Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 261149

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
649 AM CDT Thu Apr 26 2018

.SHORT TERM...  (Through Late Tonight)
Issued at 313 AM CDT Thu Apr 26 2018

Primary focus in the short term is this morning`s lingering shower
activity. The upper low and better lift associated with the H8 vort
will continue to pivot southeast through Arkansas into W. Tennessee.
Dry east/northeast surface flow will cut into the low level
moisture, effectively lowering rain chances from north to south.
It`s reasonable that much of the area will be done with the rain in
the next 4-6 hours. Far southern fringes of the forecast area may
see scattered showers linger through 13z-14z. Otherwise, measurable
precip is pretty much long gone for most locations by 12z. We will
recover pretty well with returning sunshine and temperatures pushing
well into the 60s to near 70 degrees.

It remains pretty quiet through the remainder of the short term.
Just a few days ago, it looked as though a decent frontal boundary
would move through late Thursday into Friday morning to give us
another chance of rain. Trends have really backed off on any chances
with very limited moisture. Instead, the elongated trough axis will
extend from the Great Lakes through the Plains, serving better as a
wind shift. I see much less in the way of CAA with 850 temps
momentarily dropping 2-3C. The boundary washes out as it pushes
through Friday morning with limited noticeable change.


.LONG TERM...  (Friday through Wednesday)
Issued at 313 AM CDT Thu Apr 26 2018

High amplitude weather pattern with a deep longwave trof over the
eastern U.S. and a strong ridge stretching from northern Mexico up
across the Rockies into southwestern Canada is forecast to remain
nearly stationary into Saturday.  Northwest flow aloft over the
Mississippi Valley will drive another cold front south through the
Midwest on Friday and Friday night.  Guidance seems to be in good
agreement on the movement of the front through Missouri and the
southern 1/2 of Illinois on Friday night.  There may be a few
showers over south central Illinois as the front moves through, but
it looks dry over Missouri.

The front should be down over the boot heel of Missouri and into
western Kentucky and northern Tennessee by 12Z Saturday morning.
Models are very consistently showing strong cold advection behind
the front as a 1025+ Canadian high drops into the Midwest.  Ensemble
MOS temperatures for Saturday and Saturday night keep falling a few
degrees with each run. Originally looked like highs around 70
Saturday afternoon...but latest guidance struggles to push us out of
the low to mid 60s.  2 meter temps off the raw models are even
colder in the mid 50s to low 60s!  It doesn`t look like we`ll set
any records for coolest maximum temperature for April 28, but it may
get within 5-10 degrees. Regardless, it does look like there will be
plenty of strong late April sunshine which will make it feel warmer
than it really is.

The high will shift southeast Saturday night.  Light wind and clear
sky should produce excellent radiational cooling conditions.  2
meter temperatures at 12Z on Sunday morning are approaching levels
where we might need to be concerned with frost.  Will hold off
mentioning frost in the forecast at this time since this is still 4
days away and there`s some variability in the forecast.

The upper level pattern gets in gear and shifts east Sunday into
Monday.  This will push the surface ridge east across the
Appalachians and just off the East Coast by Tuesday.  The warm
southerly flow on the western side of the ridge will push
temperatures up to near normal on Sunday and then above normal for
Monday through Wednesday next week.  The next longwave trof in the
pattern is forecast to dig into the western CONUS and move very
slowly eastward through the middle of next week.  This will set up a
more Spring-like warm and wet weather pattern with southwest flow
aloft and an active south-southwesterly low level jet over the mid
Mississippi Valley. It should certainly feel a lot more like late
April/early May with the potential for thunderstorms and
temperatures in the upper 70s to low 80s.



.AVIATION...  (For the 12z TAFs through 12z Friday Morning)
Issued at 649 AM CDT Thu Apr 26 2018

Localized areas of IFR/LIFR fog developed overnight, mainly around
KCOU and KCPS. Remnant fog will dissipate with sunrise and drier
working into the region. MVFR cigs south of St. Louis continue to
depart to the southeast through the morning. VFR conditions are
expected for the remainder of the forecast period. Wind will
remain relatively light today out of the northwest.


OVC/BKN cloud deck will continue to depart to the southeast this
morning. VFR conditions are expected throughout the forecast
period with few/sct high clouds. Northwest winds will be
relatively light today through early Friday. Northwest wind will
briefly shift out of the west-southwest overnight ahead of a
boundary, then back to the northwest Friday morning. Simplified
the TAF to just a couple of lines given the VFR conditions and
winds remaining below 10 kts.





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