Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1042 AM CDT Tue Aug 27 2024
KEY MESSAGES...
- One more day of very hot/humid weather with near record warmth
possible. Heat headlines remain unchanged.
- Potential for scattered strong to severe thunderstorms late
today and this evening, with strong damaging winds the
primary severe weather threat.
- Chances for showers and storms Wednesday and Thursday, then
again with a cold frontal passage on Friday
- Cool and dry expected over Labor Day weekend
&&
UPDATE...
Issued at 1042 AM CDT Tue Aug 27 2024
Very little has changed with the forecast in terms of the heat
and humidity expected this afternoon. Temperatures continue to
warm under mostly sunny skies with readings already in the mid
to upper 80s and should have no issue toping out in the upper
90s this afternoon. Dew points have remained elevated in the
mid-70s with higher readings noted across eastern IA that are
trying to advect into northern IL. While there remains some
uncertainty as to how high dew points will get this afternoon,
the combination of 70 degree dew points and the hot temperatures
will yield heat indices in the 105 to 115 range areawide. As a
result, Excessive Heat Warnings remain in effect for all of our
northern IL counties with a Heat Advisory in effect for
northwest IN until 10 PM this evening.
However, there remains the potential for thunderstorms to
develop this afternoon and evening. In fact, we have been
closely watching a remnant outflow boundary that is currently
racing east across northwest IL which may try to interact with
an area of elevated instability near and east of I-39. As these
two features interact the question becomes whether or not they
will be able to generate scattered thunderstorms given the
modest capping noted in aircraft and forecast soundings. Recent
CAM guidance has unfortunately been struggling with correctly
resolving these features this morning and therefore have little
confidence on their completely dry solutions for this afternoon.
For now have decided to introduce some "silent" (10-14%) POPs
to the forecast starting around midday and continuing through
the afternoon. Though will continue to monitor trends in case
more mentionable POPs are needed.
Regardless of how the afternoon pans out, a better coverage of
thunderstorms is expected to develop this evening as a frontal
boundary moves through. Given the aforementioned heat and
humidity, MLCAPE is forecast to be upwards of 3500 to 4000 J/kg
by early evening which will generate a threat for any storms to
become strong to severe. While the main severe threat will be
damaging wind gusts to 60 mph, the stout instability will also
support the potential for instances of hail possibly up to
quarter size. Thus, the Storm Prediction Center has expanded
the level 2 of 5 severe risk to encompass virtually all of
northern IL and northwest IN for this threat.
Yack
&&
DISCUSSION...
Issued at 341 AM CDT Tue Aug 27 2024
Through Wednesday:
Main forecast concerns continue to be on very hot and humid
conditions today, along with any convective outflow impacts on
the heat as well as increasing strong to severe thunderstorm
potential late in the day and this evening.
Early morning regional radar mosaic depicts a forward-
propagating MCS tracking east-southeast across central and
northern Lake Michigan. The southern portion of the MCS is not
expected to directly impact the forecast area this morning, as
it propagates into southwest lower Michigan, though a southward-
moving outflow boundary trails the convection from near
Milwaukee to Madison as of 3 AM. This boundary is expected to
push south into northern IL, though is moving into a strongly
capped thermodynamic environment as noted in recent AMDAR
aircraft soundings from KRFD which show a substantial warm nose
(+26C) around 840 mb. The expectation is that this boundary will
stall somewhere across our northern cwa this morning, and
should largely wash out with diurnal mixing.
Farther to the west, another linear MCS was occurring across
the mid- Missouri Valley, with strong storms lifting northeast
across northwest IA and southwest MN. Current CAM guidance does
not appear to be handling this MCS very well, with 07Z HRRR and
RAP, as well as the new 06Z NAM Nest and NSSL WRF too far west
compared to current radar depiction. While the general track of
these storms is to the northeast of the forecast area today,
models are in decent agreement in an eastward-moving outflow
boundary reaching the forecast area this afternoon. Thus, while
we should heat up quickly under mainly sunny skies through
midday, the arrival of this decaying outflow boundary does lend
somewhat lower confidence in maximizing our temperatures and
heat indices across parts of northern IL. Am not making changes
to headlines at this time, with temperatures in the mid-upper
90s and heat indices in the 105-110 range still expected over
much of the area, though it`s possible some of our western cwa
locations could be a little less hot. Also, a weaker wind field
in the eastern cwa may support lake breeze development later
this afternoon or evening, which could also lower temps and heat
indices, though recent high-res trends suggest this may be
after peak- heating.
Scattered strong to severe thunderstorm chances do look to
increase across the area late this afternoon and especially this
evening. A mid-level short wave tracking into the upper
Midwest/northern Great Lakes through tonight will result in
height falls and flattening of the northern periphery of the
upper level ridge in place across the Midwest. An associated
area of surface low pressure is progged to track across WI this
evening, trailing a slow-moving surface cold front into the
forecast area. Ascent ahead of the short wave will work to erode
our cap, while the approaching front provides low- level
forcing for convective initiation. Increasing mid-level flow
(35-40 kts around 700 mb) will supply 25-30 kts of deep shear,
atop strong instability characterized by MLCAPEs in the
4000-5000 J/kg range, will support scattered severe
thunderstorms with localized damaging winds and hail across the
area by this evening. Aforementioned remnant outflow boundaries
may be foci for locally stronger severe threats. The severe risk
should diminish after midnight, though additional showers and
thunderstorms will remain possible as the frontal boundary slows
across the forecast area, and eventually stalls across the
southern/southeastern cwa. P-wats in the 1.95"+ range will make
for a locally heavy rainfall threat as well, as storms may be
capable of producing torrential rainfall especially if training
occurs.
