Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 011756
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
1256 PM CDT SUN MAY 1 2016

.UPDATE...
Issued at 1102 AM CDT Sun May 1 2016

Made some updates to the forecast earlier this morning to drop
PoPs across the board during the morning and to go with mostly
sunny skies south of I-70. Latest visible satellite imagery
continues to show overcast conditions across much of the KILX CWA,
with areas south of I-70 still seeing plenty of sunshine. As a
result, a strong temperature gradient has developed across the
area...with readings ranging from the upper 40s far north around
Lacon to close to 70 degrees far southeast. HRRR suggests the
clouds will gradually thin this afternoon, but think this might be
too optimistic based on prevailing northeast flow north of frontal
boundary in the Ohio River Valley. As a result, will continue with
a mostly cloudy forecast across much of the area. As an upper low
currently over southern Iowa shears northeastward, scattered
showers and a few thunderstorms will develop during the afternoon.
Think the best areal coverage of precip will remain focused
across the central and northern CWA, where high chance PoPs are
warranted. PoPs will decrease further southeast away from the
stronger forcing. Severe threat will remain minimal today, as
best forcing and strongest instability will not coincide anywhere
across the CWA. Afternoon high temperatures will range from the
lower 50s north of Peoria, to the lower to middle 70s south of
I-70.

&&

.SHORT TERM...(Today)
ISSUED AT 305 AM CDT Sun May 1 2016

A rather complex forecast exists across central and southeast
Illinois today, with many factors to consider. A nearly stationary
surface front cuts across the forecast area generally a little south
of, and nearly parallel to, the I-72 corridor. To the south of the
boundary, partly cloudy skies exist, with scattered convection to
the south of I-70. To the north of the boundary, extensive
fog/stratus exists, with visibilities significantly reduced in some
areas. The fog is most dense near the edge of the stratus, and
surface obs suggest a Dense Fog Advisory should be considered.
However, the cloud edge is slowly sinking to the south and this
would suggest that dense fog will not remain in any one location for
a significant period of time. Plan to hold off on Dense Fog Advisory
for now with this in mind, but will continue to monitor visibility
trends closely through sunrise.

Other main concern is the expected precipitation coverage today. A
broad northeast to southwest oriented upper-level trof stretches
across the lower 48 states, with several smaller scale upper lows
embedded within it. The feature we need to key in on today is the
upper low spinning over the central Plains. This feature is expected
to shear northeast into the Great Lakes region today. The forcing
associated with this weakening feature, and associated wave on the
surface front, will be our primary precipitation trigger. There is
currently very little on radar upstream, and expect the bulk of our
precipitation will occur through a combination of diurnal heating
and mid-level cooling with the approach of the upper low remnants.
This thinking supports scattered showers across the area by
afternoon, with a few thunderstorms also possible. The thunderstorms
should mostly occur along/south of the surface front. A few of the
storms along/south of I-70 may be strong given the modest
instability (CAPEs from 1000-2000 j/kg) and shear profiles (bulk
shear to 45 kts) anticipated by afternoon.

A significant N-S temperatures gradient is likely across the area
today with the surface front in the area. Daytime highs should range
from the mid 50s NW around Galesburg to mid 70s SE around
Lawrenceville.

.LONG TERM...(Tonight through Saturday)
ISSUED AT 305 AM CDT Sun May 1 2016

Persistent upper troughing pattern expected over the eastern U.S.
much of the upcoming week, as a significant ridge builds west of the
Rockies. This will send lobes of cooler air southward into the Great
Lakes region, with 850 mb temperatures over our area dipping to
around zero Celsius on Tuesday, and Wednesday night through
Thursday. This should result in highs several degrees below normal
for early May. Late in the week, an omega blocking pattern will
evolve, although the latest runs of the ECMWF and GFS do show the
ridge edging eastward, bringing a significant warming trend for next
weekend.

Remnants of current upper low over Nebraska will drift eastward
early this week, and stretch out into a trough which will drop
toward the Ohio Valley on Tuesday. With these features around, it
will be difficult to eliminate mention of PoP`s. Have increased rain
chances to around 30% on Monday over the southeast CWA and went with
slight chances as far north as Bloomington and Havana, with the
threat diminishing with loss of daytime heating. With the trough
further south on Tuesday, slight chance PoP`s were kept over the
areas south of I-70. A more significant rain chance is expected late
Tuesday night into Wednesday, as a large upper low drops southward
into the Great Lakes. Have increased PoP`s to around 35-40% over the
eastern CWA on Wednesday. Rather significant cold pocket at 500 mb
will accompany this low, and would not be surprised to see some
isolated thunder. However, the coldest part of this will be tracking
into Indiana, so will hold off on any thunder mention for now. Dry
weather is expected late in the week, as high pressure dominates the
Mississippi Valley as the upper ridge arrives.

&&

.AVIATION...(For the 18Z TAFS through 18Z Monday afternoon)
Issued at 1256 PM CDT Sun May 1 2016

IFR ceilings prevail at the central Illinois terminals early this
afternoon: however, latest visible satellite imagery is showing
an appreciable thinning of the overcast. HRRR has consistently
suggested ceilings rising into the MVFR category over the next
couple of hours and have followed this trend accordingly. Think
KPIA/KBMI will be the last sites to improve as cloud cover appears
thickest in this area. With partial sunshine breaking through and
an upper low providing modest synoptic lift, think scattered
showers/thunder will develop this afternoon/evening. Have included
VCTS to cover this possibility. Once daytime instability wanes,
any showers will come to an end after sunset. After that, the low
overcast will spread/develop back southward...with the HRRR
showing IFR ceilings back at all TAF sites between 04z and 06z.

&&

.ILX Watches/Warnings/Advisories...
NONE.
&&

$$

UPDATE...Barnes
SHORT TERM...BAK
LONG TERM...Geelhart
AVIATION...Barnes


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