Forecast Discussions mentioning any of
"ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 05/03/21
Area Forecast Discussion...UPDATED
National Weather Service Chicago/Romeoville, IL
912 PM CDT Sun May 2 2021
912 PM CDT
Minor tweaks to the going forecast for the rest of tonight, with
the message remaining showers mainly overnight for areas
along/east of I-55, and a chance for a few showers or a storm to
sneak into far northern Illinois. The severe threat is low if
that were to happen. Confidence has further increased in
convection, some probably on the stronger to possibly severe end
with gusty winds, moving into northern Illinois mid to late
First for the rest of tonight, the system responsible for the
rain in the southeast CWA -- an upper low over eastern Arkansas --
has its broad upper diffluence creeping northward across the
southern halves of Illinois and Indiana. Light showers in that
area will continue to slowly expand north, favored to reach
southern parts of the Chicago metro after 2 A.M. This rain should
primarily be light, lingering for a few hours after sunrise before
To the north, convection lit up along the frontal zone from
northern Iowa into central Wisconsin. Convective prorogation
will continue to favor more of an eastward movement, especially
with the offsetting strength of the southerly low-level winds.
However, there are multiple southward-moving outflows seen on
radar, satellite, and surface obs in southern Wisconsin, and
scattered convection on this is probable at least for the next few
hours. This should result in the outflow getting close to the
state line after midnight. While MUCAPE tapers quickly with
southern extent (a stretched out 200 J/kg on the 00Z DVN
sounding), there is enough low-level moisture transport into these
outflows to possibly force a few showers or weak storms along/north
of I-90 late.
Looking ahead to Monday, the below AFD remains representative to
expectations and the potential for some severe storms. One key
thing we have been seeing with incoming observational data and CAM
solutions is the likely presence of an MCV currently along the
Nebraska/Kansas border that several high-res solutions pick up on
and track into/across northern Illinois Monday mid-afternoon into
mid-evening. With little in the way of convection expected to our
south (upstream in the low-levels) through peak heating Monday,
this more convectively-enhanced wave than it previously looked
should have a decent air mass to work with. A convective uptick is
favored during peak heating in eastern Iowa to north central
Illinois mid to late afternoon. While morning clouds look like a
good bet, RAP soundings continue to support the afternoon cap
eroded with dew points peaking into the mid 60s up to I-80 and
possibly I-88. Mid 60s dew points are presently up to the
Missouri/Arkansas border, so that`s definitely doable.
The overnight shift will assess convective evolution further, and
try to lock down timing that still has 3 to 4 hour differences in
various NWP solutions.
242 PM CDT
Through Monday night...
Another day of breezy and warm conditions here, with temperatures
early this afternoon generally in the low to mid 80s north of an
encroaching region of higher cloud cover. Regional visible
satellite loops show a bit more in the way of lumpy cumulus
development ongoing north of I-88 where surface dewpoints are in
the upper 50s with gradually eroding convective inhibition.
Currently, things are still quite capped to surface-based parcels
with a lingering pocket of +8 to +9 C air around 750 mb per recent
AMDAR soundings. With surface temperatures rising into the mid
80s, it`s possible we erode enough of the lingering MLCIN before
sunset to pop a few showers--maybe a thunderstorm--north of I-88.
Any activity that does develop shouldn`t be too long for this
world with the loss of daytime heating, but 30 degree dewpoint
depressions, 0-3 km lapse rates approaching 9 C/km, and DCAPE
values near 900 J/kg could support some gustier winds should any
deeper showers/storms manage to develop.
Additional--more organized/vigorous--convection is expected to
develop farther to our north and west through this afternoon and
evening ahead of a quasistationary front which stretches across
central Wisconsin and back into southern Minnesota. This activity
will build south and eastward with time, but is expected to wane
with time. It`s possible some of this activity (in a very weakened
state) presses into northwest Illinois later this evening, and
have left some low-grade PoPs in place there to capture this
potential. To the south, light showers associated with the
divergent upper-level flow ahead of a strung-out shortwave will
push into our forecast area, with the most likely areas to receive
some light precipitation those that are south and east of I-55.
This activity will peel east of the area through the mid-morning
hours on Monday, leaving behind generally dry conditions over the
region but with plenty of low and mid-level cloud cover which will
stunt our temperature climb and also the destabilization process.
However, model guidance is in pretty good agreement driving low
to even mid 60 degree dewpoints into north-central Illinois
through the afternoon hours ahead of a pretty vigorous shortwave.
