Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 101545
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1045 AM CDT Mon Aug 10 2020

.MESOSCALE DISCUSSION...
1040 AM CDT

Main focus here remains locked squarely on convective evolution
this afternoon with it appearing increasingly plausible that a
corridor of severe weather--some potentially significant--may
unfold across the region. The main time frame of concern continues
to be about 1 to 7 PM, earliest near I-39, latest closer to I-57
and 65.

We`ll get to the ongoing convective complex across western Iowa in
a minute, but currently monitoring an area of ACCAS/enhanced mid-
level cloud cover rolling across the I-39 corridor. This is
associated with very modest warm advection/isentropic ascent above
about 800 mb. This likely demarcates the eastern periphery of a
rather impressive elevated mixed layer, with Davenport`s 12z
sounding sampling a region of nearly 9 C/km lapse rates in the
700-500 mb layer. Things still look pretty well capped to
surface-based parcels, but probably will see some spits of rain
falling out of this elevated cloud deck as it continues eastward
this morning.

When looking at regional satellite loops, the eye is immediately
drawn to the developing complex across central Iowa, which
continues to accelerate eastward, aided along by a deepening cold
pool, modestly-fast mid-level westerlies, and a wound-up upstream
shortwave across the arrowhead of Minnesota. Impressive to see the
amount of measured near and above-severe wind gusts with this
activity at this time, with the downstream environment primed for
continued regenerative development. The one limiting factor here
is that mid-level flow is fairly modest, but does look sufficient
to support 35 kts or so of effective deep layer shear oriented
somewhat orthogonal to the eastward-advancing complex, which is
important to continually augment updrafts at the leading edge of
the cold pool. Even with a lack of stronger winds aloft, these
are certainly sufficient combined with the rather large theta-e
reservoir and steep lapse rate environment overhead to support a
severe threat--again potentially a fairly widespread one--into our
region.

While the main threat looks to be severe/damaging wind gusts
(perhaps locally towards 80 mph), there will be an attendant
threat for large hail (half dollar to ping pong ball) given the
lapse rates and augmentation of mid-level updraft accelerations,
as well as a non-zero threat for embedded QLCS tornadoes, mainly
south of I-88 or so. While low-level flow is fairly weak and
veered, there is a fair deal of low-level hodograph curvature and
length present, certainly sufficient when combined with the
amount of instability to support mesovortexgenesis at the leading
edge of the cold pool.

Interestingly enough, hi-res guidance seems to be handling ongoing
trends rather well, if perhaps an hour or two slow with things.
Have bumped PoPs up and sped things up a bit based on upstream
trends, as well as a commensurate ramp-up in our severe weather
messaging with the morning update. The main message here is to
stay weather-aware today, especially during the roughly 1-7 pm
timeframe.

Finally, will address a potential for a brief period of high rip
current risk tonight into Tuesday across northwest Indiana beaches
with the full afternoon package. Updated products have been
transmitted.

Carlaw

&&

.SHORT TERM...

Through Tuesday...

311 AM...Main forecast concern is the potential for severe
thunderstorms this afternoon.

Confidence is increasing for a line of thunderstorms to move
across the area this afternoon. Prior to that time period...there
does remain some uncertainty. Activity across northwest Indiana
appears to be on the back side of the wave that moved across
northern IL/southern WI Sunday evening and is now shifting across
southern lower MI. Some isolated activity may continue across
northwest IN through sunrise but confidence is low. There may also
be some isolated activity across central IL this morning that may
reach into the far southern cwa.

Dewpoints remain in the lower 70s early this morning with temps
steady in the low/mid 70s which will likely be the lows for this
morning. Some patchy fog will be possible early this morning...
especially across northwest IL. With temps starting in the 70s and
some sunshine expected through midday...temps are likely to rise
quickly...into the upper 80s and some lower 90s are certainly
possible which will push heat index values to near 100 by late
morning.

