Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 210837
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
237 AM CST Sat Jan 21 2017

.SHORT TERM...
230 AM CST

Through Today...

After 11 of the past 12 days have officially been documented as
cloudy on the Chicago climate summary, a welcome (for many) brief
change will occur today, with scattering of the clouds this
morning. The sunshine with southerly winds and such a balmy
starting point will support afternoon highs at or above 60 along
and east of Interstate 55, which will be near or at daily records.

Another closed upper low this morning over western Iowa,
basically in the same place as its predecessor yesterday morning,
has a broad 996 mb low across the Upper Midwest to Plains states.
This is tapping into an already warm and moist air mass and
drawing it poleward into the area, with a warm front to clear the
northern CWA by mid-morning. Confidence in the warm front
continuing to lift rapidly northward is high, as persistent
observed pressure falls are being observed across Wisconsin and
northern Illinois. The fog is disappearing as winds turn
southerly with the warm frontal passage. The dense fog and some
drizzle after daybreak will likely only be confined to the far
northern Illinois counties and possibly the immediate Illinois
lakeshore. This should dissipate by mid-morning, and some stratus
may lag it just a bit, but do expect most of the CWA to be mostly
sunny/partly cloudy by 9 a.m. While fog may hold on over the lake
due to high dew points over the cold waters, the flow will be
offshore by late morning.

Temperatures at daybreak will be 15 to 20 degrees above normal
highs for this time of year, with the system warm sector only then
moving in. The 00Z NAM and especially GFS were behind on the
925mb temperatures compared to the upstream SGF raob, and the 06Z
NAM has caught on much better. This has 925mb temperatures of 14C
poking into the southern CWA this morning, which is at record
levels for late January compared to the central Illinois upper air
climatology. With 925mb temperatures around 10-11C over Chicago,
that has correlated to highs on average of 57 at ORD in late
January, but this looks like an even better timed synoptic
pattern, so certainly could be several degrees warmer. Have highs
ranging from the mid 50s toward Rockford, where cloud cover may
hang on/fester the longest, to lower to mid 60s southeast. For
more on records, see the climate discussion below.

MTF

&&

.LONG TERM...
259 PM CST

Saturday night through Friday...

An active weather pattern is expected at times during the period,
with several chances for precipitation. The main concerns
will be the threat for additional periods of fog Saturday night
into Sunday, as well as a small chance of thunderstorms over my
southeast Saturday night. Attention will then focus on what
appears to be the beginnings towards a turn in the direction to
colder conditions, along with periods of snow showers across the
area by the middle to end of next week.

At the start of the period (Saturday night), an upper level
disturbance is expected to lift northeastward over the Upper Great
Lakes, while a stronger Pacific system begins to shift eastward
across the Ozarks. The northern system will likely push a weak
cold front over the area during the night, and this could act as
the focus for a few showers and thunderstorms, mainly for areas
east of the I-55 corridor. Otherwise, it appears that surface
winds will weaken over the area overnight as a weak surface
gradient sets up over the area. This could end up setting the
stage for yet another night of fog across the region.

Late Sunday through Monday, the surface flow is expected to
become northerly across the region in response to the stout
southern CONUS storm system, which should be approaching the
Central Atlantic area by late Monday. Precipitation chances over
the area look low during this period, with the main precipitation
with this system likely to remain south and east of the area.
Temperatures will cool off into the lower 40s for Monday, but
this is still a good 10 degrees above average for this time of
year.

Model and ensemble forecast guidance continues to advertise the
transition to a colder and active weather pattern by the middle of
the week. As this transition occurs, it appears that another
storm system will develop over over the Plains, then shift
eastwards towards the Lower Great Lakes Region by midweek. The
current forecast track of this system (which could still change),
may take it across southern Wisconsin, which would favor northern
Illinois residing in the warm sector of the storm system into
Wednesday morning. As a result, rain would be the primary P-type
into Wednesday. However, thereafter colder air will be shifting
over the area, and this should result to a change over to snow at
some point by Wednesday night. Periods of snow showers and colder
weather will be the main story for the later half of the week.

KJB

&&

.CLIMATE...
230 AM CST

High temperatures will approach or possibly reach daily record
levels across the southern and eastern forecast area, and it is
possible Chicago reaches their daily record.

Record highs for today, January 20th:

Chicago...62 (1906)
Rockford...60 (1906)

The last 60 degree day in Chicago in January was January 29, 2013
(63).

MTF

&&

.AVIATION...
For the 06Z TAFs...

Main forecast concerns remain vis/cigs through Saturday morning
and again Saturday night into Sunday morning.

Clearing across eastern MO and southern IL is steadily lifting
northeast and models have this clearing lifting across the
terminals Saturday morning and trends will need to be monitored as
this may occur faster than currently advertised. In addition...the
low visibilities have improved at many locations and lower cigs
have scattered out at mdw/gyy. Confidence if lower cigs/vis return
to mdw/gyy is low but some guidance still suggesting it may return
before sunrise. Even if it does...the trend will be improvement
after sunrise...from south to north and likely to vfr by late
morning or early afternoon.

Dense fog is still likely at rfd and possible at dpa and
maintained 1/4sm vis at both locations...but trends here will need
to be monitored and its possible that vis continues its slow
improvement and does not drop back down by morning. Although cigs
will improve through ifr/mvfr Saturday morning...mvfr cigs may
remain over portions of far northern IL including rfd/dpa/ord.

Southeasterly winds will turn more southerly by morning as a warm
front moves across the terminals. Forecast soundings still suggest
south/southwest winds gusting to 20kts from late Saturday
afternoon through sunset. Winds will then turn light southerly
Saturday evening...become variable for a time and then turn
northeasterly early Sunday morning. As winds turn to the northeast
expect both low cigs/vis to redevelop later Saturday evening into
Saturday night. Confidence for this is medium to high but still
some uncertainty on how low and timing. cms

&&

.MARINE...
230 AM CST

Dense fog continues over the lake early this morning, although
improvement has been seen across the far south and should continue
through the morning as a warm front lifts north. With offshore
flow developing across both the Illinois and Indiana nearshore
waters, the fog should be kept away from immediate shore areas.
Given the high dew points over the water, there is likelihood of
dense fog persisting away from the eastern and southern shores.

Southerly winds today will eventually turn northerly across the
south half on Sunday. With a fairly strong low to the south of the
region, the north winds should increase quite a bit, and Small
Craft Advisory conditions are looking more likely along the
Illinois and Indiana nearshores later Sunday into Monday.

MTF

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Dense Fog Advisory...ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-
     ILZ011-ILZ012-ILZ013-ILZ014 UNTIL 9 AM Saturday.

IN...NONE.
LM...Dense Fog Advisory...NEARSHORE WATERS
     UNTIL 10 AM Saturday.

     Dense Fog Advisory...LMZ777-LMZ779 UNTIL 9 AM Saturday.

&&

$$

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