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Area forecast discussion...updated aviation
National Weather Service Saint Louis MO
1147 PM CDT Wed Sep 18 2019

issued at 618 PM CDT Wed Sep 18 2019

Minor updates were made to pop, wx, and sky grids to reflect the
latest radar and satellite trends. Isolated to scattered rain showers/thunderstorms and rain
are still expected to percolate for a little while longer before
dissipating with the loss of daytime heating.



Short term... (through late Thursday night)
issued at 325 PM CDT Wed Sep 18 2019

Persistence will continue to be the main theme weather wise through
Thursday night. The remnant backdoor cold front from a couple of
days ago may help initiate a few isolated showers/thunderstorms
mainly Thursday afternoon for portions of central and northeastern
Missouri as well as west-central Illinois. Otherwise, dry weather is
expected across the area along with continued hot/humid conditions
for the middle of September. Lows both for tonight and Thursday
night are forecast to be in the mid 60s to near 70 degrees with
highs on Thursday afternoon ranging from the mid 80s to low 90s.
These values will continue to be about 10 degrees above normal for
this time of year.


Long term... (friday through next wednesday)
issued at 325 PM CDT Wed Sep 18 2019

(friday - Sunday night)

Attention on Friday and Friday night will be on the remnants of
tropical storm Imelda. Models continue to diverge with respect to
how much it will or will not hold together as it attempts to move
out of the arklatex region. Most of the guidance including the
GFS/European model (ecmwf) basically dissipate this system entirely by midday Friday
leading to fairly minimal impacts to our County Warning Area (increase in mid/high
level cloudiness and a small increase in chances of showers/storms).
The new 1200 UTC NAM however keeps a coherent vorticity maxima
through the Mississippi Valley coincident with an increase in low-
level moisture convergence. Consequently, the NAM has a widespread
round of showers and storms for parts of the County Warning Area. The atmosphere
will be supportive of heavy rainfall rates due to very deep warm
cloud depths and precipitable water values near two inches so if a
solution similar to the NAM actually were to occur, locally heavy
rainfall would be possible. The good news is that most areas have
been quite dry so far in the month of September. In fact, there are
pockets in southeast Missouri which have received very little (under
a tenth of an inch) to no rainfall. Given the dry antecedent
conditions, flash flood guidance will be very high and difficult to
reach, at least initially. Due to the uncertainty with how imelda's
remnants evolve through Friday night, leaned toward a blend of model
guidance which resulted in high chance pops across portions of the
County Warning Area.

The concern increases however for flash flood potential Saturday
night through Sunday night as a slow moving cold front moves through
the County Warning Area. There are still some timing differences in this frontal
passage, but it appears a bit slower than yesterday. Consequently,
the most likely period of widespread showers/storms with the
potential for heavy rainfall now looks to be more Sunday/Sunday
night. As alluded to in the previous paragraph, the flash flood
potential Sunday/Sunday night will depend on what happens with the
potential "round 1" associated with imelda's remnants due to
preexisting dry conditions.

Due to the increasing cloud cover and precipitation chances, high
temperatures will cool a couple of degrees each day through Sunday.
Meanwhile, low temperatures will remain near persistence values and
on the mild side.

(Monday - next wednesday)

Ensemble guidance suggests the shortwave trough should exit the
Great Lakes region by Monday night with northwest flow aloft across
the mid-Mississippi Valley. At the surface, the cold front should
make it at least into the mid south before potentially stalling and
working back northward as a warm front. North of the boundary,
surface high pressure should reside and likely lead to a dry period
most likely Monday night into Tuesday. By Tuesday night and
Wednesday, chances for showers and thunderstorms may increase ahead
of the next system attempting to come out of The Four Corners region
of the Continental U.S.. however, given model biases trying to eject cutoff
lows too quickly out of the Desert Southwest, the European model (ecmwf) solution
which keeps things dry through the middle of next week probably has
some credence.

Temperatures early/mid next week will likely be slightly above
normal day and night, but still quite a bit cooler than this past
week so far. Look for highs in the upper 70s to low 80s for the most
part along with lows generally in the 60s.



Aviation... (for the 06z tafs through 06z Thursday night)
issued at 1121 PM CDT Wed Sep 18 2019

VFR conditions are expected at all terminals for the majority of
the valid taf period. The exceptions may be some possible light
fog between 08-14z and isolated to scattered rain showers/thunderstorms and rain between
18-24z near a stalled cold front. The expected sparse coverage of
rain showers/thunderstorms and rain precludes a mention in the tafs at this time. Winds
will remain generally southeasterly through the period.



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