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Area Forecast Discussion

FXUS62 KCHS 271729

Area Forecast Discussion
National Weather Service Charleston SC
129 PM 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.


No changes with the afternoon update. Radar is showing showers
developing over Charleston County and that matches up well with
the ongoing forecast.

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. Even without the cap the
soundings indicate puny lapse rates and CAPE values struggling to
reach 1,000 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.

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.


Mainly VFR through 18Z Tuesday. Convection is developing along
the sea breeze in SC, so added TSRA to the KCHS TAF. Brief MVFR
is possible as the convection passes over the airport. No
convection is expected for KSAV. There is also a small
chance for late night fog and/or stratus at KCHS and KSAV. The
probabilities are too low to include in the current TAFs.

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 just to our east will gradually dissipate through
the day. Winds along the land/sea interface are increasing this
afternoon with diurnal heating and could reach 15-20 knots for
the Charleston Harbor. Then overnight winds will diminish to 10
knots or less and become southerly. 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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