Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 200400
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
1100 PM CDT TUE JUL 19 2016

.UPDATE...
Issued at 830 PM CDT Tue Jul 19 2016

The strong convection which affected mainly the western half of
the forecast area late this afternoon and early this evening has
weakened considerably over the past hour. The SPC mesoanalysis
indicated an MCV near or just south of Springfield, edging slowly
south. Latest ILX objective analysis was showing the better
instability over our southern counties with the complex of storms
shifting south-southeast into that higher instability. Further
north, in the wake of the MCS, a much more stable air mass is in
place. Latest surface analysis indicates a weak meso-high from the
complex of storms situated north of Bloomington effectively
pushing the synoptic scale boundary further south and west again
this evening. This boundary is forecast to edge back north later
tonight into Wednesday, which will once again set up the
possibility for more scattered storms before the heat builds in
for real on Thu and Fri. Have made some minor adjustments to the
precip chances overnight as well, with some tweaks to sky condition
and dew points. Updated zones out by 845 pm.

&&

.SHORT TERM...(This evening through Wednesday)
ISSUED AT 258 PM CDT Tue Jul 19 2016

Diffuse, nearly stationary front extended from just north of Quincy
to about Taylorville and Effingham at 200 pm. Scattered, slow moving
showers and isolated thunderstorms have developed along and just to
the north of this front in parts of central IL. A larger complex of
storms in southern Iowa - which the short range models do not have a
good handle on - will be making a turn to the southeast along an
instability axis from southeast IA into west central IL early this
evening.

What this all adds up to in the forecast is likely t-storms from
Rushville to Jacksonville through 9 pm, with showers and
thunderstorms of a more scattered nature in other parts of central
IL. With a steering wind of only 20 knots from the NW, the storms
will be fairly slow moving, so areas of heavy rain are possible
early this evening.

The aforementioned front is expected to move quite slowly to the
northeast overnight and into Wednesday. This will keep the potential
for period of showers and t-storms in the forecast for late tonight
around the IL River Valley to the north of the front, and across
much of the forecast area during the day Wednesday as the storms
become more surface based. Overall, the heaviest rain Wednesday
should be close to the I-74 corridor where 1.00 to 2.00 inches of
rain are possible.

.LONG TERM...(Wednesday night through Tuesday)
ISSUED AT 258 PM CDT Tue Jul 19 2016

Heat and humidity look on track to produce the hottest conditions we
have seen in quite some time later this week. Dangerous heat
conditions are likely at least Thursday and Friday, with Saturday
not far behind. As a result, have decided to extend the Excessive
Heat Watch to include Saturday.

The ingredients are coming together for substantial heat and
humidity later this week. A massive 598dm upper high is projected to
shift east from the Plains on Thursday and continue to affect
Illinois through Friday and most likely Saturday. Surface
moisture is being boosted by evapo-transpiration from expansive
corn and soybean fields. That, in combination with recent rains
will support surface dewpoints in the mid to upper 70s, if not
some lower 80s. Mixing of 22-24C 850 mb temps to the surface will
push high temps into the 94 to 98 deg range Thur and Friday, with
Saturday in the 92-95 range. Heat indices Thur and Fri should top
out between 108-113, with 102-107 on Saturday. Northern areas on
Saturday may see some increased cloud cover from storms approach
from the north, but we will still include all counties in the
excessive heat watch on Saturday for now.

Prior to the heat wave setting in, we expect some lingering storms
into Wednesday evening, as `ridge-riding` storms remain close enough
to our NE counties before the ridge axis gets closer to IL later Wed
night. An active weather pattern is still expected to continue just
to our northeast in the Great Lakes Thursday through Saturday, as
storms develop along the periphery of the ridge. The ECMWF even
retrogrades some storms into northern IL on Friday, progressing them
from the northeast toward our northern counties. A capping
inversion should help to keep our area free from storms on Friday.
Saturday has increased convection indicated across most of IL in
both the GFS and ECMWF, but we still expect very warm 700mb temps
to suppress convection for most of the day.

Sunday looks to be the start of an airmass change for at least
northern IL, as a shortwave progresses along the Canadian border and
drags a cold front southward into IL. At this point, it looks like
the front will progress through our counties Sunday night, so Monday
will be the first day with a noticeable improvement in the heat
and humidity. Highs on Monday and Tuesday look to be much closer
to normal, in the mid to upper 80s. However, surface dewpoints may
only retreat back into the upper 60s, which is still on the muggy
side.

&&

.AVIATION...(For the 06Z TAFS through 06Z Wednesday night)
Issued at 1100 PM CDT Tue Jul 19 2016

Convection continues to slowly shift south and east with CMI
possibly seeing a round or two of TSRA over the next few hours.
Otherwise, additional showers and scattered storms will be
possible towards dawn along a northward moving warm front that
was temporarily pushes south of our area earlier this evening
by all the convection that occurred over the western half of
the forecast area. The aforementioned frontal boundary may take
until later tomorrow night to clear our forecast area, as a
result, at least a chance for scattered TSRA will be with us into
late Wednesday/Wed evening. Just where the storms will reignite
and the coverage area is still quite uncertain at this time into
the daylight hours of Wednesday.

Some patchy MVFR fog will develop with all the low level moisture
in place from the rainfall and a very light wind regime forecast
to hold thru Wednesday. We should see surface winds tend to become
more southeast to south during the day on Wednesday with speeds of
10 kts or less. Any TAF location that does see thunderstorm move
across the area, will see a tempo lowering to MVFR or even IFR for
a brief time before returning to VFR.

&&

.ILX Watches/Warnings/Advisories...
Excessive Heat Watch from Thursday afternoon through Saturday
evening for ILZ027>031-036>038-040>057-061>063-066>068-071>073.

&&

$$

UPDATE...Smith
SHORT TERM...Miller
LONG TERM...Shimon
AVIATION...Smith


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