Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 210525

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1125 PM CST Tue Feb 20 2018

844 PM CST

Main update this evening was to expand the winter weather advisory
into the Chicago area and into portions of northwestern Indiana
for tonight.

We have a fairly classic Ana-front precipitation event underway
across the region. Strong deep southerly flow aloft continues to
transport plenty of moisture into the region atop the surface cold
front, which is now shifting across portions of northern Indiana.
Meanwhile, the area resides within the right entrance region of a
170+ kt upper level jet. This in combination with the presence of
a band of mid-level frontogenesis, associated with the frontal
boundary aloft, is likely to keep precipitation going for several
more hours. This is mainly true across portions of northeastern
IL, and into northwestern Indiana. Farther north and west towards
the Rockford area, it appears the threat for more precipitation
tonight has come to an end.

Given temperatures are quickly approaching the freezing mark (now
33 at KARR) it appears that the rain will transition over to
freezing rain into portions of the Chicago area within the next
couple hours. While the warm temperatures today may keep ice from
initially accumulating on area roads, it does appear that
elevated surfaces, including overpasses and bridges could ice up
a bit later tonight. While this will not be a big ice event for
the area, the increasing threat for a several hour period of
freezing rain and sleet, we felt it prudent to go ahead and expand
the advisory for most of the Chicago area tonight. It does appear
that the highest precipitation amounts tonight will fall from
Chicago southward. Up to a tenth of an inch of ice is possible on
elevated surfaces, and possibly some minor sleet accumulation as
well. Things should quite down from northwest to southeast towards
Wednesday morning.



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 06Z TAFs...

Primary forecast concern remains freezing rain and mixed wintry
precip through the early morning hours.

The back edge of the precip extends roughly along a vys/dkb/ugn
line but is expected to only slowly move east through the early
morning hours as another wave of precip lifts northeast. As colder
air continues to spread underneath this precip...the light rain is
expected to transition to light freezing rain for a 2-3 hour
period and may end with a mix of sleet or light snow. Precip will
gradually end from northwest to southeast during the predawn hours
but end times may need to some tweaking. Once this precip ends...
the rest of the period is expected to be dry.

Generally low mvfr cigs are expected while precip is falling
though some patchy ifr is possible. Mvfr cigs are beginning to
scatter out across eastern IA so once precip ends...any mvfr cigs
likely to only last a few hours before scattering out.

Gusty northwest winds overnight will turn more north/northwest
after sunrise...then northerly by late morning and finally to the
north/northeast Wednesday afternoon. Prevailing speeds 10-12kts
for much of Wednesday. cms


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.



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

     Winter Weather Advisory...ILZ033...2 AM Wednesday to 9 AM

     Winter Weather Advisory...ILZ010-ILZ011-ILZ012-ILZ019-ILZ020-
     ILZ021-ILZ032 until 3 AM Wednesday.

     Winter Weather Advisory...ILZ006-ILZ013-ILZ014-ILZ022-ILZ023-
     ILZ039 until 6 AM Wednesday.

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

     Winter Weather Advisory...INZ001-INZ002-INZ010-INZ011...2 AM
     Wednesday to 9 AM Wednesday.

LM...Small Craft Advisory...LMZ743-LMZ744-LMZ745...3 AM Wednesday to
     9 PM Wednesday.




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