513
FXUS63 KILX 281031
AFDILX

Area Forecast Discussion...Updated Aviation
National Weather Service Lincoln IL
531 AM CDT Fri Jun 28 2024

.KEY MESSAGES...

- Widespread rain returns to the area today and tonight, though
  the risk for severe weather and flash flooding has decreased.

- Drier, cooler and less humid conditions evolve Saturday night
  through Monday; ideal for outdoor activities.

- Seasonably hot and humid weather returns by the middle of next
  week, extending through the holiday weekend, with near daily
  chances for showers and storms.

&&

.DISCUSSION...
Issued at 141 AM CDT Fri Jun 28 2024

A quick peek at radar this morning reveals an area of showers
pushing into far western Illinois. This activity is high-based and
struggling to reach the ground, with quite a bit of dry air to
overcome in the lowest 10 kft (see 00z ILX sounding).

The better opportunity for measurable rain arrives this afternoon
as a compact mid-level shortwave lifts across the Mid-
Mississippi Valley and provides life-support to a decaying
MCS/MCV. By that time, thunderstorm potential appears low with the
decaying MCS/MCV outpacing the axis of instability. Leaning on
the latest hi-res guidance, this first round of measurable
rainfall looks to stay north of I-72 -- perhaps well north.

The best opportunity for measurable rain arrives late tonight as
strong shortwave energy pivots across the Upper Mississippi Valley
and helps drive a surface cold front into portions of the Upper
Midwest and Central Plains. A 30-40 kt LLJ will veer ahead of the
front and provide the focus for nocturnal convection to blossom
overnight.

Most of the 00z CAMs suggest storms initializing over central
Missouri overnight as the LLJ moves atop some sort of west-east
surface boundary. As the cold pool matures with eastward extent,
the MCS will stake a greater claim of the LLJ -- effectively
robbing central Illinois of its share of rich theta-e air needed
for strong convection. The net effect, if this narrative comes to
fruition, will be a diminished threat for severe weather and flash
flooding for central Illinois as storm coverage will be more
sparse with higher inflow bases.

One final note about the setup for today/tonight is that we appear
to have a column moisture and instability problem. And, it`s
quite possible that the culprit is the cut-off low positioned
along the Gulf coast. Without a good fetch of western-Gulf air
surging northward into Illinois, we are left with recycled
moisture and what is looking more-and-more like a disjointed LLJ.
This most likely adds up to a very non-uniform QPF field across
central and southeast Illinois through Saturday afternoon. For
what it`s worth, the 10th-90th percentile 24-hr QPF from the 00z HREF
offers something like 0.05" - 1.25" as a reasonable range of
expected rainfall -- most of which is expected north of I-70.

We will need to keep an eye on model trends for Saturday with the
cold front pushing across the forecast area. The current thinking
is that convective debris from Friday night will mitigate boundary
layer recovery Saturday afternoon -- every 00z CAM omitted
convective initiation over our forecast area.

After a dry, secondary cold front makes its passage Saturday,
temperatures will cool down in a big way. Afternoon highs on
Sunday and Monday are forecast to be in the upper 70s, with
overnight lows cooling into the 50s Sunday night, per
deterministic NBM guidance. This modified Canadian air mass will
not stick around long, though.

By the middle of next week, typical summertime heat and humidity
will return across the region. Mid-range global guidance is in
fair agreement that another cold front sinks south into central
Illinois sometime Tuesday night or Wednesday, with it stalling-out
over or just south of the forecast area. This could put us in a
position for daily thunderstorm chances Tuesday-Friday depending
mostly on where the 594mb heat dome sets up. The NBM continues to
hedge this time period with daily 40% PoPs.

MJA

&&

.AVIATION...  (For the 12z TAFs through 12z Saturday Morning)
Issued at 516 AM CDT Fri Jun 28 2024

Overcast mid-level clouds will be accompanied by scattered showers
through this TAF period. Mention of VCTS has been retained for
tonight, with both confidence and coverage still too uncertain to
upgrade to prevailing TSRA. While cigs are forecast to remain VFR
through the period, some guidance suggests a reduction to MVFR is
possible near the end of the period as a cold front draws closer
to the regional terminals.

MJA

&&

.ILX WATCHES/WARNINGS/ADVISORIES...
None.
&&

$$