Special Weather Statement, NWS Chicago, IL

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
216 AM CDT Fri Jun 18 2021

.SHORT TERM...
1251 AM CDT

Through Tonight...

Early-morning surface analyses place a surface cold front from near
Lincoln, Nebraska to Ames, Iowa to the Wisconsin Dells with a
north/south moisture axis continuing to inch eastward into Illinois.
Robust convection continues to blossom in southern Wisconsin
along the nose of a 50 kt low-level jet (sampled nicely by the DMX
VWP), except across far northern Iowa where a stout cap has kept
storm coverage less than previously expected. At least scattered
development remains probable along the IL/WI state line over the
next few hours, and especially between 3AM and daybreak, as the
LLJ continues to interact with the cold front and increasing
outflow from the convection in Wisconsin. With impressively steep
mid-level lapse rates in place (9.7 K/km sampled by the 00Z DVN
RAOB), any storm this morning will keep still have the potential
to become severe with damaging hail and winds. The highest
coverage of thunderstorms this morning should be north of I-88.

As the convection pulls away later this morning, attention will then
turn to what is shaping up to be the most humid and hot day so far
this year. Surface dew-points climbing into the upper 60s to lower
70s combined with surface temperatures in the low to mid 90s should
lead to heat index values at or just above 100, especially south of
I-80. Periodic upper-level clouds should prevent temperatures (and
hence heat index values) from getting too out of hand, though it is
not out of question it feels more like 105 than 100 in a few spots
this afternoon (again, mainly south of I-80). If planning to be
outdoors today, be sure to take breaks in cooled areas and hydrate
frequently. Don't forget the sunscreen, too!

With the building humidity levels will be building instability,
with forecast MLCAPE expected to build >3000 J/kg by early evening.
With an upper-level jet max expected to swing through the Great
Lakes, deep-layer shear will also increase, altogether affording an
environment supportive of robust if not explosive severe weather.
However, with indications that the low-level surface flow will
veered this afternoon, sources of low-level confluence may be hard
to come by likely preventing widespread convection from
developing across the area. (And, that's not to mention a stout
cap beneath the impressive EML). The lake breeze this afternoon
and evening will be an area to watch, as well as any outflow
boundary that is spit out by the convection this morning. A
better chance of thunderstorms may arrive after dark tonight as a
renewed LLJ develops and terminates in far eastern Iowa. Like this
morning, a conditional threat for severe weather will exist if
the coverage of thunderstorms is high, as well as a localized
threat for flooding of low-lying areas given healthy PWATs >1.5".

Borchardt

&&

.LONG TERM...

Saturday through Thursday...

216 AM...Main forecast concern is thunderstorm chances Saturday
through Monday.

Overall, fairly low confidence for convective trends Saturday and
Saturday night. Much of the 00z guidance is dry across the area,
with the best chance which is still low generally across the
southeast cwa Saturday into Saturday evening. These trends are in
line with the previous forecast, keeping the bulk of the area dry
and continued these trends. High temps on Saturday are uncertain.
There is expected to be a lake breeze in the afternoon with an
overall shift to northerly by late in the afternoon. Previous
forecast highs were in the lower 90s which is supported by the
ecmwf but other models are cooler. Opted to keep these warmer
temps for now but highs for Saturday will need to be revisited
with the next forecast.

A strong cold front is expected to move across the area Sunday
night and this is likely to be the next best chance for showers
and thunderstorms across the entire area. Best timing for
convection seems to be focused on the late afternoon through the
evening and if this timing materializes, some potential for
strong to severe storms would exist. Highs on Sunday may possibly
be back into the lower 90s, perhaps a bit cooler along the IL
shore with south/southeast winds possible.

Precipitation on Monday is dependent on how fast the cold front
moves through the area Sunday night into Monday morning and its
location when new convection redevelops on Monday. Have maintained
chance pops for this time period but the best chance will be
across the southeast cwa. Dry and cooler weather is expected
through midweek with morning lows in the lower 50s Tuesday and
Wednesday mornings with perhaps a few upper 40s possible in the
outlying areas. cms

&&

.AVIATION...
For the 06Z TAFs...

Aviation Forecast Concerns:

* Thunderstorm potential through daybreak then again later this afternoon.

* Wind speed and direction during the day and influence from potential
  thunderstorms and lake breeze.

Low pressure continues to move across northern Iowa with an
attendant cold front moving southeast. The low level jet is
focused into southern Wisconsin where a corridor of showers and
thunderstorms continues to move east-southeastward. Additional
showers and isolated thunderstorms have begun to develop to the
south of the main corridor and should increase in coverage over
the next few hours with more activity expected to work into far
northern IL. Questions still remain with regard to how much
coverage will occur across the terminals as we head into the early
morning hours. Have made some small changes to the going
forecast, though suspect that the best organized thunderstorm
activity will remain to the north of the terminals, with isolated
to scattered coverage locally. Will reassess the tempo thunder
groups in another 1-2 hours. Still expect whatever activity does
occur to wind down by mid morning. The next question becomes what
thunderstorm coverage looks like this afternoon and this will
depend on what happens through the morning. Large scale coherent
forcing is lacking along with the presence of some other negative
factors so will maintain the prob30 mention for TS this afternoon.
The lake breeze could end up being the more likely focus should
it develop.

Winds are the other challenge. Outflow from storms to the north
may shift winds northerly for a time and if there is a better
coverage of storms yet this morning then winds in their wake may
favor a southeasterly direction. Without much in the way of
storms, then southwesterly winds would likely prevail. Storm
evolution of the next 2-3 hours should help paint a better picture
of what wind fields will be later this morning.

MDB

&&

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

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Special Weather Statement
National Weather Service Chicago/Romeoville
241 AM CDT Fri Jun 18 2021

ILZ003>005-180830-
Boone IL-McHenry IL-Winnebago IL-
241 AM CDT Fri Jun 18 2021

...SIGNIFICANT WEATHER ADVISORY FOR WINNEBAGO...WESTERN MCHENRY AND
BOONE COUNTIES UNTIL 330 AM CDT...

At 241 AM CDT, Doppler radar was tracking strong thunderstorms along
a line extending from near Footville to Ridott. Movement was east at
55 mph.

Winds in excess of 40 mph will be possible with these storms.

Locations impacted include...
Rockford, Belvidere, Woodstock, Loves Park, Machesney Park, Roscoe,
Harvard, South Beloit, Rockton, Marengo, Poplar Grove, Rockford
Airport, Candlewick Lake, Cherry Valley, Winnebago, Davis Junction,
Pecatonica, Sharon, Durand and Capron.

Including the following interstates...
 I-39 between mile markers 115 and 123.
 I-90 between mile markers 0 and 41.

This includes...  Boone County Fairgrounds, Rock Cut State Park, Rock
Valley College, Rockford Rivets Baseball, Rockford Speedway, and
Winnebago County Fairgrounds.

LAT...LON 4249 8840 4215 8852 4215 8917 4221 8917
      4220 8940 4250 8940
TIME...MOT...LOC 0741Z 274DEG 46KT 4267 8913 4229 8950

CMS