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Area forecast discussion
National Weather Service Saint Louis MO
359 am CDT Mon Aug 19 2019

Short term... (through tuesday)
issued at 356 am CDT Mon Aug 19 2019

A weak cold front over Iowa is moving slowly south this
morning...and may get as far as northeast Missouri and west central
Illinois before stalling later today. There probably won't be much
of a change in the sensible weather as the front moves south, other
than a slow wind shift to the north or northwest and perhaps a
slight drop in dew point temperature. Forecast soundings show quite
a bit of instability building up today with a warm and moist
boundary layer and 2m dew point temperatures in the mid 70s across
much of the forecast area. The forecast dew point temperatures may
be a little high as most current observations are in the upper 60s
to low 70s...and yesterday afternoon's dew points were mainly in the
low 70s as well. Additionally, forecast soundings are showing an
inversion around 750mb across Central, Northeast and east central
Missouri as well as west central and parts of southwest Illinois
this afternoon. This inversion should keep a lid on convection
today across much of the area...if the model soundings are correct.
The inversion weakens the further south and southeast in Missouri
and Illinois that you look, so I think that chance pops from
southeast Missouri and south central Illinois fading to slight
chance to the northwest remains a reasonable forecast today. The
current forecast has afternoon highs ranging from the mid 80s in
northeast Missouri to the low 90s in southeast Missouri and
southwest Illinois. The latest MOS guidance is a little warmer in
northeast Missouri and west central Illinois so I think i'll tweak
up there, but the the low 90s still look good further south.

Attention turns to tonight and Tuesday. Any convection over the
area this afternoon should dissipate during the early evening with
the loss of diurnal heating. Short range guidance shows the low
level jet re-focusing over eastern Nebraska or western Iowa
overnight. Strong moisture convergence along with a shortwave
moving into the Midwest on northwest flow aloft ignite nocturnal
convection. There is some disagreement in the guidance on exactly
where and when the storms will fire, but both the GFS and NAM have
4000+ j/kg MUCAPE over eastern Nebraska and western Illinois around
06z. 30-40kts of 0-3km and 0-6km shear along with the strong
instability will promote a well-organized mesoscale convective system. Models show a strong
cold pool developing after 06z as the storms get organized, and
forward propagating corfidi vectors are pointing to the southeast
which would bring the mesoscale convective system down into southern Iowa/northern Missouri
between 09-12z. The GFS, and the convection allowing models
including the hrrr, NAM 4km, and arw/nmm WRF are in pretty
remarkable agreement on this scenario. These models all push the
slowly decaying mesoscale convective system southeast through our forecast area Tuesday
morning into the early afternoon. The operational NAM is an outlier
with the mesoscale convective system moving east-southeast through Iowa into Illinois and
then just clipping eastern Missouri as it turns more southeastward
during the day. Am leaning toward the model consensus for Tuesday
as the more eastward solution of the NAM is not supported by the
corfidi vectors.

An mesoscale convective system moving through the area on Tuesday could put
the kibosh on the excessive heat potential for Tuesday afternoon
depending on how fast it moves and how much convective debris it
leaves behind. Will decrease temperatures slightly from previous
forecast...which may just barely keep heat index values below
advisory criteria. Will therefore hold off issuing any heat
advisory this morning for Tuesday due to the uncertainty.

Carney

Long term... (wednesday through next sunday)
issued at 356 am CDT Mon Aug 19 2019

Medium range guidance shows the upper level pattern slowly
amplifying for Wednesday through Friday. An upper level low digs in
and deepens over southeast Canada Wednesday which forces a surface
cold front through the mid Mississippi Valley. Models are hinting
at another mesoscale convective system ahead of the front on Wednesday, and then more
convection Wednesday night into Thursday as the front continues
drifting south through Missouri and Illinois. The European model (ecmwf) is more
aggressive in moving the front into the lower Mississippi Valley and
eastern Great Plains, while the GFS hangs the front up over southern
and western Missouri. The GFS is therefore much wetter than the
European model (ecmwf) as dry air from the high pressure system behind the front
doesn't drive as far west/southwest into the area. Given the
uncertainty I kept an ensemble blend of chance/slight chance pops in
the forecast through the weekend. Temperatures should be more
moderate on the "cool" side of the front though with highs mainly in
the low to mid 80s and lows mainly in the 60s.

Carney

&&

Aviation... (for the 06z tafs through 06z Monday night)
issued at 1114 PM CDT sun Aug 18 2019

Precip is looking rather unlikely through the overnight hours so
attention turns to fog potential. Given clearer skies than
previously expected, dewpoint depressions have dropped, currently
sitting between 0 and 10 degrees. With additional cooling
expected in the early morning, coupled with weak and variable
winds, there seems to be a reasonable chance for patchy fog across
the area. This is especially likely in the typically foggy areas
like river valleys and east of the Mississippi where current
dewpoints depressions are near 0. Guidance agrees with this
assessment, painting patchy fog across those areas. All that said,
i'm not confident the patchy fog will actually form over a
terminal, and i'm not sure how low the visbys will drop. For now,
have stuck close to guidance with just 6sm fog at all terminals
save cou through just after sunrise.

Beyond that, the primary focus will be for shower and storm
chances along the cold front expected to sag south through the day
tomorrow. Forcing along the front will be fairly weak, so I don't
anticipate much more than initially isolated storms across our
north, turning scattered in our south in the afternoon. Generally
VFR conditions will prevail, though if a terminal is hit by one
of the storms, conditions will obviously drop.

Bsh

&&

Preliminary point temps/pops...
Saint Louis 93 76 93 77 / 30 20 40 20
Quincy 89 73 89 74 / 20 10 50 30
Columbia 91 73 92 72 / 20 5 50 30
Jefferson City 93 73 93 72 / 20 10 40 20
Salem 91 72 90 74 / 40 20 50 20
Farmington 93 72 93 72 / 40 20 50 10

&&

Lsx watches/warnings/advisories...
MO...none.
Illinois...none.

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