Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 172030
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
330 PM CDT Thu Aug 17 2017

.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
afternnon. 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 exected 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 cooldown 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 cooldown 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 18z TAFs through 18z Friday Afternoon)
Issued at 1258 PM CDT Thu Aug 17 2017

Cold front continues to roll throught he area. Front is through
UIN and COU and is moing through SUS now. The wind will become
more west to northwest and a bit gusty with the gusts generally 20
kts or less. Drier air moving in will lead to a clearing sky and
VRR coditions.

SPECIFICS FOR KSTL: Front moving through about now with the wind
becoming more westerly. Short range models try to bubble up a few
showers late this afternoon. If they do form they will be well
south of the terminal. VFR forecast.

JPK

&&

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

$$

WFO LSX



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