Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 220545
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
1145 PM CST Sun Jan 21 2018

.UPDATE...
Issued at 900 PM CST Sun Jan 21 2018

A focused band of forcing for precip is advancing from Missouri
into Illinois, and will continue to advance across our forecast
area the rest of the night and into Monday morning. At 9 pm, the
surface warm front extends across Illinois roughly along a line
from Galesburg to Kankakee, with a slight northward drift over the
last few hours. We expect the warm front to continue to lift
northward as the surface low in eastern Kansas progresses toward
NW Illinois overnight.

Instability params continue to look marginal, but moderate speed
shear upstream appears to be fueling some lightning strikes across
Missouri. Will continue with isolated thunder the rest of the
night and tomorrow morning as this initial wave of precip advances
eastward.

A break in the rain looks on track, due to a dry slot cutting into
the southwest side of the surface low. A re-development of rain
and storms still appears possible tomorrow afternoon, as a 60KT
LLJ progresses across IL. Once again, marginal instability is
expected, but wind shear could fuel some strong storms, or at
least storms that produce damaging wind gusts.

Temps tonight look on track for a steady or slowly rising
scenario. Temps are generally in the upper 40s and low 50s, with
little movement, and mainly upward, expected through sunrise.

Wind gusts may need to be bumped up tomorrow morning, when daytime
mixing drags down some of very strong low level winds.

&&

.SHORT TERM...(This evening through Monday)
ISSUED AT 250 PM CST Sun Jan 21 2018
A deep trough and upper low over the southern High Plains this
afternoon will be the main issue for the forecast through tonight
and Monday. A warm front stretching out into the Midwest today has
brought significant moisture and very warm temperatures up into
Central IL. Persistent fog along the boundary continued into the
early evening hours to the north...as well as east central and
southeastern portions of the state. Moisture to the east and
southeast not properly handled in most of the models early...and
now models are having issue with the much warmer sfc temps that
are starting to erode the southern edge of the more limited
visibilities. Later tonight, precip develops along and ahead of a
cold frontal feature as the low further occludes and moves into
the Central Plains. Precip moves into the forecast area mainly
later tonight and into tomorrow morning. Although the instability
is somewhat limited in the early hours, there is still some
assistance offered by a LLJ, particularly in the southern half of
the state. Strong winds will be the main threat with the early
storms. Gusts in the overnight hours remain high mainly for just
that issue, even with only moderate tightening of the pressure
gradient...it won`t take much to pull some of those winds down
with a shower. Some isolated thunder will remain possible through
the overnight hours.

As for Monday, a significant dry slot has been consistent in
models for tomorrow into the early afternoon. In the past few
runs, NAM/4km WRF have been developing scattered thunderstorms in
the expansive dry slot. With lingering surface moisture, cooler
air moving in aloft, the lapse rates increase considerably in that
environment...with instability parameters less than impressive.
Any thunderstorms that do develop in that environment will have a
vertical assist from the proximity of the low, as well as offering
some shear to help with potential for mini/low topped supercells.
Threat for any severe weather will end quite quickly in the early
evening as the low moves into the Great Lakes.

.LONG TERM...(Monday night through Sunday)
ISSUED AT 250 PM CST Sun Jan 21 2018
Cold air moving into the region behind the low will result in
potential for light snow for late Monday night...but little more
than a dusting on already wet ground will limit accumulations.
Forecast settles into a northwesterly flow regime with
temperatures closer to seasonal normals. 12Z runs of ECMWF and GFS
both pointing to a wave moving through aloft on Tues night/Wed
morning, but so far the wave is moisture starved aloft and sliding
over a surface high...producing zero qpf. Behind that wave, high
pressure ridge slips eastward, starting another warm advection
pattern through the end of the week. Highs are back to the low 50s
by Friday. A developing storm system in the desert SW will push
moisture and precip chances northeast into the Midwest to start
the weekend. Another rainy Saturday shaping up so far...warmer but
rainy.

&&

.AVIATION...(For the 06Z TAFS through 06Z Monday night)
Issued at 1145 PM CST Sun Jan 21 2018

The warm front has finally shifted north of all the terminal sites
as of 0530Z. Dense fog still lingers at PIA at 05Z, but should
improve shortly based on HRRR output and upstream observations.
The next concern turns to potential for some isolated thunder with
this initial wave of showers the rest of the night. Instability
remains less than 100 J/Kg, but wind shear looks favorable enough
for thunder. Lightning strikes have been detected into Illinois in
the last 30 mins, and that trend should continue as the north-
south axis of forcing progresses into central Illinois. Have kept
the VCTS in all TAFs for 3-4 hours later tonight.

A dry slot closely trailing the back edge of precip will push
across IL this morning, with the showers/storms departing into
Indiana by 16z or so. That will leave an 3-5 hours of dry
conditions, before the afternoon instability and wind shear
combine with a shortwave to produce a line of convection between
20z-24z. Some of those storms could contain strong winds. The
afternoon LLJ is forecast to peak at 40kt or so, while the LLJ
late tonight looks to reach 60KT.

Forecast soundings indicate that ceilings may improve to MVFR
with the line of showers, while IFR conditions remain scattered
across the area. Overall conditions look MVFR tomorrow and
possibly even VFR in the dry slot, before showers/storms develop
again.

&&

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

$$

UPDATE...Shimon
SHORT TERM...HJS
LONG TERM...HJS
AVIATION...Shimon



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