Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

fxus63 klsx 181015 
afdlsx

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

Short term... (through late Thursday afternoon)
issued at 404 am CDT Wed Sep 18 2019

Early this morning, an upper level ridge dominated the mid-
Mississippi Valley, while tropical depression Imelda was now inland
near Houston, Texas. At the surface, a large area of high pressure
continued to be centered over the Saint Lawrence valley with its
influence extending all the way westward into our region. The
extreme western periphery of this high pressure area continued to be
represented by a "backdoor" frontal boundary draped over much of
eastern MO, just west of the Mississippi River. A few thin high
cirrus clouds were passing through, with mainly clear skies.
Temperatures are once again 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.

The upper ridge is expected to maintain its strength and remain
centered near western Kentucky through Thursday afternoon. At the
surface, the "backdoor" frontal boundary is expected to remain
nearly stationary today, oriented northwest-southeast across eastern MO, and will
result in a similar setup from Tuesday afternoon: MLCAPES between
1500-2000 j/kg, very weak 0-6km bulk shear of less than 20kts, no
cinh, and weak moisture convergence near the frontal boundary. Will
maintain the slight chance pops for eastern and central MO for this
afternoon. There is even some signs of this frontal boundary
maintaining enough integrity to do a repeat on Thursday afternoon.
This should be the only chances for rain during this period.

The remnants of "imelda" are expected to drift north into the
arklatex region by the end of this period, but this position is
expected to have no impact on our region...yet.

MOS continues to exhibit a cool bias with respect to Max temps in
this current pattern and have forecasted maxes a few degrees above
MOS values, resulting in upper 80s to lower 90s today (very close to
persistence), and similar values again on Thursday. Min
temperatures tonight will likewise reflect persistence, ranging from
the mid 60s to around 70.

Tes

Long term... (thursday night through tuesday)
issued at 404 am CDT Wed Sep 18 2019

Will begin to see a gradual cooling trend with high temperatures
beginning on Friday and increasing pops as the upper level ridge
shifts southeast of the region. The remnants of tropical depression
Imelda will lift northeastward into our area Friday afternoon and
night. Low level moisture convergence will be increasing over east
central and southeast MO on the nose of a southwesterly low level
jet Friday night. A moist southwesterly flow will become
established this weekend with low-mid level warm air advection and
increasing upper level divergence ahead of weak southwest flow
shortwaves. The highest pops and quantitative precipitation forecast will likely occur from
Saturday night through Sunday evening as the European model (ecmwf) model has a
deepening upper level trough approaching the region with an
associated cold front dropping southeastward through our forecast
area Sunday night into Monday morning. Although the operational GFS
model is weaker with the upper level trough and associated cold
front, it still depicts relatively high quantitative precipitation forecast across our area. With
high precipitable water values over the region there will be the
potential for heavy rainfall possibly resulting in flooding,
although flash flood guidance values will be initially quite high
due to the lack of rainfall the past week over our area. Still
uncertainty this far out where the axis of heaviest rainfall will
set up and the highest rainfall amounts expected. Cooler high
temperatures can be expected this weekend due to the cloud cover and
the precipitation with highs close to seasonal normals by Sunday.
There is a lot of forecast uncertainty Monday and Tuesday due to
significant model differences. The European model (ecmwf) model keeps it dry as a
surface ridge moves eastward through our area after fropa. The
operational GFS keeps low quantitative precipitation forecast over parts of our region along and
north of a weak warm front. For now will lower the pops by Monday,
but keep slight chance to low chance pops going for most of our area
through Tuesday.

Gks

&&

Aviation... (for the 12z tafs through 12z Thursday morning)
issued at 515 am CDT Wed Sep 18 2019

VFR conditions, dry weather, and light southeast surface winds will
prevail at the taf sites through the valid period. There may be a
brief instance of minor visibility reductions in mist at cps and
sus. Once again, there is some small potential for a few showers
and storms this afternoon but probs remain too low to mention at
this time.

Tes

&&

Lsx watches/warnings/advisories...
MO...none.
Illinois...none.
&&

National Weather Service Glossary of Abbreviations