Scattered shower and thunderstorm threat will linger into
Wednesday, as the mid-level wave completes its transit of the
upper Midwest. Greatest precip chances will generally be across
our south and southeastern cwa closer to the stalled frontal
zone, especially during the diurnally more favorable hours of
Wednesday afternoon. Considerable cloud cover and northeasterly
low-level winds north of the boundary will make for cooler, but
sill warm and muggy conditions. Afternoon highs should range
from the low-mid 80s across northeast IL near Lake Michigan, to
the upper 80s along/south of the front.
Ratzer
Wednesday Night through Monday:
Weak ridging out of ahead of the next deeper upper level trough
is projected overnight on Wednesday into Thursday. Models are
showing fairly decent consistency with an east-west instability
axis setting up in Central Illinois and Indiana. With ample
moisture remaining in place, low level southerly winds
providing low level warm air advection, and mid level lapse
rates around 6.5C/km, there could potentially be periods of
showers and thunderstorms on Thursday. Without a strong forcing
mechanism, the forecast maintained 15 to 35 percent slight
chance to chance probabilities.
That previously mentioned upper level trough is expected to
move from the northern Plains to Hudson Bay Thursday through
Friday. Its reflect surface front will move across the region.
Considering just 24 hours ago it was suggesting it would arrive
near Rockford at 18Z Thursday and now is projected around or
after 06Z Friday, there is lower confidence in exact timing, as
models are struggling on the fronts movement. However, this
trough has been in long range models since early in the weekend.
While timing can still be adjusted in later forecast, the
forecast kept the "likely" PoPs that was put in for showers and
thunderstorms to pass over the area thursday night into Friday.
Projected lapse rates aloft look okay but not overly exciting
and wind shear values are around or less than 20 knots. While
showers and storms are expected, the risk for severe weather is
low for current model runs.
850 mb temperatures ahead of the front will likely stay in the
20C range helping to maintain warmer surface temperatures in the
mid to upper 80s through the end of the week. A drier air mass
is ushered in with the cold frontal passage as well as northwest
flow that will help decrease high temperatures into the 70s
early next week which can set-up quite the favorable Labor Day
Weekend for outdoor plans.
DK
&&
AVIATION /12Z TAFS THROUGH 18Z WEDNESDAY/...
Issued at 623 AM CDT Tue Aug 27 2024
Key Aviation Impacts for the current TAF period:
- Low confidence in wind trends
- Chances (30 to 50 percent) for severe thunderstorms after 00Z
Tuesday night, but confidence is low on exact timing and wind
shift to the northwest behind the storms
The south to southwest winds around 10 knots and will slowly
rotate to the southwest to west by late morning and into the
afternoon. There is a storm complex in west central Iowa that is
being monitored. If that were to continue along the outflow due
east, there could be thunderstorms arriving around KRFD in the
early afternoon, but confidence is low so TAFs remain dry
through the day.
Latest guidance continues to suggest a front moving over the
region after 00Z, though each model run nudges it slightly later
into the overnight. There is a chance that winds turn northeast
or east out of ahead of the boundary, but as model guidance as
struggled to depict this event, it was removed from the TAF for
the time being, especially as the more impactful winds will be
the gusty northwest winds associated with the front itself. Low
confidence on the exact timing of the boundary to pass through.
so VCSH was maintained through the early morning hours. There is
a signal for another weak boundary to provide showers between
12Z and 18Z Wednesday, but the probability was less than 30
percent so it was kept out for now and will be revisited later.
Winds will diminish behind the front and be out of the northwest
and slowly veer to the north Wednesday morning.
DK
&&
CLIMATE...
Issued at 330 PM CDT Mon Aug 26 2024
Record Max Temperatures and High Minimum Temperatures for early
Tuesday and Wednesday this week:
Record Max/YearRecord High Min/Year
Chicago:
Tue Aug 27th.........97 in 1973...........78 in 2020.
Wed Aug 28th.........97 in 1955...........77 in 2020.
Record Max/YearRecord High Min/Year
Rockford:
Tue Aug 27th.........97 in 1953...........74 in 2018.
Wed Aug 28th.........97 in 1955...........76 in 1928.
Ratzer/Yack
&&
LOT WATCHES/WARNINGS/ADVISORIES...
IL...Excessive Heat Warning until 10 PM CDT this evening for ILZ003-
ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-ILZ011-ILZ012-ILZ013-
ILZ019-ILZ020-ILZ021-ILZ023-ILZ032-ILZ033-ILZ039-ILZ106-
ILZ107-ILZ108.
Air Quality Alert until midnight CDT tonight for ILZ005-ILZ006-
ILZ012-ILZ013-ILZ020-ILZ021-ILZ103-ILZ104-ILZ105-ILZ106-
ILZ107-ILZ108.
Excessive Heat Warning until 10 PM CDT this evening for ILZ103-
ILZ104-ILZ105.
IN...Air Quality Alert until midnight CDT tonight for INZ001-INZ002-
INZ010-INZ011.
Heat Advisory until 10 PM CDT /11 PM EDT/ this evening for
INZ001-INZ002-INZ010-INZ011-INZ019.
LM...None.
&&
$$
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