Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 11/01/20

Area Forecast Discussion

National Weather Service Chicago/Romeoville, IL

 816 PM CDT Sat Oct 31 2020

.UPDATE...
815 PM CDT

The main updates to the forecast were to gently tone down wind
gusts a bit through the late evening here as things have managed
to decouple a bit with the loss of daytime heating. Upstream
surface observations show slackening speeds as winds veer to the
southwest and west-southwest ahead of the cold front, and
anticipate this trend will continue into our region. This reprieve
will be short-lived, however, as winds will pick up in earnest
with the arrival of the northwesterly wind shift and deepening
cold advection behind the main surface cold front which still
seems to just now be pushing into northwest Iowa. Did contemplate
a Wind Advisory here with the mid-evening update based on progged
PBL depths tapping into mid 40 kt flow in the cold advection
regime, but upstream observations would suggest that the
pervasiveness of 45-55 mph gusts has waned a bit compared to what
was occurring earlier this afternoon. Still, it`s possible that
wind gusts locally tag Wind Advisory criteria for a period
immediately behind the main front as the core of 7-8 mb/6-hr
pressure rises slides into northwest and central Illinois after
midnight, but it looks like things may then settle into more of a
40-45 mph range through Sunday morning before the strongest gusts
start to gradually ease through the afternoon hours, and the plan
is to continue to handle this with active Special Weather
Statement.

Have also slowed the temperature fall this evening and overnight
with upstream temperatures still in the upper 40s ahead of the
main cold front where temperatures quickly drop into the 30s. No
changes to the inherited (low) PoP forecast here with mainly just
some spotty sprinkles possible across northeast Illinois and
northwest Indiana as this system scoops up some additional low-
level moisture. Forecast soundings do support the potential for us
to rattle out some flurries on Sunday but that`ll be about the
extent of things. Updated products will be out shortly.

Carlaw

&&

.SHORT TERM...
233 PM CDT

Through Sunday...

The forecast concerns in the short term center on wind, as they
often do in these parts as October transitions into November. Peak
gusts the rest of this afternoon into this evening are looking 35
to possibly 40 mph, and overnight through Sunday primarily in the
40-45 mph range. We will have at least sporadic gusts to over that
on Sunday morning, and the collaborative decision was that we may
be just a tad below confidence in frequent enough gusts to that
level. However the message continues of a windy day and to secure
loose objects (if not already blowing around today!). The Wind
Advisory and Lakeshore Flood Warning for Porter County, where
effects will be maximized from off the lake, continue as is.

The driver of our weather through All Hallows Eve and into Sunday
is a strong trough digging southeast from the Canadian Prairie
Provinces this afternoon. The associated quick-moving 988 mb
surface low has tightened the pressure gradient over the forecast
area between itself and a 1034 mb high over New England / Mid
Atlantic. The inversion overhead here has lifted to around 2,000
ft on latest ACARS data, allowing for southerly winds to
sporadically gust to 30 to 40 mph, especially the northwest half
of the CWA (Rockford recently gusted 37 mph). The pressure falls
of 5 to 7 mb per 3 hours will continue through the first half of
the evening, which will basically keep the sustained winds near
what they are this afternoon. Given temperatures will respond in
the warm advection regime dropping only to the lower 50s or so,
the boundary layer should stay somewhat mixed despite shrinking in
depth. As noted in previous AFDs, an after dark strong warm
advection regime this time of year is always challenging for
gauging how much if any of the low-level jet can be sporadically
mixed to the surface. Feel the 35+ mph gusts are possible but if
so more than likely infrequent.

As some eastern Pacific moisture is drawn into the system this
evening, clouds along and within pre-frontal convergence should
increase. While the brief forecast MUCAPE is less than 50 J/kg,
it does show some light shower or sprinkle possibility, namely 9
pm to 2 am, and convection allowing models support this idea. This
will be of little to no impact, but will signify a change is
about to happen.

