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-- Discussion containing changed information from previous version are highlighted. --
000
FXUS63 KLOT 092016
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
316 PM CDT Thu Jul 9 2020

.SHORT TERM...
257 PM CDT

Through Friday night...

The chances for some severe thunderstorms and heavy rainfall
continue to be the primary concern for northern IL late this
afternoon through the evening.

Thunderstorm activity over our area has remained isolated to this
point. However, thunderstorms have been on the uptick to our west
over IA early this afternoon in advance of an approaching MCV
over central IA. The MCV will track eastward into northwestern IL
this evening, and over northeastern IL tonight. As it does so, it
will continue to interact with an uncapped and unstable environment
(1800 MLCAPE per the 18z DVN RAOB) thus leading to more numerous
showers and storm development in a band or two over northern IL
into this evening. The threat of severe thunderstorms will
continue to be primarily focused on some isolated instances of
damaging wind gusts over northern central, and perhaps parts of
northeastern IL this evening. This threat should wane across the
area later this evening and overnight.

The other threat with these storms this evening into tonight is
the threat for some periods of very heavy rainfall. Since the
storms are expected to develop into north to south oriented
bands, which will be nearly parallel to the steering layer flow,
the potential exists for a period of training storms over northern
IL this evening. This adds some concern for the potential for
some areas of flash flooding as the rain rates in these storms
could easily exceed 2" per hour. So, even though some of the area
has been on the dry side lately, I cannot rule out this
possibility especially in more urban areas. We have opted to hold
off on issuing a flash flood watch, but this will need to be
monitored closely for portions of northern IL this evening.

Some showers and storms could linger over eastern IL and
northwestern IL late tonight, but this activity should begin to
shift out of the area Friday morning. Thereafter, it appears that
Friday afternoon could be mainly dry across the area. Temperatures
should also be cooler on Friday as some breezy northwesterly winds
develop and usher in a bit cooler airmass. Expect high
temperatures to be in the mid to upper 80s.

KJB

&&

.LONG TERM...
316 PM CDT

Saturday through Thursday...

Northwest flow expected over the weekend with some model
discrepancies regarding potential northwest flow shortwave. NAM
and GFS both have a more amplified shortwave (likely result of
convective enhancement) with an attendant greater convective
risk, though with differing timing (GFS Saturday night vs NAM
Sunday). The ECMWF doesn`t really have any meaningful shortwave
with the northwest flow this weekend and would support largely a
dry weekend. While a more amplified solution cannot be ruled out,
leaning toward the less amplified ECMWF solution. NBM pops were
fairly aggressive for the weekend, so reverted back to previous
shift`s lower pops. Certainly the NAM/GFS would support hanging
on to these pops, and should these more amplified solutions
verify, then pops would likely need to be bumped up once
confidence in timing increases.

Would appear the massive/intense heat dome over the central and
western U.S. looks like it will briefly bulge northeastward into
the area Tues/Wed time frame in advance of a northern stream
shortwave trough. This should allow temps to rebound back into the
90s by Tuesday with NBM keeping 90s going the remainder of the
week. Worth noting that if convection/convective debris were to
linger during the afternoon, then there could be an unscheduled
break in the 90 degree heat.

Tightening geopotential height gradient between the strong upper
ridge and northern stream trough will result in a corridor of
stronger flow aloft, which could certainly support a ring of fire
type pattern with attendant strong/severe convective threat on the
northern flanks of the upper ridge. At this distance, hard to say
where exactly this will set-up and if it will affect our CWA. Our
best chances look to be in the midweek Tue-Thu time frame, which
is where NBM pops were targeting the best chances. Beyond that
time frame, medium range ensemble guidance continues to suggest
the heart of the upper ridge will build northeast into the Midwest
next weekend. If this pans out, an even more intense spell of
heat would be possible at that time.

- Izzi

&&

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

Warm, unstable air mass over the area is prone to allow for
isolated to scattered coverage SHRA/TSRA to develop this afternoon
as they have the past couple of days. Most likely focusing
mechanism is a weak lake breeze, though already seeing some storms
develop just south of the WI/IL border. May need to amend to
eventually include a short 1-2 hour long TEMPO for TSRA mid
afternoon at ORD and MDW, depending on eventual development along
the lake breeze. If coverage of storms is pretty good along the
lake breeze, not out of the question that outflow could push the
lake breeze westward and result in a wind shift to an easterly
direction later this afternoon, but confidence is too low to
include this in the TAF at this time.

Potential for somewhat more organized SHRA/TSRA this evening as
disturbance over IA moves eastward, with timing into immediate
Chicago area terminals looking to be in the 01-04z time frame. In
the wake of this activity look for a wind shift to northwest
Friday morning with drier and more tranquil conditions expected
Friday.

- Izzi

&&

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

$$

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