Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 08/07/19

Area Forecast Discussion

National Weather Service Chicago/Romeoville, IL

 623 PM CDT Tue Aug 6 2019

.SHORT TERM...
310 PM CDT

Through Wednesday night...

Through Tonight:
Pesky low clouds are finally scattering this afternoon, allowing
for temps to warm. However, despite lingering moisture dew points
in the middle to upper 60s, there are still very weak mid-level
lapse rates over the area, as seen on latest aircraft soundings
from MDW. Thus, suspect the weak to moderate uncapped SB/ML CAPE
shown on SPC mesoanalysis remains a bit overdone. Latest visible
satellite suggests this to be the case, with any towering Cu in
the Cu field well northwest of the CWA.

With the main forcing for convection well east of the area,
little/no low-level convergence inland, and weak lapse rate
environment not expected to improve much through early evening, it
continues to appear coverage of any showers through early evening
will be quite low/isolated to perhaps widely scattered. Weak
short-wave(s) moving overhead would be the source of larger scale
forcing, which did kick off a few showers well north of us over
southwest Wisconsin in the better Cu field. One possible area of
focused convergence and perhaps a few showers would be on the
synoptically enhanced lake breeze moving inland through early
evening, from roughly just south of latitude of downtown Chicago
to points south. For areas well inland, best chance for isolated
development looks to be mainly I-80 and south though can`t
completely rule it out north. Thunder wise, given aforementioned
weak forcing and convergence (aside from near lake breeze) and
weak mid-level lapse rates limiting destabilization, think chances
are isolated/slight chance at most.

Turning to later tonight, clear to partly cloudy skies and light
to calm winds should enable low temperatures outside of Chicago
to cool to at or below current dew point temperatures. Low dew
point depressions could support patchy fog development, so added
patchy fog mention in the grids after midnight through early
Wednesday outside of Chicago. Doesn`t appear fog would become
widespread, but some patchy visibility under 1 mile can`t be ruled
out in favored low-lying areas.

Wednesday-Wednesday Night:
The rest of the daytime hours Wednesday should be primarily dry
and warm. Forecast soundings indicate good mixing potential with
steady west-southwest to westerly winds. As has been the case in
the drier pattern we`ve been in lately, suspect the afternoon dew
points shown on the NAM and GFS will be well overdone and likely
closer to the better verifying ECMWF, so expectation is mixing out
to the upper 50s over the city and lower to perhaps mid 60s
elsewhere. Progged 925 mb temps in anticipated well mixed air mass
should support highs in the mid to upper 80s (85-89), with upper
80s potentially widespread. With no low-level/surface convergence,
and maybe a weak wave moving over the area in the afternoon, would
expect mostly developing Cu and little chance of shower/t-storm
development (except maybe far N/NW CWA late afternoon).

Convective trends Wednesday evening-night are uncertain. A cold
front will be approaching at an unfavorable diurnal time and
instability should be fairly minimal with continued weak mid-level
lapse rates. It looks like slightly better chance for widely
scattered showers and storms will be north of I-80 closer to
glancing blow from short-wave forcing over Wisconsin. Confidence
on any strong-severe storms is low and if any risk, would most
likely be for gusty/strong winds in parts of far north/north
central IL where SPC has a level 1 (Marginal) risk. Low chances
for showers and perhaps storms may then swing southeast after
midnight.

Castro

&&

.LONG TERM...
310 PM CDT

Thursday through Tuesday...

Drier and slightly cooler air will filter in behind the front on
Thursday, so should be a pleasant day for this time of year with
highs in the lower to mid 80s inland and upper 70s to around 80
lakeside. Only slight chance PoPs PM hours are well south of IL
and Kankakee Rivers closer to better moisture and instability near
the front. After a comfortable Thursday night, Friday will be
similar to Thursday if maybe a degree or two cooler and again
feature a bit cooler temps lakeside.

Relatively benign northwest flow aloft expected over the region
through the upcoming weekend with continued comfortable dewpoints
will spell little threat of organized rain. Some of the 12z
guidance, namely the ECMWF and Canadian, introduced the potential
for building heat by Sunday, so mid 80s in official forecast
would be underdone if this occurs. Indications in the medium range
guidance point toward a warm and humid and more active stretch
early next week as disturbances ride over northern periphery of
mid-upper ridge centered over the southern Plains and mid South.

Castro/Izzi

&&

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

VFR conditions are expected through the period. At 00Z TAF issuance,
a few -SHRA near RFD will dissipate over the next couple hours.
Meanwhile, a lake breeze boundary has stalled near ORD and may
oscillate over the terminal for the next hour before pushing farther
inland and weakening after sunset. Light and sometimes variable
winds are then expected tonight ahead of steady W winds on
Wednesday. There is a low chance that an isolated TS could affect
ORD/MDW near the very end of the period (03-06Z Thursday), but
decided to keep the TAF dry for now given the high uncertainty.

Kluber

&&

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

$$

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