NWS Chicago, IL wrote its last area forecast discussion before the first event at Jun 14, 9:21 AM CST, 2 hours 12 minutes earlier.
Area forecast discussion, Jun 14, 9:21 AM CST, 2 hours 12 minutes before
Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1021 AM CDT Wed Jun 14 2017
.UPDATE...
1020 AM CDT
Still monitoring the high likelihood of thunderstorms across much
of northern Illinois and northwest Indiana this afternoon into the
evening, some of which will be capable of becoming strong/severe.
In the near term, early morning upstream convection has since
diminished, with any lingering development currently staying
confined to eastern Iowa or far western Illinois. However, it has
produced a well defined outflow boundary which had been
progressing east southeast through north central Illinois but is
now beginning to slow up. It's this remnant boundary along with
upstream large scale ascent which will help drive an increase in
thunderstorm activity as early as late morning or midday. Highly
amplified upper level pattern with closed system over the northern
Plains and southern Canada, will allow for upstream mid level
energy to move overhead by midday/early afternoon and have timed
this energy into the western CWA by the 17-18z.
As this moves across the entire CWA today, should see convection
blossom during this time especially along the remnant boundary.
CIN is already eroding and will likely be gone over the next hour,
with an uncapped environment of moderate/strong instability in
place by late morning. Scattered strong to severe storms will be
possible by midday area wide as this forcing/development continue
to the east. Additional development is probable during this time
over eastern Iowa and northwest Illinois along approaching mid
level energy and MCV currently over central Iowa. This will
support a broken line/multicell clusters to develop, likely
pushing through portions of northern Illinois and northwest
Indiana through mid to late afternoon.
Main hazards expected today will be for damaging winds and large
hail. Lack of deep layer shear will limit overall organization to
development today, meaning could quickly get into a "messy"
pattern of slow moving thunderstorms. However, the approaching MCV
is a little concerning given that it could support a slight
increase in shear across north central Illinois. If this were to
occur, could not rule out an isolated tornado is this location.
With the slow storm motion and high pwat axis shifting overhead,
heavy downpours and localized flooding will also be a threat.
Should see all convection diminish by early to mid evening.
Rodriguez
&&
.SHORT TERM...
325 AM CDT
Through Tonight...
After the first dozen dry and sunny days of the month, yesterday
kicked off a period of unsettled weather that continues today and
resumes Friday and early this weekend. There is scattered severe
weather potential today and tonight, though the forecast
evolution of storms and their coverage/organization, has lower
confidence.
Early this morning, a well-defined stacked low over North Dakota
is tracking northeastward and will continue to today. Scattered
warm advection thunderstorms are over Lake Michigan into parts of
northwest Indiana, while areas of storms stretch from western
Wisconsin through the central Plains ahead of the system cold
front and tied to a couple minor impulses. In-between these two
convective areas is a forcing lull over the CWA and should
continue past sunrise for several hours. It is possible much of
the area remains dry all morning, though do expect isolated
showers/storms to likely start to pop in north central Illinois
later this morning as the low-level jet, while weakening, veers
into the region coincident with steeper lapse rates above.
The higher dew points temporarily scoured out of the CWA last
night by convective cold pools, but will return this morning with
the air mass just to the south/southwest having upper 60s and
lower 70s. The boundary layer thermal characteristics again are
similar to the past several days, with 925mb temperatures in the
mid to upper 20s and 850mb temperatures in the upper teens. This
can support widespread mid 90s if convection or widespread clouds
do not impact areas prior to that, and yesterday showed that for
southern CWA locations. Have gone with 90-94 for most areas,
keeping in mind that if storms do start early in north central
Illinois those areas could just peak in the 80s. Heat index
readings will near or even briefly tag 100 degrees in some areas,
similar to yesterday.
It is a challenging forecast with storms this afternoon and
tonight, particularly with specifics in evolution and timing,
especially when considering CAM solutions that have varied greatly
in their solutions for today. That disparity is not surprising
considering the weaker-type forcing today, given the area only
seeing a glancing of height falls and mid-level impulses being
quite low amplitude. So have stuck close to synoptic forcing and
peak instability time, with mid-late afternoon into the evening
probably being the most favored for areas/clusters of storms. As
noted above, earlier isolated convective activity is probable
mainly in north central Illinois, but do not see strong enough
forcing to move rapidly east as some CAMs suggest, unless a
better-defined MCV were to emanate eastward.
