Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 152036

Area Forecast Discussion
National Weather Service Saint Louis MO
336 PM CDT Tue Aug 15 2017

.SHORT TERM...  (Through Late Wednesday Afternoon)
Issued at 330 PM CDT Tue Aug 15 2017

A weak stationary boundary over northern Missouri will slowly push
north tonight. Scattered showers and thunderstorms are possible
through early this evening. Afterward, the combination of scattered
thin clouds, light and variable winds, and radiational cooling
will lead to patchy valley and river fog overnight mainly over
southeast Missouri and southwestern Illinois. A weak warm front
is expected to move north Wednesday with scattered showers and
thunderstorms possible. Temperatures will likely be slightly below
MOS guidance due to the combination of early morning fog and more
cloudiness. Winds will be light south to southwest on Wednesday.


.LONG TERM...  (Wednesday Night through Next Tuesday)
Issued at 330 PM CDT Tue Aug 15 2017

(Wednesday Night - Thursday)

There may be ongoing showers and thunderstorms early Wednesday
evening, mainly across portions of southeast Missouri and southwest
Illinois associated with an area of low-level moisture convergence
and diffluent flow aloft. Main threat of showers and thunderstorms
however continues to look to be with the synoptic cold front. Have
highest PoPs in the northwestern CWA Wednesday night where better
upper-air forcing and instability will reside. Further to the
southeast, combination of weaker upper-level forcing for ascent,
lessening instability, and less favorable surface convergence all
suggest a weakening convective trend overnight Wednesday night into
Thursday morning. Cannot rule out a few strong/severe storms across
portions of central and northeastern Missouri Wednesday evening,
though not expecting widespread severe weather at this time due to
modest midlevel lapse rates, and moderate deep-layer shear and
instability. Severe threat will likely diminish overnight Wednesday
night with weakening instability.

Best chance of thunderstorms on Thursday likely will be in the
morning hours across portions of southeast Missouri and southwest
Illinois along and ahead of the cold front. This front should clear
the CWA during the late afternoon or early evening hours on
Thursday, effectively ending the threat of precipitation.

Temperatures are expected to remain seasonably warm Wednesday night
and Thursday, with a bit of a lag behind the front to the
drier/cooler airmass. Forecast lows Wednesday night are expected to
be in the low to mid 70s, with highs on Thursday in the mid 80s to
near 90 degrees. Stuck fairly close to MOS values as they seemed
very reasonable.

(Thursday Night - Sunday Night)

Cooler, drier weather is likely from Thursday night through Friday.
Temperatures should be near normal for the middle of August, with
lows in the 60s and highs in the 80s. Medium-range guidance is in
much better agreement with track/timing of a shortwave trough
embedded within northwest flow aloft. This feature is expected to
bring a chance of showers and thunderstorms to much of the
area Friday night and early Saturday. The exact timing of this wave
and associated cloud cover/precipitation coverage will likely have a
large impact on temperatures both Friday night and Saturday.

Much of the rest of the weekend is expected to be dry with
moderating temperatures as 850-hPa temperatures climb toward +20C
and mid/upper level ridge amplifies across the southern CONUS.
Slightly above normal temperatures are favored Sunday and Sunday

(Monday August 21st)

The trend over the past 24 hours in medium-range NWP guidance is for
the mid/upper level ridge across the southern CONUS to continue to
build a bit northward through the day on Monday. This is a good
trend for prospective viewers of the total solar eclipse passing
through early Monday afternoon. Have lowered PoPs and sky cover a
bit from yesterday`s forecast as the better chance of thunderstorms
will be tied to the northern periphery of the ridge across the mid-
Missouri Valley eastward toward the lower Great Lakes. However, some
opaque cirrus blowoff from these storms to the north may be
possible. In addition, there are numerous other mechanisms for the
formation of clouds, many of which are very difficult to discern
past 36-48 hours. That being said, current synoptic pattern
forecast for Monday afternoon does look supportive of a partly
cloudy to mostly sunny sky, with the most likely types of
potential clouds being the aforementioned high-level cirrus
blowoff from thunderstorms to the north/northwest of the area as
well as diurnal cumulus. However, past total solar eclipses have
noted that the cooling induced by the eclipse itself helps to
reduce diurnal cumulus. Speaking of cooling, it certainly will be
interesting to see exactly how much air temperatures are affected
from the beginning to end of the eclipse. Attempted to add some
detail in hourly temperatures by cooling readings several degrees
nearest to totality (1800 UTC/100 PM) and 1-3 degrees in the hour
immediately before and after during the partial phase of the
eclipse. Actual temperatures nearest totality between ~1810 and
~1820 UTC will likely dip 5-15 degrees more, at least briefly.

(Next Tuesday)

Deterministic GFS/ECMWF, as well as GEFS all suggest upper-level
ridge axis is expected to shift westward toward the intermountain
west. Downstream, northwest flow aloft is expected across the mid-
Mississippi Valley. A cold front should pass through the area,
bringing a chance of showers and thunderstorms. Slightly above
normal temperatures are favored, but clouds/precipitation may keep
conditions cooler Tuesday afternoon if they materialize.



.AVIATION...  (For the 18z TAFs through 18z Wednesday Afternoon)
Issued at 1250 PM CDT Tue Aug 15 2017

A weak stationary over northern Missouri will slowly lift north
the rest of the afternoon into this evening. A complex of showers
and thunderstorms over southern Iowa will move east along the
Missouri and Iowa border through this afternoon. Some scattered
showers and thunderstorms will be possible in northern Missouri
this afternoon with brief MVFR ceilings and conditions possible.
Scattered showers are also possible with weakening complex of
storms in extreme southeast Missouri and southwestern Illinois
with brief MVFR conditions possible. VFR conditions will persist
through tonight with river valley fog possible and patchy fog
elsewhere. A warm front will lift north through southeast
Missouri and southwestern Illinois with brief MVFR ceilings and
conditions possible with scattered showers and thunderstorms.

VFR ceilings and conditions expected through early tonight. Patchy
fog near the terminal with river valley fog possible tonight could
lead to brief periods of MVFR ceilings through early tomorrow
morning. Otherwise VFR conditions and ceilings through rest of the





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