Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 08/24/20

Area Forecast Discussion

National Weather Service Chicago/Romeoville, IL

 900 PM CDT Sun Aug 23 2020

.UPDATE...
900 PM CDT

Earlier strong to locally severe convection has waned with the
loss of daytime heating, leaving behind just a few spotty showers.
Impressive little vort max showing up on moisture channel loops
this evening spiraling southward into east-central Illinois. Even
though we`ll continue to lose access to any remaining surface-
based parcels with CINH nocturnally increasing, have kept some low
(20%) PoPs going through much of the night roughly east of I-57
which coincides with the main region of lingering ascent in the
vicinity of the aforementioned vort max. Recent VAPOR aircraft
soundings out of MDW still show about 1500 J/kg of MUCAPE for
parcels rooted above 900 mb or so. As a result, some renegade
showers and storms will remain possible tonight.

Have expanded the areal extent of the patchy fog wording in the
gridded forecast to encompass most of the area except the
immediate heart of Chicago. Dewpoints remain elevated at this time
in the upper 60s and lower 70s, with clear skies looking to
prevail tonight (outside of any isolated showers/storms). The main
limiting factor for more of a widespread/dense fog threat will be
just a hint of wind immediately off the deck with recent forecast
soundings depicting about 10 kts of flow persisting around 950 mb
which could help keep things mixed up just enough to preclude
more of a widespread fog potential. That said, modified Richardson
numbers off recent BUFKIT output do indicate at least some
potential for decoupling to occur which will support a potential
for at least patchy dense fog development tonight. Updated
products have been transmitted.

Carlaw

&&

.SHORT TERM...
218 PM CDT

Through Monday night...

Forecast concerns center around afternoon/evening thunderstorm
coverage, fog tonight, and then on a convective complex Monday
night that could sneak into portions of the local area.

The upper low that will be our focus for showers and storm
development is moving through the area this afternoon, currently
centered over the I-39 corridor. With some subsidence behind the
upper low, the clouds along and north of the low track have
struggled to grow but as the instability axis spreads in from the
west, expect additional storms may fill in, largely along and south
of Interstate 80 ahead of the low where there is a bit more surface
convergence. Localized heavy rainfall would be the main concern
with these storms. Isolated shower activity will still fester
farther into northeast Illinois. We will hang onto some low precip
chances into the evening as we wait for the the low and
associated weak frontal boundary to clear the area northwest to
southeast.

Last night there was dense fog upstream of the area in Iowa.
Guidance is less excited about the depth of moisture tonight per
forecast soundings. There it appears fog may be patchy or just
localized, though can`t rule out some localized thicker fog along
and west of the Fox River valley.

Monday during the day and most of the evening appear uneventful in
the precipitation department, though after a fairly hot day today,
we find ourselves on the northeast quadrant of an expanding thermal
ridge to the west. With 90/near 90 in the Chicago area today, we see
no reason why we will not tack on at least a few degrees to highs
on Monday, jumping into the lower 90s.

SPC does have a Marginal Risk cutting the northeast tier of the area
into Lake and McHenry IL counties Monday night. Fast flow to the
south of a cold front across the upper Midwest in an fairly
unstable airmass will likely lead to a convective complex that
given the mean wind field and forward propagation would track
southeast through Wisconsin, and based on the ridge position,
could track into a portion of northeast Illinois later Monday
night into early Tuesday. Should this occur, a risk of damaging
winds would exist.

KMD

&&

.LONG TERM...
230 PM CDT

Tuesday through Sunday...

The forecast message remains unchanged for midweek, with hot and
dry conditions expected across the area, likely peaking on
Wednesday with near record highs in the mid to upper 90s. The heat
will continue through at least Thursday, with the chances for
showers and storms returning to the forecast into Friday.
Thereafter, cooler and dry weather looks likely for next weekend.

