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Area Forecast Discussion

FXUS62 KCHS 270738

Area Forecast Discussion
National Weather Service Charleston SC
338 AM EDT MON JUN 27 2016

A cold front will stall and dissipate south of the area today.
Another cold front will approach from the northwest Tuesday,
becoming nearly stationary and lingering nearby Wednesday and
Thursday, before dissipating by Friday. Atlantic high pressure
will then extend across the area next weekend, before another cold
front approaches early next week.


Today: The morning will start off with a few showers and/or
thunderstorms potentially brushing the Charleston County coast.
This shower and thunderstorm activity is originating offshore
within a subtle surface trough and will continue to mainly impact
the adjacent coastal waters. For the remainder of the day, the
upper level pattern will feature upper level ridging that
continues to extend eastward across the area from the large
anticyclone that resides near the four corners region. For the
most part, NVA will prevail, though there are some model progs
that show a very weak embedded shortwave that pushes mainly off
the North Carolina coast. At the surface, the weak surface low and
associated trough will gradually dissipate leaving behind a rather
nebulous pattern dominated by mainly onshore easterly flow. Model
soundings and time heights reveal a very dry mid/upper level
atmosphere. Also, temperature profiles in the mid/upper levels are
quite warm and serve to effectively cap the atmosphere from a few
models. Even on the soundings that aren`t capped, lapse rates are
putrid and CAPE values struggle to reach 1000 J/kg. As such,
convective activity is expected to be quite limited and pops have
been limited to just slight chance for a small area mainly along
the South Carolina coast. For temperatures, the onshore flow will
help to keep areas further east a touch cooler and highs there
should top out right around 90. Further inland, highs up to around
94 or 95 will be possible for Allendale to Statesboro and other
similar points.

Tonight: The forecast area will be solidly positioned within a
large region of NVA aloft ahead of the approaching trough from the
northwest. An associated cold front will begin to approach late,
but the forecast is dry. A few showers and/or thunderstorms may
make a run for the far western zones by daybreak Tuesday.
Otherwise, a quiet night with lows mainly in the mid 70s.


Tuesday: Mid and upper level ridging will break down atop the local
area as the east coast trough begins to re-establish itself as
strong short wave energy moves through the Great Lakes. This forces
a cold front near the spine of the Appalachians early in the period
to approach from the northwest. Compressional heating in advance of
the cold front will allow for temps of 90F north to 95F south,
before cloud cover and rain chances climb in response to the
approaching front. Heat indices will peak at 100-103F south of I-16
in Georgia, but not high enough for our pre-July 1 criteria of 105F
or greater for a Heat Advisory. Lift and convergence as the front
draw closer will have ample moisture to work with as PWAT reaches as
high as 2 inches, or near the 90th percentile for this time of year.
Convective rain chances will build during the heating of the
afternoon, climbing into the 50-60% range northwest tier, with 30-
40% probabilities elsewhere. A sluggish storm motion and the elevated
moisture content supports a risk for localized flooding concerns in
persistent showers/t-storms. While the overall thermodynamics are
not overly impressive, dew points in the lower and middle 70s will
support steep low level lapse rates and that along with 20-25 kt of
0-6km shear might supply just enough of a boost to some of the
updrafts to support gusty winds with as much DCAPE as 800-1000 J/kg.

Wednesday through Thursday: The large scale pattern aloft will
feature a broad trough in the east and ridging in the west. The
surface cold front will struggle to make too much progress to the
south-southeast as it starts to become aligned parallel to to flow
upstairs. Thus we look for that front to waver over or near the
local zones during the middle of the week. While it is certainly
difficult to pinpoint if and when any short waves will slide through
the trough, better mid level lapse rates with the lower heights
aloft and at least seasonable PWAT will support a better than
average risk for showers and t-storms. Dependent upon where the cold
front is able to establish itself Wednesday and Thursday, there is
also the nearby proximity to the RRQ of the upper level jet and
indications of modest QG forcing per various models. That could
support even higher probabilities than the 40-50 Pop now shown in
the forecast. With the higher rain chances does come slightly lower
temps, with highs both days before the onset of convection to peak
a degree or two either side of 90F.


The pattern aloft flattens out with a quasi-zonal flow to develop
over the local vicinity by late Friday into Sunday, with the cold
front at the surface becomes diffuse and dissipates. This allows for
an expansion westward of the sub-tropical Atlantic ridge, while
Piedmont trough develops inland. This equates to a return to a more
typical pattern for summer, featuring mainly diurnal scattered
showers/t-storms. This allows for hotter temperatures and more
uncomfortable heat indices to occur. For early next week another
cold front may approach as the east coast trough begins to develop
yet again.


VFR conditions are expected to prevail at the terminals through
06z Tuesday. Through daybreak, we could see some patches of MVFR
ceilings or even some light fog, but probabilities appear to be
too low to include in the forecast. A few surrounding observations
have reported some restrictions, but overall the fog and stratus
has been rather transient. Then for the afternoon, very low
chances for showers and thunderstorms. Best chance for impacts
would be at KCHS, but even there the probabilities are too low to
include in the forecast.

Extended Aviation Outlook: Flight restrictions are possible at
least periodically in SHRA/TSRA Tuesday afternoon through
Thursday. Less chance of direct impacts on Friday, but still a
risk for SHRA/TSRA.


Today through tonight: A weak area of low pressure and associated
surface trough will gradually dissipate through the day just to
the east. Winds will generally top out in the 10-15 knot range out
of the northeast early and will veer around to easterly through
the afternoon, and more southerly by the overnight. Winds along
the land/sea interface will increase this afternoon with diurnal
heating, and could increase to as high as 15-20 knots for the
Charleston Harbor for a few hours. Then overnight winds will
diminish to 10 knots or less. Seas will be 1-3 feet, highest at 20
nm and further out.

Tuesday through Thursday: A slow moving cold front will move into or
near the marine area during the middle of the week, generating a
fairly nebulous pressure pattern with winds and seas far below any
Advisory thresholds. The proximity to the front will cause an
increased chance of showers and t-storms, and the lighter wind
fields plus sufficient instability, moisture and shear could support
an increased risk for waterspouts.

Friday through Saturday: The cold front will have dissipated and the
Bermuda High will expand west across the local waters, allowing for
a more typical pattern for early July, with slightly higher south-
southwest winds, seas of 2-3 ft and a lower potential for showers
and t-storms.






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