Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 221120

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
620 AM CDT Fri Mar 22 2019

.SHORT TERM...  (Through Late Tonight)
Issued at 333 AM CDT Fri Mar 22 2019

High pressure currently centered over Manitoba will slide south
through the upper and middle Mississippi Valley over the next 24
hours.  This surface high combined with an upper level ridge
building east from the Great Plains into the Mississippi Valley will
keep dry and tranquil weather over the forecast area through
tonight.  With little to no cloud cover today, I expect a strong
diurnal warmup in spite of northerly flow ahead of the incoming high.
A 20-25 degree rise looks reasonable after looking at yesterday`s
temperatures, particularly over western Missouri and eastern Kansas
which were closer to the upper level ridge much like our area will
be today.  Therefore, I leaned toward warmer MOS guidance and away
from some of the blended model guidance temperatures which are more
affected by bias correction and the cooler temperatures we`ve seen
over the past month.  This yielded highs ranging from the mid 50s in
south central Illinois to the mid 60s in central Missouri.  Lows
tonight will be highly influenced by the surface high as it drops
into eastern Illinois/western Indiana.  Light wind and a mostly clear
sky east of the Mississippi River will allow for excellent
radiational cooling so temperatures will likely drop below
freezing into the mid 20s to around 30.  There`s more uncertainty
across central and southeast Missouri where high clouds will be a
bit more prevalent and 5-8kt southeast flow will limit the
radiational cooling potential.  Temperatures should therefore be a
little warmer in the mid 30s in these areas.


.LONG TERM...  (Saturday through Thursday)
Issued at 333 AM CDT Fri Mar 22 2019

Split upper flow will prevail over the CONUS into early Monday.  The
main feature of note for the upcoming weekend is a storm system that
is currently over the Great Basin and should be over the central
High Plains by the start of the period.  The initial wave of
precipitation on Saturday should be from the increasing WAA and
moisture kicked out ahead of the slowly approaching storm system.
Higher probabilities for rain are then expected heading into late
Saturday night through Sunday night from west to east as the main
upper component of this storm system tracks just north of our region
and the surface low pressure reflection and its attendant cold front
slowly track through.  The best thunder potential still looks set
for Sunday, especially in the afternoon and evening, as the cold
front presence and peak heating during the afternoon combine to
maximize instability.

Passage of the surface cold front and upper level component to east
of the forecast area should occur Monday morning, ending rain

The split flow pattern is then expected to end by Monday morning,
transitioning to a progressive amplified flow, with a dampening
upper RIDGE sliding through by Wednesday and a digging TROF over the
western CONUS, effectively turning the NW upper flow over our region
for early next week into SW upper flow for late next week.

The NW upper flow Monday afternoon through Wednesday morning should
result in a period of quiet, dry weather.  The main highlight will
be a moderately strong and chilly Canadian airmass dropping down on
Monday and lingering into Tuesday, with its ridge axis located just
north of the forecast area early Tuesday morning.  There is some
minor disagreement amongst of the models on how strong the high
pressure will be, but good agreement on the bottom line, with
widespread min temps in the 20s expected.  If the growing season is
determined to be established then, this would constitute a freeze.
Max temps on Monday and Tuesday are expected to be the only two days
of below normal values for the next seven days, with the remainder
to be above normal.

The advent of SW upper flow for late next week should also herald
more active weather heading into the next weekend, already beginning
by Thursday with the return of mentionable PoPs.



.AVIATION...  (For the 12z TAFs through 12z Saturday Morning)
Issued at 559 AM CDT Fri Mar 22 2019

High pressure will control the weather through 12Z Saturday
morning. Therefore, VFR flight conditions are expected to prevail
across the area through early Saturday morning. Wind will turn
from the northwest today, to the east tonight and then the
southeast by early Saturday morning as high pressure moves into
the mid Mississippi and Ohio Valley.


VFR flight conditions are expected at Lambert through Saturday
morning. Northwest flow will turn slowly clockwise over the next
30 hours, settling down to a southeasterly direction by 12Z
Saturday morning.





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