Area Forecast Discussion
Issued by NWS Chicago, IL
FXUS63 KLOT 230943
Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
443 AM CDT SAT JUL 23 2016
335 AM CDT
Decided to keep heat headlines as is today with another hot and
muggy day expected. A few isolated thunderstorms popped up
overnight and have very low slight chance PoPs through this morning
for any additional isolated storms that may form. Confidence is low
in thunderstorm coverage this morning, but increases as forcing
develops this afternoon. An upper level wave shifts east and may
kick off storms mainly west of a Rockford to Fowler IN line early
this afternoon. A lake breeze is expected and confidence is
increasing in storms forming along it early this afternoon. While
CAPE is abundant, shear is not. The steering flow is also weak, but
PWATs remain high at 1.5-2 inches. Therefore expecting storms to
move slowly and produce locally heavy rainfall. Could see an
isolated storm produce damaging winds as well but widespread severe
weather is not expected.
Hi-res guidance then suggests a convective complex will pass over
Wisconsin tonight. Guidance differs on the complex`s exact path, but
a CAPE gradient could direct it southeast through the warning area.
The complex should be deteriorating, so once again not expecting
widespread high winds, but heavy rain is possible.
Cloud cover and precip will have a direct impact on temps today so
opted to not make any changes to the going heat headlines. Many
locations are beginning the day in the mid 70s to around 80 with
dewpoints in the mid 70s. So regardless of how hot we actually get,
it will feel muggy and icky outside.
335 AM CDT
Sunday through Friday...
Debated issuing a heat advisory for Sunday as many areas may have
heat indices in excess of 105 degrees. However, a cold front shifts
through and storms are possible ahead of it mainly south of I-88.
The storms and associated clouds could lead to cooler temps and heat
indices, so opted to hold off on the advisory for now. One could
also argue that a heat advisory may be needed due to the cumulative
effects of multiple days of heat and humidity. Will pass these
concerns on to the day shift.
Widespread severe weather is not expected Sunday, but do have
concerns about locally heavy rainfall.
Outside of morning storms well south of I-80, Monday looks dry and
cooler. Highs will be in the mid 80s, but only the low 80s along
the lake. Tuesday looks very similar to Monday with continued dry
Mid to late next week looks to have near normal temps, but multiple
chances of showers and storms. Another cold front moves through
Wednesday followed by a surface low Thursday and then an upper level
wave Saturday. There will be dry hours, but the end of the week
looks wet and busy.
For the 06Z TAFs...
SPC analysis depicts we are unstable and only slightly capped if
not at all. We do not have a trigger quite yet but there are some
weaker upstream shortwaves along with a wind shift coming near the
lake. Hi res guidance including the rap which was way too low on
dewpoints this morning and now depicts more in the way of
scattered thunderstorms this morning. These could produce heavy
rain and gusty winds. They could be slow moving and send outflows
given the slow storm motion as well creating erratic winds.
Confidence on coverage is still not super high, but high enough to
at least carry a VCTS for now for the Chicago terminals is
focusing on an axis along and south of ORD/MDW. Expect lower
VFR/MVFR cigs to fill in a bit more as well this morning.
Previous discussion from 06z...
A few isolated showers may pop up from time to time overnight and
then dive southeastward. Otherwise the concern initially is for
occasional mvfr cigs and possibly some lower visibility. MVFR
cigs have been bouncing back and forth between broken and
scattered and is likely to be the case overnight. Regarding
visibility, low dewpoint depressions are in place away from the
urban terminals and winds are little to none, but there is a weak
wind component that may keep visbys from dropping too much tonight
especially towards the Chicago terminals. Favored areas would be
under the surface high from KVYS/KRPJ southeast to KRZL, but even
these may remain these areas may see stratus coupled with cirrus.
Will continue the mvfr vsby mention away from KORD/KMDW with
infrequent ifr possible.
Next concern is on thunder chances. There are a few subtle
features that may contribute to shower and t-storm chances. The
first is aforementioned, with KGYY favored before daybreak as
forcing is subtle but there is a weak cap in place over the area
and more so over east central IL and northwest IN where some quick
developing pop-up showers may form. Expect quite a bit of
convective development to occur across Iowa tonight where the LLJ
is focused. Convective debris may limit destabilization over our
area where subtle shortwaves in the coupled possibly with the lake
breeze may be enough to break the cap which Confidence is low on
occurrence even though a few short term guidance sources depict
some isolated development to fester at times through the day, so
kept tafs dry at this point for uncertainty in coverage.
Best chances of thunderstorms are tied to the slow moving compact
shortwave across the central plains that will approach the
Mississippi River late this afternoon/early evening. With best
forcing headed north initially, it may just graze our area (KRFD)
this afternoon and evening and may be more of an issue on late
tonight into Sunday.
309 AM CDT
High pressure will build over the Western Great Lakes today
resulting in generally light winds. A weak cold front will shift
winds to northerly before continuing to veer east-southeasterly.
The pressure gradient will increase Saturday night as a trough of
a low pressure system moving across Ontario approaches. This will
result in south to southwest winds/gusts up to 25 kt on Sunday. A
cold front trailing from the low pressure system will move across
the lake Sunday night, with fairly light west and northwest winds
on Monday behind the front. High pressure spreading to the Eastern
Great Lakes by mid week will bring light winds Monday night
Thunderstorms are possible across the lake over the weekend,
especially tonight into Sunday, with locally higher winds and
waves possible with any stronger thunderstorms.
IL...Excessive Heat Warning...ILZ008-ILZ010-ILZ019-ILZ021-ILZ023-
ILZ032-ILZ033-ILZ039 UNTIL 7 PM Saturday.
Heat Advisory...ILZ020-ILZ022 UNTIL 7 PM Saturday.
IN...Heat Advisory...INZ010-INZ011-INZ019 UNTIL 7 PM Saturday.
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