Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 181142

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
642 AM CDT Fri Aug 18 2017

.SHORT TERM...  (Through Late Tonight)
Issued at 335 AM CDT Fri Aug 18 2017

Focus through tonight will be on chances of thunderstorms beginning
late this afternoon. Weak cold front across the central Plains will
slowly move southeastward through tonight, and is currently
producing some disorganized scattered showers and thunderstorms
nearing the Missouri River. Much of today will be dry with
increasing mid and high-level cloudiness. Scattered thunderstorms
are expected to redevelop along the frontal boundary mid-late this
afternoon and move southeastward toward portions of central and
northeastern Missouri. CAM`s not too excited with development late
this afternoon into early this evening, which does make sense given
very weak surface convergence along the frontal boundary and an
inversion roughly between 750-800 hPa helping to supress convective
development. Have highest PoPs early this evening along the front
across far northern sections of the area in an area where stronger
midlevel cooling will take place. Thunderstorms should gradually
weaken overnight tonight and lessen in coverage as instability

Assuming at least isolated convection affects the area between late
this afternoon and early tonight, there does appear to be a chance
for a few strong to severe thunderstorms. Mid-Mississippi Valley
will be beneath a 40-45 knot midlevel speed maxima this evening
helping yield strong deep-layer shear for the middle of August. In
addition, steep midlevel lapse rates are forecast across the area
with plenty of CAPE within the hail growth zone. Lots of dry air at
low and midlevels as well which could help pose a wind threat via
strong precipitation loading. Unfavorable atmospheric conditions for
strong/severe thunderstorms include the aforementioned temperature
inversion aloft/high convective inhibition, weak surface convergence
along frontal boundary, and modest instability (MLCAPE values ~500-
1000 J/kg).

Slightly above normal tempeatures are expected for this afternoon
ahead of the cold front. Leaned aoa MOS guidance for high
temperatures today with plenty of sunshine expected for at least the
first half of the day and strong low-level warm advection. Expect
highs in the upper 80s to low 90s across the area. Lows tonight look
to be near to slightly above normal. Coolest conditions will be
across portions of northeast Missouri and west-central Illinois
behind the cold front with warmest conditions for portions of
southeast Missouri, southwest Illinois, and the St. Louis urban
area. MOS low temperatures looked reasonable and did not deviate
much from these values for tonight.


.LONG TERM...  (Saturday through Thursday)
Issued at 335 AM CDT Fri Aug 18 2017

(Saturday - Sunday)

Overall this period looks rather tranquil. A cold front will be
bi-secting the CWA early Saturday morning in association with an
upper trof digging into the OH/TN Valley region. A few showers and
thunderstorms will be possible across parts of southern IL and
southeast MO in the morning as the cold front continues to move
southeastward through the CWA. Otherwise high pressure will settle
east-southeast through the upper-mid MS Valley and into the OH
Valley throughout the day in the wake of the front, bringing
drier air into the area.

The flow aloft across the region will flatten some Saturday night
as the eastern upper trof continues to progress through the
northeast U.S. and into the western Atlantic, with heights aloft
rising on Sunday as an upper ridge begins building from the
southwest Atlantic into the Nation`s midsection. Surface high
pressure while continuing to retreat east away from the area will
continue to exert an influence, keeping low level moisture at bay
to the south along that portion of the old frontal boundary.
Further west across eastern KS/far western MO into eastern
NE/western IA, the western part of the old front will return north
and will be somewhat ill-defined as it assumes a more north-south
orientation on the far western periphery of the retreating
surface high. Moisture spanning this frontal zone to our south and
well to our west and the accompanying instability will support
the potential for showers and thundestorms in those areas.
Meanwhile the influence of the surface high in the LSX CWA will
bring dry conditions with lows Saturday night near average and a
bit above average high temps on Sunday.

(Sunday Night - Monday/Eclipse Day)

Lots of attention on this period encompassing the eclipse and the
viewing potential. Unfortunately until we get inside of 24 hours,
its hard to give the specifics of hourly cloud forecasts for eclipse
viewing. Thus the forecast at this time is largely based on the
features and the trends of the latest model runs. The good news is
the upper ridge will continue to build west across the mid MS Valley
and into the southern Plains Sunday night into Monday, with the main
band of westerlies retreating to the northern tier of the CONUS.
This would suggest the main threat of organized showers and
thunderstorms will also be well to the north from the central Plains
into upper MS Valley in the vicinity of the primary baroclinic zone,
southwest LLJ, and short wave track.

However there are some caveats to consider for our area. The
previously discussed ill-defined warm front will be sliding east
across the region as the surface high continues to retreat and low
level flow becomes south-southwesterly. Both the NAM and GFS
suggest that there could be some elevated showers/thunderstorms
overnight Sunday and into Monday morning to the east of the warm
front in response to low-level WAA via the LLJ. The favored
region at this time would be a north-south zone through central
MO. Typically this type of activity peaks 09-13Z or so then
dissipates and the clouds diminish. The QPF fields from the GFS
would support that with a dry afternoon. Alternatively the ECMWF
does not show this signal but instead suggests spotty diurnally
driven showers/storms across northern MO. Present indications
would suggest less weather caveats and the opportunity for the
best viewing conditions will reside south/southeast of St. Louis
into southern IL. Overall it looks like a seasonably hot day with
highs generally in the lower 90s.

Monday Night - Thursday)

The threat of showers and thunderstorms will begin to ramp-up across
northern sections late Monday night, and across the remainder of the
CWA Tuesday and Tuesday night. A series of short wave trofs will dig
across the upper MS Valley and into the Great Lakes marking the
onset of large scale amplification, and also sending a cold front
into and across the area. Numerous showers and thundestorms are
expected as a favorably moist and unstable air mass will reside
ahead of the front, along with a well-defined west-southwest LLJ,
and modest upper support.

A deep trof evolves across eastern NOAM during the later part of the
week supporting a rather decent surface high in the wake of the cold
fropa. Temperatures will once again cool to below average values
with humidity levels below normal for later August as well.



.AVIATION...  (For the 12z TAFs through 12z Saturday Morning)
Issued at 637 AM CDT Fri Aug 18 2017

Area will see increasing mid/high level cloudiness ahead through
the day today. A weak cold front will move through the region this
evening and early overnight, bringing the possibility of scattered
showers and thunderstorms. Visibilities may be reduced to IFR/MVFR
in heavier downpours. Some patchy fog is possible behind the
front, particularly for KCOU and KUIN.


Expect an increase in mid/high level cloudiness through this
evening ahead of weak cold front. Isolated-scattered
showers/storms possible along the front, though probability too
low to mention in TAF. Winds will remain very light through the
period, but switch from the west/southwest to the northwest behind
the frontal passage. VFR conditions are expected barring any
showers or storms affecting the terminal tonight.





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