Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 150446
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1146 PM CDT Fri Jul 14 2017

.SHORT TERM...  (Through Late Saturday Afternoon)
Issued at 330 PM CDT Fri Jul 14 2017

High pressure over Minnesota will drop southeast into Illinois over
the next 24 hours.  Drier air is filtering south out of the high
this afternoon...but dewpoints remain in the low to mid 70s along
and south of the I-70 corridor this afternoon.  The northerly flow
pushing into the warm and humid airmass over southern Missouri is
producing scattered showers and thunderstorms...primarily over our
eastern Ozark counties.  This activity will continue moving slowly
southeast through the remainder of the afternoon.  Expect the
convection to dissipate this evening after we lose daytime heating.
Sky should clear out over most of the area tonight and with drier
air pushing in the temperatures should drop into the low to mid 60s
over much of the area.  Think some fog is likely across the Ozarks
tonight where the drier air will have less penetration...especially
where it rains this afternoon and evening.  Saturday still looks dry
and seasonably warm under the influence of high pressure with highs
mainly in the mid 80s with low humidity and light northeast flow.

Carney

.LONG TERM...  (Saturday Night through Next Friday)
Issued at 330 PM CDT Fri Jul 14 2017

Upper level HIGH will begin the period over the western CONUS with
NW flow aloft over our region, with this pattern expected to
continue into most of Monday.  Few if any upper level disturbances
are expected during this time.  However, this flow will encourage a
backdoor cold front to drop into our region from the northeast
beginning late Sunday afternoon and continue to edge into the MS
river area thru most of Monday.

Temps will already begin rebounding on Sunday back to seasonal
averages after what looks to be the peach day on Saturday as the
airmass that dropped in begins to modify and exit to the east.

The initial intrusion of the backdoor cold front late Sunday
afternoon should carry with it enough low level convergence to
justify mentionable PoPs and have expanded their coverage further SW
in response to a better push in that direction indicated by the more
recent model guidance.  The backdoor front is expected to stall and
begin to lose its identity on Monday at some point around the MS
river and still questionable whether the low level convergence will
be enough to warrant mentionable PoPs on Monday afternoon.

This beginning of loss of identify of the cold front Monday is in
response to changes aloft with heights building as the upper HIGH
over the western CONUS drifts east and expands.  Areas behind (to
the east) of the front on Monday will probably see temps comparable
to Saturday while areas ahead (to the west) of the front will see
temps at or slightly higher than they were on Sunday, and this will
create a temp gradient somewhere near or just east of the MS river.

The big story remains the return of dangerous heat and humidity for
much of next week as the upper level HIGH builds overhead for
Tuesday and continues into Friday and continues to have strong model
consensus.  The result will be air temps in the 95 to 102 range with
afternoon and early evening heat index values peaking between 103
and 110 for many areas.  The highest values will be in the STL heat
island.

A change from yesterday was that the models delayed any real change
to the RIDGE aloft for late week by a day, backing it off to at
least late Friday now, effectively extending the period of dangerous
heat and humidity (and delaying relief via a front and its
associated rain) by 24 hours.  The EC from yesterday led this delay
with the other models following and it continues to show no frontal
passage thru the end of the forecast period.

TES

&&

.AVIATION...  (For the 06z TAFs through 06z Saturday Night)
Issued at 1145 PM CDT Fri Jul 14 2017

VFR and dry conditions expected thru the TAF period. Winds will be
light and nely to enely. With an ely wind and drier air advecting
into the area, do not currently expect fog to be an issue at KSUS.

Tilly

&&

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

$$

WFO LSX



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