Area Forecast Discussion
Issued by NWS Central Illinois

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414
FXUS63 KILX 210136
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
836 PM CDT Sun May 20 2018

.UPDATE...
Issued at 830 PM CDT Sun May 20 2018

Complex of thunderstorms over central Missouri continues to track
east and northeast towards our area. Instability not that great to
its east early this evening so we expect the storms to weaken some
before they get into our area towards Midnight. The storms are
associated with a weak upper level wave over west central Missouri
and that feature will continue to track northeast towards the
lower Great Lakes on Monday. Current forecast seems to have a good
handle on the timing of the storms late this evening through the
early morning hours. May need to fine tune the arrival time over
our far west and southwest counties, but overall, the current
forecast is in good shape. Not planning on any ZFP update at this
time.

The upper level wave and the associated showers and storms should
push off to our east by afternoon but models were slow to get the
cool front to our east until tomorrow night. As a result, there is
the chance for storms to redevelop tomorrow afternoon, especially
along and east of I-55. However, if clouds and rain linger longer
than expected, any redevelopment may take place just to our east
and southeast late tomorrow afternoon.

With a weak frontal boundary stretched from west to east across
the heart of the forecast this evening, quite a contrast in
temperatures and dew points with temperatures over our far
northern counties in the low 60s while along the I-70 corridor,
temperatures were near 80 degrees. Coolest readings across the
north tonight as the front will not make any significant track
to the north overnight, with rather mild readings expected across
the south where the mercury may not get much lower than the upper
60s. Highs Monday afternoon will range from the middle 70s north
of I-74 to the middle 80s over southeast Illinois.

&&

.SHORT TERM...(This evening through Monday)
ISSUED AT 302 PM CDT Sun May 20 2018
Many mesoscale features making for a very complicated forecast in
the short term, made worse by the lack of clarity between models.
The 500mb shortwave to the west that was progged to slowly move
out over the region is slowing further still, limiting the upper
support for the MCS remnants as they drift into the region. The
same lack of support seen overnight as well as in the day time
today may continue...suppressing severe threat somewhat. A lack of
steep lapse rates in the midlevels as well as a lack of deep shear
are so far dulling convection. CAPEs even limited throughout the
day from blow off cloud cover/remnant precip from western MCS
activity. Short term models are a mess, with the HRRR providing
some of the better solutions to work with. Forecast is vague at
best, trying to follow the trend in the HRRR of a break in the
activity this evening, then the remnants of a developing MCS to
the west moving in after 06z tonight. Lack of support should keep
the thunderstorms in check, but how recharged the atmosphere is
over Central IL is a bit of a question mark considering the
moisture and warm air advection, coupled with limited daytime
heating. BUFKit soundings do not look promising, and NAMNest,
admittedly off at initiation, is marginal at best. Have several
complicating factors in place with a weak sfc low, mesoscale
boundaries, and moisture convergence...but little dynamics to
initiate or sustain. Hopefully 00Z runs will provide more detail. With
the potential for the precip not materializing before midnight,
have added a patchy fog mention as the sfc dwpts in the 60s
already, and continuity from the last couple nights.

Tomorrow is another conflict with ending precip early, but chances
continue throughout the day with boundaries and plenty of buoyant
air in place. Very vague mention of potentially higher pops to the
north as the wind fields back up a passage of a weak low to the
north and northeast in the afternoon hours.

.LONG TERM...(Monday night through Sunday)
ISSUED AT 302 PM CDT Sun May 20 2018
Once the weak upper low clears out of the region tomorrow night,
500mb ridge starts to develop over the Plains, keeping the axis
just to the east of the Miss River Valley. This results in a dry
period at least through midweek until the next low dives into a
more northerly stream aloft, skirting the US/CAN border and
driving precip slowly into the region to wrap up the week. With
the ridge axis just to the west, weak NW flow does not cut off the
region from some moderate WAA from the desert SW. Midlevels
maintain access to the warmer temps of the swrn CONUS through the
end of the forecast, keeping temps about 10F above seasonal norms.
Very low pops work into the forecast about Thursday as that open
upper wave drifts into the Upper Midwest. Higher pops in place
with approach Friday through Saturday.

&&

.AVIATION...(For the 00Z TAFS through 00Z Monday evening)
Issued at 600 PM CDT Sun May 20 2018

A weather disturbance over southwest Missouri will track northeast
towards the forecast area late this evening and during the
overnight hours bringing an increasing threat for showers and
thunderstorms, especially at SPI and PIA. That should bring low
VFR to MVFR cigs and vsbys to most of our area late tonight and
into the early morning hours of Monday. The main forecast concern
will be the timing of the MVFR cigs and vsbys and based on the the
current radar trends to our west and southwest, it appears the
time frame from 06z-09z will be when the lower clouds and vsbys
push in with the showers. The shower threat will be with us
through the morning with a gradual increase in cig bases to VFR
after 19z Monday.

Surface winds will be east to northeast tonight at 10 to 15 kts,
with winds veering into a southerly direction by tomorrow aftn
with speeds again in the 10 to 15 kt range.


&&

.ILX WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

UPDATE...Smith
SHORT TERM...HJS
LONG TERM...HJS
AVIATION...Smith



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