Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 201953
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
153 PM CST Wed Feb 20 2019

.SHORT TERM...  (Through Late Thursday Afternoon)
Issued at 153 PM CST Wed Feb 20 2019

Area remains under SW flow aloft through Thurs with some weak
ridging aloft tomorrow. At the surface, a weak ridge will build into
the area tonight, with the axis lifting through the region Thurs.

Low clouds remain in place this afternoon, other than a few holes in
the clouds that has allowed a brief period of insolation. Expect low
clouds to clear out of the area early this evening. However, model
solns continue to suggest high clouds moving back into the region
this evening and becoming more widespread and thicker late tonight
into Thurs. Believe models also continue to hold on to too much snow
cover today, with this lingering into tonight. Have therefore
trended toward warmer guidance tonight.

As the surface ridge settles over the area tonight, expect winds to
become calm. With dewpoints above freezing, would expect any snow to
continue to melt this evening, adding to the low level moisture.
Currently, the best conditions appear to be across the southern half
or so of the CWA. However, if northern portions can remain free of
clouds longer, some fog may develop here as well. While not a great
setup, have mention of patchy fog for these areas late tonight into
Thurs morning.

For Thursday, believe many of the model 2m temps are again too cold
due to the snow cover mentioned above. MOS is also in good agreement
across the region for Thurs. Regardless, with the ridge axis lifting
north of the area, temps should rebound. Have therefore trended just
below the MOS guidance for highs on Thurs despite not being a great
warming setup.

Tilly

.LONG TERM...  (Thursday through Tuesday)
Issued at 324 AM CST Wed Feb 20 2019

(Thursday - Friday)

An area of surface high pressure will be centered over the mid-
Mississippi Valley Thursday morning. This high should slide
northeastward into the Great Lakes region Thursday night and Friday,
allowing for a slight moderation in temperatures by Friday
afternoon. Seasonable temperatures along with a relatively dry
period of weather is likely, though a small chance of showers
returns to southern sections of the forecast area Friday afternoon
as low-level warm air advection intensifies ahead of the next
approaching storm system.


(Friday Night - Saturday Night)

The main focus in the extended portion of the forecast continues to
be on a round of showers and possible strong to severe thunderstorms
Saturday afternoon due to a strong low pressure system moving just
to the northwest of the CWA. Many severe weather parameters are very
strong including low and deep-layer shear and low-level helicity.
The key will be the amount of destabilization realized during the
afternoon hours ahead of a dryline moving in from the west. With
such a dynamic system, it probably will not take much instability to
get severe weather. Currently, models do not suggest very much
surface-based CAPE (a couple hundred J/kg) due to ongoing
precipitation. In addition, model guidance suggests that the system
may quickly occlude late Saturday, which would help pinch off the
poleward advection of higher surface dewpoints into the CWA. At this
point in time, the severe weather threat appears higher to the
southeast of the CWA, but this system will need to be monitored as
high shear/low CAPE severe cases on the cool season can be very
impactful.

The other main story will be the very strong winds/wind gusts out of
the northwest immediately behind the departing surface low. Wind
gusts of 40-45 knots appear possible across northeast Missouri and
west-central Illinois Saturday night where the strongest pressure
gradient will reside.

Temperatures within the warm sector of this system are expected to
be quite warm, with highs in the 50s and 60s on Saturday. These
values are about 15 degrees above normal for late February. Cold
advection in the wake of the departing low should get lows Saturday
night back closer to normal, with temperatures bottoming out in the
upper 20s to 30s. This may lead to rain mixing with or changing over
to light snow before ending across far northern areas before the
precipitation shifts off to the north and east.


(Sunday - Tuesday)

Dry weather with seasonable temperatures is likely Sunday through
Monday. Another system moves from the central Plains across the
Mississippi River Valley between Monday night and Tuesday, which
will bring a chance of rain/snow to the region. Currently, the track
of this system will likely bring the highest chances of measurable
precipitation to parts of northeast Missouri and west-central
Illinois.

Gosselin

&&

.AVIATION...  (For the 18z TAFs through 18z Thursday Afternoon)
Issued at 1127 AM CST Wed Feb 20 2019

Expect conditions to improve throughout the afternoon and into the
evening hours. The DZ has also come to an end across most of the
area. Model guidance suggests some fog development possible at
KCOU and KUIN late tonight into Thurs morning. There are some
questions regarding cloud cover over night that may prevent the
fog development. Have only added a tempo group for now and will
continue to monitor.

SPECIFICS FOR KSTL/KSUS/KCPS: Expect conditions to continue to
improve this afternoon and into the evening. Terminals will
probably become scattered briefly this afternoon before more MVFR
cigs move back into the region. Otherwise, expect cigs to lift
above 2 kft late this afternoon, then VFR tonight.

Tilly

&&

.PRELIMINARY POINT TEMPS/POPS...
Saint Louis     44  31  48  32 /   5   0   5   5
Quincy          37  27  40  27 /   5   0   0   5
Columbia        38  28  48  30 /   5   0   5   5
Jefferson City  41  28  49  30 /   5   0   5   5
Salem           44  30  46  32 /  20   5   5   5
Farmington      45  29  47  34 /  10   0   5  10

&&

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

$$

WFO LSX


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