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Area forecast discussion...updated 
National Weather Service Chicago/Romeoville, Illinois
852 PM CDT Sat Jul 23 2016

Mesoscale discussion...
851 PM CDT

Storms moving into DeKalb and far northern LaSalle counties have
shown some up and down in intensity. The northern area of these
storms is moving into rain-modified air from late afternoon storms
and this may cause a weakening as it approaches the western
Chicago Metro. There remains pockets of stronger wind gusts
however, including 58 mph at 3000 ft just north of Creston,
warranting a Severe Thunderstorm Warning for DeKalb County. The
southern portion of the storms, including any new storms into
LaSalle County, will have a more untapped air mass and may have a
little better potential for severe winds if storms can develop.
Earlier signs of leading line rotation have ended, likely because
the apex of these storms is now over the rain cooled air, and
overall the ambient low-level flow is light (850-925mb at 15-20 kt
and surface flow at 10 kt). So the main threat remains isolated to
scattered severe winds across DeKalb and possibly LaSalle counties
through 10 PM. As long as the trend of storms is slightly
downward, the watch should be allowed to end at 10 PM despite
thunderstorm activity ongoing at that time.

Mtf

&&

Short term...
347 PM CDT

Through Sunday...

A Severe Thunderstorm Watch has been issued for northern Illinois,
generally along and north of I-80, through 10 PM this evening. No
changes made to the heat headline or any issuance for Sunday due
to uncertainty and residual effects of nighttime convection.

Convective cluster in northeast Iowa has raised concerns as cloud
tops have rapidly cooled and is further showing signs of
organization. Mesoscale convective vortex (mcv) across central
Iowa is likely the driving feature and has become more clear this
afternoon on satellite with mid-level speed Max supporting
effective shear of 35 to 50 kt. Also there is support for a cold
pool to develop and grow. The effective surface front and
instability gradient, affected by outflow from storms to the
north, is draped across far northeast Iowa and far near/just south
of the WI/Illinois state border. This likely would be the path that
organized storms would travel eastward, though propagation would
indicate a slight south of eastward movement too.

There is high uncertainty on just how organized these storms will
become as well as how far south this evening. There is capping in
place the further south into the area and the storms are
immediately moving into somewhat worked over air due east of
them. But given the degree of instability (mlcapes of 3500 to 5000
j/kg) and moisture (pwats around 2 inches) over a large part of
the dvn County Warning Area and ours, at least some convective cluster(s) are
likely to make it into our area. The best timing estimate of these
reaching the Rockford area appears to be around 7 PM and the
Chicago area 830 to 11 PM. Again this is assuming the storms have
some form or organization.

The primary hazards with these evening storms would be pockets of
severe winds due to high downdraft convective available potential energy and precipitation
loading, especially any more organized activity. Near the outflow
boundary there may be a localized tornado threat due to backed
low-level wind flow, and the boundary layer is quite moist for
maintaining low-level instability into the evening. Storm motion
could be fairly slow especially anything that develops ahead of
the main cluster(s) and would be of greatest concern for localized
flooding.

There is now naturally less confidence on late night/daybreak
Sunday storms but maintaining likely in northern areas where
vort Max is pegged to move eastward across Wisconsin and the low-
level jet points over our area. The less organized evening storms
are the more likelihood of overnight storms and that they could
bring a marginal severe weather risk.

For Sunday, high dew points in the mid to upper 70s are expected.
The thermal ridge immediately ahead of the cold front will support
mid 90s for highs if we can clear and have full heating. There is
high uncertainty of this so that is why no heat advisory at this
time. One may be needed for a large portion of the cwa, including
Chicago, if the going forecast continues. The front itself will
probably spark scattered storms although how much coverage will
likely be impacted by how storms behave tonight.

Mtf

&&

Long term...
213 PM CDT

Sunday night through Saturday...

