Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 222342
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
642 PM CDT Sun Apr 22 2018

.SHORT TERM...  (Through Late Monday Afternoon)
Issued at 252 PM CDT Sun Apr 22 2018

Upper low currently centered over north central AR to continue
slowly tracking to the east northeast. Associated surface low just
pinwheeling around this circulation through Monday before finally
lifting further northeast away from region.

So for tonight will see several waves of energy rotating around
upper low. Current activity to continue a slow track to the north
and northwest, constantly fighting the dry air. Eventually expect
increasing light rain over STL metro area this evening, with best
chances over portions of southeast MO and southwestern IL. Lows
tonight will be in the mid 40s to low 50s.

On Monday, light rain to persist through morning hours before slowly
diminishing from west to east. As for high temps on Monday, the
coldest temps will be over the southeastern half of forecast area
where there is more cloud cover and precipitation, in the upper 50s
to low 60s, further northwest highs will be in the mid to upper 60s.

Byrd

.LONG TERM...  (Monday Night through Next Sunday)
Issued at 252 PM CDT Sun Apr 22 2018

A mean upper trof will be located through the eastern U.S. this
upcoming week and this will result in predominately below normal
temperatures through this period.

The first system to contend with is the pesky vertically stacked low
that will be exiting the area. By early Monday evening the upper low
will be centered over the western OH Valley and it will eject slowly
northeastward and into OH Valley by Tuesday evening in response
to a series of upstream trofs digging into the Plains and Great
Lakes region. Weak low level convergence/lift within the moist
cyclonic flow could produce some lingering light showers/rain
across parts of east-central/southeast MO into southern IL on
Monday night. Otherwise clouds hang tough across the region with
maybe some clearing in the far western CWA late Monday night.
North-northeast low level flow in the wake of the departing low
continues on Tuesday and low-level RH progs from the models
suggest that at least over eastern MO into IL clouds will remain
persistent with slow eastward clearing thru the day. In fact, my
high temps on Tuesday may be a little ambitious from St. Louis
eastward.

The next aforementioned digging trof will remain centered to the
west of the area, digging southeast through the central Plains and
into the lower MS Valley Wednesday into Wednesday night. The NAM
seems a bit aggressive with its eastward extent of forcing and
precipitation with this upper low/trof and brings precipitation
well into western IL. This appears too far east and is an outlier.
The GFS and ECMWF are in much better agreement with the main
thrust of forcing across eastern KS/western MO shifting
southeastward in concert with the trof motion. This would then
bring the eastern extent of forcing and thus precipitation threat
into only parts of central and southeast MO which seems more
reasonable.

As we get into the Thursday-Friday time frame there is yet another
digging and deepening trof within the northwest flow aloft. The
ECWMF and GFS differ with both the position and intensity of the
trof and attendant cold front and this results in low pops probably
spread out over a longer period than reality. Some time in this
period there will be a greater threat of showers however the
current model timing varies by 12 hours.

Finally it appears that by next weekend we will begin to see a
pattern change with the mean eastern upper trof shifting away
into the Atlantic, while a new long wave trof develops in the
western U.S.. Rising heights and ridging aloft should ensue within
the MS Valley with a return of low level south-southwesterly
flow, WAA, and much warmer temps.

Glass

&&

.AVIATION...  (For the 00z TAFs through 00z Monday Evening)
Issued at 631 PM CDT Sun Apr 22 2018

A low pressure system will continue to slowly move east across srn
MO. Very light showers will continue across the srn half of MO
into southwest IL, but expect to remain south of KUIN. Easterly
winds will gradually become nely on Mon. The showers are expected
to slowly shift east during the day on Mon. Expect KUIN/KCOU to
remain VFR thru the period.

SPECIFICS FOR KSTL/KSUS/KCPS: Main question is if and when non-VFR
cigs move into the terminals. Lower cigs remain just south of the
terminals, as expected, but believe these lower cigs will be slow
to move north, if they do at all. Have pushed back onset of MVFR
cigs, but kept a sct deck as there is some uncertainty. Believe
light showers will persist overnight and gradually move east out
of the region Mon morning.

Tilly

&&

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

$$

WFO LSX


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