Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 151201

701 AM CDT Mon Sep 15 2014

.SHORT TERM:  (Through Late This Afternoon)
Issued at 329 AM CDT Mon Sep 15 2014

Shower and thunderstorm trends are the primary forecast problem for
today.  Definite lack of activity so far in our CWA as of 08z,
however impact of the veering low level jet and moisture advection
is just now starting in earnest over our CWA, so still believe that
elevated storms that developed in sw IA around 06z will be trying to
build south with time with some of this precipitation area impacting
northern sections of our CWA.  The latest HRRR, local WRF, as well
as some of the 00z synoptic models are also suggesting lesser/much
spottier development is also possible further south along and south
of the I-70 corridor over the next few hours, and don`t think this
consensus can be totally discounted with some showers currently
south of SGF.

With the primary zone of warm advection/isentropic ascent shifting
to our east during the afternoon, southward pushing cold front
should be the primary trigger for additional showers and
thunderstorms.  I have held onto a zone of likely PoPs in the
vicinity of the front, but it may be these values will be a bit too
high, especially if the warm layer depicted by the GFS between
700-750mb comes to fruition.

Have used a variety of 3 hourly temperature guidance trying to
capture impacts of the aforementioned precip trends as well as that
of the cold front over northern sections of the CWA.  Highs are
expected to range from the middle 60s in our n counties to the 75-80
degree range south of the Missouri River.


.LONG TERM:  (Tonight through Sunday)
Issued at 329 AM CDT Mon Sep 15 2014

Cold front should make it southeastward to around STL by 00z
Tuesday. There will be scattered showers along and just behind
this front as it continues to move southeastward through southeast
MO and southwest IL this evening. It does not appear that there
will be much precipitation left by late tonight as a surface ridge
builds southeastward into our area behind the front with cooler
and drier air filtering into MO and IL. It should be dry for
Tuesday and Tuesday evening, then showers may return to at least
portions of central and southeast MO late Tuesday night and
Wednesday as weak low-mid level warm air advection ahead of a weak
northwest flow shortwave intercepts return low level moisture on
the backside of the surface ridge shifting east of our area. The
eastward extent of this precipitation is in question. The models
keep most of their qpf west of the Mississippi river. Warmer
temperatures are expected for Friday as as upper level ridge over
the southwestern US shifts eastward into our area along with
strengthening surface/low level southerly winds. Convection should
return to the region Saturday afternoon and night as a strong
shortwave breaks down the upper level ridge and sends a cold front
southeastward through our forecast area Saturday night and Sunday
morning. Prefer the ECMWF model during this time period as the GFS
has some feedback problems and generates a surface low along the
front that is likely too deep and has too much qpf.



.AVIATION:  (For the 12z TAFs through 12z Tuesday Morning)
Issued at 700 AM CDT Mon Sep 15 2014

Convection has been on a slow but steady increase since the
329am issuance of the short term AFD. Greatest area of development
has been over north central MO in zone of strongest WAA, and this
more concentrated area of showers and thunderstorms should impact
UIN during the morning. Elsewhere have gone with a threat of
elevated shower-type activity, although will need to monitor radar
and lightning trends as a few lightning strikes have occurred
along I-44 southwest of STL metro area. This warm advection precip
should slide east of most TAF sites by midday, with a secondary
threat of convection occurring as the frontal boundary slips
across the CWA. MOS, forecast soundings, as well as SREF cig
probability progs are all suggesting a band of MVFR cigs along and
behind the front for several hours, and this appears reasonable
given the ceiling trends over the last few hours north of the
frontal boundary from se NE into s IA. These lower ceilings should
then sweep out of the area during the overnight hours as much
drier air surges south behind the front. Given the intensity of
the drying, do not believe we`ll have a repeat of last week when
low clouds became locked over the area.

Specifics for KSTL:

Ceilings around 5kft can be expected this morning along with a
few spotty showers. A thunderstorm cannot totally be ruled out,
but lightning trends over the last hour or so strongly suggest
that thunder threat will remain south of terminal forecast area. Cold
front is expected to pass through in the 20-22z time frame with a
chance of thunderstorms at that time, and also expected a rapid
drop in ceilings to around 2kft with the arrival of the colder air.
These MVFR ceilings should persist throughout the evening hours,
then clear out shortly after 06z as much drier air advects into
the area from the north.





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