Scientific Forecaster Discussion
fxus63 klot 200814
Area forecast discussion
National Weather Service Chicago/Romeoville, Illinois
314 am CDT Fri Sep 20 2019
314 am CDT
Main forecast concerns/challenges are with thunderstorm potential
along/south of I-80, and then with increasing chances for more
widespread showers/storms on Saturday. Severe threat through the
period appears low, with heavy downpours the main threat.
Latest radar imagery depicting scattered showers/storms
continuing to fester across parts of north central and east
central Illinois early this morning. These storms are developing along
lingering weak boundary/surface trough stretching across parts of
northern Illinois. Earlier development was also supported with a weak
mid level vort, but with its departure, additional support is
stemming from weak low/mid level flow over this boundary/trough.
Convergence and flow both expected to remain relatively weak, and
should assist with keeping coverage limited with additional
development staying confined to areas along/south of I-80 early
this morning. Additionally, intensity and storm organization will
also remain limited with lacking flow and bulk shear, and with
weak lapse rates in place. However, with precipitable water axis overhead and
with weak steering flow, heavy rainfall with localized flooding
will be a concern this morning.
Should see any remaining convection diminish through mid morning,
as boundary/trough becomes more diffuse. An isolated storm can't
be ruled though, with this potential once again staying confined
to areas along/south of I-80 in Illinois. Skies will start off partly
sunny to cloudy for most areas, as cloud cover from the early
morning precip remains overhead. Will see this cloud cover scatter
through midday/early afternoon, with more partly cloudy skies
anticipated this afternoon. As these skies clear, guidance showing
that instability will be on the rise with little to no cap this
afternoon. Chances for isolated storms this afternoon appear low
at this time, with lacking focus. However, have continued isolated
wording in the forecast given the moist and unstable conditions
likely in place this afternoon. Despite early morning cloud cover,
still expect temps to rebound with highs well into the 80s. Any
thunderstorm development later tonight expected to remain to the
west of the area, with a dry forecast still appearing likely.
Will see moisture really ramp up on Saturday, with high pwats
approaching two inches expected to move overhead throughout the
day. Meanwhile, more energetic flow and surface trough/boundary
approaching from the west will provide increasing chances for
storms. Potential exist for storms across north central Illinois
Saturday morning, but more widespread storms look to arrive later
in the afternoon. Highest chances Saturday afternoon will remain
across north central Illinois. Can't rule out an isolated stronger storm
or two, but severe weather appears low at this time. The main
threat will once again be heavy downpours and flooding. Will need
to monitor this period for possible flooding/flash flooding,
especially as additional storms are appearing likely into the
remainder of the weekend.
213 PM CDT
Friday night through Thursday...
Forecast concerns during the long term period continue to focus
on the increasing threat of very heavy rainfall and possible flash
flooding across the area, particularly late Saturday through
An upper level ridge axis, currently in place across the area,
will be shunted to the east by Saturday, as a large upper trough
over the western Continental U.S. Shifts across The Rockies. As this occurs,
strong and deep south-southwesterly flow will be induced across
much of the Mississippi River valley and western Great Lakes
region on Saturday. This will thus allow tropical moisture from
the remnants of Imelda (featuring precipitable water values
around 2') to be transported northward into our area this weekend
setting up the stage for some very heavy rainfall, and possibly
some significant flooding.
While we will begin to see chances for some showers and storms
early on Saturday our best chances for more widespread showers
and storms looks to be Saturday night and especially Sunday. This
is the period in which we will see the large scale dynamic
forcing for ascent maximize as the upper trough and surface cold
front approaches the area. With deep tropical moisture over our
area ahead of this front, deep warm cloud depths will result in
some very efficient rainfall producing showers and storms. As a
result, it appears that at least localized rainfall rates
exceeding 1.5" per hour will be possible at times into Sunday.
Total rainfall amounts across much of northern Illinois and portions of
northwestern in through Sunday evening could exceed 2", with
locally higher amounts in excess of 3" possible. While there does
continue to be some uncertainty with where the heaviest rainfall
axis will occur, trends continue to suggest that much of northern
Illinois could be near this heavy rainfall axis. For this reason, flash
flooding and river flooding will be a concern that we will need
to monitor as we get closer to this event. Since it is too early
to consider any type of watch, we will issue an esf (hydro
outlook) to increase awareness of this increasing potential for
very heavy rainfall.
Surface high pressure is expected to build across the area Monday
following Sunday evenings frontal passage. This should result in a
couple of quiet and pleasant weather days early next week before
the possibility for showers and storms returns to the area mid to
late next week.
for the 06z tafs...
The aviation weather forecast for today is a bit muddled due to
the diffuse nature of a multitude of nearby boundaries and mid-
level waves. As a result, forecast confidence is overall on the
lower end of the spectrum today.
One notable 700-600 mb shortwave is pressing towards the I-55
corridor late this evening and seems to be mostly responsible for
the ongoing area of light shower activity pushing into the
Chicago-area terminals. Instability is limited and, as a result,
do not expect much (if any) in the way of lightning impacts from
this activity. Will show a combination of vcsh and tempo -shra for
a few more hours as this wave presses eastward. While pockets of
showers and possibly some embedded ts may persist through the
mid-late morning hours, currently expect the bulk of this activity
to remain generally south of I-80 and below mentionable in the
Ord/mdw/gyy taf sites as a result.
Light and variable winds will trend towards light south to south-
southeasterly this afternoon. Some potential exists for a lake
breeze push this afternoon, but am beginning to think the chances
of this making it fully to mdw and Ord is decreasing due to the
potential for linger cloud cover and about 15 kts of southwesterly
flow immediately off the surface. Will simply tighten up the
inherited east-southeast wind shift a bit, but it's possible we may be able
to remove this mention as trends become more clear.
Given a lack of identifiable forcing mechanisms this
afternoon/evening, have left the tafs dry for the time being.
There may be a potential for some showers/storms to pop along the
lake-enhanced convergence zone, but forecast soundings look just a
bit too capped to support this potential.
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