Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 201807

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1207 PM CST Tue Feb 20 2018


1049 AM CST

Several concerns this morning through this afternoon and into
tonight remain that include: patchy dense fog over northern IL,
heavy rainfall with flooding/river flooding, additional showers
and thunderstorms this afternoon/evening, along with the potential
for a wintry mix tonight.

One main concern to address in the near term was patchy dense fog
which is in place for mainly areas along/north of I-88. Lowest
vis under this dense fog will be around one quarter mile. Did
issue an SPS to address this hazard and think this should suffice
as the arrival of rain to the west should help vis improve here
within the next hour.

Did extend the areal flood warning through tonight and early
Wednesday morning. Flooding is occurring over much of northern
Illinois and northwest Indiana from the snow melt and the two to
four inches of rain that fell over the last 24 hours. With
additional rainfall of one to three inches, with locally higher
amounts, expected with the arrival of the showers and
thunderstorms to the west/southwest, feel that any flooding will
likely worsen early this afternoon into tonight. Will also need
to monitor the possibility for flash flooding during this time, as
more convective elements will likely be in place. Despite some
slight increase in instability today, don`t have any concerns for
widespread severe weather. Weak low/mid level lapse rates and the
lack of any surface based instability should limit this threat.
However, do think a stronger storm could be possible with small
hail and gusty winds possible.

Continuing to assess the possibility of a wintry mix later this
evening/tonight. Surface front will push through the area tonight,
with surface temps cooling. However, no change to the pattern
aloft will support continued precip development and the likelihood
for mainly liquid. This will provide freezing rain concerns, that
I am continuing to assess. Will have more on this with later
AFD/forecast update today.



306 AM CST

Today through Wednesday...

Heavy rainfall and flooding concerns remain the primary forecast
concern through tonight, along with the potential for a period of
freezing rain/sleet late this evening through the overnight hours
as colder air behind the surface cold front undercuts the elevated
baroclinic zone.

Surface low pressure was analyzed over northeastern Iowa early
this morning, with a baggy warm frontal trough extending across
far southern WI, and a nearly stationary cold front trailing into
central Missouri and the Southern Plains. An unseasonably warm and
moist warm sector air mass was across most of IL and all of IN,
with temps in the upper 50`s to mid 60`s, and dew points of 55-60
F. An extensive area of moderate to heavy rain and embedded
thunderstorms persisted across the forecast area, in response to
strong moisture convergence and transport on 45-55 kt low level
jet. Heavy rain and embedded thunder is expected to continue to
persist through the day, as forcing and moisture remain plentiful
in advance of the slow moving surface cold front. With time, the
axis of the heaviest rainfall should shift to the east/southeast
through later this afternoon/evening as the front begins to push
across the forecast area. Radar rainfall estimates indicate 1.5+
inches have already fallen across a large portion of the forecast
area, with some 2-2.50" amounts in spots. Flooding has already
begun to become an issue in a few places, and area rivers and
creeks are showing significant rises. Obviously, no changes
planned to the flood watch which is in effect through tonight.

As the front pushes east tonight, precipitation will continue to
shift slowly east across the forecast area, and is expected to
clear all but the farthest southeast counties by Wednesday
morning. As colder air moves in behind the front however, there is
some concern for a period of freezing rain/sleet which may
produce some ice accumulation or glazing from mid-evening out west
through the overnight hours farther to the east. Confidence is
fairly low in the details of this however, as some guidance such
as the GFS depicts precip ending fairly quickly as the colder air
arrives, and rapid drying in the lower and middle levels. The NAM
is most bullish in maintaining precipitation into the cooling low
levels, but also eventually drys things out quickly afterwards.
Have generally used a blended guidance approach with thermal
profiles/precip end timing based on the uncertainty, and do have
some light ice accumulations overnight. Will need to continue to
monitor this closely later today, when guidance will hopefully
have a better handle on the fine details for later tonight.

Precipitation should move out of the southeastern counties of the
cwa by late morning Wednesday, with high pressure spreading
across the Upper Midwest to our north. While clouds may linger
across the southern parts of the forecast area, closer to the
frontal zone, at least partial sunshine is expected across far
northern IL.



306 AM CST

Wednesday Night through Monday...

Large scale pattern of a long wave trough in the west and an
upper ridge off the southeast Atlantic coast persists through most
of the extended period. While the front which moves through early
in the day Wednesday settles south of the area, southwest flow
aloft will persist aloft, with several small-amplitude
disturbances potentially brining a few additional rounds of light-
moderate precip to the area into the weekend.

Surface high pressure slides north of the region Wednesday night,
while the cold front sags south of the Ohio River. The front ends
up becoming nearly stationary south of our area as it parallels
the upper flow, but the upper jet remains in place across the
upper Midwest. Guidance is in fair agreement in depicting a small
amplitude short wave rippling through the flow aloft, which
briefly veers 850 mb flow to the south within the northern
periphery of the elevated baroclinic zone. While the NAM and GFS
keep precipitation out of the forecast area through Thursday
morning, the ECMWF, SREF and particularly the GEM do develop some
light QPF across the south half of the cwa after midnight. Will
generally limit this to a slight chance mention late Wednesday
night into Thursday morning, given relatively dry profiles
depicted in the NAM and GFS forecast soundings. Precipitation type
forecast is of low confidence, as 850 mb temps are just above 0C
across the southern cwa at that time, though dry air below would
potentially support wet-bulb cooling for a snow/sleet mix.

