Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 190739

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
139 AM CST Sun Feb 19 2017

918 PM CST

Main update this evening was to increase cloud cover and include
areas of Fog over the far southern CWA, mainly along and south of
a line from Pontiac IL to Rensselaer IN. Short term guidance has
been indicating the possibility for low stratus and fog, already
present over central IL and IN, to spread northward overnight into
early Sunday morning. Although confidence is low with trends
tonight, several factors in place which could support expansion
and northward movement of this stratus/fog. Boundary which has
sagged south over the region is lingering in place, while
southerly flow with mid/upper 40s dewpoints are in place ahead
of it. So did increase cloud cover to at least make mention of
partly cloudy skies, and have include areas of fog. Will need to
monitor trends, as a more concern to keep an eye on will be for
dense fog. If this were to move into the CWA tonight, it will
likely persist through mid morning Sunday.



245 PM CST

Through Sunday...

Impressive, record-breaking warmth for mid-February continues,
with top five all time February temperatures having been
reached/tied today at Chicago and Rockford, including the second
earliest 70 on record for Chicago. A lake breeze has developed but
is hugging the lake shore as anticipated, with downtown Chicago
presently 51 degrees. Wind speeds will be diminishing fairly
quickly this evening enabling quicker radiational cooling than
last night, but still a plenty mild evening for February.

After lows in the mid to upper 30s, another quick rebound will
occur on Sunday. Have gone above all guidance for non-lake
adjacent areas with 925mb temperatures still near-record at 10C-
11C. Have highs near daily record levels again with mid to upper
60s for areas not near the lake.

For the lakeside counties, Sunday looks to bring a more
traditional lake breeze, which adds a challenge to the forecast
and reduces certainty in this area. The lake breeze should
progress through the lake adjacent counties by early to mid
afternoon given experience with a surface high directly over the
area and a stout temperature difference between land and marine
layers. For immediate lakeside communities including downtown
Chicago, temperatures are favored to remain in the 40s. For areas
near O`Hare and Midway, we presently have the lake breeze pushing
through in the early to mid afternoon, likely bringing a quick 10
degree drop or so after attaining forecast highs in the lower to
mid 60s.



205 PM CST

Sunday night through Saturday...

The high amplitude upper level pattern will become more progressive
by Sunday night as the axis of the upper ridges moves across the
upper midwest with longwave troughing moving across the Rockies and
over the western plains.  Swly flow will persist though Monday as a
cold front pushes across the plains.  Unseasonably warm conditions
will persist through much of next week.  Low pressure is still
expected to drop out of the northern rockies Sunday night and lift
northeast across the northern plains and into south central Canada
Monday.  As the low continues to track northeast, an associated cold
front will push across the region Monday night into early Tuesday
morning.  The longer range guidance has trended toward the slower
solution indicated by previous ECMWF runs and the the general model
consensus is for the the frontal passage to occur a little later
than previously anticipated.  However, there is little change to the
going forecast as far as pcpn and temperature trends, with a swath
of showers pushing into the Rockford area late Monday afternoon or
early evening and then steadily spreading east Monday night and
Tuesday.  There should be sufficient forcing and instability along
and ahead of the front to carry at least a slight chance for
thunderstorms.  However, the strongest forcing will be to the north
of the wisconsin border while the strongest instability should be to
the south, across the lower Mississippi and Ohio Valleys. Therefore,
confidence is low in the coverage and timing of thunderstorms
impacting the region.  Highs in the 60s should continue through at
least mid week.  There is some potential for lake cooling impacting
the max temps along the lake front, keeping temps close to the lake
a little lower.

Temperature trends will change little through early next week with
highs in the lower to middle 60s across much of the area.  Another
southern stream system is expected to lift across the central plains
Wednesday night and Thursday.  The longer range guidance is in good
agreement on the development and track of the system, there are
characteristic timing differences between the GFS and ECMWF in the
speed of the system.  The GFS is the faster solution, which is
fairly typical, while the ECMWF is trending 12-18 hours slower.
Following the passage of this system, temperatures should trend back
to closer to seasonal normal levels by late next week with lows in
the middle 20s to around 30 F and highs in the middle to upper 30s.



245 PM CST

A highly anomalous warm stretch that has already broke daily
records on Friday February 17th and Saturday February 18th will
continue, with potential for more breaking of records. Here are
daily records.

Chicago:                         Rockford:
      High:      High Min:       High:      High Min:
2/19: 65 (1930)  51 (1994)       63 (1930)  42 (1994)
2/20: 64 (1930)  49 (1930)       61 (1983)  46 (1930)
2/21: 67 (1930)  47 (1930)       64 (1930)  42 (1930)

For more, including February warmth on consecutive days, see our
web top news headline or Public Information Statement.


For the 06Z TAFs...

Aviation concerns remain few into Sunday as weak high pressure
drifts east across the region. Light/variable winds (NNW if any
particular direction) overnight will eventually turn to the east-
southeast Sunday morning as the surface ridge moves east. Light
easterly gradient and another unseasonably warm day will support
an easterly lake breeze by early afternoon, moving into ORD/MDW by
mid-late afternoon. Winds are expected to remain light east or
southeast Sunday night.

VRF conditions are expected to prevail into Sunday night, with
only occasional patchy high clouds. South of the terminals (PNT-
IKK-OXI southward), IFR/LIFR fog and low stratus is possible late
tonight into Sunday morning, though this should remain well south
of the northern IL/IN terminals. Gradual increase in low level
moisture late Sunday night/toward Monday morning may result in
development of some haze/fog toward the very end of ORD`s current
30 hour TAF.



138 AM CST

The main periods of interest through the forecast period will be
Monday and Monday night, then later in the week as a series of
storm systems impact the region.

A surface ridge of high pressure will build over the lake later
this morning, and this will result in a period of light and
variable winds, before becoming east-southeasterly by tonight.
Southeasterly winds are expected to increase markable Monday
afternoon and night as potent area of low pressure shifts
northward into South Central Canada. While the airmass over the
lake will be quite mild and stable, wind speeds of 25 to 30 KT
appear likely by Monday evening as the pressure gradient
strengthens ahead of an approaching cold front. The cold front is
expected to shift over the lake by early Tuesday morning,
resulting in a period of lighter westerly winds on Tuesday.

By mid to late week, the weather pattern is expected to become
active over the region again, with at least two weather systems
expected to have an impact on the Great Lakes Region. The first
looks to move over the region by Wednesday evening. Ahead of this
low, southerly winds will likely increase for a period Wednesday,
then a another cold frontal passage Wednesday night will result in
northerly winds over the lake by early Thursday. Finally, an even
more impressive storm system could impact the region Thursday
night and Friday. This second storm system could be a strong low
that tracks northeastward from the Plains to the Great Lakes by
Friday afternoon/evening. Given the potential strength of this
storm system, a period of east-northeasterly gales will be
possible into Friday, followed by the potential for high end
west-northwesterly gales (~45 KT) late Friday into Saturday.
While there is uncertainty in the timing and track of this storm
system, this is certainly a storm system to keep an eye on in the
coming days.






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