Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 271715
AFDILX

Area Forecast Discussion...Updated Aviation
National Weather Service Lincoln IL
1215 PM CDT Wed Mar 27 2024

.KEY MESSAGES...

- Wednesday night lows are forecast to fall to around or below
  freezing once again, with a 30-40% chance for a hard freeze
  area-wide. Precautions should again be taken to protect tender
  vegetation.

- A warm and stormy pattern is expected beginning Friday night and continuing
  through at least Monday. Severe potential appears low (less
  than 5%) Friday into Saturday, but becomes more uncertain Sunday
  and Monday.

&&

.UPDATE...
Issued at 934 AM CDT Wed Mar 27 2024

A 1023-mb sfc high pressure is positioned over the KS/NE border
this morning. Visible satellite imagery reveals mostly cloudy
skies still present across most of the local area as of
0930am/1430z. The cloud cover is gradually thinning, and the
expectation remains that skies improve to mostly sunny by this
afternoon. Made some minor adjustments to the cloud cover, but
otherwise no notable forecast changes were necessary at this
time.

Erwin

&&

.DISCUSSION...
Issued at 216 AM CDT Wed Mar 27 2024

Latest nighttime microphysics satellite imagery shows stratus which
sfc obs reveal is between 2-4kft AGL extends northwestward all the
way across much of Iowa and Wisconsin. HREF suggests this mass of
clouds will shift east as drier air over the Plains overspreads the
Prairie State, and 80-90% of its membership has cloud cover gone
west of I-57 by 11am. West winds should perpetuate cool advection,
but the lack of clouds, along with dry low levels featuring dew
points in the 20s, will allow radiational warming to bring temps
into the mid 40s to around 50 north (low 50s south) of I-70 this
afternoon. Highres guidance, including the HRRR, RDPS, RRFS, and
ARW, suggest winds will ease to around 5 mph or perhaps less this
evening/tonight, so depending on how far south the low-mid clouds
depicted by the HREF wind up (currently projected to stay
predominantly north of I-72) we could have another hard freeze. I`m
a little surprised HREF probabilities for sub 28 degF low temps
tomorrow morning are only running around 5-20%, as the 27.00z HRRR
has pockets of mid-20s temps during the 3-8am timeframe; the NBM
suggests a 40-50% chance for a hard freeze, which seems more
reasonable, though perhaps the truth is somewhere in the middle(?).
We`ll keep an eye on model trends to determine whether another SPS
might be needed to highlight the potential for sensitive vegetation
damage in the absence of protective action, and for now message
probabilities around 30-40%.

Despite the cold start to the day, the dry airmass in place and low-
level warm advection with the incoming ridge will support highs in
the mid 50s north of I-74 to low 60s southwest of a Macomb to
Robinson line tomorrow, and as 850 temps continue to warm into
Friday (nearly 60% of LREF membership is over 8 degC by Friday
evening) we`ll add another 5-10 degrees for highs in the 60s to
close the work week. A shortwave trough developing along and
flattening the crest of the ridge is slated to pass just to our
north Friday night, providing some support for precip where 700-
500mb lapse rates under -7C/km will foster a plume of elevated
instability. Models have trended north with the precip and axis of
maximum instability - around 500-1000 J/kg MUCAPE, if you believe
the NAM - though it`s close enough to our area that we felt it best
to keep at least a slight (15+%) chance for thunder in the forecast
north of I-70 Friday night into Saturday morning.

Thereafter, the forecast becomes a little harder to pin down, as the
deterministic models exhibit relatively small but
meteorologically significant differences in how quickly - and how
far north - the ridge expands back into the region. Behind the
departing shortwave trough, we should (in theory) be too stable
for more than isolated showers/storms Saturday afternoon and
evening, but on Sunday the GFS and ECMWF bring the next warm front
- a focus for convection - into the region. Both models have a
distinctly different track in the sfc low and consequently the
warm front (and thus, thunderstorm activity), however, which has
implications for the severe weather risk in our area. At this
point, the highest confidence in sufficient overlap between
instability and shear is south of I-74 Monday afternoon and
evening when there is a roughly 30-35% joint probability for
0m-500mb shear > 30 kt and SBCAPE > 500 J/kg. Sunday will still
bear watching, though, and, if the system slows down at all, so
will Tuesday across our east. The CSU MLP brings a 15% contour
across the southern half of our area Sunday, and across nearly the
entire area Monday. In other news, temperatures will trend about
10 degrees above normal through the weekend and into the start of
next week, with highs ranging from the mid 60s to mid 70s on any
given day - looking warmest Monday at this point.

Whenever that system`s cold front finally clears our area, we`ll get
another blast of cooler conditions with 40-60% of LREF membership
bringing 500mb heights to less than 540 decameters (dm) at some
point late Monday through early Thursday. North of I-70, there will
be a 30-50% chance for sub freezing lows Wednesday morning, so
delicate vegetation might once again be at risk for damage.

Bumgardner

&&

.AVIATION...  (For the 18z TAFs through 18z Thursday Afternoon)
Issued at 1214 PM CDT Wed Mar 27 2024

Visible satellite shows an MVFR cloud deck thinning, but periods
of broken MVFR ceilings could linger at KDEC/KCMI during the first
hour of the period. Otherwise, VFR condition should prevail, with
westerly winds turning to southwesterly overnight. Winds become
light overnight, then increase to around 10 knots by mid-morning
on Thursday.

Erwin

&&

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

$$


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