Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

fxus63 klsx 221716 

Area forecast discussion...updated aviation
National Weather Service Saint Louis MO
1116 am CST Fri Nov 22 2019

Short term... (through late tonight)
issued at 334 am CST Fri Nov 22 2019

High pressure over the upper Midwest has pushed a cold front through
southeast Missouri this morning. The remaining precipitation north
of the front over south central and southeast Missouri should be
coming to an end between now and 12-14z as drier air in the lower
levels works its way south. Models show a shortwave ridge currently
over the east-central Great Plains drifting eastward over Missouri
and western Illinois today. This ridging should keep most of the
forecast area dry today...with the possible exception of
southeastern Missouri late this afternoon as the next low level wave
drifts toward the mid Mississippi Valley from the southeast plains.
Short-range deterministic guidance is showing increasing 850mb
moisture convergence ahead of the wave as the flow turns back to the
south ahead of the wave late this afternoon and this evening. While
models show little to no quantitative precipitation forecast in our area before 00-03z, I cannot
rule out a little light rain late this afternoon/early this evening
mainly south of I-70/I-44.

Areas of light rain will likely overspread most of central and
eastern Missouri as well as southwest and south central Illinois
from late evening into the overnight hours. Forecast soundings show
that temperatures in the column are generally supportive of frozen
precip north of I-70, with a mix to rain from the I-70 corridor
southward. That being said, 850mb temperatures will be warming
overnight due to warm advection ahead of the wave. Think any snow
that falls will have a high moisture content and will be light
enough that accumulations will likely be limited to grassy areas,
and will be less than 1/2 inch.


Long term... (saturday through thursday)
issued at 334 am CST Fri Nov 22 2019

The closed upper low is expected to be located near Kansas City by
12z/Sat and track eastward to near Quincy, Illinois by 18z/Sat and east of
our region later Saturday afternoon as it slowly becomes more open
as it edges closer to rejoining the main upper flow. The main
region of lift should be located on the southeast flank of the upper
low and this is expected to result in high pops for southeast MO, the
stl Metro area, and into southwest Illinois during Saturday morning. What
is less certain is how well developed precipitation will be in the
weakening deformation/trowal region further north, but the better
chances of seeing precipitation in this region will exist earlier on
Saturday than later as the system slowly becomes less closed with
time. Despite this, surface temperatures are expected to be around
freezing 12z/Sat for areas north and west of stl Metro, with other
areas likely remaining above this, and should limit any snow
potential. That said, stated snow accums look to be limited to
grassy areas only and less than an inch. Max temps will only be in
the upper 30s and low 40s as clouds are expected to be slow to clear

Northwest upper flow will continue into Sunday with another shortwave
dropping through this flow, but the track of this feature has
trended more northerly and with an already pretty dry column to
work on in the wake of the Saturday system, will have very limited
to no impact on our region. Max temps on Sunday are expected to be
in the low-mid 50s as surface flow becomes SW for a day with some

A pattern shift takes place on Monday and becomes more established
by Tuesday, with flow aloft more from the SW as a longwave trough sets
up over the western Continental U.S.. there is reasonable agreement on one
storm system affecting our region on Tuesday with much greater
uncertainty surrounding another possible system in the Wednesday
night-Thursday time frame.

The system on Tuesday appears to be a strong open wave structurally
and could carry some instability with it to result in thunderstorms
but the overall signal is less than it was 24hrs ago. With such a
progressive system it will be hard to get much in the way of quantitative precipitation forecast and
the complete lack of cold air, as supported by the decent clustering
of gefs ensemble members, will strongly support rain as the dominant

The Wednesday night-Thursday system appears to be a result of how
well the current storm system off the Southern California coast can get
absorbed into the developing western upper trough undertow and there
is considerable divergence in ensembles on how to handle this both
with temperatures and pops. Handled this with chance category pops
for now with a preference for rain over snow again given the overall
lack of cold air intrusions with this pattern.

Temps for much of next week after Sunday will be at or above normal.



Aviation... (for the 18z tafs through 18z Saturday afternoon)
issued at 1107 am CST Fri Nov 22 2019

MVFR cigs along the Mississippi have been a bit slower to scatter
out than previously forecast. That said, current satellite data
does show a slow clearing trend in these low clouds, so have only
kept MVFR in place for a hour or two beyond 18z. VFR conditions
will then persist into the evening before another area of
precipitation moves into the region from the southeast. This precip will
sweep west to east across the area into tomorrow morning, and it's
likely we see a little bit of snow mix in with the rain late
tonight and tomorrow morning. Cigs will also drop off as the
precip moves into the region, falling back to MVFR overnight. Uin
will be the exception, as they are likely to be a bit too far
north to see reduced flight conditions.



Lsx watches/warnings/advisories...

National Weather Service Glossary of Abbreviations