Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 160854

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
354 AM CDT Sun Oct 16 2016

329 AM CDT

Through tonight...

Primary concern in the near term is the area of showers and
scattered thunderstorms that will gradually progress southeastward
through the morning. An axis of anomalously high moisture, with
PWAT values of 1.5" to 1.6" per SPC mesoanalysis, will support
heavy downpours, though fast mean storm motion of individual cells
will likely preclude a flooding threat. Precipitation will exit
most of the CWA, except perhaps parts of NW IN by early afternoon.
Main hazards will be the aforementioned heavy downpours which
could cause localized ponding on some roads, as well as occasional
cloud to ground lightning strikes.

Stratus will linger behind the precipitation as parent cold front
washes out over the area, leaving a mild and unseasonably moist
air mass in its wake. This is the main uncertainty with
temperatures today, whether any breaks in the overcast develop.
Have cloudy/mostly cloudy conditions in the grids, so warming
should be tempered some, though with readings as of this writing
incredibly warm for mid October (65-70), it won`t take much to
reach the low-mid 70s most areas. Other concern is with potential
for a back door cold front to slip down the lake and flip winds
northeast near the lake in northeast Illinois. Have trended wind
forecast toward this expectation. With slightly cooler and more
dense marine air bumping up against mild and moist air over land,
there may be some patchy fog development. Also, highs along
immediate IL shore may not get out of the upper 60s.

Warm front will be lifting north tonight, and think that a
primarily dry night is in store, with shortwave energy passing to
our north, though cannot rule out an isolated shower far northern
Illinois. Have some concern for patchy fog with such an
unseasonably warm (temps well into 60s) and moist air mass in
place. Increasing southerly winds as front lifts north overnight
should start to scour any fog however.



329 AM CDT

Monday through Saturday...

A blast of near record, mid summer warmth will be driven northward
into the area on Monday with top of the chart 925 mb temps in the
lower 20s Celsius by the afternoon. Primary uncertainty is how
long it takes the morning stratus to erode and lift northward in
strong southwest flow. Prefer the depiction of the 00z ECMWF for
Monday, which favors the stratus lifting northward fairly quickly
and also slightly warmer thermal profile and deeper mixing.
Adjusted temperatures upward from guidance blend, especially south
of I-88 where confidence is highest in full sun during the
afternoon. This yields mid to even high 80s south of I-80. To the
north, have low to mid 80s, except a few upper 70s readings along
WI border accounting for potential for clouds to still limit
things slightly there. The other concern on Monday is the
potential for southwest wind gusts to approach or even exceed 35
mph, which along with dew points mixing out some could make it
easier for brush fires to spread.

Another extremely warm and breezy night is in store Monday night
as strong low pressure lifts toward Lake Superior. Temperatures
will likely stay in the 70s area wide during the evening hours, so
it will be July in October. Cold front trailing from the low
pressure will move across the area on Tuesday, putting an end to
the extreme warmth, but not before highs reach well into the 70s
north of I-80 and around 80/low 80s south. The front looks to move
through the area dry, with wind shift to west main sensible
weather impact.

Guidance continues to exhibit substantial differences Tuesday
night through the end of the work week, lowering confidence.
First, ECMWF remained consistent with previous forecast, with
stalled baroclinic zone, weak surface wave along quasi-stationary
front to the south and approaching upper trough causing widespread
rain to blossom and spread north. Operational GFS and Canadian are
dry during the same period. This is followed by additional
differences in handling of upper trough digging southeast through
the end of the week. Maintained PoPs from guidance blend, which
yields highest chances south of I-80 Wednesday through early
Thursday. Confidence will increase as this period draws closer.

For temperatures, it will become at least closer to seasonal for a
few days to close the week, with potential for colder if deeper
mid level trough and associated thermal trough occurs.




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.

October 16th
Record High Min for Rockford: 65 in 1968

October 17th
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...

Aviation concerns initially focus on timing a period of
showers/thunderstorms early this morning, then shifts to the
potential for IFR conditions with low cigs and fog through late
morning. Also of concern later this afternoon is how far south a
cold front sags into northern IL, with the potential for winds to
shift to the east-northeast and the associated fog/low cig

Surface low tracking well north of the Great Lakes early this
morning was trailing a cold front across WI and IA. An area of
showers and thunderstorms was occurring ahead of the front as
expected, and these showers and a few thunderstorms are expected
to move slowly east across the terminals through sunrise as the
front continues to push slowly southeast. VFR conditions ahead of
the precipitation are expected to fill in with lower cigs/light
fog as rain develops, with model guidance in good general
agreement in lingering IFR conditions through the morning in moist
and weakly convergent low level flow near the front. Showers or
drizzle may linger into mid-late morning especially across Chicago
area. Winds, which continue to gust near 20 kts at midnight from
the southwest, will diminish as the frontal trough approaches.
Cold front itself then is progged to settle across northwest and
far northern IL during the afternoon, with some spread in
solutions as to how far into metro area it sags before winds turn
southeasterly this evening. Some of the high-res pushes the front
across ORD/MDW by mid-afternoon, turning winds NE which would
likely also present an IFR/LIFR cig threat. Other guidance more
bullish with keeping some sort of southerly component to wind,
though looking more like front will move through Chicago
terminals and have trended that way in TAF. IFR/LIFR conditions
could linger through this evening in that event, until front
begins to lift back north with winds becoming south and increasing
late tonight.



221 AM CDT

Winds across northern Lake Michigan continue to diminish early
this morning as cold frontal trough approaches from the northwest.
The gale warning will be allowed to expire as scheduled, with
winds already sub-gale as of 07z/2 am CDT. The cold front will
continue to push southeast across the lake today, allowing
southwest winds to diminish on the southern part of the lake as
well by midday, and veer to the north-northeast. The front will
stall briefly across the far south end of the lake before lifting
back north as a warm front late tonight and Monday, with winds
again becoming southerly and increasing. Warmer air over still
warm waters look to limit the depth of the mixed layer over the
water, so winds are expected to remain in the 25-30 kt range over
open waters, while gusts may approach gale force along the western
shore Monday afternoon where mixing over land is deeper. This will
be in response to another deep low which will lift northeast
across the northern lakes Monday night, eventually trailing
another cold front across the lake Tuesday. Winds will diminish
behind the front Tuesday evening, as weak high pressure spreads
into the region behind the front.

For the IL and IN nearshore zones, a small craft advisory remains
in effect early this morning until 4 am. Winds should diminish
quickly after sunrise with weaker gradient associated with
approaching cold frontal trough. Will evaluate a possible short
extension based on observational trends closer to expiration. In
any case, offshore flow should fall below 20 kts over most areas
by mid-morning.



LM...Small Craft Advisory...NEARSHORE WATERS
     UNTIL 4 AM Sunday.




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