Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 270814

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
314 AM CDT Tue Jul 27 2021

313 AM CDT

Through Thursday...

A plethora of forecast goodies to diagnose in the short term this
morning. The main messages are:

- Two more hot days areawide (three more south of I-80), including
  a Heat Advisory maybe being needed for north central Illinois
  for Wednesday afternoon

- A chance for spotty storms tonight and during the day
  Wednesday, mainly north of I-80

- Forward propogating MCS potential overnight Wednesday into
  Thursday morning, especially northeast Illinois into northwest
  Indiana, that could have a severe wind threat although
  confidence on whether brunt hits the CWA is still low

- Strong cold front moving through Thursday afternoon, shifting
  storm chances into the south and bringing a breezy afternoon

A mid-summer hot weather setup in the western and central U.S.,
with a 500 mb ridge of 594 dm to strengthen some as it expands
east-southeast through Wednesday. The northeastern periphery of
this ridge is over the Upper Midwest into the Great Lakes and is a
zone of low to mid-level baroclinicity underneath a strong upper
jet and multiple short wave disturbances. Ahead of any of these
impulses are a breeding ground for convection. Such was the case
last night into early this morning with multiple MCSs having
transpired in Wisconsin, with the second one ongoing in central
Wisconsin as of 3 A.M. With 35 kt of 850-925 mb flow on the MPX
and ARX VWPs, along with ample MUCAPE and precipitable water,
would expect this MCS to be slow to weaken as it moves southeast.
As the short wave moves more east than south though, it will start
to feel the effects of less forcing and a weakening low- level
jet after daybreak. While expecting cloud cover from this to make
it into the northern CWA this morning, and possibly outflow, do
not think any of the storms themselves will.

By midday into afternoon, much of the clouds should be gone and
warming will again push temperatures to or above 90. Yesterday
saw multiple 94 readings over the drier northern CWA under full
sun. With some morning clouds, we probably will fall a bit shy of
that, although difficult to precisely hit the maximum temperatures
in recent days. Dew points will be inching up some today, aided
by evapotranspiration from mature crops (as noticed yesterday in
Td observations over more agricultural areas), but still look to
be only peaking around 70 in those locations. That will keep heat
index readings only 2-3 degrees above the afternoon temperatures.

For tonight into the day Wednesday, that old mantra of "cannot
rule out a storm" in a moist and high MUCAPE atmosphere will be
the rule. There will be a less significant short wave moving over
the Upper Midwest than seen last night, so overall convection
upstream should be less. There is projected upglide in the
310-315K layer as tonight progresses, and corresponding gradual
moistening near the base of the MUCAPE layer. This will be
1,000-2,000 J/kg over northeast Iowa to south central Wisconsin.
Some CAMs show scattered activity there, and mean flow would
support potential for this to maybe inch into far northern
Illinois late tonight or Wednesday morning. Deep layer shear is
on the order of 30-35 kt, so if some cells mature there could be
an isolated wind/hail threat (consistent with SPC level 1 risk).

For Wednesday afternoon, there will be some hinging of
temperatures to any morning clouds or if there is spotty
convection. Also a majority of high resolution guidance wants to
show onshore flow into lake adjacent counties, with an
outflow/lake breeze composite. Confidence is not the highest with
that, but that could also spring a storm or two with forecast
soundings from the RAP, NAM, and GFS showing the cap eroded. Have
cooled forecast readings some in that lake adjacent area. The
area with the most unimpeded heating looks to be north central
Illinois. A consistent signal in guidance has been for dew points
to rise into the mid 70s (where they were this past Saturday).
This exponentially increases the heat index from today, with
forecast values of 100-103 in north central Illinois. While true
Heat Advisory criteria for there is 105, day shift will have to
consider a headline with further evaluation of any debris clouds
and just how warm temperatures will reach.

The prime time for convection remains Wednesday night,
particularly late evening through early Thursday morning. An MCS
is favored to develop across Wisconsin with a short wave
propagating on an impressive 110 kt upper level jet streak to its
north. Strong instability in Wisconsin in the evening (MLCAPE
values >3,000 J/kg) and deep layer shear values of 50-60 kt,
signal convective organization and an eventual forward
propagation. The steering flow favors this to at least clip
northeast Illinois and northwest Indiana with more of a track
into southwest Michigan, yet there is certainly potential it
could move directly at the northeast CWA (including the Chicago
area). Whether the MCS can be of a more enhanced severe threat
into the area during overnight will depend on exactly how the
cold pool evolves and the orientation and propagation of the apex
of convection, as well as timing. There is an increasing
likelihood that north central and east central Illinois see little
if any convection Wednesday night into Thursday morning, and
that`s especially true if an eastward track.

For the day Thursday, there will be a domino effect of where the
convective outflow is to how any festering or redeveloping
convection will be and how it will evolve. To the south of the
boundary (south to well south of I-80), heat index readings could
top 100 in the afternoon. If enough heating can occur, there would
be some conditional severe threat with any convection and that
would probably only be the far southern CWA. Elsewhere, the
synoptic front will be passing late morning into early afternoon,
turning winds north to northeast and providing gradual drying and
cooling. Some wind gusts of 20-25 mph to potentially 30 mph in
lake adjacent counties, will be seen behind the front.




Thursday night through Monday...

212 AM...Only two forecast concerns in the extended, high waves
Thursday into Friday and precip chances this weekend.

The cold front will be pushing south of the area Thursday night.
Blended pops have slight chance pops across the far south and held
onto those for now due to timing uncertainty, but current faster
trends would support dry conditions across the entire area
Thursday night. This front will bring a break from the heat and
humidity with highs on Friday only in the lower 80s and cooler
along the Lake Michigan shore. Temps may rebound back into the
mid/upper 80s Saturday ahead of another cold front which will move
across the area Saturday night. Low confidence for precip coverage
with this front but slight chance pops warranted Saturday and
again Sunday across the southeast, for timing uncertainty. High
temps then settle back into the upper 70s/lower 80s into early
next week along with lower humidity levels.

Strong northeasterly winds will develop on Thursday behind the
cold front and this will allow high waves to affect the IL and IN
Lake Michigan beaches creating dangerous swimming conditions. A
little too early for swim risk headlines but given current trends
they will likely be needed Thursday into Friday. cms


For the 06Z TAFs...

1228 AM...No forecast concerns expected this period. Light
southwest winds early this morning may become light and variable
or calm for many areas. Southwest winds will increase into the
10kt range by late morning/early afternoon with some gusts into
the mid teen kt range. Speeds will diminish well under 10kts after
sunset this evening. A lake breeze is possible today and if one
forms, it will likely remain near the Lake Michigan shore.

Scattered thunderstorms across northeast WI currently along with
additional thunderstorms expected across WI and Lake Michigan
later today and tonight, may produce an outflow/wind shift that
reaches northeast IL later this afternoon or this evening. Fairly
low confidence of this reaching the terminals and no mention for
now but this will need to be monitored later today. Convection is
expected to remain north/northeast of the terminals through the
period but it may reach as far south as the IL/WI state line by
daybreak Wednesday morning. cms




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