Forecast Discussions mentioning any of
"ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 08/25/23
Area Forecast Discussion
National Weather Service Indianapolis IN
942 PM EDT Thu Aug 24 2023
.Mesoscale Update...
Issued at 942 PM EDT Thu Aug 24 2023
A steambath of an evening continues with heat indices dropping back
slowly with the sunset. Skies were clear with temperatures in the
mid and upper 80s at 01Z after the hot day where much of the area
saw low and mid 90s with dewpoints hovering near 80.
Yet again this evening...our focus takes us to convection organizing
outside the forecast area and any indirect impacts the storms may
bring to parts of central Indiana late tonight. This evening...
western and central lower Michigan has been the location where
convection has taken off rapidly over the last few hours and at this
hour continues to grow upscale into an MCS. The bulk of the
significant convection will remain well to our north and
northeast...tracking with the mean flow to the southeast into
northern Ohio into the overnight.
Am more interested in the tail end of the convective complex which
while noticeably weaker than just 1-2 hours ago...still has a few
stronger cells dropping into northeast Illinois. Strengthening
westerly flow within the 925-850mb layer supports at least some
continuation of broken convection dropping southeast across the
Chicago metro over the next few hours and after midnight...
potentially into northeast portions of the forecast area. The
atmosphere becomes increasingly hostile over central Indiana
underneath the retreating ridge with very warm temps in the mid
levels. And while there remains a lingering cap within the 850-700mb
layer...the current IND ACARS sounding also shows a tremendous
amount of instability through the column that may aid the low level
jet in maintaining at least a few cells into the area after
midnight. Will maintain an isolated mention for storms essentially
northeast of a Lafayette-Anderson/Muncie line for a few hours
overnight. While any convection should be subsevere...could see
gusty winds with stronger cells.
Otherwise expect another sultry night with lows in the mid and upper
70s. The cold front poised to finally bring an end to the hot and
humid conditions will be on our doorstep near daybreak and ready to
swing through the region on Friday. Will be cancelling the excessive
heat warning over northern portions of central Indiana originally
set to expire at 04Z. Further south...the warning will continue into
Friday.
Zone and grid updates out.
&&
.Short Term...(This evening through Friday)
Issued at 250 PM EDT Thu Aug 24 2023
...Dangerous Heat Continues...
Tonight.
For information on the ongoing dangerous heat through the rest of
the afternoon, see the mesoscale discussion above.
Quiet conditions are expected for the overnight hours tonight with
very warm temperatures continuing through the night. Some mid and
high level clouds will pass overhead with near saturation around
500mb. This cloud field will be most dense across the northern
counties where additional moisture associated with the recent series
of storm complexes across the Great Lakes has begun to seep into the
region.
There is still a very conditional and unlikely threat for convection
late this evening into the early overnight hours associated with
another wave in the Great Lakes, but any thunderstorms are expected
to stay to the north and east of the forecast area with any
instability across Indiana well into the mid levels. Models continue
are undercapturing the morning low stratus/fog with such rich
moisture across the area, so will include pockets of patchy fog
again tonight.
Friday.
Relief from the heat will arrive for some on Friday as a weak cold
front associated with the aforementioned system allows the surface
flow to finally shift to the north and with it some cooler
temperatures. There remains a fair amount of uncertainty as to how
far the cooler air will be able to push during the daytime hours,
but current thoughts are it situating somewhere approximately along
the I-74 corridor to slightly north of that corridor. Areas to the
southwest of this boundary will see a return to the mid 90s and
oppressive humidity while areas to the north will end up much cooler
with some spots closer to 80.
Will be expanding the Excessive Heat warning further to the
northeast to cover this area of higher uncertainty but with the
recent extreme heat and the length of the ongoing heat wave, impacts
will be high regardless of whether the final heat index values are
105 or 110.
&&
.Long Term...(Friday night through Thursday)
Issued at 250 PM EDT Thu Aug 24 2023
The prominent and anomalously strong ridge that has dominated our
weather for the past week is in the process of de-amplifying and
will retreat westward. A synoptic pattern characterized by mean
troughing over the eastern third of CONUS then takes hold, leading
to more seasonable weather for central Indiana.
Some residual heat lingers across our far southern counties on
Saturday, but not nearly as intense as it has been. High
temperatures around 90 are possible from Vincennes southward. Dry
air associated with the continental airmass behind Friday`s front
may not be fully through the region until later Saturday evening. As
such, some higher heat indices are possible across southern Indiana,
ranging from 95 to 100. While this is not up to the heat advisory
threshold, it could exacerbate any heat stress caused by the long
duration of our recent heat wave.
