Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 160901

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
401 AM CDT Mon Jul 16 2018

303 AM CDT

Through tonight...

Early this morning, a cold front stretched from the Upper
Peninsula of Michigan southwest into the Central Plains. As the
parent low well north in Canada tracks east it will drag this cold
front east and south across our CWA today. Frontal convergence is
weak with calm/light variable winds ahead of the front and just a
light northwest behind the front. In addition, the primary
shortwave trough will pass well north of the area with just some
very subtle height falls and no much in the way of large scale
ascent. Despite these mitigating factors, ambient air mass ahead
of the front is very moist and potentially unstable, so cannot
rule out some isolated to scattered showers/storms today as the
front moves through. Some of the high-res convective allowing
models are favoring the lake breeze for convective development
this afternoon, likely due to the more focused low level
convergence. Generally kept pops fairly low, but in a nod to the
HRRR, HRRRx, and RAP did carry slightly higher (still just chance)
pops near and downstream of the lake breeze.

As is typical with summer fronts, the theta-e gradient is fairly
weak, so not looking for a significant drop in dewpoints or temps
this afternoon. However, a secondary cold front is progged to move
across the CWA late tonight and early Tuesday morning. It is in
the wake of this front that we will see a more pronounced drop in
dewpoints and temps, along with a freshening up of the northerly
winds in its wake.

- Izzi


303 AM CDT

Tuesday through Sunday...

Tuesday and Wednesday will feature very comfortable humidity
levels (dewpoints in 50s to near 60), ample sunshine, and slightly
below average temps.

Late in the week, we look to get into a more unsettled pattern.
Medium range models remain in good agreement on the general
synoptic pattern with northwest flow aloft over the area as a
vigorous northwest flow shortwave/closed mid-upper level low
digging southeast into the Midwest/Upper Mississippi Valley
regions. There remain some noteworthy differences in timing,
track and convective details with this system, however, it does
appear as though this system could marry stronger flow aloft and
resultant favorable deep layer shear with at least moderate
instability to result in an increasing threat for strong to severe
storms somewhere in the Midwest region. Still a little early to
have a good handle on how much if any severe weather threat will
exist in our CWA, but certainly the pattern late in the week
(Thursday night/Friday time frame) bears watching the next couple
of day.

- Izzi


For the 06Z TAFs...

VFR/dry conditions with generally light and variable winds in
place early this morning. Expect this to remain through early this
morning, though a very low chance for an isolated shower is
expected across the terminals. Low pressure and frontal boundary
will move across the area later this morning, and provide a
slightly higher chance for showers and thunderstorms through
midday and early/mid afternoon. Scattered thunderstorm development
is possible, but with confidence lower on exact timing/coverage
at this time. After FROPA later this afternoon, precip chances
will end from north to the south. Southwest to west wind expected
ahead of front, with a wind shift to the northwest likely later
this afternoon. Do expect lake breeze to remain just east of ORD
and MDW later today, but will continue to monitor trends for a
possible wind shift to the east northeast this afternoon/evening.



401 AM CDT

Area of low pressure situated across Ontario this morning, while
a trough and front from this low extend into the western Great
Lakes. These features in place are helping to provide southerly
winds across the lake, with slightly stronger winds in place over
the northern portions of the lake. This southerly flow is
supporting fog development, with the more widespread dense fog
still likely across the far northern portions of the lake. With
this pattern likely not changing until later this morning, think
this dense fog will persist slightly longer than previously
thought and so have extended the dense fog advisory through
early/mid morning. This fog should clear out later this morning,
as the front pushes across the lake. The arrival of the front will
turn the winds northerly, though this will likely only be brief
with this first boundary. Expect another boundary to move across
the lake later tonight into Tuesday morning, which will support
stronger northerly winds in the 15 to 25 kt range over much of the
lake. These strengthening winds will support building waves which
could reach conditions hazardous for small craft late tonight
into Tuesday. At this time, the Indiana nearshore looks to see the
higher waves, but the Illinois nearshore could at least briefly
observe these higher waves as well.






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