Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 270554

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1254 AM CDT Sat May 27 2017


The final showers, along with a couple elevated storms, should
depart the eastern CWA shortly. Attention for the rest of tonight
then turns to fog potential. Apart from the present cloud cover,
it`s a fairly good setup given earlier rain, low temperature/dew
point spreads, and light winds. GOES-16 short wave infrared
channel is indicating some holes in the cloud cover. Difficult to
say how long any will persist or how they will evolve, but it may
not take any lengthy clearing for at least shallow fog to develop.
Patchy fog has been added to the forecast for most outlying
areas. While climatologically not favored in late May, cannot rule
out some pockets of dense shallow fog, maybe most favored in the
southern CWA.

Given the aforementioned uncertainty with cloud cover, it is
possible that Saturday morning starts mostly cloudy, or with fog,
that should erode quickly given the shallow nature of the low-
level moisture.



300 PM CDT

Tonight through Saturday...

For near term convective/severe thunderstorm trends, check for the
latest Mesoscale Discussions above. The weather will quiet down
tonight with the effective warm front shunted well south behind
the MCV. Showers and thunderstorms should exit quickly by the
early to mid evening hours. This will set the stage for a quiet
and pleasant start to the Memorial Day weekend, at least for the
daylight period. Strong heating will take place away from lake
influence on Saturday with plenty of sun followed by increasing
mid and high clouds during the afternoon. Low level thermal progs
support highs in the mid 70s to around 80 away from the lakeshore
and few miles inland, which will be kept in the 60s.



300 PM CDT

Saturday night through Friday...

The effective warm front will gradually lift north in advance of a
likely robust MCS, but mainly stay confined to the southern CWA or
points south. There is uncertainty on timing and trajectory of
this MCS, with the an increasing likelihood for most significant
impacts to remain well south of I-80 or even south of the CWA
counties. Portions of the area are likely to remain for outdoor
activities during the evening hours with likely PoPs for most
widespread shower and thunder coverage after midnight (and highest
thunder coverage along/south of I-80). Deep layer shear and MUCAPE
may be sufficient to support an isolated hail threat and perhaps
localized strong to damaging winds.

A surface low will take shape and drag a cold front across the
area on Sunday with perhaps an isolated risk for gusty to damaging
thunderstorm winds and marginally severe hail southeast of I-55.
There is plenty of uncertainty with the convective scenario on
Sunday and much of the afternoon could certainly be dry northwest
of I-55 or I-57. Expecting breezy west-northwest winds in the
afternoon behind the front, with only modest cool advection so
highs should reach the lower to mid 70s.

Memorial Day Monday could be fairly similar to Sunday temperature
wise. Main feature of note is a large upper low over the northern
Great Lakes, from which some isolated diurnally driven showers
and possibly thunderstorms could form during the afternoon. Again,
most of the day should be dry, so no need to cancel outdoor
plans- just keep an eye on the forecast and the radar. The upper
level low pressure will only slowly slide east through the week,
keeping area under northwest flow aloft and generally slightly
below normal temperatures for the end of May and beginning of
June. Can`t completely rule out any shower activity in the mid to
late week, but it looks to be an overall drier regime.



For the 06Z TAFs...

Concerns are on existing MVFR and IFR clouds, and then on
IFR/LIFR clouds and or fog expanding once the lingering clouds
erode. The later periods will then need to address showers and
embedded storms.

An area of lower clouds has begun to erode from the northwest, but
may slow its progression in the overnight hours and likely expand
now that we are in the overnight hours with moist conditions and
light northeast winds under the ridge. In addition where clearing
has occurred, dewpoint depressions are low and not expected to
climb, thus IFR or even LIFR clouds will likely return and/or low
to very low visibilities. Confidence is not high on how low and
how long conditions stay down but at least occasional reductions
if not longer and the pattern is favorable for low conditions of
some sort for many areas to start the day. What does form may be
initially slow to clear with the light profile, but then just
expect east winds with a lake breeze likely.

An area of showers and thunderstorms across the central and
southern plains will lift northeast tonight. Expect south of the
main terminals where the more active thunderstorms will be given a
significant gradient in instability from south to north, but
expect a period of showers and embedded thunder area wide as the
forcing with the wave is decent. More details with this in the
future TAFs and just hinted at for now in ORD 30 hr. General timing
is medium-high confidence.



227 PM CDT

Will keep the marine fog advisory going as web cams indicate
haze/fog remains over the open waters.

A high pressure ridge over the lake will continue east. A weak
surface low over the plains will pass south of the lake tonight and
continue across the Ohio Valley Saturday. The low may cause wind
speeds to increase to 10-20 kt over the far southern end of the lake
this evening. One model would suggest gusts up to 25 kt, but do not
have enough confidence to go that high.  High pressure passes over
the lake early Saturday morning and light winds will vary in
direction across the lake.  Variable winds are expected over the
southern end and southerly winds over the northern end.

The next low forms over the plains Saturday and reaches the Great
Lakes region Sunday morning.  The low moves over Michigan Sunday
afternoon and winds become west 10-20 kt behind the low.  The low
over Michigan merges with another Canadian low to the north over
Ontario Sunday night. The low is forecast to remain relatively
stationary over Ontario early next week which may lead to a an
extended period of westerly winds of 15-25 kt. High pressure moves
in behind the low mid to late next week.






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