Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 161750

1250 PM CDT Thu Apr 16 2015

Issued at 1250 PM CDT Thu Apr 16 2015

Low clouds have eroded and are being shunted to the far southern
and eastern areas of our region and they should continue to break
up during the afternoon hours as diurnal cumulus clouds where
clearing has already occurred. Still should see decent sunshine
all-in-all, and this should propel temps well into the 70s, with
some upper 70s readings expected by day`s end.

Heading into tonight, should see mostly clear skies initially with
only some diurnal leftover debris clouds. But with little in the
way of mixing today due to light winds and therefore no turnover
of the airmass, a stagnant pattern strongly favoring a persistence
forecast looks to be in our future with expected re-development of
low stratus and fog once again late tonight. The preferred areas
are similar to last night, where the highest dewpoints are: STL
metro and areas to the south and east. The fog will further
facilitate temps dropping into the 50s.



.SHORT TERM:  (Today and Tonight)
Issued at 331 AM CDT Thu Apr 16 2015

Shortwave ridging will develop and amplify aloft today and tonight,
providing the area with warmer and drier weather than the past few
days.  Forecast soundings show us mixing up to around 900mb this
afternoon, so used a healthy dose of mix-down temperatures for highs
today, blended with MOS guidance.  This yielded highs at or just
slightly above warmest MOS guidance.  With ridging holding in
tonight, don`t expect much if any precip...although some of the
high-res WRF members do spit out some sprinkles.  Again...this seems
unlikely given the subsidence provided by the ridge aloft.  As a
result, stuck with a dry forecast for tonight.  It looks like
mid-high clouds may be increasing toward morning, and therefore some
decent insulation so I used a blend of guidance which ended up
producing lows close to the warmer end of MOS numbers across the


.LONG TERM:  (Tonight through Wednesday)
Issued at 419 AM CDT Thu Apr 16 2015

Models continue to slow the advancement of the Four Corners low out
into the Great Plains as the week draws to a close.  With the upper
level ridge locked in ahead of the low, it looks like Friday and
most of Friday night should be dry across the CWFA.  Kept some low
chance/slight chance PoPs across parts of central and southeast
Missouri, primarily after midnight as the upper ridge begins to
shift east and the low level jet picks up a bit in response to the
approaching low.  Should see a general increase in shower and
thunderstorm activity through Saturday afternoon into Saturday
evening.  Temperatures Friday and Saturday should stay above normal,
with Friday topping out in the mid to upper 70s.  Saturday looks a
bit cooler due to increasing clouds and shower activity.

The surface reflection of the wave is now forecast to move through
Sunday afternoon/evening with another round of showers and
thunderstorms.  SPC has us outlooked in a 15% probability for severe
weather in their DY4 forecast.  GFS forecasts 1000-1500 J/kg MLCAPE
and 30-40kts of 0-6km shear at 18Z some threat for
severe storms isn`t unreasonable.  However, this forecast is still
far from clear cut with timing uncertainties and the strength of the
upper system as it opens up and is absorbed into the next system
diving in from the north.  Regardless, Sunday still looks a bit
cooler, though with temperatures still near normal, and wet as the
system moves through.

Should see a cool start to the week as another deep cutoff low moves
slowly along the U.S./Canadian border.  This will open the door for
cool Canadian air to filter down through the Plains into the
Midwest.  Below normal highs in the low to mid 60s look reasonable
with lows generally in the 40s.  Don`t see much chance for
precipitation until Wednesday when low level flow turns to the
south-southwest in response to a shortwave moving across the area on
the southern periphery of the larger low to our north.  Resulting
warm advection and moisture convergence could set off some scattered



.AVIATION:  (For the 18z TAFs through 18z Friday Afternoon)
Issued at 1250 PM CDT Thu Apr 16 2015

Little in the way of change for the airmass today so will follow
persistence as a primary guide on the forecast. This will mean VFR
conditions thru this evening for most areas, with some sub-VFR VSBY
reductions in fog beginning late this evening and expanding
overnight. The favored areas will be STL metro sites where they
are closer to the higher dewpoints and since they all dropped into
IFR last night, will forecast it again tonight, allowing room for
subsequent shifts to hit it harder as confidence builds. An east
flow will develop on Friday with a return to VFR anticipated by
late morning.

Specifics for KSTL: VFR into early overnight with a return to IFR
expected late and into Friday morning. Certainly some room for
error here with crossover temps in the mid-upper 50s and around
the min temp forecast for STL but even if it doesn`t form directly
at the airport, it still looks likely they will have it advect
there from where it was able to form nearby. Otherwise, east winds
developing on Friday as conditions return to VFR.





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