Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Chicago/Romeoville, Illinois
347 PM CDT Wednesday Aug 24 2016

Short term...
233 PM CDT

Through tonight...

For the remainder of the afternoon and into early evening,
conditions across the region will be generally quiet. However, an
old mesoscale convective vortex is moving across nrn IL, generating some isold shra. While
lightning data currently does not show any activity, there is a
chance that any of these showers could develop enough to produce
some lightning. Also, with a very moist environment, with pwats up
to 2.2 inches, these showers will produce brief, heavy downpours
with up to 1.5 inches of rain in 30 min or less. Also, the mesoscale convective vortex is
enhancing low level cloud cover that has held down temps in areas
that have not seen much sunshine today. But, for areas with breaks
in the overcast,M temps have been observed to shoot up into the
lower 80s.

With sunset, expect that the isold/sct shra activity will diminish
and focus will shift to the next round of tsra/shra, which will
also have the potential to produce locally heavy rainfall. The
high res and larger scale model guidance is generally coming into
decent agreement on tracking a strong short wave out of the cntrl
plains this afternoon, through the middle Mississippi Valley and
and across nrn Illinois/srn WI late tonight. The larger scale
environment will be primed for additional ts development late
tonight with strong mid-level swly flow continuing to stream a
plume of very moist air into the region. Complicating matters is a
weak cold front moving through the upper Mississippi Valley this
afternoon. There is weak push behind the front, but progression
should be relatively steady, with the boundary pushing into nwrn
Illinois durg the early morning hours. This will add sfc focus to the
upper level focus associated with the short wave crossing the
area. Expect that periods of heavy downpours will accompany the
thunderstorms late tonight into early tomorrow morning.

For the above reasons, Flash Flood Watch is being issued for Lee,
LaSalle, Kendall, Grundy and Livingston counties from late tonight
through mid Thursday morning (1am-10am). Trends will need to be
monitored this evening for flash flooding potential farther east.

Krein/rc

&&

Long term...
335 PM CDT

Thursday through Wednesday...

Stout short wave that will be responsible for the flooding threat
overnight into the early morning will be centered over lower
Michigan by mid morning, ending the threat for heavy rain. Main
uncertainty is on timing the back edge of the widespread showers
and thunderstorms and how much if any of the morning commute in
Chicago is affected. The overnight and morning activity should
force the effective cold front farther southeast and keep the
better threat for redevelopment of scattered showers and
thunderstorms mainly to areas along and east of I-55 in the
afternoon. Confined mid to high chance pops to these areas. As
such, Storm Prediction Center did shift the marginal risk area eastward on the updated
day 2 outlook. Nonetheless, there may be a small window for an
isolated strong to severe thunderstorm threat in the southeast 1/3
of the County Warning Area (east of I-57) in the afternoon prior to cold front
passage. With low level winds veering, main threat if there are
any stronger storms would be gusty winds and perhaps hail as deep
shear will be adequate. Expecting highs of about 80-85.

Still expecting a dry period Thursday night through Friday as high
pressure spreads across the region, keeping warm
front/quasi-stationary front to our south. However, with
disturbed southwest mid-level flow overhead, there could be
fairly widespread mid and high cloud debris from convection to
our southwest, especially southern half of the County Warning Area. We'll have a
brief respite from the humid conditions as well with highs in the
upper 70s to around 80. The Lake Shore will be kept slightly
cooler by persistent onshore winds.

High pressure will quickly scoot east Friday night with a surface
trough or weak low pressure approaching and beginning to lift warm
front north. Latest guidance has sped up timing of shower and
thunderstorm potential as short wave moves overhead. Confidence is
low on evolution, but may need to increase pops overnight and then
Saturday now looking like a decent threat for showers and
thunderstorms (at least morning through mid-day), so did increase
pops from previous forecast. Next surface high will build in
behind cold front Saturday night into Sunday, shifting better
shower and thunderstorm potential south. It could very well be
primarily dry later Sunday through Wednesday. Temperatures appear
they will be seasonably warm in the 80s, except cooler at times
near the shore with lake breeze occurrence.

