Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 201655
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1155 AM CDT Fri Apr 20 2018

.SHORT TERM...  (Through Late Tonight)
Issued at 320 AM CDT Fri Apr 20 2018

A frost advisory continues for portions of Southeast Missouri and
Southwest Illinois. Latest surface analysis reflects temperatures
ranging from the lower 30s to the lower 40s. As of 07z, the
coldest locations are in Southeast Missouri, where winds are
calm/light, temperatures are in the lower 30s and dewpoint
depressions are only 1-3 degrees. This may be the best potential
for frost, as most locations outside of Southeast Missouri are
still several degrees milder. Light and variable wind may stay up
just enough the limit any widespread frost through early morning,
especially in warmer areas.

Surface high pressure over the Upper Midwest will remain in
control through Friday. As the high continues its slow eastward
progress, surface flow will shift out of the east-southeast with
an upper ridge building overhead through the course of the day.
Plenty of dry air is in place, so the main question is related to
any high clouds for late Friday afternoon. Should clouds hold
off, given lower surface dewpoints, dry surface and limited
green- up, it is conceivable that high temperatures could go a
few degrees above the going forecast. Overall, a pleasant spring
day is in store for the bi-state region.

Maples

.LONG TERM...  (Saturday through Thursday)
Issued at 320 AM CDT Fri Apr 20 2018

Limited concerns exist in the extended forecast with the potential
for a couple of quick hits of light rain and slightly below normal
temperatures.

The longwave pattern shifts east, bringing an upper low out of the
Southwest CONUS and into the plains by Saturday afternoon. A
strong surface high over the Great Lakes could play a role in
keeping the northern extent of the rain shield from making it too
far north. Plenty of dry, easterly flow continues to pump into
the region on the northern periphery of the moisture through the
weekend as rain develops over Southern Missouri and Illinois
Sunday. Measurable precipitation should remain south of I-70 with
the deepest moisture profiles lined up near and south of
Farmington in Missouri. The main question beyond this will be in
reference to the slow eastward exit of the low, as it remains cut
off from the main flow. Low chances for showers could extend into
early Monday over southern sections of Missouri and Illinois.

Further north, a ribbon of drier air could keep Northeast Missouri
and Central Illinois rain-free Monday through the first half of
Tuesday. We then become enveloped in northwest flow as a longwave
trough develops over the eastern CONUS. The next potential for
widespread, light rain originates out of the Pacific Northwest in
the form of a shortwave, which dives southeast through the
western side of the longwave trough. Chances for showers are
greatest late Tuesday into Wednesday, which may linger into early
Thursday.

Maples

&&

.AVIATION...  (For the 18z TAFs through 18z Saturday Afternoon)
Issued at 1146 AM CDT Fri Apr 20 2018

Ridge of high pressure centered over northwest IL will continue
moving slowly eastward. An east-southeast surface wind can be
expected through the period. VFR conditions will continue with
just an increase in mid-high level cloudiness expected tonight and
Saturday, well ahead of a low pressure system moving into the
southern Plains.

SPECIFICS FOR KSTL: Ridge of high pressure centered over northwest
IL will continue moving slowly eastward. An east-southeast
surface wind can be expected through the period. VFR conditions
will continue with just an increase in mid-high level cloudiness
expected tonight and Saturday, well ahead of a low pressure system
moving into the southern Plains.

GKS

&&

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

$$

WFO LSX


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