Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 261953

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
253 PM CDT Wed Apr 26 2017

245 AM CDT

Through Thursday...

A bit more active day is expected today across the region with a
series of upper level lows spinning across the plains with heavy
rain a considerable concern, along with some risk for severe
weather especially later today/this evening.

The presence of altocumulus castellanus clouds Tuesday afternoon
is well understood looking at the DVN/ILX 0z soundings. While
these are still very dry downstairs they depict the presence of
the elevated mixed layer with 700-500 mb lapse rates very steep,
8-9 deg/km but very high based above 700 mb. Instability therefore
was pretty small, but weak shortwave energy was able to tap into
this to trigger a few storms across northwest Indiana Tuesday
evening. The SGF sounding, indicative of the airmass to some of
the area headed our direction, depicts 1000 J/KG of cape and near
50 kt of deep layer shear. This coupled with -20C height of about
21K ft suggests that there are several thunderstorms concerns
later today.

Water vapor satellite depicts a fairly strong wave across
Oklahoma with some weaker waves ahead of it, and these will
continue northeast through north central Illinois this morning
where the best t-storm threat would be. Current showers to our
west are in a minimum of mid level lapse rates and thus no
thunder. RAP forecasts of instability creep up as this stronger
wave well southwest shifts in the coming hours, but does not get
too high with this wave. The airmass is still on the drier side
with this lead wave, so expect a decent cut off to the eastern
extent of thunderstorms this morning, but expect there could
certainly be some high based showers farther east as well. There
could be some hail with the stronger storms, but the main storms
should be through before instability increases to the levels of
severe concerns. Expect some weakening as the low level jet
wanes through the morning with a downtick likely.

More significant height falls will occur this afternoon as the
upper lows spread eastward, the upper level jet will also undergo
some strengthening indicating a system getting stronger, and
providing good forcing for ascent ahead of an advancing cold
front. This forcing will come atop and increasingly unstable
airmass mainly due to advection, but it will limited farther
north and especially west of I-55 due likely significant cloud
cover which could keep convection coverage less initially. Plus,
there are some weaker lapse rates aloft that will creep in from
the west across Illinois this afternoon and some stability with
the approaching cold front which will mitigate the severe threat
for areas northwest, but these do look to recover some later. Still will
need to watch any leading cells that remain discrete as they could
pose a localized severe threat.

The moisture transport kicks into a bit of higher gear tonight as
instability is waning, though instability will get nudged up some
at least before late evening, especially along the I-55 and
eastward corridor. This is the period where any line/bowing
segments lifting up from likely more active convection to the
south pose a more significant severe concern, as when this occurs
the low level shear really increases. The low level shear/helicity
values get fairly large as well. Instability will weaken later
this evening as we trend to a heavy rain threat as soundings
depict a very deep and saturated profile with PW values from
1.3-1.5" for several hours but little much less instability. There
will also be a very strong frontogenesis with the encroaching
cold front that it could really dump rain for a few hours. This
may end up for many areas being a more significant concern than
the severe threat. Overnight the upper low will still shift
through with some lingering fgen, so in spite of the cold front
stabilizing things overnight, modest rains could still fall late
into early Thursday in a band farther west, with some showers
underneath the low itself that could still have a little kick.
Fortunately the dynamic cooling does not look like enough to get
over to a snow profile (had to find a way to sneak snow into this
AFD) as hinted at by a NAMBufr sounding near RFD early Thursday.

The moisture axis and low will shift north in the afternoon ending
precip as much colder and drier air filters back in from the



245 AM CDT

Thursday through Monday...

The concern for the weekend will be heavy rainfall and periods of
thunderstorms. The lowest GEFS ensemble member from 0z suite has
2.25 inches of QPF for ORD with Wednesdays and the weekend
precipitation, so this certainly indicates flooding concerns
exist, including a re- aggravation of rivers that have just
recently subsided.

The northern stream upper low will linger across the northern
Great Lakes Friday with a massive longwave trough enveloping much
of the country, but there is a small ridge axis that will setup
across the upper midwest ahead of developing low pressure out
west. Southwest flow ahead of the developing low will send waves
of precipitation through at times until this upper low really
gains some strength and eventually shifts northeastward through
our region. The first wave appears to be on Friday with a leading
warm advective wing. Precipitation could be initially light, but
NAM/GFS project a strong and strengthening upper jet with a
northward moving warm front to lead to some better rains, though
the locations are a bit hairy to project and could split us a bit
north and south. At this point thunder looks like a minimal

The northward moving front gets shoved south by the high pressure
area mentioned earlier. This will keep the area locked in a
cooler northeast wind pattern and could keep precipitation on the
lighter side on Saturday. Confidence is not high though as the EC
is still somewhat wet Saturday afternoon, but is possible the high
to our north keeps the better activity south.

The big issues appear to come Saturday night into Sunday. This
period holds the highest concern for very wet conditions and
the aforementioned flood concerns with significant warm advection
induced precip beginning Saturday night, which could the wettest
period, but a lot to still iron out here. It will also be windy
at times, with thunderstorms re-entering the picture as well.
More to come on this once we get through Wednesday`s active



For the 18Z TAFs...

Forecast concerns include thunderstorm potential this evening,
wind trends through the period and cigs overnight tonight through
tomorrow morning.

Pcpn potential has greatly diminished over the area following a
batch of shra/tsra moving through nwrn/ncntrl IL earlier this
morning. At issuance time, the airmass over the region has
slightly stabilized and any ongoing convection is well south of
the area. For the remainder of the afternoon, expect conditions to
remain dry, though some isold shra could develop as temperautres
rise into the middle 70s with dewpoints in the upper 50s. Winds
will be the primary concern for the afternoon, with swly winds
gusting to arnd 25 kt. Expect winds to remain gusty well into the

Pcpn chances will be on the rise into early evening as a wave
along a cold front moving through the region ripples northward and
warm/moist advection sets up a broad region of isentropic lift.
This should set up the environment for initialtion of shra/tsra.
Expect coverage and intensity to increase overnight, and have
highlights the 03-07z timeframe with a higher confidence of
impactful thunderstorms at the terminals. Expect that will trend
toward prevailing tsra as timing becomes more certain. Certainly,
there will likely be pcpn across the region tonight and into early
tomorrow morning, but not confident enough to carry an overly long
period of thunder at the terminals at this time. As persistent
pcpn continues through the night, expect that cigs should
eventually lower to ifr into the morning hours tomorrow. Pcpn
should eventually lift northeast of the region, with pcpn ending
durg the late morning hours. At that point, winds should veer to
more westerly with gusts arnd 25 kt through much of the day.


251 PM CDT

A very active period of weather is expected across the Great Lakes
region through the weekend and into next week as a series of lows
lift across the region. An elongated trough that extends along
much of the Mississippi Valley this afternoon will shift east and
consolidate into a low that lifts north across Lake Michigan
tonight and eventually to James Bay early Friday. An area of
weaker flow (col area between the departing low and new low over
the Southern Plains) will lift north across Lake Michigan Friday
into Friday evening. North to northeast winds increase again
Friday night into Saturday as another low lifts from the Southern
Plains into the eastern Great Lakes. Yet another and deeper low is
expected to develop over the lower Mississippi Valley early
Sunday and lift north to Lake Michigan early Monday. Gales appear
possible out ahead and behind this low early Sunday though



LM...Small Craft Advisory...nearshore waters
     until 11 PM Wednesday.




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