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Area forecast discussion 
National Weather Service Chicago/Romeoville, Illinois
330 am CDT Sat Jul 30 2016

Short term...
328 am CDT

Through tonight...

Main forecast concerns/challenges are with shower/thunderstorm
trends later this morning into mid afternoon, with heavy rainfall
and localized flooding possible once again.

Am concerned this morning that the area will see a repeat of slow
moving thunderstorms later this morning into the afternoon, with
heavy rainfall and localized flooding. Similar pattern is expected
to remain in place today with upper level trough still slowly
moving through the region, with surface trough axis still situated
over the area while a secondary surface focus also still remains
in place. Outside of a few stray and brief showers, the majority
of the County Warning Area is dry early this morning and think this dry period
will likely persist over the next several hours. Don't think the
County Warning Area will observe blossoming showers and thunderstorms until more
towards mid morning, when current speed Max/short wave energy
currently pushing into Iowa reaches northern Illinois. Have this
additional forcing timed into northern Illinois by around the mid
morning time frame, and this is consistent with most short term
and hires guidance indicating an increase in precip coverage
across northern Illinois.

With high dewpoint air in place, higher precipitable water axis overhead, and
with instability axis still situated through the cwa, feel a
similar situation with scattered thunderstorms developing across
the County Warning Area will occur today. Have increased pops today with likely
pops in place by mid to late morning over a majority of north
central Illinois and Northwest Indiana, and continue these higher
pops through late in the afternoon. Timing of highest coverage and
intensity of thunderstorms will be slightly sooner given the
quicker arrival of the additional ascent. Guidance continues to
vary to the placement of this precip axis today, but do feel that
this axis could be very near where the heaviest rainfall occurred
yesterday or just slightly to the southwest. A slightly further
southwest shift would make sense given the current surface
pattern. Although somewhat lower confidence with exact location of
the heaviest rainfall, best thoughts right now off of current
surface pattern, would suggest the corridor extending from areas
near/around Amboy to Manhattan could see the higher amounts.
Guidance still once again bouncing around today, and its possible
that locations in Kane, dupage, and Cook County that observed the
highest amounts yesterday could see additional heavy rainfall
today. Had contemplated some type of flood headline for today but
with the lower confidence on placement, have opted not to issue a
headline at this time. With departing ascent later this evening,
should see precip weaken/diminish through the evening with dry
conditions then expected through the overnight hours.

Regarding the beach hazards statement...the persistent northeast
winds today bring another day of near 4 ft waves for the northeast
Illinois shore, though conditions are likely to be more marginal
for rip currents. Waves heights have been around 3.5 to 4 ft at
the Wilmette buoy since yesterday. A slight drop is probable late
this morning and early afternoon, but a slight rise might follow
this up as the pressure gradient tightens and the wind direction
aligns better with the lake fetch. So will waves will remain
marginal, feel better keeping the beach hazards statement for rip
currents going at this time, and allowing day shift to see trends.

Rodriguez

&&

Long term...
328 am CDT

Sunday through Friday...

The end to the weekend will remain generally dry. Guidance
continues to bounce around on the idea for some possible isolated
showers/thunderstorms in the afternoon across the area. However,
given lower confidence, have decided to keep the forecast dry at
this time. Although some cloud cover expected Sunday, shouldn't be
enough to keep temps from rising to the low 80s. The beginning of
the work week will see a start to a warming and more humid trend
through much of the week with high temps slightly warmer each day
through Thursday. Increasing dewpoints during this time are
expected to provide hot/humid conditions, especially Wednesday and
Thursday. However, this pattern will also support increasing
chances for thunderstorms. Monday will likely remain dry for most
areas but do think higher potential will be Monday night into
Tuesday, when guidance is indicating the arrival of more energetic
flow.

Rodriguez

&&

Aviation...
for the 06z tafs...

Once again the challenges are potential for low clouds near
daybreak, mainly MVFR though a small chance for IFR, and scattered
to numerous showers and some storms late this morning through the
afternoon. There also will be isolated showers through mid-
morning.

A challenging pattern continues with a slow moving, somewhat broad
upper low over the area. Sporadic, widely isolated showers have
been seen on radar early this morning and would expect that to
continue with several high-resolution models indicating a
concentration in far northeast Illinois within the 08z-13z
period. While some of these may be overdone and confidence is not
particularly high at all, there are some observational trends to
indicate this so have included shra in the 06z Ord taf in the pre-
dawn hours.

Otherwise the main threat for showers will return again late this
morning and afternoon. There are less ingredients in place than
yesterday but still plenty for at least scattered showers and
some thunder. The focus today for initialization of showers/storms
and better coverage is thought to be a rough line just south of
Rockford to just south of midway during 16z-17z, and then slowly
evolving southward. Confidence in this is medium, and just a
little difference in placement could impact the airports more
significantly or quite a bit less and not even Worth a vcsh.

Northeast low-level flow continues through the taf period with
again the possibility of any nearby showers/storms influence the
wind direction. There is potential for fog Saturday night into
early Sunday morning as well and will assess that further for 12z
tafs.

Mtf

&&

Marine...
200 am CDT

Persistent low pressure to the south of Lake Michigan will begin
to slowly move east this afternoon into tonight. Overall east-
northeast to northeast winds will continue over the lake today and
this evening. Waves have been consistently around 3.5 ft at
Wilmette since yesterday. While a slight drop will probably occur
late this morning into early afternoon, a rise back to around 4 ft
should be seen later into the day into evening across the Illinois
nearshore. Conditions look to remain below Small Craft Advisory
criteria though. A high pressure ridge will move across the lake
late Sunday night into Monday before southerly flow returns to the
lake. This should persist through at least early Thursday. Some
15 to 25 kt winds may be seen ahead of a cold front later in the
week.

Mtf

&&

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

In...none.
Lm...none.
&&

$$

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