158
FXUS62 KRAH 171740
AFDRAH

Area Forecast Discussion
National Weather Service Raleigh NC
140 PM EDT Tue Sep 17 2024

.SYNOPSIS...
An area of surface low pressure will drift west across
northwestern South Carolina through tonight. The remnants of
this system will linger and gradually dissipate across the mid-
Atlantic through the end of the work week.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 1035 AM Tuesday...

Regional surface observations and SPC mesoanalysis show the center
of the surface low over upstate SC with an occluded boundary
extending north-northeast towards the Triad and then back east-
southeast through the Triangle and to Goldsboro and off the NC
coast. This boundary is supporting a train of light showers with
embedded moderate showers along and to its north where some weak
instability (250-500 J/kg of MUCAPE) and strong low-level moisture
transport exists. Satellite imagery confirms mostly low-topped
updrafts comprised mostly of water droplets and some dry mid-level
air spreading north of the eastern periphery of the stacked low to
our west. Hi-Res guidance shows this regime should slowly spread
northward towards VA through the morning into the early afternoon,
but based on current observations, it may become stationary as
diurnal heating to the south and diabatic cooling to the north lock
in the occluded front; continuing to steady light rain and embedded
moderate showers over the Triangle for most of the afternoon.

Latest model guidance shows a tongue of 500-1000 MLCAPE spreading
west along and north of the boundary this afternoon and may result
in some localized deeper updrafts capable of moderate to perhaps
locally heavy rainfall at times. The training nature of showers and
isolated storms within the streamer of moisture may result in narrow
swaths of higher rainfall totals and localized flooding mainly in
urban and poor drainage locations along and north of the boundary.
Rain rates may struggle today to produce any flooding within an
hour, but steady moderate rain with pockets of heavier rates my
produce flooding concerns where multiple hours of rain occurs. Some
shower activity may continue into the overnight hours towards the
Triad as the occluded low and mid-level forcing begins to wobble
back east.

Highs today will be tricky and depend on the frontal position and
associated steady rain, but generally expect low 70s in the west and
perhaps into the low 80s in the south and east. Lows should settle
in the mid 60s overnight for most locations.

&&

.SHORT TERM /WEDNESDAY/...
As of 355 AM Tuesday...

The surface and mid/upper low impacting the region over the past few
days will weaken into Wednesday as it shifts north and east. A
weak surface trough will extend from southeastern VA southwest
into the remnant low circulation across western SC on Wednesday
afternoon. Abundant moisture across the region will result in
widespread low clouds and perhaps some fog to start the day on
Wednesday morning. With some breaks in the overcast developing,
especially in the southern and eastern areas, the air mass will
becoming moderately unstable during the afternoon and support
scattered showers and thunderstorms. With a very weak low level
wind field and a unidirectional southwesterly flow in the mid
levels don`t expect much in the way of strong storms. Afternoon
highs will range from the upper 70s in the Triad to the lower
80s in the Coastal Plain. -Blaes

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/...
As of 345 AM Tuesday...

Mid/upper troughing will extend down the Eastern Seaboard including
into central NC from Wednesday night into Thursday. Meanwhile a
surface low will linger off the northern Mid-Atlantic coast. This
will bring one last day of upper forcing for ascent and above-normal
PW values, so POPs on Thursday are generally in the 30-40% range,
highest east and maximized during peak heating. There could be
enough instability for a few storms. QPF amounts in the models are
not impressive, so not concerned about an additional heavy rain
threat, and looks fairly low impact overall.

As a mid/upper ridge over the Plains begins to move east on Friday,
it will begin to push the trough east and offshore. This will result
in NW flow and much drier air moving into central NC, with PW values
trending below normal. At the surface, high pressure will gradually
nose down from SE Canada into the Mid-Atlantic from Friday into
Monday. Overall this pattern supports limited rain chances at best,
which is a significant change from last night when the models
depicted the mid/upper low moving back south across the area. Now,
they keep it and the associated surface low well to our east in the
western Atlantic. Instability looks minimal to non-existent as well
given the cool wedge of high pressure and NE flow near the surface.
The best chance for clouds and isolated showers looks to be in the
Coastal Plain closer to the low and where there will still be moist
onshore flow in the low levels, but even there POPs are only slight.
The GFS does try to bring a shortwave through the area on Sunday, so
POPs are slight that day areawide.

As for temperatures, highs should be within a few degrees of normal
from Thursday through Saturday, in the upper-70s to lower-80s. Lows
will gradually trend downward as the air mass turns drier, from mid-
to-upper-60s on Wednesday night to upper-50s to lower-60s by
Saturday night. On Sunday and Monday when the cool wedge of high
pressure really takes hold, forecast highs are only in the 70s with
lows in the mid-50s to lower-60s. If the raw GFS is to be believed,
some places won`t even get above the 60s on these days, but would
like to see more support from ensemble guidance before buying into
this too much.

&&

.AVIATION /18Z TUESDAY THROUGH SUNDAY/...
As of 135 PM Tuesday...

Light rain, with moderate embedded showers, and IFR cigs have been
forming along a nearly stationary boundary stretching from the
INT/GSO to RDU and south of RWI through early afternoon. Some brief
improvement to low-end MVFR is possible at RDU, but IFR cigs would
quickly return after sunset. Isolated thunder is possible at RWI for
the next few hours as shower/storm coverage increases. Cigs along
and north of the stalled boundary will likely lower well below
alternate minimums and approach landing minimums by early Wed
morning. Confidence is low on if/when cigs will lower at FAY.
Gradual improvement is expected through the morning hours, slowest
at GSO/INT. Scattered showers/storms will again be possible Wed
afternoon with greatest confidence in the vicinity of INT and GSO.

Outlook: An unsettled period will continue through Thursday with
areas of overnight and early morning stratus and a risk of some
afternoon showers and possibly a storm. Aviation conditions will
generally improve for the weekend. -Blaes

&&

.RAH WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

SYNOPSIS...Blaes
NEAR TERM...Swiggett
SHORT TERM...Blaes
LONG TERM...Danco
AVIATION...Swiggett/Blaes