Additionally, Friday`s front may be trending a bit slower and could
still be close enough to our area for a shower or thunderstorm. Some
models are hinting at a vort max (some of the remnant energy from
Tropical Storm Harold) ejecting from the Rockies and interacting
with the slowly advancing front, leading to some convection across
southern Indiana. Current suite of guidance has the bulk of this
activity south of our CWA, though this trend will need to be
monitored.
Dry northerly flow continues and cooler less humid air finally
arrives. Sunday looks to begin rather cool and crisp with morning
lows in the upper 50s to low 60s. Highs are expected to only rise
into the upper 70s and lower 80s. With little change in the overall
synoptic pattern, the cool and crisp weather is expected to continue
through the long range. Some patchy morning fog / ground fog is
possible at times, given any lingering moisture and clear calm
nights.
One potential period of active weather may arrive on Tuesday.
Guidance is honing in on a trough and associated cold front sweeping
in from the northwest. One key question is how much instability will
be available in the pre-frontal environment. Guidance shows
generally northerly flow continuing this weekend into Monday,
briefly switching to southwesterly as the front nears. This may not
be enough time for sufficient moisture / instability advection.
Nevertheless, enough instability may develop that a few showers or
thunderstorms cannot be ruled out.
&&
.Aviation...(00Z TAF Issuance)
Issued at 623 PM EDT Thu Aug 24 2023
Impacts:
* Small threat for a shower or storm impacting particularly KLAF
overnight
* Winds veering to northwest on Friday
Discussion:
Diurnal cu already diminishing early this evening with lingering
cirrus from early morning convection over Ohio still drifting over
the region. Some uncertainty as to whether convection developing
over southeast Wisconsin and northeast Illinois currently may track
into north central Indiana late tonight in a weakened and more
scattered state...and might briefly impact KLAF and even perhaps
KIND overnight. Right now...confidence is far too low to introduce
even a VCSH or VCTS but this will be something to monitor over the
next few hours and depending on how convection evolves to our
N/NW...may need to make further adjustments later this evening.
Even if storms remain to the north of the terminals which is the
likelier scenario at this time...the southern flank of high cloud
debris associated with the convection will expand south overnight
and may produce some broken ceilings. Brief patchy fog is again
possible near daybreak at the outlying terminals.
A cold front will sag south on Friday...veering winds to northwest
at all terminals by the afternoon. Any threat for convection is low
during the day Friday.
&&
.IND WATCHES/WARNINGS/ADVISORIES...
Excessive Heat Warning until 8 PM EDT Friday for INZ043>048-
051>057-060>065-067>072.
&&
$$
Mesoscale...Ryan
Short Term...White
Long Term...Eckhoff
Aviation...Ryan
Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
650 PM CDT Thu Aug 24 2023
.MESOSCALE DISCUSSION...
Issued at 650 PM CDT Thu Aug 24 2023
Two severe thunderstorms continue across southeastern Wisconsin, the
first near Whitewater and the other just offshore of Kenosha. Both
storms are just behind merging synoptic and lake-enhanced backdoor
cold front, and along the southern periphery of a subtle mid-level
wave moving into Lower Michigan. The environment across far southern
Wisconsin and northern Illinois is exceptionally unstable with a
deep layer of anomalously high moisture (surface dew points in the
upper 70s to around 80) leading to SBCAPE of 6000-7000 J/kg.
Meanwhile, modest mid-level flow along the southern side of the wave
is leading to 25-30 kt of effective bulk shear (which is plenty when
dealing with such extreme instability). However, subsidence behind
the departing wave is leading to an increasingly strong inversion
(via subsidence) based between 900 and 800 mb, as confirmed by AMDAR
soundings sampled within the past hour. As a result, unless
convection in Wisconsin can quickly "grow upscale" and develop a
deep, robust cold pool, it may struggle to maintain intensity as it
moves southeast into northern Illinois over the next few hours. In
other words, the environment across northern Illinois is not
necessarily favorable to maintain a discrete storm mode.
A Severe Thunderstorm Watch remains in effect for Boone, McHenry,
and Lake counties in Illinois, which remains appropriate given the
storms may be able to survive to the state line before succumbing to
increasingly hostile environment across northern Illinois.
