Area Forecast Discussion
Issued by NWS Chicago, IL
FXUS63 KLOT 170545
Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1245 AM CDT Mon Oct 17 2016
559 PM CDT
Made some tweaks to near term grids to reflect some small scale
things likely to occur this evening. Lake enhanced front can be
seen on TORD terminal doppler stretching from near Montrose Harbor
northwest to the Chain `O Lakes region. Synoptic flow over
northern IL has become light east to east-southeast which should
allow this lake enhanced boundary to make some more progress
inland this evening, particularly from Chicago north. Marine fog,
locally very dense, could seep inland a couple miles this evening
from Chicago north across Lake County IL before warm front lifts
north late this evening and overnight quickly shoving fog back out
over the lake and north out of our area. Will closely monitor
trends this evening, but at this point suspect the fog coverage
and duration will be such that it can be handled with just an SPS
over land and have already issued a marine fog advisory for the
nearshore waters from Northerly Island northward.
Low temperatures will likely be reach this evening, particularly
behind the lake front where temps could dip into the lower 60s.
Following the warm frontal passage after midnight look for temps
to begin rising with most areas probably near or above 70 degrees
by sunrise. Just to put that into perspective...the sunrise
temperatures are forecast to be around 10 degrees WARMER than the
average high for this time of year! Obviously this will set the
stage for a windy and unseasonably warm day tomorrow which current
grids look to have a good handle on.
146 PM CDT
The area of showers and thunderstorms have pushed well east of the
region. for the remainder of the afternoon, main concern will be
the progression of a back door cold front pushing quickly down
Lake Michigan this afternoon. As of 2:00 PM CDT, the front is
pushing through the UGN area. Expect that the front should make a
little more swd progress, reaching the Chicago area, but likely
hanging up closer the IL/WI border further inland. The short range
guidance is in a bit of disarray in handling the swd progression
of the front, but feel that the general trend should be that the
front will not make much more swd progression well inland, but
should reach the Chicago metro area. The main effect with this
would be to limit any further warming over far nern IL this
afternoon while allowing the remainder of the area to increase a
couple more degrees before sunset. Even so, mat temps today will
top out well above normal for mid October. With the front pushing
to the Chicago Metro area, low stratus and patchy fog are likely
to hang around with perhaps a few sprinkles near the lake.
However, additional wetting pcpn chances for the remainder of this
afternoon and evening are low enough to just go with a dry
forecast. The next concern will be timing of the front moving back
north as a warm front. for the past several days, the models have
been consistently advertising this idea and still feel that the
front will surge north again late this evening, with winds turning
back to more southerly. Wind speeds should increase through the
night, and become more gusty into the early morning hours. With
the front surging back north overnight, the temperature trend
overnight should be fairly flat, with temps only dropping into the
middle to upper 60s. Dewpoints will also be unseasonably high,
closely following the temperature trend. Would normally anticipate
warm advection fog overnight under more typical conditions, but
with a warm, moist air mass already in place and winds increasing
overnight, keeping the boundary layer stirred up, do not expect
dense fog to form, but there could be some misty conditions.
304 PM CDT
Monday through Sunday...
The main forecast concerns continue to center around the very
warm (near record warmth) and windy conditions expected over the
area Monday and Monday night.
A strong 130+ KT upper-level jet max, associated with an upper-
level trough currently shifting over the western CONUS, is
expected to induce another surface low over the central Plains
tonight. The low will then deepen to a stout sub 990 mb low as it
shifts northeastward over the Upper Great Lakes Monday evening. As
this occurs, a surface frontal boundary currently across southern
Wisconsin and far northern Illinois will be driven northward out
of the area as a warm front. While this system is not expected to
produce any precipitation over our area on Monday, it will
produce windy and very warm conditions over the area. Expect
south-southwesterly winds to develop area-wide Monday morning,
with increasing speeds through the afternoon and evening in
response to strong surface pressure falls developing over the
Western Great Lakes. Winds look to become gusty as high as 35 to
40 MPH over the area by Monday afternoon with breezy conditions
likely to continue during the evening. These strong winds will
also allow for unseasonably warm conditions to shift northward
over the area. As a result, expect high temperatures on Monday
look to be in the low to perhaps mid 80s, which is not far off the
records for the time of year (see climo section for the records).
The storm system is expected to shift into western Quebec on
Tuesday. As this occurs, an attendant cold front will likely shift
over the area, with some breezy westerly winds likely in its wake.
It appears that the front will come through mainly precipitation
free. However, there is a small chance that a few showers could
develop with the front over portions of the area. However, at this
time we have opted to continue a dry forecast at this time.
Expect temperatures to become more seasonal following this
frontal passage, especially for Wednesday and Thursday, when
highs will be in the 60s.
Another weather disturbance is expected to impact the portions of
the Mid-Mississippi valley and into the Ohio valley by mid to late
week. This system could produce another period of rain for
portions of the area. However, there are still uncertainties on
how far north into Illinois this rain could end up. For this
reason, were have continued to only mention small chances of rain.
At this time it appears the main rain threat could stay over
central Illinois and Indiana, in closer proximity to the remnant
surface frontal boundary.
415 AM CDT October 15th
Record/near record high minimum and maximum temperatures are
possible Sunday and Monday based off the current forecast. Below
are the current records that could be threatened.
Record High Min for Rockford: 65 in 1968
Record High Max for Chicago: 86 in 1950
Record High Max for Rockford: 87 in 1950
Record High Min for Chicago: 63 in 1998
Record High Min for Rockford: 61 in 1998
For the 06Z TAFs...
Primary aviation concern today and tonight is with strong and
gusty south-southwest winds. Latest observational trends suggest
that most of the forecast guidance is far too pessimistic with
CIGs this morning. A warm front is in the process of lifting north
across Illinois, which will result in increasing south winds. With
there not being upstream CIG development and observed soundings
around the region not showing the lower level moisture to
saturate, have increasing confidence that the CIGs on most of the
guidance will not materialize. This will enable deeper mixing
quicker, and as strong low pressure passes to the northwest and
north through tonight, the result will be strong south-southwest
winds. Gusts could approach or even exceed 30 kt this afternoon
and evening, which could cause some cross wind issues as
directions look to range between 190 and 210 degrees.
Have almost removed the VCSH shower mention from the TAFs, as
radar trends suggest shower activity will remain northwest of all
terminals. Will continue to monitor CIG trends this morning.
327 PM CDT
A rather active period of weather is expected over the lake
through Tuesday. Currently a surface boundary over southern
sections of the lake will begin to shift northward over the lake
tonight as a trough of low pressure develops over the Upper
Midwest. This will lead to an increase in southeast winds tonight
over the northern half of the lake, with south winds developing
over southern lake Michigan. Wind speeds of 25 to 30 KT will be
possible late tonight into Monday morning. Wind speeds may then
ease a bit into Monday afternoon, before increasing again late
Monday and Monday evening as another strengthening area of low
pressure shifts over the Upper Great Lakes. This second low will
likely result in 30 KT southerly winds over the lake, with the
possibility for some gale force gusts as well, especially Monday
Small craft advisory winds are likely in the near shore
waters Monday through early Tuesday. Therefore a small craft
advisory has been issued for this period.
LM...Dense Fog Advisory...LMZ740-LMZ777-LMZ779 UNTIL 3 AM Monday.
Small Craft Advisory...NEARSHORE WATERS
UNTIL 10 AM Tuesday.
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