Scientific Forecaster Discussion
fxus63 klot 181059
Area forecast discussion
National Weather Service Chicago/Romeoville, Illinois
559 am CDT Fri Oct 18 2019
228 am CDT
Through Saturday night...
Today and tonight: broad mid-level ridging is approaching the area
from the west as surface high pressure over Indiana continues to
drift eastward. Light southeast/south-southeast winds will gradually increase today,
particularly across the western County Warning Area where some gusts may surpass 20
mph. Temps will be near normal in the upper 50s north to low 60s
south under passing upper-level clouds. Another mostly clear night
is in store, though S/south-southeast winds of 5-10 mph should keep temps in the
low to mid 40s.
Saturday and Saturday night: a mid-level shortwave exiting The Four
Corners region today will lift northeastward across the mid-
Mississippi River valley Saturday afternoon and evening. Overall
forcing is limited as the trough becomes increasingly stretched and
the mid to upper-level wind field is not favorable. Additionally, an
associated cold front will be weakening as it interacts with modest
low-level moisture return. Only isolated to scattered showers are
expected to spread across the area, beginning in the northwest County Warning Area
around noon and reaching the Chicago Metro late in the afternoon.
Coverage may decrease while spreading east through the afternoon,
with just widely isolated showers surviving into Northwest Indiana
by the early to mid evening. Any shower activity should be clear of
the entire County Warning Area by late Saturday evening.
300 am CDT
Sunday through Thursday...
Sunday will not be too bad of day for mid to late October. High
pressure will transit the area. We will have mild southwest flow
aloft ahead of our next weather system. We could have some
lingering morning clouds with a warm front in the area and onshore
flow, and we will also see some increasing late day clouds.
Southeast surface flow will prevent any significant moisture
return. Highs should reach the 60s nearly area wide, coolest
Lakeside and far northeast Illinois, and pushing the upper 60s
Our next more organized rain maker is still well offshore and not
well sampled by the radiosonde network. This system will have a
bit more time to advect Gulf moisture northward ahead of it.
Guidance is still hinting that very late Sunday night into the
first half of Monday for the most favor time for rain when
moisture transport under strong southwest flow is maximized. We
could some some precip lingering into the afternoon given that the
system, while still deepening to the northwest and nearing the
process of occlusion. Some weak destabilization is possible with
near 60f dewpoints advecting north in advance of the cold front,
so will maintain a slight chance of thunderstorms. The better
instability axis appears to remain off to our south. Rain could
fall heavily with this system, though all indications are that it
should be fairly progressive. Plus, our period of modest rains
appears limited given current guidance as the stronger forcing
heads to the upper Midwest and we are limited to the warm air advection phase,
and it appears we will get into the dry slot of the deep cyclone.
Monday/early Tuesday appear to be quite breezy both ahead of the
surface low Monday and in the cold advection on Tuesday. Gusts to
30 to 35 mph, locally higher appear plausible.
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.
for the 12z tafs...
Fairly quiet aviation weather conditions will persist through the
High pressure over the terminals will spread east today. Some
patchy outlying MVFR fog will quickly ease VFR conditions
expected to persist through Friday. Expect high clouds to pass
overhead at times.
Southeast winds will increase, as gradient tightens ahead of low
pressure trough lifting northeast from the plains to the upper
Midwest. Gusts in the 15-20 kt range are possible by midday and
early afternoon, with strongest winds across western Illinois. Gusts
will likely diminish quickly with sunset this evening, though
30-40 kt winds above the decoupling boundary layer may produce
weak llws, with a better chance at rfd closer to 6z or more so
overnight for a brief period.
Precip appears to hold off until after the 30 hr taf cycle. Winds
will eventually shift to south and at some point southwest
Saturday morning, occurring first west of the Chicago terminals.
Illinois...frost advisory...ilz023-ilz033-ilz039 until 8 am Friday.
In...frost advisory...inz001-inz002-inz010-inz011-inz019 until 8 am
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