Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 180446
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1146 PM CDT Thu Aug 17 2017

.UPDATE...
Issued at 950 PM CDT Thu Aug 17 2017

Skies are expected to be mostly clear the rest of the night.
Latest satellite imagery is showing cirrus exiting southeast
Missouri and southern Illinois with only scattered low clouds
over far northeast Missouri and west central Illinois. Did make
some minor adjustments to lows based on current temperature
trends.

Britt

&&

.SHORT TERM...  (Through Late Friday Afternoon)
Issued at 330 PM CDT Thu Aug 17 2017

Cold front will continue to move through the forecast area this
afternoon. Drier air will flow in on the west to northwest wind
with dew points across the MO/IA border in the upper 50 to lower
60s. Have shaded low temperatures tonight to the low side of
guidance. Dry air, a clear sky and light wind by morning should
lead to a pretty good drop off in temperatures overnight. Likewise
for Friday, the dry air will heat up quickly so have shaded
toward the warmer MET guidance.

JPK

.LONG TERM...  (Friday Night through Next Thursday)
Issued at 330 PM CDT Thu Aug 17 2017

(Friday night - Sunday)

There is good model consensus on timing and track of a shortwave
TROF that will swing thru Friday night that will be coincident with
a weak surface cold front.  Most of the upper level energy will miss
our region to the north and northeast but certainly worthy to
maintain chance PoPs for this event with the possibility of a boost
into likely category once more confidence in the details is known.
Rain chances should quickly diminish Saturday morning.  With a
disturbance-free NW flow aloft for the remainder of Saturday and
rising heights aloft due to a retrograding RIDGE from the SE CONUS
on Sunday, rain chances look minimal after Friday night for this
period.  One potential item is some model hints at MCS development
in the Mid-MO river valley Saturday night that may track into
portions of our area on Sunday but this is low confidence at this
time.

In the wake of the weak cold front on Saturday, near seasonable max
temps can be expected.  Temps are then expected to edge a few degs
above seasonal averages on Sunday with flow already from the south
again.

(Monday - ECLIPSE DAY)

The trends of the overall pattern look more promising than what was
seen a couple days ago, with a developing, decent consensus on a
retrograded RIDGE aloft dominating areas just to our south, the
primary MCS track, should they develop, well to our north in IA, and
the main surface cold front also well to our north.  In addition,
any pcpn and cloud cover associated with what would develop in the
preferred region to our north should stay to the north with flow at
H250-300 from the due west.

But (and you knew this was coming) a warm front will be draped from
eastern Nebraska to southwest MO early Monday morning and as it
slowly pushes northeastward towards central and northeast MO, is
anticipated to weaken and even dissipate by later on Monday.  While
a MCS event is expected to occur over the Mid-MO river valley
Sunday night and track eastward across IA on Monday (and keep its
associated cirrostratus shield to our north as well), there should
be enough lift near the warm front at a time that traditionally
has a weaker cap setup late Sunday night into Monday morning to
pop a few TSRA to our west. As the front weakens and lifts towards
central MO by late Monday morning, convective activity is
expected to diminish. The trick will be will there be convective
activity close enough upstream in the westerly flow aloft to also
bring the associated high clouds to disrupt eclipse viewing around
18z.

All in all, in our CWA, there is better support for good eclipse
viewing for areas further east into east-central and southeast MO
and southern IL which should be further away from what could be
isolated TSRA associated with the weakening warm front further west,
with central MO still looking to be in the worst spot in our CWA.

The other eclipse caveats mentioned in the previous discussion still
apply with what could be a brief cool down in and around totality
of up to 10-20 degrees, some dissipation of lower convective
based cloud cover (no effect on higher clouds) due to temps
lowering below convective values. The brief cool down will
probably have minimal effect on the bottom line on max temps with
a fast recovery expected into the upper 80s and low 90s.

(Tuesday - Thursday)

Another upper shortwave TROF is set to track thru on Tuesday with a
strong surface cold front late Tuesday.  This should represent our
best and most widespread rain chances of the next week.  How fast
convection kicks off Tuesday will determine how warm to go.  A delay
until late afternoon will likely result in easily the warmest day on
Tuesday with temps well into the 90s and possibly a heat headline.
Timing issues being so far out are preventing any significant
adjustments from guidance with confidence.

Otherwise, NW flow aloft will prevail Wednesday and Thursday with a
decently strong high pressure area building in and NE flow at the
surface.  Should see a pair of dry days for most locations with
below average temps.

TES

&&

.AVIATION...  (For the 06z TAFs through 06z Friday Night)
Issued at 1130 PM CDT Thu Aug 17 2017

Mainly dry and VFR conditions are expected through the end of the
period. Only exception will be at KSUS where some river fog may
develop between 09-13Z. Otherwise winds have become light as high
pressure moves across the Middle Mississippi Valley.

SPECIFICS FOR KSTL: Dry and VFR conditions are expected through
the period. Winds will become light and variable as high pressure
moves through the area.

Britt

&&

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

$$

WFO LSX



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