Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 200401

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1001 PM CST Sun Feb 19 2017

952 PM CST

Low stratus and dense fog are concerns once again tonight into
Monday morning, and have adjusted sky cover and weather to reflect
current trends. This stratus and fog are currently situated over
central IL, and do think this will spread north into the CWA later
tonight, likely in the 6Z-9Z time frame. Given the boundary layer
flow and moisture, the western half of the CWA would be the
favored location to observe this stratus/fog. However, with
complicating factors such as high cloud cover and lower confidence
to the northward extent of this dense fog in place, have held off
issuing any headlines. Will continue to monitor trends for the
more dense fog over the next couple of hours, but could see most
locations observing at least patchy fog by late tonight into early
Monday morning. As was observed today, should see improving
trends with any fog by mid to late morning on Monday.



215 PM CST

Through Monday...

Another unseasonably warm day today with record setting high temps
at ORD and RFD. A lake breeze pushing quickly inland will put the
brakes on much more warming this afternoon for far nern IL and
nwrn IN, though inland locations could still see a couple more
degrees of warming this afternoon under sunny skies. With mostly
clear skies likely into the overnight hours, radiative cooling
should be rather quick, but with sfc dewpoints on an upward trend,
already in the lower 40s over much of the area and expected to
increase through the night, more widespread fog development is
likely tonight, with some sheltered, low-lying areas possibly
seeing some dense fog. Overnight lows should be in the middle to
upper 40s, which is still 10 to 15 degrees higher than seasonably
normal high temps for this time of year. Tomorrow should be
another unseasonably warm day. After any morning fog burns off,
expect that temperatures should reach into the middle 60s to
around 70F across much of the area. This would be sufficient to
break records again at ORD and RFD. However, with strengthening
SSELY winds tomorrow as sfc ridging moves off to the east and a
sfc trough and associated cold front push east across the plains,
any lake cooling will likely be confined to the IL shore from
downtown Chicago northward...where temps should top out in the
middle 50s.

As the cold front tracks ewd overnight and tomorrow, increasing
cloud cover is expected to build in from west to east. Chances for
pcpn ahead of the front should be on the increase through the
afternoon. There should be adequate instability and forcing along
and just in advance of the front to carry slight chance TS with
the pcpn, though coverage of any TS is still a question mark.


340 PM CST

Monday night through Sunday...

The cold front should be slow to progress through the area Monday
night given the strength of the ridge in advance, and as such have
1.) provided added forecast resolution to rain and thunder and 2.)
increased Monday evening temperatures considerably.

With an increasing southwest wind Monday evening, temperatures
should hold steady or only drop slightly. In fact, near the lake
front temperatures are likely to increase as winds turn more
offshore. Have temperatures around or a little above 60 through
early overnight across a lot of the area. Amazing for February.

Immediately ahead of the front, there will be a plume of near to
record precipitable water values for February (1.30 inches). The
moisture transport nose steering this in will provide a focused
area of moist convergence along the boundary supporting a swath
of showers progressing east-northeast. The right entrance region
of a jet looks to support better coverage across the northern half
of the area, but most locations should still see rain showers.
Elevated lapse rates while not strongly unstable are certainly
enough with the moisture to support isolated/scattered thunder
and have continued that in the forecast. Feel that blended
guidance is likely underdone on rainfall amounts, with some
locations likely to see between one half to one inch...most
favored northwest.

Attention then turns to the temperatures Tuesday and Wednesday as
we look to enter a sunny pattern with west to southwest winds once
again. Due to the nature of the cold front tomorrow night being a
Pacific front, the cool air behind it is quite weak. If indeed we
can clear out quickly Tuesday morning, temperatures should rebound
to near if not record highs given 925mb temperatures 10C-11C. Of
greater note is Wednesday, which with a weak low to the north,
could focus an enhanced warm corridor directly over the area with
a favorable southwest wind. Synoptically and looking at local
climatology, it points to potential to near or exceed all time
February highs (lower-mid 70s in the CWA). Looking back at a
couple other late winter/early spring record to near-record
warm stretches like we are in, the last or second to last day can
be the highest. So that further supports going a ways above
guidance. Have collaboratively done that, with upper 60s to low
70s forecast.