That change will be the air behind the cold front. The 925 mb
temperatures will fall from 10C to -6C in a little over 12 hours,
with significant pressure rises associated with the cold advection
and quick movement of the synoptic system. The regime is favorable
for near Wind Advisory criteria winds and soundings, and depending
on model do support that at times overnight into Sunday morning.
Upstream conditions in the Dakotas show quite a few sites gusting
40-50 mph, though that is maximized during sunshine. The frontal
passage here will be more near or just after midnight.
Nonetheless, the favorable synoptic regime will carry on into the
post daybreak hours on Sunday and should see frequent gusts
exceeding 40 mph during the morning and early afternoon.
Temperatures will be falling into mid-morning with very limited
recovery, and afternoon highs of only mid to upper 30s.

The west-northwest wind direction behind the front will become
more northwesterly after sunrise Sunday. This will increase the
orthogonal component of the wind to the Porter County shore. Winds
driven by lake-induced instability and lack of friction should
push 50 mph gusts. The fetch will be enough to result in double
digit wave heights for the Porter County shore with high
confidence, and this results in a clear lakeshore flood risk
especially for the northeastern shore. So Lakeshore Flood Warning
and Wind Advisory for Porter County continue, with a Lakeshore
Flood Advisory for Lake County, Indiana where there is slightly
less of a fetch.

MTF

&&

.LONG TERM...
255 PM CDT

Sunday Night through Saturday...

After a windy Sunday, winds will ease in the evening and
overnight hours. The breezy northwesterly winds will continue to
advect cold air into the area and result in a chilly night with
lows in the 20s area-wide.

The big story of the next several days is the expected warming
trend through mid-week with well above normal temperatures then
continuing through the end of the week. Monday will see high
temperatures warm back into the 50s with southwesterly flow
returning as the surface high shifts off to the south and east of
the area. Although not nearly as windy as over the weekend, Monday
could be fairly breezy again in the afternoon with gusts in the
20-25mph range.

As upper trough continues to shift eastward on Tuesday the jet
stream lifts well north into Canada and ridging extends across
the central CONUS. By Wednesday high temperatures will likely be
10+ degrees warmer than normal in the mid 60s. This pattern will
remain in place through the end of the week which would support a
continuation of these warmer temperatures. Some areas may even
approach 70 degrees late in the week, especially south of I-80.

A cut-off low currently off the southwest coast of California
will slowly meander eastward through the period as it merges into
the upper flow. This could result in increased cloud cover
Wednesday night into the day on Thursday would could help moderate
temperatures a bit. No precipitation is expected with this
disturbance with dry and mild weather continuing through the end
of the week.

Petr

&&

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

623 PM...Forecast concerns include...

Gusty southerly winds this evening.
Low level wind shear this evening.
Cold front/wind shift to northwest tonight.
Strong/gusty northwest winds overnight and Sunday.

A cold front will move across the area tonight shifting southerly
winds to the northwest. Gusts this evening will likely remain in
the mid 20kt range...perhaps higher at times but low level winds
may be near 50kts and with some locations likely to lose gusts at
times this evening...added low level wind shear to this forecast.
Winds will shift more southwesterly ahead of the front and then to
the northwest behind the cold front. A period of gusts into the
mid 30kt range is possible with and just behind the front. Gusts
may settle into the 30kt range through daybreak and then gusts
into the mid/upper 30kt range are possible Sunday. Speeds/gusts
will begin to diminish mid/late Sunday afternoon but will likely
still remain gusty into early Sunday evening...eventually
diminishing into Sunday evening.

There is a chance for some light rain showers this evening ahead
of the front...though the best chances will remain north and
northeast of the terminals but maintained vicinity mention. cms

&&

.MARINE...
230 PM CDT

Southerly winds this afternoon over the water have been somewhat
slow to increase, likely due to the strength of the inversion
overhead. Gusts still are on track though to reach low end gale
force (35 kt) by early evening. Thereafter, a strong cold front
will shift eastward over the lake late tonight. In its wake,
westerly gale force winds to 40 kt are likely through Sunday, with
temporary 45 kt gusts possible. A Gale Warning is in effect for
all of the IL and IN near shore waters through Sunday evening.
Winds should abate across the lake late Sunday night.

MTF/KJB

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...Lakeshore Flood Warning...INZ002...3 AM Sunday to 3 AM Monday.

     Wind Advisory...INZ002...3 AM Sunday to 4 PM Sunday.

     Lakeshore Flood Advisory...INZ001...3 AM Sunday to 3 AM Monday.

LM...Gale Warning...nearshore waters until
     midnight Monday.

&&

$$

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