Forecast Discussions mentioning any of
"ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 08/24/23
Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
901 PM CDT Wed Aug 23 2023
.UPDATE...
Issued at 900 PM CDT Wed Aug 23 2023
Made some minor adjustments to going forecast to increase dew
point temps a bit overnight based on current obs trends. As of 830
pm, area ASOS/AWOS sites still reporting dew point temps in the
upper 70s to around 80 degrees. Combined with air temps still
ranging from the mid-80s outside of the Chicago metro to around 90
within the core of it, heat index values remain in the 100-110
degree range in many spots. An uncomfortable night is in store for
those without AC, with those heat indices only gradually easing
to the upper 70s (outlying) and 80s (in the core of the metro) by
morning.
No changes to forecast into tomorrow at this point, with another
dangerously hot day expected. Late day frontal passage and
possible convection looks to be late enough as to not impact max
temps/heat indices, thus current heat headlines appear on track.
Air temps tagging 100 somewhere not out of the question with the
low level thermal axis more directly over the forecast area.
Ratzer
&&
.SHORT TERM...
Issued at 301 PM CDT Wed Aug 23 2023
Through Thursday night...
Mid-afternoon observational trends and MDW ACARS soundings
indicate that dew points are struggling to mix out under a stout
subsidence inversion around 3kft, even as SW winds gust up to 30
mph. Dew points as of 2-3pm have decreased by only a degree or two
for most locations. With the low-level temperature profile, the
dew points generally in the low 80s have maybe shaved a degree off
high temps, but still over-performed on heat index values. Most
area AWOS/ASOS sites as of 3pm are in the 112-118F range, with a
few locations touching 120F. Needless to say, this is on the
upper-echelon with the combination of heat and humidity in our
region.
Little relief is expected tonight, especially given the high
dewpoint temps and continued SW flow. Min temps may not fall much
lower than the upper 70s inland and the lower 80s in Chicago.
Corresponding heat index values in Chicago may also struggle to
fall much below 90F. Upstream trajectories for our area on
Thursday favor a slightly warmer thermo profile with some
potential for slightly more low-level mixing of drier air. Along
with the several degree head start on temps Thursday morning
compared this morning, the slightly lower dew points may allow for
some locations to make a run toward 100F in the afternoon. But as
has been the case for the past few days, max temps in far
northern/northeast Illinois will be highly dependent on timing of
an incoming cold/backdoor front. Most CAMs have been overly
zealous with upstream convection over the past 24 hours, so any
effective front/outflow in guidance may be a little quick in its
arrival. Overall, have opted to keep temps and heat index values
roughly the same as previous forecasts, with max heat index values
generally 110-115F for the entire area.
Forecasting convection over the next 24-36 hours remains a
difficult undertaking. As noted before, several CAMs have been far
overdone with convection by over-moistening the inversion layer
and eroding capping. The downhill effect has allowed for various
residual boundaries to focus new convection through the period.
Overall, have convection potential fairly minimal through Thursday
night outside of a couple points. First, an increasingly sheared
subtle wave entering northwest Illinois has largely lost much of
the remaining mid-level cloud cover to entrainment. Surface-based
convection appears unlikely with the current cap, so have
maintained dry conditions through early evening. The next period
of concern will be the arrival of the cold front late Thursday
afternoon. Mixing ahead of the front may keep ML profiles
sufficiently capped to convection, but deep-layer thermal and
kinematic profiles support isolated severe convection (primarily
damaging wind) along the front late afternoon through the evening.
Kluber
&&
.LONG TERM...
Issued at 300 PM CDT Wed Aug 23 2023
Friday through Wednesday...
Key Forecast Messages:
* Cooler Friday but still very warm and humid with a slight chance
of showers and thunderstorms in the evening
* Comfortable near to below normal temperatures and primarily dry
Saturday onward
* Dangerous swimming conditions likely at Lake Michigan beaches on
Saturday-Saturday night
The backdoor cold front passage on Thursday discussed in the short
term section above will be the start of a significant pattern
change that will result in several days of pleasant and primarily
dry conditions through mid next week.
Friday will still have very warm 850 mb temperatures peaking in
the lower 20s Celsius, while 925 mb temps peak in the mid 20s C.
