Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 220823

Area Forecast Discussion
National Weather Service Saint Louis MO
323 AM 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.


.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

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.



.AVIATION...  (For the 06z TAFs through 06z Sunday Night)
Issued at 1153 PM CDT Sat Apr 21 2018

Light rain showers are expected to spread east and northward
overnight and into Sun. KUIN will remain VFR and dry thru the
period with ely winds. Main question will be how far north the
precip will spread. Have delayed precip somewhat, but believe
that showers will move north to impact KCOU/KSUS/KCPS toward
sunrise. The precip shield shud sink back south thru the morning
hours, with a second round possible during the afternoon hours.
That said, can not rule out showers moving into and impacting the
terminals off and on throughout much of the day. A second
question will be how far north MVFR cigs will develop. Latest
guidance suggests MVFR cigs shud remain south of the terminals,
albeit not far south. Best chances for lower cigs are during the
Sun evening hours and will continue to monitor as new data
becomes available.

SPECIFICS FOR KSTL: Have backed off timing of onset of precip to
around sunrise. As this initial band dissipates, the focus shud
return south of the terminal. However, isod to sct showers shud be
possible thru the afternoon hours. Questions still remain how far
north MVFR cigs will develop. Have kept a sct deck in when MVFR
cigs are more likely, but lower cigs may remain south as dry air
continues to filter into the region.



Saint Louis     64  50  61  50 /  40  60  60  20
Quincy          66  46  65  48 /  10  10  10  10
Columbia        62  48  63  48 /  50  20  30  10
Jefferson City  62  49  64  49 /  70  40  30  10
Salem           66  51  60  50 /  40  80  60  30
Farmington      58  49  59  48 /  90  90  60  30




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