Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 06/02/19

Area Forecast Discussion

National Weather Service Chicago/Romeoville, IL

 647 PM CDT Sat Jun 1 2019

.MESOSCALE DISCUSSION...
608 PM CDT

Since 5 pm, storms have remained similar in intensity with
isolated strong/near-severe. There may be just enough "messy"
convective coverage in this environment favoring outflows, that
it very well could be hampering widespread robust, sustained
storms. In addition, the strongest effective deep layer shear of
near 40 knots remains a little north of where the current highest
convective coverage is (south Chicago metro to Streator). That all
said, the scattered severe threat continues, and we continue to
some semi-discrete convective, at times resembling some
supercellular structure. Recently on the immediate south side of
Chicago there have been a flurry of hail reports with the storms
there. With the mid-level winds steady or even slightly
increasing in tandem with still uncapped and non-turned over air
to the south, the scattered severe weather threat will continue
across mainly the south Chicago metro and far northwest Indiana
through 7-8 p.m.

Primary threats remain hail, especially 1-1.5 inch hail, and
isolated gusty winds. The threat of a more congealed wind threat
is lowering with each hour. Some locations have received multiple
rounds of storms in the Chicago metro and are at a little higher
risk to have localized flooding, maybe even flash flooding of a
favorable location given temporary heavy rainfall rates. The
overall flood threat though remains somewhat low.

MTF

&&

.SHORT TERM...
155 PM CDT

Through Tonight...

An outflow/lake breeze composite boundary has pushed inland to
near a Woodstock to Naperville to Wheeler line based on latest
surface observations. The airmass to the east and north of this
boundary has been stabilized, with dewpoints in the upper 50s and
temperatures in the upper 60s. This stabilization is also
demarcated by stable wave clouds on GOES-16 visible satellite
imagery. Latest guidance does indicate the breezier synoptic
southwesterly flow may force this boundary back east a bit through
the afternoon hours, and how this modified airmass impacts the
severe potential into portions of Lake (IL) and Cook counties
remains a bit uncertain at this time. Recent ACARS soundings out
of Midway show this stable layer is pretty robust, with a notable
temperature inversion present near the surface. With additional
diabatic heating, would expect at least some airmass recovery on
the cool side of this boundary with baroclinicity starting to wash
out a bit over the next 1 to 3 hours. If this boundary remains
robust, however, it could locally mute the severe threat into Cook
and Lake counties later this evening by limiting the potential
for strong downdrafts and associated winds to reach the surface.

The other question stems around a notable pocket of dry air now
being analyzed in the 850-700 mb layer with 850 mb dewpoints under
0 C now being analyzed across portions of northwestern Illinois
and into our far western counties. This could tend to work against
updraft intensity--at least initially--although an axis of much
higher moisture is analyzed near I-90. With surface temperatures
having warmed into the 80s, dewpoints in the lower 60s, and steep
mid-level lapse rates in place overhead, MLCAPE values are,
nonetheless, inching past 2000 J/kg early this afternoon,
especially near and west of the I-90 corridor. Mid-level winds
are also increasing ahead of a small but sharp shortwave, and
effective deep layer shear values are pushing north of 30-35 kts
across the region (lower south of I-80, highest north). 500 mb
height falls on the order of 50-60 m/12 hr have been noted
spreading towards southern Wisconsin. Associated increases in
forcing for ascent will be responsible for expanding showers and
thunderstorms along and incoming cold front through the evening
hours. All of this taken together spells a continued threat for
strong to severe storms across our region this afternoon. Hi-res
guidance continues to struggle with a rather wide disparity in
solutions still noted this afternoon.

