Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 141114

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
614 AM CDT Tue Jul 14 2020

.SHORT TERM... (Through Wednesday Night)
Issued at 326 AM CDT Tue Jul 14 2020


High pressure is currently situated over the Great Lakes and will
continue to slide slowly eastward through the day today.
Southerly winds are forecast to gradually increase across the bi-
state area as the pressure gradient increases between the
aforementioned area of high pressure to our northeast and a cold
front draped from the south-central Plains into the Upper Midwest.
Dry weather is favored today as the cold front stays well
northwest of the area. Humidity is expected to slowly increase
through the afternoon, but stay on the low side for most of the
area. The exception will be northwestern sections of the region as
dewpoints approach 70 degrees closer to the incoming cold front.
High temperatures today should be about 2-3 degrees warmer than
yesterday due to weak low-level cold advection beneath plentiful
sunshine. Look for highs to range from the upper 80s to low 90s.
Highest heat index values are most likely this afternoon across
portions of central Missouri where upper 90s to near 100 degrees
are possible.

(Tonight - Wednesday Night)

The main foci for tonight through Wednesday will be on convective
trends associated with the cold frontal passage and possible high
heat index values Wednesday afternoon ahead of the boundary.

Thunderstorms that are likely to develop by late this afternoon
across the mid-Missouri Valley are expected to congeal and become
more widespread overnight. The convection should move
east/southeast due to a veering low-level jet and southeastward
progress of the cold front. Storms are likely to reach portions of
northeastern Missouri toward dawn Wednesday morning. Locally
heavy rainfall appears possible in this region due to the slow-
moving nature of the cold front in a favorable environment for
very efficient rainfall characterized by precipitable water values
1-2 standard deviations above normal and deep warm cloud depths
(>12 kft). Some sort of weakening trend of storms is more likely
than not on Wednesday morning due to low-level moisture
convergence waning slightly, though would not expect total
dissipation as mid/upper level forcing for ascent remains fairly
strong. Exactly how convection evolves Wednesday morning will
have big impacts on the potential heat and possibility of severe
weather Wednesday afternoon. Definitely not a high confidence
forecast for specifics with respect to either of these potential
hazards, but given the anticipation of morning storms from central
Missouri into west-central Illinois, believe odds favor east-
central and southeast Missouri as well as southern Illinois for
heat and severe weather potential along/ahead of the cold front.

More specifically regarding the heat potential, it will come down
to how far south and east convection gets by late Monday morning
and how well organized it remains. A more well organized complex
would have thicker debris clouds further downstream limiting the
heat potential, whereas weakening or dying convection would
easily allow temperatures into the low to mid 90s. Those
temperatures combined with dewpoints in the low to mid 70s would
yield peak heat index values near magnitude heat advisory
criteria (105F). Given the uncertainties with how convection
evolves Wednesday morning, not confident enough to issue a heat
advisory with this package, but one could be needed down the road.

The main area of concern for severe thunderstorms will be
along/ahead of the southeastward moving cold front. Degree of
instability is unknown ahead of this front due to the
aforementioned uncertainty with convective trends Wednesday
morning. With the anticipation that storms likely will not totally
die out and percolate given the favorable mid/upper level forcing
for ascent, best guess is that more likely area to see strong-
severe convection would be around the I-44/I-70 corridors in
Missouri and Illinois respectively and points southeast. Multicell
clusters and possible bowing segments make damaging winds the
primary threat Wednesday afternoon and evening. Storms may
continue along the slow-moving front much of Wednesday night,
mainly across parts of southeast Missouri and southwest Illinois.
However, storms should weaken with loss of daytime instability
and subsidence setting in aloft behind the departing shortwave


.LONG TERM...  (Wednesday Night through Monday)
Issued at 326 AM CDT Tue Jul 14 2020

(Thursday - Friday)

The focus will remain on heat potential and thunderstorms through
the end of the work week.

Models have come into better agreement with the position of the cold
front Thursday morning, generally draped from southwest to northeast
near the confluence of the Ohio and Mississippi Rivers. This
front is forecast to stall out on Thursday. A shortwave trough on
the northern periphery of the midlevel anticyclone is expected to
help initiate at least scattered thunderstorms in the vicinity of
the front, with a focus on the Thursday afternoon timeframe.
Given the warm, moist and unstable airmass, some of these storms
may be strong to severe across southeast Missouri and southwest

This front looks to be a problem through at least Friday as models
now suggest a slow retreat of it to the north as a warm front. This
boundary could continue to provide the focus for showers and
thunderstorms within zonal flow aloft, which may also act to delay
its northward progress.

The other potential area of concern remains the heat. High dewpoints
will remain in and around the frontal zone, with low to mid 70s
likely. However, the main uncertainty lies with high temperatures
each day. Given that the front should be located south of most of
the CWA on Thursday, have shaved off a degree or two off of
Thursday`s highs. However, peak heat index values of 100-105F still
are possible Thursday afternoon, particularly for southeast
Missouri and southwest Illinois. If storms form early enough
however, these values may be a bit too high as temperatures could
stay a few degrees cooler than forecast.

Models have also backed off slightly on high temperatures on
Friday with the possibility of convection near the retreating warm
front and associated cloud cover. However, dewpoints may continue
to pool along this front, so the expectation is for more
widespread peak heat index values of 100-105F across the area.

(Friday Night - Monday)

The main thrust of the heat wave appears to be focused from this
weekend into (at least) early next week as the warm front finally
surges north of the CWA. In addition, the midlevel ridge builds
poleward more into the mid-Mississippi Valley coincident with
850-hPa temperatures climbing into the +20-25C range. The airmass
itself does not appear too extreme as both 850-hPa temperatures
and 500-hPa heights are around 1 standard deviation above normal
on the NAEFS, helping to yield high temperatures generally in the
mid to upper 90s each day. However, these temperatures combined
with dewpoints in the low 70s are expected to result in max heat
index values each day around or slightly above 105 degrees for
large portions of the area. What could be quite noteworthy is the
longevity of this heat wave. No signs of the mid/upper level
pattern changing very fast across the CONUS so the heat/humidity
is more likely than not to continue well into next week. Of
course, this longevity is of high concern. Previous research has
shown increases in mortality as a heat wave continues in large
part due to a lack of relief/cooling down period in buildings
without air conditioning.



.AVIATION...  (For the 12z TAFs through 12z Wednesday Morning)
Issued at 608 AM CDT Tue Jul 14 2020

Dry/VFR conditions with a southeast wind around 10 knots is
forecast across the area through this evening. A cold front is
forecast to approach from the northwest toward dawn on Wednesday
bringing a chance of thunderstorms. The best chance of seeing
storms late tonight is across portions of central Missouri
northeastward into west-central Illinois. Not sure how far
southeast the storms will get (and how fast) so left VCTS mention
out for now at KCOU and KUIN.


Dry weather forecast through tonight with southeast winds around
10 knots. A cold front will begin to approach late Wednesday
morning. This front should bring scattered storms but not
confident on how far south and east the storms will get at this
point in time so left out VCTS mention.





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