Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 212032

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

1154 AM CST

Midday Update...

Low stratus and fog have eroded quickly from south to north during
the morning, with breezy south winds and mostly sunny skies
helping to warm temperatures to near record levels for late
January. Main adjustments to going forecast have been to decrease
cloud cover (except for far northwest IL where lower clouds will
likely develop slowly eastward this afternoon), and increase
temps based on 925 mb mixing climatology. This places lower 60s
across the southeast half or so of the cwa this afternoon where
best mixing/low-level thermal ridging will be coincident. Record
high for today in Chicago is 63, and it looks like we`ll be just
short of that this afternoon. Otherwise, had updated earlier to
remove dense fog advisory for remaining northern counties.

Updated digital grids/text forecasts already available.



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.



231 PM CST

Sunday night through Saturday...

The main challenges during the period will be with the timing, and
associated local impacts, of a storm system expected to impact
the Lower Great Lakes region by midweek.

Prior to the midweek storm system, a potent storm system will
move over the Ozarks on Sunday, than to the Central Atlantic Coast
on Monday. While this storm system will be too far to our south to
produce much precipitation over the area, a few rain showers will
still be possible on Monday over far eastern Illinois and into
northwestern Indiana. Otherwise, this system will result in a
period of breezy cooler northerly winds Sunday night and Monday.
Expect high temperatures for Monday and Tuesday to be several
degrees cooler (low to mid 40s) than this weekend temperatures.

Attention then turns to what appears to be a quick larger scale
pattern shift towards a +PNA pattern (Upper ridge in the west,
Upper trough in the east) over North America by mid to late week.
As this pattern shift occurs, the eastern Pacific storm system
will be driven eastward over the central CONUS and into the Lower
Great Lakes Region sometime around midweek. While model and
ensemble forecast guidance is in fair agreement with the
evolution of the larger scale pattern supportive of this storm
system, there are still some rather large differences in the
timing and strengthen of an associated surface low. Most notable
has been the faster trend in the GFS solutions over the past 24 to
36 hours. The 12Z GFS now appears to be on the fast side of all
the guidance, with the forecast surface low already shifting
towards Lake Huron by 12z Wednesday, which even outpaces the GEFS
mean. Meanwhile, on the other end of the envelop is the 12Z
Canadian, which has the surface low all the way back over
Northwest Missouri at 12z Wednesday. Finally there`s the 12Z
ECMWF, which seems to fall right in the middle of the pack, with
the low over Northwest Illinois during this time.

All in all these large differences in timing still result in low
confidence with the specifics of this system, especially in terms
of timing a change from rain to snow over the area. Because of
this, we have not made any major changes to the forecast during
this period. If the track of the low does go over far northern
Illinois or southern Wisconsin, the P-type would likely be mainly
rain Tuesday night. The question is than when and how much of the
precipitation could change to snow on the back side of the storm
system late Wednesday or Wednesday night. Also, in addition to
the precipitation, if the surface low ends up on the stronger
side, and tracks just north of the CWA, it could support a period
of warm temperatures followed by strong westerly winds over the

Colder weather and some lingering periods of snow showers or
flurries look to continue during the later portion of the week.



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



For the 18Z TAFs...

Main aviation concerns are for the potential redevelopment of
IFR/LIFR conditions tonight and which could linger through Sunday

Surface low pressure was lifting northeast along the
Iowa/Minnesota border at midday, with a warm front extending east
along the Illinois/Wisconsin border per surface analysis. Low
clouds and fog have given way to VFR conditions with the warm
front now north of the terminals, with breezy south winds and
mainly clear skies to continue through early this evening. The
exception to this will be at KRFD however, as an area of IFR/MVFR
stratus is slowly moving east across eastern Iowa and far
northwest Illinois. This is expected to move back into the KRFD
area by late afternoon with development of MVFR cigs initially on
the eastern periphery of the stratus deck.

Concerns increase this evening as the low over MN fills and allows
the winds to weaken significantly with sunset, with moist low-
levels likely allowing for redevelopment of fog/low stratus and/or
expansion from IA/WI/NW IL overnight in the weak gradient.
Forecast soundings suggest the potential for LIFR cigs in 200-500
ft range late tonight/early Sunday, with only gradual improvement
likely as surface winds shift to the north-northeast in response
to another low pressure system moving through the Ohio Valley.

Confidence in overall trend is medium-high, though timing of how
quickly low stuff develops this evening/tonight is low as is
timing of improvement Sunday.



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.






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