Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 092330

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
630 PM CDT Sun May 9 2021

255 PM CDT

Through Monday night...

Quieter weather than the past 24 hours ahead, with the main
interest being chilly nights and the likelihood of frost and
potentially for a freeze mainly on Monday night. For tonight, a
Frost Advisory has been issued for along and south of I-80.

The low pressure that brought a decent swath of two inches of
rain across central to southern parts of the forecast area is now
over Ohio and heading quickly east. Clearing has spread over much
of the north of I-80 CWA with clouds to be shoved out of the far
southern and eastern forecast area by sunset. An upper level
trough over Ontario is continuing to dig south-southeastward with
what`s basically an associated secondary/backdoor cold front
moving southward through central Wisconsin. This feature`s cold
advection -- primarily at 850-925 mb -- has induced some high
based closed-celled cumulus across Wisconsin. Short term model
moisture field solutions are in generally good agreement with
slowly moving these clouds southward into the area, and look to
reach I-80 in the pre-dawn hours. Cannot completely rule out some
sprinkles or a light shower toward the Wisconsin state line later
tonight with these clouds either.

With the late day clearing south of I-80 and the resulting
limited insolation, temperatures will have a lower starting point
going into the night. Dew points are presently around 40 in much
of the southeast forecast area with T/Td spreads less than 10
degrees, making for some concern of fog. Would envision that being
just in the far south (along/south of U.S. Highway 24) if that
occurs, as drier air from the northeast continues to work in the
rest of this afternoon into evening. With the clearing and drying,
minimum temperatures look to drop to 33 to 36, and probably would
go colder if it wasn`t for the elevated dew points slowing the
fall a bit. So frost looks to be the concern and confidence is
high in this developing along/south of I-80, with patchy frost
in places north of I-80.

For Monday, the secondary cold front will be continuing to move
southward and as it enters the southern CWA, a short wave in the
northwest flow is forecast to override it. This will be during
peak heating with highs expected to reach the mid to upper 50s
(upper 40s near the lake). Dew points south of the front are
predicted to be in the lower to mid 40s by the RAP, while more
around 40 by the NAM. Profiles show meager instability, but enough
that there may be spotty afternoon showers from western to
central Illinois, and in our area that would include south of
I-80. While a non-zero lightning threat if these can develop
thanks to low -10C levels, any CAPE is awfully thin, and CAM
output of 40+ dBZ is marginal to none on various output.

Monday night should be clear for the northern CWA with high
confidence, while trending clear as the night progresses for the
south. At least a Frost Advisory for much of the CWA is likely,
and there will need to be consideration given to a Freeze Warning



1257 PM CDT

Tuesday through Sunday...

Sprawling surface high pressure will settle into the region during
the long term portion of the forecast which looks to greatly limit
our local precipitation chances, at least until towards the end of
the week and upcoming weekend.

We look to start things off Wednesday morning on another cool
note, with a generally clear sky and slackening winds affording
temperatures a chance to fall into the low to mid 30s (away from
Chicago). Dewpoints will remain high enough to support the
development of frost, and it`s looking probable that we`ll be
headed for some additional frost headlines for this period.

Lake breezes look to make a daily appearance with the weak
gradient flow in place, and this will help keep our immediate
lakeside locales a few degrees cooler than inland areas, at least
until the background flow turns more south and southwesterly by
the end of the week. Outside of a healthier cumulus field near
the lake on Tuesday, cloud cover should also be fairly minimal
during this time.

We`ll start to navigate our way back into the return flow on
Friday, and more son over the weekend as high pressure builds into
the Northeastern US. Depending on the degree of moisture return
here, there could be a thunder threat, but at this time the
extended instability profiles look a bit too meager to support a
mention in the extended grids.



For the 00Z TAFs...

Aviation Forecast Highlights:

* Northeast surface winds through the period.

* Period of isolated to scattered showers possible Monday morning.
  VFR conditions expected.

Surface low pressure associated with cloud cover and rains of last
night and this morning has moved well off to the east of the
region. Gusty north-northeast flow has helped to dry out the low
levels and erode cloud cover. Gradient continues to weaken this
evening, and winds will continue to diminish through sunset.

A pair of upper level disturbances will track southeast across
the area later tonight and Monday, which will bring VFR cloud
cover back to the region along with the potential for isolated to
scattered showers. Shower potential appears to be greatest for the
terminals during the morning hours, with the focus then shifting
south of the terminals Monday afternoon. Confidence in coverage is
somewhat low, though there is fairly decent model agreement in
showers across far north/northeast IL during this time.
Thunderstorm potential appears low, likely a little better off to
the south of the terminals during the afternoon. By afternoon,
northeast onshore winds should help to stabilize conditions and
decrease cloud cover across the Chicago metro terminals.

Wind direction may become somewhat variable this evening/tonight
as the gradient remains weak in the vicinity of a secondary
surface trough/cold front. Overall however, synoptic pressure
field should continue to support generally modest northeast flow
through the period.



IL...Frost Advisory...ILZ019-ILZ021-ILZ023-ILZ032-ILZ033-ILZ039-
     ILZ107-ILZ108...midnight Monday to 8 AM Monday.

IN...Frost Advisory...INZ001-INZ002-INZ010-INZ011-INZ019...midnight
     Monday to 8 AM Monday.

LM...Small Craft Advisory...nearshore waters
     until 4 AM Monday.



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