Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 150543
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1143 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 06z TAFs through 06z Thursday Night)
Issued at 1130 PM CST Wed Feb 14 2018

Tough forecast for the overnight period into Thursday morning.
Non-uniformity in the surface winds is resulting in areas of
radiational fog dotted across the region, and these could come and
go in response to changes in surface wind speeds. My initial
thinking outside of the radiational fog is that stratus across
southern MO will both develop and advect north-northeastward
overnight bringing MVFR flight conditions to the terminals. A
southwest LLJ centered through western MO is also expected to
bring LLWS conditions to KCOU. Present thinking is MVFR flight
conditions will persist through mid-morning, after which
increasing surface winds and scattered showers should lead to
improving flight conditions. Showers could impact the terminals
off and on anytime from 14-15Z until the cold frontal passage and
accompanying west to northwest wind shift. Gusty northwest winds
are expected to develop in the wake of the cold front and during
the late evening and overnight, a new round of stratus/MVFR cigs
should spread southeast across much of the area.

SPECIFICS FOR KSTL:

Stratus across southern MO will both develop and advect north-
northeastward into the terminal overnight bringing MVFR flight
conditions. Present thinking is MVFR flight conditions will
persist through mid-morning, after which increasing surface winds
and scattered showers should lead to improving flight conditions.
Showers could impact KSTL off and on anytime from 14-15Z until
the cold frontal passage near 00Z, and the accompanying west to
northwest wind shift. Gusty northwest winds are expected to
develop in the wake of the cold front and overnight, a new round
of stratus/MVFR cigs should spread southeast into the terminal.

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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