Storms are beginning to develop across southwestern Wisconsin at
this hour, near a developing weak surface low and at the leading
edge of stronger ascent from the incoming shortwave. Latest
thinking is that storms will continue to develop south and
westward over the next hour or two, eventually nearing Winnebago,
Ogle, and Lee counties towards 4 PM or so. Additional convection
is also attempting to develop along the aforementioned
outflow/lake breeze across southern Cook and into Lake (IN) and
Porter counties. The airmass immediately ahead of this boundary is
unstable and only weakly capped, so we will need to keep an eye on
this area for potential storm development as well. Initial
discrete/isolated activity will carry a risk for large hail
(potentially up to ping pong-golf ball sized), but storms should
quickly begin to congeal into line segments as robust forcing
overspreads the area. As a result, the severe threat will begin to
transition to mainly wind damage, with a potential for 60-70 mph
wind gusts. At this time, think the main severe corridor may end
up developing near and north of I-80, and particularly focusing
along I-88 and I-90 where instability will be maximized. The
tornado threat is low from a synoptic standpoint with minimal
low-level hodograph curvature, but we will be keeping an eye on
any stronger updrafts which manage to develop near this feature
later this afternoon. A brief tornado spin-up or two can`t be
ruled out in the vicinity of this boundary, but the main severe
risk still appears to be from hail and damaging winds.

Looks like a surging line of storms will move out of our south and
eastern counties by mid-evening with a continued threat for strong
to locally damaging wind gusts. Will be holding onto some low PoPs
behind this line of storms with lingering moisture and some
instability to work with, but the main severe window will come to
a close by about 9-10 pm area-wide.

Carlaw

&&

.LONG TERM...
155 PM CDT

Sunday through Saturday...

Amazingly, it does indeed look like we will enter a briefly
quieter weather pattern on Sunday and into early next week.
Unfortunately, it does appear as if we`ll have to contend with
some lingering low precipitation potential on Sunday as low-level
moisture will not be entirely scoured out by today`s shortwave and
cold front. Forecast soundings reveal a sliver of lingering
instability, trapped between a building subsidence inversion and a
strongly mixed boundary layer. Certainly doesn`t look like things
would be able to cool enough for charge separation so will not be
indicating any thunder in the grids, but have added some very low
(10%) PoPs to the region to account for showers bubbling with
daytime heating. Weak northwesterly gradient flow should allow a
lake breeze to push inland Sunday afternoon, keeping things
generally in the 60s lakeside while upper 60s and lower 70s become
more prevalent inland.

A more subsident regime will begin to build in overhead on Monday
as high pressure starts to really nose into the region. Will be
showing precipitation-free conditions on Monday with highs ranging
from the lower 60s near the lake to the lower 70s inland. The dry
weather looks to continue through most of Tuesday, although low
to mid-level moisture will begin to increase during the afternoon
hours as southerly return flow re-establishes itself with our
transient surface high shifting eastward into the Mid Atlantic.
This will allow instability to diurnally build which could support
some low storm chances mainly across our western locales during
the afternoon and evening.

The next cold front will be approaching the area Tuesday night
but there are plenty of questions on convective evolution into
Wednesday. Forcing Tuesday night may be somewhat disjointed over
the region and possibly focused more south. Then on Wednesday, the
front may stall or wash out over the area. In west-northwest flow
aloft, bulk shear up to 30 kt and steep lapse rate plume could
enable some storm organization for aforementioned lower end
strong/severe risk sometime during Tuesday eve- Wednesday. Also,
PWAT of 1.5"+ could support localized flooding risk.

Finally, looking ahead to late next week, plenty of uncertainty on
synoptic evolution. Question on Thursday looks to be whether front
sags south of the area or stalls overhead/lifts slowly north,
which will drive convective chances. Then for Friday into next
weekend, it looks like a closed low emerging from the southwest
will completely cut off from the mid-upper jet as large scale
ridging over central and eastern NOAM builds into Canada. Models
struggle with handling of upper lows, but in general the pattern,
should this possible closed low affect our region, would spell
cloudy, humid and unsettled conditions, with slow moving showers
and thunderstorms at times. This could be a period to watch for
more widespread significant rainfall totals, as PWAT values may
approach 2", or near/around 200% of normal.

Carlaw/Castro

&&

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

Multiple outflow boundaries have lead to variable and occasionally
strong/gusty winds at the Chicago area terminals this afternoon.
Latest radar imagery indicates that convective activity is
becoming more scattered as the cold front pushes southeastward
across the region. As the front pushes south of the terminals
early this evening, the potential for showers and thunderstorms
will diminish while winds weaken and back from prevailing nely to
more nwly. While there may be the potential for some isold showers
later into the evening, potential will quickly diminish with
sunset.  The remainder of the period from late this evening
through tomorrow should be relatively quiet.

&&

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

$$

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