000
FXUS63 KLOT 261802
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
102 PM CDT Wed Jul 26 2017

.SHORT TERM...
352 AM CDT

Today through Thursday Night...

Main short term forecast concerns focus on timing of thunderstorm
potential, severe threat, and location of axis of heaviest
rainfall through tonight.

Surface high pressure was centered across the Northeast early
this morning, with weaker ridging trailing west into northern
IN/IL. Well to the north of the area, deep low pressure was moving
across northern Ontario, in association with a vigorous mid-level
short wave trough. A cold front trailed across the Upper
Mississippi Valley and into the Central Plains, where another
weaker low was noted over northwestern KS. Modest low-level
southerly return flow had become established around the western
periphery of the surface ridge over the Midwest, from the lower
and middle Missouri Valley to the Upper Mississippi Valley and
northern Lakes.

The cold front will move very slowly to the southeast today, with
the weaker KS surface low expected to ripple slowly east-
northeast along the front into IL by Thursday morning in response
to a weaker short-wave trough propagating through westerly flow
aloft. This will result in an increase in low level moisture and
instability as the nose of a modest south-southwesterly low level
jet spreads east across the region. Convection currently occurring
across northeast KS and eastern NE is expected to expand eastward
as this occurs, reaching western portions of the cwa by late this
afternoon or early this evening. Guidance is generally in
agreement in focusing the strongest instability, low level jet
convergence and moisture transport from southeast IA into central
IL, which would support the greatest threat for heavy rainfall and
isolated severe thunderstorms (SPC slight risk west with
wind/hail threat mainly during the evening before low-level lapse
rates weaken after sunset) primarily west and southwest of the
cwa. While heavier amounts are expected south/west of the cwa, a
blend of guidance and WPC qpf still suggests the potential for
locally heavy rainfall of 1-2 inches across the southern tier or
so of LOT counties overnight.

The weak surface wave continues to ripple east along the front
Thursday, with the front slowly sagging off to the southeast of
the cwa by early evening. Forcing and lapse rates appear
significantly weaker during the day, with the primary focus for
additional convection tied closely to the front/warm sector over
the southern portion of the forecast area. SPC maintains a
marginal risk across the southeast half of the cwa for Thursday.
Rain and thunderstorm threat tails off Thursday night as diurnal
instability wanes and mid-upper level drying strengthens.

Warmer and gradually more humid conditions expected today. High
clouds off of convection west/northwest of the area will likely
impact temps a bit especially north today, with highs expected
from the mid-80`s far north to the upper 80`s most other
locations. Cooler northeasterly flow and residual cloud cover
Thursday will hold temps a bit below normal around 80, and in the
mid-70`s near the lake.

Ratzer

&&

.LONG TERM...
352 AM CDT

Friday through Tuesday...

Cooler and drier weather conditions are expected to end the work
week and persist into the weekend, with a gradual moderation in
temps back to about normal by early next week.

A fairly vigorous mid-level short wave trough is progged to be
digging southeast across the area early Friday, helping to amplify
the departing eastern CONUS upper trough as it propagates into
the Ohio Valley by evening. Despite 500 mb height falls on the
order of 40 meters ahead of the mid-level trough axis, indicative
of focused large scale ascent, north-northeasterly winds and drier
air through the lower and middle troposphere imply little support
for organized precipitation. Low level raw thermal fields from
models support afternoon temps from the mid-70`s north to around
80 south, with breezy north-northeast winds 20-25 mph and a
scattered stratocu deck.

Subsidence continues across the Upper Midwest and Great Lakes
regions Friday night into the weekend, as mid-level ridge
gradually builds into the area. At the surface, an expansive area
of high pressure is expected to provide fair weather conditions,
with light winds and daily lake breezes near Lake Michigan.
Pleasant, though cooler than average temps are expected, with
highs generally 75-80 Saturday moderating to near 80/low 80`s on
Sunday.

Medium-range guidance indicates a cold front approaches or pushes
across the area from the north late Monday or sometime Tuesday,
with the GFS being the most bullish with a stronger jet streak
digging southeast across the western Lakes region. While scenario
is plausible with both the GFS and the ECMWF bringing a short-wave
across the northern Lakes, confidence in the details of timing
the front are quite low at this distance. Greater confidence in a
continued gradual moderation of temps back into the low-mid 80`s
Monday and Tuesday.

Ratzer

&&

.AVIATION...
For the 18Z TAFs...

Main concerns for the 18z update will focus on timing and coverage
of ts this evening and then wind direction, with a wind shift to
nely tomorrow morning.

Have been monitoring convective trends with the system moving
through srn Wisconsin to cntrl Iowa into nrn Missouri. latest
radar trends show some sct ts developing with the passage of an
outflow boundary moving through srn Wisconsin. Cell movement is
to the northeast and expect that this activity should remain north
of the WI/IL border, but an isold cell developing south of the
border is not out of the question. Main TS concern will be with an
MCV moving through cntrl Iowa. Expect that this feature will
maintain some TS potential, and the high res guidance is trending
a little faster with the ewd progression of the system in general.
However, convective development within the pcpn shield is
becoming more questionable and a solution with isold/embedded TS
within the pcpn may be more likely. So, have gone with a vcts
mention for thunder as confidence is low on trying to time TS
impacts at the terminals. Larger scale guidance would also suggest
that the heaviest pcpn may be suppressed south of the terminals
with a large area of high pressure building into the nrn
plains/upper Mississippi Valley overnight tonight.

Winds should become light and variable this evening, persisting
into the early morning hours. With the light flow and moisture in
the area, fog/low clouds could develop. Northeast winds will
increase into afternoon hours as the center of the high tracks
east to the upper Great Lakes and low pressure and a weak, quasi-
stationary frontal boundary sets up across cntrl IL/IN.

&&

.MARINE...
132 AM CDT

Southerly winds will continue over the lake today, with
morning gusts around 30 kt in the north. Showers and some
thunderstorms are expected across the north and central this
morning sinking slowly south later today into tonight. This is
ahead of a trough/cold front that will move into the northern part
of the lake late this evening into overnight. Low pressure moving
due east from the central Plains to the Ohio River Valley tonight
into Thursday will help usher this front southward Thursday
morning. As the pressure gradient tightens in its wake, northerly
winds will increase Thursday afternoon, with 20-25 kt gusts
across much of the lake by late day into Thursday night. This
along with building waves will bring Small Craft Advisory criteria
to the Illinois and Indiana nearshores at some point Thursday
afternoon that will continue likely through Friday night. High
pressure will then move over the lake for the weekend bringing
light winds and calm seas.

MTF

&&

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

$$

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