Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 01/28/20

Area Forecast Discussion

National Weather Service Chicago/Romeoville, IL

 709 PM CST Mon Jan 27 2020

.UPDATE...
709 PM CST

Scattered light radar returns have developed across a decent portion
of the CWA over the past hour, likely in response to some weak
ascent from a sheared 850 hPa wave shifting south from southeast WI
and associated weak isentropic ascent on the 275K theta surface
early this evening. ACARS soundings from ORD/MDW, PIREPs and METARs
indicate cloud depths of 2-2.5kft, with bases roughly around 2500`.
Meanwhile cloud top temps are around -8C. This environment supports
the mix of light snow and drizzle/freezing drizzle reports we have
received in the past hour under the more noticeable radar returns
over Aurora and Lansing. A brief period of very fine mist was also
observed at our office in Romeoville.

Given the less favorable high cloud bases and shallow cloud depths,
any precip should remain light and patchy this evening before a
better chance of icy conditions develops tonight as outlined in the
short term discussion and current Special Weather Statement.

Kluber

&&

.SHORT TERM...
149 PM CST

Through Tuesday night...

Forecast concerns center around identifying periods of potential
impact from freezing drizzle along with lake enhanced snow
showers.

A blanket of low clouds exists across much of the northern half of
the country from the upper Great Lakes down to the Ohio River
valley. Therefore expect these cool and cloudy conditions to
persist for several days. Temperatures in the cloud layer are for
the most part below the -10C level where ice nucleation
probabilities increase enough to suggest better chances of snow
mixed with the supercooled liquid level, but scientifically
probabilities for snow do still exist below -10C they are just
lower. And observations across the region support both precip
types with both drizzle/snow being observed from time to time.
Some of this is lake enhancement. This lake enhancement should
become more focused across northwest IN as the low level turns
more NW. Identifying periods of increased chances for freezing
drizzle therefore becomes the main forecast challenge.

Our main forcing mechanisms will be the subtle waves in WNW flow
through tomorrow. After the first sheared zone across southern
WI/northern IL (which is less impressive than it was before)
moves through there is a bit more organized wave across northwest
WI (with subtle features embedded in the flow). It looks like we
may have a relative minimum per much drier low level water vapor
features and elevated cloud bases across central WI. Hi resolution
for impacts from freezing drizzle still show signal tied to this
next wave combined with subtle low level isentropic lift. The
signal is still not high enough to imply more than a continued
patchy nature to the precipitation, but the overnight into early
morning period is usually more of a concern for at least localized
impacts to non treated areas, including sidewalks and driveways,
possibly favored northwest of the Chicago metro where surface
temps are a touch cooler. We have not made to many changes to the
going forecast given the Detroit probabilities have shown a slight
downward trend, though these subtle signals may fluctuate.

Regarding lake effect snow showers, with the marginal
temperatures profiles, equilibrium levels only about 4-5Kft, and
weak low level Lake induced CAPE, expect accumulations would also
be limited this afternoon and again later tonight into early
Tuesday.

Upper level ridging and surface high pressure is suggested again
briefly later Tuesday afternoon into Tuesday night, but we will
await the approach of a Pacific origin trough later Tuesday night
into Wednesday.

KMD

&&

.LONG TERM...

Wednesday through Monday...

112 PM...Cloudy conditions look to persist for the middle to end
of the week with another weak wave moving across the region on
Wednesday that may bring some light snow or flurries. Low level
winds turn easterly on Wednesday which may also allow for some
flurries and added a chance of flurries to the forecast for this
time period. Another weak wave is possible Thursday night into
Friday with a chance of light snow. And depending on how deep the
cold layer is with these waves...some patchy freezing drizzle is
also possible. A bit stronger wave appears to move across the
region Friday night into Saturday which may have the best chance
to produce some minor snow accumulations though there are still
some placement/timing differences. Models then show a warmup
Sunday into Monday with highs well into the 40s by Monday and if
dewpoints materialize as currently shown...these temps could be
low. But cloud cover would be a concern and potential limiting
factor for highs. cms

&&

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

Main aviation weather interests for this forecast cycle are:
/1/ Prevailing MVFR cigs through the next 24-30 hours.

/2/ Potential for -FZDZ or -FZDZSN late this evening into early
Tuesday morning.

/3/ Light northeast wind shift and another potential for light
snow and/or freezing drizzle Tuesday night.

Cigs early this evening have pretty uniformly lifted to high-MVFR,
above 2 kft, and anticipate that this will remain the case through
the rest of the evening hours based on upstream observations.
While there are some very spotty radar returns showing up, it
looks like the sub-cloud layer will stay just dry enough to
preclude any precipitation until later.

The main features we`re monitoring are a series of disturbances
dropping south through northern Wisconsin at this time, and should
be pushing into northern Illinois later tonight. As this occurs,
some light precipitation could get squeezed out from the slowly
deepening (and lowering) cloud layer. The better low-level
saturation looks to remain mostly south and west of the immediate
Chicago- area terminals. As a result, have moved the precip
mention into TEMPO groups during the most-likely time periods
based on latest model guidance. It looks like cloud top
temperatures may remain near -9 or -10 degrees C, which may be
just cold enough to keep some cloud ice present. Will indicate
-FZDZSN in the TEMPO groups as opposed to -FZDZ as a result. Will
monitor upstream observations and model trends closely this
evening, and update as necessary.

Once this disturbance moves through, clouds may lift back above
about 2 kft on Tuesday afternoon. During the ORD/MDW extended
TAFs, have indicated a light north-northeast wind shift. There is
some potential for an additional batch of light snow and/or
freezing drizzle to push off the lake, but this is just beyond the
current TAF window.

Carlaw

&&

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

$$

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