Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 191654
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1154 AM CDT Thu Apr 19 2018

.SHORT TERM...  (Through Late Tonight)
Issued at 303 AM CDT Thu Apr 19 2018

Have issued another freeze warning and frost advisory for the
southern half of the CWA tonight into early Friday morning.

GOES-16 imagery showed that there were considerable breaks in the
clouds early this morning. Low level flow will become less
cyclonic through mid morning and drier air will move into the area
through early this afternoon per the RAP. This should cause
decreasing clouds through the late morning into the afternoon
hours. With the cold start and cold air advection today, highs
will still be 10-15 degrees below normal.

Skies will turn clear tonight and winds will be light as a
surface ridge moves through the area. This will set up strong
radiational cooling with temperatures falling into the lower to
middle 30s. Went with a freeze warning over south central Illinois
where I expect sub-freezing lows, and a frost advisory in all of
the other counties that the growing season has started.

Britt

.LONG TERM...  (Friday through Wednesday)
Issued at 303 AM CDT Thu Apr 19 2018

Quiet weather is expected through Saturday evening. A closed
upper low will move from the Four Corners region into the Southern
Plains and then towards the southeastern CONUS over the next few
days. Although the surface low associated with this feature will
pass south of the LSX CWA, it will be close enough to spread
isolated to scattered showers into parts of southern and
southeastern MO as well as southern IL this weekend. Given the
limited instability this far north, at this time only rain
showers (and not thunderstorms) are expected on Sat/Sun. Since
there will be several smaller PV anomalies rotating around the
upper low, it is possible that the additional lift could support
sprinkles farther north than is currently forecast. The entire
low pressure system will be moving into an upper ridge, and given
that the upper low will also be closed rather than an open wave,
it will tend to progress slowly. This will keep a chance of rain
lingering into Monday across the far southern CWA.

By the middle of next week, a northern stream PV anomaly is
forecast to move into the Northern Plains. The trailing cold front
associated with this system will bring a chance of rain to the
area during the middle of the week, although there is still
sufficient variability between models to muddy the details.

After a cool start to the morning on Friday, temperatures should
warm into the upper 50s to lower 60s by afternoon due to mostly
clear skies and strong April insolation. Easterly surface flow as
well as cloud cover associated with the closed low passing south
of the area will tend to keep temperatures cooler than seasonal
normals over the weekend. Highs should be around 5-10 degrees
colder than average. By early next week, decreasing clouds and
weak south to southwest flow ahead of an approaching trough should
help boost temperatures into the mid/upper 60s, which is much
closer to typical values for this time of year.

Kanofsky

&&

.AVIATION...  (For the 18z TAFs through 18z Friday Afternoon)
Issued at 1137 AM CDT Thu Apr 19 2018

A ridge of high pressure extending from eastern SD south through
eastern KS will move eastward. Northerly surface winds will become
light this evening, then east-southeasterly on Friday as the
ridge axis shifts east of the taf sites. MVFR, stratocumulus
field in the St Louis metro area will gradually lift in height to
the VFR category this afternoon, while at the same time
scattering out as drier air advects southward into the area.

SPECIFICS FOR KSTL: A ridge of high pressure extending from
eastern SD south through eastern KS will move eastward. Northerly
surface winds will become light this evening, then easterly on
Friday as the ridge axis shifts east of the taf sites. MVFR,
stratocumulus field in STL will gradually lift in height to the
VFR category this afternoon, while at the same time scattering out
as drier air advects southward into the area.

GKS

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Frost Advisory from 1 AM to 9 AM CDT Friday for Boone MO-
     Callaway MO-Cole MO-Crawford MO-Franklin MO-Gasconade MO-
     Iron MO-Jefferson MO-Lincoln MO-Madison MO-Moniteau MO-
     Montgomery MO-Osage MO-Reynolds MO-Saint Charles MO-Saint
     Francois MO-Saint Louis City MO-Saint Louis MO-Sainte
     Genevieve MO-Warren MO-Washington MO.

IL...Frost Advisory from 1 AM to 9 AM CDT Friday for Bond IL-Calhoun
     IL-Clinton IL-Jersey IL-Madison IL-Monroe IL-Randolph IL-
     Saint Clair IL-Washington IL.

     Freeze Warning from 1 AM to 9 AM CDT Friday for Fayette IL-
     Marion IL.

&&

$$

WFO LSX



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