315
FXUS63 KLOT 142320
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
620 PM CDT Sat Jun 14 2025

.KEY MESSAGES...

- Temperatures will increase into next week, accompanied by
  increasing shower/storm chances by midweek.

- Multiple rounds of thunderstorms may occur in the general
  region Tuesday through Wednesday evening. Some storms may be
  severe, particularly on Wednesday.

- There is an emerging signal for the warmest and most humid air
  of the season to arrive next weekend.

&&

.DISCUSSION...
Issued at 248 PM CDT Sat Jun 14 2025

Through Sunday Night:

Northern Illinois and northwestern Indiana are on the northern
periphery of a slow-moving upper-level trough drifting east-
northeastward along the Ohio River beneath a strengthening ridge
along the US/Canadian border. Closer to the surface, a broad
high pressure system is centered over southern Ontario, leading
to a slight onshore breeze around the Lake Michigan basin. The
near annual maximum sun angle quickly eroded the overcast skies
that started this morning, giving way to mostly sunny to partly
cloudy skies. Aside from somewhat humid air lingering in the
region, it`s a fairly nice mid June afternoon out there.

Tonight, the relatively relaxed surface pressure gradient along
the southern flank of the surface high to our north will lead
to light winds. Decreasing humidity levels (thanks for incoming
air from Lower Michigan) will set the stage for a comfortable
night with lows falling into the upper 50s to around 60.

Tomorrow, a couple of convectively invigorated upper-level
shortwaves will meander from the northern Plains and into the
Midwest. With Canadian surface high continuing to feed a steady
supply of dry low- to mid-level air into the Lower Great Lakes
region, do have to think that any convection or convective
cluster that attempts to work southeastward toward northern
Illinois will struggle and ultimately decay. All things
considered, opted to lower the inherited 20% PoPs to 10% and
convert mentionable rain to sprinkles, altogether along and west
of I-39 tomorrow. Otherwise, tomorrow is shaping up to be
another pleasant mid-June day with partly cloudy skies. Lower
humidity levels tomorrow should allow for highs to be some 5
degrees warmer than today and in the low to mid 80s. The
exception will be along the Lake Michigan shoreline where
onshore flow will hold temperatures in the upper 60s.

Borchardt


Monday through Saturday:

As we approach Friday`s summer solstice, the weather will
certainly be summer-like this upcoming workweek (very warm and
humid with occasional rounds of storms), likely followed by hot
and humid conditions next weekend. The main timeframe of
concern from a convective standpoint is Tuesday into Wednesday.

The week will start on a mainly dry note with shallow moisture
progged and very weak flow. A weak remnant MCV crossing the MS
River late in the day could kick off some isolated diurnal
pulse-type convection primarily to the west of I-39. Highs will
reach the mid to upper 80s inland, with moderately humid
feeling dew points (60s). A lake breeze will keep highs near the
lake in the 70s, or perhaps lower on the immediate shore.

On Tuesday, thunderstorm chances will uptick in the afternoon-
evening (30-50% PoPs) as another MCV or convectively modulated
mid- level impulse progresses eastward. Marginal deep layer
shear suggests generally pulse-type to perhaps loosely organized
multi-cells with downpours, localized gusty winds, and small
hail. Highs will be in the upper 80s to around 90F, with ~65-70F
dew points yielding peak heat indices in the low-mid 90s.

Later Tuesday night, there`s a rather strong signal for a MCS
moving across the mid-MS Valley. Overnight timing is on the late
side to support an appreciable severe threat given weakening
mid-level flow (marginal bulk shear) locally. However, PWATs
pushing near 2" (150-200% of normal) and modest steering flow
may result in a localized flash flooding threat if thunderstorms
aren`t on a rapid decaying trend.

The signal for severe storms on Wednesday has become a bit less
apparent on the last couple model cycles. Ongoing convection
from the overnight MCS and resulting debris cloud may
delay/limit stronger destabilization. In addition, while some
previous cycles suggested a stronger, more amplified trough
ejection across the region, recent runs have trended toward a
weaker, positively tilted trough and weaker surface low over the
area. There may even be a play for suppressed convective
coverage during the afternoon if subsidence is strong enough
behind the morning MCS. With all this being said, we`ll still
need to monitor this period closely. Deep layer shear of 35 kts
or more and the presence of frontal boundaries and remnant
outflow could yield a severe threat contingent upon sufficient
destabilization.

Very dry mid-level air and height rises in the wake of
Wednesday night`s cold front passage will result in primarily
dry conditions Thursday-Friday. Thursday`s highs will be near
seasonable in the lower 80s, along with lower dew points/less
humid conditions. A warm front will lift north on Friday,
bringing a return to very warm (mid-upper 80s) and humid
conditions. More amplified mid-level ridging over the eastern US
next weekend (500 mb heights in the 590s DaM) will carry 850 mb
temps into the lower +20s Celsius and 925 mb temps in the mid-
upper 20s C, supporting highs around 90F/lower 90s on Saturday
and likely even warmer on Sunday. Heat indices could flirt with
100F amidst dew points in the 70s. If the more amplified ridging
comes to fruition, convective chances will likely be minimal due
to strong capping.

Castro

&&

.AVIATION /00Z TAFS THROUGH 06Z SUNDAY/...
Issued at 620 PM CDT Sat Jun 14 2025

There are no aviation weather concerns for the 00Z TAF period.

NE winds below 10 kt this evening will go light and variable
(favoring NNE) overnight, then onto ENE near or below 10 kt
during the day on Sunday. VFR can be expected throughout the
period.

Doom

&&

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

&&

$$

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