Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 211751

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1251 PM CDT Mon Aug 21 2017

Issued at 1026 AM CDT Mon Aug 21 2017

Convection over west central/northwest Missouri continues to hang
on with isolated storms as far northeast as Kirksville. Current
thinking is that the ongoing convection should weaken over the
next couple of hours, but there will be plenty of debris clouds
drifting into central Missouri from these storms. Additionally,
earlier convection north of the forecast area has generated an
outflow boundary along with a series of gravity waves which is
moving south through west central Illinois and northeast Missouri.
These waves will reach east central Missouri between 1730-18Z if
they maintain their current speed. All of this adds up to some
pretty large uncertainty with respect to cloud cover along and
north of the I-70 corridor from St. Louis west through Columbia.
Best eclipse viewing should be south of these areas across
southeast Missouri into southwest and southern Illinois.



.SHORT TERM...  (Through Late Tonight)
Issued at 351 AM CDT Mon Aug 21 2017

Water vapor imagery shows the upper ridge is firmly established from
the southwest Atlantic into the lower MS Valley, with quasi-zonal
flow to its north. Satellite shows a good deal of high level
moisture within the zonal flow from the central Plains through the
MS Valley which is presenting itself in cirrus clouds of varying
degrees of cover and opacity. Overall this changes very little today
so I think we will continue to cirrus stream across the region
throughout the day.

The biggest nuisance to the forecast right now is the MCS centered
in western IA. This is occurring in response to strong lift/MCON
associated with a stout southwesterly LLJ. Recent radar has shown
some south-southeastward sag and a decrease in organization. Corfidi
vectors, the thickness gradient, and the deep layer shear all
suggest that the MCS motion should become more east-southeast within
the next few hours and by daybreak they would suggest a decided east
motion. So the question is how far south it may move along with the
rather extensive cloud shield. Given the aforementioned change in
steering factors and a veering LLJ, it would appear that northeast
MO and west central IL would be most vulnerable this morning and
this is where the highest pops will be focused along with mostly-
cloudy to cloudy skies through mid-morning. Hopefully this system
behaves and doesn`t have a mind of its own like the 00Z NAM would
suggest. Keep in mind the general zone north of I-70 will retain a
chance of showers and storms this morning, separate from the MCS,
where WAA associated with the west-southerly H85 flow might
support elevated activity

For areas in the eclipse path, as I mentioned above, I think cirrus
is the main culprit and while its not optimal for the best viewing,
I don`t think it will be a show stopper either. Some diurnal cu will
also start to develop mid-late morning as the boundary layer heats.
Honestly I am not sure how its development will progress when
cooling associated with the eclipse impacts the normal boundary
layer deepening processes. I would think it would slow or halt it
for a few hour window, and thus this would suggest the development
of diurnal convection would also be delayed. My best guess is
pending no ongoing showers or storms once the eclipse begins (and I
think that threat is quite low), there won`t be any development
until after it ends or after 20z. Temperatures should also rebound
and top out by late afternoon in the upper 80s to lower 90s.

An active night still appears on tap. The models are in good
agreement and consistent with previous runs showing a short wave
trof digging through the upper/mid MS Valley. In the process a
prominent short wave/vort max moving through the central Plains is
"absorbed" into the trof moving into MO and western IL overnight.
Height falls and large scale forcing, good instability, and a broad
southwesterly LLJ  will support extensive thunderstorms both ahead
of the advancing cold front and within the LLJ axis from northeast
KS/NW MO spreading east. This activity will spread in earnest
across the northern CWA during the overnight hours with the
southern edge from near St. Louis through mid MO at daybreak
Tuesday. Any severe threat and some heavy rain threat as well
should be confined to northeast MO and west central IL when the
better instability and shear can be realized.


.LONG TERM...  (Tuesday through Sunday)
Issued at 351 AM CDT Mon Aug 21 2017

(Tuesday - Tuesday Night)

Showers and thunderstorms will likely be ongoing first thing Tuesday
morning across the central CWA ahead of a cold front. Continued
weakening of this area of precipitation is expected as low-level jet
and associated moisture convergence wanes. Atmosphere shouldn`t have
much time to recover Tuesday afternoon either as there should be
plenty of cloud cover associated with the overnight storms. Current
thinking is that coverage of storms Thursday afternoon will be more
scattered, primarily focused across portions of southeast
Missouri and southwest Illinois. Could be a case where you
actually get little to development along the actual cold front as
effective boundary gets pushed well southeastward due to
antecedent convection. By early Tuesday night, the synoptic cold
front is expected to clear the CWA with drier, cooler air
filtering into the bi-state area.

Temperatures Tuesday afternoon look to be heavily impacted by
clouds/precipitation. Expect highs to reach only the low to mid 80s,
or about 7-10 degrees cooler than that expected of today. Exact
highs could be a bit cooler or warmer than currently forecast
depending on how convection and associated cloud cover evolve
through early Tuesday afternoon.

(Wednesday - Sunday)

The rest of the extended forecast period will feature predominantly
northwest flow aloft and strong surface high pressure across the
Upper Midwest slowing moving eastward toward New England. Flow aloft
briefly attempts to go more zonal with some signs of a weak
disturbance trying to work its way eastward toward the mid-
Mississippi Valley. However, continued long, deep fetch of dry air
and weak forcing for large-scale ascent at upper levels should
leave the area dry.

All in all, a seasonably cool and dry 5-day period is expected from
mid week through this weekend. Expect highs in the upper 70s to mid
80s and lows in the mid 50s to low 60s. In essence, very similar
weather to what the area has experienced earlier this month.



.AVIATION...  (For the 18z TAFs through 18z Tuesday Afternoon)
Issued at 1242 PM CDT Mon Aug 21 2017

Primary concern is thunderstorms through the period. Current
thinking is that scattered storms will continue to develop north
of the I-70 corridor this afternoon. Some isolated storms are also
possible across parts of southeast Missouri over the eastern
Ozarks. Afternoon storms should diminish this evening. Attention
turns to a cold front tonight. The thunderstorms will develop
along the front and drop southeast across the forecast area after
05-06Z. MVFR flight conditions and possibly IFR conditions in
heavier storms will be possible until late Tuesday morning.


Isolated storms are possible this afternoon as an outflow boundary
moves through the terminal. Any storms that form will be capable
of brief MVFR/IFR conditions in heavy rain. Afternoon storms
should dissipate during after sunset. A cold front will move into
Missouri tonight with thunderstorms along and ahead of it. Current
thinking is that the best chance for thunderstorms will be between
10Z and 12Z. MVFR and possibly IFR conditions will be possible
with the strongest storms. Should see flight conditions improve in
the late morning.





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