Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 192340

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
640 PM CDT Sat Aug 19 2017

.SHORT TERM...  (Through Late Sunday Afternoon)
Issued at 343 PM CDT Sat Aug 19 2017

A weak ridge of high pressure over the area will drift east tonight
and Sunday.  The next upstream shortwave is moving into the western
Great Plains at this time, and will continue to progress eastward
tonight.  The low level reflection of this wave strengthens over
Nebraska and northern Kansas tonight resulting in a 30-45kt low level
jet over the east central Plains.  Guidance is in pretty good
agreement in developing an MCS over eastern Nebraska/northeast
Kansas tonight.  This MCS isn`t expected to move far enough east to
affect us directly tonight, but the models are indicating that it
will leave behind an MCV which will drift across northern
Missouri/southern Iowa on Sunday.  This MCV will provide a focus for
convection to continue or for new development to occur during the
late morning/early afternoon.  Have accordingly introduced some low
chance PoPs across northeast Missouri into west Central Illinois.
Temperatures through the period will be seasonably warm with lows
tonight in the mid 60s to around 70 and highs near 90 on Sunday.


.LONG TERM...  (Sunday Night through Next Saturday)
Issued at 343 PM CDT Sat Aug 19 2017

(Monday - ECLIPSE DAY)

The overall pattern for this day continues to show a strong upper
RIDGE dominating areas just to our south, an anticipated primary MCS
track of better baroclinicity near the MO/IA border into southern
IA, and a synoptic cold front a bit further north.  This would
suggest the highest rain chances will be to our north and northwest.
H250-300 heights would support most of the thick high cloudiness
generated staying north.

However, models continue to show a weak warm front oriented north-
south across western MO early in the day with this feature tracking
slowly eastward during the day.  With what should be good moisture
content in the column across much of our region this day, the warm
front will only serve to encourage some TSRA development further
south during late Sunday night and into Monday with some of these
thunderstorms edging eastward during the day.

The bad news is that it does not look as good for eclipse viewing
with a greater part, if not all, of our region expected to be under
this chance of thunderstorms and its associated cloud cover provided
by this slow moving warm front.  The good news is that the coverage
should be spotty enough that it should not be a solid overcast for
all areas and these anticipated breaks will provide some hope for
those hoping to catch a glimpse.

It is also not fully known how well the approach of the umbra of the
moon will impact the warm front`s ability to generate new convection
but it will likely only be for a short time window if it does.
However, the cooler temps for an hour or so will have little to no
impact on pre-existing mid and high level clouds typical of
convective outflow or debris--only on diurnally generated low

Putting it together, it is much harder now to determine any better
eclipse viewing region in our CWA than before as all areas seem to
have the same problems now with respect to pcpn chances and clouds.
Hopefully some details with regards to temporal and spacial
resolution can be achieved in the next day or two, but in all
honesty, with a setup like this, this may not be attained until one
gets within a few HOURS of the event.

Despite a brief cooldown around midday and early afternoon due to
the eclipse, a rapid recovery during the mid-late afternoon should
allow temps to reach the upper 80s and low 90s.


A significantly strong upper shortwave TROF still looks set to move
thru on Tuesday along with a surface cold front.  Plenty of moisture
available for this to work with and will continue to justify high
PoPs for the entire region.  Lots of clouds and what looks to be a
fairly early onset of high rain probs should keep temps in check
with low-mid 80s for maxes for most areas.

(Wednesday - Saturday)

NW flow aloft will prevail Wednesday and Thursday and then
transition some as a weak upper RIDGE builds in.  At the surface,
mild Canadian high pressure will build in and dominate our region
for much of this period.

While there are some hints from the models at bringing weak, fast
moving upper level disturbances thru Wednesday and Thursday in the
wake of the main Tuesday storm, it will need to overcome what should
be a dry column in place.  With forward speed too much and strength
too little, it is unlikely any disturbance of this type will be able
to produce sensible wx in our CWA and so opted for a dry forecast.
There is also some indication of another disturbance on Saturday,
but enough timing issues and being at the end of the forecast period
to also forego pcpn mention until this gets better resolved.

Otherwise, the big story here will be a little taste of early autumn
with daytime max temps around 80 with lo humidity and nighttime mins
in the 50s.



.AVIATION...  (For the 00z TAFs through 00z Sunday Evening)
Issued at 622 PM CDT Sat Aug 19 2017

Dry and VFR thru the period. Light and vrb winds tonight will
become sely to sly on Sun afternoon. There is a chance for TSRA,
mainly at KUIN, Sun afternoon. However, too much uncertainty
remains to mention in TAF attm.





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