Scientific Forecaster Discussion
fxus62 kchs 291409
Area forecast discussion
National Weather Service Charleston SC
1009 am EDT Thu Jun 29 2017
high pressure to the north will move into the Atlantic today,
where it will persist into next week. A surface trough will
develop inland on Friday and prevail through the weekend.
Near term /through tonight/...
today: shortwave ridging aloft will begin to give way this
afternoon as the baggy trough and associated mid-level low
begin to track across portions of the southeast. At the surface,
high pressure situated to the northeast will move further out
over the Atlantic leaving the forecast area in a large area of
prevailing east to southeasterly low-level flow.
This onshore flow will allow for a steady moistening of the
lower troposphere today, with pwat's 1.5-2.0 inches south of the
Savannah River. Further north it'll take longer for vertical
moistening, as pwat's are as low as 1.0 inches near the Santee
river. Isentropic ascent on the 305k surface and a subtle mid
level perturbation that ripples through the ridge will supply
the necessary lift, with additional forcing from the sea breeze
and differential heating boundaries. Combined with the greater
moisture will support 30-50% pop's south of the Savannah River,
greatest to the south of I-16. Further north there is more of a
cap and less moisture and forcing, so no more than 20-30%
chances south of the Charleston tri-County, which looks to be
void of any convection through days end.
Convective temps are in the lower and middle 80s, and these
lower values have already allowed for a quick onset of showers
across parts of southeast Georgia. We held off on mention of any thunder
until afternoon, as there isn't much instability or cape. This
is typical in a low-topped easterly flow regime, and combined
with weak lapse rates and no shear suggests that any severe risk
is close to zero.
Our 850 mb temps are still a little below late June normals, and
combined with the influx of maritime air and increasing cloud
cover will prevent temps from reaching 90f yet again.
Tonight: diurnal convection will diminish in the evening with
the loss of heating, but deep moisture will be in place in the
presence of increasing mid/upper level support. This setup
should allow for more nocturnal showers and storms than we have
seen in previous nights, especially across the adjacent coastal
waters and the beaches. 30-40 percent chances are in place, but
it is likely some locations will eventually need higher values
depending on how things evolve with residual boundaries. We
could even see some fog, mainly well inland, but it has not been
added to the forecast as of yet. Increasing clouds will help
result in a warmer night with lows in the mid 70s in most
Short term /Friday through Sunday/...
a ridge will gradually build over our area during this time
period. At the surface, high pressure will move into the
Atlantic where it will persist into the weekend. Flow around the
high will allow deep moisture to advect into the area and
remain in place through the weekend. Plenty of instability will
also be in place each day. Convective initiation will mainly be
along the sea breeze corridor each afternoon. Models still hint
that a weak mid-level wave may move from the Gulf of Mexico over
our area late Friday. This could lead to increased convective
coverage and intensity. However, if it arrives in the evening,
following peak heating, then the effects won't be as noticeable.
Though, it could carry with it stronger storms from inland
towards our coast, which could interact with ongoing
thunderstorm outflow in our area and sea breeze circulations
during the evening hours to generate more convection. It's still
too early to determine the details, so we have chance pops. The
typically waning of convective activity overnight is expected.
Pops on Sunday are a bit lower due to slightly lower pwat's and
instability. But we're still going to keep them in the chance
Temps will remain below normal Friday owing to onshore flow and
ample cloud cover, before returning to near normal this weekend
as a more humid airmass fills in.
Long term /Sunday night through Wednesday/...
high pressure will prevail in the Atlantic while a surface
trough persists inland. Southerly flow around the high will keep
plenty of moisture advecting into the southeast during this
time period. The result will be the typical summertime
Aviation /14z Thursday through Monday/...
the first concern for the upcoming taf period is the potential
for thunderstorms to impact ksav this afternoon. It does not
look like thunderstorms will be a concern at kchs where drier
air is in place. Most model solutions at least show
thunderstorms in the vicinity of ksav this afternoon with many
depicting a direct hit at the terminal. Introduced a tempo for
MVFR thunderstorms conditions. This will likely need to be
refined as the afternoon approaches. The next issue is the
potential for ceiling development at both sites late tonight.
Model guidance has been hinting at this for a few days now and
confidence is high enough to introduce high end MVFR ceilings at
both sites in the 07-08z time frame. There could also be some
fog around, though it is expected to be well inland if it forms
Extended aviation outlook: brief flight restrictions are
possible in afternoon showers/thunderstorms at both terminals
today and tonight: high pressure situated to the northeast will
slip further over the Atlantic through the period. This will
result in a prevailing east to southeast flow through the
period. Winds this morning and afternoon will once again be
elevated, mainly around 15 knots though we could see some
periods of 15-20 knots just about everywhere. Then, as the
gradient relaxes late, wind speeds will diminish into the 10
knot range. Seas will generally range 2-3 feet, with some 4
footers possible in the outer Georgia waters.
Friday through monday: Atlantic high pressure will prevail
offshore while a surface trough persists inland. The interaction
between these two features will bring the coastal waters light
to moderate breezes, with the usual summertime sea breeze wind
enhancements near the coast in the afternoon, and possible brief
periods of gusty winds around daybreak. Seas will average 2-3
ft in small southeast swell and minor local windswell.