897 
FXUS63 KLSX 120439
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1139 PM CDT Thu Oct 11 2018

.SHORT TERM...  (Through Late Friday Afternoon)
Issued at 303 PM CDT Thu Oct 11 2018

A somewhat active period is expected across the area Friday into
early next week. Tonight however looks tranquil and rather chilly.
High pressure will slide into the area from the west this evening
bringing light winds. This combined with clear skies should lead
to a good temperature drop. As we head into the overnight hours
however we will see mid-high clouds overspreading the area from
the Plains, occurring in advance of our next storm system
associated with a upper trof moving into the north central Plains.
The temperature fall overnight should level out from west to east
attendant with the thickening cloud cover. Present thinking is the
coldest temperatures falling into the middle 30s will be across
northeast MO and west/central and south central IL. There is
potential for some patchy frost within this area. 

I think Friday is going to be a big weather shock to alot of
folks. The area will see a cold rain and for many areas along and
north of Interstate 70, the highs will we be in the 40s and it 
will be more reminiscent of November. As far as the sensible 
weather, an upper trof initially centered in the north central 
Plains will advance quickly eastward and into the Upper-Mid 
Mississippi Valley by early evening. Large scale ascent associated
with the upper trof along with low level WAA and low-mid level 
frontogenetic forcing will generate a large shield of rain which 
will spread from western MO into central MO during the first part 
of the morning and across the remainder of the CWA by early 
afternoon. The combination of the clouds and rain and diabatic 
cooling will result in temperatures running 20-25 degrees below 
average. The latest guidance is a bit faster with the progression 
of the upper trof and associated forcing, and by late afternoon 
the rain band should be centered across southwest IL into 
southeast MO.

Glass

.LONG TERM...  (Friday Night through Next Thursday)
Issued at 303 PM CDT Thu Oct 11 2018

Rain should come to an end by mid-evening across southwest IL and
southeast MO as the upper trof and associated forcing advances
quickly into the Ohio Valley region. The main question then for
Friday night and into early Saturday morning is the potential for
frost. I am not a frost expert but the conditions don't appear all
that favorable. Temperatures will fall into the 30s during the
night and winds will be light. The negating factors would be how 
much clearing of the clouds will occur and the ground will be 
wet, both of these inhibit decent radiational cooling. Presently 
we are forecasting overnight clearing in northeast MO and west 
central IL and I have mentioned some patchy frost, however 
confidence is low.

Temperatures will remain below average through the weekend. Saturday
will be the pick day of the weekend and then we will see several 
chances/waves of rain Saturday night into Monday. None of the rain
looks heavy and preferred areas for precipitation vary in 
location during this time frame. A broad and positively tilted 
upper trof stretching from the upper Mississippi Valley into the 
Great Basin will result in southwest flow aloft. Within this flow 
we will see a series of migratory low amplitude disturbances. The
first of these disturbances along with low level WAA and moisture
transport will bring a threat of rain, predominately to southern 
MO and southern IL Saturday night into Sunday morning. Another 
disturbance along with a cold front advancing from the north and 
frontogenetic forcing will bring yet another chance of rain to
central and northern sections of the CWA Sunday afternoon and 
evening. By Monday morning, the main focus will then shift again 
to predominately southeast MO and southern IL as the next 
disturbance and increasing frontogenesis focus rainfall in that 
area.

More tranquil weather is expected Tuesday through Thursday. Monday
night into early Tuesday morning looks like the coldest time
period of the next 7 days, possibly a freeze. A pattern shift 
which results in ridging aloft and a return to a low level WAA 
regime and southwest low level flow is forecast by Thursday. 
Translated, temperatures gradually return to near normal by late 
next week.

Glass

&&

.AVIATION...  (For the 06z TAFs through 06z Friday Night)
Issued at 1107 PM CDT Thu Oct 11 2018

Clouds continue to stream east across KS/wrn MO heading toward
terminals late this evening. These MVFR cigs will spread across
the region thru sunrise. Guidance continues to show showers
developing and spreading into the area, impacting KCOU around
sunrise. This area of rain will continue to spread east across the
region thru the day, moving south or east of the terminals during
the mid to late afternoon hours. Lower cigs shud also move into
the region accompanying the rain, eventually lowering to IFR
except at KUIN where low end MVFR is expected. That said, there is
some indication that IFR cigs are possible at KUIN as well, but
confidence is lower at this time. 

SPECIFICS FOR KSTL: Still expect rain to impact the terminal
around 18z. Activity may be more sct/showers before this time and
can not rule out a brief shower impacting before 17z. Still 
expect rain to move out of the area around 22z. With confidence
increasing, have added IFR cigs for Fri afternoon. Low cigs shud 
linger thru the evening hours and into Sat morning. 

Tilly

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...None.
IL...None.
&&

$$

WFO LSX