Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 10/24/20

Area Forecast Discussion

National Weather Service Chicago/Romeoville, IL

 903 PM CDT Fri Oct 23 2020

.UPDATE...
856 PM CDT

Only update this evening was to generally slow the temperature
fall overnight with the lingering northwesterly breezes and
stratus deck overhead. Upstream satellite imagery and surface
observations do, however, show drier air chewing away readily at
the cloud deck across eastern Wisconsin. We may hold onto a bit
more cloud cover across our western locales, but have generally
trimmed sky cover late tonight given the latest trends and
observations indicating a rather thin layer of saturation. At the
same time, have beefed up cloud cover Saturday afternoon, as it
looks like a good bet we`ll build a pretty expansive stratoCu deck
through the day. Updated products have been transmitted.

Carlaw

&&

.SHORT TERM...
300 PM CDT

Through Saturday night...

A well developed autumn cold front has passed through most of
northeast Illinois and northwest Indiana this afternoon.
Convection has taken off along and ahead of the front in a warm
and moist airmass that developed with decent shear in place.
Fortunately for the local area the front is through most locations
and thunderstorms are no longer a concern. Rain lingers post
frontal and winds should pick up in the next few hours now that we
are seeing some 3+ mb/6 hr pressure rises. Once we lose the deep
moisture in the southwest flow aloft, showers will exit the area,
though there could be a bit of drizzle in the still fairly solid
stratus deck across the region.

Drier air will filter in from the northwest, and will attempt to
go to work on a fairly stout frontal inversion under which the
stratus is trapped. Hard to say how quickly this stratus will
erode with guidance somewhat mixed, but HREF does suggest at least
some partial clearing, which seems reasonable given the subsidence
and lowering inversion. Pockets of subfreezing conditions will
result where we get clearing and continued cold advection.

We seem more confident in clearing as we head into Saturday. In
spite of the clearing, high temperatures will struggle to reach
50. High pressure will shift off to the northeast but will
continue a feed of dry air. With westerly flow aloft we will
spread in some higher clouds, but it should still be a fairly cool
night with readings in the 30s.

KMD

&&

.LONG TERM...
330 PM CDT

Sunday through Friday...

The main forecast concern is the potential for precipitation later
Sunday into or through Monday, some of which could be a rain-snow
mix or change to snow in portions of the area. Confidence is low,
however, due to variance in the guidance. Unseasonably chilly
conditions are in store through Tuesday, with moderation closer to
normal in the mid to late work week period. A few periods of
precip skirting by primarily southern CWA (central Illinois and
Indiana south of Kankakee River counties) are possible, though low
confidence so slight chance PoPs for these. Guidance is mixed on
trends into Halloween weekend, though current medium- long range
consensus suggests Halloween itself could be dry and seasonable.

Turning back to the Sunday PM-Monday period of interest, the key
players are: unseasonably strong and cold high pressure over the
northern High Plains; a fairly tight thermal gradient/baroclinic
zone over the Midwest; and strong disturbed west-southwest mid
and upper level flow spurred by troughing into the west/southwest
CONUS and western Atlantic ridging. With dry deep layer easterly
flow over the area on Sunday, there will be substantial lower
level dry air to erode before any precip can occur. Therefore,
it`s possible much of if not the entire day remains dry, with
possible exception of the far north and northwest CWA. Of the
global operational and ensemble models, the ECMWF and GFS suites
essentially switched places for this event, greatly lowering
confidence Sunday evening/night into Monday.

With the lingering lower level dry air issue possibly keeping
much of the precip at bay Sunday evening and the early overnight,
if notable precip occurs with this setup, best chances look to be
late Sunday night into or through Monday evening. As alluded to
above, the ECMWF suite (with support from outer ranges of NAM) is
now much more robust with QPF signal from large scale ascent and
mesoscale thermally driven (f-gen) forcing. Meanwhile, the GFS
suite greatly backed off PoPs for this event due to dry air
problem and weaker forcing. Should subsequent trends favor the
ECMWF (and NAM) idea, sufficient wet bulb evaporative cooling
along with otherwise cold enough profiles for snow could yield a
mix or changeover to wet snow in the northwest 1/3 or 1/2 of the
CWA, including during the day on Monday. Given the low confidence,
too early to mention any specifics with slushy accum potential,
but even if it happened, marginal boundary layer would probably
render minimal if any impacts. Finally, and on the other hand, a
GFS-like idea would yield little if any precip for Sunday PM-
Monday. Stay tuned.

Castro

&&

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

The only minor aviation weather concerns this forecast period are
timing the cessation of MVFR cigs this evening, and a flip to
light northeasterly winds on Saturday.

Lingering gusty northwesterly winds in the wake of a cold front
will continue to gradually ease this evening. At the same time,
the frontal inversion continues to dry and deepen, and cigs are
slowly lifting and should rise uniformly above FL020 over the next
few hours (except at GYY where the lake influence will likely
maintain some lower cigs for a period). Upstream satellite trends
and recent AMDAR soundings show the stratus layer is thinning
appreciably, so would anticipate a return to VFR sometime later
this evening/overnight. By Saturday afternoon, winds should veer
slightly to favor a northeasterly direction. Expansive
stratocumulus should develop with daytime heating, and while it`s
possible cigs come in just under FL030, have left things VFR with
the outgoing 00z TAFs.

Carlaw

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...Lakeshore Flood Advisory...INZ001-INZ002 until 10 AM Saturday.

LM...Small Craft Advisory...IL nearshore waters until 9 AM Saturday.

     Small Craft Advisory...IN nearshore waters until 3 PM Saturday.

&&

$$

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