Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

fxus63 klot 182341 
afdlot

Area forecast discussion
National Weather Service Chicago/Romeoville, Illinois
641 PM CDT Fri Oct 18 2019

Short term...
234 PM CDT

Through Saturday night...

Not much shaking on the weather front here this afternoon with
just some passing high level cirrus cloud cover occasionally
muddying up the sky. Just some occasional gusts to around 20 mph
as we're mixing some higher momentum aloft down to the ground, but
these will subside quickly with the loss of heating this morning.
Based on upstream satellite trends, it's looking like we may even
get rid of some of the ongoing cirrus cloud cover tonight
resulting in a period of mostly clear conditions with slackening
winds. As a result, dropped low temperatures a bit into the upper
30s and lower 40s in the typical "cool spots" outside of the
urbanized corridors. Not too worried about frost given these
temperatures, but suppose some very isolated pockets aren't out
the realm of possibility.

Tomorrow we'll be turning our attention to a lead shortwave which
will begin to press into the region during the afternoon and
evening hours. Until then, dry conditions are expected to prevail
with gradually increasing mid-level cloud cover into the
afternoon. This should allow temperatures to warm into mid and
possibly even upper 60s across the region. Rain chances will
gradually tick upwards into the mid-late afternoon hours, but
mainly for locales near and west of I-39. Overall, things not
looking overly impressive from a precipitation standpoint with
very dry mid-level air in place ahead of this system likely to
work against widespread precipitation. As a result, have pared
pops back mainly across our western locales into the early
evening, but will leave some slight chances for showers into the
Chicago area during the early-mid evening hours.

Winds look to fall off notably Saturday night with the passage of
this initial wave, but low-level moisture will remain in great
supply. Forecast soundings depict a favorable environment for low
stratus formation into Sunday morning. Things may stay mixed up
just enough to keep this stratus deck from working down into a
dense area of fog and, as a result, no mention in the gridded
forecast at this time due to uncertainties, but something to keep
an eye on.

Carlaw

&&

Long term...
234 PM CDT

Sunday through Friday...

Next system of interest is a potent vort Max and associated area
of low pressure which is expected to pinwheel into the region
Sunday night and into Monday. Developing gusty southeasterly
surface flow ahead of this system will encourage additional
poleward moisture transport. That said, the best upper-level
dynamics associated with this system from an upper-jet perspective
look to remain displaced both immediately south and north of the
region. This, combined with a lack of appreciable surface-based
instability should help greatly curtail the threat for strong-
severe storms with this system. Greatest precipitation chances
look to move in into Monday morning and early afternoon hours
(with a threat for some embedded thunder), but this will be a
quick hit as the warm conveyor rapidly peels east of the area
through Monday afternoon and evening. As a result, while there
will be a window for some locally more moderate to even heavier
rainfall at times, the quick-hitting nature of this system should
keep any associated flood threat to a minimum.

The main concern with this latest system will be from the wind,
with a potential for wind gusts Monday and into Tuesday to rise
into the 30-40 mph range for a period. At this time, gusts look to
remain perhaps just under Wind Advisory criteria, but either way
looks like a period of rather breezy conditions to start off next
week.

There could be some light wrap around precip, though this does
appear to just graze the northwest portion of our area. Given
the more amplified upper flow, brief colder air and generally
dry weather is expected in the wake of this system outside of
any NW flow shortwave activity that will drive another weaker
surface reflection/front across the region mid week.

Another system looks to impact the area during the later part of
next week, but still a lot of uncertainty with regards to
track/intensity and resultant impacts to our weather. It does
appear to bring another brief cool shot.

Carlaw/kmd

&&

Aviation...
for the 00z tafs...

Quiet VFR aviation continues this evening, and will persist
through Saturday as well. Only forecast concern is the potential
for some light showers late Saturday/Saturday evening, though
models continue to trend drier and less enthusiastic with
potential for any significant precip at that time.

In the near term, surface high pressure has moved east of the
forecast area, with breezy south-southeast winds across the
Midwest in advance of a surface cold frontal trough slowly
advancing across the plains. Winds have already diminished with
sunset this evening, and will remain from the south-southeast
through the night under mainly clear skies. 30-40 kt low level jet
will remain focused into northwest Illinois overnight, limiting low level wind shear potential
largely to areas west of rfd and Chicago terminals. Mid-clouds
will gradually spread into the region Saturday ahead of the cold
front, with south winds generally around 10 kts. VFR mid-deck will
thicken up by Saturday evening, with the potential for a few
showers or sprinkles into rfd toward 00z, and for the Chicago
terminals after dark. As indicated above, dry air mass and
weakening forcing with a damping short wave disturbance do not
look to support a significant rain threat, especially for Chicago
terminals, and at this distance have maintained dry tafs.

Surface cold front is expected to move through the terminals mid-
late Saturday evening, with a wind shift to the west, but with not
much push behind it.

Ratzer

&&

Lot watches/warnings/advisories...
Illinois...none.
In...none.
Lm...none.
&&

$$

Visit US at http://weather.Gov/Chicago (all lowercase)

National Weather Service Glossary of Abbreviations