Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 212121
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
321 PM CST Wed Feb 21 2018

.SHORT TERM...
320 PM CST

Through Thursday...

For the remainder of this afternoon and into the evening, conditions
will be relatively quiet across the region, with only a canopy of
cirrus overhead.  As high pressure builds across the Upper
Mississippi Valley, colder air will continue to filer across the
region.  While the high pressure should continue to build east
across the Great Lakes region, skies should remain mostly cloudy as
winds turn more easterly from off of Lake Michigan.  Broad southwest
flow will remain in place aloft, as an upper high continues to
slowly build east off the south Atlantic coast and longwave troughing
deepens over the western CONUS.  a series of weak shortwaves are
expected to lift through the swly flow aloft, bringing an increased
chance for some precipitation late tonight.  Latest guidance would
suggest that much or all of the pcpn should be confined to the far
southern portions of the CWA, mostly south of the Illinois and
Kankakee Rivers.  With the ongoing river flooding on the Illinois,
Kankakee and Iroquois River, any additional precipitation will only
negatively impact the situation.  At this point, it looks like any
pcpn should be very light.  A greater concern with this
precipitation will be more focused on precipitation type.  While
surface temperatures will be below freezing, some warm advection
aloft could set up thermal profiles that might be conducive to
either light freezing rain or sleet mixing with the snow. Confidence
in the northward extend of the freezing or frozen precipitation is
low at this time, but there is moderate confidence that any wintry
mix pcpn should be of a relatively short duration.

For Thursday, weak upper ridging building across the area should
help allow temperatures to rebound back into the upper 30s to lower
40s and any lingering pcpn tomorrow morning would likely change over
to all rain.  Since precipitation amounts are expected to remain
light, any pcpn should not have an impact on forecast river
stages.

&&

.LONG TERM...
228 PM CST

Thursday night through Wednesday...

Main forecast concerns/challenges are with precip chances
Thursday night into Friday with a small chance of freezing rain
mainly for locations north of I-88 near the IL/WI border. Also
monitoring a potentially more robust system over the weekend that
could bring both a period of winter weather and a period of
showers and thunderstorms.

Initial dry conditions in place should be overcome area wide
during the early evening hours on Thursday ahead of approaching
shortwave energy. With strong WAA continuing Thursday night,
thermal profiles will support all liquid across the entire area.
In this setup, temps should remain just above freezing. However,
it is possible for locations north of I-88 near the IL/WI border
to see temps right around freezing for a time in the evening. With
this possibility, did convey this in the min temp/hourly temp
forecast as well as leaving chance freezing rain for this
location. It is possible though that the higher threat of freezing
rain will be further to the north in WI. Will continue to monitor
this possibility though. Rainfall amounts Thursday night into
Friday should be on the lighter side, under one half inch.

Expect precip to depart quickly Friday morning with high pressure
briefly building across the region through Friday evening.
Quickly turn attention to mid/upper level trough expected to push
through the central CONUS into the weekend. Low pressure expected
to develop over the central Plains Friday night into Saturday and
then lift northeast throughout the day. As this occurs should see
precip develop along and ahead of an associated warm front that
should push into the southern portions of the area late Friday
night into Saturday morning. Some uncertainty as to the placement
of this precip shield but given the potential strength of this
system, there is a decent chance of this precip to at least reach
the southern half of the CWA by Saturday morning. Will need to
monitor this potential as another period of a wintry mix will be
possible across parts of the area, and it is also possible for a
period of all snow with initial cooling/saturation. A lull in the
precip may occur but as this highly amplified trough and deepening
surface low move into the region late Saturday into Saturday
night, additional periods of all rain will be possible. A couple
of items to monitor with this time frame will be with potentially
higher amounts of rainfall, as well as the threat of
thunderstorms. Have not included thunder quite yet, but with the
potential strength of this system and for higher dewpoint air to
be in place, it is appearing that thunder could be possible.

Rodriguez

&&

.AVIATION...
For the 18Z TAFs...

Relatively quiet conditions through tonight and into tomorrow
morning as the rainfall that impacted the region the past couple
days has finally moved well east and south of the terminals.

VFR conditions will likely prevail through early tomorrow
afternoon as high pressure spreads across the Middle Mississippi
Valley and mostly high level cloud covers the region. North winds
will gradually veer to the northeast this afternoon and then
closer to easterly overnight and into tomorrow. Winds should then
maintain an easterly component through remainder of the period. By
late afternoon tomorrow, the latest guidance is suggesting that
mvfr cigs may move into the Chicago area terminals as easterly
flow continues to advect some lake moisture inland.

&&

.MARINE...
417 PM CST

Areas of dense fog over the southern portion of the lake should
clear this evening. Brisk northwest winds up to 30 kt behind cold
front passage this evening will diminish on Wednesday as strong
high pressure overspreads the lake. Speeds will be fastest to
diminish on the northern portion of the lake. On the south part
of the lake, winds will shift to northerly and will be slower to
subside. Hazardous conditions for small craft are expected in the
Indiana nearshore from the pre dawn hours of Wednesday through
Wednesday evening. Depending on how strong winds are when they
shift northerly, it`s possible that a Small Craft Advisory may be
needed for the Illinois shore as well. The next period of
heightened concern for strong winds over the lake will be
primarily over the weekend, particularly Sunday, as a strong low
pressure system may affect the region.

Castro

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory...IN nearshore waters until 9 PM Wednesday.

&&

$$

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