Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 262018
AFDLOT

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

.SHORT TERM...
309 PM CDT

Through Thursday...

Main concern for the short term forecast period will focus on heavy
rainfall associated with showers and thunderstorms moving into the
region by late this afternoon or early evening and continuing
through the night.

Latest guidance and observed trends indicate that much of the
remainder of the afternoon should be dry.  While vis satellite
imagery indicates an area of towering cu developing in the warm
sector air not covered by layered mid and high cloud, there is no
indication of pcpn development on radar and SPC meso analysis still
indicates a small amount of CIN where MU CAPE levels are 500 j/kg or
higher.  Sfc dewpoints are slowly creeping up into the upper 50s and
expect dewpoints to climb further to around 60 F, especially for
locations east of the I-55 corridor.  Latest pwat analysis are the
region is around 1.2 inches and model guidance suggests that pwats
should increase to 1.5 inches or slightly higher overnight. Ongoing
convection is still well south of the CWA, in the St. Louis area and
spreading east into central IL.  Latest guidance suggests that the
convective activity will gradually build northward, possibly
reaching the Pontiac area by 6:00 - 7:00 PM and then continuing to
spread north and east overnight.  While the models had been
advertising that the main long wave trough aloft would be fairly
progressive, the latest guidance suggests that the progression of
the upper long wave and associated sfc front should be relatively
slow.  While there is still some severe thunderstorm threat tonight,
currently feel that heavy rainfall and the potential for flooding
with be the greater threat.  Expecting widespread rainfall amounts
of 1.5 to 2.0 inches storm total QPF from late this afternoon
through early tomorrow afternoon.  Locally higher amounts will be
possible invof stronger storms.  With upper level winds remaining
sly through the night, with a 50 kt low level jet right on top of
there area, some echo training will also be possible.

The models remain relatively consistent in lifting the system newd
away from the region, with some wrap-around rain possibly lingering
through late morning or early afternoon tomorrow.  Following the
passage of the associated cold front, temperatures will be much
lower tomorrow, with highs only in the lower to middle 50s or even
some upper 40s close to the WI border.

&&

.LONG TERM...
317 PM CDT

Thursday night through Wednesday...

Very wet and at times stormy conditions expected over the weekend
for northern Illinois and northwest Indiana resulting in renewed
flooding concerns.

A broad upper trough that covers the western 2/3 of the country is
expected to become increasingly amplified late in the week through
the weekend a strong shortwave digs across the Intermountain West
and into the Desert Southwest over the weekend. A wide corridor
of deep southerly flow will develop from the Gulf of Mexico into
the Midwest transporting high theta-e air into the region. PWats
start to increase to around an inch during the day Friday as a
lead low amplitude shortwave trough moves across the region. This
will support at least a chance of showers though instability at
this point isn`t impressive with only an off chances for a
thunderstorm or two. Meanwhile, as the strong shortwave rounds the
base of the longwave trough Saturday, cyclogenesis will occur
with a surface low lifting from the Southern Plains into the mid
Mississippi Valley Saturday night. A 50kt low level jet out ahead
of the low will send another surge of moisture into the local area
with PWats further increasing to around 1.5-1.7 inches Saturday
night while a corridor of strong moisture convergence on the nose
of the low level jet slowly lifts across northern Illinois and
northwest Indiana and serves as the focus for potentially strong
thunderstorms and heavy rainfall. Both the GFS and ECMWF paint a
corridor of 2-3 inches 24 hour QPF through midday Sunday which
raises concerns for renewed rises on areas streams and rivers,
especially given a potential round of heavy rainfall tonight and
additional rain Friday. Guidance still needs to work out some of
the fine details on the axis of heaviest rainfall, but those with
hydro interests or in flood prone areas should stay in tune with
forecast updates over the next few days.

Low pressure begins to occlude over the Upper Midwest Sunday
evening and night while the dry slot wraps across northern
Illinois and northwest Indiana. Still have some potential to see
some additional light rain or drizzle within the low level
cyclonic flow which persists into Monday. High pressure is
expected to build across portions of the Great Lakes by midweek
and should provide a couple days of respite from the
precipitation.

Deubelbeiss

&&

.AVIATION...
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 temperatures
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
evening.

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 initiation 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.

&&

.MARINE...
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
Monday.

BMD

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory...nearshore waters
     until 11 PM Wednesday.

&&

$$

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