Borchardt
&&
.SHORT TERM...
Issued at 253 PM CDT Thu Aug 24 2023
Through Tonight...
It`s another remarkably hot and humid day with areawide
temperatures in the mid to upper 90s. Chicago O`hare officially
logged a 100F degree temperature at approximately 2:51 PM, the
first time since July 6th, 2012. In addition, August 24, 2023 is
now tied for the 5th latest 100 degree reading ever on record in
Chicago during the calendar year. If the air temperatures weren`t
hot enough, low-level moisture has struggled to mix downward this
afternoon in spite of full sunshine and southwest winds of 15-20
mph. Surface dew points remain in the upper 70s to lower 80s
areawide, including over the urban core of Chicago where moisture
tends to mix more aggressively compared to outlying areas. As a
result, heat indices range from 110F to locally 120F degrees at
press time, with an earlier hourly heat index observation of 118F
at Chicago O`Hare (100T/79Td). With a peak heat index near 116F
yesterday, Chicago O`hare has officially logged 2 consecutive days
with heat indices above 115F. This is an incredible (and frankly,
somber) feat that has only occurred a few times in recorded
history with the last time during the historic heat wave of 1995.
All in all, we`re experiencing a historic and dangerous heat wave.
Continue to check in with loved ones and neighbors and dial 911
if you or someone else is experiencing symptoms of heat related
illness.
A recent hand surface analysis places a weak 1008 surface low
pressure system in southern Wisconsin with a surface cold front
stretching southwestward into central Iowa. The surface low is
moving eastward along a stationary front draped across Lower
Michigan, and ahead of a subtle/weak 700mb vort max moving
southeastward through southern Wisconsin. As the 700mb wave moves
into Lower Michigan, thunderstorms are expected to rapidly develop
and grow upscale into a cluster with a pronounced damaging wind
threat into southeastern Lower Michigan. However, thunderstorms may
also develop as far west as Milwaukee or even Madison along the
trailing cold front, though confidence remains lower than average
owing to competing factors.
For one, low-level confluence behind the wave this evening will be
modest at best, except perhaps near merging backdoor cold front and
the synoptic front near Milwaukee. In addition, subsidence behind
the 700mb wave will lead to a weak low-level inversion, against
which updrafts will struggle as they grow in the weakly confluent
regime. It is for these reasons we believe available CAM guidance
continues to show little to no convection across southern Wisconsin
this afternoon and subsequently northern Illinois this evening.
However, the airmass along and south of the front is exceptionally
unstable with surface dew points in the upper 70s to lower 80s
anchoring a deep layer of anomalously high moisture and SBCAPE
nearing 7000 J/kg. Surface dew points continue to run 3 to 5 degrees
higher than nearly all available guidance, signaling that there is
more instability than CAM guidance is "using" to simulate (or not
simulate) convection. Accounting for the observed low-level
moisture, there may be enough instability to simply overcome the
weak subsidence inversion and arguably weak low-level confluence
along the front. Hence, our take is that there will likely be at
least one, if not several, storms develop in southern Wisconsin this
afternoon which will track toward if not into northern Illinois this
evening.
Should storms develop, mid- to upper-level steering flow would track
them southeastward across far northern to northeastern Illinois,
more or less east of I-39. Now, even with modest deep-layer shear,
thunderstorms in environments with extreme instability can pretty
much do whatever they want. In other words, any storm that develops
would almost certainly become severe with a threat for damaging
winds and even hail at the least, not to mention heavy, soaking
downpours. Timing the position of the front, the threat for severe
weather continues to look like 5 to 10 or 11 pm or so, again mainly
east of I-39. Needless to say, we`ll keep everyone updated as the
afternoon/evening wears on as trends become established.
Finally, the heat will break tonight behind the cold front
(regardless of if there are storms). Overnight lows will drop down
into the upper 60s to lower 70s by sunrise...which will feel quite
comfortable after the heat wave.
Borchardt
&&
.LONG TERM...
Issued at 300 PM CDT Thu Aug 24 2023
Friday through Thursday...
