Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 221755
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1255 PM CDT Sun Apr 22 2018

.SHORT TERM...  (Through Late Tonight)
Issued at 322 AM CDT Sun Apr 22 2018

Impressive closed low continues to spin its way slowly eastward this
morning.  GOES-16 water vapor imagery is showing a well developed
warm conveyor belt rising up through the lower Mississippi Valley
into the Midwest ahead of the low.  What`s been less than impressive
is the performance of some of the short range model consensus PoPs.
Think this is due to their heavy skew toward HRRR simulated
reflectivity and other CAMs for short range PoPs.  Simulated
reflectivity products would lead one to believe that there should be
scattered/widely scattered showers across a large portion of the
area, however surface dew point depressions are 20+ degrees and
forecast soundings show a very dry layer in the low levels from the
surface up to around 5000 feet.  That dry layer is forecast to
moisten up quickly across southeast and parts of central Missouri
over the next few hours...and indeed a few sprinkles have already
been reported in central Missouri.  At any rate, what seems to be
working the best for accumulating precip at this time is the RAP QPF
so have based my PoPs through the short term on it.  The RAP slings
two distinct waves of low level moisture convergence around the
northeast quadrant of the low as it moves east through Arkansas
today and into the evening.  The waves of moisture convergence lose
strength as they move further north-northwest around the low, so the
vast majority of the QPF is across central and southeast Missouri,
and southwest/south central Illinois so this is where I have
concentrated the highest PoPs for the next 24 hours.  The column
does moisten up further north as well, so have mid-high chance
wording spreading up along and north of I-70 as well...though think
precip amounts will be pretty anemic.  Temperatures will be very
dependent on precipitation for the next 24 hours, but should stay
near seasonal normals with highs in the 60s and lows in the mid 40s
to around 50.

Carney

.LONG TERM...  (Monday through Saturday)
Issued at 322 AM CDT Sun Apr 22 2018

A formidable area of high pressure situated over the east coast will
act as a temporary blocking mechanism Monday. It will slow the
eastward progress of a closed upper low and its coinciding surface
low, both of which will pivot around the bootheel of Missouri/
Southern Illinois. Showers will be likely through Monday morning,
especially east of the Mississippi River and could linger into
early Tuesday morning.

The upper low eventually moves into the Ohio Valley Region Tuesday
and evolves into a longwave feature, as the surface low briefly
stalls before broadening over the southeast U.S. Mid and low level
moisture evacuates the area for a brief time through the day
Tuesday, which will serve up some dry time for much of the day
Tuesday.

Guidance continues to point to an upper level shortwave dropping in
from the northwest and onto the back side of the previous system. A
weak cold front and light showers accompany this wave through the
first half of Wednesday. Trends would suggest this to be a quick hit
of light precip, which will be out of here by Wednesday afternoon.

A second and stronger front approaches the region on Thursday. This
brings another quick round of showers Thursday afternoon with a
reinforcing shot of cold air for Friday. While high temperatures
most of the week will be in the 60s, northwest flow aloft will take
over Friday, as we may struggle to reach 60.

Cooler conditions will not last long. Surface high pressure and
upper ridging begins to build in from the west late in the period.
Sunshine returns next weekend with temperatures pushing well into
the 60s.

Maples
&&

.AVIATION...  (For the 18z TAFs through 18z Monday Afternoon)
Issued at 1247 PM CDT Sun Apr 22 2018

Upper low centered over north central AR will continue to slide to
the east northeast through the forecast period. Will see several
rounds of light rain rotate around system through taf sites,
mainly ones along I-70 corridor, while KUIN to stay dry, though a
few sprinkles not out of the question for there. As for cigs, to
remain VFR til better chances of rain moves in early this evening,
so will see cigs dip down to MVFR after 04z Monday. Winds to
persist from the east, then gradually back to the northeast
towards end of forecast period.

SPECIFICS FOR KSTL:
Upper low centered over north central AR will continue to slide to
the east northeast through the forecast period. Will see several
rounds of light rain rotate around system through metro area. As
for cigs, to remain VFR til better chances of rain moves in early
this evening, so will see cigs dip down to MVFR after 04z Monday.
Winds to persist from the east, then gradually back to the
northeast after 12z Monday.

Byrd
&&

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

$$
WFO LSX


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