Area Forecast Discussion
Issued by NWS St. Louis, MO

Current Version | Previous Version | Text Only | Print | Product List | Glossary Off
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50

FXUS63 KLSX 281134

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
634 AM CDT Wed Oct 28 2020

.SHORT TERM...  (Through Late Thursday Afternoon)
Issued at 349 AM CDT Wed Oct 28 2020

The main focus for this period of the forecast is the potential for
heavy rainfall across the southern portion of the CWA.

Currently aloft, a low is positioned over the southwestern U.S.,
placing the CWA under west/northwest flow. This has allowed for
the baroclinic zone and weak frontogenesis to remain in place
over the CWA overnight and this morning, spawning occasional light
rain and drizzle. This activity will diminish through the morning
as frontogenesis weakens.

Our attention then turns to the aforementioned upper-level low as it
ejects into the Southern Plains and the Mississippi Valley. Ahead of
this low, low-level southerly flow will advect warmer air into our
CWA and bump afternoon highs into the 50s - something we haven`t
experienced in several days. This low-level southerly flow will also
advect quite a bit of moisture into the Middle Mississippi Valley.
In response, rain chances will increase from south to north this
afternoon and evening ahead of the approaching surface low. The
heaviest rain will fall primarily along and south of I-44 in MO and
I-70 in IL through tonight into Thursday as the surface low passes
just south of the CWA into the Ohio Valley. Even though the low will
be to our east on Thursday, rain will continue along the deformation
zone through the day and into the evening primarily south of I-70.
Max PWAT values associated with this system seen in SPC HREF, NAEFS,
EPS guidance range from 1.5-2 inches across the southern portion of
the CWA - very anomalous for this time of year. The resulting
rainfall for the entire event will range from around an inch along I-
44 in MO and I-70 in IL, with amounts diminishing northward and
increasing to 2.5-3 inches across our southern and southeastern
counties. In these counties, guidance has the soil moisture in the
range of 40-70%. Given the duration over which this rain will fall
and that it is falling on sub-saturated soils, confidence in a flood
threat is not particularly high at the moment. However, isolated
flooding in low areas and on creeks and streams is possible and will
bear watching.


.LONG TERM...  (Thursday Night through Tuesday)
Issued at 349 AM CDT Wed Oct 28 2020

The main focus for this portion of the forecast period is
temperature trends.

Aloft, the shortwave will be departing the Middle Mississippi Valley
to the east, placing the region under northwest flow. Closer to the
surface, cold, dry air will advect into the CWA, keeping overnight
lows and highs on Friday below climatological norms. However, this
regime will be short lived, as broad, upper-level ridging develops
over the southern U.S. This in turn will shift low-level winds to
out of the south/southwest late Friday, which will usher in warmer
air and help raise temperatures on Saturday.

Our attention then turns to an upper-level trough that will quickly
dig southward across the central U.S. Saturday afternoon. This
trough`s associated low will traverse southern Canada, but still
swing a cold front through the Middle Mississippi Valley. Model
soundings continue to have the lower atmosphere quite dry, and both
the EPS and GEFS are void of precipitation with this FROPA. Thus, I
have confidence the only notable weather with this front will be
winds shifting from southerly to northwesterly.

Behind the front, high pressure settles into the central U.S. Spread
among ensemble guidance is still high in regards to temperatures
associated with this airmass on Sunday and Monday. However, even
with the degree of spread, ensemble members are still on the cool
side of climatological norms, and CIPS Extended Analog Threat
Guidance shows temperatures below normal, building confidence in
such a cool outcome. Also noted in that guidance and in ensemble
guidance is the return of more seasonable temperatures at the end of
the period thanks to the previously mentioned surface high pushing
eastward and southerly low-level flow returning to the CWA.



.AVIATION...  (For the 12z TAFs through 12z Thursday Morning)
Issued at 632 AM CDT Wed Oct 28 2020

IFR to LIFR flight conditions will be present at the St. Louis metro
terminals (KSTL, KSUS, and KCPS) through the morning hours before
ceilings improved to MVFR around mid-day. These same conditions will
be present at the KCOU terminal to start the period, but will take a
little longer to improve. There is some uncertainty as to when
exactly ceilings will improve at these four terminals, as all four
will be on the northern edge of a low stratus deck through the day.
As evening approaches, the chance of rain and low stratus returns
and continues through the overnight hours for KSTL, KCPS, KSUS, and
KCOU. The KUIN terminal is likely to remain VFR through the period,
and the greatest chance of rain should remain just south of the
terminal. However, I`ve added VCSH to the period in which rain will
come the closest to the terminal.



Saint Louis     55  43  50  35 /  20  90  70  10
Quincy          52  39  49  30 /   5  30  10   0
Columbia        50  40  48  30 /  30  80  30   5
Jefferson City  52  41  50  31 /  40  90  40   5
Salem           56  44  51  35 /  20 100  90  30
Farmington      54  43  49  32 /  70 100  80  30




WFO LSX is the U.S. government's official web portal to all federal, state and local government web resources and services.