Beyond the focus will be on the large scale weather system that
will shift the weather pattern into a more seasonable one for the
weekend into next week. Guidance has been consistently showing a
deep low tracking northeast across the middle of the country
during late week. There have been wobbles in time and could very
well see a slowing. This is of note because it could keep
temperatures/dew points elevated over the CWA on Friday,
potentially bringing a stronger thunderstorm threat given the
forecast synoptic anomalies in comparison with the typical cool
season/early spring severe weather setups. Overall though, this
is too many days out yet to be able to say what part of the system
will impact the area. Confidence is highest in gusty winds and at
some point a robust temperature drop, most favored now late
Friday into Friday night.

Within the northwest sector of this system there will likely be
snow and some of that could wrap into the area, although at this
time the system speed forecast does not look to favor an
accumulating snow threat in our area. A more active and
seasonable temperature pattern is forecast the final days of the
month into the first days of March, and somewhere in there could
present a better snow threat.



350 PM CST

A highly anomalous warm stretch that has already broke daily
records will continue to threaten records this week. Here are
daily records.

Chicago:                         Rockford:
      High:      High Min:       High:      High Min:
2/20: 64 (1930)  49 (1930)       61 (1983)  46 (1930)
2/21: 67 (1930)  47 (1930)       64 (1930)  42 (1930)
2/22: 68 (1922)  52 (1930)       61 (1922)  48 (1930)

For more, including all-time February warmth and consecutive
days of such warmth this time of year, please see our web top
news headline or Public Information Statement.


For the 00Z TAFs...

After several quiet aviation weather days, many concerns develop
during this TAF period. These include possible LIFR/VLIFR fog and
low stratus late tonight and early Monday, gusty southeast winds
late Monday and Monday night, a period of LLWS Monday evening, and
showers developing toward Monday evening.

Surface low pressure over the northern High Plains this evening
will lift northeast across the Dakotas and into Ontario through
Monday night. In response, the weak front which pass across the
forecast area Saturday will lift slowly back north as a warm front
tonight and Monday. Light convergent winds and increasing low
level moisture is expected to result in development of dense fog
and low stratus across portions of the area later tonight, with
high-res model guidance focusing especially on the DBQ-JOT-IKK
axis for the lowest conditions, similar to what occurred last
night across central IL/IN. Some uncertainty with how widespread
these conditions will be, with increasing high cloud cover and a
more organized surface pressure gradient being negative factors,
though surface dew points in the low 40`s to near 50 advecting
from the south/southeast will not need nearly as much radiational
cooling as last night. Thus have indicated lowest (1/2SM) vsby and
cigs at RFD-DPA-GYY while keeping ORD/MDW IFR where warmer urban
conditions should help vis/cigs higher, though some potential
exists to see lower conditions advect in from the south/southeast
early Monday morning.

Southeast winds increase through the day Monday which along with
diurnal warming should improve cig/vis during the day. Southwest
low level jet increases to around 50 kts above 2000 ft toward
evening, which may produce a LLWS situation during the evening,
though the gradient should be tight enough and temps warm enough
to continue gusting above 20 kts at the surface. Rain chances
increase toward evening for RFD, and by late evening in Chicago
area as band of pre-cold frontal precipitation spreads in from the
west. Potential for embedded thunder develops, though primarily
just beyond the end of the TAF period.



355 PM CST

A somewhat active week is expected over the lake. Southeast winds
will be on the increase late Monday afternoon into Monday night
across the lake, with occasional gales possible across the north.
Also, small craft advisory conditions will be possible for a
period Monday evening with stiffening offshore flow. A cool front
will then pass near daybreak Tuesday, but with limited westerly
wind speeds behind it.

A low is forecast to pass east over the lake during midweek, before
a stronger low is forecast to track northeast into the western Great
Lakes region on Friday. This system looks to support gales
possibly both ahead of the low and behind it. This will depend on
the exact low center track and thus where the strongest wind
field and eventual cold advection is. Given the strength of the
system forecast in several of the available model guidance we look
at, 45 kt gales are certainly plausible.






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