Deep layer shear does inch up in the more favored convective time
of mid-afternoon and a tad more in the evening, though it still
is on the weaker end of the spectrum, at 20-30 kt. Moisture and
instability will not be a problem for deep convection, and as we
saw yesterday, MLCAPE values in excess of 2000 J/kg in the
presence of little shear can still support quite a bit of
convective activity and some hail threat. One notable difference
was we will not have a sharp low-level focus today, so mid-level
impulses will be key for initiation and organization of
convective clusters, which then could drive themselves more so on
congealed downdrafts. CAMs do hint at these but not seeing any
overly well- defined structures in high temporal solutions or wind
fields that would hint at a a more enhanced wind threat, and that
makes sense. So think that threat is scattered as well as the
hail risk, though a couple larger hail producing storms...to
possibly golf balls...are possible in the region with instability
on the higher end. With true summer PWAT air (1.60-1.90 in.) it
will lead to some efficient rain producers with still limited
storm motion.
All that said, uncertainty on evolution and timing is high, and so
not really screaming a widespread severe weather event, and
somewhat marginal shear too complicates things. Have gone likely
wording for the afternoon and evening, but certainly will not be
storming that entire time in any one area, and more temporal
resolution will likely be able to be added to the forecast today.
Convective chances should ease from west-to-east late this evening
into overnight.
MTF
&&
.LONG TERM...
336 AM CDT
Thursday through Tuesday...
The "cold" front will be progressing through the area Thursday
morning scouring out any early morning rain chances and offering
sun by late morning into the afternoon. Temperatures look to
rebound to around if not a little above 90 based on 850-925mb
temperatures forecast. Dew points will drop some behind the
boundary making it not as humid, but certainly another day added
to the early season hot stretch.
The next very low amplitude impulse is forecast to move east over
the region late Thursday night and early Friday, possibly bringing
a few showers and storms. A slightly better wave is forecast
later Friday, after dark it appears, that brings stronger mid-
level flow with it. This could have some stronger storms mainly
west of the area during the evening. There is uncertainty in cloud
cover and some rain chances Friday afternoon, but if those are not
an issue then 90 again for a high in many areas.
As the zonal flow increases and starts to dig ahead of a trough
later Saturday, storm chances return. The warm advection on
Saturday seems like it could be quite strong, and if we do remain
dry during the day, temperature profiles look like they would
support mid 90s. So going forecast may be a bit cool depending on
timing of system features. The deepening low to our north on
Saturday night has trended more significant in recent global model
solutions, thus bringing a sharper defined cold front through the
area later that night or early Sunday. This could have organized,
probably linear convection along it. Sunday looks like the
coolest day for a good part of the CWA in over a week, but still
with temperatures basically at normal for mid-June.
MTF
&&
.AVIATION...
For the 12Z TAFs...
The main forecast concern today continues to center around the
best timing for thunderstorms to impact the terminals today, and
the winds associated with the storms, and then following the
storms.
The main area of focus this morning is with the line of storms to
our west over Iowa. These thunderstorms have been continually
redeveloping through the night along an eastward moving outflow
boundary. It appears likely that this line of storms will become
diurnally reinvigorated through the morning as the remnant storms
and the outflow shifts eastward into northern Illinois. As a
result, I expect storms to shift across the KRFD area anytime from
roughly 14-17z at KRFD, and from mid to late morning through
midday/early afternoon at the Chicago terminals. Given the
history of this long lived outflow boundary, and the likelihood of
reinvigorated downdrafts with new storms this morning, it appears
that there could be a period of very gusty westerly winds as the
storms shift over the terminals.
The other concern will be the possibility for the development of
breezy easterly winds for a couple hours at the terminals
following the passage of the storms. There has been some signs at
this occurring over Iowa, indicating some good origination of the
storms. Considering this is likely to occur into this afternoon
at the Chicago area terminals, I have added the mention of
easterly winds to the TAF. Confidence is not the highest on how
long they will last, but a couple hours is certainly plausible.
With time, the winds should trend back to a southerly direction
later this afternoon and evening.
Another round of thunderstorms is possible either later this
afternoon or tonight. However, confidence on this second round,
and its timing is low at this time. At the present I have added a
PROB30 for this possibility.
KJB
&&
.MARINE...
207 AM CDT
An active weather pattern is likely to continue over the lakes
region into the weekend as several disturbances approach the
region. These disturbances are likely to result in additional
periods of thunderstorms through the period. While some isolated
to scattered storms will be possible this morning, mainly over
southern portions of the lake, it appears that more thunderstorms
will impact the lake later today and tonight, and some of these
storms may produce strong winds in excess of 40 to 50 KT.
Additional periods of storms this weekend may also produce very
gusty and erratic winds on the lake at times.
In addition to the thunderstorm threat over the lake today and
this weekend, it appears that a unseasonably strong area of low
pressure may shift over the Upper Great Lakes region on Sunday.
There is still a large amount of uncertainty in how this storm
system will evolve over the weekend. However, there are some signs
that a potent storm system could produce a period of strong south
to southwest winds on the lake Saturday night into Sunday,
outside of any thunderstorms.
KJB
&&
.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.
&&
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