Area Forecast Discussion
Issued by NWS Columbia, SC

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FXUS62 KCAE 191926
AFDCAE

Area Forecast Discussion
National Weather Service Columbia SC
226 PM EST Thu Jan 19 2017

.SYNOPSIS...
A warm front will move into the region tonight and lift north
of the area Friday. Another warm front will develop and move
into the region Saturday ahead of strong low pressure. The low
will be near the forecast area Sunday and Monday bringing the
possibility of strong thunderstorms to the region.

&&

.NEAR TERM /THROUGH TONIGHT/...
A warm front extending from southeastern Georgia to northern
Alabama will gradually move northeastward tonight and be
located along the Savannah River late tonight, continuing into
the midlands by sunrise Friday. Meanwhile, a strong upper level
disturbance will also move toward the area, but will be
weakening overnight. The combination of the two will promote
thickening clouds this evening, with rain showers moving into
the area overnight. Many areas will see some rain overnight,
but isentropic lift will not be particularly strong, so
rainfall amounts will be modest, with less than 1 inch
expected. It is possible that little to no rain falls near I-95
due to the aformentioned weakening, so I have lowered the pops
a bit in our far eastern areas.

The other possible issue toward morning is fog in the south
side of the warm front. Both NAM and GFS based guidance produce
some low vsbys in the CSRA after the front passes very late
tonight. I have added in patchy fog to account for this
possibility, but don`t have enough confidence in dense fog to
be any more aggressive than that.

&&

.SHORT TERM /FRIDAY THROUGH SATURDAY/...
An enhanced area of isentropic lift associated with warm
advection will be over the forecast area around 12z Friday as a
warm front pushes northeast across the forecast area along with
a weakening upper trough. The highest pops will be early Friday
then decreasing pops through the day with dry weather expected
late Friday through Friday night. Some lower level moisture
will remain over the area Friday afternoon despite the loss of
mid and upper moisture which may support isolated showers with
some daytime heating. Precipitable water values decrease
through the day and 500mb height rises associated with a
building upper ridge support a dry forecast Friday night which
is supported by latest MOS guidance and hi-res models.

Temperatures on Friday will remain well above normal with highs
in the 70s and it should be noted that the MET/MAV MOS has
continued to trend warmer over the past several runs. The only
fly in the ointment could be if lower clouds do not break by
early to mid afternoon. Lows Friday night will be mild in the
50s.

&&

.LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/...
Active weather expected over the weekend with the possibility
of some severe weather impacting the forecast area.

Highly amplified 500mb pattern features a northward moving
upper trough over the Northern Plains on Saturday with the
upper ridge over the southeastern states shifting offshore. A
vigorous shortwave moving across the desert southwest will
emerge over the Southern Plains as a closed low with a surface
low developing over Oklahoma Saturday night. Expect another
surge in atmospheric moisture Saturday afternoon and night over
the forecast area with precipitable water values increasing to
near 2-3 standard deviations above normal and values well over
1 inch. There are indications of some available instability
combined with increasing shear (NAM more impressive than GFS)
during peak heating Saturday which could support some
thunderstorms and isolated severe potential. Will continue to
highlight likely to categorical pops late Saturday and Saturday
night.

There is some uncertainty among the model guidance by Sunday
regarding the location and movement of the surface low pressure
system and the associated upper trough/closed low. The NAM is
much stronger in deepening the surface low across TN and
lifting it into central KY Sunday afternoon and brings very
strong low level shear over the forecast area (850mb winds
50-60 knots) along with significant instability with LI values
-6C to -8C. This appears to be a  bit of an outlier at the
moment as the GFS/ECMWF/CMC are weaker with the surface low and
track it further south along with a further south track of the
upper low moving it over our forecast area Sun night into
Monday. This scenario keeps the strongest shear a bit further
south across southern GA and northern Florida and along the SC
coastal plain and instability is not quite as unstable with LI
values around -4C to -5C.

Despite the uncertainty of the guidance it appears possible
that we could have severe weather over the weekend and we will
continue to monitor forecast trends. The most favorable time
frame for severe weather at this time appears to be late
Saturday through Sunday evening (but not likely the entire
period, just sometime within that window). SPC has the area
outlooked in general thunder with a marginal risk in the CSRA
Sat/Sat night and slight risk on Sunday.

The upper low will cross the region on Monday and depart to the
northeast by Monday night with quiet weather expected Tues/Wed
as surface high pressure builds over the region and weak
shortwave ridging moves overhead. Another digging upper trough
across the Northern Plains/Midwest will push a cold front into
the area sometime Wednesday night or Thursday bringing another
increased chance for rain. Temperatures will remain above
normal throughout this period.

&&

.AVIATION /19Z THURSDAY THROUGH TUESDAY/...
VFR conditions are expected for the remainder of the afternoon
into the evening. High clouds will be on the increase from the
west, but cigs will remain above troublesome levels.

Showers will start moving into the area from west to east along
a warm front after midnight, and with the showers will come
lower cigs and vsbys, first at AGS/DNL, then the midlands
terminals. There are some questions on the timing depending on
the model you believe, so I went with a similar start time at
all terminals for now, with some adjustments likely needed with
later packages. Given the the forcing for the rain will be
moving northeast rapidly, I have left rain out of OGB for now
as I don`t have enough confidence yet in the rain for a tempo
or prevailing. However, I do think low clouds and fog will
develop toward morning, so there are likely going to be some
restrictions anyway.

Both MET and MAV guidance are now giving IFR conditions to
AGS/DNL Friday morning in fog and low clouds, but I was not yet
confident enough to go with the 1/4-1/2 mile the guidance is
suggesting. This may also need to be updated with later
packages.

EXTENDED AVIATION OUTLOOK...The chance for showers will
continue into the day Friday. More showers...and associated
restrictions...are likely Saturday through Monday as a low
pressure system crosses the region.

&&

.CAE WATCHES/WARNINGS/ADVISORIES...
GA...None.
SC...None.

&&

$$


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