Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 242328

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
628 PM CDT Thu May 24 2018

219 PM CDT

Through Friday night...

Summer-like warmth will be the main weather story to finish out
the week. Scattered thunderstorm chances will increase late in the
day Friday, especially across western portions of the forecast

Surface high pressure continues to retreat off to the southeast
across the upper Ohio Valley and mid-Atlantic regions this
afternoon. Temperatures have warmed into the low-mid 80`s in most
areas, with a lake breeze spreading inland across far northeast IL
as noted in ORD TDWR imagery. The above normal temps, combined
with modest south gradient winds tonight, will result in a mild
overnight with lows in the 60`s expected in most spots under
mainly clear skies.

Mid-level short wave trough over the northern high plains will
lead to some flattening of the upper level ridge across the area
Friday. Within the approaching trough, guidance hints at at least
some increased mid/high cloud cover during the day, in association
with weaker vortcity/convective debris spreading into the area
from parts of Iowa/Missouri. Little focus for renewed convective
development is noted across our area through much of the day
however, with the primary low level instability axis located west
of the Mississippi River ahead of the cold frontal trough trailing
from surface low pressure along the Canadian border. Mid/high
cloud cover may modulate temp rise during the day, though some sun
is expected and with 850 mb temps warming to around +16C and 925
mb temps progged +22/23C, highs are once again expected to reach
the mid-upper 80`s.

Several high-res CAMs indicate thunderstorm development over
portions of the upper Mississippi Valley Friday afternoon, with a
resultant MCS building south-southeastward along the eastern
periphery of the stronger low level instability axis into
northwest/north central IL into early Friday evening. The 3km WRF,
NMM and ARW all depict this progression. Deep layer vertical wind
shear is pretty weak however, with 0-6 km bulk shear generally 20
kts or less over northwest IL, and diminishing farther
south/east. Thus while storms may push into northern IL Friday
evening, and could present some local severe wind threat for a
time, the overall trend should be for these storms to weaken with
loss of stronger diurnal instability and progression away from
better shear. SPC has maintained a marginal day 2 severe risk
across roughly the northwest half of the LOT cwa for this.



253 PM CDT

Friday night through Thursday...

The very warm conditions continues to be the main weather story
for the holiday weekend and into next week.

The upper level ridge is expected to rebuild across the central
CONUS on Saturday and remain in place across the area through
early next week. Overall, this will result in rather warm and
somewhat humid conditions across the area during the period.
Precipitation chances will also be limited through early early
next week with this upper ridge overhead. Expect temperatures each
day to top out in the upper 80s to lower 90s, with heat indices
climbing into the middle 90s by Sunday. Areas along the shores of
Lake Michigan will likely be the only areas to see some cooler
afternoon temperatures as lake breezes are likely to develop each
afternoon due to the light west-southwest synoptic winds over the

The chances for thunderstorms will return to the area by midweek
as the weather pattern becomes a bit more active. Overall, a
tropical system likely to impact portions of the Gulf Coast
during the early part of the week. This feature will help lock
the upper ridge in place over the area for a few days early next
week. Its still a bit uncertain if we will see any precipitation
from this system locally. However, during the mid to late part of
the week it appears that the upper ridge will break down across
the area an allow a more active belt of westerlies to develop
across the central CONUS. So either way it does appear that some
storms will be possible at times later in the week.



For the 00Z TAFs...

VFR conditions with generally light winds are expected through the
period. Light SE winds this evening will transition to persistent
SSW winds Friday. Slightly stronger winds (10-15kts) than today
should keep the Lake Michigan lake breeze east of ORD/MDW/DPA Friday
afternoon. Near to slightly beyond the end of forecast period, there
is a chance for a decaying southward-propogating thunderstorm
complex to cross northern IL after 00Z Saturday, nearing RFD around
02-05Z, and approaching ORD/MDW/DPA within an hour or two of 06Z.
Confidence on this complex reaching ORD/MDW is very low, so mention
was not included with this TAF issuance.



148 PM CDT

A period of modestly breezy south-southwest winds of 20 to 25 kt
is expected across Lake Michigan tonight through Friday night, as
high pressure slides off to the southeast across the upper Ohio
Valley and mid-Atlantic region. An area of low pressure will move
across northwest Ontario and the far northern Lakes Saturday,
which will trail a weakening cold front across the upper Midwest.
The gradient will weaken and become baggy in the vicinity of this
cold frontal trough, with winds becoming light and somewhat
variable in direction at times as the front becomes diffuse over
the lake into Sunday. South to southeast winds will re-establish
across the upper Midwest Monday and Tuesday, as another low
develops across the Northern Plains and tracks north of the Lakes.

Thunderstorms may affect portions of the lake later Friday and
Friday night, and again late Saturday and Saturday evening.






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