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Area forecast discussion...updated aviation
National Weather Service Saint Louis MO
1145 am CST Wed Nov 20 2019

Short term... (through late Thursday afternoon)
issued at 312 am CST Wed Nov 20 2019

Patchy dense fog is expected to dissipate by mid/late morning as the
boundary layer becomes mixes out. In addition, some improvement
may occur due to increasing midlevel cloudiness advecting
southeastward early this morning out of southern Iowa. No changes
made at this time to the ongoing dense fog advisory which expires
at 9 am.

Mild and dry weather is forecast for today with plenty of sunshine
early, giving way to increasing cloudiness by afternoon hours.
Warmed high temperatures up today compared to the previous
forecast given favorable synoptic pattern as the midlevel ridge
crests across the mid-Mississippi Valley and 850-hpa temperatures
climb above +10c. In addition, model guidance the past few days
has generally been at least a couple of degrees too cool.
Therefore, preferred the warmer mav MOS values and went above
these numbers is some locations. This resulted in high
temperatures this afternoon ranging from the upper 50s to mid 60s
across the region. Similar to yesterday, the warmest temperatures
are expected to reside across portions of central Missouri.
Southeasterly winds will also pick up through this afternoon as
the surface pressure gradient tightens between the departing
surface high in the Ohio Valley and an approaching surface low in
the south-Central Plains. Sustained wind speeds of up to 15-20
mph with gusts as high as 25-30 mph are likely.

The timing of the onset of shower activity looks an hour or two
slower than 24 hours ago, which continues the slowing trend of the
past few days. Widespread rain showers remain likely areawide
overnight tonight moving from west to east. This rainfall will be
primarily supported by strong low-level moisture advection. The
widespread shower activity will be well ahead of the actual cold
front, with the main batch of showers exiting the County Warning Area early Thursday
morning. The trailing cold front at this time will still be across
western Missouri which is expected to force scattered showers right
along the boundary. The cold front should progress toward the
southeastern County Warning Area border very late in the afternoon on Thursday, with
dry weather favored for areas behind the front.

Temperatures will remain very mild during this period as the County Warning Area
will remain within the warm sector all night tonight and most of
the day on Thursday. As a result, lows tonight should only drop
into the upper 40s to mid 50s with highs on Thursday in the upper
50s to mid 60s. There is more uncertainty for thursday's highs due
to slight differences in the timing of the synoptic cold front
and the amount of cloud cover ahead of the boundary. Some model
guidance hints at some breaks in the cloud deck behind the
departing widespread shower activity and the trailing cold front.
If sufficient sunshine does occur, cannot rule out some areas
reaching the upper 60s.


Long term... (thursday night through tuesday)
issued at 312 am CST Wed Nov 20 2019

(thursday night - Friday night)

Another round of widespread rain still is in the offing for Thursday
night focused on the southern portions of the County Warning Area. This rainfall
will fall in response to strengthening low-level moisture advection
centered along the 850-hpa baroclinic zone, upper-level jet support,
and increasing mid/upper level forcing for ascent via vorticity
advection. The rainfall will fall predominantly northward of the
slow-moving cold front, but the atmosphere is much too warm for
anything to fall but a cool rain.

The rain should mostly shutoff by early Friday morning as shortwave
ridging and subsidence set in behind the departing midlevel trough.
There remains quite a bit of uncertainty with how precipitation
trends will evolve Friday afternoon through Friday night as
deterministic and ensemble guidance is quite dispersive. The
midlevel closed low near the Colorado-Kansas border is forecast to
move largely due eastward, but weaken as it does due to
increasingly confluent flow downstream. In addition, any forcing
for large-scale ascent will have to overcome some weak low-level
dry air advection. Some models (cmc/ecmwf) continue to produce
some light quantitative precipitation forecast aided by a subtle surface inverted trough and
increasing mid/upper level diffluence ahead of the closed low
while others (gfs/nam) are largely or completely dry. At this
time, both scenarios are plausible and it will likely come down to
a battle of the dry low-level air vs. The strength of the
mid/upper level forcing. The current forecast maintains the chance
pops for Friday afternoon and night due to the high amount of
uncertainty. If it precipitates Friday night, there remains the
possibility that rain mixes with and eventually changes over to
light snow during the overnight hours, mainly north and west of
St. Louis. Cannot rule out at least a low chance of some minor
accumulations, but at this time any accumulating snow appears

A return to cooler than normal temperatures is forecast to occur
Friday and Friday night behind the cold front expected to move
through the County Warning Area by early Thursday evening. Highs on Friday are
likely to be in the low to mid 40s with lows Friday night ranging
from the upper 20s to low 30s. These values would be about 5-10
degrees below normal for this time of year.

(Saturday - tuesday)

The theme for the weekend will be dry, with seasonably cold
temperatures moderating toward Sunday. The general flow pattern
aloft will be northwesterly, with some type of secondary cold
front forecast to move through the bi-state area Saturday evening.
There does not appear to be any cold air behind this frontal
boundary however. Discounted the GFS solution of a deep, closed
midlevel low swinging through the area on Sunday. While this
system comes through with no precipitation, it is much cooler
behind this system. This solution is an outlier though with little
to no support from other deterministic guidance nor its own
ensemble membership.

Speaking of differences in model guidance, they continue to grow
heading into early next week, but especially by Tuesday. There have
been hints of a significant storm system somewhere across the
central Continental U.S. Bringing a variety of impactful weather just in time
for peak Holiday travel. However, the variability in the
strength/track/timing of this system is very large. Look no further
than the differences between the deterministic 0000 UTC GFS/ECMWF.
The GFS has blizzard conditions for the mid-Missouri Valley into
the upper Midwest with showers and thunderstorms within the warm
sector. Meanwhile, the European model (ecmwf) has a weak surface low moving through
the mid south into the lower Ohio Valley with just some light
rain near our neck of the Woods. Plenty of time to wait and see
how this all plays out, but be sure to stay tuned to the latest
forecast information especially if travailing to anywhere in the
central part of the country leading up to the Thanksgiving Holiday.



Aviation... (for the 18z tafs through 18z Thursday afternoon)
issued at 1134 am CST Wed Nov 20 2019

Rain is likely to move into central MO late this evening before
spreading across the region tonight. The rain will persist through
much of the night, with cigs gradually dropping to MVFR by the
early morning. Winds will remain gusty through this afternoon,
with sustained winds speeds remaining around 10-15 kts through
the night. Winds will also be in excess of 40 kts just off the
deck tonight, but the elevated surface winds will preclude the
mention of low level wind shear.

Widespread rain will taper off tomorrow morning, with more spotty
showers expected through the afternoon as a cold front swings
through the region. Ahead of the front, breezy SW winds will
persist until the front shifts winds around to the west and
eventually northwest.



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