387
FXUS63 KLOT 211732
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
1232 PM CDT MON APR 21 2025
KEY MESSAGES
- WESTERLY WINDS COULD GUST TO 35-40 MPH TODAY, ESPECIALLY THIS
MORNING.
- SHOWER/T-STORM CHANCES RETURN TUESDAY AND ESPECIALLY TUESDAY
NIGHT, MAINLY NORTH OF I-80.
- WELL ABOVE NORMAL TEMPERATURES EXPECTED INLAND OF THE LAKE
TUESDAY THROUGH WEEKS END WITH MAYBE ANOTHER ROUND OF SHOWERS
LATE WEEK.
UPDATE
ISSUED AT 901 AM CDT MON APR 21 2025
WEST TO SOUTHWEST WINDS HAVE BEEN EFFICIENTLY MIXING BEHIND A
COLD FRONT AND ASSOCIATED LOW PRESSURE SYSTEM LIFTING INTO
SOUTHERN ONTARIO THIS MORNING. GUSTS IN THE PAST FEW HOURS HAVE
PEAKED ANYWHERE FROM FROM 40 TO 50 MPH, INCLUDING A 59 MPH GUST
MEASURED AT THE CHICAGO HARRISON-DEVER CRIB. AS THE LOW
CONTINUES PULLING AWAY FROM OUR AREA THIS MORNING, PRESSURE
RISES WILL EASE, ALLOWING FOR WIND GUST MAGNITUDES TO GRADUALLY
TAPER INTO THE AFTERNOON.
THE HIGH TEMPERATURE FORECAST THIS AFTERNOON IS CONTINGENT ON
HOW FAST THE ONGOING LOW STRATUS CLEARS. OBSERVED 12Z RAOBS AS
WELL AS AMDAR SOUNDINGS SUGGEST THE LOW-LEVEL SATURATED LAYER
ISN'T EXCEPTIONALLY DEEP (PERHAPS 1500FT DEEP). GIVEN THE STRONG
SUN ANGLE AND COLD-AIR ADVECTION REGIME (WHICH ENCOURAGES
SINKING AIR TO FACILITATE MIXING), THE HOLES IN THE STRATUS
UPSTREAM OF OUR AREA IN IOWA SHOULD TEND TO GROW WITH TIME.
HOWEVER, STEEP LOW-LEVEL LAPSE RATES AMIDST A RELATIVELY MOIST
BOUNDARY LAYER MAY ENCOURAGE STRATOCUMULUS CLOUDS TO DEVELOP
WHERE THERE IS ANY CLEARING. ALL THINGS CONSIDERED, WILL
CONTINUE TO FAVOR A MOSTLY CLOUDY DAY AND MUTED HIGHS RANGING
FROM THE MID 50S TO AROUND 60 (WARMEST NEAR US-24). IF CLOUDS DO
END UP CLEARING, HIGHS IN THE LOWER 60S WILL BE IN PLAY.
UPDATED PRODUCTS WILL BE SENT SHORTLY.
BORCHARDT
DISCUSSION
ISSUED AT 303 AM CDT MON APR 21 2025
THROUGH TUESDAY:
A 1001 MB SFC LOW WAS LOCATED OVER WESTERN WI AT 08Z WITH COLD
FRONT TRAILING SOUTH ALONG THE MISSISSIPPI RIVER BETWEEN IL AND
IA, THEN SOUTHWEST ACROSS MISSOURI. THIS LOW AND THE ASSOCIATED
VIGOROUS UPPER TROUGH WILL MOVE NORTHEASTWARD INTO ONTARIO BY
LATER THIS AFTERNOON WITH TRAILING COLD FRONT MOVING EAST OF OUR
CWA BY MID MORNING.
COOLER AND LESS HUMID AIR WILL FILTER INTO THE AREA TODAY IN THE
WAKE OF THE FRONT. POST FRONTAL STRATUS SHOULD KEEP SKIES PRETTY
CLOUDY THROUGH MIDDAY FOLLOWED BY GRADUAL CLEARING FROM THE
SOUTHWEST THIS AFTERNOON. GUSTY WESTERLY WINDS ARE EXPECTED
TODAY, ESPECIALLY THIS MORNING WHEN A FEW GUSTS OF 35 TO 40 MPH
ARE POSSIBLE. WINDS WILL EVER SO SLOWLY BEGIN TO EASE THIS
AFTERNOON, FOLLOWED BY A MUCH MORE RAPID DECREASE IN WINDS
TOWARD EVENING.
