Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 221958
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
258 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 agressive 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 18z TAFs through 18z Monday Afternoon)
Issued at 1247 PM CDT Sun Apr 22 2018

Upper low centered over north central AR will continue to slide to
the east northeast through the forecast period. Will see several
rounds of light rain rotate around system through taf sites,
mainly ones along I-70 corridor, while KUIN to stay dry, though a
few sprinkles not out of the question for there. As for cigs, to
remain VFR til better chances of rain moves in early this evening,
so will see cigs dip down to MVFR after 04z Monday. Winds to
persist from the east, then gradually back to the northeast
towards end of forecast period.

SPECIFICS FOR KSTL:
Upper low centered over north central AR will continue to slide to
the east northeast through the forecast period. Will see several
rounds of light rain rotate around system through metro area. As
for cigs, to remain VFR til better chances of rain moves in early
this evening, so will see cigs dip down to MVFR after 04z Monday.
Winds to persist from the east, then gradually back to the
northeast after 12z Monday.

Byrd

&&

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

$$

WFO LSX



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