Despite post-frontal northeast winds across the area, the low-
level thermal fields will support highs away from the lake in the
mid 80s to around 90, warmest in far interior sections of the CWA
in northwestern and central Illinois. Highs near the lake will top
out in the upper 70s to around 80F.
With forecast dew points in the lower to locally mid 70s where
forecast highs are around 90F, this yields peak heat indices
around 100F, with 90s elsewhere well inland of the lake. So again
another very warm and humid day, but not nearly to the extent of
today and what we will experience on Thursday. If dew points pool
a bit higher across central Illinois, there may be some consideration
of a heat advisory (or extending heat warning) for parts of our
far southern counties in Illinois, though this isn`t currently the
most likely scenario.
A secondary cold front associated with a stout mid-level short-
wave trough tracking across the northern Great Lakes into Friday
evening may be able to touch off a few spotty showers and
thunderstorms Friday evening into the early overnight. Main
limiting factors will be lacking large scale support and weak
lower level convergence and with these, slight chance (20%) PoPs
from the NBM appear reasonable.
The secondary cold front passage will set the stage for persistent
brisk northerly winds down the length of Lake Michigan during the
day Saturday into Saturday evening, with building waves likely to
produce strong currents. As things stand, we`ll likely need a
Beach Hazards Statement for beaches along the entire Illinois and
Indiana shoreline. Aside from this concern, Saturday will be much
more comfortable, with highs in the upper 70s to lower 80s
(locally mid 80s well south) away from the lake and mid 70s
lakeside.
Sunday will have temperatures and dew points a few degrees lower
than Saturday, so even more comfortable than Saturday and a far
cry from Wednesday-Thursday this week. Should see a modest warm-up
to start the work week (lower 80s away from the lake) as winds
briefly turn westerly inland. Another backdoor cold front will
move across the area later Monday, which due to minimal
instability and lacking moisture, may at most touch off a few
spotter showers. An instability starved short-wave may then touch
off a few spotty showers on Tuesday, though any spotter showers
will likely be the exception to the primarily dry rule that will
persist to or even beyond the current day 7 (Wednesday 8/30).
Forecast highs are in the 70s area-wide for Tuesday and Wednesday.
Castro
&&
.AVIATION...
For the 00Z TAFs...
Aviation Forecast Concerns:
* Breezy west-southwest winds continue Thursday, prior to a cold
front which will shift winds northeast by early evening.
* Low, but non-zero chance of thunderstorms late Thursday
afternoon/early evening with the cold front. If storms were to
form, they could be strong to severe.
Early evening surface map depicts a nearly stationary frontal
boundary from southern MN east across central WI and central lower
MI. South of this front, breezy west-southwest surface winds were
in place within an impressively hot/humid air mass across the
forecast area. Winds continue to gust 20-25 kts especially across
our warmer Chicago metro terminals, though these gusts should ease
and become more sporadic with time through sunset. Similar
conditions are expected on Thursday, with winds again becoming
gusty as the boundary layer deepens after sunrise.
The main TAF impacts will be focused on the front to our north,
which is forecast to push into the area late in the day Thursday
as a cold front. Current high-res model guidance indicates fropa
across the ORD/MDW terminals in the 00-01Z time frame with a
corresponding shift to north-northeast winds. A couple of high-
res CAMs do indicate the potential for isolated to scattered
thunderstorm development along the front over southern Lake
Michigan and parts of the Chicago metro along the front early
Thursday evening. Boundary layer mixing ahead of the front may
lower dew points enough to maintain a cap for near-surface based
updrafts, though is storms were to develop they could be strong to
severe. At this point probability is in the 15-20% range, below
TAF mention, but is something to keep an eye on Thursday for the
possibility of TSRA in the Chicago terminal area early Thursday
evening.
Ratzer
&&
.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Excessive Heat Warning...ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-
ILZ010-ILZ011-ILZ012-ILZ013-ILZ019-ILZ020-ILZ021-ILZ023-
ILZ032-ILZ033-ILZ039-ILZ103-ILZ104-ILZ105-ILZ106-ILZ107-
ILZ108 until 8 PM Thursday.
IN...Excessive Heat Warning...INZ001-INZ002-INZ010-INZ011-INZ019
until 8 PM Thursday.
LM...None.
&&
$$
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