Area Forecast Discussion
Issued by NWS St. Louis, MO

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000
FXUS63 KLSX 210831
AFDLSX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ST LOUIS MO
331 AM CDT Thu Aug 21 2014

.SHORT TERM:  (Through Tonight)
Issued at 200 AM CDT Thu Aug 21 2014

Focus continues to be precip chances influencing temps thru the
period.

Mdls are in good agreement regarding synoptic features and mass
fields. However, subtle differences in thermal profiles and
mesoscale features will be important over the next few days. Main
question is centered around ongoing convection over NEB and nrn
KS. Believe the GFS handles this s/w the best tracking it thru
cntl IA today along the sfc boundary. Any subsequent outflow
boundaries will determine how far S convection will propagate this
afternoon and tonight. Believe the area will remain dry this
afternoon due to timing of the s/w as well as a lack of stronger
forcing. A broad LLJ tonight shud be enuf forcing to support TSRA
that may impact nrn portions of the CWA. The most likely timing
will be late tonight at will at least in part depend upon how
convection over IA region plays out today.

As for temps, have trended twd the cooler guidance which has
verified well even in places that did not see TSRA over the past
couple of days. Mdls are slightly cooler with 850mb temps over the
region. For nrn counties, forecast soundings suggest those regions
will not mix to 850mb anyway. Prelim forecast of 90 degrees at UIN
may be too warm due to convective debris. Further S, warmer temps
will be offset by better mixing and therefore, cooler dewpoint
temps.

Overall, the resulting heat index values thru this afternoon are
similar to what was forecast yesterday. Have therefore not made
any changes to the going headlines.

.LONG TERM:  (Friday through Wednesday)
Issued at 309 AM CDT Thu Aug 21 2014

Focus continues to be temps thru the period with ongoing heat
headlines.

Overall, not many changes to the prev forecast. The GEM has come
into better agreement with other mdl solns, but does still break
down the upper ridge quicker. Questions regarding the need to
expand the current advisory remain the same as they did yesterday.
Mdls depict a weaker cap across much of the region, but especially
across nrn portions of the CWA. This area is also closer to the
sfc boundary and may therefore experience more convective debris
into the weekend, if not storms. Have therefore kept low PoPs
going for this area and trended temps cooler. Elsewhere, continued
a trend twd the cooler MAV guidance as mentioned above.

Mdl solns begin to diverge early next week with the ECMWF now
reinforcing the upper ridge, keeping it in place thru the middle
of the week. However, the GFS/GEM suggest a weaker upper ridge
across the cntl U.S. Regardless, all mdl guidance suggests cooler
mid level temps which may allow for afternoon/nocturnal convection
across the region. Have kept PoPs on the low end until mdls come
into better agreement and confidence builds.

Due to questions mentioned, have not made any changes to the
ending of the going headlines. However, headlines may need to be
extended if the upper ridge holds into next week.

&&

.AVIATION:  (For the 06z TAFs through 06z Thursday Night)
Issued at 1043 PM CDT Wed Aug 20 2014

Complex of showers and thunderstorms which brought locally heavy
rain and strong wind gusts is currently moving into south-central
Illinois away from TAF sites. Rest of the tonight and tomorrow
period will likely be dry...though secondary warm front which
helped trigger organized convection earlier is still forecast to
eventually move back north and east tonight. Also cannot rule out
some diurnal airmass type thunderstorms tomorrow as capping does
not look too strong. Will keep TAFs though dry for now due to a
low confidence in either one of these scenarios actually
occurring. Other issue in the short term is fog potential as skies
are already clearing out behind the MCS in Illinois. Will have to
monitor this close through the night as combination of clear skies
and high dewpoints may cause fog development overnight
tonight...especially at KSUS and KCPS which received some heavy
rainfall earlier this evening. One factor that may preclude fog
development is winds look to stay up aoa 5 knots.


Specifics for KSTL:

Rest of the tonight and tomorrow period will likely be
dry at the terminal...though secondary warm front which helped trigger
organized convection earlier is still forecast to eventually move
back north and east tonight. Also cannot rule out some diurnal
airmass type thunderstorms tomorrow as capping does not look too
strong. Will keep TAF though dry for now due to a low confidence
in either one of these scenarios actually occurring. Other
potential in the short term is fog...but believe winds should stay
up enough so will not include in TAF for now but will monitor
overnight as it is a possibility due to clearing skies and very
high dewpoints/low level moisture.


Gosselin

&&

.PRELIMINARY POINT TEMPS/POPS:
Saint Louis     95  77  97  77 /   5  10  10  10
Quincy          90  73  92  73 /  10  20  20  20
Columbia        95  74  96  74 /  10  10  10  10
Jefferson City  95  74  97  75 /   5  10  10  10
Salem           93  74  92  74 /  20  10  10  10
Farmington      93  73  95  73 /  20  10  10   5

&&

.LSX WATCHES/WARNINGS/ADVISORIES:
MO...HEAT ADVISORY until 7 PM CDT Sunday FOR Audrain MO-Boone MO-
     Callaway MO-Cole MO-Franklin MO-Gasconade MO-Jefferson MO-
     Lincoln MO-Madison MO-Moniteau MO-Montgomery MO-Osage MO-
     St. Charles MO-St. Francois MO-Ste. Genevieve MO-Warren MO.

     EXCESSIVE HEAT WARNING until 7 PM CDT Sunday FOR St. Louis City
     MO-St. Louis MO.

IL...HEAT ADVISORY until 7 PM CDT Sunday FOR Bond IL-Calhoun IL-
     Clinton IL-Greene IL-Jersey IL-Macoupin IL-Marion IL-Monroe
     IL-Randolph IL-Washington IL.

     EXCESSIVE HEAT WARNING until 7 PM CDT Sunday FOR Madison IL-St.
     Clair IL.

&&

$$

WFO LSX






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