Area Forecast Discussion
Issued by NWS Columbia, SC

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FXUS62 KCAE 280609

Area Forecast Discussion
National Weather Service Columbia SC
209 AM EDT Mon May 28 2018

Tropical moisture will stream north into our region from the
south tonight. Heavy rain possible. Expect a moist pattern to
remain through late week.


Deep tropical moisture will continue to advect northward into
the region between Subtropical Storm Alberto and a strong ridge
over the western Atlantic. Precipitable water values have
increased to around 2 inches and may reach 2.4 inches by sunrise,
or about 175% of normal, especially across the eastern
Midlands. This will result in a threat for heavy rainfall
capable of producing flash flooding, and a Flash Flood Watch is
in effect for the entire area beginning at midnight.

A band of showers and isolated thunderstorms capable of
producing very high rainfall rates should move in from the south
well after midnight...maybe even closer to daybreak and lift
north across the area. Models are also in agreement showing a
general 1-2 inch rainfall event tonight, but some locations that
experience the heaviest rainfall could see isolated amounts up
around 3-4 inches. The greatest concern appears to be across the
eastern Midlands where precipitable water values are highest.
Given the later timing, we have delayed the start of the Flash
Flood Watch until midnight. Due to recent heavy rains and
already saturated soils, flooding of low-lying, poor drainage
and urban areas is possible with any heavy rainfall.

The other concern late tonight is the small potential for an
isolated/brief tornado. There appears to be a weak low-level
mesoscale low that forms, and an area of enhanced 0-1 km
helicity spreads northward across the eastern Midlands from 09Z
and persisting beyond daybreak. Weak instability would be a
limiting factor, but still potentially supportive of the
isolated tornado threat.


Alberto is still forecast to be located over the far northern
Gulf of Mexico, just off the Florida panhandle coast early
Monday morning. Official forecast keeps the storm well west of
the area. However, plenty of moisture will be moving inland off
the Atlantic on the eastern fringes of the storm early in the
day. Models show a batch of heavy rainfall possible across the
central Midlands early Monday, then moving northward into NC by
late morning. Heavy rainfall potential will exist with that area
of rain. Also with moderate shear and instability through the
day, the possibility will exist for a brief/isolated tornado in
any stronger activity. Later in the day Monday and even into
Monday night, activity remains scattered showers and storms as
some dry slotting moves into the area. SPC does place much of
the cwa under a marginal risk during the day and night hours
Monday. Heavy rain will continue to be the primary threat Monday
with precipitable water values remaining at or above 2 inches,
although coverage may be more scattered at times. WPC continues
to advertise a slight risk of excessive rainfall over the
forecast area Monday and Monday night.

By Tuesday, models still consistent with moving the remnants of
Alberto northward and into the Tennessee Valley. Region remains
in the right quadrant of the circulation on much of Tuesday,
so still cannot rule out the possibility of an isolated brief
tornado. Western portions of the area are under a marginal risk
on Tuesday.

Temperatures will be near normal for highs in the mid to upper 80s
and well above normal for lows in the lower 70s.


Alberto`s remnants will lift towards the Great Lakes region
through the end of the week. Models then bring those remnants
back towards the southeast by the weekend and into early next

Chances of rain should become more diurnally driven each
afternoon. Driest days may be Friday and Saturday ahead of the
upper low due to drier air aloft pushing into the area. Still
should see some sort of afternoon activity across the region
though even into the weekend, with coverage more limited. Near
normal high temperatures expected early in the extended forecast
period, but as the atmosphere dries out a bit later in the
period, temperatures are expected to rise above normal with
highs pushing into the 90s Fri/Sat. Overnight lows expected to
be well above normal the entire period in the lower 70s.


Regional radar tonight shows light rain across the northern half
of the forecast area with moderate to occasionally heavy rain
across the eastern Midlands into the CSRA. VSBYS are currently
mainly MVFR/VFR across the taf sites, but will gradually
deteriorate towards morning with heavier rainfall and lower
clouds moving into the area. Expect IFR conditions to settle in
at all taf sites between 06-08Z. The heaviest rain is expected
to push north of the terminals by 13-14Z this morning. Beyond
that period, have included VCSH with VFR visibilities after that
time. Models indicate some improvement to ceilings back to MVFR
and possibly VFR late in the period. With dry slot moving over
the area during the afternoon hours and some breaks in the
clouds also mentioned a tempo group for thunderstorms between
18-24Z across the Midlands and CSRA. Winds will generally be
from the east-southeast to east-northeast around 5 to 8 knots
through the period.

EXTENDED AVIATION OUTLOOK... A moist atmosphere is expected to
remain in place, providing late night and morning IFR to MVFR
ceilings and possibly visibilities, along with mainly diurnal
showers and thunderstorms through the period.


Concerns for flooding on some of the area rivers continue for
later in the period. Rainfall projections show highest amounts
will fall in the mountains/piedmont of the Carolinas. Rainfall
amounts expected to range between 3 and 4 inches with locally
higher amounts across the upper portion of the Saluda...Broad
and Catawba River Basins. Area lakes are near full pool for
summer recreation and will have little excess storage. The main
areas for concern will be along the Congaree River. However,
expect area streams and rivers to have elevated flows.


GA...Flash Flood Watch through this evening for GAZ040-063>065-077.
SC...Flash Flood Watch through this evening for SCZ015-016-018-


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