Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 210853

Area Forecast Discussion
National Weather Service Saint Louis MO
353 AM CDT Mon Aug 21 2017

.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 00z TAFs through 00z Monday Evening)
Issued at 615 PM CDT Sun Aug 20 2017

Ongoing TSRA is expected to just miss KSUS to the west, tho a
brief shower is possible. Ongoing TSRA into SW MO are expected to
remain west of KCOU, although impacts can not be ruled out.
Otherwise, winds will be light tonight and pick up Mon from the
SW. Storms are expected to develop across IA tonight and sink
south into nrn MO Mon morning. Confidence in events tomorrow are
too low to add more than a VCTS attm.

SPECIFICS FOR KSTL: Outflow from storms will start the area with a
wly to wnwly wind, tho shud swing around to a more sly direction
overnight. Can not rule out a shower, but expect bulk of the
precip to remain west of the terminal.





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