Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 151750
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1150 AM CST Thu Feb 15 2018

.SHORT TERM...  (Through Late Tonight)
Issued at 411 AM CST Thu Feb 15 2018

The synoptic pattern early this morning features west/southwest flow
aloft over the region. This flow will turn more zonal by the end of
the period as a trough slides through the region, bringing a surface
cold front across the area tonight.

A bit of a tricky forecast is on tap today as many factors are in
play. First is the patchy dense fog this morning, along and north of
the effective surface boundary which continues to slowly lift
northward this morning. The dense fog has been mostly confined to
western IL, and even that has been transient at times with thicker
high clouds passing through. Will continue to keep an eye on it, but
for now thinking a Dense Fog Advisory is not warranted unless this
fog expands and becomes more widespread through the morning hours.

It appears there were be two general forcing mechanisms for showers
today. The first will come this morning and is already seen on radar
with elevated returns over portions of southwest MO and western KS.
This precipitation is associated with low/mid level warm air and
moisture advection which will spread northeast across mainly central
and northeast MO into western IL this morning. Soundings suggest
this precipitation is currently fighting dry air between about 3,000-
10,000 feet, thus much of it is currently not hitting the ground.
However, with time that wedge of dry air should erode which should
lead to increasing coverage of showers this morning especially over
northeast MO and western IL.

The nose of the low-level jet and associated moisture advection
should push northeast of the region by late this morning into the
early afternoon. This should bring a general lull in the precip late
this morning into early this afternoon which most CAMs support,
although the HRRR continues to consistently show a solid, widespread
band of rain along the stalled boundary this morning through the
early afternoon. Have generally thrown this solution out given none
of the other hi-res models are as bullish (including the NSSL-WRF
ensemble). By late this afternoon into this evening, the surface
boundary over northeast MO/western IL will begin to surge southeast
as a cold front as a surface low passes to the east. As convergence
increases along the boundary and as height falls overspread the
boundary, increasing ascent should lead to a developing band of
showers and even a few embedded thunderstorms especially as the
front approaches the I-70 corridor and areas southeast. Much of the
precipitation will exit the area by sunrise Friday morning.

Temperatures today will be tricky. It appears most locations will
have a fairly thick deck of mid/high clouds, with areas along/north
of the boundary (northeast MO and western IL) likely to have low
clouds. Despite the clouds, strong surface warm air advection should
get temperatures at least near the guidance consensus especially
across central and southeast MO. If the clouds thin out more than
expected, going high temperatures likely will not be high enough.
Expect values in the mid 70s generally along/south of I-70 in MO and
low/mid 60s elsewhere, although far northeast MO will likely remain
in the 50s north of the boundary. Much cooler air will filter in
tonight behind the cold front as temperatures dip into the 20s and
30s.

KD

.LONG TERM...  (Friday through Wednesday)
Issued at 411 AM CST Thu Feb 15 2018

Colder conditions can be expected for Friday as a surface ridge,
cold air mass builds southeastward into our area from the northern
Plains.  There may be lingering light rain or light snow across
parts of southeast MO and southwest IL Friday morning, but most of
the post frontal precipitation should be southeast of our forecast
area.  Although there could be a little light snow or flurries
Friday morning, no accumulation is expected with air and
surface/ground temperatures above the freezing mark.  The sky will
clear out from northwest to southeast on Friday.  Highs on Friday
will be much colder, and about 5 to 10 degrees below normal.  An
upper level trough will move eastward through the region on Saturday
and may bring precipitation back into at least southeast MO and
southwest IL on Saturday.  The NAM and ECMWF models were stronger
with this upper level trough and did have QPF mainly across central
and southeast MO Saturday morning, and from the Mississippi River
eastward Saturday afternoon.  The operational GFS model kept the
precipitation south of the forecast area on Saturday.  Still some
uncertainty as to the strength of the upper level trough and the
amount of low level moisture return into our area.  For now will
include the highest pops in southeast MO Saturday morning and in
southeast MO and southwest IL Saturday afternoon.  With initially
cold temperatures Saturday morning there is the potential of a
wintry mix with up to around one half inch of snow accumulation
possible, mainly across southeast MO.  The precipitation type will
change to liquid rain by Saturday afternoon as the freezing level
rises. Warmer temperatures will return Sunday afternoon due to
rising upper level heights, and as the surface wind becomes
southerly on the backside of the retreating surface high.
Precipitation will return to the area Sunday night due to low-mid
level warm air advection and increasing low level moisture.  The
temperature profile will be warm enough that the precipitation type
will be liquid rain.  Showers and even a few thunderstorms can be
expected Monday through Tuesday with southwest flow shortwaves
moving through our area along with a moist and unstable atmosphere
for February. Despite the cloud cover and rain high temperatures and
surface dew points will be unseasonably warm or high for February on
Monday.  Widespread showers with significant QPF is likely Monday
night through Tuesday night as a slow moving cold front moves
southeastward through our forecast area with weak surface waves
traveling along it.  The operational GFS was a little quicker or
more progressive with this front than the ECMWF model, but has a
surface wave on the front as it shifts southeast of our area and
still keeps precipitation going across east central and southeast MO
and southwest IL Tuesday night into Wednesday.  The precipitation
should finally shift southeast of our forecast area by Wednesday
afternoon, and hopefully will put a dent in our drought. With colder
air filtering into northeast and central MO and west central IL
behind the strong cold front there will be the potential for a
wintry mix of precipitation in this area mainly Tuesday night.

GKS

&&

.AVIATION...  (For the 18z TAFs through 18z Friday Afternoon)
Issued at 1150 AM CST Thu Feb 15 2018

VFR conditions will prevail until convection develops ahead and
along a cold front dropping down this afternoon. This front will
pass thru UIN this afternoon and COU and STL metro sites this
evening, with the best chances for SHRA immediately ahead of it.
Some thunder is possible for areas around STL metro and have
included VCTS as a result with the best chances to the SE and E.
Some non-VFR conditions are expected with SHRA, but more
widespread MVFR is expected later tonight with cloud field
dropping down behind front. Otherwise, gusty SW winds will shift
NW behind front and maintain gustiness for much of the valid
period.

TES

&&

.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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