Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 161142

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
642 AM 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 12Z TAFs...

The main concern is SHRA/TSRA potential, as early as late this
afternoon, through the overnight and possibly into Thursday
morning. Furthermore, lake influence is likely to increease
easterly winds to around 10 kt at ORD and MDW early this
afternoon. Have included a short VCSH this morning at RFD for a
disturbance producing SHRA activity. Another lead disturbance
currently over MO and downstate IL could spread scattered
SHRA/TSRA north of a warm frontal boundary over the region as soon
as the late afternoon/early evening. Confidence continues to be on
lower side in coverage of this activity and whether it will
contain much TSRA especially eastern terminals, but did include a
VCSH early this evening. Started VCTS for RFD at 23z where greater
instability and potentially less capping could enable for a better
chance of TSRA with the initial lead activity. This period will
certainly need refinement based on trends.

A strong surface low over the upper Mississippi Valley will then
lift the warm front north this evening and better forcing with it
could enable additional rounds of SHRA/TSRA over the region from
the mid-late evening through the overnight. Have maintained VCTS
mention for this period and also PROB30 groups for direct impacts
at the terminals. Primary uncertainty is whether SHRA/TSRA that
form to the west weaken, particularly eastern terminals, given
unfavorable diurnal timing. Increased low level moisture may also
support a lowering of CIGs with the SHRA/TSRA tonight. Winds will
shift to due southerly or south-southwest overnight after the
warm front passage, with gusty southwest winds developing Thursday
morning ahead of the system cold front. Confidence was too low
with this issuance to include additional precip mention beyond 12z
Thursday in the ORD 30-hr TAF.



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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