567 
FXUS63 KILX 050415
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
1115 PM CDT Sun Aug 4 2013

.DISCUSSION...
ISSUED 845 PM CDT Sun Aug 4 2013

High pressure continues to slowly edge east of our area early this
evening. Very pleasant conditions prevail across the region with
mild temperatures and comfortable humidities. As the fair weather
system continues to move away tonight, our winds will turn more into
the south by morning which should allow dew points to rise back into
the 60s most of the area by afternoon. Satellite data depicting a
rather vigorous shortwave over North Dakaota, which is forecast
to track east into the Great Lakes on Monday increasing the chances
for showers and thunderstorms, especially across the north. Further
to our southwest, another convective complex is expected to form
on the nose of the low level jet late tonight and track east and then
southeast across parts of southern Missouri. 00Z ILX sounding still
indicating quite a bit of dry air in place thanks to the departing
area of high pressure with most models holding off any threat for rain
until Monday morning at the earliest. Current zone forecast handling
the evening trends quite well, so no zone update will be needed.

&&

.AVIATION...
ISSUED 1110 PM CDT Sun Aug 4 2013

Little change from previous discussion. Except for the 08z-12z
time frame where some lower cigs/vsbys will be possible acrs our
southern TAF sites, VFR conditions are expected through the
remainder of this forecast period (06z Tue). Patches of mid and
high level clouds continue to stream southeast into the area late
this evening. Larger band of stratocu seen on satellite edging
northeast with bases of cigs in the 4000-6000 foot range. In
addition, the past few runs of the HRRR model indicating some
lower vsbys in fog and even some stratus developing just south of
SPI in the 09z-11z time frame. Current temp-dew point spreads
suggest the possibility late tonight...after 08z in SPI. Remnants
of MCS which passed acrs SE Mo into extreme southern IL continues
to track away from area but will have to watch for some stratus
and possibly some lower cigs to form on its northern periphery
and drift northwest as the boundary layer winds turn more into
an east-southeast direction. At this point will keep vsbys around
5 miles and introduce some lower clouds with bases around 1000
feet late tonight and for now keep this threat in SPI.

Scattered-broken cigs at 4000-6000 feet will continue during the 
day as an upper level system pushes to our north. Will continue
with VCSH mainly across our northern TAF sites but coverage appears
too limited at this time to go with any tempo groups. Surface winds
will continue to be light and variable tonight and then swing into
a southerly direction on Monday with speeds of 8 to 13 kts. 

Smith
&&

.PREV DISCUSSION...
ISSUED 250 PM CDT Sun Aug 4 2013

High pressure slowly drifting across the Great Lakes early this
afternoon, producing some diurnal cumulus. High clouds continue to
stream across the south half of the state, from convection over
southwest Missouri that has been on the wane the last few hours.
Have had some good mixing of the air aloft which has brought
surface dew points down into the lower to mid 50s, as early
afternoon air temperatures reached the 75-80 degree range. Aloft,
water vapor imagery showing a shortwave tracking along the
periphery of an upper high centered over Texas, with the wave
moving through eastern Nebraska and western Iowa. A second wave
was tracking along the Montana/Saskatchewan border. 

The primary forecast challenge continues to be with timing of
shower and thunderstorm activity the next few days.

SHORT TERM...Tonight through Wednesday...

Model guidance has generally settled into the thought of producing
two separate MCS clusters overnight, one with the Montana wave and
tracking to our north, and the other sliding southeast across
Missouri in association with the Nebraska wave. Not looking
terribly promising in between at the moment, with the best chances
of seeing anything in our area being with the northern cluster on
Monday into early evening, so have focused most of the rain
chances with that feature. Have limited rain chances to slights
at best over the remainder of the area into early Tuesday. 

The main rain maker in our area will be with the primary cold
front toward midweek, as a strong upper wave swings across the
Great Lakes. Some convective feedback on the GFS is resulting in a
slower frontal passage, but the general model consensus a passage
through our forecast area late Tuesday night through about midday
Wednesday. Will have the highest rain chances over the entire area
Tuesday night, and lingering into Wednesday across the southeast
quarter. Frontal boundary will not be going through at an optimal
time for severe weather, but some stronger winds will be possible
with a few of the storms. Greatest potential for heavy rain will
be during this period though, as precipitable water values rise
to around 2-2.25 inches. 

LONG TERM...Wednesday night through Sunday...

Frontal boundary to slow down Wednesday night as it starts to
parallel the upper flow. Latest ECMWF more generous with spreading
the rain back northward as ripples of low pressure ride along the
boundary, while the GFS keeps most of the precip to our south
Wednesday night and Thursday. Have kept some low chances of around
30% across southeast Illinois to account for this. 

The large upper low which will be spinning over south central
Canada much of the week will finally swing eastward late in the
week, with an associated frontal boundary sweeping through the
state Friday night. With the old boundary south of us intercepting
any moisture return, this frontal passage will likely be dry.
These dry conditions are expected to linger into next weekend, as
high pressure builds southeast into the Midwest.

GEELHART


&&

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

$$