Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 221918

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
218 PM CDT Mon May 22 2017

214 PM CDT

Through Tuesday...

Northern Illinois and northwest Indiana experienced mostly sunny
skies a good portions of the day thus far, but cloud cover is
increasing from the west late this afternoon with showers and a
few thunderstorms expected this evening and overnight. Broad upper
low is rotating across portions of the far Upper Midwest and
northern Great Lakes region while axis of mid level height rises
is pivoting across the local area this afternoon helping to
provide the fair weather conditions. Water vapor imagery reveals
several upper level disturbances poised to rotate through the
region as the height rises begin to lift to our northeast. Up
first for this evening, a low amplitude sheared vort max over Iowa
will move east across northern Illinois. Already some showers
showing up on the scope out in advance of this wave, and could see
some uptick in convective activity as we near our diurnal peak.
That said, instability is progged to remain fairly marginal this
afternoon and evening, peaking around 200 J/kg or so owing to weak
mid level lapse rates which should help to limit coverage and
intensity. Deep convection will be a struggle given the modest
forcing so have only a slight chance/low chance mention of thunder
favored over the western half of the CWA where instability is
best and expect coverage of showers to remain on the scattered
side. Most models also indicate that coverage will diminish late
this evening and overnight, though will maintain at least slight
chance PoPs as there could be some festering activity late.

Meanwhile, two stronger upper level disturbances, one moving
south into far northern Minnesota and a second moving east across
South Dakota, are expected to merge over the midwest early
tomorrow with a weak surface reflection developing across far
eastern Iowa and northwestern Illinois. Showers and thunderstorms
are expected to redevelop and continue off and on throughout the
day. Temperatures will be a little cooler tomorrow, topping out in
the mid to upper 60s. An afternoon lake breeze is possible given
the relatively weak surface flow which would further cool down
temperatures along the lake front.



205 PM CDT

Tuesday night through Monday...

The unsettled pattern will continue into Wednesday as deep low
pressure across the corn belt region will shift south to the Ohio
valley and lower Mississippi. Areas of showers will pinwheel
around the periphery of the upper low. The core of the coldest
temperatures aloft will drift to our south and west which would
suggest the thunder potential here is small, but 700 mb low and
just to our east suggests the deformation band of precipitation
associated with this system will get into at least a portion of
our area, with model guidance depicting more reasonable agreement
across the I-57 corridor southeastward across northwest Indiana.
The Kankakee and Iroquois river basins which are somewhat swollen
may be impacted by a period of moderate rain for a time as the
trowal axis extends back into the area and contains a modest PWAT
axis in excess of 1.1" PW. While individual model disagreement
exists, GFS ensemble measure of predictability is fairly high with
this system, but certainly these upper lows can provide fits for
forecasts with minor shifts in track having significant impacts on
the location of the moderate to possibly heavier rainfall, with
the EC and NAM being on opposite envelopes regarding the western
extent of precip.

While some lighter precipitation will linger Wednesday night, the
low will slowly pull east across the Ohio valley. While we get
some weak height rises on the back side of the low which will lead
to column drying and some warming, we maintain a northerly low
level flow which will keep things a bit cooler for Thursday and
its possible we still have some lingering lower level cloudiness
into the day.

Normally in an amplified pattern the big ridge to our west brings
a break from the cool and showery weather. Well we will get a
ridge from the west, but it appears to be dampening as it slides
eastward. We will warm up due to southerly flow but our dry period
may be somewhat short lived thanks to another deep upper level
low currently well off in the Gulf of Alaska that will slide
southeast across the northern Rockies Friday. Energy ahead of the
low will interact with southern stream jet energy leading to
another chance of showers and storms later Friday with unsettled
weather continuing into the holiday weekend with the upper level
low making more headway into the region. Depending on the Friday
system areas north could once again get stuck in cool northeast
flow for the first half of the weekend with a warm front draped
across our southern areas and a big temperature disparity, but
model predictability for the weekend low is not that great.
Therefore the weekend forecast confidence while favorable for
unsettled conditions is still not super high on specifics thus
the broad chances of rain at times.



For the 18Z TAFs...

Breezy southwest winds along with VFR conditions will be in place
through sunset. Primary forecast challenge for today will be
timing and placement of scattered convection from mid/late this
afternoon on. Some festering showers continue to move east across
Iowa early this afternoon and will move across northern Illinois
mid afternoon. Leading edge of this precipitation is expected to
continue to slowly erode, however, modest forcing will develop
starting mid afternoon and very weak instability in the area will
allow for some additional widely scattered shower development.
Models have been honing in on axis of low level convergence that
will serve as the primary focus of better precip coverage which
will stretch from west central Illinois into northern and
northeastern Illinois. As mentioned, instability is fairly weak
today, thus removed any thunder/vicinity thunder mention for the
terminals except for RFD where more marginal instability is

Expect a lull in precipitation overnight, but showers are expected
to increase in coverage again on Tuesday as series of pinwheeling
upper level disturbances rotate through the region. Confidence in
specific timing tomorrow is low, but expect off and on showers
throughout much of the day. Winds may also be a bit tricky
tomorrow with models suggesting a weak surface low developing
over northern Illinois resulting in backed, albeit relatively
light, winds during the afternoon. Ceilings are also expected to
lower to MVFR by early tomorrow morning and persist into the early



205 PM CDT

Breezy southwest winds courtesy of high pressure across
the Ohio valley and Low pressure north of Lake Superior, most
notably across the nearshore waters, will ease as a secondary area
of low pressure across the corn belt will shift to the south and
west of the southern tip of Lake Michigan through Tuesday. Weak
high pressure will build across north central Ontario, which will
allow a weak cold front to shift south across Lake Michigan
beginning tonight across the north half, and eventually across the
south half Tuesday night. The low south of Lake Michigan will
strengthen through mid week which will lead to a several day
period of increasing northerly winds across the entire lake. Gales
appear unlikely though expect winds to pick back up to 25 kt or
briefly higher especially Wednesday, which will build modest
waves across southern Lake Michigan and lead to hazardous
conditions for small craft.



LM...Small Craft Advisory...nearshore waters
     until 9 PM Monday.




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