Area Forecast Discussion
Issued by NWS Chicago, IL

Home | Current Version | Previous Version | Graphics & Text | Print | Product List | Glossary Off
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50

FXUS63 KLOT 190226

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
926 PM CDT Mon Sep 18 2017

920 PM CDT

Didn`t make any big changes to the going forecast, as the the idea
of showers and thunderstorms moving across the area still appears
to be on track. Did increase pops given overall trends, and made
the bigger changes across the southern CWA along/south of I80.
This is where surface trough and boundary, along with approaching
mid level wave will help to focus the greater ascent and more
widespread precip tonight. Precip chances will lower the further
north, but with locations in the I80 to I88 corridor still having
a chance at seeing precip. Instability axis will stay mainly over
the southern CWA, and have continued the higher chances of thunder
here. With higher PWATs shifting overhead, brief heavy downpours
are likely, and have increased QPF overnight as well.



228 PM CDT

Through Tuesday...

The main weather story through the period will be with the shower
and thunderstorm chances over the area tonight into early Tuesday.

Early this afternoon, a decent mid-level disturbance was noted in
the water vapor imagery shifting eastward over western Missouri.
This feature has been driving an area of showers, with some
thunderstorms, since last night. Therefore, it appears likely now
that a period of some much needed rain will occur over most of
the area tonight. Currently it appears the best timing will be
from later this evening through early Tuesday morning. During this
period, a corridor of better moisture (featuring Precipitable
water values up around 1.6") is expected to shift over the region.
While lapse rates will be marginal for charge separation, it does
appear that a few thunderstorms will be possible within the area
of showers overnight, especially over my southern CWA. Given the
likely presence of decent moisture over the area, it appears that
at least some areas could see a quarter to possibly a half inch
of rain with this activity tonight, especially in areas of any
better convective activity in my south tonight.

Some of these showers may linger over eastern sections of the area
early Tuesday. It also appears that a period of low clouds and
possibly some fog will be possible, especially during the early
morning on Tuesday. Clouds may linger across the area into the
afternoon, and as a result, this could result in slightly cooler
high temperatures than previously forecast over northern Illinois.
As a result, temperatures may remain in the 70s for daytime highs
on Tuesday.



207 PM CDT

Tuesday night through Monday...

As the active weather continues out west thanks to a
strong upper level jet stream and deep upper trough, the midwest
will remain on the warm and drier side of things as the strong
upper jet will dig the backside of the upstream trough and
amplify the upper flow in our area, though there will be period of
showers and storms at least close by if not in some areas for a
few days.

Tuesday night, weak upper ridging will develop, while a strong low
level jet will stir up convection to our west. This ridge appears
like it will amplify some as this convection nears, and with the
main upper forcing expected to shift northward with the surface
low, the convection will likely be in a decaying state as it gets
close to the area. Have to hold onto some lower precip chances
across north central IL Wednesday morning. PWATs and dewpoints
soar to summer like levels with unimpeded south-southwest flow.
Any debris cloud cover may cap temps a bit, but otherwise
Wednesday will be warm and humid.

Storms may again initiate to our northwest ahead of a cold front
Wednesday afternoon. With the high PWAT air in place and the low
level jet expected to flare back up to our west Wednesday night,
there is a bit better chance that some of these may get into the
area late Wednesday, but model guidance is still trying to hold
onto the ridge. We will continue the highest storm chances during
this period of time (Wed night - Thurs morning) for this
potential, but still only in the chance range, and much lower the
farther east you may be.

If we get a bit more convection or the front sags south like the
NAM shows, Thurs could start with some stratus northeast, but will
continue with the global models idea that this remains
north. After this point the moisture convergence axis will shift
even farther north and west as the deep western upper low will dig
into the four corners region through the weekend, and we will
remain well under the strong ridge. No precipitation is expected
with continued well above normal temperatures through the 80s.
The most uncomfortable air will be Thursday-Friday, with some
decrease in dewpoints through the weekend, but still very warm.
The pattern will lend itself to some cooling right near the IL
lakeshore each day. KMD


For the 00Z TAFs...

Aviation forecast concerns are focused on a period of showers
overnight into early Tuesday morning, and the potential for a
period of MVFR ceilings to linger for a time during the morning.

Weak area of low pressure over northeast MO/western IL will
continue to lift east-northeast along a warm front overnight in
response to a mid-level trough propagating across the region.
Warm/moist air lifting across the low level frontal zone will lead
to lowering VFR cloud deck tonight, with showers developing into
the region mainly after midnight. While a few thunderstorms are
likely mainly south of the terminals overnight, mid-level lapse
rates are weaker farther north and thunderstorm coverage should
be isolated over the terminals. Timing from previous forecast of
roughly 06-07Z through about 11Z appears good based on recent
high-res guidance runs.

Forecast soundings indicate that while drying occurs in the mid
and high levels during the day Tuesday, low level moisture will
linger. This would suggest some potential for MVFR ceilings to
linger longer than some of the statistical guidance would support.
For now have hinted with a SCT MVFR deck mention. Winds generally
east-northeast 10 kts or less through the period, though may be a
bit of variability late tonight as the weak surface low passes
just south of the area.



207 PM CDT

Once high pressure north of Lake Michigan shifts east to
Quebec, its associated ridge axis will remain in place across the
eastern third of the country through the remainder of the week. A
A series of low pressure systems will form across the plains, but
with the high in place, most of these will track north and west of
the lake, resulting in an extended period of southerly winds. The
pressure gradient will tighten at times when these lows pass by to
the north, with occasional winds up to 25 kt, generally less. KMD





WWW.YOUTUBE.COM/NWSCHICAGO is the U.S. government's official web portal to all federal, state and local government web resources and services.