Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 162010
AFDLSX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ST LOUIS MO
310 PM CDT Thu Apr 16 2015

.SHORT TERM:  (Through Late Tonight)
Issued at 307 PM CDT Thu Apr 16 2015

A weak upper level RIDGE is currently overhead with a very weak
pressure pattern in place at the surface yielding light and variable
winds across our region.  While the regional radar mosaic is
rain-free, the visible satellite imagery shows lots of diurnal
cumulus that has developed with the heating of the day--indicative
of the same moisture that also played a role in the morning stratus
and fog last night.  Temperatures have risen into the 70s at most
locations, with some 60s still lingering in areas where they
retained the morning clouds the best--across southeast MO and far
southern IL.

The weak upper ridge and very weak surface pressure pattern will
persist thru tonight and with little anticipated change in the
airmass composition in the boundary layer from minimal mixing, am
heavily favoring a persistence forecast with temps expected to drop
into the 50s once again tonight with fog formation likely,
especially in areas south and east of STL metro.  May ultimately
need a fog advisory for some of these areas but prefer to let
evening shift monitor to see how it develops.

TES

.LONG TERM:  (Friday through Next Thursday)
Issued at 307 PM CDT Thu Apr 16 2015

Upper ridge will remain over region on Friday, so kept dry
conditions going with highs in the upper 70s. Then as upper ridge
begins to move east, surface warm front develops south of forecast
area. So will see showers and thunderstorms develop over western MO
and increase in coverage across central MO Friday night and across
rest of forecast area Saturday and Saturday night as warm front
lifts north through region.

Then main upper level shortwave to begin to lift out Sunday with
cold front sliding through region. Decent instability with capes
between 500 and 1500 J/kg as well as decent shear, so SPC still has
15 percent probability of severe weather on Day 4. Precipitation to
taper off Sunday night as front exits region.

Beyond that, weak surface ridge builds with dry and colder weather
expected for the first part of the work week with highs in the 60s.
Then extended models have differing timing and solutions for next
weather system. For now kept slight chance/chance pops for Wednesday
through Thursday.

Byrd
&&

.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.

TES
&&

.LSX WATCHES/WARNINGS/ADVISORIES:
MO...NONE.
IL...NONE.
&&

$$
WFO LSX





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