Area Forecast Discussion
Issued by NWS St. Louis, MO
FXUS63 KLSX 260443
AREA FORECAST DISCUSSION...UPDATED AVIATION
NATIONAL WEATHER SERVICE ST LOUIS MO
1143 PM CDT Fri Jul 25 2014
Issued at 938 PM CDT Fri Jul 25 2014
The 00z mdl data is still just rolling in, so is difficult to
justify any radical changes to the going forecast. However, the
SREF/local WRF suggest TSRA will be possible over nrn and central
portions of the CWA late tonight and thru sunrise. The HRRR
suggests this too to some degree as new cycles come in. Believe
enuf confidence exists to add slight chance PoPs Sat morning after
sunrise. PoPs late tonight and into Sat morning may need to be
raised again after more mdl data arrives. Best scenario is that
TSRA will form just N of the CWA on the nose of the LLJ and
gradually work swd to swwd twd sunrise as the LLJ veers. Mdl
soundings suggest a fair amount of elevated CAPE with little to no
CIN by around 09z tonight over at least central portions of the
CWA. All of that said, the lack of convection is troubling given
that mdls suggest that at least something shud be going on by now.
However, the cu field developing over far sern IA does give some
hope that the mdls may not be far off.
.SHORT TERM: (Through Late Tonight)
Issued at 403 PM CDT Fri Jul 25 2014
A thunderstorm complex, which began the day over far northeastern MO
and west-central IL, never fully died and has been working its way
inexorably southeast with several waves of precipitation. The final
wave is progressing thru STL metro now and will deliver a quick shot
of rain. It is weakening, despite being more successful at pushing
this far south than the other waves, and doubtful how much will
survive into southeast MO and far southern IL.
A warm front has been effectively held at bay over western and parts
of central MO today thanks to the rain-cooled air and thick cloud
cover downstream which has been steadily reinforced by the
persistent storms. Temps vary from near 90 just south and west of
COU/JEF to the mid 60s in central IL.
TSRA complex should be out of the CWA by 00z early this evening, if
not dissipated, and at that point, warm front should begin its
advance. If we clear out enough, could see a drop in the evening to
where overnight mins may occur around midnight before a steady or
slow rise occurs after midnight as warm front edges through. Would
not be surprised to see another round of TSRA fire to the north
ahead of the warm front and where better upper support is, with what
does develop then trying to push southeast into our region very late
tonight again or Saturday morning. The expected path of this pcpn
should be further east than what we saw with this event today due to
the anticipated position of the warm front.
Prefer something along the lines of the cooler MAV temps in the
east, and approaching the warmer MET MOS in central MO.
.LONG TERM: (Saturday through Next Friday)
Issued at 403 PM CDT Fri Jul 25 2014
Two items of concern for Saturday. First: the 850mb thermal ridge
is still forecast to to build almost directly across the I-70
corridor Saturday. MOS temperatures in the mid to upper 90s still
look very reasonable under the thermal ridge with low to mid 90s on
either side. With low to mid 70s dewpoints entrenched over the
area, would issue a wide ranging heat advisory along and either side
the I-70 corridor, however it looks like the atmosphere will mix
fairly deeply allowing drier air to mix to the surface, especially
east-northeast of the Ozark Plateau in central Missouri. This
should keep heat index values below 105 degrees even though it will
be fairly hot. However, St. Louis looks to be far enough to the
east northeast that dewpoint temperatures will remain a bit higher
and the heat island of St. Louis and surrounding urban areas will
likely keep ambient temperatures higher as well. Heat index
values kiss 105 for a couple of hours Saturday afternoon, so will go
ahead and issue a Metro Area heat advisory.
The second concern is severe weather for Saturday afternoon and
night. Models continue to show impressive instability between
4000-5000 J/Kg developing Saturday afternoon above a capping
inversion in the low levels. Am leaning toward the GFS solution
which would initiate convection during the late morning or early
afternoon Saturday somewhere over western Iowa as a shortwave
ripples by and wipes out the cap. Think storms should grow upscale
into a severe MCS and push east southeast with the primary threat
being damaging winds. I think the greatest area of concern is
actually across southeast Iowa into central Illinois. However,
given the degree of instability and uncertainty regarding the
strength of the cap on the northern edge of the low level baroclinic
zone, I cannot totally rule out severe weather over our area
Saturday afternoon and Saturday night.
Cold front will move thought the area Sunday morning and should be
down across the Ozarks by 18Z. Expect there will be isolated to
scattered convection continuing along the front as it passes.
Should also see a decent warm up on Sunday as cool air lags behind
the actual wind shift by a bit. However, it should get noticeably
drier fairly rapidly after FROPA so no heat advisory should be
needed for Sunday.
Medium range models remain locked on a highly amplified solution
next week with an abnormally deep longwave trof over the eastern 1/2
of North America and a very sharp ridge over the west. This will
keep the Mississippi Valley locked in northwest flow aloft. This
will allow cool and dry Canadian air to push across the eastern 1/2
of the CONUS. The deep trof should allow steep lapse rates to
develop over the area which will likely produce isolated showers and
possibly a few thunderstorms in the afternoon as the airmass begins
to modify later in the week.
.AVIATION: (For the 06z TAFs through 06z Saturday Night)
Issued at 1131 PM CDT Fri Jul 25 2014
VFR fcst with a couple chances of TSTMs. The first being a low
probability of precip around the metro area around sunrise. There
is a chance that the LLJ could aid in convection breaking out along
a warm front across sthrn IA and nthrn MO overnight. The bndry stretches
from S cntrl IA SE along the MS rvr and into the lower MS vly. Feel
chance of occurrence is too low to include mention attm. The next
chance is late Saturday aftn...thru the evng and into the
overnight hrs...esp for KUIN...as a cold front sags into the
region. With concerns over coverage and timing...have left the
fcst for now. Otherwise...expect winds aob 12kts with a srly component.
Specifics for KSTL:
VFR fcst with minimal chances for precip thru the prd. First
chance is around sunrise...but is too low to include attm. The
other chance is during the evng and into the overnight...but
coverage and timing issues preclude inclusion in the fcst attm.
MO...HEAT ADVISORY from 1 PM to 7 PM CDT Saturday FOR St. Charles MO-
St. Louis City MO-St. Louis MO.
IL...HEAT ADVISORY from 1 PM to 7 PM CDT Saturday FOR Madison IL-St.