NWS Chicago, IL wrote its last area forecast discussion before the first event at Jul 29, 11:43 AM CST, 57 minutes earlier.
Area forecast discussion, Jul 29, 11:43 AM CST, 57 minutes before
Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1243 PM CDT FRI JUL 29 2016
.SHORT TERM...
324 AM CDT
Through Saturday...
Main forecast concerns/challenges are with shower/thunderstorm
trends through the period, with isolated to scattered precip
expected over much of the CWA.
In the near term, fog is in place over much of the area with some
locations observing patchy dense fog. At this time, only a few
locations have fallen below one mile, mainly south of I-88. Will
need to continue to monitor fog trends over the next couple of
hours as expansion of the more dense fog will likely occur. Likely
locations would still be south of I-88, away from the more solid
stratus which is in place further north. Not much precip
development at the moment, but radar imagery showing some
isolated precip developing along the Illinois nearshore. These
showers are forming along convergent axis situated over the
southern end of the lake within main surface trough axis. Most
guidance indicating an increasing trend with regards to precip
coverage early this morning through mid/late morning. Focus will
come from increasing large scale ascent ahead of approaching upper
level trough, while convergence within surface trough axis
remains. Did increase pops today, with slight chance increasing to
chance this morning into the afternoon. Although confidence is
high with regard to precip occurrence today, it is lower on exact
placement/duration, and have capped pops at high chance. However,
once trends are likely noted later this morning, would expect a
trend to at least likely pops to occur. Greatest coverage and
intensity will be around midday into mid afternoon, with this
scattered development likely over much of the entire CWA. Flow
aloft will remain weak and support slow movement of any precip
once again today. Although lightning is currently limited across
the area, instability axis is overhead and will remain in place
today, with lightning likely increasing this morning. Not
anticipating any widespread severe weather at this time with
lacking shear, however, an isolated stronger storm is possible.
This will especially be the case given the trough axis in place as
well as the potential for other boundaries to develop. PWATs have
increased across the area, and with these higher PWATs likely
remaining today, do think heavy rainfall will also occur.
Do expect coverage and intensity of showers/thunderstorms to
diminish this evening into the overnight, but with this upper
level trough expected to slowly move through the region, similar
pattern supportive of additional precip development will remain
overhead tonight. Similar setup for another chance of scattered
showers/thunderstorms will once again occur on Saturday, but as
better focus will begin to slide away from the area, think highest
chances for this precip Saturday will be south of I-80. Temps
today and Saturday will be limited due to precip/cloud cover with
that being felt more today. Have kept high temps in the 70s across
much of the area with highs around 80 expected over the southern
CWA. Maintained highs around 80 on Saturday, with the possibility
for more breaks of the clouds to occur.
Regarding Beach Hazards Statement...A Beach Hazards Statement for
dangerous rip currents, especially at non-protected beaches,
continues for today and tonight. Waves are already a little over 3
ft along the northeast Illinois shore early this morning. Expect
that continued modest northeast flow down a sizable fetch of the
lake will build these in excess of 4 ft with an orientation
favoring rip currents along some northeast Illinois and very far
northwest Indiana beaches. Continued modest winds on Saturday,
with a slightly more northerly direction, may require a statement
for all lake adjacent counties.
Rodriguez
&&
.LONG TERM...
324 AM CDT
Saturday night through Thursday...
Drying trend expected to occur Saturday night into Sunday, while
temps get back into the low 80s on Sunday. Some guidance hinting
at possible isolated precip development Sunday afternoon but with
low confidence, have left chances out at this time. Conditions
should continue to be dry across the CWA Sunday night, while more
energetic flow remains to the west. However, upper level ridging
expected across the region early next week into midweek. While
this occurs, guidance indicating more energetic flow to move over
the area and bring a return to more active weather. Along with
this active weather, a trend towards hotter and more humid
conditions will occur through the end of the work week. Have kept
highs in the upper 80s but do think highs in the low 90s will
become probable Wednesday and Thursday. Will need to monitor this
period, especially as guidance indicating the possibility for mid
to upper 70s dewpoints to spread back across the area.
Rodriguez
&&
.AVIATION...
For the 18Z TAFs...
The concern for the next few hours with the TAFs will center on
scattered showers and thunderstorms rapidly developing across the
region.
At issuance time, development is largely driven by diurnal heating
in a moist, unstable environment. There is no strong focussing
mechanism across nrn IL/IN so activity invof the terminals should
remain scattered. So, have gone with the prevailing VCTS with a 4
hour TEMPO group through 22z. With time, expect that the TS may
focus more to the south of the terminals where old outflow
boundaries may help focus activity there and tsra/shra potential
may diminish with time. The 4 hour TEMPO group may end up a bit
pesimistically long, but confidence is too low to narrow the
timing any more.
A series of mid level impulses moving across the region will keep
some shower potential across the area through the remainder of the
late afternoon hours, through the night and, possibly even into
tomorrow morning. This shra activity should also be scattered,
with no significant sfc focus, so will maintain the VCSH from the
going forecast.
Winds may be a bit problematic through the afternoon. On the
larger scale, winds should remain generally northeasterly at arnd
10kt, but there will likely be some random direction gustiness
with outflow from the thunderstorms.
There will be potential for lower cigs and visibility again
tonight into Saturday morning, but should be less prevalent than
last night since winds might remain a bit stronger and not
necessarily light and variable. However, given the low confidence
in the reduced cig/vis potential, will maintain the going trends.
&&
.MARINE...
214 AM CDT
Mainly northeast winds will continue across the lake through
Saturday evening. The speeds will undulate through that time with
sporadic gusts exceeding 20 kt especially in the open water and
the western nearshores. The highest nearshore waves today can be
expected in the Illinois nearshore where they will be close to
Small Craft Advisory criteria of over a 4 ft average. Given that
waves at the Wilmette buoy are over 3 ft early this morning, may
end up leaning on issuing one but will watch trends for a couple
more hours. The winds turn a little more north-northeast on
Saturday spreading more of the 3 to 5 ft waves into the Indiana
nearshore.
High pressure will move over the lake during the last half of the
weekend into Monday before southerly flow resumes.
MTF
&&
.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Beach Hazards Statement...ILZ006-ILZ014 UNTIL 4 AM Saturday.
IN...Beach Hazards Statement...INZ001 UNTIL 4 AM Saturday.
LM...NONE.
&&
VISIT US AT HTTP://WEATHER.GOV/CHICAGO (ALL LOWERCASE)
FOLLOW US ON FACEBOOK...TWITTER...AND YOUTUBE AT:
WWW.FACEBOOK.COM/NWSCHICAGO
WWW.TWITTER.COM/NWSCHICAGO
WWW.YOUTUBE.COM/NWSCHICAGO