT FOR SEVERE
WEATHER. STRONG DEEP LAYER SHEAR AND MLCAPE VALUES 1000-2000
J/KG WOULD BE MORE THAN SUPPORTIVE SEVERE THUNDERSTORMS,
ASSUMING THE CAP IS BREACHED. IF CONVECTION WERE TO REMAIN MORE
ISOLATED, THEN SUPERCELLS, CAPABLE OF LARGE HAIL (1-2" IN
DIAMETER), WOULD BE POSSIBLE. IF CONVECTION WERE TO BECOME MORE
LINEAR, THEN DAMAGING WIND THREAT WOULD INCREASE, BUT GIVEN THE
SOMEWHAT MODEST FORCING AND STRONG CAP, CONFIDENCE IN A SOLID
LINE DEVELOPING IS ON THE LOWER END OF THE SPECTRUM. GIVEN THE
UNCERTAINTIES IN REGARDS TO FORCING VS CAP STRENGTH, THE LEVEL 2
OF 5 RISK IN LATEST SWODY1 IS REASONABLE. IF EVIDENCE GROWS
THAT CAP WILL BE BREACHED, THEN HIGHER SEVERE PROBS COULD BE
NEEDED FOR PARTS OF OUR CWA FOR THIS EVENING, ESPECIALLY EASTERN
CWA.
CONFIDENCE IS HIGHER IN MORE WIDESPREAD CONVECTION DEVELOPING
FARTHER SOUTHWEST OVER THE OZARKS THIS EVENING. THE NORTHERN
FLANKS OF THAT MCS WILL LIKELY BRING SHOWERS AND PERHAPS SOME
OCCASIONAL LIGHTNING TO OUR SOUTHERN CWA OVERNIGHT AND MAYBE
LINGER INTO SATURDAY MORNING. WHILE CANNOT RULE OUT SOME SHOWERS
EVEN INTO THE AFTERNOON HOURS NEAR/SOUTH OF HWY 24, THE MORE
LIKELY SCENARIO IS NOT MUCH OF A PRECIP THREAT DURING THE DAY
SATURDAY. TEMPERATURES SATURDAY WILL BE MUCH COOLER,
PARTICULARLY NEAR THE LAKE WHERE AFTERNOON TEMPS WILL PROBABLY
BE IN THE 40S!
- IZZI
SATURDAY NIGHT THROUGH THURSDAY:
THE UPPER TROUGH CURRENTLY OVER THE MOUNTAIN WEST IS EXPECTED
TO DIG INTO THE SOUTHWESTERN CONUS OVER THE WEEKEND AND BEGIN TO
EJECT INTO THE SOUTHERN PLAINS ON SUNDAY. AT THE SAME TIME, A
STATIONARY FRONT WILL BE STALLED ACROSS THE OHIO RIVER VALLEY
AND SOUTHERN PLAINS. WHILE THE FRONTAL POSITION SHOULD KEEP THE
MAJORITY OF ANY PRECIPITATION TO OUR SOUTH SATURDAY NIGHT, THE
FRONT IS FORECAST TO LIFT BACK NORTH AS A WARM FRONT ON SUNDAY
AS A DEVELOPING SURFACE LOW IN THE SOUTHERN PLAINS LIFTS INTO
THE MISSISSIPPI VALLEY. THUS, PERIODS OF RAIN SHOWERS ARE
EXPECTED FOR SUNDAY WITH A CHANCE THAT SOME EMBEDDED
THUNDERSTORMS MAY DEVELOP SUNDAY AFTERNOON DEPENDING ON THE
DEGREE OF DESTABILIZATION. THAT SAID, A 50-60 KT 500MB JET
STREAK IS FORECAST TO BE OVERHEAD SUNDAY AFTERNOON AND SHOULD
GENERATE 40-50 KTS OF DEEP LAYER WIND SHEAR FOR STORMS TO WORK
WITH. IF SUFFICIENT DESTABILIZATION IS ABLE TO OCCUR A
CONDITIONAL THREAT FOR SEVERE WEATHER MAY MATERIALIZE SUNDAY
AFTERNOON AND EVENING. AT THIS TIME, THE BETTER THREAT FOR
SEVERE WEATHER LOOKS TO REMAIN JUST SOUTH OF NORTHERN IL AND
NORTHWEST IN, WITH THE LATEST DAY 3 SPC OUTLOOK KEEPING THE
SEVERE WEATHER PROBS JUST BARELY SOUTH OF OUR CWA. THOUGH, THIS
WILL BE A PERIOD TO KEEP AN EYE ON AS WE GET CLOSER, ESPECIALLY
FOR THOSE IN OUR SOUTHERN CWA.
THE AFOREMENTIONED SURFACE LOW AND ASSOCIATED UPPER TROUGH ARE
FORECAST TO PIVOT ACROSS NORTHERN IL SUNDAY NIGHT WHICH WILL
BRING MORE WIDESPREAD RAIN TO THE AREA. WHILE THERE IS A NOTABLE
SIGNAL IN GUIDANCE THAT RAINFALL TOTALS TO PEAK IN THE 0.5 TO
1.0 INCH RANGE ESPECIALLY IN THE DEFORMATION AXIS, THE
PROGRESSIVE NATURE OF THE SYSTEM SHOULD LIMIT THE THREAT FOR ANY
FLOODING. THE RAIN WILL GRADUALLY TAPER MONDAY MORNING AS THE
SURFACE LOW MOVES INTO NORTHERN LOWER MI. HOWEVER, WINDS WILL
BECOME BREEZY BEHIND THE LOW AS THE PRESSURE GRADIENT TIGHTENS
AND DEEPER MIXING COMMENCES IN THE COLD ADVECTION REGIME. THUS
EXPECT WEST-NORTHWEST GUSTS IN THE 25-35 MPH RANGE ON MONDAY.