NARROW SURFACE RIDGE AXIS WILL MOVE ACROSS THE REGION TONIGHT
WITH SOUTHERLY FLOW BECOMING REESTABLISHED ON THE BACKSIDE OF
THE RIDGE TUESDAY. THE SOUTHERLY FLOW SHOULD ALLOW TEMPS TO
REBOUND NICELY WITH HIGHS MOSTLY IN THE 70S, THOUGH A SOUTH
SOUTHEAST SYNOPTIC GRADIENT SHOULD ALLOW A LAKE BREEZE TO
PENETRATE INTO NORTHEAST IL TUESDAY AFTERNOON RESULTING IN
COOLER CONDITIONS THERE.
A TRANSITION TO A ZONAL MID-UPPER LEVEL FLOW IS EXPECTED
TUESDAY WITH VERY LOW AMPLITUDE PERTURBATIONS IN THE FLOW AND
LOW LEVEL THETA-E ADVECTION LEADING TO SOME POTENTIAL FOR
SHOWERS AND THUNDERSTORMS. UNFORTUNATELY, IT ISN'T UNCOMMON FOR
GUIDANCE TO STRUGGLE WITH LOW AMPLITUDE SHORTWAVES LIKE THIS,
WHICH IS RESULTING A WIDE RANGE OF SOLUTIONS WITH REGARDS TO
PLACEMENT AND TIMING OF POTENTIAL CONVECTION TUESDAY/TUESDAY
NIGHT. CANNOT RULE OUT SOME DISSIPATING ELEVATED CONVECTION
MAKING IT INTO OUR NORTHERN CWA TUESDAY MORNING, SO HAVE SOME
SLIGHT CHANCE POPS FOR THAT. DEVELOPING SURFACE FRONT COULD ALSO
BECOME A POSSIBLE FOCUS FOR AFTERNOON CONVECTION OVER SOUTHERN
WI OR FAR NORTHERN IL, BUT CONFIDENCE REMAINS LOW, SO STUCK WITH
JUST LOW-MID RANGE CHANCE POPS IN OUR NORTHERN TIER OR TWO OF
COUNTIES.
- IZZI
TUESDAY NIGHT THROUGH SUNDAY:
THE PREVIOUSLY ADVERTISED ZONAL (WEST-EAST ORIENTED) UPPER
PATTERN IS EXPECTED TO BE IN PLACE BY TUESDAY NIGHT AND WILL
PERSIST THROUGH THE END OF THE WEEK. THEREFORE, MOSTLY DRY
CONDITIONS AND ABOVE NORMAL TEMPERATURES CAN BE EXPECTED EACH
DAY WITH HIGHS IN THE 70S (POSSIBLY NEAR 80 IN SPOTS WEDNESDAY
AND THURSDAY) INLAND OF LAKE MICHIGAN. HOWEVER, THERE WILL BE A
COUPLE PERIODS WHERE CHANCES FOR SHOWERS AND THUNDERSTORMS CAN
BE EXPECTED.
THE FIRST PERIOD OF POSSIBLE RAIN IS EXPECT TO OCCUR TUESDAY
NIGHT INTO WEDNESDAY MORNING AS A SHORTWAVE TROUGH PIVOTS ACROSS
THE NORTHERN PLAINS. AT THE SURFACE A STALLED FRONTAL BOUNDARY
IS FORECAST TO BE IN PLACE FROM NORTHEAST KS TO SOUTHERN WI
WHICH WILL BE THE FOCUS FOR SHOWERS AND THUNDERSTORMS DURING
THIS TIME. SINCE THE RAIN WILL BE TIED TO THE BOUNDARY, IT
APPEARS THAT ONLY THE NORTHERN THIRD TO HALF OF OUR AREA WILL
ACTUALLY SEE PRECIPITATION WHILE AREAS SOUTH OF GENERALLY I-80
REMAIN DRY. THAT SAID, THE NEARLY STATIONARY RAIN WOULD NORMALLY
POSE A CONCERN FOR FLOODING, BUT THE GENERALLY DRY SOILS SHOULD
BE ABLE TO SOAK UP THE 0.25-0.5 INCH AMOUNTS AND LIMIT THE
FLOODING THREAT. WHILE SHOWERS SHOULD TAPER BY MID-MORNING ON
WEDNESDAY, SOME GUIDANCE TRIES TO REDEVELOP SHOWERS AND STORMS
WEDNESDAY AFTERNOON ALONG THE LINGERING BOUNDARY. THOUGH, THE
DRY LOWER ATMOSPHERE AND LACK OF BROAD FORCING SHOULD LIMIT THE
COVERAGE OF SHOWERS/STORMS AND KEEP THE BULK OF THIS ACTIVITY
MORE INTO NORTHWEST IL AND NEAR THE IL-WI LINE.