There remains some degree of disparity regarding the location of
the northern terminus of the advancing EML plume tomorrow, but if
anything, guidance has trended a bit farther north into the CWA
with this. What this means for us is a greater degree of capping
through the morning/early-mid afternoon hours, but a somewhat
greater supply of instability ahead of the aforementioned
shortwave, especially as you work farther south of I-80 and into
Deep layer shear will remain in short supply through much of the
morning hours on Monday, but looks to increase rapidly with the
approach of the aforementioned shortwave through the late-
afternoon. As a result, a gradual intensification of convection
along an incoming cold front is possible as it approaches our
region from the west. Thunderstorms appear likely with this
feature as it traverses our forecast area, and some could become
severe should MLCAPE values indeed claw their way towards 1000-1500
J/kg as some guidance suggests into an increasingly-sheared
environment. Largely boundary-orthogonal shear may help keep some
degree of discrete-ness going with a gradual clustering of cells
with time yielding a damaging wind threat given the fairly fast H7
flow and some degree of mid-level drying with time. The steepened
mid-level lapse rates may also afford storms the potential to
produce some large hail, and, as with any system sporting a
sub-1000 mb surface reflection traversing eastern Iowa, the
tornado potential will be non-zero. Do think the tornado potential
is probably on the lower side at this time given the
sufficiently-veered look to our surface winds and overall pretty
small hodographs in the 0-3 km layer, but still something to keep
an eye on. The main window of concern is a later one with
everything mentioned, closer to 5-9 PM or so. The main threat axis
may align mainly south of I-80 (into the corridor of better
instability), but will extent across the entire area and may
increase north towards the I-88/I-90 corridors if instability
increases sufficiently. Activity will clear the area through late
217 PM CDT
Tuesday through Sunday...
A cooler stretch of weather is expected to starting Tuesday with
generally dry conditions expected outside of a couple systems
that may bring precip chances. On Tuesday, a cold frontal
boundary will be positioned across central IL with the low/mid
level baroclinic zone still across the local area. A longwave
trough will move across the area into Tuesday evening while a
surface low tracks up the Ohio Valley ahead of a more amplified
portion of the upper trough. As a result, Tuesday appears to be a
showery day, especially across the southern part of the area
closer to the front and better larger scale forcing. It will be
cooler with highs in the mid to upper 50s north to around 60
south, with winds off Lake Michigan keeping lakeshore areas in the
High pressure moves across Wednesday leaving dry conditions and highs
in the lower 60s for all but lakeshore areas. Light winds will
likely support lake breeze development so afternoon temps will
likely only be in the low to mid 50s lakeside. By this time, large
scale ridging will have developed across western North America
with a broad upper trough over the eastern half. A shortwave
trough is expected to traverse the flow and cross the area
Thursday bringing chance for showers and maybe a few
thunderstorms. The wind field should remain light enough to
support another lake breeze. High temps should range from the
upper 50s north to lower 60s south, with mid 50s near the lake.
The pattern begins to shift east Friday allowing for somewhat
milder air to return, except for areas near the lake once again.
While the upper flow temporarily flattens into Saturday another
trough is expected to pivot southward toward the region with a
second trough ejecting from the southwest triggering surface low
pressure which will move east or northeastward across the Ohio
Valley. This would support another chance for
showers/thunderstorms some time Saturday into Sunday.
For the 00Z TAFs...
636 PM...Primary forecast concerns include...
Scattered thunderstorms Monday afternoon/Monday evening.
Ifr cigs Monday morning.
Gusty southwest winds tonight.
Chance of showers early Monday morning.
Southwest winds will remain gusty this evening, into the mid 20 kt
range and will slowly diminish after midnight. Winds will turn
more southerly early Monday morning and may briefly drift to the
south/southeast after sunrise, but will turn back to the southwest
in the 10-12kt range by late Monday morning.
An area of rain across central IL and central IN will slowly drift
to the northeast later tonight and a period of showers is possible
during the predawn hours. Confidence is low for how widespread
this activity will become as well as its duration and its possible
that the bulk of this shower activity will remain east of the
Chicago area terminals with a few lingering showers possible
through mid morning Monday.
A large area of mvfr cigs will spread north across the area early
Monday morning and there is good agreement that these will lower
to ifr around or just after daybreak for a few hours before then
lifting back through mvfr by late morning and then a low vfr deck
by early afternoon. Though its possible high mvfr cigs may remain
prevailing across northwest IL and at rfd in the afternoon.
Scattered thunderstorms are expected to develop across eastern IA
and northwest IL Monday afternoon and quickly move east across the
terminals through early evening. Models have been handling this
system and these convective trends well for several days. Still
some uncertainty on timing, but current tempo timing covers the
expected time period well, with some timing adjustments possible
as trends emerge. Left thunder mention out of the gyy taf for now,
as best current timing could be at or just after the end of this
forecast. Expect some weakening of this activity as it spreads
east into the Chicago area and for now have only included higher
wind speeds/gusts in the rfd tempo but these may be needed at the
rest of the terminals as winds will likely shift from the
prevailing southwest to a northwest direction as the storms move
through. Prevailing winds expected to remain north/northwest after
this activity shifts to the east. cms
LM...Small Craft Advisory...nearshore waters
until 3 AM Monday.
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