Area Forecast Discussion
Issued by NWS Chicago, IL

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285
FXUS63 KLOT 281757
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1257 PM CDT Tue Mar 28 2017

.SHORT TERM...
319 AM CDT

Through tonight...

Main forecast concerns/challenges are with precip chances
returning Wednesday night through Thursday night, with a low
chance of thunder mainly south of I80 Thursday afternoon.

Upper level low and surface low/trough continue to depart to the
east this morning, while high pressure to the northwest shifts
southeast. Any associated precip now well east of the area and
expect dry conditions today, outside of some patchy light drizzle
early this morning. There have been some breaks in the stratus
this morning, but think this will be short lived and isolated with
expansive cloud deck still in place over much of the region. The
northern edge of these clouds will steadily drop south today but
the CWA staying in mostly cloudy conditions into the afternoon.
With this cloud cover in place and with a steady northerly wind,
do anticipate slightly cooler conditions today. Have lowered temps
for most locations today with highs around 50 now expected, but
with low 40s still expected for areas near the lake. Quiet/dry
conditions continue tonight with upper level ridging overhead, and
while surface high remains situated over the region.

Rodriguez

&&

.LONG TERM...
319 AM CDT

Wednesday through Monday...

Upper level system currently over the western CONUS will slowly
slide east into Wednesday, assisting for returning cloud cover
especially Wednesday. With the closed nature of this system,
expect its progression east to be on the slow side but guidance is
advertising some energy to lift northeast across the region on
Wednesday. Guidance developing QPF mainly to the west of the area
on Wednesday but some is bringing this precip into northern
Illinois by Wednesday afternoon. Given the amount of drier air
likely in place, think precip reaching northern Illinois Wednesday
may be a tad too quick. So have limited slight chance pops to
mainly west of the I39 corridor in the afternoon. Higher pops
Wednesday night reflective of increasing large scale ascent moving
overhead, and showers likely spreading into much of northern
Illinois. Instability looks really meager and so have continued to
leave out any thunder. However, strengthening LLJ could help to
offset this lacking instability, with a few rumbles of thunder
trying to reach the southern CWA late Wednesday night not
completely out of the question. With upper level trough, and
attendant low with lifting boundary approaching the area Thursday,
more widespread showers look to be in place for all areas now
including northwest Indiana. Instability axis trying to inch
closer to the southern CWA by Thursday afternoon, as surface
boundary lifts into east central Illinois. Guidance advertising
surface wave riding along this boundary close to the CWA border
and if this were to occur, could put a portion of the CWA in the
warm sector. This could support a little more of a chance for
thunder in the afternoon. Did include slight chance thunder for
areas south of I80 Thursday afternoon, but at this time, any
threat of stronger thunderstorms looks to be to the south of the
CWA. Will continue to monitor this period though. The bulk of the
precip should exit Thursday night, but with the potential for
additional energy remaining overhead, some showers are possible
Friday. Dry conditions then expected Friday night into Saturday.

Rodriguez

&&

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

Ceilings remain the primary forecast challenge and even the short
range guidance continues to have a poor handle on the details of
the trends. The observed trends this morning have been for a very
slow lifting/improvement of cigs, and anticipate that this should
be the trend for the remainder of the afternoon and through the
night. Looking at sounding information, an inversion should remain
in place through the period, which would inhibit vertical mixing
down of drier air aloft, so have maintained the mvfr cigs through
the night for the Chicago area terminals. With high pressure
building across the upper Mississippi Valley and upper Great
Lakes, winds should generally remain nely, so flow will remain off
the lake for the Chicago area terminals, helping to keep the lower
cigs in place. Latest satellite trends indicate some upstream
scattering of cloud cover over srn WI which coincides with the
drier air. So, have improved cigs at RFD to vfr, given the drier
advection. The greatest uncertainty to the forecast will be for
cigs tomorrow morning. With an inversion becoming stronger
overnight, and winds remaining off of the lake, there is a chance
that cigs will not have an opportunity to improve to vfr tomorrow
morning, but confidence is relatively low at this time to keep the
mvfr cigs through the morning hours tomorrow. So, have maintained
an improving trend for cigs for tomorrow, but there is a chance
that cigs will need to be lowered in later updates. Otherwise,
have relatively high confidence in wind trends given the
persistence of the ridge of high pressure moving across the upper
Mississippi Valley and upper Great Lakes while low pressure
deepens over the central plains tomorrow. This pattern would
suggest a trend to more ely winds tomorrow.

&&

.MARINE...
258 AM CDT

Moderate north flow will be in place across Lake Michigan today
behind low pressure departing from the Ohio Valley to the Mid
Atlantic Coast and high pressure in place across the Canadian
Prairies and Upper Midwest. Low pressure will develop over the
southern Plains Tuesday and then lift to Missouri midday Thursday.
Winds over Lake Michigan will veer to the northeast Wednesday and
to the east on Thursday as the low approaches. East winds on
Thursday are expected to be around 30 kt across the south half of
the lake, and there is some potential for a short window of
gales. Winds will back to the northeast as the low continues east
to the Mid Atlantic coast late in the week, but will remain
moderately strong gusting to 30 kt at times into Friday. Winds
should diminish some over the weekend as high pressure builds into
the western Great Lakes.

BMD

&&

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

     Small Craft Advisory...IL nearshore waters until 3 AM Wednesday.

&&

$$

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