THE SECOND AND ARGUABLY BETTER CHANCE FOR RAIN IS FORECAST TO
OCCUR DURING THE THURSDAY NIGHT THROUGH FRIDAY NIGHT TIMEFRAME
AS A BROADER TROUGH SWINGS THROUGH THE REGION. WHILE GUIDANCE
REMAINS IN GENERALLY GOOD AGREEMENT ON RAIN OCCURRING, THERE
REMAINS A FAIR AMOUNT OF UNCERTAINTY IN HOW THE SYSTEM WILL
EVOLVE, PARTICULARLY IN REGARDS TO A SOUTHERN STREAM SHORTWAVE
AND ITS ASSOCIATED SURFACE LOW. IN SHORT, IF THE SOUTHERN WAVE
AND LOW ARE STRONGER, THEN RAIN COVERAGE WILL LIKELY BE MORE
WIDESPREAD WITH HIGHER AMOUNTS AS OPPOSED TO THE WEAKER
SOLUTIONS. FOR NOW SAW NO REASON TO DEVIATE FROM THE 50-70% POPS
OFFERED BY THE NBM BUT SUSPECT FURTHER REFINEMENT WILL BE
NEEDED AS WE GET CLOSER. REGARDLESS OF THE EVOLUTION, RAIN
SHOULD TAPER ON SATURDAY LEAVING US WITH A MOSTLY DRY WEEKEND.
THOUGH, TEMPERATURES BEHIND THIS SYSTEM ARE FORECAST TO BE
NOTABLY COOLER WITH HIGHS BACK INTO THE 50S AND 60S FOR THE
WEEKEND.
YACK
AVIATION /18Z TAFS THROUGH 00Z WEDNESDAY/
ISSUED AT 1232 PM CDT MON APR 21 2025
THE KEY AVIATION MESSAGES ARE:
- BLUSTERY WESTERLY WINDS WILL CONTINUE THIS AFTERNOON BEFORE
EASING SHARPLY THIS EVENING. OCCASIONAL GUSTS TO AROUND 35
KNOTS.
- SMALL POTENTIAL FOR MIFG/BR TONIGHT, MAINLY INVOF GYY, DPA,
AND RFD. TOO LOW FOR A MENTION IN THE TAFS.
- SEVERAL THREATS FOR SHOWERS/STORMS ON TUESDAY, 1ST IN THE MID-
LATE MORNING AND THEN IN THE AFTERNOON.
WESTERLY WINDS WILL CONTINUE TO GUST ROUTINELY AROUND 30 KNOTS,
WITH OCCASIONAL GUSTS JUST ABOVE 35 KTS THROUGH THE REST OF THE
AFTERNOON. WINDS WILL THEN EASE SHARPLY THIS EVENING (AFTER
00-01Z). THEREAFTER, WINDS WILL BECOME LIGHT AND VARIABLE,
BEFORE EVENTUALLY ACQUIRING A SOUTHEASTERLY DIRECTION LATE
TONIGHT/EARLY TUESDAY MORNING. EVENTUALLY, WINDS WILL BECOME
SOUTHWESTERLY TOWARDS MIDDAY/EARLY AFTERNOON, AND MAY ALSO
BECOME GUSTY.
LATE TONIGHT/EARLY TUESDAY MORNING, SOME SHALLOW FOG/MIST MAY
DEVELOP AT OUTLYING SITES (DPA, GYY, RFD). THIS POTENTIAL
REMAINS TOO LOW FOR AN INCLUSION IN THE TAFS, HOWEVER.
THERE WILL BE A FEW PERIODS ON TUESDAY WHICH COULD FEATURE A
THREAT FOR SHOWERS AND THUNDERSTORMS. THE FIRST IS MID-LATE
MORNING INVOF RFD WHERE A PROB30 FOR TSRA HAS BEEN INTRODUCED.
WHILE THIS ACTIVITY MAY GET CLOSE TO THE CHICAGO-AREA TERMINALS,
CHANCES WITH THIS FIRST ROUND APPEAR TO BE UNDER 30 PERCENT AT
THIS TIME. THE SECOND WINDOW IS LATER IN THE AFTERNOON, FOR
WHICH A SEPARATE PROB30 HAS BEEN INTRODUCED AT ORD/MDW DURING
THE EXTENDED TAF WINDOW.
CARLAW
LOT WATCHES/WARNINGS/ADVISORIES
IL...NONE.
IN...NONE.
LM...GALE WARNING UNTIL 7 PM CDT THIS EVENING FOR THE IL AND IN
NEARSHORE WATERS.
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