Next low-amplitude short wave is progged to track across the area
Thursday night into Friday. Southerly flow develops again above
the frontal inversion, with modest moisture transport and
isentropic upglide providing ascent/saturation for another round
of light to moderate precipitation. Thickness values increase
enough during the night for much of this precipitation to be
liquid, except for far northern IL counties near the Wisconsin
border. Surface temps look to be more marginal there however,
where precipitation could begin as freezing rain before changing
over to rain Friday morning. Wave moves off to the east of the
area during the afternoon, with rain ending from the west.

Models are in decent agreement in eventually kicking western
CONUS trough eastward by the weekend, with the more-amplified
upper trough inducing surface low development across the Southern
Plains. This low then tracks to northern IL through Saturday
night, with some differences between the global models as to the
strength of the system. In any case, renewed forcing for ascent
develops across the forecast area Friday night into Saturday as
the old stalled front to our south/southwest lifts north as a warm
front. GFS, strongest of the solutions as previously noted,
brings a swath of 0.75" QPF across the southern parts of the cwa,
which would likely be largely in the form of rain. Again, far
northern IL closer to the WI border could see a period of mixed
precip Friday night before low level temperatures warm and turn
everything over to rain during the day Saturday. The cold front
moves through early Sunday, with colder low level air potentially
supporting a brief period of mixed precip before things dry out.



For the 18Z TAFs...

Multiple aviation concerns through tonight. The next wave of SHRA
and TSRA is currently overspreading the area, with the window of
occasional/temporary TSRA impacts through 21-23z. A cold front
will sweep across the area during that time, with SHRA continuing
into the evening but TSRA likely focusing southeast of the
terminals. As colder air moves into the area behind the front and
yet another wave of precipitation expands northeast into the area,
concern continues for a period of freezing rain to sleet this
evening into overnight. The timing of temperatures dropping to and
below freezing is uncertain, especially for the Chicago
terminals, as well as how long the precipitation lingers while
temperatures are at or below freezing. Confidence is medium in
precipitation continuing through 8-10z at Chicago area terminals,
latest at GYY and earliest at ORD/DPA, but confidence in precip
type trends is lower. For now, have a timing of 5-8z at ORD and
7-9z at MDW for frozen precipitation. RFD looks to be on northwest
fringe of precip and could potentially be missed by -FZRAPL, but
did maintain mention in TAF.

Exact CIG and VSBY trends into tonight are lower confidence,
though a period of IFR CIGs still looks likely this afternoon into
this evening for the eastern terminals where CIGs are currently
MVFR, with VSBY depending on precip intensity. Expecting VFR
conditions to return on Wednesday. Winds will be gusty south-
southwest ahead of the cold front and shift to gusty northwest
behind it. As strong high pressure spreads into the area on
Wednesday, winds will shift to north-northeast 10-15 kt during
the afternoon.



301 AM CST

An active weather pattern will continue across the Great Lakes
through the remainder of the week and into the weekend. An
elongated trough bisects Lake Michigan early this morning with
moderate east to northeast flow in place across northern Lake
Michigan north of the trough axis while south to southwest flow is
in place across the south end of the lake. Flow is more variable
in the mid section of the lake near the trough axis. Have the GLF
segmented into three portions to account for the differing flow
across the lake. The southerly flow has helped to transport
unusually high dewpoints into the region resulting in foggy
conditions across the south half of the lake. Have also hoisted a
dense fog advisory for southern portions of Lake Michigan until
the cold front comes through this evening.

A low is analyzed within the trough axis over southeast Wisconsin
early this morning and will meander northward across Lake
Michigan through the day today. This evening and tonight the low
will lift across Ontario into Quebec with trailing cold front
finally clearing the entire lake leaving north to northwest flow.
A cooler more seasonable air mass moves back into the region
behind the front as a strong high builds into the upper midwest.
Northwest winds will top out at around 30 kt tonight then should
diminish through the day Wednesday as the high approaches. Another
relatively weak low is expected to lift across the region Friday
followed by a stronger low over the weekend.



IL...Flood Watch...ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-ILZ011-
     ILZ012 until 6 PM Tuesday.

     Flood Watch...ILZ013-ILZ014-ILZ019-ILZ020-ILZ021-ILZ022-ILZ023-
     ILZ032-ILZ033-ILZ039 until 6 AM Wednesday.

IN...Flood Watch...INZ001-INZ002-INZ010-INZ011-INZ019 until 6 AM

LM...Dense Fog Advisory...LMZ740-LMZ741-LMZ742-LMZ777-LMZ779 until 9
     PM Tuesday.

     Dense Fog Advisory...LMZ743-LMZ744-LMZ745...6 PM Tuesday to 9 PM

     Small Craft Advisory...IN nearshore waters until 6 PM Tuesday.




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