Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 250524

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1224 AM CDT THU AUG 25 2016


834 PM CDT

No big changes planned at this time to overall forecast philosophy
for tonight. Will be tweaking pop/wx grids shortly to account for
small scale forward propagating convective cluster over eastern
Iowa. RAP analysis suggests a pocket of dryer more stable air is
in place over northern IL attm, likely reflected in part by the
lack of cloudiness/Cu late in the afternoon in the subsident
region behind the strong afternoon shortwave that has moved well
off to the east. The 00Z ILX and DVN soundings both sampled a
moist and quite buoyant airmass, which the RAP suggests should
replace the bubble of drier/more stable air as a modest
southwesterly low level nocturnal wind max develops.

In the near term, will need to monitor the small scale bow western
of DVN for a localized wind damage threat as KDVN WSR 88D shows a
well defined strong rear inflow jet developing. Given the high
dewpoints and already small sfc T/Td spreads, it seems unlikely
that a significant or quick stabilization of the boundary layer
will take place this evening. Given the organized nature of this
small line and apparent MCV, some wind damage threat could
conceivably exist as this system continued accelerating east into
our western CWA.

Of potentially greater concern is the heavy rainfall and flash
flood potential that could eventually develop tonight. Additional
convection is developing farther upstream over northwest Missouri
and it seems plausible that additional development could take
place this evening over southern IA or northern MO before taking
aim on portions of our CWA overnight. Confidence in exactly where
heaviest rain threat will develop is still low, so not planning
any changes to ongoing flash flood watch at this time. However, as
convection evolves over the next few hours, if Mother Nature tips
her hand a bit, its possible I may extend the flash flood watch
farther east into probably central/southern portions of our CWA.

Latest short range guidance does suggest precip threat could end
faster tomorrow morning than our current grids suggest, but that
may be something the midnight shift could better address with
perhaps just some small tweaks this evening.

Updated products and grids are all forthcoming.



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 MCV 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 MCV 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 IL/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
IL 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.



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, SPC 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 CWA (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 CWA. 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.



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 CWA. 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.



For the 06Z TAFs...

A period or two of thunderstorms are still expected overnight,
though it appears the better chances will be over the southern
terminals and points south.

An area of thunderstorms is currently moving towards the Chicago
area terminals. However, the most intense portion of this storm is
moving east along the I80 corridor. Therefore, it appears the KORD
may only get a brief shower with this activity hear over the next
hour. KMDW and KGYY have a better chance of getting a short period
of heavier rain and thunder with this initial line of storms.

Following this first line of storms, additional showers and
thunderstorms to the southwest over west central Illinois are
expected to track east-northeastward into portions of northern
Illinois. These storms will likely impact the terminals after 8Z
through around or shortly after daybreak today. After this second
wave of activity moves out of the area by 13z it appears that only
some widely scattered showers/storms could develop along the cold
front later this morning into the early afternoon. However,
confidence on these storms impacting the terminals is too low to
include more than a VCSH at this time.



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.



IL...Flash Flood Watch...ILZ010-ILZ019-ILZ020-ILZ021-ILZ032 UNTIL 10
     AM Thursday.




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