Area Forecast Discussion
Issued by NWS Columbia, SC

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FXUS62 KCAE 251503

Area Forecast Discussion
National Weather Service Columbia SC
1103 AM EDT Fri May 25 2018

The region will remain moist and unstable the next few days
between a ridge offshore and weak low pressure to the west.
Scattered to numerous showers and thunderstorms, mainly during
the afternoon and evening, are expected. Periods of heavy rain
are possible Sunday and Monday as a plume of tropical moisture
from Subtropical Storm Alberto is directed into the area.


Mid level short wave lifting north across the area this morning
was producing showers...mainly across the western Midlands.
Convection associated with the sea breeze already beginning.

Weak surface boundary stretched across northeastern SC expected
to slowly shift north this afternoon. Moist atmosphere to
remain in place, with PW values 1.7 to 1.9 inches and weak to
moderate instability. Convection across western SC late this
morning will continue to lift north as the mid level short wave
shifts to the north. Scattered to numerous showers with
isolated thunderstorms still expected to develop through the
afternoon into early evening. Isolated severe possible, but
main threat will again be locally heavy rain/flooding due to the
high moisture levels with any slow moving, merging or training
cells. Convection should diminish with loss of heating early

Most areas will see at least bits of sun...allowing afternoon
temperatures to warm into the lower and mid 80s. As with
previous 2 nights...will see lows in the upper 60s to lower


The lingering weak upper trough over the southeastern states
will continue to shift north of the forecast area on Saturday
being picked up by an upper trough moving east across the Ohio
Valley into the Mid-Atlantic region. There will not be much
upper forcing available on Saturday to aid in convective
development but sufficient moisture will remain in place to
interact with surface heating and result in scattered showers
and thunderstorms. Forecast instability will be moderate with LI
values around -5C but wind shear is marginal with generally
unidirectional south to southwesterly winds at 20 knots or less.
Cannot rule out the possibility of a severe storm or localized
flooding threat. Will keep pops in the chance range. Saturday
night should provide a brief break in the wet pattern we have
been stuck in as mid level drier air moves over the region and
precipitable water values drop to around 1.2 inches.

We continue to monitor the possible development of low pressure
in the Gulf of Mexico and NHC has given a 90 percent chance of
tropical cyclone formation. Whether or not the system becomes
tropical or not, a surge of tropical moisture is expected to
move into the forecast area Sunday afternoon with precipitable
water values rising well over 2 inches. Strong moisture
transport combined with atmospheric destabilization should yield
widespread showers and thunderstorms along with a heavy rain
threat which will carry into the overnight hours Sunday night.
Possible diffluent flow at 500mb will also provide support for
widespread rainfall. Temperatures during this period will be
near to above normal for highs, with Saturday being the warmer
day in the mid to upper 80s, and above normal for lows in the
upper 60s to around 70 degrees.


Active weather is expected through the extended forecast
period. Much of this part of the forecast will be dependent on
what happens with the low pressure system in the Gulf of Mexico
and where it eventually tracks and how strong it may become. The
GFS, which had been an eastward outlier compared to the
NAM/ECWMF/CMC, has come around to a more westerly solution with
the low pressure system but is still on the right side of the
other guidance. Nevertheless, there is high confidence in an
abundantly moist airmass with precipitable water values around
or above 2 inches throughout the period with persistent
southerly flow off the Gulf of Mexico.

Expect above normal chances of rainfall throughout the entire
forecast period as the surface and upper low remain to our west
through Wednesday, and then cross the forecast area on Thursday.
WPC rainfall forecast has 2-4 inches of rain across the
forecast area during this period and depending on how much rain
occurs over the weekend, a flood watch may need to be considered
early next week. Temperatures during this period will be near
normal for highs in the mid to upper 80s and above normal for
lows in the upper 60s to lower 70s.


Scattered to numerous showers with isolated thunderstorms will
bring periods of CIG/VSBY restrictions to the region through
the afternoon.

Large area of morning showers has moved north of the TAF sites
at 15Z. IFR/MVFR Cig gradually improving to VFR through early
afternoon. Convection expect to increase in coverage through
the afternoon in moist atmosphere as heating/instability
increases. Will also see convection move into the Midlands from
the east as the sea breeze moves inland.

Convective coverage expected to begin decreasing and shifting
north this evening, with deteriorating CIG/VSBY expected again
late tonight into early Saturday morning.

EXTENDED AVIATION OUTLOOK...Restrictions in late night and
early morning fog and stratus, along with mainly diurnal
showers and thunderstorms expected through the weekend as a
very moist air mass remains over the region. Tropical moisture
will increase by the late Sunday into Monday, further increasing
the frequency and intensity of showers and associated restrictions.




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