Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 161151
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
651 AM CDT Mon Apr 16 2018

.SHORT TERM...  (Through Late Tonight)
Issued at 255 AM CDT Mon Apr 16 2018

Main concerns are cold temperatures early this morning and again
tonight the potential for a frost/freeze. Will keep current freeze
warning going until 9 am. Have issued another freeze warning as
well as a frost advisory for tonight for parts of eastern Missouri
and southwest Illinois.

GOES-16 was showing clouds over much of Missouri and Illinois at
0845Z with considerable breaks developing in them in the past few
hours. Expect the clouds to begin clearing out of the area from
west to east today as the upper low over the western Great Lakes
moves off to the east. It will still be a chilly April day with
cold air advection and 850mb temperatures near -10C. Expect highs
today to only be in the 40s.

Then skies will be mainly clear tonight as a surface ridge moves
across the area with light winds and clear skies, temperatures will
fall to 32 or below along and north of I-64 in Illinois and I-70 in
east central Missouri. Have gone with a freeze warning once again
in these areas. South of the freeze warning, there will also
likely be some frost development where there will be good
radiational cooling and dewpoint depressions will be 5F or less.
Will also be watching central Missouri for an expansion of the
frost advisory, but held off for now as there may be some increase
in wind toward dawn as the surface ridge will move off to the
east and dewpoint depressions are a bit higher.

Britt

.LONG TERM...  (Tuesday through Sunday)
Issued at 255 AM CDT Mon Apr 16 2018

Quiet weather is expected on Tuesday as an upper ridge moves
across the region. Highs will range from the upper 50s (northeast
MO and west central IL) to around 70 (parts of southeast MO).
Such temperatures range from around 5-7 degrees below normal to
near normal for this time of year.

Meanwhile, an upper disturbance will have crossed the Rockies and
induced a surface reflection over the plains. The resulting low
pressure system is forecast to lift northeastward and pass north
of the LSX CWA on Tue night and Wed. Although a broad H85 LLJ is
noted overhead after 06z Wed, little pcpn is expected because the
warm front aloft will already be well to the north and east by
that time. The tight surface pressure gradient will produce breezy
conditions on Wed across the area.

The aforementioned low pressure system lifts into the Great Lakes
on Wed night, placing the LSX CWA within the cooler post-frontal
air mass in its wake. Temperatures will be cooler on Thursday
with highs only reaching the mid-50s to around 60. One item to
watch will be the potential for frost on early Friday morning. A
surface high is forecast to be over the northern plains and upper
Midwest with the ridge axis extending southward into MO/IL, and
good radiational cooling conditions would be conducive for frost
development if sufficient moisture is present. However, there are
still some questions about the amount of cloud cover due to a
developing low pressure system upstream. The 16/00z model runs
depict a very slow movement for this system as it eventually
passes south of the LSX CWA, and sChc-Chc PoPs look reasonable
for the weekend given this scenario.

Kanofsky

&&

.AVIATION...  (For the 12z TAFs through 12z Tuesday Morning)
Issued at 639 AM CDT Mon Apr 16 2018

High MVFR or low VFR ceilings are expected this morning before
clouds move out of the terminals this afternoon. Some scattered
flurries are possible at KUIN and the St. Louis are terminals
before 15Z. Then dry and VFR conditions are expected after 15Z.
North to northwest winds will gust up to 25 to 30 knots through
this afternoon before they diminish and become light and variable
tonight as high pressure moves through the area.

SPECIFICS FOR KSTL:

High MVFR or low VFR ceilings are expected through 19Z before
clouds move out of the terminals this afternoon. Some scattered
flurries are possible before 15Z. Then dry and VFR conditions are
expected after 15Z. North to northwest winds will gust up to 25
to 30 knots through this afternoon before they diminish and become
light and variable tonight as high pressure moves through the
area.

Britt

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Frost Advisory from 1 AM to 9 AM CDT Tuesday for Crawford MO-
     Franklin MO-Gasconade MO-Iron MO-Jefferson MO-Madison MO-
     Reynolds MO-Saint Francois MO-Sainte Genevieve MO-
     Washington MO.

     Freeze Warning until 9 AM CDT this morning 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.

     Freeze Warning from 1 AM to 9 AM CDT Tuesday for Lincoln MO-
     Montgomery MO-Saint Charles MO-Saint Louis City MO-Saint
     Louis MO-Warren MO.

IL...Frost Advisory from 1 AM to 9 AM CDT Tuesday for Monroe IL-
     Randolph IL.

     Freeze Warning until 9 AM CDT this morning for Bond IL-Calhoun
     IL-Clinton IL-Fayette IL-Jersey IL-Madison IL-Marion IL-
     Monroe IL-Randolph IL-Saint Clair IL-Washington IL.

     Freeze Warning from 1 AM to 9 AM CDT Tuesday for Bond IL-Calhoun
     IL-Clinton IL-Fayette IL-Jersey IL-Madison IL-Marion IL-
     Saint Clair IL-Washington IL.

&&

$$

WFO LSX



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