Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 192055
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
255 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 18z TAFs through 18z Tuesday Afternoon)
Issued at 1025 AM CST Mon Feb 19 2018

Cold front extending from a surface low in south central IA
southwest through northwestern MO and into the TX panhandle will
remain northwest of the taf sites this period, although it will
be close to UIN and COU by 18Z Tuesday. A strong southwesterly low
level jet will continue to bring abundant low level moisture into
the region with periods of showers and a few storms. Showers
continued to move through the St Louis metro area late this
morning. Another area of showers and storms moving northeastward
through southwestern MO will impact COU and the St Louis metro
area this afternoon, and possibly into UIN as well. Waves of
showers and embedded storms will continue to impact the taf sites
tonight and Tuesday. The MVFR ceilings in UIN will continue,
possibly dropping into the IFR catagory tonight. The MVFR ceilings
will move into COU early this afternoon and into the St Louis
metro area by late afternoon. South-southwesterly surface winds
will continue through the period. There will be some LLWS tonight
but with boundary layer mixing it does not look quite strong
enough to include in the tafs.

SPECIFICS FOR KSTL: Cold front extending from a surface low in south
central IA southwest through northwestern MO and into the TX
panhandle will remain northwest of STL this period, although it
will be close to STL by 00Z Wednesday. A strong southwesterly low
level jet will continue to bring abundant low level moisture into
the region with periods of showers and a few storms. Showers
continued to move through the STL area late this morning. Another
area of showers and storms moving northeastward through
southwestern MO will impact STL this afternoon. Waves of showers
and embedded storms will continue to impact the STL area tonight
and Tuesday. MVFR ceilings will move into the STL area by late
afternoon. South-southwesterly surface winds will continue
through the period. There will be some LLWS tonight but with
boundary layer mixing it does not look quite strong enough to
include in the STL taf.

GKS

&&

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

$$

WFO LSX



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