Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 230237

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

937 PM CDT

There will isolated to scattered showers/sprinkles around into
the overnight, though with plenty of dry time. A short-wave/speed
max noted on W/V imagery near KS/MO border looks to bring an
increase in shower activity spreading in from west central
Illinois and points northeast by Tuesday morning, though there is
uncertainty on timing this comparing recent runs of HRRR/RAP and
00z NAM. Thunderstorms have been confined to the north and
northwest of the area in an axis of around 500 j/kg MUCAPE per SPC
mesoanalysis. This looks to be the case for the remainder of the
night. Overnight lows will be in the mid to upper 50s.



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 00Z TAFs...

Thunderstorms along the IL/WI state line will remain north of the
terminals. Isolated showers may form ahead of the larger area of
showers over east central IL. Showers are expected to increase in
coverage this evening, and thinking the showers will mainly impact
MDW and GYY. Kept a VCSH in for ORD and DPA through 08Z in case
showers form further north than I`m expecting. Some guidance
members feature another surge of showers late tonight/early
Tuesday morning, but have low confidence in the showers occurring
so kept the TAFs dry.

Cigs become MVFR as showers redevelop ahead of the next upper
level wave mid Tuesday morning. Periods of showers are expected
through Tuesday evening. I have high confidence in showers
occurring, but medium-low confidence in exactly when as there
will be breaks in showers throughout the period. Thunderstorms are
also possible Tuesday aftn, but only had enough confidence to
include VCTS at RFD at this time. Thunderstorm chances decrease in
the evening.

Winds back to northeast by Tuesday evening with speeds less than
10 kt. Expecting MVFR cigs to lift to VFR in the late
afternoon/evening as well.



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.






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