Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 180004

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
604 PM CST Sat Feb 17 2018

.SHORT TERM...  (Through Late Sunday Afternoon)
Issued at 305 PM CST Sat Feb 17 2018

A strong upper level disturbance was located over northwestern IL
this afternoon.  This feature helped to drive pcpn development over
northeast MO and west-central IL during the late morning and early
afternoon at the same time WAA aloft was the dominant force for pcpn
development and movement further to the south across much of the
forecast area.  The forcing for all areas has since moved off to the
east, leaving behind areas of drizzle that are exiting hastily to
the east.  Widespread low cloudiness has kept temps down all day in
the 30s, but some clearing is occurring over northeast and central
MO, where temps are rising into the 40s.

Skies are expected to clear for all areas by 9pm, and with high
pressure settling in and moving thru, look for near calm winds for
much of the night as well.  With a wet ground from the wet snow and
rain that fell today and a late clearing, the calm winds and clear
skies should provide good conditions for radiational fog.  The only
item that will reduce this potential is drier air also advecting in
with the high pressure build.  Have continued the mention of patchy
to areas of fog for overnight with the evening shift monitoring any
need for any further upgrades.  High pressure is expected to be
progressive enough where southerly winds increasing late for areas
north and west of STL metro should prevent much fog there.
Otherwise, look for min temps in the 25 to 30 range.

Flow aloft already begins to back SW again on Sunday but any
disturbances to create lift will hold off until nightfall, leaving
what should be a dry day with increasing and thickening clouds late
in the day.  A tightening pressure gradient at the surface with
southerly flow will result in wind speeds of 15 to 20mph with gusts
to 35mph.  This strong south flow will propel temps well into the
50s on Sunday afternoon with 60F possible for parts of central and
east-central MO.


.LONG TERM...  (Sunday Night through Next Saturday)
Issued at 305 PM CST Sat Feb 17 2018

(Sunday Night - Tuesday Night)

***Active weather pattern early next week...beneficial rainfall

Warm and active period will begin on Sunday night as strong low-
level warm/moist advection intensifies across the area. Best chance
for shower activity (and potentially a few thunderstorms) will be
across portions of south-central Illinois where strongest forcing
is forecast to reside. Temperatures on Sunday night will be on the
mild side with a non-diurnal swing as a warm front quickly moves
through the CWA.

Chances of showers and possibly a few storms continue through Monday
and Monday night, but not expecting an all day or night rain.
Expecting there will be breaks but weak vorticity maxima will
continue to move rapidly northeastward within the deep southwest
flow aloft. Probably will be one of those cases where most areas
receive some measurable rainfall both on Monday and Monday night,
just with quite a bit of dry time between showers. Monday still
appears to be the warmest day of the forecast period with 850-hPa
temperatures near +10C and at least some breaks in the rainfall.
Shouldn`t be looking at anything as warm as observed this past
Thursday due to the possibility of rain and high chances of
overcast sky conditions. That being said, still expecting highs in
the upper 60s to low 70s across much of the area which is about
20-30 degrees above normal for mid/late February.

Main focus for heavier rainfall continues to look to be on Tuesday
into early Tuesday night. Showers with embedded thunderstorms will
focus along and slightly poleward of a cold front which will only
slowly progress southeastward in time. Still some placement and
timing differences with this cold front, but consensus is that a
couple inches of rainfall will be possible across much of the
southeastern half of the area Tuesday-Tuesday night with locally
higher amounts possible. Main result of this expected rainfall would
be very beneficial to area rivers/streams as they are very low at
the moment due to the ongoing drought. Soil moisture should also
help be recharged. Due to these antecedent conditions, not
expecting much negative impact at this time (if any), but cannot
rule out a small chance at some flash flooding or minor river
flooding. There are some signals of the potential for heavy
rainfall rates with very deep warm cloud depths (~10 kft) and
precipitable water values of 1.25-1.50" which nears or exceeds
record values for KSGF and KILX within the month of February. If
the cold front`s progress slows down more or stalls out across
southeastern sections of the area, the threat for river flooding
and flash flooding would increase.

Another potential concern is how much post-frontal precipitation
there will be and how quickly surface temperatures drop below
freezing behind the front. Large disagreement between the GFS and
ECMWF. While both have rapidly dropping temperatures as a shallow
Canadian airmass builds into the area, the ECMWF has a narrow axis
of moderate precipitation in the form of freezing rain potentially
mixing with or changing over to sleet before ending. Meanwhile, the
GFS looks to be a classic Schmocker rule scenario with precipitation
ending before the subfreezing air advects into any one location.

(Wednesday - Next Saturday)

Large model disagreement continues into the middle of next week. The
ECMWF has another wave within the southwest flow aloft which brings
another round of sleet and freezing rain to much of the area whereas
the GFS and CMC are basically dry until Thursday night. Favored the
agreement between the deterministic GFS/CMC which is also supported
by nearly every GEFS ensemble member.

Broad model agreement in another moderate rainfall event for late
week or early next weekend. Highest chances for rainfall at this
early juncture appear to be Friday/Friday night.



.AVIATION...  (For the 00z TAFs through 00z Sunday Evening)
Issued at 539 PM CST Sat Feb 17 2018

VFR and dry with west/northwest winds for all TAF sites this
evening as surface ridge has moved into the region. Light winds
associated with high pressure building in, clear skies and wet
ground from the snow/rain today look to reduce visibility through
the overnight hours for the metro sites. As fog dissipates Sunday
morning winds will back to the south and become gusty.


Patchy fog after 6z for STL. Visibilities look to stay MVFR but
could drop to IFR for some periods overnight. After sunrise fog
will dissipate and winds will back to the south and become gusty.
Potential for LLWS Sunday late afternoon into evening. Confidence
too low at this point to include in TAF. Will revisit for next





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