Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 220904

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
404 AM CDT Sat Jul 22 2017

349 AM CDT

Today through Tonight...

Surface low pressure was analyzed over western Iowa early this
morning, with a convective outflow enhanced stationary front
extending east-southeast along a roughly DSM-CMI-north of IND
line. Regional radar mosaic depicts several thunderstorms
clusters/MCS`s on the cool side of the low level boundary, from
northeastern IA into northern IL/IN and northwest OH.
Southwesterly 35-40 kt low level jet, forcing ascent across this
low level baroclinic zone, appears to be the primary forcing for
these thunderstorm clusters, within a moderately unstable and very
moist air mass characterized by MUCAPEs of 1000-2000 J/kg and
PW`s of 1.50-2.00+ inches.

The low level jet is expected to veer more westerly with time
early this morning, with the focus for additional elevated
thunderstorm development and heavier rainfall shifting primarily
south of I-88 across northern IL and perhaps even more so
along/south of the I-80 corridor. Gradual weakening of convection
and decreasing rainfall rates are expected however, with the
diurnal weakening of the low level jet. Based on this will have
highest pops across the south/southwest half of the cwa this
morning. While threat of new flooding is diminishing, will
maintain flash flood watch as through 9 am CDT is based on
coordination with WFOs DVN/MKX. Later in the day, threat of
additional thunderstorms is expected to be mainly across the south
third or so of the forecast area, close to current surface
boundary position as Iowa surface low tracks east along it this
afternoon. Some potential for a few isolated showers/thunderstorms
farther north during the afternoon as region of broad height
falls develops across the Upper Midwest in association with
stronger mid-level wave, though suspect coverage will be fairly
minimal. Isolated diurnal showers/thunderstorms should fade
quickly by evening, with a few scattered storms far south sagging
out of the cwa with the front during the early evening hours.
Decreasing cloudiness late tonight could result in some patchy fog
in some spots.

With the convective-reinforced frontal boundary across the
southern cwa, surface winds should turn north-northeasterly across
far northern IL by afternoon. This, and residual cloud cover from
ongoing convection, should result in a range of temps from the
low 80`s along the IL lake shore, to around 90 far south near the
front. Very high dew points in the mid-upper 70`s may again allow
heat indices in the 100-105 range for a short time across the far
southern cwa.



402 AM CDT

Sunday through Saturday...

After some very wet and active weather, Sunday begins a transition
to a cooler and drier airmass, at least for a couple of days.  By
Sunday morning winds should be veering west and northwest at all
levels.  While this will begin to usher in the new airmass, the
afternoon passage of an upper trough over residual warmth and
moisture at low levels may support a few additional showers during
the afternoon.

By Monday morning northerly flow has become established with the
approach of a surface high into WI and the western Great Lakes. This
high will support cooler temperatures and inhibit moisture return
through early Wednesday as it slowly makes its way through the Great
Lakes and toward New England. Highs Monday in many locations will
not make it out of the 70s, and Tuesday looks only be a few degrees

By Wednesday the high is far enough east that southeasterly return
flow again becomes established.  Models also suggest the approach of
a broad frontal zone stretching from the southern Plains into the
central Great Lakes by Wednesday afternoon or evening which then
shows signs of lingering over the area through late in the week.
Moisture convergence along this boundary would support increased
precip chances for the second half of the week, with possible
clearing by the weekend as the front finally moves south away from
the area.  Confidence decreases considerably toward later in the
week regarding the motion of this boundary.



For the 06Z TAFs...

Primary concern in the near term will be ongoing chances for
thunderstorms overnight. Strongest thunderstorms late this evening
(around 04Z) have settled south of the terminals with more
stratiform precip with embedded thunder impacting the terminals
through 05Z-06Z. More intense thunderstorms have redeveloped over
northeast Iowa, however, and are starting to work east towards
the northern Illinois terminals overnight. This newer line is
oriented parallel to its movement so expect several hours of
thunderstorms to impact the terminals overnight fairly
continuously. Hi-res convective allowing models suggest this
activity will settle southeast of the terminal and diminish
between 10Z and 12Z early Saturday morning.

Confidence in the forecast details is very low through the rest of
the day Saturday. Cannot rule out a few isolated to widely
scattered showers and thunderstorms throughout the day but
confidence in anything on station is too low to include in the
TAF. Winds also appear to be problematic and will be influenced by
smaller scale boundaries that models are unable to handle. Went
with the idea advertised by several models of winds gradually
veering to the southwest tomorrow with an afternoon lake breeze
turning winds northeast at the Chicago area terminals, but again
outflow boundaries may dictate otherwise.



400 AM CDT

A light gradient today will generally support northerly flow but
local thermal effects likely will promote lake breeze formation.
Winds remain light on Sunday and somewhat variable. A slightly
stronger gradient ahead of an approaching high will support
northerly flow again Monday and the long fetch may promote enough
wave growth on the southern end to prompt small craft advisories.



IL...Flash Flood Watch...ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-
     until 9 AM Saturday.




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