Area Forecast Discussion
Issued by NWS St. Louis, MO
FXUS63 KLSX 230341
AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE ST LOUIS MO
1041 PM CDT Fri Aug 22 2014
Issued at 1037 PM CDT Fri Aug 22 2014
Main change to forecast for overnight was to cut PoPs back below
mentionable levels. A secondary warm frontal type boundary does
remain draped over central portions of the CWFA. However...there
is a lack of forcing aloft to trigger anything as LLJ resides over
central KS with subsidence aloft from mid/upper level ridging.
Peaking into Saturday...it appears some activity may approach far
western or northwestern sections of the area as LLJ does veer to
more westerly. Did not add any PoPs at this time but mid shift
may need to take a closer look at this possibility. In
addition...latest guidance keeps aforementioned boundary nearly
stationary which could have implications on temps/dewpoints
tomorrow as well.
.SHORT TERM: (Through Late Tonight)
Issued at 303 PM CDT Fri Aug 22 2014
Similar to the past few days, isolated thunderstorms are expected
across the southern CWA due to terrain influences in the eastern
Ozarks. These storms will dissipate with the loss of daytime
Isolated to scattered thunderstorms are also possible across
northeast MO/west central IL tonight in the vicinity of a warm
front or outflow boundary. Favorable parameters overnight include
PW values near 2", H7-H5 lapse rates of 6 to 6.5 deg C/km,
sufficient MUCAPE for elevated thunderstorms, and a strengthening
LLJ. At this time, it looks like the best coverage should remain
to the north of the CWA.
.LONG TERM: (Saturday through Next Friday)
Issued at 303 PM CDT Fri Aug 22 2014
Dangerous levels of heat and humidity will remain across the
region through the weekend and into early next week. These
conditions are due to an upper level ridge of high pressure across
the central CONUS along with favorable surface winds that will
maintain high dew points. The Heat Advisory has been spatially
extended to cover additional counties in the eastern Ozarks where
dew points are no longer expected to mix out each afternoon. The
lack of substantial mixing will likely result in at least 4
consecutive days where heat index values exceed 100 degrees F.
This is supported by the Missouri Mesonet site in Crawford County
near Cook Station, which was already reporting heat index values
between 100-104 degrees F this afternoon. The Heat Advisory and
the Excessive Heat Warning have also been temporally extended
through Monday evening now that models have reached improved
agreement on the large scale upper air pattern through Tuesday.
Some portions of the Heat Advisory, especially across northern MO
and west central IL, might be canceled early if convective debris
cloudiness interferes with heating.
A cold front is forecast to end the stretch of hot/humid weather
during the middle of the upcoming week. The best opportunity for
widespread precipitation appears to be on Wed/Wed night with the
cold front and passage of the upper trough, but the details are
still uncertain this far in advance.
.AVIATION: (For the 00z TAFs through 00z Saturday Evening)
Issued at 507 PM CDT Fri Aug 22 2014
Although there may be isolated thunderstorms across much of the
area this evening in the humid and unstable environment, the best
threat for more widespread convection should be in the UIN area
this evening and overnight as a weak warm front lifts northward
tonight. Will include showers/storms in the UIN taf, but leave the
other tafs dry tonight. Should be just mid-high level clouds outside
of the convection tonight and Saturday morning, including high
level convective debris cloudiness. Mainly weak s-swly surface
winds through the forecast period.
Specifics for KSTL: The latest NAM model run appears overdone
with its precipitation forecast across east central MO tonight,
and will follow the other model guidance which keeps the
convection north and northeast of STL. Just mid-high level
cloudiness mainly tonight and Saturday morning. The swly surface
wind will weaken this evening, then only increase to 6-7 kts
MO...HEAT ADVISORY until 7 PM CDT Monday FOR Audrain MO-Boone MO-
Callaway MO-Cole MO-Crawford MO-Franklin MO-Gasconade MO-
Iron MO-Jefferson MO-Knox MO-Lewis MO-Lincoln MO-Madison MO-
Marion MO-Moniteau MO-Monroe MO-Montgomery MO-Osage MO-Pike
MO-Ralls MO-Reynolds MO-Shelby MO-St. Charles MO-St.
Francois MO-Ste. Genevieve MO-Warren MO-Washington MO.
EXCESSIVE HEAT WARNING until 7 PM CDT Monday FOR St. Louis City
MO-St. Louis MO.
IL...HEAT ADVISORY until 7 PM CDT Monday FOR Adams IL-Bond IL-Brown
IL-Calhoun IL-Clinton IL-Fayette IL-Greene IL-Jersey IL-
Macoupin IL-Marion IL-Monroe IL-Montgomery IL-Pike IL-
Randolph IL-Washington IL.
EXCESSIVE HEAT WARNING until 7 PM CDT Monday FOR Madison IL-St.