Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 150344
AFDLSX

Area Forecast Discussion...Updated
National Weather Service Saint Louis MO
944 PM CST Wed Feb 14 2018

.UPDATE...
Issued at 921 PM CST Wed Feb 14 2018

The forecast the remainder of tonight and through Thursday morning
is anything but clear cut. A defined low level WAA regime is in
place with veering south to southwesterly flow through the lower
trop. Mostly cloudy skies will be the rule tonight resulting from
both a continued increase in high clouds from the Plains and also
increasing stratus. Stratus already blankets the east-southeast
portion of the CWA and the latest GOES East Stratus and Nighttime
Microphysics products are showing the development/spread into
southwest/south-central MO. This later stratus is expected to
continue developing/spreading north-northeast through the night
across the CWA. I think the clouds as well as increasing low level
flow should keep any substantial fog at a minimum. Min temps will
be quite mild for late February and actually above normal highs
for this time of year. Another change was to increase POPs on
Thursday morning, especially for areas along and north of
Interstate 70. The last 5+ runs of the HRRR along with some
support from the NAMNEST and RAP have been showing a rather
explosive development of showers in response to low level WAA and
moisture advection associated with a southwest LLJ and weak ascent
attendant with a migratory impulse aloft. If this precip indeed
pans out and persists through much of the morning, it could have
a negative impact on the highs presently forecast.

Glass

&&

.SHORT TERM...  (Through Late Thursday Afternoon)
Issued at 310 PM CST Wed Feb 14 2018

Zonal flow aloft exists over our region this afternoon with
southerly flow at the surface.  A strong storm system was anchored
off the southern coast of California.  After a relatively mild start
to this morning, and thanks to dissipation of the expansive low
cloud field for most locations, temps have surged well into the 50s
and 60s this afternoon under a filtered sunshine from abundant
cirrus clouds.

A piece of energy will break off from the CA storm system and will
race eastward towards our region by Thursday morning with a larger
piece in tow.  This will increase deep lift across our region late
tonight into Thursday as a surface cold front also moves thru the
area.

Abundant high cloudiness from cirrus will continue for many areas
this evening as thicker clouds begin to roll back in.  Along with a
steady moderate southerly flow of wind, temperatures will not fall
much overnight, with min temps in the low-mid 50s for many areas
near and south of I-70, and mid-upper 40s for areas to the north.
The threat for rain showers is also expected to hold off until very
late tonight for parts of central MO or into Thursday morning for
most areas near and north of I-70 with this rain threat then
dropping southward during Thursday afternoon with the advent of the
cold front.

Clouds will be another factor in determining how aggressive to
handle what could be a very warm day for February tomorrow, and have
decided to continue a via media with respect to MOS on max temps:
ranging from low 70s for many areas near and south of I-70 to the
low 60s in far northern MO and central IL.

Temperatures will be mild enough to preclude any concerns on pcpn-
types other than all liquid.

TES

.LONG TERM...  (Thursday Night through Next Wednesday)
Issued at 310 PM CST Wed Feb 14 2018

(Thursday Night - Sunday)

Cold front will continue to progress southeastward Thursday evening.
Showers are likely along and ahead of the boundary. Some weak low-
level instability so kept mention of slight chance thunder during
the evening hours, mainly across portions of southwest Illinois and
southeast Missouri. An upstream shortwave may also help induce some
more stratiform light rain on the cool side of the boundary
overnight Thursday night. This area may clip portions of
southeastern Missouri.

Strong cold advection takes place most of the day on Friday with a
surface high pressure center sliding down the Missouri River Valley.
Temperatures will much much colder on Friday behind the front. Looks
for highs in the 30s and 40s across the area, or about 30 degrees
colder than Thursday!

Still plenty of uncertainty with the next chance of precipitation
late Friday night and early Saturday. Best chance of precipitation
across the area looks to be across southern sections of the CWFA
where strongest low-level warm/moist advection will reside.
Precipitation type will be problematic as classic battle between the
aforementioned warm advection vs. wetbulbing/evaporative cooling
takes place. Believe a rain/snow mix wording is the best way to go,
leaning toward more snow initially as wetbulbing takes place
transitioning to rain as boundary layer warms well above freezing.
Some minor accumulations are possible but still quite a bit of
uncertainty with respect to precipitation type and northern extent
of stratiform precipitation shield into our area.

A weak warm front is forecast to move through the bi-state area on
Saturday night into early Sunday morning. This will leave the CWA in
the warm sector of a developing cyclone across the Rocky Mountains.
High temperatures are expected to be in the 50s predominantly on
Sunday.

(Sunday Night - Next Wednesday)

Warm and active weather regime still appears likely for early next
week as persistent southwest flow aloft sets up across the mid-
Mississippi Valley. Setup appears conducive to at least a couple
rounds of showers and thunderstorms between Sunday night and Tuesday
night. Moderate to potentially heavy rainfall appears possible, but
the specifics are highly uncertain this far out. Exact placement of
the slowly sagging cold front will be critical and this feature and
its progress will be highly dependent on the strength of the
downstream mid/upper level anticyclone and the strength to the low-
level cold airmass behind the front. At any rate, appears warm and
active ahead of the cold front which should bring at least some
beneficial rainfall to the area.

Gosselin

&&

.AVIATION...  (For the 00z TAFs through 00z Thursday Evening)
Issued at 538 PM CST Wed Feb 14 2018

VFR conditions prevail at all the terminals early this evening
with high clouds overspreading the area and stratus across
southeast MO into southern IL. Current thinking is that stratus
will redevelop back to the west across MO late this evening and
overnight, with predominately MVFR flight conditions impacting
the terminals. Also overnight a southwesterly LLJ will develop
across southeast KS and western MO and this will produce LLWS
conditions thru 15Z at KCOU. I think there will be a general trend
for improving flight conditions by late morning as surface winds
increase. Precipitation trends are less defined. A cold front will
be moving through the area during the late morning-afternoon and
present indications are there will be showers ahead of the front.
Coverage and timing of the showers varies amongst the guidance.
There certainly could be higher probabilities at KCOU and earlier
in the day in the St. Louis region.

SPECIFICS FOR KSTL:

VFR flight conditions should prevail this evening then MVFR
stratus is expected to develop and move into the terminal
overnight. Improving flight conditions are expected by midday on
Thursday along with increasing southwest winds. Precipitation
trends are bit harder to define. I kept with the trend of the
previous forecast with showers during the later half of the
afternoon ahead of an advancing cold front. There is some newer
short-range guidance that suggests showers could occur as early as
15-16Z and this will be evaluated in preparation of the next TAF
issuance.

Glass

&&

.CLIMATE...
Record high temperatures for February 15th

STL...75 in 1976
COU...78 in 1911
UIN...74 in 1921


&&

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

$$

WFO LSX



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