FOR TUESDAY ONWARD, THE UPPER-LEVEL PATTERN IS FORECAST TO
BECOME MORE ZONAL (WEST-EAST ORIENTED) WHICH SHOULD FAVOR A
PERIOD OF MORE TRANQUIL WEATHER. THOUGH, BOTH DETERMINISTIC AND
ENSEMBLE GUIDANCE IS INDICATING THAT SUBTLE SHORTWAVES MAY PIVOT
THROUGH THE PATTERN AND RESULT IN ADDITIONAL PERIODS OF SHOWERS
AND PERHAPS THUNDERSTORMS. GIVEN THE UNCERTAINTY IN TIMING AND
COVERAGE OF SUCH ACTIVITY, SEE NO REASON TO ALTER THE 20-30%
POPS OFFERED BY THE NBM THROUGH THE END OF NEXT WEEK, BUT
SUSPECT THAT MANY DRY HOURS WILL OCCUR AS WELL. ON A MORE
POSITIVE NOTE, CONTINUED MID-LEVEL WARM ADVECTION (850MB TEMPS
FORECAST IN THE 10-12C RANGE) IS EXPECTED TO KEEP TEMPERATURES
IN THE ABOVE NORMAL CATEGORY FOR MID-APRIL WITH HIGHS IN THE
UPPER 60S TO 70S AND OVERNIGHT LOWS IN THE 40S TO AROUND 50. THE
EXCEPTION HOWEVER, WILL BE FOR THOSE NEAR THE LAKE WHERE THE
POTENTIAL FOR DAILY LAKE BREEZES MAY KEEP READINGS NOTABLY
COOLER.
YACK
AVIATION /12Z TAFS THROUGH 18Z SATURDAY/
ISSUED AT 621 AM CDT FRI APR 18 2025
FORECAST CONCERNS FOR THE TERMINALS...
- GUSTY SOUTHWEST WINDS TODAY WITH PEAK GUSTS IN THE 30-35 KT
RANGE.
- PERIOD OF SHOWERS AND THUNDERSTORMS LATE THIS AFTERNOON AND
EVENING. A FEW OF WHICH COULD BE STRONG TO SEVERE.
- MVFR CEILINGS TONIGHT INTO SATURDAY MORNING.
THE THUNDERSTORMS THAT HAD DEVELOPED IN IA OVERNIGHT FIZZLED
BEFORE REACHING NORTHERN IL WHICH HAS LEFT THE TERMINALS DRY
WITH VFR SKIES. HOWEVER, THE SHORTWAVE DISTURBANCE RESPONSIBLE
FOR THOSE STORMS CONTINUES TO PIVOT OVERHEAD THIS MORNING WHICH
HAS STARTED TO DEVELOP SOME LIGHT SHOWERS IN CENTRAL IL. WHILE
THESE SHOWERS ARE EXPECTED TO REMAIN SOUTH OF THE TAF SITES, WE
WILL HAVE TO CLOSELY MONITOR TRENDS IN CASE ADDITIONAL SHOWERS
DEVELOP WITH THIS WAVE THROUGH MID-MORNING. OTHERWISE, MOSTLY
DRY CONDITIONS AND VFR CONDITIONS ARE EXPECTED FOR THE DAYLIGHT
HOURS TODAY DUE TO A STRONG CAPPING INVERSION ALOFT. DESPITE
THE INVERSION, DEEP MIXING IS STILL EXPECTED TO PRODUCE GUSTY
SOUTHWEST WINDS THIS AFTERNOON THROUGH EARLY EVENING. WHILE
GUSTS SHOULD GENERALLY PEAK IN THE 30-35 KT RANGE, OCCASIONAL
GUSTS IN EXCESS OF 35 KTS WILL BE POSSIBLE ESPECIALLY DURING THE
EARLY AFTERNOON HOURS.
A COLD FRONT WILL SWING THROUGH NORTHERN IL AND NORTHWEST IN
THIS EVENING WHICH SHOULD HAVE ENOUGH FORCE TO ERODE THE CAP AND
GENERATE SCATTERED SHOWERS AND THUNDERSTORMS. GIVEN THE
PROGRESSIVE NATURE, SHOWERS/STORMS SHOULD ONLY LAST A FEW HOURS
AT ANY ONE LOCATION HENCE THE TEMPO GROUPS HAVE BEEN MAINTAINED.
ADDITIONALLY, THE STOUT INSTABILITY EXPECTED TODAY MAY ALLOW A
FEW OF THESE STORMS TO BECOME STRONG TO SEVERE WITH THE MAIN
THREAT OF DAMAGING HAIL (UP TO 2 INCHES IN DIAMETER) BUT
LOCALIZED GUSTY WINDS ARE ALSO POSSIBLE. SHOWERS/STORMS WILL
GENERALLY COME TO AN END BY 05Z THIS EVENING BUT ISOLATED
SHOWERS MAY LINGER OVER THE CHICAGO AREA TERMINALS THROUGH 08Z.
REGARDLESS, WINDS WILL BECOME WEST-NORTHWEST BEHIND THE FRONT
THIS EVENING AS SPEEDS GRADUALLY SUBSIDE OVERNIGHT. CEILINGS
WILL ALSO LOWER TO MVFR BEHIND THE FRONT THIS EVENING AND
PERSIST AS SUCH THROUGH MID-MORNING ON SATURDAY BEFORE RETURNING
TO VFR.
YACK
LOT WATCHES/WARNINGS/ADVISORIES
IL...NONE.
IN...NONE.
LM...GALE WARNING UNTIL 9 PM CDT THIS EVENING FOR THE IL AND IN
NEARSHORE WATERS.
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