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Area forecast discussion 
National Weather Service Chicago/Romeoville, Illinois
934 PM CDT Friday Jul 29 2016

Update...
934 PM CDT

Things have quieted down this evening. We opted to transition the
Flash Flood Warning for the western suburbs to an areal Flood
Warning valid into early Saturday afternoon. This time might be a
bit long, but due to 3 to 4 with isolated higher amounts falling
in this area, especially over southern dupage and portions of
Kane County, numerous roads were still reported to be impassible
this evening. Although some scattered showers are still possible
overnight, the threat of heavy rain has ended for the night.
Another area hit particularly hard this afternoon was southern
Cook County and extreme Northwestern Lake County in Indiana. This
area also experienced 3 to 4 inch amounts. The Flash Flood Warning
for this area ends just before 11 PM this evening. This will also
likely be replaced with an areal Flood Warning into Saturday.

Water vapor imagery this evening continues to indicate a slow
moving mid-level perturbation over northern Illinois. This
feature will likely continue to support some widely scattered
showers overnight, though most areas will likely be dry for most
of the night. The weather pattern changes little on Saturday,
therefore additional slow moving scattered thunderstorms are
possible again Saturday afternoon.

The other concern tonight, like last night will be fog
development, especially over the southern half or so of the area.
Some dense fog is also possible late.

Kjb

&&

Short term...

Through Saturday night...

236 PM...thunderstorms developed along the weak cold frontal
boundary earlier this afternoon and have been training over the
same areas producing torrential rain and flooding. An outflow
boundary moving south of the current activity should continue to
provide the focus for additional thunderstorm development through
early this evening. Short term/hi res guidance has all been in
fair agreement with this activity weakening and dissipating this
evening. The main threat from these storms will continue to be
heavy rain and flash flooding but isolated strong wind gusts will
also be possible.

Models show a weak wave moving across the region overnight into
Saturday morning which may allow additional showers/thunderstorms
to develop overnight toward morning. Confidence is fairly low
regarding coverage and location...which could end up mainly north
of the cwa and over the lake but maintained low chance pops thru
the night and trends will need to be monitored later this evening.

The front will slowly sag south across the region tonight into
Saturday and similar to this morning...there will likely be areas
of fog...possibly dense across the southern cwa. Added patchy fog
away from the Metro area overnight but trends will again need to
be monitored this evening and overnight.

Weak low pressure over the southern lakes region will slowly begin
to move east Saturday and Saturday night...but additional
thunderstorm development is expected by late Saturday morning and
should continue through Saturday evening...similar to today and
Thursday. Location of thunderstorm development is uncertain due
to possible ongoing precip Saturday morning but focus may end up
being over the southern cwa by Saturday afternoon. Activity should
dissipate Saturday evening with mainly dry conditions Saturday
night along with the potential for fog...especially over the
southern cwa. Cms

&&

Long term...

Sunday through Friday...

300 PM...high pressure will be building south across the lakes
region Sunday into Sunday night. This will allow a lake breeze to
develop Sunday afternoon which may allow isolated showers or
thunderstorms to develop during the afternoon. Confidence however
remains low and coverage could end up being quite isolated. As
this time period approaches...some slight chance mention will
likely be needed but opted to remain dry with this forecast and
see if future runs continue with these trends. This high appears
to depart more slowly and much of Monday continues to look dry
with any activity holding off until Monday evening or more likely
Monday night into Tuesday. Quite a bit of difference between the
GFS/ECMWF regarding precip coverage and amounts. Maintained chance
pops for now but quite a bit of uncertainty during this time
period. Uncertainty continues mid/late week with how fast the
upper ridge builds into the area...winds off the lake and how fast
an approaching cold front arrives from late Thursday afternoon
through Friday morning. Cms

&&

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

Somewhat messy aviation forecast into Saturday, with scattered
showers and the potential for IFR/MVFR ceilings to develop and
persist overnight into early Saturday. Saturday will likely be
somewhat similar to today, though some guidance suggests the focus
for showers/thunderstorms will remain just off to the south of the
terminals during the day.

Warm, humid and unstable air mass remains across the forecast area
early this evening. Weak surface low pressure was located over
central Indiana, with a trough trailing across northern Illinois
in association with a very slow moving mid-level disturbance which
will propagate across the region tonight and Saturday. In addition
to the surface trough, multiple outflow boundaries have developed
from the scattered showers/thunderstorms today, with some local
variation of wind direction within an otherwise generally synoptic
northeast flow. Currently, the greatest concentration of
thunderstorms appears to be with an embedded circulation over
Southern Lake Michigan, and across northwest and north central Illinois
beneath the larger scale upper circulation center. Current precip
minima over Ord/mdw may see a few showers work back in from the
southwest this evening, though these should remain somewhat
spotty with a relatively low coverage of ts. Very moist low levels
will be at risk for development of low clouds and some fog
however, with much of the available guidance depicting MVFR
ceilings by late night and extending into Saturday morning. Less
low-cloud currently upstream and over the lake than last night, so
not expecting prolonged IFR conditions.

Mid-level circulation still slowly shearing east across area
Saturday, with a redevelopment of scattered showers and
thunderstorms expected by mid/late morning. As noted above
however, several model runs focus the greatest threat just off to
the south of the terminals. Have maintained a vcsh at this time
with some uncertainty as to where the trough axis will be. If
convection does develop closer to the terminals, a mention of
thunder would likely be needed. Northeast winds expected through
the period.

Ratzer

&&

Marine...

434 PM...weak low pressure over the southern lakes region will
gradually move east Saturday and Saturday night while high
pressure over the northern lakes builds south Sunday into Sunday
night. A period of east/northeast winds 15-25kts is expected
tonight over the southern portions of the lake. These winds may
diminish a bit Saturday morning but then increase again and turn
more northerly Saturday afternoon. Northerly winds will continue
Sunday with winds eventually becoming light southeast by Monday
morning as the high settles across the lake. Winds will turn more
southerly Monday into Tuesday as a large trough of low pressure
develops over the plains. Cms

&&

Lot watches/warnings/advisories...
Illinois...beach hazards statement...ilz006-ilz014 until 4 am Sunday.

In...beach hazards statement...inz001 until 4 am Saturday.

Lm...none.
&&

$$

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