Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 192358
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
558 PM CST Mon Feb 19 2018

.SHORT TERM...  (Through Late Tuesday Afternoon)
Issued at 250 PM CST Mon Feb 19 2018

Much of the showers/storms were moving northeastward into northeast
MO and west central IL this afternoon.  Activity should be more
scattered across east central and southeast MO and southwest IL for
the late afternoon hours.  Strong cold front extended from a surface
low over southeastern IA southwest to just west of IRK, to just west
of DMO, and to just west of JLN.  This front may move a tad further
southeast tonight, then move southeastward through northeast and
central MO and west central IL Tuesday afternoon.  The NAM model is
a little quicker dropping the front southeastward on Tuesday versus
the operational GFS.  Waves of showers and storms will continue to
move through much of our forecast area tonight and Tuesday as
southwest flow shortwaves move through our area.  A strong
southwesterly low level jet will continue to bring abundant low
level moisture into our area.  The high resolution explicit models
do shift the best coverage of showers/storms across northeast and
central MO and west central IL for tonight just ahead of the cold
front with more scattered activity further south and east.  The
operational models appear to follow suit with their best QPF across
areas north and west of STL tonight and also on Tuesday.  Lows
tonight will be quite mild ahead of the front with lows in STL
around 30 degrees above normal.  It will be unseasonably warm again
Tuesday, but the temperatures will fall during the afternoon across
northeast and central MO and west central IL after cold frontal
passage.

GKS

.LONG TERM...  (Tuesday Night through Next Monday)
Issued at 250 PM CST Mon Feb 19 2018

(Tuesday Night - Wednesday Night)

A wide variety of potential weather exists Tuesday night through
Wednesday night. Still looking at a slowly progressing cold front to
move through the remainder of the CWA mainly Tuesday evening.
Moderate, to perhaps occassionally heavy, rainfall rates with
embedded thunder will exist along and ahead of the front. A general
1-2 inches of additional rain appears likely along and southeast of
I-44 in Missouri and I-70 in Illinois. Still not too concerned with
any kind of widespread problems as antecedant conditions suggest the
area should be able to absorb this type of rainfall quite easily.
Very high precipitable water values and deep warm cloud depths do
suggest the possibility of very high rainfall rates, which if they
were to occur over any given location long enough, could pose a
flash flood threat. Will continue to hold off for now on any
flood/flash flood headlines however given forecast rainfall amounts
and dry antecedant conditions, but if forecast for Tuesday night
changes toward a heavier rain scenario, a flash flood watch may need
to be issued for this time period.

Attention post-frontal will be on how quickly temperatures drop
below freezing and how much precipitation falls into the
subfreezing air. NAM continues to be a major outlier with moderate
to heavy precipitation for at least a few hours with temperatures
below freezing. GFS continues to by completely dry as does 12Z ESRL
HRRR. The ECMWF model is a bit of a compromise and was relied on for
this forecast. Regarding temperatures, the 12Z NAM actually verified
best with its initialization looking at the cold airmass upstream so
it may end up handling temperatures better late Tuesday night into
Wednesday and Wednesday night. However, even if the colder NAM is
true regarding temperatures, not sure how much precipitation will
occur within the subfreezing temperatures as alluded to above.

Temperatures and chances of precipitation are even more of a
question mark Wednesday afternoon and overnight wtih GFS/CMC/ECMWF
dry or mostly dry with the NAM further northwest with disturbance
bringing the southeastern 1/2-1/3 of the area another round of
freezing rain and rain. Favored the consensus of the GFS/CMC/ECMWF
which yielded mostly chance PoPs and the possibility of light
amounts of additional icing if temperatures are still below
freezing. Total ice amounts of a light glazing to as much as a tenth
of an inch are expected between late Tuesday night and Wednesday
night. Mitigating factors for ice accretion include potentially
moderate-heavy rates of precipitation Tuesday night, very warm
droplet temperatures initially, warm ground temperatures initially,
and finally air temperatures likely only slightly below freezing
(generally in the 30-32 range). All that being said, while at least
a brief period of freezing rain is likely, still lots of uncertainty
on how much will fall and what impacts there will be.


(Thursday - Saturday Night)

Models are in fairly good agreement that 2-3 more rounds of light to
moderate rainfall are likely to occur from Thursday night through
Saturday night. A general 1-3" of rain appear possible in this
timeframe with the heaviest amounts likely to be focused across far
southeastern sections of the forecast area in close proximity to the
quasi-stationary front. Models for the time being at least have
shifted slightly southward compared to yesterday, but a shift back
to the north could bring the area at least sligthly higher rainfall
amounts. With the anticipation of much more saturated soils and
rivers/streams at much higher levels, may have more of a flood
and/or flash flood threat for this period but plenty of time to sort
that all out. As has been stated the past couple of days, good news
is area should see a huge improvement in ongoing drought conditions,
especially since vast majority of impacts across area are
hydrologically focused or closely related (i.e., low streams/ponds
negatively affecting agriculture).


(Sunday - Next Monday)

A dry couple of days looks likely Sunday and for next Monday as a
weak surface ridge of high pressure will be in control of sensible
weather elements. Temperatures look to be approximately 10 degrees
above normal day and night under a clear to partly cloudy sky.

Gosselin

&&

.AVIATION...  (For the 00z TAFs through 00z Tuesday Evening)
Issued at 540 PM CST Mon Feb 19 2018

Present thinking is the main corridor of showers and scattered
thunderstorm this evening will be from southwest MO through
central and northeast MO and west central IL. Therefore the lowest
flight conditions, predominately MVFR will at KCOU and KUIN, with
potentially brief periods of IFR. I think showers elsewhere will
be more spotty and flight conditions should largely be VFR - this
includes the St. Louis metro terminals. Overnight and on Tuesday
both KCOU and KUIN will remain in the primary axis of showers and
thunderstorms with MVFR flight conditions the most prevalent.
There will be an increasing amount of showers further east
impacting the St. Louis region in the predawn hours and continuing
to increase during the day on Tuesday with gradually deteriorating
flight conditions. Gusty southerly winds will persist much of the
period, and there appears to be sufficient mixing to limit true
LLWS potential tonight into early Tuesday morning.

SPECIFICS FOR KSTL:

I think showers will be spotty and flight conditions should
largely be VFR this evening into the overnight hours. There will
be an increasing amount of showers impacting KSTL in the predawn
hours and continuing to increase during the day on Tuesday
accompanined by gradually deteriorating flight conditions. Gusty
southerly winds will persist much of the period, and there appears
to be sufficient mixing to limit true LLWS potential tonight into
early Tuesday morning.

Glass

&&

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

$$

WFO LSX



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