Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 170857
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
357 AM CDT Tue Apr 17 2018

.SHORT TERM...  (Through Late Tonight)
Issued at 320 AM CDT Tue Apr 17 2018

Temperatures are already in the lower to middle 30s across the
area under clear skies with light winds. Should see frost in most
areas this morning with this setup. The surface ridge axis was
close to the Mississippi River at 08Z, and it will move off to the
east this morning. Winds will turn out of the east and southeast
under a mostly sunny sky today. While there should be ample
sunshine today, an easterly wind is not a favorable wind direction
for warming and forecast soundings are only showing mixing up to
900mb. Given the cold start, think high temperatures, while warmer
than yesterday, will still be below normal for the middle of
April.

With the low level ridge moving off to the east tonight, there
will be an increase in the low level jet after midnight that
increase moisture convergence across northeast Missouri and west
central Illinois. This will be ahead of a cold front that enters
eastern Missouri toward sunrise on Wednesday. There will also be
an increase in mid-level ascent ahead of this front caused by an
upper trough over the Great Plains. This should be enough to cause
a few showers across northeast Missouri and west central Illinois
late tonight and early Tuesday. Lows tonight will be at or above
normal given the clouds and the warm air advection ahead of the
cold front.

Britt

.LONG TERM...  (Wednesday through Monday)
Issued at 320 AM CDT Tue Apr 17 2018

The aforementioned trough over the Great Plains will pass just to
the north of the CWA on Wed due to the steering influence of a
weakening shortwave ridge over the central CONUS. Most of the
precipitation is expected to occur on the north side of the
system, although iso-sct SHRA is possible over northeast MO and
west central IL. The proximity of the surface low will produce a
tight pressure gradient across MO/IL, leading to windy conditions
across the area on Wed with sustained winds of 15-25 mph and gusts
of 30-35 mph.

An upper ridge then builds across the central CONUS, ensuring a
period of quiet weather through Saturday. By Saturday night, a
closed low over the Four Corners region will have drifted far
enough eastward to bring a chance of SHRA to the southwestern and
southern CWA, and the chance of rain will persist into Sunday.
There are some model disagreements regarding the track of this
system as it leaves the southwestern CONUS, and further
refinements to the forecast are likely between now and Saturday.
Some models take the low pressure system as far south as central
TX while others are slightly farther north. The current forecast
for the weekend reflects an ensemble approach which is neither as
far south as the southernmost solutions nor as far north as the
northernmost solutions.

There will be a tight temperature gradient across the CWA on Wed
due to the passage of the surface low through northern MO. Parts
of northeast MO and west central IL will be on the cool side of
the system and will likely only reach the 50s during the day, but
parts of southeast MO and southern IL could easily reach the
upper 60s to low 70s. Lows on Wed night will be in the mid to
upper 30s. Although the winds will probably remain high enough to
prevent widespread frost on Wed night, frost could develop in
sheltered areas across the southern CWA where less cloud cover is
expected. Widespread frost appears more likely on Thu night/Fri
morning when overnight RH values will be higher, temperatures
will be in the mid-30s areawide, and a surface high pressure
center over IA/MN will produce light winds and mostly clear skies
across the entire area. Temperatures then gradually moderate each
day from Thu through Mon.

Kanofsky

&&

.AVIATION...  (For the 06z TAFs through 06z Tuesday Night)
Issued at 1158 PM CDT Mon Apr 16 2018

The period is expected to be uneventful. Few mid to high clouds
and light/variable wind is forecast overnight into early Tuesday.
The only real activity comes at the very tail end of the 24 hour
period, as a weak disturbance approaches. This will have fairly
limited impact with VFR anticipate to hold through Tuesday
evening.

SPECIFICS FOR KSTL:

Very little to talk about in the next 24 hours. Few passing clouds
into Tuesday with light/variable winds shifting out of the ESE
Tuesday afternoon. Low pressure will pass well to the north
through Wednesday morning. Out of ahead of it, mid-level VFR
clouds will spread into the region Tuesday night with increasing
to 10+ kts.

Maples

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Frost Advisory until 9 AM CDT this morning for Boone MO-Callaway
     MO-Cole MO-Crawford MO-Franklin MO-Gasconade MO-Iron MO-
     Madison MO-Moniteau MO-Osage MO-Reynolds MO-Saint Francois
     MO-Washington MO.

     Freeze Warning until 9 AM CDT this morning for Jefferson MO-
     Lincoln MO-Montgomery MO-Saint Charles MO-Saint Louis City
     MO-Saint Louis MO-Sainte Genevieve MO-Warren MO.

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.

&&

$$

WFO LSX



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