Overall the period looks dry through at least late Thursday.
However, we did opt to add in a slight chance for a few showers
and storms late Monday night into early Tuesday morning over far
northeastern IL near the WI state line. Showers and thunderstorms
are expected to develop into an MCS Monday night to our north of
WI in response to cold front dropping into the upper Midwest.
While the primary focus for these storms should remain to our
north, I cannot rule out the possibility for some showers and
storms to propagate south-southwestward along the 850 to 300 mb
thickness gradient. If any of these storms, and the associated
outflow make it far enough south to impact northeastern IL into
early Tuesday morning, this area could experience some gusty
winds. Otherwise, it would appear the threat for severe weather
would be low over our area.

The effective surface boundary could hang up over northeastern IL
during the day Tuesday. This thus could result in northeast winds
off the lake, and cooler temperatures near the lake during the
afternoon. It appears at this time the threat of redeveloping
thunderstorms near the boundary Tuesday afternoon will be low due
the likelihood of capping as upper heights begin to build. For
this reason, I have left the area dry after early Tuesday morning.
Temperatures inland on Tuesday will top out in the low to mid
90s, with 80s likely closer to the lake.

Winds will turn southwest again prior to Wednesday morning. This
will set the stage for the eastward expansion of the hot airmass
originating under the upper ridge over the High Plains. Forecast
guidance continues advertise temperatures at 850mb up around 21
C, and near 30C at 925 mb. These rank within the warmest 5 to 10
percentile for this time of year, so near record high temperatures
for Rockford and Chicago are looking likely as we top out in the
middle to upper 90s (record highs at both locations is 97 on
Wednesday). Offshore southwesterly winds on Wednesday should also
allow these hot conditions to reach all the way to the lakeshore
areas. Similar conditions are likely on Thursday as well, but we
will have to watch for the possibility for more cloud cover from
any remnants of Laura that approach the Ohio Valley later in the
week. If we get more cloud cover overhead on Thursday,
temperatures could be held down a couple of degrees from what is
experienced on Wednesday.

In spite of the near record heat possible Wednesday, heat indices
do not look to climb much above the actual air temperature due to
the likelihood of lowering dewpoints (in the 60s) in the
afternoon. For this reason, at this time it appears unlikely that
we would experience widespread areas with heat indices over 105
degrees.

Uncertainty increases late in the week with the timing of a cold
frontal passage, and the accompanying period of showers and
thunderstorms. We will also have to keep an eye on the evolution
of Laura as it makes land fall around midweek somewhere possibly
across the central Gulf Coast. The remnants of Laura could inch
northward and track not too far to our southeast later in the
week. As such, we will have to continue to monitor the timing of
the cold front, which could be delayed until later Friday with
the remnant tropical activity slowing things down. Once this cold
front does shift south across the area, we should see a cooler and
drier airmass shift in over the area in time for next weekend.
Highs next weekend could be in the upper 70s to low 80s.

KJB

&&

.AVIATION...
For the 00Z TAFs...

647 PM...Forecast concerns include...

Chance for thunderstorms early this evening.
Chance for fog early Monday morning.
Possible lake breeze Monday afternoon.

There is still a thunderstorm east of mdw currently and there
will be a chance of showers for the next hour or so but the threat
for thunderstorms for ord/mdw/dpa is ending with still the
potential at gyy as an outflow boundary moves south and may
provide the focus for new development. A recent thunderstorm near
mke in southeast WI will need to be monitored as it drifts south.

Guidance is showing the potential for fog especially across
northwest IL overnight into Monday morning though trends have been
backing off some regarding how widespread the fog may become and
how low visibilities may drop. Maintained previous fog mention for
rfd/dpa which looks on track. Fog may be more focused in areas
that received rain today...including areas just north of ord.

The outflow boundary has shifted winds easterly at ord and will
shift winds to the east at mdw. These winds will slowly turn more
to the southeast over the next few hours and then back to light
southerly for the rest of the night with light southerly winds
for much of the area. Winds will turn more westerly Monday morning
and a lake breeze is expected to form and likely move through gyy
but by later in the afternoon...low level winds may increase
enough to prevent the lake breeze from reaching ord/mdw though
confidence is low. cms

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.
&&

$$

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