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fxus63 klsx 222358 

Area forecast discussion...updated aviation
National Weather Service Saint Louis MO
558 PM CST Fri Nov 22 2019

Short term... (through late Saturday afternoon)
issued at 312 PM CST Fri Nov 22 2019

The mid-Mississippi Valley is currently situated beneath a weak
upper ridge with surface high pressure building in from the
northwest. This has resulted in quiet conditions today, though
this is likely to change as a broad upper low currently moving out
of The Four Corners pushes into the region overnight. The
approaching upper low will slowly deepen a surface trough to our
southwest, which in turn will help draw the upper portions of
yestreday's cold front back into the region this evening.
Isentropic analysis of forecast guidance reveals weak moist
upglide along this baroclinic zone overnight, which coupled with
weak DCVA associated with the slowly filling upper low, should be
enough to generate precip across much of the region.

While precip is quite likely tonight and tomorrow morning in
southern MO and IL, i'm not certain how far north the precip will
extend. The preponderance of ensemble members shows measurable
precip extending at least to I-70, with most deterministic
forecasts and ensemble means pushing light quantitative precipitation forecast all the way into NE
MO and west central Illinois. Therefore have continued to increase pops
across much of the area, with quantitative precipitation forecast ranging from around 0.1" in our
south to 0.01" in our north. Questions also remain with regards
to p-type, as sub-freezing surface temps will be creeping into
the region through the morning. Thermal profiles should be
supportive of snow into tomorrow morning, primarily for areas
north of I-70. However, it does appear that we may lose cloud ice
toward NE MO as dry air wraps into the upper low, which may result
in a brief period of freezing rain. It's also possible the dry
air simply cuts off the precip entirely, so confidence in any fzra
is rather low. Therefore have largely stuck with just a rain/snow
forecast, with a brief mention of fzra in NE MO. No ice
accumulations are expected, and any snow accums are likely to be
limited to the north of I-70. Where snow does occur, I would be
surprised to see much more than half an inch of snow on grassy and
elevated surfaces.


Long term... (saturday night through next friday)
issued at 312 PM CST Fri Nov 22 2019

Northwest upper level flow will take hold behind the departing
low, with a handful of embedded shortwaves forecast to push
through the region into early next week. Despite this mid level
forcing, no precip is anticipated through Monday thanks to a
steady supply dry west-northwest flow in the mid/low levels surface and high
pressure building into the region. As the surface high shifts to
our east on Sunday afternoon, surface winds will begin to swing
around to the south, drawing warmer air back into the region. With
that, temps should build into the 50s on Sunday and I expected
several locations to top 60 on Monday.

On Tuesday, ensemble means continue to show strong agreement on
anomalously low upper level heights over the central portion of
the country, increasing confidence that a potentially strong
cyclone may move through the Midwest. The 12z runs of the
operational GFS, Euro, and Canadian show remarkable consensus on
low track, timing, and intensity, further bolstering confidence
that the mid-Mississippi Valley will be impacted by this storm a
few days prior to Thanksgiving. Despite the increasing confidence,
we're still nearly 100 hours away from this event and it would be
foolhardy to try and nail down forecast specifics at this time.
Small deviations in low characteristics could on one hand result
in frozen precip, while on the other hand result in strong storms
in our region. The most responsible forecast at this time is to
say at cyclone will move through the Midwest on Tuesday, likely
bringing precipitation to the mid-Mississippi Valley. As we
approach this event and confidence increases, forecast specifics
will become more clear.

Beyond Tuesday, high pressure is expected to build into the
Midwest while yet another low pushes across the southern Continental U.S..
the surface high and upper level height rises should generally
keep a lid on precip chances in our region. However, some guidance
does push the northern fringes of the precip from the southern
Continental U.S. System into our area on Thursday. Given large differences in
forecast guidance, confidence in this period is low.



Aviation... (for the 00z tafs through 00z Saturday evening)
issued at 556 PM CST Fri Nov 22 2019

Precipitation will spread northeast across the area this evening
and continue overnight. The best time for precipitation will be
late this evening into the early morning hours. The rain will mix
with or change over to snow at kcou and KUIN with some minimal
accumulations on grassy accumulations. The precipitation is
expected to move out of the area on Saturday morning. Ceilings
will drop to MVFR after 06z and remain that way through tomorrow
afternoon before improving to VFR on Saturday afternoon.

Specifics for kstl:

Rain will move into the terminal around 06z and continue into mid
morning Saturday. The rain may mix with snow at times between
10-15z, but no accumulation is expected at this time. Ceilings
will drop to MVFR after 06z and remain that way through tomorrow
afternoon before improving to VFR on Saturday afternoon.



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