Scientific Forecaster Discussion
fxus63 klot 180127 aaa
Area forecast discussion...updated
National Weather Service Chicago/Romeoville, Illinois
827 PM CDT Thu Oct 17 2019
827 PM CDT
Had issued a frost advisory for the southeast forecast area (south
of the Kankakee river) earlier this evening. This area stands the
highest chance to have calm winds the entire night and has
already seen a sharp temperature drop in the immediate Post-sunset
time frame. Ideal radiational cooling and a favorable hydrolapse
in the near-surface air will support areas of frost in this
southern part of the forecast area with the highest confidence in
the County Warning Area. Elsewhere, at least patchy frost is expected in outlying
locations. Limiting factors for more widespread frost further
north include a light wind (not calm) expected overnight in north
central Illinois, and possibly some cirrus moving in.
242 PM CDT
Through Friday night...
Main forecast concerns/challenges are with possible frost tonight
across the area.
Relatively quiet weather in place as high pressure continues to
build across the region. A fair amount of cloud cover remains in
place as the region is on the western fringe up the upper level
trough. The mid/high cloud cover will steadily exit the area over
the next several hours, while the stratus may linger into the
evening across parts of northeast Illinois and northwest in. There is a
diurnal component to this cloud cover, with some erosion likely
this evening. Did increase cloud cover for these locations into
mid/late evening though, to account for a slower clearing trend.
Should see any remaining stratus depart to the east/northeast
later tonight as flow backs. Light winds and clearing skies for
most areas tonight will support lows in the mid to upper 30s.
Under this setup, am mainly getting patchy frost tonight and have
held off on any frost headlines at this time. It is possible that
some areas could see lower temps and with frost to be more
widespread than currently forecast. Will continue to monitor
trends into this evening for this possibility. Sunny/partly cloudy
skies with highs in the 60s expected on Friday.
242 PM CDT
Saturday through Thursday...
Dampening lead shortwave will eject out across the region
Saturday with increasing shower chances, with latest guidance
still favoring western County Warning Area. Blended model pops still seem
reasonable for Saturday with mainly chance pops in place, as some
variability among model guidance on the extent of the precip
shield remains. The cyclone progged to develop in the Gulf is
likely to result in northerly Continental air advecting into the
northern Gulf of Mexico in its wake Friday night into Saturday.
The lead shortwave impacting our area Saturday looks to limit that
scouring of the Gulf and in should allow for weak moisture return
to get underway into Texas later Saturday into Saturday night. This
should lay the ground work the powerful cyclone moving into the
northern plains Sunday to really tap into the deeper Gulf moisture
and draw it northward into the cyclone.
Guidance is starting to lock in on late Sunday night into the
first half of Monday for the most favor time for rain. Some weak
destabilization is possible with near 60f dewpoints advecting
north in advance of the cold front, so maintaining a slight chance
of thunderstorms. Rain could fall heavily with this system,
though all indications are that it should be fairly progressive
and keep rainfall amounts from getting out of hand. While stream-
flow remains generally above average, the recent dry weather will
likely allow US to absorb some healthy rainfall totals without any
significant flooding problems, rather just some renewed
rises/slower falls on main Stem rivers.
Colder air and dry weather is expected in the wake of this
system. Another system looks to impact the area during the
middle/later part of next week, but still a lot of uncertainty
with regards to track/intensity and resultant impacts to our
weather with that system.
for the 00z tafs...
Quiet aviation weather conditions will persist through the period.
Weak lake breeze finally pushing across Ord early this evening,
with winds shifting to light easterly at this time. Coincident
with surface high pressure ridge drifting east of the forecast
area, winds should settle to light east-southeast across the area
(or just calm or light/variable) overnight. Area of VFR stratocu
across far northeast Illinois looks to remain just off to the northeast
of the Chicago terminals for the most part, though could be a
brief bkn035-bkn040 ceiling this evening before winds in the
cloud-bearing layer shift south. Some patchy MVFR fog is not out
of the question by sunrise, though expect this to be limited and
mainly away from Metro Chicago. VFR conditions expected to persist
through Friday, with only patchy high clouds at times.
On Friday, 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 likely by
midday and early afternoon, with strongest winds across western
Illinois. Gusts will likely diminish quickly with sunset Friday evening,
though 30-35 kt winds above the decoupling boundary layer may
produce weak low level wind shear conditions especially west of the Chicago area
Illinois...frost advisory...ilz023-ilz033-ilz039...1 am Friday to 8 am
In...frost advisory...inz010-inz011-inz019...1 am Friday to 8 am
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