Area Forecast Discussion
Issued by NWS St. Louis, MO

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058
FXUS63 KLSX 191001
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
401 AM CST Sun Feb 19 2017

.SHORT TERM...  (Through Late Tonight)
Issued at 356 AM CST Sun Feb 19 2017

Fog and stratus have developed early this morning across parts of
the area, especially over locations which received light rainfall
yesterday and which remained beneath mid/upper clouds for most of
the day followed by clearing within a few hours of sunset. Regional
METARs show areas of dense fog at times, therefore a Dense Fog
Advisory has been issued through 8am. Visibilities should improve
rapidly after sunrise.

Upper ridging, mostly clear skies after lingering morning
fog/stratus mixes out, a relatively warm start to the day, diurnal
mixing to around 900 hPa, and favorable winds with a southerly
component will contribute to another unusually warm February day.
That said, a few items suggest that today`s highs will not be as
warm as they could be: existing fog/stratus and how quickly it mixes
out this morning, BUFKIT depictions of diurnal mixing are not quite
as deep as they were on Friday, and winds will not have a favorable
southwesterly direction which adds a downslope component.

Kanofsky

.LONG TERM...  (Monday through Saturday)
Issued at 356 AM CST Sun Feb 19 2017

The synoptic pattern at the start of the long term period will
feature a fairly amplified regime, with an upper-level ridge
situated over the eastern CONUS northward into Canada, and a trough
sliding into portions of the Plains.  However, the bulk of the
energy associated with this trough will cut off across the southern
CONUS, which will allow for mostly zonal or southwesterly flow to
persist through the period.  This regime will keep very mild
temperatures in the forecast through the end of the week.

There continues to be two main systems of note in the long term
period, the first of which will come with the trough moving through
on Monday into Monday night.  As the main PV anomaly becomes cut off
across portions of the Southern Plains, the trough axis will take on
more of a positive tilt as it starts to become sheared out.
Nevertheless, it still appears sufficient isentropic ascent and
moisture convergence ahead of it will lead to the development of
showers across central Missouri as early as Monday morning, with the
best chances areawide coming Monday afternoon into Monday night as
the trough and associated cold front move through the region.  The
latest forecast soundings continue to suggest some weak elevated
instability, but the fairly weak synoptic forcing for ascent may
tend to keep any lightning fairly isolated in nature. Given the
weakening forcing as the system passes through from west to east,
think central MO stands the best chance to see higher rainfall
amounts (0.25" or so) with lesser amounts expected over eastern MO
and western IL (0.10" or so).

Despite the passing system, temperatures will continue to remain
very mild through the upcoming week.  In fact, have upped
temperatures on Wednesday given the 850mb thermal ridge settling
over the area with good WSW surface flow.  Assuming we see mostly
sunny to partly sunny skies through much of the day (could see some
opaque cirrus start to move in late), think temperatures should be
able to climb well into the mid and even upper 70s.  Would not be
surprised if a few of the warmer sites even make a run at 80 degrees
Wednesday afternoon! While Wednesday looks the warmest, temperatures
look to climb into the 70s Monday, Tuesday and Thursday too. A very
impressive stretch of warm weather for February.

The latest guidance has come into a bit better agreement regarding a
fairly deep surface low expected to develop over the Central Plains
and move into the Midwest.  The deepening low will help sharpen a
warm front across northern MO/western IL on Thursday into Thursday
night, along and north of which elevated convection is expected to
develop.  Further to the south in the warm sector, it appears that a
mid-level cap should help suppress much of the convection until
the system passes just to the east of the CWA. Guidance is still
locking in on this system, so the details are likely to change.
One thing that is looking more likely is that it will be rather
windy Friday as the system passes through. Colder air will attempt
to filter in on the backside of the system on the strong northerly
winds, but as temperatures get cold enough for any precip type
issues, the moisture will be exiting.

KD

&&

.AVIATION...  (For the 06z TAFs through 06z Sunday Night)
Issued at 1023 PM CST Sat Feb 18 2017

Stratus and fog was expanding late this evening across parts of
southeast MO and southwest IL. This stratus and fog will likely
impact the St. Louis metro area late tonight, particularly in CPS
where there was limited diurnal heating, mixing and drying
Saturday afternoon. The ceiling may be only around 500 feet late
tonight/early Sunday morning as the stratus advects into parts of
the St Louis metro area with vsbys dropping to around 1-2SM in
fog. The stratus and fog will dissipate by late Sunday morning
with just some high level clouds expected for the rest of the day.
The light surface wind will become south-southeasterly by Sunday
afternoon as the surface ridge shifts east of the region.

SPECIFICS FOR KSTL: Stratus and fog was expanding late this evening
across parts of southeast MO and southwest IL. This stratus and
fog will likely impact the STL area late tonight. The ceiling may
be only around 500 feet late tonight/early Sunday morning as the
stratus advects into parts of the St Louis metro area with vsbys
dropping to around 2-3SM in fog. The stratus and fog will
dissipate by late Sunday morning with just some high level clouds
expected for the rest of the day. The light surface wind will
become southeasterly Sunday afternoon as the surface ridge shifts
east of the region.

GKS

&&

.CLIMATE...
Record Highs

      2/19    2/20    2/21    2/22    2/23

STL: 77/2016 78/2016 79/1935 78/1995 77/1996
COU: 76/1930 76/2016 76/1935 76/1995 73/1933
UIN: 71/1930 72/2016 69/1930 70/1922 70/1922


&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Dense Fog Advisory until 8 AM CST this morning for Crawford MO-
     Franklin MO-Iron MO-Jefferson MO-Madison MO-Reynolds MO-
     Saint Charles MO-Saint Francois MO-Saint Louis City MO-
     Saint Louis MO-Sainte Genevieve MO-Washington MO.

IL...Dense Fog Advisory until 8 AM CST this morning for Bond IL-
     Clinton IL-Fayette IL-Jersey IL-Macoupin IL-Madison IL-
     Marion IL-Monroe IL-Montgomery IL-Randolph IL-Saint Clair
     IL-Washington IL.

&&

$$

WFO LSX



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