Key Forecast Messages:
* Cooler Friday but still very warm and humid with a chance of
showers and thunderstorms in the evening into the overnight,
especially near and southwest of a Sterling-Morocco-Fowler line
* Comfortable near to below normal temperatures and primarily dry
Saturday onward
* Dangerous swimming conditions likely at Lake Michigan beaches on
Saturday-Saturday night
Overview:
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-Friday Night:
Friday will likely start with post-frontal stratus blanketing the
northeast half or 2/3 of the CWA and patchy fog over the lake may
hug the immediate shoreline. The stratus should transition to
more open cell strato-Cu due to the warm air mass aloft and
lowering but still high late August sun angle, except perhaps
right near the lake. We`ll 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 mid to upper 70s. All in
all, quite a temperature change, especially near the lake.
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 mid 80s to low 90s elsewhere well inland of the
lake. The highest heat indices of 96 to 102F are forecast near
and south of a Sterling to Kankakee to Fowler line. Did forecast dew
points on the higher side and close to better performing guidance
of late, and if these 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,
but currently not the favored 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 some scattered showers and
thunderstorms Friday evening through the early overnight, with a
few spotty storms possible as soon as the late afternoon in far
southwestern CWA sections, and some sprinkles to the north and
east. Despite modest large scale forcing well south of the main
short- wave, 2 DM 12-hr 500 mb height falls, an uptick in
convergence, and the lingering northwest to southeast oriented
axis of appreciable instability, resulted in a stronger signal for
convection in the 12z model suite than in previous forecast
cycles.
Per recent model consensus, near and southwest of a Sterling-
Morocco-Fowler line is most favored for scattered thunderstorms,
capable of localized wet microburst wind damage (owing to 30 kt of
deep layer bulk shear) and soaking downpours. Increased PoPs a
bit into the chance range in the favored corridor. Instability
will quickly diminish with north and northeastward extent, plus
expecting mid-level dry advection deeper into the evening, so only
carrying slight chance thunder mention into the Chicago and
Rockford metros in the early to mid evening hours.
Saturday-Thursday:
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 has been alluded to over the past
couple days, 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 away from the lake and lower
to 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 west-southwest inland. Another backdoor cold front
will move across the area later Monday, which due to minimal
instability and lacking moisture, may touch off a few spotty
showers, particularly in the evening over northeast IL. An
instability starved short-wave may then touch off a few more
spotty showers near the lake on Tuesday, though any spotty 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 through
Thursday, with lows in the 50s common outside of Chicago through
much of the long term period and even some upper 40s possible in
the Tuesday night-Wednesday night timeframe! Following our
historic late season oppressive heat episode, looking at a fall
preview next week as meteorological summer draws to a close.
Castro
&&
.AVIATION...
For the 00Z TAFs...
Aviation Forecast Concerns:
* Potential for strong thunderstorms across mainly the Chicago
metro terminals this evening.
* Winds shift to the northeast this evening behind cold front.
* Period of MVFR/patchy high-end IFR ceilings possible later
tonight and early Friday behind cold front.
Surface low was analyzed tracking southeast across southern Lake
Michigan early this evening, and a cold front trailing through
southern WI into IA. Scattered thunderstorms have developed in the
vicinity of the front from southeast WI eastward across LM in
association with a subtle mid-level disturbance, and these storms
will likely continue to develop south-southeastward this evening
as the front (and combined convective outflow/lake breeze)
continues to push into the forecast area. Confidence in coverage
of storms into IL/northwest IN remains somewhat low given capping
inversion still evident in recent AMDAR aircraft soundings, though
the enhanced low-level convergence supported by convective
outflow may be enough to bring storms into the Chicago metro
terminals by around 0100-0130Z (800-830 pm CDT). HRRR, RAP, RRFS
CAM guidance suggests coverage and strength may decrease over the
next few hours as storms evolve to the southeast, though hot/humid
airmass would support strong/severe windy storms if the are able
to maintain themselves. Will have to continue to monitor short
term trends, and will make strategic amendments as necessary.
Aside from convective concerns, combined outflow/lake breeze/cold
front appears to be spreading south-southwest across the southern
part of the Lake, and will likely bring a shift to northeast winds
for ORD/MDW in the 01-02Z time frame base on current 88D/TDWR
data, though it could potentially be later if convection and
associated outflow weakens. Breezy west winds will persist prior
to this.
Once the front moves through later this evening, winds should
remain north-northeast for the duration of the forecast period.
Low-level stratus will likely spread in off the lake overnight and
linger into Friday morning. IFR cigs likely over/along the lake,
and while can`t be ruled out farther inland suspect lower MVFR
more likely for ORD/MDW. These lower ceilings should lift and
scatter Friday morning. A mid-level VFR deck will persist much of
Friday, with a few sprinkles possible.
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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