Area Forecast Discussion
Issued by NWS Chicago, IL

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351
FXUS63 KLOT 160826
AFDLOT

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

.SHORT TERM...
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.

KJB

&&

.LONG TERM...
310 PM CDT

Wednesday night through Tuesday...

Emphasis in this portion of the forecast was mainly placed on
thunderstorm with heavy rain potential on Wednesday night.
Additional but low chances of storms are present at the start of
the weekend.

Two well-defined short wave troughs will be moving east across
the upper and central Plains during Wednesday and are agreed upon
in guidance to phase on Wednesday night into Thursday over the
Upper Midwest. In response, strong upper forcing for ascent and a
deep moisture plume will overlap in the region on Wednesday
evening into overnight. Would expect to see showers and storms
increase in coverage fairly quickly during the evening. While
storms should be moving within an increasing wind field, there is
potential for fairly widespread coverage and redevelopment which
could pose excessive rainfall concerns. Moisture magnitudes of
around 2" are observed upstream (1.98" at FWD this morning) and
forecast by model guidance for Wednesday night. Will continue to
note heavy rain potential in the HWO and Weather Story.

In terms of severe threat, that remains fairly low but non-zero.
The highest amount of instability in the warm sector, driving by
low-level dew points in the 70s, will arrive Thursday. So on
Wednesday night instability would likely be a limiting factor,
though a widely scattered strong to severe storm or two is
possible. On Thursday, as the system advection patterns
strengthen, immediately ahead of the cold front in a muggy air
mass should be a decent focus for at least scattered storms. This
includes northern Illinois in the morning hours, and then in the
afternoon for mainly northwest Indiana. The latter would have some
chance for scattered severe storms with mainly a wind threat.

Beyond, few changes made to what blended guidance provided. The
cold front will not provide much for a temperature drop, with
highs on Friday still around or just a bit above 80. Another
strong short wave is forecast to move through Friday night into
early Saturday. This will not have anywhere near the moisture
return than its predecessor, and its currently forecast poor time
of day passing would limit any high impact weather threat. Have a
chance of storms noted in the forecast at this time. There should
then be a period of quiet weather lasting the rest of the weekend
as a short wave upper ridge moves over.

MTF

&&

.AVIATION...
For the 06Z TAFs...

Aside from patchy ground fog/BR outside of Chicago this morning,
the main concern is SHRA/TSRA potential, particularly later this
evening through the overnight. Furthermore, lake influence is
likely to shift southeast winds to easterly around 10 kt at ORD
and MDW early this afternoon. After any BR early, conditions will
likely be VFR much if not all of the day. A lead upper
disturbance could spread scattered SHRA/TSRA north of a warm
frontal boundary over the region as soon as the late
afternoon/early evening. However, confidence is low in coverage of
this activity and whether it makes it up to the terminals, so
have left mention out of the TAFs.

A strong surface low over the upper Mississippi Valley will lift
the warm front north this evening and better forcing with it could
enable multiple rounds of SHRA/TSRA over the region from the mid-
late evening through the overnight. Have added VCTS mention for
this period and also PROB30 groups for direct impacts at the
terminals in RFD TAF and ORD 30-hour TAF. 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.

Castro

&&

.MARINE...
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.

Castro

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.
&&

$$

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