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Area Forecast Discussion (AFD)

Issued by Chicago, IL (LOT)

FXUS63 KLOT 201149

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
649 AM CDT Tue Aug 20 2019

316 AM CDT

Through Wednesday...

Main focus through the short term portion of the forecast is on the 
potential for strong to severe thunderstorms this morning followed 
by additional (but highly conditional and uncertain) storm chances 
later this afternoon and on Wednesday. 

A soupy airmass is in place across Illinois early this morning, with 
surface dewpoints analyzed in the low to mid 70s. Light winds and 
mainly clear skies (although cloud cover is increasing) will promote 
another potential for continued fog development through the rest of 
the overnight hours. We'll keep an eye on visibility trends this 
morning, but currently don't have plans to hoist a Dense Fog 
Advisory with the expectation that mid- and upper-level cloud 
cover will just continue to increase although some isolated spots
of sub-1 mile visibilities will be possible. 

Surface analysis indicates yesterday's very weak frontal boundary 
(partially enhanced a bit by the lake) has stalled out near the I-80 
corridor. Dewpoints subtly increase south of this area, with this 
boundary arcing southwest towards Quincy, Illinois before it 
continues off to the northwest towards the Iowa/Nebraska state line. 
An EML plume has advected atop this boundary, which is characterized 
by very steep mid-level lapse rates nearing 9 C/km in the 700-500 mb 
layer and shows up impressively on the most recent OAK RAOB. 
Moisture channel loops depict a robust shortwave translating into 
western Minnesota and Iowa. Noting an associated increase in the low-
level mass response ahead of this shortwave as a result, with recent 
KOAX VWP data sampling 35 kts of southwesterly 850 mb flow, veering 
to 40 kts at 700 mb. This is a bit more robust than currently 
depicted by recent RAP analyses. Either way, the net result of this 
has been the rapid development of robust convective cores across the 
western half of Iowa as enhanced warm advection has begun to 
intercept the aforementioned steep lapse rate plume aloft. An 
additional area of elevated convection--which initiated a few hours 
ago--is currently drifting into northeastern Iowa. This activity 
should continue to slowly weaken as it encounters a considerable 
pocket of dry air in the 700-500 mb layer, although this will likely 
begin the process of saturating this critical layer. 

As has been case with recent MCS's, it seems as if hi-res guidance 
becomes less and less helpful as we approach the event with recent 
guidance not really capturing the current nature of convection. 
Thus, we're sort of at the mercy of conceptual models and older CAM 
guidance here. Based on the current look of radar and satellite and 
movement of the gradually congealing storms, it does appear as if a 
cohesive MCS will begin to surge towards Illinois over the next 
several hours. Typically the cold pools associated with these MCSs 
tend to elongate along the 700 mb flow in the downshear-propagating 
portion, which should back a bit to the west-northwest through 
daybreak. As a result, have favored a somewhat more northward 
solution here and have painted essentially categorical PoPs across 
the entire CWA through the late-morning hours. Latest thinking 
(favoring a faster solution) has the leading edge of the forward-
propagation portion of the bow approaching I-39 around 9 AM and then 
racing towards the I-57 corridor by 11 AM to noon and then east of 
our CWA by about 2-3 PM.

Effective deep layer shear values do look to decrease with eastward 
extent into our CWA this morning, but 25-30 kts oriented almost 
orthogonal to the incoming line should be sufficient to keep 
updrafts propped up against the gust front resulting in a wind 
damage threat likely through most of the CWA. Currently think the 
main corridor for the strongest winds is roughly near and south of
a Rockford to Aurora to Valparaiso line. Probably will be a 
bookend vortex tracking somewhere through our CWA as well, and 
this will likely focus the greatest wind potential. Think that the
embedded QLCS tornado potential is on the lower side with this 
event due to the weaker low-level flow (and sub-optimal time of 
day) but certainly couldn't rule a spin-up out of the equation. 

We'll probably clear out pretty quickly with this system's
passage. This should allow temperatures--at least over parts of
the area--to rebound into the mid and possibly upper 80. There's a
pretty big bust potential on the high temperatures though if this
system decides to slow down. The other question is if we can fire
new convection later this afternoon. Given the quicker arrival, we
may have sufficient time to destabilize and and get away from the
main subsident regime immediately behind the departing MCS. In 
addition, there's another shortwave/coupled jet near the 
International Border which may deliver a glancing blow of ascent 
to the region during peak heating. CAMs remain unenthusiastic with
re-development potential, but we are a bit concerned that we may 
indeed be able to re-fire near the remnant outflow boundary which
--based purely on a conceptual model at this point--would stretch 
from near a Rockford to Fowler line. The kinematic and 
thermodynamic parameter space would support a severe threat with 
large hail and damaging wind gusts. Also worth mentioning that 
*should* convection fire on any remnant outflow, the flow 
orientation would support training storms with an attendant flash 
flood potential. Low confidence in this occurring and will only 
show 20-30 percent PoPs as a result, but something certainly worth

For tonight, we'll probably see another MCS develop perhaps a bit
farther southwest than the one currently. This may make a run at
our far southwestern counties towards the morning hours. We'll
maintain a stripe of 30% PoPs south of a Dixon to Rensselaer line
to account for this potential. Otherwise, a cold front will
probably push through our CWA during the afternoon hours bringing
slightly cooler and drier conditions to the area into the evening



213 AM CDT

Wednesday night through Monday...

Synoptic front expected to continue sliding south across the 
region at the start of the period, with CAA and dry advection the 
trend. However, some potential for this boundary to hang up around
the southern CWA Wednesday night. If this were to occur, 
additional upstream development to the west during this time could
ride along it and provide some chances for showers and 
thunderstorms. Main instability axis will be to the south, with 
instability lowering across the CWA through the night. So, would 
think that the bulk of any development should stay just to the 
south, but with locations along/south of the Kankakee river valley
possibly observing the northern extent of this development. Will 
be trending drier and cooler into the end of the work week, as 
high pressure builds across the region. With this high in place, 
any precip development into this weekend will stay well 
west/southwest of the area. Below normal temps are expected late 
in the work week into the weekend.



For the 12Z TAFs...

Active pattern expected to bring showers/thunderstorms 
near/across the terminals this morning into midday/early 
afternoon, and potentially again later this afternoon. Continue to
monitor area of showers/thunderstorms currently moving into 
northwest IL, with higher confidence now in place for these storms
to impact the terminals in northern IL and northwest IN this 
morning. Current timing in the TAFs still looks good, and haven't 
made any changes at this time. Did lower vis, as it's appearing 
that some of the stronger development will move overhead. Mostly 
dry conditions then expected later this afternoon through much of 
the end of the period. Will need to monitor later trends, as some 
isolated thunderstorm development could occur this afternoon. Low 
confidence on this possibility at this time. Outside of precip 
development this morning, mainly VFR conditions expected for the 
remainder of the day. Do think there is some potential for lower 
ceilings later tonight into Wednesday morning. Winds will likely 
be highly variable, with the expected precip. A wind shift to more
of a southerly direction is anticipated this morning ahead of the
storms, and think this direction will be the more preferred 
direction through much of today.