Scientific Forecaster Discussion
fxus63 klsx 192032
Area forecast discussion
National Weather Service Saint Louis MO
332 PM CDT Sat Oct 19 2019
Short term... (through late Sunday night)
issued at 329 PM CDT Sat Oct 19 2019
Area of showers continues this afternoon mainly across portions
of central and eastern Missouri. This shower activity should
continue to move east over the next few hours before weakening and
eventually dissipating by early evening. A dry forecast is
expected for the rest of the night and into the day on Sunday.
Models suggest fog development overnight tonight, mainly for
northwestern sections of the forecast area. This does make sense
if we can get some clearing this evening as light/variable winds
and some light rain earlier today could help promote the
development of fog. Dense fog does not look likely however, with
lowest visibilites likely staying above 1/4 mile.
Temperatures tonight are expected to be near normal with lows in the
mid 40s to low 50s. Cooler locations are forecast to be across
central and northeast Missouri, behind the cold front and where
there should be at least partial clearing.
High temperatures on Sunday are a bit tricky, as stratus may get
trapped beneath an inversion aloft across portions of the area.
Models do not agree well with how any lingering stratus will evolve
late Sunday morning into the early afternoon, which makes the
temperature forecast difficult. The NAM (which tends to have a low
cloud bias) keeps some stratus through much of the day helping to
keep high temperatures in the low to mid 60s for parts of the area.
The GFS meanwhile mixes out the boundary layer quickly Sunday
morning scattering out the stratus. Consequently, high temperatures
on the GFS compared to the NAM are 5+ degrees warmer. Given past
experience observing how stratus this deep into the cool season
tends to linger around in these scenarios, gave a nod to the cooler
NAM, which resulted in highs in the 60s to near 70 degrees. These
values are fairly similar to the previous forecast.
More active weather is still on track beginning late Sunday evening
as a strong cold front approaches from the west. Widespread showers
with a few embedded thunderstorms are looking increasingly likely
due to strong forcing for ascent associated with a negatively tilted
shortwave trough and jet-streak dynamics. Moisture return ahead of
the cold front continues to look slightly delayed compared to
yesterday, with near 60 degree dewpoints only making it as far
northward as southeast Missouri. Timing of this front is far from
ideal for any strong to severe thunderstorm threat, but we may have
to keep an eye on our far southern counties for the potential for
strong-severe storms given the strength of the dynamics. Cam output
is not too excited, but the shear strength of this system and the
associated environment (strong low-level shear/helicity) suggest
there is a non-zero threat late Sunday night in southeast Missouri.
Long term... (monday through next saturday)
issued at 329 PM CDT Sat Oct 19 2019
The cold front early Monday morning should be nearly bisecting the
County Warning Area from north to south. However, the effective front likely will be
a bit further east associated with the widespread showers and a few
embedded thunderstorms. Any precipitation threat should exit the
area by midday with the actual frontal passage, with decreasing
clouds and brisk westerly winds in its wake. The airmass behind this
front is Pacific in origin, so temperatures on Monday should be near
normal for the date.
(Monday night - saturday)
Slightly cooler than normal temperatures are likely behind a
secondary cold front Monday night through Tuesday night. Dry weather
is also favored during this time period.
A stronger front still looks to be in the offing, but deterministic
models have large differences in timing. The GFS is much quicker,
passing through the County Warning Area in the Wednesday/Wednesday night timeframe
whereas the European model (ecmwf) is basically 12 hours later. For now, went with a
blend of the two, with a slight nod to the faster GFS due to
the strength of the air mass.
Behind this front, much cooler temperatures are likely Friday into
Saturday. High temperatures will likely be 10+ degrees below normal,
with a hard freeze possible Thursday night and/or Friday night if
the clouds can at least partially clear out of the region. Regarding
precipitation chances, they will increase particularly behind the
front as an upstream shortwave trough ejects out of the south-
Central Plains. This helps develop a secondary surface low along the
cold front which tracks northeastward into the Ohio Valley. The
track of this system is highly uncertain at this time, not to
mention the timing. That being said, it appears a light stratiform
rain event will likely occur sometime between Thursday and Friday.
In fact, could not even rule out rain mixing with snow late Thursday
night for northern sections of the forecast area. Temperatures aloft
likely will be cold enough to support snow, but it will depend on
whether or not the boundary layer can be shallow and cold enough.
Aviation... (for the 18z tafs through 18z Sunday afternoon)
issued at 1215 PM CDT Sat Oct 19 2019
Light rain showers may affect kcou and KUIN before they dissipate
toward evening. Visibilities however should remain VFR. A weak
cold front is moving through the area late this afternoon and
evening, with light and variable winds expected tonight into
Sunday morning. Behind this front, a period of MVFR ceilings are
likely. Kcou and KUIN should see a brief period of MVFR ceilings
late this afternoon and early evening, with the Metro terminals
more likely overnight tonight into Sunday morning. MVFR
visibilities in fog are likely overnight tonight for kcou and KUIN
due in part to partial clearing. Any fog should evaporate by mid
to late morning.
Specifics for kstl:
Area of rain showers to the west should dissipate before reaching
the terminal. A weak cold front is expected to pass through this
evening, with light and variable winds expected overnight tonight
into Sunday morning. A period of MVFR ceilings is likely behind
the front, roughly from around 0800 to 1600 UTC Sunday. VFR
conditions are likely from midday through the afternoon on Sunday
after sky cover scatters out.