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fxus63 klsx 171748 

Area forecast discussion...updated aviation
National Weather Service Saint Louis MO
1248 PM CDT Tue Sep 17 2019

Short term... (through late Wednesday afternoon)
issued at 343 am CDT Tue Sep 17 2019

Early this morning, an upper level ridge dominated the lower
and mid-Mississippi Valley. At the surface, a large area of high
pressure was centered over the Saint Lawrence valley and whose
influence extended all the way westward into our region. The
extreme western periphery of this influence was represented by a
"backdoor" frontal boundary from far southern Illinois curling
northwestward near the MS river into northeast MO. Thin high cirrus
clouds were passing overhead, resulting in generally mostly clear
skies, with temperatures much above normal for this time of year,
with readings in most locales in the upper 60s to lower 70s.
Dewpoints were only a degree or two lower with patchy fog forming in
favored valley regions.

The upper ridge is expected to begin a gradual weakening on its hold
over our region, but should still retain enough influence through
Wednesday afternoon to still be the dominant force. At the surface,
the "backdoor" frontal boundary is expected to push a bit more SW by
this afternoon, and with a similar setup as yesterday, should result
in the potential for isolated showers and thunderstorms in a
corridor from northeast and central MO into southeast MO and may
also intersect the far southwestern sections of the stl Metro area.
The frontal boundary should weaken, if not dissipate, by Wednesday
afternoon and with no other discernible triggers for convection,
will keep a dry forecast in place.

MOS has had a cool bias with this current pattern, but nevertheless,
temperatures should be a few degrees cooler behind (to the
northeast) the backdoor cold front than ahead of it, where
persistence temps are expected, or from the upper 80s to the mid
90s. Heat index values today will be highest near the front where
dewpoints and temps can Max out, but even then, should peak just
below 100f. Another mild night is ahead tonight with min temps in
the mid 60s to the lower 70s. With the frontal boundary expected to
be dissipated or greatly weakened on Wednesday, temps will either be
near persistence or slightly higher, with broad upper 80s to lower
90s expected. Heat index values will be in the 90s.


Long term... (wednesday night through monday)
issued at 343 am CDT Tue Sep 17 2019

Unseasonably warm conditions will continue on Thursday with upper
level ridging over the region along with south-southeasterly low
level winds. Went a tad warmer than MOS for highs on Thursday as
MOS guidance has been running too cool. There may be isolated
showers/storms Thursday afternoon as the atmosphere destabilizes
with daytime heating. Friday should be the start of a more active,
wet weather pattern as the upper level ridge shifts east of the
region. The slow moving, tropical upper level low now over the
southeast coast of Texas will be lifting north-northeastward and
approaching our area by Friday night. The threat for convection
will continue through the weekend due to low-mid level warm
advection in a moist southwesterly low level flow, along with
upper level divergence ahead of weak southwest flow shortwaves.
The European model (ecmwf) model also has a northern stream upper level trough
which moves eastward through the northern plains Sunday, sending a
cold front slowly southeastward through our area Sunday night and
Monday. High temperatures will be trending down this weekend,
mainly due to increased cloud cover and at least scattered
convection. Highs by Sunday should be close to seasonal normals.



Aviation... (for the 18z tafs through 18z Wednesday afternoon)
issued at 1233 PM CDT Tue Sep 17 2019

Isolated showers and perhaps a rumble of thunder or two are
expected this afternoon but sparse coverage precludes any mention
at the terminals for now. This activity should dissipate quickly
this evening. Fog development is possible overnight tonight,
mainly at ksus and kcps. Added MVFR visibilities for late tonight
for these terminals but kept kstl/kcou/KUIN VFR. Any fog that does
develop should lift and dissipate by mid morning with dry/VFR
conditions through midday on Wednesday.

Specifics for kstl:

Dry/VFR conditions are likely through the day on Wednesday. Will
watch for a few isolated showers/storms this afternoon and again
on Wednesday afternoon but coverage looks too low and mostly
further west so no mention in the taf. Winds will remain light out
of the east through the period.



Lsx watches/warnings/advisories...

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