163
FXUS63 KLOT 180828
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
328 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.
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 /06Z TAFS THROUGH 12Z SATURDAY/
ISSUED AT 1258 AM CDT FRI APR 18 2025
FORECAST CONCERNS FOR THE TERMINALS...
- SCATTERED SHOWERS AND THUNDERSTORMS POSSIBLE LATE TONIGHT
MAINLY ALONG AND NORTH OF A RPJ TO ORD LINE.
- GUSTY SOUTHWEST WINDS FRIDAY WITH GUSTS IN THE 30-35 KT RANGE.
- ANOTHER ROUND OF SHOWERS AND THUNDERSTORMS FRIDAY EVENING
FOLLOWED BY MVFR CEILINGS FRIDAY NIGHT.
A SUBTLE SHORTWAVE DISTURBANCE IS TRAVERSING ACROSS IA THIS
EVENING WHICH HAS LEAD TO THE DEVELOPMENT OF SCATTERED SHOWERS
AND THUNDERSTORMS. WHILE THIS DISTURBANCE IS EXPECTED TO PIVOT
INTO NORTHERN IL AND SOUTHERN WI OVERNIGHT, CONFIDENCE ON THE
COVERAGE OF SHOWERS AND THUNDERSTORMS SURVIVING THROUGH NORTHERN
IL REMAINS LOW. THAT SAID, FORECAST SOUNDINGS DO SHOW A LARGE
AMOUNT OF ELEVATED INSTABILITY REMAINING OVER NORTHERN IL
TONIGHT INTO EARLY FRIDAY MORNING WHICH IN THE PRESENT OF THE
DISTURBANCE AND STRONG LOW-LEVEL JET MAY ALLOW FOR A FEW SHOWERS
AND/OR STORMS TO REACH THE TERMINALS PARTICULARLY THOSE ALONG
AND NORTH OF A RPJ TO ORD LINE. THEREFORE, HAVE DECIDED TO
MAINTAIN THE PROB30S AT THE CHICAGO AREA TERMINALS FOR NOW BUT
DID UPGRADE RFD TO A TEMPO SINCE CONFIDENCE IS A BIT HIGHER. ANY
STORMS THAT DO SURVIVE WILL HAVE THE POTENTIAL OF PRODUCING
REDUCED VISIBILITIES ALONG WITH LOCALLY GUSTY WINDS IN EXCESS OF
30 KTS.
THE SHORTWAVE AND ASSOCIATED PRECIPITATION WILL MOVE EAST OF THE
TERMINALS FRIDAY MORNING RESULTING IN MOSTLY DRY CONDITIONS FOR
FRIDAY. HOWEVER, A DEVELOPING SURFACE LOW WILL BEGIN TO MOVE
TOWARDS THE AREA WHICH WILL CAUSE WINDS TO INCREASE AND BECOME
GUSTY BY LATER MORNING/EARLY AFTERNOON. CONFIDENCE IS HIGH THAT
GUSTS OF 30-35 KTS WILL OCCUR THROUGH MUCH OF THE DAY ON FRIDAY,
BUT THERE IS A CHANCE THAT LOCALLY HIGHER GUSTS MAY OCCUR
ESPECIALLY IF DEEPER MIXING IS ACHIEVED. OTHERWISE, EXPECT SCT
TO BKN VFR CLOUDS THROUGH FRIDAY AFTERNOON.
THE AFOREMENTIONED SURFACE LOW WILL MOVE INTO SOUTHERN WI FRIDAY
EVENING WHICH WILL FORCE A COLD FRONT THROUGH NORTHERN IL AND
NORTHWEST IN. AS THIS OCCURS, ANOTHER PERIOD OF SHOWERS AND
THUNDERSTORMS IS EXPECTED AT THE TERMINALS A FEW OF WHICH COULD
BE ON THE STRONG SIDE WITH LOCALLY GUSTY WINDS AND HAIL.
HOWEVER, A FEW SHOWERS MAY DEVELOP AS EARLY AS 22Z NEAR RFD AS
THE FRONT BEGINS TO ENTER THE AREA. GIVEN THE QUICK MOVEMENT OF
THE FRONT, SHOWERS AND STORMS SHOULD BE SHORT LIVED AND MOVE
THROUGH BY 05Z FRIDAY NIGHT. BEHIND THE FRONT WINDS WILL BECOME
NORTHWESTERLY AND GRADUALLY LOSE THEIR GUSTINESS BUT OCCASIONAL
18 TO 20 KT GUSTS MAY LINGER THROUGH FRIDAY NIGHT. ADDITIONALLY,
CEILINGS WILL ALSO LOWER TO MVFR AND REMAIN AS SUCH THROUGH THE
REMAINDER OF THE FORECAST PERIOD.
YACK
LOT WATCHES/WARNINGS/ADVISORIES
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY UNTIL 10 PM CDT THIS EVENING FOR THE IL
AND IN NEARSHORE WATERS.
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