Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 200554

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1154 PM CST Mon Feb 19 2018

Issued at 937 PM CST Mon Feb 19 2018

The cold front has made some eastward push across northeast MO and
was located just east of a Canton-Monroe City-Columbia line
trailing back into southwest MO. The front may make a tad more
eastward push but pressure rises behind it are weak suggesting
additional motion should be minimal. In fact a weak surface wave
traveling northeast along the front overnight may result in the
front retreating a bit back to the west late before it resumes its
east motion tomorrow morning. In terms of sensible weather the
front position will only impact temps which are 10-15 degrees
colder in the immediate wake of the front. Otherwise a strong
southwest LLJ will continue to result in lift and plentiful
moisture transport supporting a continued zone of showers and
scattered thunderstorms spanning the frontal zone and focused from
southwest through central and northeast MO and west central IL.
Rainfall rates and amounts thus far have been quite managable.



.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


.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.



.AVIATION...  (For the 06z TAFs through 06z Tuesday Night)
Issued at 1140 PM CST Mon Feb 19 2018

The main corridor of showers and scattered thunderstorms will
remain from southwest MO through central and northeast MO and
west central IL overnight, with showers becoming more numerous
east of this zone across the St. Louis region in the predawn
hours. Flight conditions have been quite variable thus far
tonight. I think KUIN has the highest probability of predominately
MVFR fight conditions and elsewhere VFR may dominate. The coverage
of showers and scattered thunderstorms should increase throughout
the day on Tuesday with flight conditions deteriorating to
predominately MVFR. A cold front will advance slowly east across
the area Tuesday afternoon and into the evening and flight
conditions will lower to IFR in the wake of the front. Gusty
southerly winds will continue ahead of the front with gusty
northwest winds after the cold frontal passage.


I think predominantly VFR flight conditions will prevail overnight
until around mid-morning on Tuesday. Early on during the overnight
period, showers will be rather spotty however they will become
more numerous prior to daybreak. The coverage of showers and
scattered thunderstorms should continue to increase throughout
the day on Tuesday with flight conditions deteriorating to
predominately MVFR. A cold front will move throught the terminal
on Tuesday evening with flight conditions lowering to IFR in the
wake of the front. Gusty southerly winds will continue ahead of
the front, with gusty northwest winds after the cold frontal
passage Tuesday evening.





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