Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 161800

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
100 PM 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.


.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.



.AVIATION...  (For the 18z TAFs through 18z Tuesday Afternoon)
Issued at 1240 PM CDT Mon Apr 16 2018

MVFR ceilings at KUIN will lift to VFR around 20z. Otherwise, dry
and VFR conditions will persist through the TAF period. Gusty
northwest winds will gust up to 20 to 25 knots before weakening
just after sunset. Light northwest winds tonight as  high
pressure pushes into the area. Winds will veer to the south late
Tuesday morning as high pressure pushes east away from the region.

SPECIFICS FOR KSTL: Dry and VFR conditions expected over the
terminal through Tuesday afternoon. Gusty northwest winds of 20 to
25 knots will diminish shortly after sunset. Light northwest winds
overnight will veer to the south and remain light by late Tuesday



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 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 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.



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