Rc

&&

Hydrology...

233 PM CDT

A very moist in is place across the upper Midwest as the region
has seen a significant return of deep layer moisture. Sfc
dewpoints have returned into the lower 70s coincident with a
corridor of pwats in excess of 2 inches ahead of a long wave
trough dropping into the upper Missouri Valley. Many factors are
coming together that will be conducive to heavy rainfall across
much of the County Warning Area. Current guidance tracks a strong shortwave from
the cntrl plains through the middle Mississippi Valley late
tonight and into early tomorrow morning. The 12z dvn and ilx
soundings both indicate pwats at or slightly in excess of 2
inches. Various larger scale and high res models all indicate
slightly in excess of 2 inch pwats at 12z Thursday morning, but
suspect that even these numbers may be a little on the lower side
considering the extended stream of deep layer moisture into the
region. Also, even the weaker, isolated showers have shown 3 to 4
inch per hour rainfall rates while observations of 1.0 to 1.5
inches of rainfall in less than 30 minute periods. However, pcpn
is likely to be more widespread late tonight into tomorrow
morning, so heavy rainfall will likely not be as isolated, with
flash flooding or areal flooding becoming an increasing concern.

Krein

&&

Aviation...
for the 18z tafs...

The main concern continues to focus on thunderstorms and rain timing, tonight.

Following the passage of a mid level impulse, much of the pcpn has
moved east of the terminals and cigs at the terminals have
improved to MVFR, though a few pockets of IFR cigs persist across
the region. An old circulation generated by thunderstorms over the
Central Plains is moving across nrn Illinois this afternoon per the
latest radar imagery. This system is generating some scattered
showers and a few thunderstorms, but the chances for any of these
storms to impact the terminals is low. However, given the presence
of this system, will maintain a mention of vcsh through the
remainder of the afternoon.

Of greater concern will be another round of thunderstorms
overnight tonight. Expect that the activity will lift from
southwest to northeast across the region, reaching the terminals
shortly after midnight. Given some uncertainty of the timing of
onset, have only gone with prevailing shra and a tempo tsra, but
expect that prevailing ts will be required at some point durg the
early morning hours. Some extension of the ts chances is possible
as a cold front drops out of the upper MS valley tomorrow
afternoon. The models don't have a great handle on the timing or
coverage of ts with the front, so have gone with an optimistic
forecast for now, but updates will likely be needed to add in some
more pcpn tomorrow.

&&

Marine...
347 PM CDT

Surface frontal trough crossing the lake will diminish previously
strong south-southwest winds and cause lingering waves to subside
tonight. Winds will be lighter out of the west on Thursday,
shifting southwest late in the day ahead of a secondary cold
front. This front will result in a period of northwest winds of up
to 20 kt Thursday night, possibly briefly to 25 kt on the southern
portion of the lake. Winds will quickly diminish Friday as high
pressure spreads across the lake and then shift back to moderate
southerly on Saturday ahead of the next low pressure trough
approaching from the west. A cold front trailing from this low
will sweep across the lake late Saturday night through Sunday
morning, shifting winds to west and northwest. High pressure will
spread back across the region Sunday night into Monday.

Rc

&&

Lot watches/warnings/advisories...
Illinois...Flash Flood Watch...ilz010-ilz019-ilz020-ilz021-ilz032...1 am
Thursday to 10 am Thursday.

In...none.
Lm...none.
&&

$$

Visit US at http://weather.Gov/Chicago (all lowercase)

Follow US on facebook...twitter...and youtube at:
www.Facebook.Com/nwschicago
www.Twitter.Com/nwschicago
www.Youtube.Com/nwschicago

National Weather Service Glossary of Abbreviations