Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 170532
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1132 PM CST Sun Feb 16 2020

.UPDATE...
Issued at 909 PM CST Sun Feb 16 2020

Going forecast still looks on track. Evening upper air analysis
is showing a weak low level jet setting up over southwest Missouri
while the left exit region of a jet approaches from the Central
Plains. Still expect rain to develop west of the Mississippi River
shortly after midnight and then spread east of the river toward
morning. Made some minor adjustment to temperatures based on the
latest trends.

Britt

&&

.SHORT TERM...  (Through Late Monday Night)
Issued at 321 PM CST Sun Feb 16 2020

This evening looks to be rather tranquil with surface high
pressure retreating east through the Ohio Valley and high clouds
on the increase. Present indications are the onset of rain will be
faster tonight than previously anticipated. Nearly all the
guidance has a more aggressive onset with rain/showers developing
overnight from central into eastern MO after 06z in response to
impressive isentropic lift and poleward moisture transport via a
southwesterly LLJ. I have boosted POPS accordingly with the
highest POPS centered from eastern MO into west central IL.
Temperatures are expected to be above freezing.

The exact precipitation trends on Monday are not totally clear.
This stems from the fact that isentropic lift ahead of the
advancing cold front will be the main driver, and any large scale
ascent is weak and really doesn`t seem to come into play until
later in the afternoon. The forecast precipitation trends kind of
bare this out with the initial thrust of rain shifting
east/northeast of the CWA by mid-morning, then perhaps a short-
lived minimum in coverage, and then rain ramping up from the later
part of the morning into the afternoon. As stated yesterday, all
of this precipitation appears to be pre-frontal, thus the rain
should end west to east on Monday night as the cold front advances
eastward. In fact,  present thinking is the lionshare of rain
will probably have shifted east of the CWA by midnight, and well
before appreciable CAA.

Glass

.LONG TERM...  (Tuesday through Next Sunday)
Issued at 321 PM CST Sun Feb 16 2020

Tuesday still looks seasonably cold with weak cold advection as
the leading portion of the expansive Arctic high begins settling
into the area. The NAM no longer has its phantom snow band for
late Tuesday and Tuesday night that was advertised in yesterday`s
runs and the majority of the guidance supports a dry forecast.

Wednesday and especially Thursday are still on track to be the
real cold period as the Arctic surface high pressure system builds
southward. The main thrust of stronger CAA will occur Wednesday
night into Thursday as an upper trof in the NW flow drives
southeastward. Temperatures on Thursday will be 15 to 20 degrees
below normal.

Moderating temperatures appear on target for Friday into the
weekend as the Arctic high retreats, heights aloft rise, and a low
level WAA regime gets established.

Glass

&&

.AVIATION...  (For the 06z TAFs through 06z Monday Night)
Issued at 1116 PM CST Sun Feb 16 2020

Rain is expected to develop at the terminals between 08-12Z. The
rain is then expected to continue, at least intermittently,
through the rest of the period. MVFR ceilings and visibilities
will develop between 12-18Z, with ceilings falling to IFR at KUIN
by mid morning and by afternoon at the other terminals. Southeast
winds will turn out of the west tomorrow evening behind a cold
front.

SPECIFICS FOR KSTL:

Rain is expected to develop at the terminal around 09Z. The rain
is then expected to continue, at least intermittently, through the
rest of the period. MVFR ceilings and visibilities will develop
between 12-18Z, with ceilings falling to IFR by afternoon.
Southeast winds will turn out of the west tomorrow evening behind
a cold front.

Britt

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...None.
IL...None.
&&

$$

WFO LSX


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