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Area forecast discussion
National Weather Service Chicago/Romeoville, Illinois
319 am CDT Tue Aug 20 2019

Short term...
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 radiosonde observation.
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 mesoscale convective system 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 County Warning Area 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 County Warning Area by about 2-3 PM.

Effective deep layer shear values do look to decrease with eastward
extent into our County Warning Area 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 County Warning Area. 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 County Warning Area 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 mesoscale convective system. 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
watching.

For tonight, we'll probably see another mesoscale convective system 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 County Warning Area during the afternoon hours bringing
slightly cooler and drier conditions to the area into the evening
hours.

Carlaw

&&

Long term...
213 am CDT

Wednesday night through Monday...

Synoptic front expected to continue sliding south across the
region at the start of the period, with cold air advection and dry advection the
trend. However, some potential for this boundary to hang up around
the southern County Warning Area 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 County Warning Area 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.

Rodriguez

&&

Aviation...
for the 06z tafs...

Active pattern expected to bring showers/thunderstorms
near/across the terminals later this morning into midday, and
potentially again later this afternoon. Monitoring area of
showers/thunderstorms currently developing across
northwest/northern Iowa at this time, as these storms will likely
impact the area this morning. Expect further development over the
next several hours into more of an organized complex of storms,
which will then track east southeast. At this time, it still
appears that that the bulk of this development will stay just to
the south of the terminals. Have continued current timing of -shra
with vcts with this in mind, however, it is still possible that
slightly more thunder could be observed. Monitoring the potential
to include at least a tempo for tsra, and am monitoring trends to
see if a slightly sooner arrival time will be needed. 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
and some fog development this morning, mainly VFR conditions
expected. 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 the period.

Rodriguez

&&

Lot watches/warnings/advisories...
Illinois...none.
In...none.
Lm...none.
&&

$$

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