Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 230002
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
602 PM CST Fri Feb 22 2019

.SHORT TERM...  (Through Late Saturday Night)
Issued at 417 PM CST Fri Feb 22 2019

Focus remains the chances for storms through Sat afternoon and any
potential severe threat. Focus then turns to winds late Sat night.

Models have come into better agreement regarding the sfc low track.
The GFS now is a northern outlier, while the FV3/NAM/ECMWF and to
some extent the GEM, are closely clustered together.

Ongoing precip should slowly move north tonight. This precip
continues to be supported along the nose of stronger LLJ where some
strong moisture convergence. While models weaken this forcing
slightly, would still expect some light scattered showers to persist
into the evening. After a lull, expect rain showers to increase
again late tonight into Sat morning as a s/w lifts into the region
along with yet stronger moisture convergence further south. While
the bulk of the precip is expected to remain both north and south of
the CWA, believe a band of widely scattered showers will lift NE
through the region through mid morning.

As the upper low approaches, expect another s/w to eject into the
region during the afternoon hours coupled with a very strong LLJ.
This should support another round of showers, with a few
thunderstorms lifting NE through the region. Depending on how far
north the warm front can lift before the cold front arrives, there
may be a brief window for a threat of severe weather, mainly across
far southeast portions of the CWA. The 18z NAM has pushed the SBCAPE
slightly further south, but a threat along the SE CWA border
remains given high shear and 500+ J/kg of MUCAPE.

Precip should push north and east of the CWA during the evening
hours. As the cyclogenesis ensues and the low lifts NE into the
Great Lakes region, a strong 850mb jet and tight pressure gradient
should generate strong winds and very strong gusts. The GFS
continues to suggest higher gusts using Bufkit momentum transfer. If
these trends continue, a High Wind Warning may be needed late Sat
night for northern portions of the current Wind Advisory. Confidence
is not high enough for a watch at this time. However, the north to
northeast third of the CWA should see at least advisory criteria
winds. This advisory may also need to be expanded with future
updates.

Tilly

.LONG TERM...  (Saturday Night through Next Friday)
Issued at 430 PM CST Fri Feb 22 2019


The aforementioned low pressure system will continue to affect the
LSX CWA even after precipitation has ended. The tight pressure
gradient between a 1040+ hPa surface high building into the northern
CONUS and the sub-980 hPa surface low associated with the departing
low pressure system over the Great Lakes will yield strong and gusty
northwest winds across the entire region, and these winds will
persist through the day on Sunday. A Wind Advisory has been issued
from midnight on Sunday through 6pm on Sunday for roughly the
northeastern one-third of the LSX CWA. This is the area where the
strongest sustained winds and strongest wind gusts are currently
anticipated. Even though BUFKIT soundings show that the lowest
levels remain well-mixed through Sunday night, they also show that
the winds aloft will have decreased sufficiently by 6pm to end the
concern for widespread impacts due to gusty winds.

A secondary cold front is currently expected to move through the
region on Sunday night, bringing a temporary return of unseasonably
cold temperatures to the region. Overnight lows will range from the
lower teens to lower 20s, which would be around 5-10 degrees colder
than normal.

Model solutions then depict a quasizonal upper air pattern from
Monday through the middle of the week, although there are slight
differences between the models-- for example, the ECMWF shows an
initially stronger NW flow component than the GFS. One or more weak
disturbances will likely move through the region between Monday and
Thursday, but the current forecast keeps their influence well to the
north. That said, it is possible that one of these features could
ultimately be slightly farther south than currently forecast and
could bring a slight chance of rain or snow to parts of the northern
CWA. Overall, the current temperature forecast calls for highs in
the 30s to 40s, which would be 5-15 degrees colder than normal.
However, models also show a quasistationary boundary stretching from
southern KS through either southern MO or northern AR. Depending on
the precise location of this feature, temperatures could be warmer
than currently forecast across the southern half of the LSX CWA.

Model differences become much more apparent by 12z Thursday, and
this leads to low certainty regarding temperature and precipitation
trends for the end of next week.

Kanofsky

&&

.AVIATION...  (For the 00z TAFs through 00z Saturday Evening)
Issued at 544 PM CST Fri Feb 22 2019

A few bands of showers will move northeast through the area this
evening mainly. The heavier rain within these bands may briefly
drop the visibility to 2-3sm if there is a direct terminal impact.
Otherwise deteriorating flight conditions are expected this
evening into the overnight hours as low stratus spreads north-
northeast across the area lowering flight conditions first to MVFR
and then IFR. Another uptick in showers is expected overnight into
the morning on Saturday and at this point IFR flight conditions
will already be widespread. A band of thunderstorms may impact the
St. Louis area terminals between 19-21z. Improving flight
conditions are expected by the end of the valid TAF period along
with gusty southerly winds.

SPECIFICS FOR KSTL:

A band of showers will move northeast through the terminal this
evening. The heavier rain within the band may briefly drop the
visibility to 3sm if there is a direct terminal impact. Otherwise
deteriorating flight conditions are expected this evening into the
overnight hours as low stratus spreads north-northeast across the
area lowering flight conditions first to MVFR this evening, and
then IFR well overnight. Another uptick in showers is expected on
Saturday morning, and at this point flight conditions will already
be IFR. A band of thunderstorms may impact KSTL between 19-21z.
Improving flight conditions are expected by early Saturday
evening along with gusty southerly winds.

Glass

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Wind Advisory from midnight Saturday night to 6 PM CST Sunday
     for Knox MO-Lewis MO-Lincoln MO-Marion MO-Monroe MO-Pike MO-
     Ralls MO-Shelby MO.

IL...Wind Advisory from midnight Saturday night to 6 PM CST Sunday
     for Adams IL-Bond IL-Brown IL-Calhoun IL-Fayette IL-Greene
     IL-Jersey IL-Macoupin IL-Marion IL-Montgomery IL-Pike IL.

&&

$$

WFO LSX


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