Flagg, Illinois Weather Forecast Discussion

001
FXUS63 KLOT 140520
AFDLOT
Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1220 AM CDT Sun Sep 14 2025
.KEY MESSAGES...
- Areas of fog may develop early this morning outside of urban
  areas.
- Generally warm and dry conditions are expected through midweek.
- Temperatures turn more seasonable toward the end of the week
  with periodic shower and storm chances returning.
&&
.DISCUSSION...
Issued at 212 PM CDT Sat Sep 13 2025
Through Sunday:
Thunderstorms continue to regenerate this afternoon in
northwestern Indiana along the nose of a 40-50kt 700mb jet and
along the eastern edge of an EML plume extending eastward from
the Upper Mississippi River Valley. This region is along the
downwind side of an upper-level ridge centered over the Upper
Mississippi River Valley. The expectation is for the
regenerating thunderstorms to continue while gradually shifting
eastward this afternoon and evening. Convective-layer shear of
around 35 kt will continue support episodic bouts of dime to
locally quarter size hail as the activity scoots eastward. (In
fact, have noted periodic mid-level mesocyclones, confirming
that convection is taking advantage of the shear). Elsewhere,
it`s turned out to be a pretty nice day with filtered sunshine
and a southeasterly breeze. Without additional forcing (away
from the nose of the 700mb jet), our thinking is that additional
storms are unlikely to develop northwest of where they are now.
With that said, will hold onto low (20%) chances for storms
through this evening across much of the area just to be safe (as
anything that does develop would be prime to be strong given a
better overlap of shear and instability with westward extent).
Tonight, the center of the upper-level ridge will shift eastward
toward the Great Lakes. At the same time, a weakness in surface
pressure will develop, altogether leading to mostly clear skies
and relatively light winds. As a result, do think areas of fog
will develop especially where rain fell earlier today. The
inherited mention of fog in the forecast is in good shape.
Tomorrow, a surface high pressure system centered near the
borders of Ontario and Quebec will strengthen, causing winds in
the southern Great Lakes to be decidedly onshore/easterly. As a
result, anticipate there will be a gradient in temperatures
ranging from the mid 70s along the lakeshore to the upper 80s
far inland. Otherwise, mostly sunny skies will prevail.
Borchardt
Sunday Night through Friday:
The upper level pattern next week will feature high amplitude
ridging across the Great Lakes paired with a closed upper low
in the Southeast setting up a pseudo-Rex Block pattern across
the eastern CONUS. This should keep the various western CONUS
shortwave troughs and associated showers and storms mainly west
of the region through the first half of the upcoming week. Long-
range guidance continues to diverge slightly on the position and
strength of the upper low which has some implications for the
degree of warming each day and timing of the onset of more
unsettled weather later in the week. The GEFS has the upper low
closer to the coast allowing much warmer air to lift into the
region paired with an earlier breakdown of the ridge by midweek
and return to shower chances. Meanwhile, the EPS maintains a
further west position of the upper low (closer to the more
classic Rex Block pattern) which slows the breakdown of the
ridge and delays the arrival of precipitation to later in the
week. This still favors warmer (albeit cooler than the GEFS) and
dry conditions prevailing in the local area for the majority of
the workweek.
With all of that said, there are no major changes in
expectations through the upcoming work week with the going
forecast favoring the EPS solutions with dry and generally warm
conditions expected well inland of Lake Michigan through
midweek. Expect highs in the 80s to potentially near 90 toward
western and central Illinois. Light surface flow will allow
daily lake breezes to surge well inland each afternoon which
should hold highs in the 70s along the lakeshore with falling
temperatures in the afternoon expected for inland areas in its
wake.
The upper ridge/blocking pattern begins to breaks down toward
the end of the week with the western trough axis gradually
shifting toward and over the area into next weekend. This will
lead to increasing shower and thunderstorm chances Thursday
night onward; however, there is not a strong signal for more
organized convection or heavy rain at this time. In tandem,
temperatures will trend cooler and more autumn-esque with highs
in the 70s and lows in the 50s.
Petr
&&
.AVIATION /06Z TAFS THROUGH 12Z MONDAY/...
Issued at 1220 PM CDT Sun Sep 14 2025
- Potential for IFR to LIFR CIGs/VSBYs in fog early this morning
  at RFD and DPA
Light winds and residual low-level moisture may allow for an
expansion of fog and low stratus overnight through early this
morning, mainly outside of Chicago. So far VSBY/CIG reductions
have been confined to southern Wisconsin and along the WI-IL
state line. Opted to trend a bit more pessimistic at RFD and DPA
with lower-end IFR TEMPOs from 9-13Z for both CIGs and VSBYs
based on upstream observations and an ESE orientation of the
weak low-level flow. LIFR to near VLIFR certainly can`t be
ruled out across interior northern Illinois and will continue to
monitor trends closely. Otherwise opted to maintain the
inherited 5SM BR TEMPOs for ORD/MDW with much of the fog
expected to remain outside of Chicago.
Winds will be light easterly to at times variable through
daybreak then settling into a prevailing east direction around
10kt. Winds then ease again after sunset.
Petr
&&
.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.
&&
$$
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NWS LOT Office Area Forecast Discussion