Sunday evening through Monday...the upper trough currently moving
east across southern Saskatchewan will swing across
western/central Ontario Sunday night and Monday. The associated
cool front will be dropping southward across the southern half or
so of the local area Sunday evening. Will continue with chance
pops Sunday evening/night across the south to southeast half of
the area as the front will be slow to progress and there is a
decent chance of an embedded shortwave moving through the main
upper flow. Some activity may linger Monday morning across the far
southern counties but surface high pressure will be quick to build
in bringing drier air with it. 850 mb temps cool to 16-17c Monday
with abundant sunshine expected once any lingering clouds clear
south. This supports highs in the mid to upper 80s but onshore
flow will develop near Lake Michigan keeping areas within a few
miles of the lake in the lower 80s. Lows Monday night will fall
into the mid 60s with upper 60s/around 70 in the Chicago Metro.
High pressure will be overhead Tuesday and shift to the east
Tuesday night. High temps will be similar to Monday with
continued cooling near Lake Michigan.

Wednesday through Saturday...the generally zonal upper flow
of Monday and Tuesday will buckle a bit mid week with medium
range guidance suggesting a more pronounced larger scale trough
developing into and over the weekend. Until then...the surface
pattern is fairly ill-defined through Wednesday but the upper
flow patter supports the passage of embedded shortwaves. This will
probably lead to transient shower/storm chances Wednesday and
Thursday. A more coherent surface low may move across Thursday
with a stronger upper wave noted in guidance but this has not
been the most consistent feature. Guidance is in decent agreement
on a well defined upper trough crossing the area sometime Friday
or Saturday which will bring a more organized chance for storms
but the timing is not clear at this range. Low/mid level temps do
not fluctuate much through late week. Cloud cover may be rather
persistent from Wednesday Onward with any convection also working
to hold temps down some as well. Will continue with highs in the
lower to mid 80s...but may end up being warmer if convection is
kept to a minimum.

Mdb

&&

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

655 PM...current thunderstorms over northeast Illinois will continue to
move southeast over the next few hours. Expect a short lull before
the line of thunderstorms west of rfd to north of Quad Cities
moves east. Current timing in the tafs might be a tad early but
expect these storms to impact rfd shortly at northeast IL
terminals by/around 02z with some uncertainty regarding the
southern extent of these storms though there could continue to be
southeast development. Once these storms pass the area by late
evening...trends would suggest the overnight hours being quiet/dry
but confidence is low. Changed prevailing thunder after sunrise to
vicinity with this forecast and this time period may end up being
dry.

Additional thunderstorm development is expected Sunday afternoon
into Sunday evening ahead of an approaching cold front. Timing has
differed from various models but trend now appears later and
maintained vicinity mention for now.

Northerly winds at the Chicago area terminals with current
activity will likely diminish to a light easterly ahead of the
next round of thunderstorms which are expected to have an outflow
shifting winds back westerly. Winds will settle to light southerly
later this evening and overnight with gusty south/southwest winds
expected Sunday. Cms

&&

Marine...
213 PM CDT

Light and variable winds will transition to a steadier but still
fairly light south to southeast direction this evening and
tonight. Surface low pressure over southern Saskatchewan this
afternoon will track into western Ontario by Sunday afternoon.
This will bring a steady south to southwest breeze across the lake
by daybreak or so Sunday and persist through the afternoon. Gusts
near 20 kt are likely across the Illinois/in nearshore but not expecting
Small Craft Advisory thresholds being met at this time. A cool
frontal boundary will move east- southeast across the lake late
Sunday afternoon and Sunday evening shifting winds to the west and
eventually northwest by late Sunday evening. Thunderstorms will
become likely across much of the lake by late evening ahead of the
advancing front...then diminish in coverage from northwest to
southeast Sunday.

Wind speeds will diminish into Monday afternoon. High pressure
will spread in from the west so winds may end up coming onshore
around the lake later Monday afternoon. Light onshore flow is more
likely Tuesday with high pressure pretty much overhead. The
surface pattern becomes more ill-defined Wednesday into the
weekend keeping wind speeds generally light with erratic direction
at times...especially Wednesday and Thursday. A more persistent
north to northeast direction is favored into the weekend.
Thunderstorm chances increase again Wednesday through Saturday and
may substantially modify the wind fields depending on how
organized they are.

Mdb

&&

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

$$

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