887
FXUS63 KILX 211516
AFDILX

Area Forecast Discussion...Updated
National Weather Service Lincoln IL
1016 AM CDT Tue May 21 2024

.KEY MESSAGES...

- There is a level 4 of 5 (moderate) risk for severe weather this
  evening west of the Illinois River, with gradually lower
  chances further east. While all severe hazards are in play, the
  greatest risks will be damaging wind gusts and brief tornadoes.
  A level 1 of 5 (marginal) risk will linger into tomorrow evening
  south of I-70.

- There is a 70% chance heat indices surpass 90 this afternoon,
  posing a risk for heat-related illness in vulnerable
  populations.

- Strong south winds this afternoon are expected to frequently
  gust to 35 mph or higher, while there is a 15% chance gusts
  reach 45 mph. This may cause difficult driving conditions for
  those traveling on west-east oriented roads.

&&

.UPDATE...
Issued at 1016 AM CDT Tue May 21 2024

10 am surface map placed 996 mb low pressure along the eastern
KS/NE border with a warm front arcing northeast across southern IA
and into northern IL. This placed the local area firmly in the
warm sector where south winds were already gusting over 25 mph.
Wind speeds will increase and gust to around 40 mph this afternoon
as the low deepens and tracks into NW IA, tightening the MSLP
gradient. Deep mixing above 800 mb and abundant sunshine (with the
exception of convective debris high clouds from IA convection)
will promote a very warm day and highs in the upper 80s to lower
90s. Dewpoints may drop a couple degrees tapping into drier air
aloft, but still low/mid 60s Tds will yield low/mid 90s heat
indices.

For convective trends through late afternoon, current thinking is
the area will remain dry due to capping near the 775 mb level
evident on forecast soundings this afternoon. Batch of convection
currently over IA is expected to shift east-northeast due to mean
layer southwest flow, staying north of our area. Our main window
for severe convection still appears to be 7 pm to midnight, after
an organized convective line develops ahead of the cold front
across IA/MO later this afternoon. Damaging straight line winds
are the highest risk locally, with tornado and large hail risks
present primarily if broken line segments occur. The highest
severe risks are northwest of the IL River, as a gradual weakening
trend is forecast through the evening as storms approach I-55.

25

&&

.DISCUSSION...
Issued at 138 AM CDT Tue May 21 2024

Current radar imagery shows a few decaying thunderstorms across
northwest Indiana, with more sustained and, at times, severe
convection rippling across Nebraska and western/central Iowa on the
nose of a 45-55 kt LLJ. Ultimately, outflow boundaries from last
night`s storms will linger across portions of Iowa and northern
Illinois this morning into early afternoon, offering instability
gradients and locally enhanced low level environmental shear to
govern storm evolution.

From a big picture perspective though, surface cyclogenesis will be
fostered in the left exit region of a deepening upper trough taking
on a negative tilt as it lifts across the central/northern Plains
this afternoon. Strong warm advection ahead of that deepening low
will thus gradually overspread the Prairie State today, a warm nose
at 700mb leading to a rather strong capping inversion giving our
forecast soundings the classic "loaded gun" look. This should offset
mixing of the increasing winds aloft, but given (1) a lack of cloud
cover (we`ll likely have some altocumulus constellanus and/or
cumulus, but not the typical WAA stratus shield if you believe
HREF), (2) the height of the inversion around 850mb, and (3) steep
lapse rates in the mixed layer, I think we`ll be quite breezy this
afternoon. GFS and EC lamp guidance bring sustained speeds to around
25 kt (28 mph) at our ASOS sites this afternoon, while the EPS mean
brings gusts to around 40 mph and HREF suggests 40-70% chances for
gusts surpassing 40 mph. Thinking is that if any site manages to hit
advisory criteria (30 mph sustained or 45 mph gusts) it would be
brief, and NBM probs are only around 10-15% anyway, but it`ll be
something to keep one eye on given the set up looks similar to
(though perhaps slightly weaker than) March 3rd, April 16th, and
April 28th when south winds flirted with criteria given
overachieving afternoon mixing. At any rate, for many (if not most),
these winds will pale in comparison to those brought by
thunderstorms during the late evening/overnight hours. In addition,
it`ll be beneficial in one sense - it will make the summerlike
warmth (high temps and dewpoints approaching 90 and 70,
respectively) a bit less oppressive.

