Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1242 PM CDT Thu Aug 22 2024
KEY MESSAGES...
- Potential (30-40% chance) for some scattered showers and
thunderstorms Saturday night.
- Hot and humid conditions likely on Sunday and especially on Monday.
&&
DISCUSSION...
Issued at 301 AM CDT Thu Aug 22 2024
Through Friday:
Light winds and clear skies under a surface high centered overhead
is allowing for a bit a chilly start to the morning. As of this
witting, readings outside of Chicago are already flirting with 50
degrees. I suspect many areas away from the city will fall a few
more degrees, into the upper 40s, by daybreak.
Another pleasant late summer weather day is expected today as the
surface high overhead early this morning begins to slide east
into the Ohio Valley. As this occurs, winds will begin to settle
into a southerly direction today, thus aiding in another small
uptick in temperatures over what they were on Wednesday. Expect
readings to generally be in the upper 70s to around 80. The only
exception to this will once again be those areas right along the
Illinois Lake Michigan shore, where an afternoon lake breeze will
promote s bit cooler conditions.
Dry weather continues for Friday, though temperatures will
continue on their warming trend as the hotter airmass under an
upper ridge over the Plains begins to gradually work eastward.
While the heart of the heat is not expected into our area until
later in the weekend, temperatures for Friday are expected to warm
back into the lower 80s, which is about average for this time of
year. Increasingly mid and high-level cloudiness during the day
will also make for a partly cloudy day.
KJB
Friday Night through Wednesday:
Thinking on forecast trends this weekend into early next week
remains largely unchanged. An upper-level ridge will be in the
process of building across the Midwest and Great Lakes over the
weekend resulting in increasing in heat and humidity. However,
there continues to be the potential for a couple periods of
scattered showers and thunderstorms. The first period is during
the Friday night into Saturday timeframe as a couple of subtle
shortwaves round the northern edge of the ridge. Given that any
associated showers with these waves should be outrunning the
better moisture, the thinking remains that they should dissipate
prior to reaching the I- 39 corridor. Therefore, have decided
go with a dry forecast for this period.
The second and arguably more notable period of rain looks to be
during the Saturday night into early Sunday timeframe. While
guidance continues to show modest height rises (sinking air)
occurring across northern IL and northwest IN, the presence of a
low- level jet and strong warm advection atop a warm and humid
airmass could be sufficient for shower and perhaps storm
development. Both ensemble and deterministic guidance remain in
fairly decent agreement on at least some scattered showers and
thunderstorms materializing, but the meager aforementioned
forcing and modest instability still make this lower confidence.
The NBM has continued to come in with higher POPs during this
period which was maintained but at a lower 30-40% coverage to
account for the lingering uncertainty. Additionally, there also
continues the potential for any showers/storms that occur to be
heavy rain producers provided the 1.75 to 2.00 inch PWATs
forecast which could lead to some flooding concerns if
showers/storms train over the same area. Finally, the severe
threat noted in last night`s discussion does not look as
impressive as before due to instability being a limiting factor
but will continue to monitor trends in case things change.
Any showers/storms that develop Saturday night will conclude
early Sunday morning as the ridge axis becomes more centered
overhead. The sinking air beneath the ridge should clear out any
lingering clouds by Sunday afternoon and set up a couple of hot
and humid days for Sunday and Monday. While temperatures on
Sunday may be a bit cooler due to potential for clouds in the
morning, temperatures are still expected to be in the upper 80s
to lower 90s Sunday afternoon with highs in the low to mid-90s
looking likely for Monday. These temperatures in combination
with dew points forecast to be in the upper 60s to mid-70s
should yield peak heat indices well into the mid to upper 90s
with the potential for lower 100s on Monday. Keep in mind that
this part of the forecast is still 4 to 5 days out which means
subtle changes could still make or break the forecast, but
confidence is definitely growing with each forecast cycle. That
said, anyone with outdoor plans on Sunday and especially on
Monday should keep a close eye on the forecast and review good
heat safety prior to these conditions arriving.
Heading into Tuesday and the rest of next week the forecast
becomes a bit more uncertain. The reason is because guidance
continues to have some variability on when the aforementioned
ridge will begin to break down. While there seems to be a
growing signal towards a cold front moving through late Tuesday
into Wednesday, this is a newer trend that was not present
24-hours ago. Therefore, depending on when/if this front
arrives; Tuesday could be similarly as warm as Sunday and Monday
or notably cooler. Regardless, there continues to be a notable
signal in extended range ensembles for a pattern shift to occur
towards the end of August that may bring us the potential for
more active weather (i.e. chances for showers and thunderstorms).
Yack
&&
AVIATION /18Z TAFS THROUGH 00Z SATURDAY/...
Issued at 1241 PM CDT Thu Aug 22 2024
VFR conditions will prevail through Friday as a surface ridge
lingers over the area. SW winds 5-10 knots will continue through
much of the afternoon, but vary S/SSE up to 5 knots at times. A
weak lake breeze will slowly shift westward toward ORD/MDW.
There remains plenty of uncertainty on if it will actually reach
the terminals before dissipating early this evening. Have
delayed the associated SE wind shift to 22Z in the TAF, though
this may need to be pushed back further.
After mid-evening, winds 5 knots or less will eventually settle
SW overnight before increasing 5-10 knots Friday.
Kluber
&&
LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.
&&
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