Scientific Forecaster Discussion
fxus63 klot 182330
Area forecast discussion
National Weather Service Chicago/Romeoville, Illinois
630 PM CDT Wed Sep 18 2019
252 PM CDT
We will need to keep an eye on the thunderstorms currently
shifting eastward across Iowa as we head into early this evening.
These storms are expected to weaken as they shift towards the
Mississippi due to a less favorable thermodyamics environment
farther east into Illinois. However, there is a small chance that a
storm could approach the Rockford area around 6-7 PM this evening
Otherwise, eyes tonight will be on the development and evolution
of another batch of thunderstorms to our northwest. These storms
are expected to initiate over portions of southeastern Minnesota and Iowa
in the vicinity of a surface frontal boundary later this evening.
Given the orientation of the 850-300 mb thickness field, this
complex of storms is likely to shift southward along the
instability gradient, which will be nosing into north central Illinois.
For this reason, some of these storms could make it into the
Rockford area around daybreak Thursday before weakening. Due to
the expectation of a quick weakening trend with these storms in
the morning, it is unlikely that they would survive into the
Chicago area. However, some increase in high level cloudiness is
likely over the area during the morning.
Expect temperature to once again warm into the lower to middle
257 PM CDT
Thursday night through Tuesday...
Not much has changed with the thinking for the heavy rainfall
potential this weekend. Please reference the previous discussion
below for more info.
Thursday night and friday: a rather pronounced mid-level ridge will
remain entrenched across the Great Lakes region during this period,
with continued unseasonably warm conditions into Friday, when temps
in the mid to potentially upper 80s run 10f+ above normal.
Weak disturbances will continue to round the western extent of this
ridge over a moderately unstable air mass. Convection over Iowa on
Thursday afternoon may sneak into the northwest County Warning Area in a decaying Mode as
it outruns much better instability and into a more capped
environment. Depending on how far east this convection reaches, a few
storms may develop on a resultant outflow boundary over the northwest County Warning Area
Friday as capping weakens from minor cooling in the 925-850 hpa
layer. Overall, dry weather for much of the area for a significant
portion of this time period can be expected.
Saturday through sunday: deep troughing currently over the Pacific
northwest will quickly erode the ridge across the Great Lakes this
weekend. A cold front well removed from its parent low over Hudson
Bay will then approach the area on Saturday, slowly crossing the County Warning Area
on Sunday. This occurs as abundant moisture associated now tropical
depression Imelda is advected into the region ahead of the front.
Precipitable water values of around 1.8" will be near the climatological Max for
the third week of September. Putting all this together, multiple
rounds of heavy showers and/or thunderstorms will be possible
Saturday night through Sunday, though isolated to scattered activity
may precede the front on Saturday. Will continue to monitor this
period for a renewed flooding risk.
By early next week, guidance diverges with the handling of a western
Continental U.S. Trough, though it looks as if it will become cutoff before
reaching the Great Plains. Modest mid-level ridging would then be
the dominate feature across the area, producing generally quiet
for the 00z tafs...
630 PM...main forecast concern is a chance of thunderstorms
Thursday morning across northwest Illinois.
A lake breeze will dissipate with sunset this evening with light
south/southeast winds overnight turning more southerly Thursday
morning. Wind directions may turn south/southwest for a time into
the early afternoon. Wind directions are expected to shift back to
the southeast Thursday afternoon and a lake breeze is possible but
confidence is low.
Scattered thunderstorms are possible across northwest Illinois Thursday
morning and may reach rfd but there is a good consensus that this
activity will dissipate before reaching the Chicago area terminals.
So for now have maintained prob mention at rfd and remained dry at
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