Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 270303
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
903 PM CST Sun Feb 26 2017

.UPDATE...
Issued at 849 PM CST Sun Feb 26 2017

A couple forecast concerns exist across central and southeast
Illinois for the rest of the night. The first revolves around
rainfall chances across the northwest and southeast portions of
the region with two separate disturbances. The second revolves
around fog potential later tonight.

The convective looking returns in the vicinity of the northwest
portion of the forecast area are located near a weak low-level
convergence zone, and within a mid-level shear axis. Most of these
returns have not been reaching the ground and have been showing
signs of weakening. This trend should continue as their driving
forcing weakens.

The disturbance driving the showers over southern Illinois still
appears on track to bring some light rain to locations along/south
of the I-70 corridor. However, these showers will be not be
significant.

Some of the high-resolution guidance develops fog across much of
the forecast area overnight, especially along/west of the Illinois
River Valley. Surface temperature/dew point spreads at sunset were
pretty substantial. However, dew points have climbed considerably
in some areas over the past few hours, particularly near the areas
where rain may fall. Added patchy fog to basically the northwest
half of the forecast area, especially west of the Illinois River.
This area has higher surface dew points, lighter winds, and will
have limited cloud cover once the area of sprinkles moves
out/dissipates.

&&

.SHORT TERM...(This evening through Monday)
ISSUED AT 250 PM CST Sun Feb 26 2017

High pressure ridge dominating the eastern coast of the country,
putting Central Illinois in the path of southerly flow bringing some
warmer temperatures.  Overnight a few degrees warmer than last
night, with lows mainly in the 30s, upper 20s in the northwest.
Increasing clouds over the south overnight as an approaching storm
system spreads rain into southern IL...keeping the pops south of
I-70. Some moisture moving into the region after sunset under the
inversion potentially bringing some very patchy fog in low lying
areas in the west. However, the dewpoints this afternoon have a
way to go before reaching that point after midnight. Tomorrow
another partly to mostly cloudy day as the system to the south
moves through the region...but temperatures will still be able to
climb into the 50s by the afternoon.

.LONG TERM...(Monday night through Sunday)
ISSUED AT 250 PM CST Sun Feb 26 2017

Southwesterly flow aloft increases across the area for the first
half of the work week resulting in temperatures well above normal
once again. Embedded in the flow are several shortwaves that will
affect the forecast area through Wednesday. The first of these
upper waves will track across the Midwest later Monday night into
Tuesday morning bringing scattered showers to parts of central
Illinois with the more vigorous wave associated with the longwave
trof and attendant frontal boundary edging closer to the area late
Tuesday into Tuesday night. This will be main time frame of
concern for severe storms along with the threat for locally heavy
rainfall. Several days of southerly flow across the forecast area
will lead not only to an increase in temperatures but also surface
dew points with most models suggesting dew points from the mid 50s
north to the lower 60s south. Combine the increase in low level
moisture with steep mid level lapse rates, MUCAPEs of 500-1000
J/KG and impressive deep layer shear (0-6km effective shear values
around 60 kts!) brings about the threat for scattered severe
storms later Tuesday afternoon into Tuesday night with the threat
shifting southeast with the frontal boundary late Tuesday night
into Wednesday. Several models hold off much in the way of
convection due to weak subsidence in the wake of the upper level
wave that pushes across the area in the morning putting a
temporary end to the rain threat along with a capping inversion
through at least the middle portion of the afternoon, with the
main show Tuesday night into Wednesday morning. With the frontal
boundary just to our west and a surface wave approaching west
central Illinois late in the afternoon, surface convergence should
increase enough across the area later in the day to break through
the weakening cap, so we will continue with at least high chance
POPs across the area Tuesday afternoon.

Current indications suggest the primary severe threat later Tuesday
into Tuesday night would be roughly along and south of a Springfield
to Champaign line, and as mentioned above, as the convection
congeals into one or two MCSs Tuesday night, a general southeast
drift to the showers and storms will occur along with the cold
front. Precipitable Water values approaching one inch, which is well
above normal for the end of February, will bring about the threat
for some locally heavy rainfall with some of the stronger cells.
Likely to categorical POPs are included across most of the forecast
area during this time frame with lingering thunder chances over far
southeast Illinois Wednesday morning as the boundary shifts along
the Interstate 70 corridor. Some of the models continue to show a
deformation band setting up to our north as the surface wave shifts
off to our east later Wednesday, with the higher POPs Wednesday
afternoon over our northern and eastern counties to account for this
moisture wrapping back over parts of central Illinois. Low chance
POPs will be confined to the far east Wednesday evening with all the
precip chances shifting off to our east Wednesday night. In the wake
of the storm system, colder temperatures will settle across the area
with readings returning closer to normal for several days as our
upper flow turns northwesterly.

With the northwesterly flow prevailing for the end of the week, a
fast moving clipper system will drop southeast across the north
half of the state Thursday afternoon/evening. While starting as
rain, a transition to snow is possible in the evening across
extreme east central Illinois before the system and its
precipitation quickly moves away from central Illinois. The upper
flow will begin to become more zonal again next weekend resulting
in above normal temperatures next weekend with highs in the upper
50s to lower 60s Saturday and in the low to mid 60s next Sunday.
An upper level wave and frontal system is forecast to approach the
area late Sunday into Sunday night with an increasing chance for
showers.

&&

.AVIATION...(For the 00Z TAFS through 00Z Monday evening)
Issued at 558 PM CST Sun Feb 26 2017

Southerly winds will persist across the central Illinois terminals
through the 00Z TAF valid time. VFR conditions should prevail for
most of the forecast period, but there are some model signals of
stratus/fog developing by late tonight (especially in the
vicinity of KPIA) as the southerly winds bring in additional
moisture. Have kept mention out for now, but will watch low-level
moisture trends closely over the next few hours. Periods of VFR
CIGs are likely through the period.


&&

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

$$

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



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