Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

fxus63 klot 192348 

Area forecast discussion
National Weather Service Chicago/Romeoville, Illinois
648 PM CDT Fri Jul 19 2019

Short term...
316 PM CDT

Through Saturday night...

The heat is on across the area this afternoon. While actual air
temperatures have been slow to warm into the lower 90s, and may
fall a couple of degrees short of the forecast high temperature.
While this is the case, the end result is the same, that being the
heat indices topping our around, or even a bit above 110 degrees.
Expect temperatures to hover in the lower to middle 90s late this
afternoon, with heat indices holding in the 110 to 115 range.

As mentioned previously, little relieve is expected across the
area tonight, as overnight lows remain in the upper 70s in the
outlining areas, and in the lower 80s in the urban areas. This
will keep the minimum heat index in the city in the 90s for much
of the night. There is once again a very small threat of some
showers and storms overnight over far northern Illinois. This as
thunderstorms likely to occur to the north of the area try to lay
another outflow boundary over southern WI or far northern Illinois
overnight. Similar to this morning, it appears any activity would
likely dissipate quickly as it approaches the area.

Otherwise, more of the same is expected tomorrow. I did lower
temps slightly, given the struggle for them to warm as much in
this very most airmass. Nevertheless, expect dangerous heat to
continue Saturday. Highs in the low to mid 90s and dew points in
the upper 70s to around 80 will again put heat indices up around,
or a bit above 110. The good news is that there is light at the
end of the Tunnel as a cold front will be approaching the area
from the north late in the day. Once this front passes Saturday
evening, this will bring an end to the excessive heat across the
area. It will, however, come at the cost of more thunderstorms.

In terms of the thunderstorms there remains uncertainties in how
quickly they will develop over the area later Saturday into
Saturday evening. There remains a decent possibility that
a complex of thunderstorms, originating well west of the area
could try to shift over northern Illinois and southern WI late tomorrow
afternoon. If these storms do make it into the area, we will
likely see the winds shift northward over much of far northern Illinois
by late afternoon. We could also have a wind threat with these
storms later in the day. Otherwise, it appears that elevated
thunderstorms chances will increase across northern Illinois Saturday
evening and night as the elevated frontal boundary sets up right
across southern WI and northern Illinois. This could set the stage for
some periods of very heavy rainfall for portions of northern IL,
and could result in some flash flooding potential. Confidence in
the overall convective evolution Saturday night remains a bit
uncertain. However, there are signs that these storms will have
an eastward movement parallel to the frontal boundary,
potentially setting up some training cells. If this occurs, this
could easily set up a flash flooding threat over northern IL,
especially considering that these storms will likely be prolific
rainfall producers, with precipitable water values expected to be
at or above 2". This threat could continue into Saturday night,
though the focus may gradually sag southward across the area
through the night.



Long term...
332 PM CDT

Sunday through Friday...

A period of much quieter and pleasant weather is expected for
much of next week following the Saturday night cold front passage.
While thunderstorms will still be likely on Sunday as an upper
trough dig southeastward towards the area, the focus for storms
will gradually shift south of the area by Monday. The upper tough
will set up overhead late Monday, and this should result in a
rather cool day across the area, with highs only in the upper 70s
to around 80. A large area of surface high pressure is then likely
to setup over the region for the middle part of the week, and
this will result in mainly dry and cool conditions over the area
through much of the week. Temperatures will gradually moderate
back into the mid to upper 80s later in the week as this surface
high shifts east.



for the 00z tafs...

648 PM...forecast concerns include...

Possible wind shift/outflow early Saturday morning.
Patchy fog early Saturday morning.
Wind shift/cold front late Saturday afternoon/early evening.
Chance of thunderstorms...mainly Saturday evening.

Southwest winds will remain occasionally gusty through sunset with
speeds diminishing to 10kts or less later this evening. Convection
currently over northern WI is expected to remain north of the
Illinois/WI state line overnight...though trends will need to be
monitored. But the further south this activity trends tonight...
the better the chance of an outflow moving across the terminals
toward daybreak Saturday morning. Confidence is low and currently
do not have any outflow/wind shift in this forecast.

Given the very moist airmass...there is a chance of fog/haze
overnight/Saturday morning and maintained mention at rfd/dpa but
confidence is low.

Southwest winds will continue...or return if there is an early
morning outflow...on Saturday with gusts likely from mid morning
into the afternoon. A wind shift/cold front is expected late in
the afternoon or early Saturday evening...shifting winds to the
northeast. Confidence on the timing of this front is low and if
the coverage of thunderstorms across WI is large enough Saturday
afternoon...this could push the outflow/wind shift through faster.
Eventually...winds are expected to turn northeast Saturday

Thunderstorm trends Saturday afternoon into Saturday night are
uncertain. The above mentioned wind shift/front may be the focus
for thunder development at the terminals. With the later wind
shift timing...went with later thunder timing. Some of the models
are actually rather dry even with the wind shift. Thus confidence
remains low for this time period. Cms


Lot watches/warnings/advisories...
Illinois...excessive heat warning...ilz003-ilz004-ilz005-ilz006-ilz008-
ilz022-ilz023-ilz032-ilz033-ilz039 until 7 PM Saturday.

In...excessive heat warning...inz001-inz002-inz010-inz011-inz019
until 7 PM Saturday.



Visit US at http://weather.Gov/Chicago (all lowercase)

National Weather Service Glossary of Abbreviations