Focus then shifts to developing convection across southern South
Dakota and northern Nebraska with a strong wave moving across the
Plains. There has been very good agreement for several runs of the
hrrr showing this activity growing into a large complex and racing
east into the cwa during the early/mid afternoon. This solution is
also supported by several other models including the ECMWF/GEM/NAM.
The 06Z 4km NAM is also now very similar to the hrrr. And based
on this forecast have bumped up pops well into likely for this
afternoon. Confidence is only medium for the exact track of this
complex and thus where the strongest activity will focus. The
convection currently over the Twin Cities may push an outflow into
northern IA that may nudge the complex a bit further south though
its a bit early for this detail until trends emerge later this
morning.

Given the very moist/unstable airmass in place ahead of this
expected line of thunderstorms...storms could easily become severe
with damaging winds the main and likely only severe threat.
Difficult to say how strong the highest winds will be but just the
expected speed of the storms would suggest isolated 70 mph gusts
are possible. If a broken line of thunderstorms does move across
the cwa...there could be some potential for more widespread severe
winds.

Locally heavy rain will be possible but the faster movement of the
storms should limit the duration of the heavy rain. However...if
heavy rain does persist...especially over urban areas...there will
be some potential for localized flooding. Precipitation is
expected to be ending from northwest to southeast by early evening
and lowered pops accordingly tonight but its possible an even
faster end to precipitation will be needed.

The cold front should be south of the area on Tuesday with some
uncertainty as to how fast it clears the cwa and maintained low
pops in the far south Tuesday morning. Dewpoints will be dropping
into the 60s and perhaps some upper 50s north...thus lower
humidity and dry for most of the area. cms

&&

.LONG TERM...
244 AM CDT

Tuesday night through Sunday...

A somewhat stagnant pattern looks to develop mid-week into the
upcoming weekend as mid-level ridging builds northward into Hudson
Bay. Meanwhile, abundant MCS activity across the central CONUS
over the next day or so may create a broad minimum in mid-level
heights across the Ohio Valley in otherwise zonal flow. The cold
front crossing the area this evening should ultimately wash out
well south of the CWA. With broad surface high pressure settling
over the Great Lakes region mid-week before drifting northeast
through the end of the week, a period of seasonable temps, lower
dew points, and generally east flow is expected Wednesday through
Saturday. Any precip potential during this time will hinge on the
evolution of convection within stronger westerlies across the
northern Great Plains. Some guidance hints at the potential for a
convectively- induced wave to shift into the Upper Mississippi
Valley late Thursday into Friday, ultimately amplifying the height
minimum across the Ohio Valley by the weekend. This would develop
a weak Rex block with the Hudson Bay ridge, placing a lot of
uncertainty in precip potential beginning late Thursday as any
slight changes in the possible stalled mid-level flow could mean
the difference between an extended dry period and a potential
upper low settling near the CWA.

Kluber

&&

.AVIATION...
For the 12Z TAFs...

Primary concerns through today revolve around the potential for a
line of severe thunderstorms this afternoon.

Pockets of IFR, MVFR, and VFR ceilings are scattered around
northern Illinois and northwest Indiana at TAF issuance, with IFR
ceilings favored for at least some time at RFD and DPA over the
next couple hours. The IFR ceilings may briefly affect
ORD/MDW/GYY. Otherwise, ceilings should lift into a low-end VFR
deck by late morning.

Confidence continues to increase in the potential for a line of
severe thunderstorms tracking ESE into northern Illinois and
northwest Indiana this afternoon. Upscale growth of convection is
ongoing across southern South Dakota as of 12Z. This convection is
expected to accelerate across Iowa this morning and into northern
Illinois this afternoon. Arrival time into the terminals will
depend on how quickly the line accelerates, but focus for arrival
time at ORD/MDW is in the 20-22Z window with a slight chance of
arrival time as early as 19Z. Quick forward motion >40 knots is
expected, with thunderstorm wind potential of >50 knots.

After any potential convection passes by early evening, a cold
front will shift SSE across the terminals late this evening. NNW
winds with clearing skies can be expected behind the front.

Kluber

&&

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

$$

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