As the surface low draws nearer during the evening, temps aloft will
cool leading to steepening 500-700mb lapse rates conducive to strong
thunderstorm updrafts. In addition, the kinematic profile will
become increasingly supportive of sustained convection, 0-3km shear
values nearing 40 kt - highest west of I-55. While the greatest
shear will be well to our north across Iowa, northern Illinois, and
Wisconsin, our area should still have enough for sustained
storms...though storm mode might be semidiscrete (vs a more solid
line further north), particularly as the LLJ strengthens after 11pm
or so. This means that all severe hazards will be possible,
particularly with any isolated storm that develops as the line
starts to break up, which most models suggest will happen between
10pm and midnight; however, the most widespread hazard still appears
to be from straight-line winds, with a QLCS tornado or two possible
given the aforementioned 0-3km bulk shear values are over 30 kt.
Timing of storms looks similar between CAMs, with the line entering
our west around 7pm and storms becoming more scattered if and when
(guidance is having this happen around 11pm..ish) it breaks up,
after which time isolated wind and hail will pose a continued risk.
The straight-line wind (and QLCS tornado) risk will be highest
across our north and west counties where shear is strongest due to
closer proximity to the surface low. And some of these winds
could be significant, as implied by HREF`s ensemble max which has
small blotches of 80 mph or higher gusts, most of which are near
or west of the IL River where SPC has a level 4 of 5 (moderate)
risk for severe weather driven by the wind hazard. This event
could be classified as a derecho if enough significant (75+mph)
gusts are measured and its path is sufficiently long, which
appears distinctly possible at this point.

Now, one thing we`ll be keeping an eye on this afternoon is where
outflow boundaries from the upstream early morning storms linger.
The stiff southerly winds in the boundary layer should help rid our
area of such boundaries, but a couple CAMs have been hinting one
stalls right on the edge of our northwest CWA, across/near Knox
County, and that storms fire along it ahead of the main line during
the 3-6pm timeframe. The 18z WRF-ARW had this, and of the hires
models it handled convection the best yesterday; it`s 00z
iteration, however, which again matches radar the best of the HREF
ensemble, does not...So needless to say our confidence is low in
this scenario, though if it did materialize storm mode would be
supercellular and hence all severe hazards would be expected.

The upper low will lift well to the northwest as the cold front
stalls near or just south of I-70 tomorrow, and as another mid level
wave lifts northeast along/ahead of it additional storms are
forecast to fire there. SPC maintains a level 2 of 5, slight, risk
for severe weather in that area for wind and hail. Everywhere else,
temps will fall to near/below normal as cold advection overspreads
the area. Thereafter, guidance is murky on the location of a
quasistationary boundary which will linger just to our southwest,
lending to low confidence in the timing and placement of MCS
activity riding along it amidst WNW flow. Anytime between Friday and
Sunday looks conducive to episodes of severe weather across the
Midwest, but whether we`ll be close enough to the instability
gradient to get in on any one/more of those MCSs remains yet to be
seen. Confidence is also low in temps throughout the upcoming
weekend, with NBM`s 10th-90th percentile maxT range extending from
69 to 90 degF at the Decatur airport (arbitrarily-chosen point near
the middle of the CWA) by Sunday. The lower and upper bounds of the
10-90th percentile range gives a reasonable low end and high end
value, capturing the true high temp 80% of the time; in this case,
this suggests there is a 90% chance high temps in Decatur surpass 69
degF on Sunday, and 10% chance they exceed 90 degF.

Bumgardner

&&

.AVIATION...  (For the 12z TAFs through 12z Wednesday Morning)
Issued at 536 AM CDT Tue May 21 2024

South winds under 10 kt will increase sharply this morning,
becoming sustained over 20 kt with frequent gusts near 30 kt by
mid afternoon. A broken line of thunderstorms will bring brief
torrential rain, frequent lightning, and widespread gusty winds
which may locally exceed 50 kt (greatest chance at KPIA) between
7pm and 1am Tuesday night-Wednesday morning (00-06z Wednesday).
Behind that line of storms, clouds should become scattered to few
and winds ease.

Bumgardner

&&

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

$$