Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 210938
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
338 AM CST Thu Feb 21 2019

.SHORT TERM...  (Through Late Tonight)
Issued at 334 AM CST Thu Feb 21 2019

A longwave upper TROF remains firmly entrenched over the western
CONUS early this morning resulting in SW flow aloft over our region.
A sheared region of vorticity from Indiana to Oklahoma and the right
entrance region of a 140kt H300 jet was aiding the streaming of
cirrus clouds into our region from the southwest.  The column below
20kft was quite dry across the forecast area as an area of high
pressure dominated at the surface. Temperatures for most locations
were in the upper 20s and lower 30s.

A rare dry day is expected today as the column will remain dry
except for continued streaming of cirrus clouds from the southwest.
They should get thicker as the day wears on, resulting in partly
cloudy skies currently for many areas becoming mostly cloudy or
cloudy by sunset.  Conditional climatology suggests a 15-20F diurnal
rise for max temps this afternoon, and this strongly favors the
cooler MET MOS for COU and STL metro, and a blended approach to MOS
elsewhere.  The result is max temps in the 40s, or within a couple
degrees of seasonal normals.

H850 WAA and moisture convergence will begin increasing across
southern MO and far southern IL this evening and is worth continuing
a low PoP for the far southern CWA.  As a weak 30kt low level jet
begins to take shape late tonight and noses into southeast MO, this
may be able to either justify a higher PoP or push mentionable PoPs
a bit further north prior to 12z/Tue, but the general consensus is
this will hold off until then.  Precipitation-types should be rain,
but a quicker northward surge would result in a light wintry mix--
something to keep in mind on updates.  Min temps overnight will be
close to persistence for much of the forecast area (areas near and
north of I-70), with readings bottoming out between 25 and 30. This
is close to MOS values.  Where thicker clouds reside in southeast
MO, min temps will be the highest in the mid 30s, and this is
several degrees above MOS.

TES


.LONG TERM...  (Friday through Wednesday)
Issued at 334 AM CST Thu Feb 21 2019


(Friday)

There is a chance of rain showers on Friday across much of the
area as a vorticity maxima transverses the mid-Mississippi Valley
embedded in southwest flow aloft. Temperatures should be near
persistence with highs in the low to mid 40s.


(Friday Night - Saturday Night)

Chances for showers and elevated convection will ramp up Friday
night as low-level warm/moist advection strengthens in conjunction
with increasing mid/upper level diffluence. Temperatures Friday
night will be well above normal with persistent southeasterly
surface winds and cloudy conditions. Lows are expected to be in the
mid 30s to low 40s, with these minimums being reached around
midnight. Temperatures should slowly warm late Friday night.

The main concern in the extended portion of the forecast continues
to be on the potential for strong to severe thunderstorms Saturday
afternoon and possibly into the early evening hours. There is still
uncertainty with the track of this system, with the GFS/ECMWF about
50 miles further north/northwest than the CMC/NAM. The further
northwest solutions of the GFS/ECMWF probably have more merit
because rapidly deepening systems historically trend a bit northwest
than modeled several days out. Because of this factor, leaned toward
the GFS/ECMWF for the track of this system and boosted high
temperatures on Saturday for the southeastern half of the CWA. In
addition to the track of this system, the other key will be how far
north (and how rapidly) the warm front gets Saturday afternoon.
There is a high consensus in model guidance of a narrow warm sector
characterized by marginal instability for surface-based convection
(100-400 J/kg) and very strong low and deep-layer shear. How far
north this warm sector gets into our area, if at all, remains to be
seen. One main mitigating factor for severe weather is that elevated
convection should be prevalent along and poleward of the retreating
warm front. This precipitation activity very well could retard or
delay its northward progress. However, south of this boundary,
strong to severe wind gusts would be the primary threat associated
with what likely will be a QLCS that develops in Arkansas and moves
northeastward. Embedded vortices would also be favored within the
QLCS given 0-3km shear vectors orthogonal to the linear convection.
In conclusion, while the severe setup is highly conditional across
parts of southeast Missouri and southwest Illinois, if severe
weather were to occur, it may be of high impact.

The chances of thunderstorms will end by late Saturday evening, with
much of the CWA within the dry slot of the mature midlatitude
cyclone. There is still a low chance of getting clipped by the
deformation zone in parts of northeast Missouri and west-central
Illinois late Saturday night, with light rain possibly mixing with
or briefly changing over to snow before ending. At this time, even
the further south track of the NAM would likely not result in any
accumulations across northwestern sections of the forecast area.


(Sunday - Wednesday)

Seasonable temperatures with mostly dry weather is forecast from
Sunday through Tuesday. A midlevel shortwave trough moves through
the Upper Midwest Tuesday night with an associated cold front moving
through the CWA. Light rain is possible with this feature, with rain
changing to snow possibly behind the boundary which appears to
become more anafrontal in nature with time.

Temperatures on Wednesday should return to below normal behind this
aforementioned cold front. Expect highs to be mainly in the 30s with
coldest conditions in northern portions of the CWA.


Gosselin


&&

.AVIATION...  (For the 06z TAFs through 06z Thursday Night)
Issued at 1131 PM CST Wed Feb 20 2019

There remains limited potential for some patchy radiational fog
for a short time from around 10-13z, therefore have maintained the
TEMPO for 5SM BR for all terminals but KSTL. Otherwise VFR flight
conditions are expected through the valid TAF period with high
clouds across the region.

SPECIFICS FOR KSTL:

VFR flight conditions are expected through the valid TAF period
with high clouds across the region.

Glass

&&

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

$$

WFO LSX


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