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Area forecast discussion
National Weather Service Chicago/Romeoville, Illinois
934 PM CST Wed Nov 20 2019

918 PM CST

Main updates this evening were to speed up pops late this evening
and again early Thursday morning, as well as include thunder.

As mid level energy lifts across the mid/upper Mississippi
Valley, will continue to see precip Blossom and approach northern
Illinois late this evening into the overnight hours. This is already
beginning to occur, with an arc of precip, focused along 700mb warm air advection
Wing, quickly moving into northwest Illinois. This initial arc of
showers will clip locations along the I-39 corridor over the next
several hours, and then continue into WI. Have noted some
lightning strikes with these showers this past hour, with the warm air advection
and steepening mid level lapse rates likely supporting this.
Despite some lessening in coverage of thunder at this time, do
think a few lightning strikes will be possible over the next
several hours, once again along the I-39 corridor, and have
included thunder in the forecast.

No real big changes to the pops and timing later tonight into
Thursday morning, other than to account for the steadily
progressing showers, and slightly speed up timing. Did include
thunder for Thursday morning, mainly along/south of I-80, as
currently upstream instability axis looks to swing through while
forcing increases. Highest potential is south of I-80, but will
monitor the need to expand thunder further northward, into the
I-88 corridor. Severe weather not expected, however, the
convective nature of the precip could support isolated higher
rainfall amounts. In this situation, flooding should still not be
an issue, but may support more pronounced rises in some rivers.



Short term...
220 PM CST

Through Thursday night...

The short term forecast concerns are centered on rain timing and
intensity late tonight and Thursday, with the main message being a
period of moderate rain encompassing most of the area during the
morning commute. While some decent rain amounts for late November
may result in north central Illinois, this does not look to cause
noteworthy flooding or substantial river rises (see the hydrology
discussion below for more).

As upper level ridging quickly moves across the area this evening
it will be replaced by strengthening southwest flow aloft. A short
wave circulation within this flow will result in both a
strengthening surface low from the plains tonight to Wisconsin
Thursday morning and a strong poleward surge of moisture via a
low-level jet tonight. Arcs of showers should eventually become
areas of more moderate showers into the County Warning Area by the pre-dawn hours.
Embedded isolated lightning may even result near or into
northwest/north central Illinois near daybreak due to the elevated
warm-advection induced steepening lapse rates. The period with
the greatest quasi-ageostrophic lift atop the 55-65 kt moisture
transport looks to be between 3 and 9 am, and the highest
moderate rain coverage should unfold then. An impact will likely
be a slowed commute in places. By afternoon, deeper moisture and
forcing is forecast to have moved past with the system dry slot,
but enough low-level isentropic ascent is forecast to continue
for likely drizzle and patches of shallow rain showers especially
through mid-afternoon.

While the strongest warm advection will be off the deck, the
surface will see its fair share of warming tonight with upward
temperatures by late evening all the way through midday Thursday.
Readings should reach mid to upper 50s area wide thanks to
increasing southerly winds. Despite the robust flow not far from
the surface, limited mixing should keep gusts in check. As the
cold front moves through, deeper mixing in the afternoon should
result in some of the higher gusts to 35-40 mph. Temperatures will
steadily fall behind the cold front, not too extreme for the time
of year but still enough of a change, with lows by Friday morning
dipping to mid 20s north to mid 30s south.



Long term...
240 PM CST

Friday through Wednesday...

The long term looks to hold a gradual evolution of the upper
pattern to a general trough over western North America, though how
quickly that develops toward Thanksgiving has quite a lot of
variance in guidance. It would place the middle of the country in
a continued active zone of weather, with potential for one or two
modest to strong Lee-side to plains systems (sub 1000 mb lows).
With the time distance from now in this pattern, and the solution
variance between models and run-to-run, the confidence on any
high impact weather occurring is low, though there are some
synoptic features in place that point toward potential of gusty
winds and possibly a couple episodes of heavy precipitation
somewhere in the Midwest and Great Lakes.

For Friday, there is enough of a push behind the cold front that
sun is expected with highs about five degrees below normal. On
Saturday a southern stream progressive, opening wave is forecast
to move eastward into the Ohio River valley. The European model (ecmwf) continues
consistent with almost all precipitation (likely light snow on
the system's northern periphery) remaining south of the area.
Clouds though are possible and temperatures on Saturday will be
dependent on that.

As already referenced, large differences exist in global model
guidance solutions for next week. The pattern on the European model (ecmwf) stays
more progressive, with lower amplitude disturbances through
Wednesday while the GFS becomes more amplified quicker. This
results in even wider difference by the Holiday weekend. For now
for the midweek pattern have kept the chance of rain 50 percent
of less for Tuesday and Wednesday. Temperatures as a baseline look
to be near normal, but obviously once more amplified, then larger
swings are more probable.



253 am CST

Rivers across the area continue to run abnormally high, so a
response is likely from the rain late tonight into early Thursday
morning. Heaviest rainfall looks to occur over the rock and upper
Fox and upper Des Plaines basins, where isolated totals near 1
inch are possible in about a 6 hour period late tonight. If this
heavy rain materializes as expected, then there will probably be
some river points experiencing minor flooding. Contingency
forecasts do support this idea, in particular in the Rock River
basin. The fast movement of the system should keep rainfall
amounts from getting out of hand, so not a significant problem on
main Stem rivers. Otherwise, there could be some ponding of water
in poor drainage areas, particularly where gutters and storm
drains may be clogged with leaves. Given the expected minor
impacts, no plans for a Flood Watch or esf.

- Izzi


for the 00z tafs...

Cigs/vis have improved to VFR and winds have settled into a sely
direction as the sfc high center has moved to the upper Ohio
Valley and low pressure deepens over the Central Plains. This
system, which will move into the region later overnight and
tomorrow will bring the next significant precipitation to the
region. Rain will overspread the area after midnight tonight, and
continue into tomorrow morning. A period of heavier showers,
potentially reducing visibility into IFR range at times, is likely
during the late morning hours, in association with the strongest
forcing. Some drizzle will likely persist into the afternoon,
before the associated cold front pushes through the area.
Precipitation and a moist low level air mass will most likely
support ceilings in the low MVFR/high IFR range during the
morning/early afternoon prior to cold fropa with MVFR cigs
possibly lingering through the afternoon.

Forecast soundings indicate that winds above the boundary layer
to increase into the 60-70kt range as low as 2 kft agl, resulting
in low level wind shear conditions. Surface wind direction will become sswly
through the morning and become increasingly gusty as the sfc based
inversion breaks and allows stronger winds to mix down to the
surface. Winds will then shift to wly behind the cold front by
mid-afternoon, with gusts in the 25-30 kt range. Winds will
continue to shift nwly by the early evening hours.


Lot watches/warnings/advisories...
Lm...Small Craft Advisory...Gary to Michigan City in until 3 PM Friday.

Small Craft Advisory...Winthrop Harbor to Gary in until 10 am



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