Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 122322
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
622 PM CDT Wed Aug 12 2020

.SHORT TERM...  (Through Late Thursday Night)
Issued at 240 PM CDT Wed Aug 12 2020

Early this afternoon, an upper LO was located over southern
Illinois, near KMVN, slowly drifting eastward.  An upper level
disturbance was located on the northern flank of this upper LO and
the lift generated from this disturbance as it slowly rotates to the
southwest on the backside of the LO was the principal agitator for
the slow-moving, heavy rainfall producing thunderstorms that have
been occurring since late this morning.  The main area of storms,
and the heavy rainfall threat, is slowly exiting Metro East and
edging more into southern Illinois and portions of southeast
Missouri.  At the surface, a quasi-stationary frontal boundary was
located from far southern Indiana westward to just south of STL City
then turning northwestward through north-central Missouri into Iowa.
Temperatures outside of the storms have risen into the low-mid 80s
but remain quite cool in the lower 70s where storms have persisted.
Otherwise, there was a low-amplitude upper RIDGE that prevailed over
much of the central CONUS with its core over southern New Mexico.

The upper RIDGE is expected to amplify some over the next 36 hours
as the axis becomes more positively tilted, from New Mexico through
the western Great Lakes.  Unfortunately, this will also maintain a
very light steering flow over our way, and only serve to nudge the
upper LO that has been such as weather-maker to the east at a
snail`s pace.  Likewise, the surface frontal boundary, assuming it
maintains its identity, will linger across the southern and western
forecast area through this period.

What this all means is that the main source of lift for
thunderstorms will continue to be the upper LO and where it tracks,
particularly with respect to any upper level disturbances that
rotate around it.  Model guidance suggests this should slowly edge
east or southeast each day from where it impacted the day before.
Environmental parameters will follow this feature, with high
precipitable water near 2", moderate instability (MLCAPE < 2k), weak
shear (< 20kts 0-6km bulk), and deep warm cloud layers in excess of
4km.  This all points to the potential for another round of locally
heavy rainfall producing slow-moving thunderstorms in southern IL on
Thursday, likely displaced more to the E or SE from where it
occurred today affecting more of the far east or southeast sections
of the forecast area and probably away from the STL Metro area.  As
with today, storm severity risk will remain low into Thursday.

Additional chances for thunderstorms will be possible further to the
northwest and look somewhat better than they did today, with
guidance suggesting another, more migratory, shortwave tracking in
from the west Thursday afternoon and is expected to settle in or
reinforce the upper LO already in southern IL.  However, there is
some disagreement on how to merge and track this feature in.

Otherwise, seasonable temperatures will continue during this period.

TES

.LONG TERM...  (Friday through Next Wednesday)
Issued at 240 PM CDT Wed Aug 12 2020

There is good agreement between the GEFS and ENS H500 means on
general pattern evolution during this period.  The upper LO
begins the period not too far off in southwest Indiana, but should
be far enough away to no longer have influence on our sensible
weather.  Otherwise, an upper RIDGE looks set to amplify over the
western CONUS, resulting in a prolonged period of NW flow aloft over
our region from late this weekend and through the middle of next
week.

Rain chances look near CLIMO Friday considering a very warm and
humid airmass in place with no fronts, and should ramp to above
CLIMO for cold FROPA during the weekend.  The ensemble means have
leaned faster on this front dropping through, now late Saturday
instead of Sunday.  This should give us our best rain chances after
the SHORT TERM period on Saturday.  In addition, with higher
confidence we can now say Saturday should be the last hot day for a
while, but even then, it looks like peak heat index values should
stay below 100F in most places.

From Sunday on, we look to get into a dry weather pattern for
several days with below normal temperatures, as GEFS and ENS means
support a Canadian airmass building in and dominating our region for
a good portion of next week. This is further reinforced by the NBM
temperature spreads.

TES

&&

.AVIATION...  (For the 00z TAFs through 00z Thursday Evening)
Issued at 602 PM CDT Wed Aug 12 2020

Any residual showers and storms this evening are expected to
remain well away from the terminals with VFR flight conditions
expected this evening. There is a bit of uncertainty in flight
conditions overnight into Thursday morning. There seems to be
increasing potential for fog and low stratus developing. Given the
uncertainty at this time I have opted for a window of MVFR flight
conditions in the 10-14z time frame due to fog, however I can`t
discount lower flight conditions as there is potential for IFR
stratus/fog. Stratus/stratocu resulting in MVFR flight conditions
are expected at the St. Louis terminals on Thursday morning with
conditions improving to VFR by midday. I think the chance for
showers and storms on Thursday morning will remain over south
central and southwest IL with low probabilities, but too low to
mention at this time, in metro St. Louis on Thursday afternoon.

SPECIFICS FOR KSTL:

VFR flight conditions are expected this evening. There is a bit
of uncertainty in flight conditions overnight into Thursday
morning. There seems to be increasing potential for stratus
developing. At this point I have introduced MVFR stratus starting
at 12z and lasting through the morning. I can`t discount lower
flight conditions as there is potential for IFR stratus, however
my confidence is too low to mention at this point. MVFR flight
conditions are expected to improve to VFR by midday. I think the
chance for showers and storms on Thursday morning will remain over
south central and southwest IL with low probabilities, but too
low to mention at this time, at KSTL on Thursday afternoon.

Glass

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...None.
IL...None.
&&

$$

WFO LSX


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