Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 231137
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
537 AM CST Thu Feb 23 2017

.SHORT TERM...  (Through Late Tonight)
Issued at 418 AM CST Thu Feb 23 2017

The synoptic pattern early this morning features a trough
deepening across the western CONUS, which has allowed southwest
flow to envelop the Plains and Midwest ahead of it. Two main PV
anomalies of note will pass through the base of this trough as it
ejects out into the Plains today, the first of which will affect
us today and the other more potent anomaly will push through on
Friday.

Latest water vapor imagery shows the first shortwave ejecting out
of the western trough, currently located across southwest KS.
This perturbation will rapidly race northeastward later this
morning into early this afternoon. It will zipper along a
sharpening surface warm front likely to be oriented just north of
the I-70 corridor by early this afternoon. Ascent with this
feature will mostly be located in the mid/upper levels and will
be ahead of the steeper lapse rates expected to move in this
evening and tonight, thus expect only some showers across
northeast MO and western IL as this system quickly passes by.

Temperatures today will be quite tricky given the sharpening warm
front across the area, the expected clouds/showers north of the
front in NE MO and W IL, and also low stratus/fog expected to
build northward from south-central MO. High clouds will also be
thickening through the day. Have lowered temps quite a bit over
northeast MO where clouds and ENE surface flow should keep things
in the mid/upper 50s. South of the front, low stratus should
temper warming initially over central MO, thus readings in the low
70s are likely there. Far east-central MO and southwestern IL have
the best chance of staying both south of the warm front and away
from the low stratus this morning, thus have gone with temps in
the mid 70s there.

For tonight, the aforementioned warm front should begin to lift
northward. As it does, a 50+ kt low-level jet will ramp up
between 00-03Z which will help provide ascent atop the sloped
warm frontal surface. The latest cams suggest that by the time
any convection is able to initiate on the cool side of this
boundary, it will have lifted far enough north to keep precip out
of our area. Therefore, while we can`t completely rule out a stray
storm across far northeast MO mainly between 00-04Z this evening,
think it is likely that conditions will be dry. If a storm is able
to develop, can`t rule out a large hail threat given the steep
mid-level lapse rates advecting into the region from the
southwest. Again, this scenario looks unlikely at this time.

It will be very mild tonight ahead of the approaching low
pressure system and cold front. Given dewpoints in the mid 50s and
surface winds keeping the low-levels mixed, temperatures likely
won`t drop below the upper 50s and even low 60s in many areas!

KD

.LONG TERM...  (Friday through Wednesday)
Issued at 418 AM CST Thu Feb 23 2017

A mature low pressure system will bring a variety of weather to
the central CONUS on Friday. Scattered thunderstorms are expected
on Friday afternoon across the eastern half of the CWA once a
stout warm sector capping inversion begins to erode and
convection begins to develop ahead of a dryline. A few of the
storms across southwest IL could be strong to severe, but the
greatest potential for severe weather is located well to the east
of the area where conditions are more favorable. Initially
discrete storms will quickly grow upscale into linear modes based
on the shear/boundary orientation, but this upscale growth may
not happen until the storms have moved out of the CWA.

The tight pressure gradient near the aforementioned mature low
pressure system will boost sustained surface winds on Friday and
Friday night. A strong wind field aloft will also readily mix
down to the surface given the limited directional shear within
the lowest levels of the atmosphere. Wind speeds and gusts for
Friday and Friday night were increased with this forecast
package, and a Wind Advisory may eventually be needed.

The recent stretch of unseasonably warm weather will end abruptly
on Friday night when temperatures fall into the 20s and 30s behind
a strong cold front. The near-normal temperatures expected for the
upcoming weekend will feel chilly by comparison. Highs on Saturday
and Sunday will range from the upper 30s to around 50.
Temperatures are expected to moderate again during the early and
middle part of next week.

There is a chance of rain or snow from Sunday through Monday due to
isentropic ascent ahead of an approaching shortwave, but there is
poor model agreement between the ECMWF and GFS regarding the
placement and intensity of the upper disturbance.

There is a chance of rain on Tuesday and Tuesday night ahead of an
approaching trough axis, but there is poor model agreement regarding
the speed and location of a lead shortwave.

Kanofsky

&&

.AVIATION...  (For the 12z TAFs through 12z Friday Morning)
Issued at 523 AM CST Thu Feb 23 2017

Low-confidence TAF forecast this morning as a weakening cold
front becomes stationary over the region, then begins to lift
northward as a warm front this afternoon. As the front has become
stationary, dense fog has developed at COU/SUS. Fog will also be
possible at STL and CPS as the front stalls near these sites, but
think the most dense fog will remain just off to the north. UIN is
VFR for now, but IFR stratus is expected to move in by 13Z. As the
front stalls and begins to lift northward later this morning into
this afternoon, fog will likely lift to a high-end IFR or low-end
MVFR stratus deck. As the front lifts northward, ENE winds this
morning will shift to more southerly this afternoon and especially
into this evening. Gusts are likely to increase this evening as
well ahead of a developing area of low pressure.

SPECIFICS FOR KSTL:
Low-confidence forecast this cycle given a stalling boundary in
the area. This boundary is currently just to the north of STL,
with dense fog reported at ALN and SUS. Given local urban effects,
do not expect the fog to become too dense at STL, but could see
some MVFR or even high-end IFR vsbys this morning. Any fog is then
expected to develop more into a stratus deck as the front retreats back
to the north. Winds will increase out of the ESE as the front
lifts north, becoming southerly this evening into the overnight
hours with gusts to around 20 knots likely.

KD

&&

.FIRE WEATHER...
Issued at 256 PM CST Wed Feb 22 2017

Very deep mixing Friday afternoon and evening is likely with
winds at the top of the mixed layer of 35-50 knots. These winds
aloft should mix down fairly efficiently so wind gusts at the sfc
in the 30-35+ knot range are expected. Combination of strong WSW
surface winds and the warm/dry airmass behind dryline should yield
very low relative humidity values as well of 20-30 percent. These
two factors in conjunction with dry fuels should lead to at least
elevated fire danger for portions of the area with the possibility
of red flag warning criteria possible.

Gosselin


&&

.CLIMATE...
Record Highs

       2/22    2/23    2/24

STL: 78/1995 77/1996 81/1930
COU: 76/1995 73/1933 81/1930
UIN: 70/1922 70/1922 73/1930

Record High Minimums

       2/22    2/23    2/24

STL: 55/1985 55/1930 58/2000
COU: 53/1922 53/2000 61/1930
UIN: 57/1922 49/2000 55/1930


&&

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

$$

WFO LSX



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