Severe Thunderstorm Warning 117, NWS Raleigh, NC

AREA FORECAST DISCUSSION
National Weather Service Raleigh NC
349 PM EDT Sat Jul 18 2020

.SYNOPSIS...
Hot, subtropical high pressure will generally remain in place
through the next week.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 300 PM Saturday...

Water vapor satellite showed a broad, upper-level ridge in
place across the region stretching from Alabama to the eastern
Carolinas. Surface observations showed a weak surface trough
stretching from the north Atlantic coast down across central
North Carolina and a Bermuda high to our east. Radar displayed
widely scattered thunderstorms across the region thanks to
MLCAPE of 2000-2500 J/kg, diurnal heating and sufficient
lower/mid-level moisture. Slow-moving storms could produce an
isolated flash flooding threat this afternoon, but the main
threat with stronger storms is wind gusts of 30-40 mph, with
some storms producing severe wind gusts to 60 mph. SPC has
upgraded the western portion of our area to a Marginal Risk of
severe storms. Southwesterly flow near the surface will keep the
moist airmass in place across the region, with afternoon dew
point temperatures remaining in the low to mid 70s. Combined
with highs in the low to mid 90s, the heat indices have reached
the low to mid 100s again today for most areas. Lows tonight
will be in the low to mid 70s, with an isolated
shower/thunderstorm possible mainly across the Coastal Plain.

&&

.SHORT TERM /SUNDAY AND SUNDAY NIGHT/...
As of 330 PM Saturday...

...Heat Advisory Now in Effect for Most Areas Sunday...

High pressure aloft and high surface dewpoints will create
another dangerously hot day across the region on Sunday, with
temperatures a few degrees higher than Saturday. This has
prompted the issuance of the first Heat Advisory of the year for
all of the region from noon to 9PM. Heat indices above 102 will
be widespread, with portions of the Coastal Plain and northeast
Piedmont reaching around 105 degrees. Isolated areas could feel
as hot as 107. Some areas near the Triad will have the lowest
heat indices closer to 100. Scattered convection will develop
again in the afternoon and evening, however shear remains weak
while MLCAPE remains high. Storm threats Sunday will be similar
to Saturday, with isolated flooding and gusty winds possible.

Most convection should end not long after sunset, with an
isolated shower possible along the Coastal Plain. Lows will
remain above seasonal normals, only lowering to the mid 70s
across the region.

&&

.LONG TERM /MONDAY THROUGH SATURDAY/...
As of 300 PM Saturday...

...A prolonged period of heat expected through the upcoming week...

The large 592-595dm 500mb ridge will remain the dominant driver of
our weather for the upcoming week. The ridge axis will move little
this week and as such, temperatures will be consistently in the mid
90s each day through the end of the week with lows generally in the
low to mid 70s. Statistical guidance, as mentioned by the previous
shift, is trending downward with temps late in the forecast but this
is unlikely as the deterministic mass fields suggest steady state
temperatures through the end of the week. The only relief in sight
is in the form of a weak cold front/trough that is expected to move
through the Great Lakes and into portions of the Mid Atlantic next
Saturday. These fronts often struggle to make it this far south
before washing out and I'm not entirely sold on a post-frontal
regime setting up across central NC with such a strong ridge nearby.
Thus, hazy hot and humid weather is here for the foreseeable future.
Daytime heat index readings in the 102-105 degree range are expected
and Heat Advisories are likely in the cards this week.

Not to be forgotten, precip chances will be very typical for summer
this week. Surface high pressure near Bermuda will continue to
advect anomalously high PW's into the area with values around 1.5 to
1.7 inches common this week. Afternoon showers and thunderstorms are
expected each day, some of which will be capable of producing
locally heavy rain. However with weak steering flow aloft and the
lack of any discernible organized forcing features, the threat of
widespread severe weather or flooding rains appears to be low.

&&

.AVIATION /19Z SATURDAY THROUGH THURSDAY/...
As of 200 PM Saturday...

24 hour TAF period: Scattered TSRA will continue through the evening
across most of the area, especially over portions of the Piedmont
and Sandhills. In and around storms, MVFR ceilings and VIS, frequent
lightning, gusty and erratic surface winds and strong up/downdrafts
are likely. Otherwise, VFR conditions are expected through the
period with winds generally less than 10 kt.

Beyond the TAF period: Afternoon/evening TSRA/SHRA and the potential
for early morning FG/low-stratus will continue each day through the
period. Otherwise, VFR conditions are expected.

&&

.RAH WATCHES/WARNINGS/ADVISORIES...
Heat Advisory from noon to 9 PM EDT Sunday for NCZ007>011-
021>028-038>043-073>078-083>086-088-089.

&&

SYNOPSIS...MWS
NEAR TERM...JJT
SHORT TERM...JJT
LONG TERM...DL
AVIATION...JJT

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