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Area forecast discussion
National Weather Service Chicago/Romeoville, Illinois
918 PM CDT Tue Jul 23 2019

Update...
913 PM CDT

Shortwave driving out of WI is now moving into a more capped
environment with loss of daytime heating, but the better
colocation of the wave and slightly better moisture remains off
to our west with water vapor satelitte very dry in the low levels
over NE Illinois and Northwest Indiana. Therefore any showers would be
minimal this evening.

With high pressure nosing in some patchy fog is possible out west
though guidance is generally not hitting it very hard.

Kmd

&&

Short term...
216 PM CDT

Through Wednesday night...

Really can't ask for a much better early afternoon for the end of
July. Temperatures are in the upper 70s and lower 80s across the
region with dewpoints mainly in the mid to upper 50s with a light
northwesterly breeze. A lake breeze continues to push steadily
westward at this hour, and this will ensure that immediate lake
side locales remain in the lower to mid 70s today.

A couple radar blips have appeared out of an area of more
congested cumulus which stretches more-or-less along I-57 along an
axis of enhanced low-level instability. Through about 5 PM or so,
diurnally-enhanced planetary boundary layer circulations and enhanced convergence in
the vicinity of the aforementioned lake breeze will be the main
forcing mechanisms for spotty showers and thunderstorms. As a
result, have confined pops through this afternoon along and west
of the lake breeze and nudged them up a bit near I-55/57 near the
more developed cu field. While mesoanalysis currently shows
equilibrium temperatures under -40 c which would support
more widespread storm electrification, amdar soundings reveal the
subsidence inversion noted on 12z soundings is still present, with
a relative maxima in the thermal profile around 600 mb. This
"cape" robber will tend to limit diurnal gains in instability, so
don't think we'll see quite as much lightning activity as is
occurring across Michigan (where mid-level temperatures are
colder).

After 5 PM or so, a shortwave which can be seen clearly in upper-
level water vapor imagery across northern Wisconsin will begin to
scrape our County Warning Area. This should allow an additional area of isolated
to scattered showers and storms to develop near another axis of
relatively maximized instability which is progged to develop
across south central and southwestern Wisconsin. This activity
should drift southward into our County Warning Area through the early to mid-
evening hours. Have nudged pops up in the vicinity of Rockford to
account for this additional activity. If instability/moisture were
in greater supply, we'd be talking about more of a strong-severe
thunderstorm threat given roughly 35-40 kts of flow at 500 mb.
Given the dearth of instability today, we're not expecting severe
weather, but some gustier winds to 40-50 mph can't be ruled out
with any of the taller storms. Activity should gradually cease
through the mid-late evening hours.

For tonight, there's a low potential for some patchy light fog
across our western counties, but given the rather dry low-level
airmass in place, opted not to add a mention to the official
forecast at this point.

Wednesday is shaping up to be another nice day as dry air lingers
across the region. Prevailing northeasterly winds off the lake
will keep the Lakeshore once again in the mid 70s, while lower to
possibly mid 80s prevail inland. There will be a very low chance
for some showers or storms across our northwest in the afternoon
in the vicinity of another instability axis, but coverage is not
expected to be more than 15-20%.

Carlaw

&&

Long term...
216 PM CDT

Thursday through Tuesday...

Generally quiescent conditions are expected to prevail through most
of the long term portion of the forecast as temperatures gradually
warm back to near and above normal values over the weekend and into
next week.

Surface high pressure will begin to slide east of the region on
Thursday, and this will allow light southwesterly flow to return.
Moisture will take some time to slosh back into the area, however,
so Thursday and Friday are still shaping up to be pretty comfortable
days with dewpoints in the lower 60s with highs in the upper 80s to
near 90 degrees. The low-level flow should remain light enough on
Thursday to allow a lake breeze to sneak inland during the afternoon
hours, so we've cooled highs just a tad along the immediate Lake
Shore. By Friday afternoon, 20-25 kt winds just off the surface and
a tightening pressure gradient should preclude any lake breeze push.
The GFS has been producing some light quantitative precipitation forecast across portions of the
area on Friday afternoon ahead of a sharp mid-level shortwave.
Forcing for ascent from this next feature looks to remain displaced
well to our north, however, with persistent height rises through the
day. Forecast soundings depict a dry and warm subsident 850-600 mb
layer overhead as well, so we'll continue to discount guidance
producing precipitation at this time and message a continued dry
forecast into Friday evening.

Breezy and warm conditions will be the rule over the weekend with
negligible precipitation chances. The next appreciable chances for
showers and thunderstorms looks to move in sometime during the
Sunday night-Tuesday timeframe as the next cold front sweeps through
the area.

Carlaw

&&

Aviation...
for the 00z tafs...

638 PM...only forecast concern this period is a chance of showers
and possibly thunder...mainly west of the Chicago terminals this
evening and again Wednesday afternoon.

Scattered showers and thunderstorms across southwest WI will
continue to move generally southerly this evening and slowly
weaken after sunset. This activity will remain well west of the
Chicago terminals and may remain west of rfd as well. A few
showers are possible across eastern Illinois...south of the terminals
through sunset. There is another chance of isolated showers and
possibly a thunderstorm Wednesday afternoon...mainly west of the
Chicago terminals. Confidence is too low for mention at rfd with
this forecast.

Northeast winds east of the lake breeze will become light
northwest later this evening with winds possibly going light and
variable or even calm overnight. Winds will turn northeasterly for
the Chicago terminals Wednesday with a possible Early Lake breeze
but only medium confidence for timing. Cms

&&

Lot watches/warnings/advisories...
Illinois...none.
In...none.
Lm...none.
&&

$$

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