Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 140530
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1230 AM CDT Mon Aug 14 2017

.UPDATE...

840 PM CDT

Evening Update...

A few sprinkles had traversed the far north/northwestern parts of
the cwa into early this evening and had updated pops/wx for
isolated mention earlier. Otherwise, no significant changes needed
to going forecast this evening.

Surface high pressure continues to drift slowly off to the east
of the forecast area this evening, with generally calm or light
east winds across the cwa. While a few weak sprinkles developed
from eastern IA into northwest IL earlier in association with a
minor mid-level short wave as seen in GOES vapor imagery, stronger
low level moist return flow is still focused well west of the
region across the Missouri Valley and into southwestern MN. Thus
additional precip is not anticipated until mid-day Monday or
Monday afternoon, and even that may be of low coverage and is
mainly expected to be across far north/northwest cwa.

For the remainder of the overnight hours, expect mid-high level
cloud cover to persist into the pre-dawn hours. This will help
keep temps milder than last night, generally mid 50`s to low 60`s
away from warmer urban Chicago. Going forecast has this well in
hand, so no significant changes made.

Ratzer

&&

.SHORT TERM...
209 PM CDT

Through Monday...

A relatively quiet period of weather is expect for the short term
forecast period, though there should be increasing chances for
precipitation through the day tomorrow.

For the remainder of the afternoon and into early evening,
conditions should be dry with temperatures still slightly below
normal.  With increasing mid and high cloud streaming across the
region associated a northern stream short wave over Minnesota and an
associated weak cold front extending southwest into Colorado.
Otherwise, high pressure ridging will be in place across the region
through the night which should help delay the eastward progression
of the system to the west.  High temperatures this afternoon should
generally top out around 80 F which is a couple degrees below
normal.  A lake breeze boundary developed early this afternoon in
the weak pressure gradient and light wind pattern across the region.
 The lake breeze quickly pushed inland, keeping lakefront locations
a bit cooler, with highs only in the lower to middle 70s.  The lake
breeze should dissipate with sunset and winds should go light and
variable again overnight.  While winds will be light, expect less of
a chance for patchy fog development as there will still be some
layered cloud over the region with dewpoints in the lower to middle
50s.  If any patchy fog were to develop, it would likely be in
sheltered, low-lying areas.

For Monday, the center of the high will begin to shift to the east,
over the eastern Great Lakes.  The northern stream shortwave
generating some showers and thunderstorms over the northern plains
this afternoon will track east, crossing the middle and upper
Mississippi Valley through the day tomorrow.  An associated weak sfc
low should move into srn Minnesota by early afternoon with a weak
sfc trough/quasi-stationary frontal boundary extending west into the
central plains.  This system may bring a chance for some showers or
a few isolated thunderstorms late tomorrow afternoon, mainly for
locations west of the I-39 corridor.  However, confidence is fairly
low in the timing and coverage of pcpn tomorrow afternoon as this is
a significant change from earlier guidance which indicated that the
high pressure ridge should persist longer across the region, with
pcpn not expected until later in the period.  The one factor that
does give some credence to this faster pcpn chance is that the nrn
stream short wave currently over Minnesota is stronger than
previously forecast, so the faster pcpn trend may have some
validity.

&&

.LONG TERM...
226 PM CDT

Monday night through Sunday...

The surface ridge that has been parked overhead and providing
very mild and dry early-mid August weather will continue moving
east and allow return flow to begin in earnest Monday night.
Increasing low level moisture through the week will support
increased chances of showers and thunderstorms at least until late
Thursday when the passage of a cold front again dries things out
and allows ridging to develop for Friday and perhaps early
Saturday.

Models of course disagree on the timing and intensity of the
various upper shortwaves that would focus support for
precipitation. The best chance would appear to be on Wednesday
afternoon and evening when a more pronounced shortwave aloft
interacts with ample warm and moist advection at low levels. Hints
of a warm front in model guidance would further focus low level
moisture convergence early Wednesday afternoon, and the
approaching cold front would be favorable late Wednesday. For
Monday night into Tuesday night, and again Wednesday night into
Thursday, chances are lower given overall weaker forcing but
certainly worth continuing a mention of in the forecast. If the
cold front slows down then POPs may need to be raised for
Thursday.

It is not clear at this time range whether the entire upcoming
weekend would be dry, but the aforementioned ridging for Friday
and early Saturday would at least provide a start. It does appear
the longevity of the ridge would be less than what we are
presently experiencing.

Lenning

&&

.AVIATION...
For the 06Z TAFs...

Light and variable winds will turn south-southwesterly during the
day. However, the winds should be light enough to allow another
lake breeze to develop over portions of northeastern Illinois by
early afternoon. In spite of this, it appears that it will
struggle to move inland over cook county. As a result, it may
remain east of the terminal sites for much, if not all of the
day. Even if the lake breeze was to make it to ORD or MDW late in
the day, wind speeds in its wake would likely remain below 10 kt.
For these reasons, I have opted to keep the wind shift out of the
latest TAFs.

While the weather will remain quiet across the area through the
day, there will be a potential for a few showers and thunderstorms
over portions of northern IL and southern Wisconsin later this
evening and into the overnight hours as an upper level disturbance
approaches the area. However, due to the low probability for
thunder at the Chicago area terminals this far out, I have opted
to leave the mention out of all but KRFD.

KJB

&&

.MARINE...
226 PM CDT

Light flow over the lake in the near term will continue to allow
local thermal effects to dominate, with land breezes at night and
lake breezes in the afternoon. Stronger southerly flow develops by
midweek before a cold front crosses the lake late Wednesday or
early Thursday and veers winds westerly and perhaps all the way
to northerly for the second half of the week.

Lenning

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.
&&

$$

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