Area Forecast Discussion
Issued by NWS Chicago, IL

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349
FXUS63 KLOT 152345
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
645 PM CDT Wed Aug 15 2018

.SHORT TERM...
242 PM CDT

Through Tonight...

The main items of note for the short term forecast are scattered
showers and a few storms continuing into early evening along with
the potential for overnight and early morning fog.

The broad surface low and a fairly broad parent mid-level trough
are slowly drifting east across the mid-Mississippi Valley this
afternoon. Forcing for ascent within a high absolute moisture and
non-capped airmass are generating convection, but limited focus
and moisture transport/replenishment are keeping showers
scattered. Envision this trend continuing through early to mid-
evening, especially as a mid-level vorticity maximum in northwest
Illinois per water vapor imagery shifts east over the area. This
feature may continue showers and even a storm or two into the
overnight.

Locally heavy downpours with the slow-moving cells will continue
to be common, but the convection has not been deep enough to
warrant any concerns on flooding. Also, a non-zero chance for a
funnel cloud or two in north central Illinois and points west
through early evening, given high values of low-level CAPE and
vorticity.

The combination of the showers, residual high dew point air around
70, and the passage of the broad surface low over the area
overnight set the stage for stratus and/or fog. Confidence is
fairly low on which it will be, especially for north central
Illinois. This is the area that has a slightly better potential to
clear toward sunset and the area that is being hit hardest by
high-resolution guidance for potential dense fog. Have areas of
fog mentioned into the morning for this region. Feel that the
eastern half of the CWA including Chicago, which has had less
shower coverage and currently under more close-celled clouds, is
likely to stratify more so than fog in.

MTF

&&

.LONG TERM...
220 PM CDT

Thursday through Wednesday...

By Thursday morning, the first of a series of shortwaves will track
to the eastern Great Lakes while the second digs into the Upper
Missouri Valley.  At the surface, a diffuse low will move across nrn
IL.  Pcpn chances will continue into the morning hours, with the
greater thunder chances to the southeast of the sfc low track, in
the zone of stronger warm/moist advection.  Northeast of the sfc
low, expect more sct showers, and while some isold ts can`t be rules
out, lingering light nely sfc flow will be less conducive to ts
chances.  Into the afternoon hours, the second shortwave will cross
the Middle/Upper Mississippi Valley, increasing upper level forcing
in concert with increasing diurnal sfc instability.  So, expect
lingering sfc shra with a chance for more widespread, but still
isolated ts potential.  The models are in generally good agreement
on the idea that this second upper wave will be slowly progressive,
and will not push east of the region until at least Friday
afternoon.  So, there will be lingering chances for sct showers and
isold ts until the upper trough moves off to the east late Friday
and upper ridging builds across the Middle/Upper Mississippi Valley.
 Extensive cloud cover and sct showers will help limit temperatures
temperatures for Thursday and Friday for much of the area to the low
80s, while onshore flow will limit temperatures to the upper 70s
along the Lake Michigan shore.  For the weekend, as the upper ridge
continues to build east, decreasing cloud cover and rising heights
aloft should allow for some warming with highs getting closer to the
middle 80s.  However, a weak sfc pressure gradient could allow for
lake breeze development, keeping conditions a bit cooler near the
lake.  For early next week, the longer range models are advertising
another upper low tracking across the region, which would bring
another round of precipitation along with a period of lower
temperatures, with highs, perhaps, only in the middle to upper
70s.

&&

.AVIATION...
For the 00Z TAFs...

645 pm...Forecast concerns include scattered thunderstorms this
evening...fog/low clouds Thursday morning and chance of showers/
isolated thunderstorms Thursday afternoon.

Scattered thunderstorms are forming on multiple outflow boundaries
late this afternoon which then produce more outflow boundaries.
Coverage is expected to decrease with sunset but there will still
likely be showers and perhaps isolated thunderstorms through mid/
late evening. The outflow boundaries also may produce brief wind
shifts if they move across the terminals...likely from a southerly
direction since the general storm motion is slowly northward.

Winds will eventually become light and variable tonight with a
combination of fog and low clouds expected. Previous forecast has
trends handled fairly well given latest guidance. The best chance
for low visibility with fog will be across northwest IL and rfd.
However...fog is still expected across the rest of the area and
the Chicago terminals but confidence decreases further east. Ifr
cigs seem on track across much of the area but lifr is also
possible.

Winds will shift north/northeast Thursday morning and there is
quite a bit of uncertainty regarding precip coverage Thursday
afternoon. Coverage is likely to be less than this evening but
may still be worthy of tempo mention but from this distance...
vicinity mention seems on track. cms

&&

.MARINE...
220 PM CDT

Except for a chance for some scattered thunderstorms into Thursday,
conditions over Lake Michigan will be relatively quiet for the next
several days.

North to northeast winds are in place across the north end of the
lake while south to southwest winds are in place across the south as
a very diffuse cold front has settled over the mid-section of the
lake.  Winds will trend to northerly over the entire lake as a sfc
low lifts northeast across Lower Michigan Thursday night and across
the eastern Great Lakes Friday.  With strong high pressure parked
over James Bay and the weak low continuing east across New England,
generally north winds will persist over the lake into the weekend.

&&

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

$$

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