Forecast Discussions mentioning any of
"ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 10/04/20
Area Forecast Discussion...UPDATED
National Weather Service Chicago/Romeoville, IL
858 PM CDT Sat Oct 3 2020
857 PM CDT
The forecast message remains a wet one through a good part of
tonight with the going high PoPs, rainfall amounts, and timing
all looking good. Changes to the forecast have all been merely
tweaks. There is the chance some lake effect showers could graze
the northeast Illinois shore, including downtown Chicago, midday
Sunday, while likely some will reach the northwest Indiana shore.
The impressive trough for early October continues to dig in on the
synoptic scale across the Upper Midwest and Great Lakes, ushered
in by a due southward oriented 125 kt upper jet from central
Canada into the central United States. The cold air aloft is
certainly present, with the 00Z DVN sounding sampling 0C at 850
mb, and that supports the KDVN radar correlation coefficient
indicating a mix of rain and snow as low as 2,500 ft in northwest
Illinois. The boundary layer will remain sufficiently warm for the
rest of the night for all the precip to fall as rain, so no
worries there! The primary vorticity maximum center will pass the
area during overnight, so will see rain tapering fast in north
central Illinois around midnight and in Chicago near 3 a.m.
Higher resolution guidance, basically that of convection-allowing
scale or close, is depicting a slight higher signal in lake effect
showers on Sunday. The steering flow would still support more of
far northwest Indiana to see these, but it`s worth noting the HRRR
and RAP this evening have trended slightly further west, even
grazing the northeast Illinois shore with QPF. RAP soundings do
show 200-500 J/kg of lake-induced CAPE with equilibrium levels
around 10,000 ft, so certainly sufficient for some showers to
develop. However there will be a race against drier air working
into the area especially mid-late afternoon. For now no changes to
the going forecast PoPs but will note to the overnight shift.
226 PM CDT
Through Sunday night...
We`re finally starting to see the influence of a rather sharp
shortwave which is slicing through Minnesota and north-central
Iowa with a gradual expansion to a region of light rain spreading
towards parts of central and north-central Illinois. This is
occurring as the remaining region of dry air in the 850-700 mb
layer is more fully saturating, as evidenced by recent VAPOR AMDAR
soundings. We`ll continue to see this precipitation shield expand
and fill in through the afternoon and evening hours as large
scale forcing for ascent gradually increases.
Model cross sections depict the development of at least a modest
f-gen circulation late this evening in response to the deepening
larger-scale indirect ageostrophic circulation under a departing
120+ kt upper-level jet. So while most of this event will feature
only light rain, periods of locally more moderate activity appear
probable late this evening and into the early-overnight hours
near locales straddling the I-55/57 corridors where this
frontogenetic circulation is maximized. There`s actually a brief
window where some non-zero instability to upright convection
develops in our southeastern zones as lapse rates steepen up a
bit. Probably not enough to worry about electrification, but
moreso assisting in the production of briefly more moderate
rainfall. Precipitation and cloud cover may clear our far
northwestern locales early enough to support at least some threat
for fog and/or low cloud development towards sunrise on Sunday.
Think that winds just off the deck look too robust for much of a
dense fog threat, however with more of just a shallow mist and
low stratus potential. Temperatures will also fall into the upper
30s with the arrival of a northwest wind shift, but the low-
levels will dry out quickly enough to end any precipitation well
before the column cools enough to support any mixing of p-types.
Northwest winds remaining up around 5-10 mph will likely curtail
the potential for much frost development in our northwest early
Have generally sped up the departure of precipitation chances
Sunday morning with this forecast package, confining and
straggling chances to northwest Indiana. Lake effect parameters,
while marginal, will support carrying chance PoPs through Sunday
afternoon into parts of Porter, Lake, and Jasper Counties. Breezy
north to northwest winds will then develop through the morning
Sunday and afternoon, with some occasional gusts into the 25 to
perhaps 30 mph range.
The other story will be the period of near-freezing temperatures
Sunday night as the spine of an encroaching area of high pressure
crests overhead and winds go nearly calm. We`ll likely have to
contend with just a bit of high-level cloud cover at times, but
the radiational cooling potential looks solid, especially with
dewpoints in the lower 30s. We do show a few 31-32 degree
readings in our typical cool spots (Woodstock, Aurora, Rochelle
areas for example), but the sub-freezing potential looks pretty
localized and insufficient in areal extent to warrant any Freeze
Headlines. We will, however, likely need a Frost Advisory for much
of the region away from the immediate urbanized Chicago
corridors, and will let the midnight shift take a look at this
245 PM CDT
Monday through Saturday...
The primary Storm track is expected to remain north of the area
though the period. For this reason, there are no good chances for
precipitation through the forecast period. Expect temperatures
to be more seasonable next week, with highs generally in the 60s,
with low 70s at times.
A surface high that move move across the area Sunday night will
shift to our southeast during the day Monday. This will allow the
winds to turn southerly in its wake for Monday into Tuesday.
Moderating temperatures in this warm air advection pattern will
push highs back into the lower 60s for Monday, and potentially up
around 70 for Tuesday. A weather disturbance passing mainly to our
north on Tuesday may drop a weak cold front over the area during
the day, but this looks to do little to the temperatures as the
flow quickly shift back out of the southwest Tuesday night into
Wednesday ahead of a fast moving clipper system dropping
southeastward over Ontario. This system will result in a mild and
breezy day Wednesday, with a stronger cold front expected to shift
southward over the area during the day. It continues to look dry
across the area with this frontal passage, so we continue to
advertise a dry forecast.
We only look to get a glancing blow at some cooler air in the wake
of this front, so we can expect temperatures to be in the lower to
mid 60s for Thursday. Thereafter, we look to get back into a mild
weather pattern, supportive of highs around 70 for next Friday.
Even warmer temperatures, potently well into the 70s, are
possible for a period next weekend.
For the 00Z TAFs...
627 PM...Forecast concerns include...
Light rain through early Sunday morning.
Mvfr cigs tonight.
Gusty northerly winds Sunday.
Light rain will continue across the entire area this evening and
slowly end from northwest to southeast early Sunday morning. The
rain may briefly reduce visibilities into mvfr. As the low levels
saturate...cigs are expected to lower into mvfr across much of
central IL and northern IN. Confidence is decreasing for how low
cigs will become across the terminals. Mvfr cigs are still
expected but may remain in the 2kft range across most areas. The
best chance for 1kft cigs is likely along an arr/jot/gyy line with
ifr possible south of that line. Cigs are expect to slowly lift
into vfr Sunday morning.
If clouds scatter out by Sunday morning across northwest IL there
is some potential for either light fog or a low stratus deck but
confidence remains low.
Light westerly winds will shift light north/northwest this evening
with speeds increasing after the rain ends overnight. Speeds into
the 12-16kt range are expected Sunday morning with gusts into the
lower/mid 20kt range. Speeds and gusts will slowly diminish Sunday
afternoon and a lake breeze/wind shift to the north/northeast is
expected for ord/mdw. cms
LM...Small Craft Advisory...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-
LMZ745...7 AM Sunday to 4 AM Monday.
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