Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 170233
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
833 PM CST Thu Feb 16 2017

.UPDATE...
833 PM CST

Evening update...

Only minor tweaks to going forecast this evening, primarily to
adjust hourly sky cover higher this evening per satellite trends
as well as hourly temps which are falling more slowly this evening
with cloud cover and modest winds.

North-northwesterly mid-upper level flow persists across the
region this evening, on the trailing side of the deep upper trough
slowly moving off the Atlantic coast. Mid-level short wave seen in
water vapor imagery digging southeast across Lake Michigan will
slide southeast of the area overnight, allowing much of the mid
and high cloud cover to decrease from the northwest after
midnight. This cloud cover has helped keep temps from falling very
quickly this evening, with readings generally in the 35-40 range
at 02Z/8 pm CST. Surface analysis depicts a warm-frontal trough
from central Iowa into central Illinois and Indiana, and this warm
front is progged to lift slowly northeast across the forecast area
overnight. Temps are not expected to fall more than a couple of
degrees across the southern cwa with this front moving in, while
evening clouds and eventual northward movement of the front are
expected to keep temps at or above the freezing mark in most areas
further north through morning. Thus have raised overnight mins
just a bit from previous, especially across the southern counties.
Decrease in cloud cover, some diurnal cooling and slowly
increasing dew point temps late tonight may result in some patchy
light fog toward morning, though high-res guidance generally
limits vis to 4-5 miles not worth mention in grids/text forecasts.

Record warmth appears on track to spread across the area Friday,
as the warm front lifts north of the area and breezy south winds
develop. Model 925-950 mb temps support afternoon highs from
around 60 far northeast near Waukegan, to the low-mid 60s across
the south/southwestern counties. Records for both Rockford and
Chicago (60 and 59 respectively) will likely be achieved/broken
with lower 60s expected at those locations. Did nudge Friday maxes
up a degree or so in a few spots across the southwest half of the
cwa, and could even be a little underdone with highs in boundary
layer mixing is deeper than forecast soundings indicate.

Updated digital grids/text forecasts available.

Ratzer

&&

.SHORT TERM...

Main weather story for the upcoming week is the unseasonable
warmth. High amplitude pattern driven by a deep upper level
trough moving on shore over California the next 24-48 hrs will
help to build a strong upper level ridge over the central and
eastern CONUS which... minus a couple of progressive shortwave
troughs moving quickly through... will remain relatively in place
through the middle of next week. The unseasonably high upper level
heights sets the stage for the unseasonable warmth.

In the short term... the dry airmass currently in place over the
area per forecast sounding data combined with an area of good
surface pressure falls in the wake of the departing surface ridge
helping winds to occasionally gust to around 15-17kt this
afternoon. Look for these winds to ease back by dusk before the
wind field goes relatively flat overnight in advance of
approaching surface trough/warm front. With a flattening of the
pressure gradient and hence the wind field... can`t rule out some
patchy light fog develop in the predawn hours.

Ed f

Friday...

The big story on Friday will be the near record or record breaking
warmth. Models are now all in agreement that warm front will clear
our area well to the north by Friday morning. Extreme warm for
February air mass at 850 mb and 925 mb will continue to be
advected eastward from the Plains as anomalous mid-upper ridge
builds northeastward over the Midwest. The height rises will work
to shunt the mid and high cloud cover that we`ve been dealing with
today off to the northeast, yielding full sunshine during peak
heating.

Relatively tight pressure gradient and good mixing up to
just under 900 mb will result in southwest winds gusting up to 20
mph, potentially higher during the afternoon. This should prevent
a lake breeze from sneaking onto the far northeast Illinois
shore. Dry conditions of late also should aid in preventing from a
faster dew point rise, which would then give another assist to
boundary layer warming. Regarding the progged 850 mb and 925 mb
temperatures tomorrow, they are basically off the charts for mid
February, near or above the daily max around this time of the
month for ILX and DVN. The guidance progged thermal profiles are
well supported by the low-mid teens Celsius at 850/925 mb sampled
by 12z RAOBS from across Nebraska and Kansas that will be advected
into our area.

Taking all of the above into account, have increased forecast
high temperatures slightly from previous forecast to around 60/low
60s area wide. It`s possible that a few locations in far northern
Illinois come in just shy of 60. Would also not be surprised to
see a few locations in the far west and southwest CWA counties tag
the mid 60s even. The record highs for February 17th in Chicago
and Rockford are 60 (1880) and 59 (1981) respectively, and these
look to be in serious jeopardy.

Castro

&&

.LONG TERM...
253 PM CST

Friday night through Thursday...

