Forecast Discussions mentioning any of
"ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 07/23/21
Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
936 PM CDT Thu Jul 22 2021
936 PM CDT
For the rest of tonight, it appears the chances for any convection
of note are diminishing. In the warm and moist air mass, we`ve
seen widely isolated showers pop up at times, though as of this
writing, only seeing a lone cell south of Pontiac. A weak short-
wave centered near the southeast MN, northeast IA border will
drift south-southeast through the night. Meanwhile, a slightly
stronger wave over north central WI, responsible for higher
coverage of showers and thunderstorms will give only a glancing
blow of forcing to northeast IL and northwest IN overnight, with
the stronger forcing passing into lower MI.
00z RAOBs showed a pronounced dry layer above 700 mb and most
recent AMDAR soundings also indicated a weak inversion around 600
mb. Meanwhile, despite the very muggy boundary layer with low to
mid 70s Td, lacking better saturation in the 900-800 mb layer on
aforementioned soundings, where a theoretical elevated parcel
could lift from to tap into MUCAPE, also appears to point toward a
struggle to generate robust enough updrafts for lightning
generation. Thus, with only modest forcing and rather unfavorable
profiles, kept thunder mention at slight chance through the night.
For this update, kept the 30-40% PoPs into northeast IL and far
northwest IN, though looking like that may be a bit generous. Some
widely scattered lighter showers may be all that ends up occurring,
if that. Recent HRRR runs have fully backed off on activity, while
3km NAMnest has a few isolated cells over Chicagoland overnight.
The only other forecast change of note was to have haze mention
from effects of lower level wildfire smoke to areas where there
were recent minor visibility reductions. On Friday, southwest flow
looks to bring a higher concentration of low-level smoke to the
region per the 00z HRRR surface smoke forecast, so added haze
mention through Friday night to the grids. Subtle forcing toward
mid day Friday into the afternoon and lake breeze convergence
hugging the shore may be just enough to pop isolated to widely
scattered showers and thunderstorms.
248 PM CDT
Through Friday night...
Southwesterly flow is pumping in warm. moist air, with dewpoints
jumping from the mid-60s this morning to the lower 70s as of
writing. Clouds and haze from smoke has scattered out in many areas,
allowing temperatures to climb into the low to mid 80s. Temperature
differential has allowed the lake breeze to set up and move inland,
just east of O`Hare at this moment, and is expected to move just
west of the main airports, shifting winds to the southeast, and
helping keep locations near Lake Michigan a bit cooler in the upper
Increased heat and moisture is reflected in isentropic upglide,
allowing for a slightly unstable atmosphere, as reflected with
occasional showers moving through the forecast area. With CAPE
values in excess of 1000 J/kg, the potential for isolated
thunderstorms to fire this afternoon exists, although they should be
isolated in nature.
A short wave/vorticity max will move out of northern Iowa and may
provide focus for increased convective activity Thursday night into
early Friday morning. Guidance is not in agreement with the location
of this increased chances, but presently appears to be focused along
the Wisconsin/Illinois border, especially northeast Illinois. The
short wave will move east Friday morning, but with continued
southerly flow pumping in warm, moist air, instability will remain
throughout Friday for a continued slight chance of isolated showers
and maybe even a thunderstorm or two to fire Friday afternoon. High
temperatures will be in the mid to upper 80s, although the heat
indices will be in the lower 90s.
Chances for precipitation decrease Friday overnight, but continued
southerly flow will keep dewpoints in the lower 70s, with lows
Saturday morning in the lower to mid 70s.
256 PM CDT
Saturday through Thursday...
No substantial changes to the forecast for this weekend into the
middle of next week as seasonably summertime conditions with
intermittent chances of thunderstorms continue.
Broad ridging centered across the central and southern High
Plains will remain established through the forecast period,
keeping the western Great Lakes on its northeast fringe.
