Area Forecast Discussion
Issued by NWS Chicago, IL

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970
FXUS63 KLOT 251734
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1234 PM CDT Fri May 25 2018

.UPDATE...
1058 AM CDT

Morning Update...

No significant changes made to going forecast, with main concern
through this afternoon being the potential for isolated to scattered
thunderstorms to develop this afternoon across western portions
of the cwa.

Mid-level short wave trough was evident in morning water vapor
imagery across IA/MO, with a remnant MCV from overnight convection
apparent over central MO. A pseudo-warm frontal feature was
evident at the surface ahead of this wave, roughly from eastern IA
southeast into southern IL. Scattered thunderstorms have developed
along this feature south of the Quad Cities this morning, and over
far southern IL. While storm motion was generally north-
northeastward, some east-northeastward expansion of scattered
storms is expected as atmosphere diurnally destabilizes and is
aided by mid-level cooling associated with the short wave. High-
res CAM guidance limits most of this activity to areas along/west
of the I-39 corridor this afternoon, though spreading east into
this evening. While the MCV will produce some organization to
these storms, deep layer wind shear is very weak (less than 20 kts
0-6 km), and mid-level lapse rates are not very impressive. Thus
there should be a strong diurnal component to the storms with
weakening/dissipation later this evening. Severe threat should be
fairly low, and likely limited to isolated wind gusts and marginal
hail from mid afternoon into early evening when low level lapse
rates are steepest. Greatest coverage of storms will likely be
across the southern half or two thirds of the cwa in closer
proximity to the track of the MCV.

Otherwise, summer-like warmth persists with temps already in the
mid-80`s in many spots at 11 am. A few 90`s are likely in some
spots.

Ratzer

&&

.SHORT TERM...
249 AM CDT

Through tonight...

The upper level ridge will shift east today allowing an upper
level trough to pass over the region this afternoon and evening.
Despite increasing cloud cover this afternoon, I think
temperatures will be warmer than guidance indicates. Upper 80s and
a few 90s are expected. A weak lake breeze is expected in the
mid-afternoon, but I think locations along the lake will get into
the low 80s before winds turn on shore.

Convection is the next concern. Convection currently over
Missouri and MN/WI will dissipate this morning. If the convection
over Missouri produces a MCV, I think the MCV will pass south of
the forecast area. There`s a small chance that storms could clip
the far southern portion of the forecast area, but I think
stronger convection in central IL will be the main show.

Most CAMs are focusing convection along the upper level wave and
cold front in MN, WI, and IA this afternoon. Upper level flow
becomes west to west-northwest late this aftn/early eve directing
bowing segments of storms toward northern IL. Forecast soundings
feature pretty dry low levels and a rapidly forming stable layer,
so I agree with SPC`s outlook that storms will be weakening as
they approach northern IL. I still kept a chance of storms in the
forecast through tonight, but my confidence is low in coverage
and how far storms will make it across the forecast area. As
such, I kept precip chances in the chance range or lower.

JEE

&&

.LONG TERM...
249 AM CDT

Saturday through Thursday...

Overview...Continued warmth with a couple chances of storms. 90s
are still forecast for Sunday with heat indices in the low to mid
90s away from the lake.

I kept a slight chance of storms Saturday, but I have low
confidence in storms actually occurring and coverage. Soundings
feature a good cap, rather dry, and there isn`t a clear source of
forcing outside of daytime heating. High temps will once again be
in the upper 80s to around 90 away from the lake.

Low 90s are expected Sunday, but a stronger lake breeze is
possible leading to lake side cooling. Monday and Tuesday look
similar to Sunday with lake cooling and highs in the 80s to 90s
away from the lake.

Temps remain in the 80s for the remainder of the forecast with
periodic chances of thunderstorms mid to late in the week.

JEE

&&

.AVIATION...
For the 18Z TAFs...

Isolated/scattered late afternoon/evening TSRA potential is the
primary aviation forecast concern.

Weak low pressure wave was crossing the Mississippi River into
west central IL at midday. Isolated to scattered TSRA were
expanding in coverage west and southwest of Bradford IL, and would
expect this activity to continue to spread east-northeastward this
afternoon as atmosphere continues to destabilize in very warm air
mass. Expect isolated activity near RFD by 21-22Z, several hours
later into Chicago. Current extrapolation would suggest 02-03Z for
ORD and MDW, although some eastward expansion would likely bring
activity into the vicinity earlier. Have indicated VCTS for DPA by
23Z, ORD/MDW 00Z. Should eventually be a decrease in intensity and
coverage this evening with approach of sunset, so confidence in
coverage is medium at this point with respect to Chicago
terminals.

Potential also exists for showers/storms mainly over northwest IL
after midnight tonight, though this activity should be dissipating
and confidence is low in it making it into IL. Have maintained
prob30 for RFD.

Southwest winds 10-15 kts will persist through the afternoon. Lake
breeze may form along the shore, but is not expected to make it to
MDW/ORD. GYY could see a brief period of variable or light
northeast winds, though winds should shift back to southwest by
evening. Southwest or west-southwest winds around 10 kts expected
during the day Saturday.

Ratzer

&&

.MARINE...
203 AM CDT

South-southwest winds will be on the increase today over the open
waters of Lake Michigan, especially across the northern half,
where gusts of 20-25 kt should be frequent. Winds will be less on
Saturday but still south-southwesterly over the open waters.
Throughout the holiday weekend, the Illinois and Indiana
nearshores will experience afternoon lake breezes, and these
should have a little more gusto Sunday and Monday, but no small
craft advisory conditions are expected.

Thunderstorms are possible by late day across the north, and
across the entire lake tonight. Severe weather is not anticipated,
but a few storms could have gusty winds. Chances for storms are
less for the rest of the weekend, though cannot rule out isolated
storms on Saturday and Saturday evening.

MTF

&&

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

$$

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