Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 161738

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1238 PM CDT Wed Aug 16 2017

325 AM CDT

Through Thursday...

The main forecast challenges/concerns continue to focus on the
chances and timing of potentially a couple of rounds of showers
and thunderstorms later today through Thursday.

Surface low pressure is taking shape across the Central High
Plains early this morning in response to the increasing
atmospheric dynamics associated with a couple of well defined
short waves shifting across the Central Rockies. The storm system
is expected to track northeastward over the Upper Midwest by
tonight. As it does so, the stationary surface frontal boundary to
our south, currently separating the area from a very moist
airmass (featuring nearly 2" precipitable water values) to the
south, will surge northward over the area later today and this
evening. As a result, expect the chances for showers and
thunderstorms with very heavy rainfall to increase across the
area, especially tonight.

While the best chances for thunderstorms looks to be tonight,
some scattered showers and thunderstorms may end up shifting
northeast across at least southwestern portions of the area this
afternoon. as the warm front, and a lead mid-level disturbance,
shift into the area. Otherwise, expect partly cloudy skies across
the area this morning, with increasing cloudiness this afternoon.
High temperatures should again warm into the middle 80s for most
areas. However, onshore flow will result in cooler conditions
along the lakeshore.

Showers and thunderstorms look to increase in coverage this
evening, especially to the west in eastern Iowa and into western
and north central Illinois, in response to an increasing low-
level jet and upper-level height falls associated with the
approaching main mid-level short wave disturbance. Some of this
activity, possibly in a weakening phase, may then shift eastward
across much of northern Illinois tonight. While there will be a
small risk for a strong storm or two, possible into north central
Illinois during the evening, it appears very heavy rain would be
the main threat with these storms given the near 2" precipitable
water values. Some good news is that it does appear that the
storms will be moving to the northeast at a decent clip, so this
looks to limit the overall amounts of rainfall and likely the
overall threat for significant flash flooding.

Some showers and thunderstorms may be ongoing across portions of
the area early Thursday morning ahead of the approaching cold
front. The cold front is expected to shift eastward across the
area during the late morning and into the afternoon. While new
thunderstorm development will be possible ahead of the front on
Thursday, the likelihood for cloud cover over the area during the
morning may limit destabilization, and thus possibly
significantly reduce the threat of renewed strong convection.
However, better flow aloft is expected to overspread the area
ahead of the front on Thursday. So if storms are able to redevelop
on the front, some of these could be strong. At the present time
the best potential for this would be over my eastern areas in
northwestern Indiana.



340 AM CDT

Friday through Tuesday...

Things should quiet down and cool off for a period on Friday.
However, another mid-level disturbance is expected to quickly dig
southeastward towards the area for late Friday night into at least
early Saturday. As a result, it appears that there could be
another period of showers/storms around the area, but due to the
poor diurnal timing and weaker moisture return no significant rain
or storms are expected. Surface high pressure then looks to
produce another period of quieter weather for Sunday.

For early next week, it appears that the weather could remain
quiet for the eclipse on Monday, with any active weather possibly
remaining to our north until Monday night or Tuesday. Temperatures
do look to warm, possibly well into the mid to upper 80s on
Monday as southerly flow advects in a warmer airmass. It then
appears that another cold front may try to shift southward towards
the area next Tuesday. This looks to result in our next chance for
thunderstorms. Some of these storms could be strong.



For the 18Z TAFs...

Thunderstorm chances and timing make up the primary forecast
challenges today. There appear to be a couple windows of
opportunity for thunderstorms this afternoon through tonight.
First, a warm front lifting across the region this afternoon and
early evening is expected to kick off widely scattered
thunderstorms across northern Illinois and northwest Indiana. Mid
afternoon through early evening seems to be the best likelihood
for seeing thunderstorms at or near the terminals, but for now
expect coverage to remain low enough to preclude any prevailing
mention on station in the TAFs, and will maintain a VCTS in the
meantime. Confidence in specific timing remains low, and a shift
of an hour or two in either direction appears plausible.

A second round of thunderstorms is expected to develop in the
vicinity of the mid Missouri Valley this evening with
thunderstorms tracking east across central and northern Illinois
late this evening and overnight. Hi-resolution models have been
trending towards the best coverage being farther south of the
terminals overnight, so confidence in this second round of
thunderstorms is diminishing. Will maintain a prob30 for now but
would not be surprised if we remove thunder mention overnight if
model trends continue to hold.

A cold front will sweep across the region Thursday with winds
turning westerly and becoming gusty. Gusts are expected to
increase into the mid and potentially high 20kt range during the
afternoon Thursday.



241 AM CDT

After high pressure exits east today, strengthening low pressure
will lift from the central Plains to the northern Lakes by
Thursday evening and then track slowly northeastward through
Friday. This will impact the lake with fairly strong winds.
Southeast winds will increase tonight, with speeds/gusts up to 25
kt after midnight. A warm front will lift north ahead of the low
path on Thursday, with a period of 30 kt speeds/gusts
southeasterlies on the north half through mid day and up to 25 kt
southerlies on the south half.

As the low lifts north of the lake later Thursday through
Thursday night, the trailing cold front will sweep across the lake
and result in winds shifting to brisk west to west-northwest into
Friday. 30 kt speeds/gusts are probable over the open waters.
Pattern recognition of such a strong low just northeast of the
lake suggests that there may be a threat for at least occasional
gale gusts late Thursday night into Friday morning. Periods of
wind speeds hazardous to small craft are probable in the
Thursday-Friday timeframe. Winds will diminish later Friday, with
a lighter wind regime over the weekend due to weak high pressure
over the region.






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