Seasonably warm and humid conditions will be the rule for northern
Illinois and northwest Indiana. While the greatest convective
chances through the period will remain northeast of the forecast
area, the most favorable period for storms to edge southwest into
the area will be late Saturday afternoon and night, and possibly
Saturday and Sunday: Low-level moisture will continue to increase
through the day Saturday on southwest winds sporadically gusting
to 25 mph. While some guidance may be overdone with forecast dew
points into the upper 70s, widespread dew points solidly in the
low 70s and edging into the mid 70s is likely. With these dew
points, the environment should remain capped through much of the
day under an 850 hPa warm nose. We will have to monitor any
potential upstream convection on Friday night, however, as there
are some signals a residual MCV may track within faster WNW mid-
level flow just north of the area and bring either enhanced mid-
level clouds/decaying showers (assuming the expected stronger cap)
to northern Illinois late morning/early afternoon.
The increase in dew points combined with temps potentially into
the low 90s will result in heat index values around 100F in the
afternoon. If mid-level cloud cover and any potential smoke aloft
is minimal, slightly higher temps and heat index values nearing
the local Heat Advisory criteria of 105F are possible.
Capping may erode enough ahead of an approaching cold front to
allow for isolated to scattered convection to develop over
primarily northeast Illinois and northwest Indiana Saturday
evening. A highly unstable environment with modest 0-8km shear
(better metric due to potentially very tall storms) of 25-30kts
will support explosive convective growth and a resultant severe
thunderstorm wind risk with any convection that does develop.
The southward push of the cold front or effective outflow boundary
will depend on the amount of upstream convection on
Saturday/Saturday night. While guidance has been fairly persistent
on clearing the boundary through most, if not all, of the CWA by
Sunday afternoon, any slower progression will support another
afternoon of isolated explosive severe thunderstorm potential
across southern portions of the forecast area. Additionally, heat
index values will near or surpass 100F again around and even
behind the boundary ahead of somewhat drier air advecting in from
Monday through Thursday: The latest GFS is a bit of an outlier in
bringing additional rounds of convection across the area with WNW
flow aloft on Monday and again on Tuesday, likely owing to
unrealistic amounts moisture (dew points nearing 80F) in then
lowest 500ft of modeled profiles. With surface high pressure
setting over the region and the weekend`s boundary drifting south
on renewed convection each day, Monday and Tuesday will likely
remain dry and warm. A more pronounced wave should bring
increasing chances for convection Wednesday into Thursday.
For the 00Z TAFs...
Aviation Forecast Concerns:
* Some low VFR/high MVFR visibility in haze for metro terminals
* Low probability for SHRA/TSRA late tonight/pre-dawn Friday.
Somewhat ill-defined surface warm front extended from southern
WI, southeast along the lake breeze boundary west of ORD/MDW and
then southward near the IL/IN state line early this evening. This
front will continue to evolve slowly northward overnight. While a
few isolated showers had developed along this boundary across the
Chicago metro this afternoon, a lack of stronger forcing should
keep things quiet across the terminals through this evening.
Various model guidance continues to depict the potential for some
convective development primarily across southeastern WI later
tonight along/north of the warm frontal boundary, on the nose of a
modest southwesterly low level jet and in association with a weak
mid-level short wave which will track southeast across the
region. With a southeasterly propagation expected to any
showers/storms which develop over southeast WI, the potential does
exist for some of this to affect the Chicago metro terminals
after midnight, generally in the 06-11Z period based on CAM
solutions. Confidence in any significant coverage this far south
across the terminals is fairly low, with better chances north and
northeast of the terminals across WI/southern Lake Michigan on the
nose of the jet and with a little better mid-level moisture
profile. Felt it prudent to leave some mention for now though and
since can`t use the inherited prob30 (only allowed beyond first 9
hours of TAF) have carried a VCSH mention at this time and will
monitor for development across WI later this evening to assess
the potential for our forecast area.
On Friday, the mid-level wave will track off to the east and an
upper level short wave ridge is progged to build across the
western Lakes region. Warming mid-level temps due to subsidence
beneath this ridge looks as if it will generally provide a capping
inversion with respect to renewed convection. Some guidance
weakens this cap a bit late in the day and suggests some isolated
convective threat, though feel that this is a fairly low coverage
and low probability occurrence at any particular terminial and
thus will maintain dry TAF Friday/Friday evening.
East-southeast winds for ORD and MDW will become light southeast
later this evening and then light south-southwest overnight.
Modest south-southwest winds Friday should keep lake breeze
boundary east of ORD/MDW. Only other concern is with current
patchy low-end VFR visibility in haze across parts of the Chicago
metro. Expect this may improve as winds turn southerly overnight.
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