Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 202323

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
623 PM CDT Sun Aug 20 2017

.SHORT TERM...  (Through Late Monday Afternoon)
Issued at 348 PM CDT Sun Aug 20 2017

The outflow boundary from last night`s MCS has pushed almost all the
way through southeast through the forecast area, and the synoptic
warm front is lifting back up through west central and northwest
Missouri.  A line of showers and thunderstorms is moving along the
front from near Moberly up to near Maryville.  The eastern end of
the line appears to be weakening as it moves into more stable air
behind the outflow boundary over our area.  That being said...low
level moisture convergence continues along the front through the
rest of the I expect at least isolated to widely
scattered convection to continue until at least 01-02Z this evening.
across parts of central and northeast Missouri.

The front will continue to lift northeast into Iowa tonight.  All
short range guidance is now showing moderate to strong moisture
convergence over Iowa on the nose of a 30-40kt low level jet with
precip breaking out between 04-06Z either over eastern Nebraska or
northern/northwestern Iowa.  CAMs also have some flavor of MCS over
Iowa tonight.  The strength and movement of the MCS tonight will be
critical to what happens on Monday for the eclipse.

The most likely scenario is that the MCS will push east-southeast
tonight and early Monday morning as the low level jet veers.  Even
the forward propagating Corfidi vectors show a southerly component
late tonight...and the highest MUCAPE will be over southern
Iowa/northern Missouri which will help the overall movement to have
a southerly component.  Think the MCS will lay down an outflow
boundary which will push south into the area on Monday
morning...with isolated to widely scattered convection forming on it
as it intersects higher CAPE air across eastern Missouri and
southwestern Illinois.  The 4km NAM nest has the boundary drifting
all the way south to near the I-70 corridor by around noon on Monday
where it lights up with thunderstorms right around eclipse time at
18Z.  All this makes good synoptic sense given the current set up
and is very similar to what happened today.  Unfortunately this will
make eclipse viewing problematic.  Convective debris from overnight
storms, and new development on the outflow boundary will likely
produce considerable cloudiness...but like today there should also
be breaks in the clouds, and the clouds may also be partially
translucent.  Additionally...mesoscale features that are not well
represented in the models could change the forecast for the
better...or the worse.  Unfortunately there`s simply no way to
precisely predict how cloudy it will be tomorrow at 18Z right now.


.LONG TERM...  (Monday Night through Next Sunday)
Issued at 348 PM CDT Sun Aug 20 2017


A strong upper level disturbance en route to carving out a
longwave TROF over eastern North America will move thru this
period.  It will be largely coincident with passage of a strong
surface cold front.  Plenty of moisture to work with, as
precipitable water (PW) values thru the column will be over 2 inches
and will justify high PoPs, especially north of I-70.  Relatively
high warm cloud thicknesses at or just above 4km along with the PW
values will support a marginal threat for localized heavy rainfall
that could lead to an isolated flash flooding event.  Time to watch
how this evolves and pans out as it will be somewhat dependent on
how events the day before play out for specifics and so no plans on
any Flood Watch at this time.

Lots of clouds and the early onset of decent rain probs for most
areas should keep temps in check with low-mid 80s for maxes.

(Wednesday - Sunday)

NW flow aloft will then prevail Wednesday and Thursday in the
wake of passage of the digging shortwave disturbance, with this
flow then transitioning some as a weak upper RIDGE slides thru
Friday and Saturday.  This will all be happening as a strong
area of Canadian high pressure dominates the region thru at least
Friday and probably into Saturday.

While there are some hints from the models at bringing weak, fast
moving upper level disturbances thru Wednesday and Thursday, 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 the track looks to edge our
area with marginal moisture and so trended to a dry forecast for
this day as well until this can get better resolved.

Another shortwave could drop down again on Sunday to help
reinvigorate the longwave eastern North American upper TROF but the
passage of this feature also looks far enough west at this time to
preclude pcpn chances but should set us up to keep the hot temps
away as we reach the doorstep of September.

Otherwise, the big story here will be a 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 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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