Average or normal highs over the next week are in the mid to upper
30s for both Chicago and Rockford.  After tonight, at least until
late next week it is not clear when our overnight lows might be much
cooler than that.  And the forecast highs will be close to breaking
records on multiple days.  See the Climate Discussion below for
additional details. Along with the mild air, the area will stay
mostly dry through the period.

For Saturday, pronounced upper level ridging and southerly flow at
low levels will support unseasonably mild temperatures along with a
dry and mostly sunny start to the weekend.  A weak trough embedded
in the upper ridge will move across the area during the day Saturday
and turn winds northwesterly but should not provide substantial
cooling.

On Sunday the upper ridge continues to build overhead and low level
southerly flow becomes reestablished, though winds could be light
and variable for the early part of the day. The mild air and
relatively light winds might even support the development of a lake
breeze Sunday afternoon.

By Monday and especially Monday afternoon the axis of the upper
ridge will shift east of the area and allow a narrow but relatively
deep trough to approach from the Plains.  A surface low associated
with this trough will lift well north of the Great Lakes on Monday
night.  This low will drag a weak cold front toward the area but it
is not clear if there will be enough moisture and convergence along
this boundary to support much in the way of precipitation Monday
night and early Tuesday.  Have elected to maintain chance POPs for
this period.

Models suggest this front could stall near the area for Tuesday and
Wednesday, thus supporting areas of chance POPs both days but
continued mild temperatures.  A stronger push of cooler air may
arrive by next Thursday, but temperature trends still appear to be
considerably above normal, with highs only dropping a few degrees.

Lenning

&&

.CLIMATE...

427 AM CST

We will be heading into an unseasonably warm stretch of weather with
near to possibly record breaking warmth at times. Here are some
statistics on daily records, and consecutive days of 50+, 60+ degrees.

DAILY RECORDS

Chicago:                         Rockford:
      High:      High Min:       High:      High Min:
2/17: 60 (1880)  42 (1890)       59 (1981)  39 (1961)
2/18: 62 (1981)  45 (1981)       58 (1981)  38 (1997)
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)

CONSECUTIVE DAYS OF 50+ DEGREES

Chicago:                         Rockford:
Longest in February              Longest in February
8 days: 2/22-29/2000             8 days: 2/22-29/2000
        2/18-25/1930                     2/18-25/1930
7 days: 2/22-28/1998             5 days: 2/23-28/1998
6 days: 2/24-29/1976                     2/16-20/1981
        2/11-16/1882                     2/25-29/1932
5 days: 2/19-23/1943             4 days: 2/12-15/1984
        2/25-29/1932                     2/24-27/1976
        2/9-13/1876                      2/21-24/1927

Longest in Meteorological Winter
12 days: 12/14-25/1877           8 days: 2/22-29/2000
                                         12/2-10/1946
                                         2/18-25/1930

CONSECUTIVE DAYS OF 60+ DEGREES

Chicago:                         Rockford:
Longest in February              Longest in February
4 days: 2/24-27/1976             3 days: 2/19-21/1930
3 days: 2/19-21/1930                     2/23-25/1930
                                 2 days: 2/25-26/2000
                                         2/24-25/1976

Longest in Meteorological Winter
5 days: 12/2-6/1998              3 days: 2/19-21/1930
                                         2/23-25/1930

KMD/RC

&&

.AVIATION...
For the 00Z TAFs...

527 pm...Only forecast concern are winds Friday. Southeast winds
under 10kts this evening will turn more southerly overnight with
speeds diminishing to 5kts or less. Some locations may end up
becoming light and variable or calm overnight. Southwesterly winds
will increase to 10-15kts with some higher gusts on Friday and
there remains some uncertainty regarding how strong gusts may
become. If the low levels mix deeper then expected...some gusts
into the lower/mid 20kt range will be possible.

Mid clouds this evening will lift/scatter to high clouds
overnight with thin high clouds Friday...possibly becoming skc
Friday afternoon or evening. With the light winds tonight and
some increase in low level moisture...its possible for some light
fog/haze to develop Friday morning but confidence is low. cms

&&

.MARINE...
253 PM CST

Mild and relatively quiet weather will persist over the next few
days.  Winds also should be fairly light, and the favorably stable
southerly flow in place for most of the period will not support
much in the way of wave growth. Even after a weak frontal passage
Saturday when winds veer a bit more westerly to northwesterly,
wind speeds will remain fairly light. The best chance for stronger
winds and higher wave growth, though only to about 6-9 ft at mid
lake, appears to come Tuesday with a tighter pressure gradient
ahead of an approaching cold front.

Lenning

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...NONE.
&&

$$

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