Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 202126

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

325 PM CST

Through Wednesday...

Main forecast concerns/challenges are with additional rainfall
through tonight with flooding/river flooding, possible strong
thunderstorms in northwest Indiana, as well as a wintry mix of
freezing rain and sleet likely across much of northern Illinois
and northwest Indiana. Have issued a Winter Weather Advisory over
portions of central Illinois for tonight, where confidence is
highest for a longer duration of this wintry mix.

In the near term, showers and thunderstorms continue to move over
much of the region this afternoon as surface low/trough and front
push through. Do still have some concerns for isolated stronger
storms ahead of the front, primarily over northwest Indiana. This
location is still in a warm moist environment, with high dewpoint
air. Its conceivable that some surface based instability is
present and am monitoring a higher wind threat. These storms and
this threat should push east of the area further into northern
Indiana over the next 1-2 hours, as cold/dry advection persists.
Despite cold/dry advection continuing this afternoon, don`t
anticipate any change to the large scale pattern aloft, with
showers and an isolated thunderstorm to likely continue area wide.
Have high confidence of this additional light to at times
moderate rainfall to continue through much of this evening, given
upstream satellite/radar trends. From this point forward, an
additional one to three inches of rain will be likely. Focus does
begin to shift further to the south later this evening, with
highest amounts later this evening likely focusing over the
southern half of the area. This additional rain will likely worsen
the ongoing flooding across the area, that does include flooding
along area rivers/streams. Although flash flooding is still a
possibility and a concern, with the departure of the line of
storms in the near term, do think this threat will lower some.

The trend this afternoon has been for temps to quickly fall post
fropa with some locations observing around twenty degree drops in
temps. This quick fall will likely continue with the initial
passage of the front, with temps then to slowly fall through this
evening. Once again, with colder air pushing in at the surface but
with precip development to likely move overhead tonight concerns
for a wintry mix do increase by early/mid evening. Overall
confidence is still lower with exact amounts and impacts of the
possible freezing rain and sleet, but do think most locations will
see at least a one to three hour window of this wintry mix.
Locations across central Illinois look to be in a place where the
colder air will move in sooner, and the precip development
continues longer. In this location, where my confidence is higher
for a longer window of freezing rain and sleet, did issue a winter
weather advisory. In this location, minor ice amounts would
create hazardous conditions, especially on elevated surfaces.
Elsewhere, once again, freezing rain/sleet is definitely a
possibility but the window of this occurrence could be smaller as
well as the impacts lower. These lower impacts will be tied to
actual surface temps which will need to be monitored. If colder
air moves in quicker and precip lasts longer outside of the
advisory, then will need to monitor for the potential to expand
the advisory. All precip should end by early Wednesday morning.



210 PM CST

Wednesday Night through Monday...

Due to abundance of immediate concerns have left the previous long
term AFD in tact below as it still largely reflects latest
thinking. Parade of subtle systems still on track with occasional
lighter precip amounts and precip type issues. Details still
unclear with weekend system as model guidance differs quite a bit
with placement and strength of features.


Previous long term discussion...

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.

     Winter Weather Advisory...ILZ010-ILZ019 until 3 AM Wednesday.

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

     Winter Weather Advisory...ILZ021-ILZ032-ILZ039...midnight
     Wednesday to 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...IN nearshore waters until 9 PM Tuesday.

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




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