Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 211815

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1215 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.



242 AM CST

Saturday Night through Friday...

A high amplitude, quickly progressive pattern will bring an active
weather pattern through the long term forecast period, with periodic
chances for precipitation.

Unseasonably warm, moist conditions will continue into the long
term forecast period. Temperatures Sunday morning will start out
above the normal high temps for late January with lows ranging
from the middle 30s over the Rockford area to the low 40s south if
the IL/Kankakee rivers, which is 5 to 10 degrees higher than the
normal highs. While max temps Sunday won`t be quite as high as
Saturday, highs will still range from the middle 40s to lower 50s,
about 15 degrees above normal. with the warm, moist air mass in
place, patchy fog is likely through the night. For Sunday, one
deep upper low will be crossing the lower Mississippi Valley while
another moves to the western Great Lakes. An inverted trough
from low pressure to the south will be the focus for some rain as
moisture continues to stream north in advance of this strong low
to the south. Chances for rain will linger through Sunday night
and into Monday morning, The deep low to the south will lift into
the mid atlantic region as upper ridging builds across the plains
into the middle MS valley. The upper ridge will steadily move east
as a low drops through the Pacific Northwest and moves out over
the plains Tuesday night into Wednesday. Unseasonably mild
conditions will persist until this system passes of to the east
and broad long wave troughing develops over the cntrl CONUS. This
will signal a change to the pattern with a transition back to more
seasonable conditions. The longer range guidance is in
relatively good agreement on the evolution of the longwave
pattern, though there are differences in the timing of nrn stream
short wave energy dropping out of Canada. While these differences
may impact timing of individual pcpn events for the middle to
latter portions of next week, the general signal is relatively
clear. Wednesday should be a transitional day with highs in the
low 40s. A strong nrn stream shortwave will bring another wave of
pcpn to the area from tuesday night into Wednesday night. Some
colder air filtering in as the system lifts away from the area my
allow pcpn to transition to a rain/snow mix. Pcpn amounts with
this system will also be a cause for concern as area rivers are
already running high from recent rainfall and ice jam flooding.
The pcpn from Tuesday night through Wednesday will likely lead to
additional rises on area rivers and streams with additional river
flooding possible. Broad upper troughing will persist through the
remainder of the period, into next weekend. By this point, the
pattern will become less progressive with increasing amplitude as
upper riding builds over the west coast and broad troughing
persists east of the Rockies. This will bring the return to more
seasonable temperatures by Thursday and into the weaken with highs
generally in the lower 30s. The steady stream of nrn stream
shortwaves dropping out of Canada will keep periodic chances for
pcpn in the forecast through late next week, though the
predominant p-type should be snow rather than rain. While there
will be some chances for snow, no particularly strong system is
being advertised by any of the longer range models, so do not
expect any sgfnt snowfall for the period.


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.



LM...Dense Fog Advisory...LMZ740-LMZ777 UNTIL NOON Saturday.




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