Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 201201
AFDLOT

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

.SHORT TERM...
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.

Ratzer

&&

.LONG TERM...
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.

Ratzer

&&

.AVIATION...
For the 12Z TAFs...

Showers and thunderstorms, low ceilings, and low visibility all
remain concerns throughout the day today with the potential for a
wintry mix as a cold front pushes across the terminals late this
evening into the overnight hours.

CEILINGS & VISIBILITY:
Early this morning, a trough of low pressure stretches from
central Wisconsin across NW Illinois into west-central Illinois.
This corridor is generally the focus of the lowest conditions
early this morning where LIFR conditions are present. As you head
east from that region, conditions gradually improve with
widespread VFR conditions along/south of a PNT-VPZ line where the
gradient is much stronger resulting in gusty southerly winds.
Chicago area terminals generally lie between these two extremes
this morning and think that they should predominately remain MVFR
or better through the remainder of the morning and into the early
afternoon, with only Tempo IFR conditions. Meanwhile, closer to
the trough axis, RFD has the best chance to stay IFR through the
morning. In general, and similar to yesterday, models have been
hitting the low conditions too hard across much of the area and
have backed off some for the rest of today`s forecast.

As we head into the late morning and early afternoon hours, the
trough begins to shift east and expect conditions at the Chicago
area terminals will begin to lower with IFR becoming more likely
as the cold front nears the terminals by early to mid afternoon.
Not terribly confident how quickly conditions will improve behind
the front and it may take some time, but eventually should begin
to scour out some of the low clouds and fog/mist late this evening
and overnight as drier air begins to filter into the region.

PRECIPITATION:
Showers will likely be off and on through the day. Cannot pin down
any likely or lasting breaks in the precip at this point, though
as of 12Z there does seem to be lower coverage upstream that will
be moving over the terminals by around 13Z. There could be a
relative lull with only a spotty shower or two through mid
morning. Instability increases again in the vicinity of the front
by late morning into the early afternoon. Looks like the early
afternoon hours will be the best chance for thunder on station.
Otherwise, cannot rule out a few isolated embedded lightning
strikes, but nothing TAF worthy as far as coverage or likelihood
of occurring on station.

Behind the slow moving front, colder air begins to move into the
region. Guidance has been consistent over the past couple days
indicating a narrow window when precipitation may change over to
freezing rain and/or sleet before ending altogether. Will have to
keep a close eye on temperatures late this evening to get a better
sense of prevailing p-type.

BMD

&&

.MARINE...
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.

BMD

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
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
     Wednesday.

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
     Tuesday.

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

&&

$$

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