Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 191155

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
655 AM CDT Sat May 19 2018

211 AM CDT

Through tonight...

Early this morning, water vapor imagery reveals an upper low over
the lower Ohio Valley with a pinwheeling vort max lifting north
into northern Indiana and northeast Illinois. This is the focus
for showers early this morning with the greatest coverage along
and east of the I-55 corridor. Patchy light rain and drizzle is
occurring farther west, but the heaviest showers will remain
focused across the eastern CWA this morning. Precip chances should
diminish by around mid morning as the upper low shifts east and
shortwave ridging builds overhead this afternoon. This should
allow for the remainder of the daytime hours to be dry. Weak
surface low will move towards the central Great Lakes today
leaving a modest northwest gradient over the CWA. Winds will
remain light enough for a lake breeze to develop this afternoon.
Temperatures will top out in the mid 70s inland, but only in the
low to mid 60s near the lake before winds turn off the lake and
temps hold steady or fall back into the 50s.

Forecast for this evening and overnight remains complex and models
still not in terribly good agreement on how thunderstorms will
evolve. General idea is for strong to severe thunderstorms to
develop across the mid Missouri Valley this evening in response to a
strengthening nocturnal low level jet and grow upscale as they shift
east-southeast late this evening and overnight. Strong instability
will be in place across northern Missouri into central and
downstate Illinois. Strongest thunderstorms should follow the
instability gradient and the greatest forcing along the nose of
the veering LLJ. General consensus from the models is to keep
this severe activity south of the CWA tonight, though some of the
convective allowing models (ARW/NMM) do take these stronger storms
slightly farther north. For now these are northern outliers with
a very low likelihood of panning out.

Meanwhile, farther north, modest low/mid level convergence is
expected to develop across portions of north central and far
northeast Illinois in response to developing low over the plains.
This area may support showers and thunderstorms as a lead shortwave
lifts across the region this evening. The NAM is particularly
bullish developing 1000+ J/kg MUCAPE while the GFS is much more
reserved with only a couple hundred joules. Mid level flow does
increase enough (again most bullish in the NAM solution) for
effective shear to possibly support a more organized storm or two
that is capable of producing large hail. Given the model differences
there isn`t a lot of confidence in severe weather occurring, but
there is general agreement that there will at least be some
showers/thunderstorms in the vicinity of the boundary.



211 AM CDT

Sunday through Friday...

Active weather looks to be in store for portions of the region if
not the local area to close out the weekend and continue into at
least Tuesday. Broad surface high will build across the Great
Lakes Sunday into Monday and gradually spread east over New
England by Tuesday. Through this time frame, several rounds of
convection appear likely for portions of the Great Plains and
Midwest on the southern periphery of the high.

A disorganized mid-level low is progged to lift across the
central Plains on Sunday while a weak surface reflection develops
across portions of northern Missouri during the day and drifts
east into Central Illinois Sunday evening. Thunderstorms are
expected to develop in advance of these features on Sunday. Will
have to closely monitor the position of the warm front which will
likely be influenced by the evolution of convection from Saturday
night. Given the overall weak synoptic setup, it`s quite possible
the effective warm front is shunted much farther south by
tonight`sconvection which will limit the severe threat for the
local CWA on Sunday. Still, the latest GFS indicates moderately
strong MLCAPE of around 1500 J/kg will lift into the southern tier
of counties which coupled with around 40kt of deep layer shear
would result in some threat for severe storms near the warm front.

Expect a large temperature gradient across the CWA if the warm front
is able to lift into the area. Northern portions of the CWA with
northeast flow off of Lake Michigan under the influence of the high
will experience chilly conditions with highs only in the upper 50s
to mid 60s. South of the front, expect temperatures into the 80s.

Aforementioned surface low will move from central Illinois into
Indiana on Monday. Models continue to hint at additional chances for
showers and thunderstorms for the local forecast area near the low,
though wouldn`t be surprised to see this threat also shift south.
Temperatures remain on the cooler side again Monday for much of
the area, topping out 60s and low 70s north with onshore flow
through the day. Far southern counties may again tag 80 depending
on the low track.

Upper ridge will build across the central Plains and Mississippi
Valley Tuesday into Wednesday while at the surface, high pressure
takes hold over the western Great Lakes. Expect primarily dry
weather through this period, though showers and thunderstorms won`t
be too far off to our west. Also expect a gradual warming trend
through midweek with widespread 70s Tuesday eventually reaching
widespread 80s Thursday and Friday. Upper ridge is progged to buckle
late in the week with increased precip chances overspreading the
region once more.



For the 12Z TAFs...

Low ceilings, periodic showers, and reduced vis continue this
morning. Latest radar imagery showing persistent scattered
showers over far northeast IL and northwest IN. Expect this to
mainly impact MDW and GYY for a couple more hours, before this
precip axis shifts to the east. Fog is still in place this morning
and will reduce vis, possibly in the 1-2sm range through mid
morning. Lowest ceilings of 300-500ft should see a slow trend
upward through mid to late morning, with MVFR ceilings expected
by midday. A north northeast is in place this morning but as low
pressure moves through the region, will see winds briefly go more
north. A northwest wind is then expected soon there after, but
will monitor sites near the lake where an additional shift to the
northeast could be observed quicker than forecast later this
afternoon. Have continued previous wind forecast, but will
monitor a possible sooner shift over to the northeast later today.
A front is expected to push south through the area later tonight,
that will increase the northeast winds and bring additional
chances for showers. Although any thunder chances should be south
of the terminals tonight, there is a possibility that some of this
development could track slightly further to the north.



315 AM CDT

Two separate areas of low pressure will continue to move through
the great lakes region. In this pattern, winds have seen a slight
diminishing trend, with speeds mainly staying in the 10 to 20 kt
range for most of the night. The trend for the remainder of the
morning into midday and early afternoon will be for continued
diminishing speeds. This will quickly change late this afternoon
into tonight with the arrival of low pressure to the northwest.
This low and front will shift south down the lake later this
evening into the overnight hours, with strengthening northerly
winds expected over much of the lake. Expect speeds to generally
stay in the 15 to 25 kt range, however, do think there will be
periods of speeds approaching 30 kt at times tonight. Across the
nearshore, this northerly shift and increase is expected late
tonight into early Sunday morning. Although speeds will increase
over the IN nearshore, at this time, it is looking as if the
strongest winds will be observed over IL nearshore. These
increasing winds and likely building waves could bring a period of
hazardous conditions for small craft late tonight into Sunday,
especially across the IL nearshore.






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