Scientific Forecaster Discussion
fxus63 klot 230726
Area forecast discussion
National Weather Service Chicago/Romeoville, Illinois
226 am CDT Mon Sep 23 2019
207 am CDT
Cold front and much drier air steadily progressing through the
region early this morning, and will continue to push through the
area over the next couple of hours ushering in a precip free
period for the start of the day. The showers which have been
developing along and ahead of surface trough/boundary in east
central Illinois and northwest in will likely continue for the next 1-2
hours. This area is still rather moist and unstable, but this air
mass and focus will shift east and end this development across the
area, with dry conditions the trend. Then expect clearing skies
to continue with sunny/clear to partly cloudy skies expected
through the period, with a return to more normal temps.
224 am CDT
Tuesday night through Sunday...
Next chance of precip arrives at the start of the period, late
Tuesday night into Wednesday morning. Surface ridge which is
expected to be in place early in the work week will shift to the
east on Tuesday, as a mid/upper level trough dips south through
the northern plains and while a surface trough takes shape. This
should allow a return to southerly flow, and with warm air advection/moist
advection the trend. While warm air advection is anticipated, it is appearing as
if the better moisture return later in the day Tuesday into
Tuesday night will be to the west of the area. Dewpoints are
forecast to be well into the 60s across IA, with only 50s
dewpoints across northern Illinois and northwest in. This will help keep
the main instability axis also to the west Tuesday night, with
little to none overhead. This will be key with shower/thunderstorm
chances late Tuesday night into early Wednesday, and how intense
any convection could be when it arrives. If the drier trend with
less instability continues, then this would definitely limit both
coverage and intensity during this time.
Will see a strong 140kt upper level jet spread across the
northern plains and upper Midwest late Tuesday night, while rather
vigorous mid levels also approach from the northwest. This large
scale support along with approaching surface trough and
strengthening low level jet will help support shower/thunderstorm
development Tuesday evening well west of the area. This
east/southeastward moving surface trough/front will then help
steer this development towards the County Warning Area late Tuesday night. Higher
moisture air and instability axis ahead of this trough is also
expected to swing closer to the County Warning Area during this time. However, do
still have some lower confidence to the extent of the precip
coverage and intensity, with even latest guidance varying on these
items. The larger scale lift is anticipated to lift northeast by
early Wednesday while the approaching moisture/instability axis
possibly gets shunted to the south, once again both of which will
impact coverage and intensity. So, at this time, do think precip
chances remain warranted, especially across north central Illinois.
Surface trough/front and focused low level jet could still support at least
scattered development. Intensity will have to continue to be
monitored as the approaching instability could briefly support a
few stronger storms. At this time though, think the threat of any
widespread severe weather is on the lower side.
Trough/front swing through the area during the day on Wednesday
with some potential for scattered showers and storms to continue
area wide. With stronger flow/shear to the north and higher
instability to the south, think the threat of severe storms is
also low on Wednesday. A dry period with normal temps then
expected, before precip chances arrive again by the end of the
work week into the weekend. Previous guidance was varying on
pattern/setup, especially for Thursday night into Friday. However,
latest guidance is showing more of a consistent signal for active
weather during this period, and will need to continue to monitor
the thunderstorm threat. Pattern into the weekend still looking
active with precip chances warranted in the forecast.
for the 06z tafs...
The only aviation weather concerns this period are lingering
spotty cigs late this evening.
Cigs vary from VFR to IFR across the region late this evening as
we haven't quite scoured out all of the low-level moisture. Think
that any periodically reduced cigs will be temporary with region-
wide improvement after 08-09z as upstream drier air finally pushes
into the area.
Looks like a deck of cu/strato-cu may develop later this afternoon
as we mix into moisture lingering in the 3-5 kft layer. As this
occurs, we'll mix down some slightly stronger flow off the deck
in the form of occasional wind gusts to 20 kts or so through the
afternoon. Tonight, surface high pressure will drift south of the
area, which will allow winds to become either light and variable
or turn out of the southwest around 4-8 kts.
In...beach hazards statement...inz002 until 4 PM Monday.
Lm...Small Craft Advisory...in nearshore waters until 4 PM Monday.
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