Scientific Forecaster Discussion
fxus63 klsx 201117
Area forecast discussion...updated aviation
National Weather Service Saint Louis MO
617 am CDT sun Oct 20 2019
Short term... (through late Monday afternoon)
issued at 347 am CDT sun Oct 20 2019
Weak frontal boundary has exited the forecast area and washed out.
So will have dry conditions today. In the meantime, still dealing
with dense fog, with SC clearing out a bit further to the south and
east early this morning, so have expanded the dense fog advisory a
couple of counties further to the east, including portions of west
central Illinois. Should see fog begin to lift and dissipate by 11
Otherwise, high temperatures will be a bit tricky today for some
locations, depending on how long it will take for the stratus to
lift and scatter out. Will go with the warmer side of guidance as
the NAM is an outlier on the much colder end of the spectrum with
the 00z run. Highs will be in the mid 60s to low 70s.
Weak surface ridge will move off to the east later today as upper
level trof over the Central Plains continues to deepen, then
eventually lift northeastward towards Great Lakes region tonight. As
trof lifts northeastward, it will drag a strong cold front towards
region. Widespread showers and a few storms will increase in
coverage after 06z Monday and spread eastward across forecast area.
With surface based convective available potential energy at least 500 j/kg, 0-3km bulk shear values
near 50kts and low local values, especially over portions of southeast
MO, could see qlcs development as front moves through with damaging
winds and possible brief tornadoes as the main threats. Further
north, the activity will be more elevated in nature with large hail
possible. The main window for the severe threat will be between 2 am
and 7 am early Monday morning.
Then front will quickly exit region on Monday with rain coming to an
end by early afternoon. Otherwise, despite gusty west winds, cold air advection
will lag behind system, so highs will rise into the 60s through
early afternoon before beginning to fall as colder air finally makes
its way in.
Long term... (monday night through saturday)
issued at 347 am CDT sun Oct 20 2019
An upper longwave trough is expected to prevail over the central and
eastern Continental U.S. Through Thursday, serving to steer storm systems into
our region every few days. One storm system will have just exited
at the beginning of this period with the next one slated for mid-
late week. The accuracy of the timing for the next storm system
will depend heavily on this pattern persisting through the week.
While this pattern looked like it would remain in place until next
weekend 24hrs ago, model guidance has begun to shift away from this
solution to a second Camp favoring a split flow.
In the meantime, a pair of dry days are expected for Tuesday and
More active weather returns Wednesday night and Thursday with
approach and passage of the next surface cold front.
It is here where divergence of guidance occurs. The ec and CMC Camp
prefers an earlier onset of the split flow pattern change, which
causes the next storm system for late week to then become severed
from the main flow while it is still west of our region and then
drift back into our region for next weekend for what looks like a
rainy weekend. The GFS delays the split flow pattern change and
therefore keeps the next storm system entrained within the main
flow, progressively sliding through by late Thursday with the cold
front and favoring a dry weekend. With such a disparity in
solutions so far out, it will be hard to completely go with one over
the other and thus will have a prolonged period of mentionable pops
from Wednesday night Onward until this can get sorted out a bit
Airmasses of a mild Pacific origin will prevail through Wednesday
with temperatures within 5 degrees of normal for most locations.
With passage of the next cold front Wednesday night and Thursday, a
chillier airmass of a more Canadian origin will expand its influence
into our region with a more decisive shift to below normal temps.
Whether or not this continues into Saturday depends on whether the
ec/CMC Camp or GFS Camp wins out.
Aviation... (for the 12z tafs through 12z Monday morning)
issued at 601 am CDT sun Oct 20 2019
Dense fog over kcou and KUIN will persist through 15z before
lifting and dissipating. Otherwise, IFR/MVFR ceilings will
gradually lift and scatter out from north to south by mid morning.
By late tonight, next system will approach taf sites with showers
and some storms moving in as cold front slides through. Light and
variable winds will persist through the day today, before picking
up from the southeast to south late tonight.
Specifics for kstl:
some patchy fog in the river valleys, especially at ksus.
Otherwise, MVFR ceilings will lift and scatter out by 16z. By late
tonight, next system will approach St. Louis Metro area with
showers and some storms moving in ahead of a strong cold front.
Light and variable winds will persist through the day today,
before picking up from the southeast then veer to the southwest as
cold front moves through by 17z Monday.
MO...dense fog advisory until 11 am CDT this morning for Audrain MO-
Boone MO-Callaway MO-Cole MO-Crawford MO-Franklin MO-
Gasconade MO-Knox MO-Lewis MO-Lincoln MO-Marion MO-Moniteau
MO-Monroe MO-Montgomery MO-Osage MO-Pike MO-Ralls MO-Shelby
Illinois...dense fog advisory until 11 am CDT this morning for Adams Illinois-
Brown Illinois-Pike Illinois.