-- Discussion containing changed information from previous version are highlighted. --
371
FXUS62 KRAH 090626
AFDRAH

Area Forecast Discussion
National Weather Service Raleigh NC
225 AM EDT Thu May 9 2024

.SYNOPSIS...
Multiple upper level disturbances will allow for unsettled weather
to continue through Friday. A cold front moving through the region
on Friday will bring below normal temperatures for the weekend.

&&

.NEAR TERM /TODAY THROUGH TONIGHT/...
-- Changed Discussion --
As of 225 AM Thursday...

An upper level shortwave is supporting a MCS that is just about to
enter the forecast area. While the nocturnal timing will help to
limit overall instability, the line of thunderstorms is expected to
hold together through most of the forecast area. High-resolution
models are not in total agreement as to the evolution, but with
greater instability to the south, the odds of the line holding
together across the forecast area will be higher across the south.
The bulk of the rain appears to be near the I-95 corridor around
sunrise and pushing east of the area by mid-morning. Unfortunately,
models diverge as to the evolution of precipitation for the rest of
the day. Will go with scattered thunderstorms through the afternoon,
but models might have a better handle on the environment across the
area once the current line of showers/thunderstorms moves through.
The entire forecast area is under a slight (level 2 of 5) risk for
severe thunderstorms, with the primary threat coming from damaging
wind gusts. The greatest threat is likely to come in the morning
with the overnight line of thunderstorms departing the region, but
the scattered thunderstorms in the afternoon could also generate
strong winds. Model guidance has come in a few degrees cooler than
the previous forecast, which removes the 90s from the forecast, but
still has highs in the 80s everywhere.

Scattered thunderstorms could continue east of US-1 into the
evening, but conditions should dry out overnight. Lows will range
from the upper 50s to the upper 60s.
-- End Changed Discussion --

&&

.SHORT TERM /THURSDAY THROUGH FRIDAY NIGHT/...
As of 245 pm Wednesday...

* Slight/Level 2 Threat continues into Thursday...

The chance for severe storms on Thursday hinges highly on how
convection evolves tonight. A couple rounds of storms are possible
tonight, the latter of which is expected to be a larger MCS moving
into NC overnight.  That deep convection should overturn the
atmosphere and reduce the currently steep lapse rates aloft and
ultimately reduce updraft strength on Thursday.  However, guidance
isn`t exactly consist on how far east the line will track or how far
north it will extend.  The 12z NAM suggests the storms may weaken
significantly before reaching the coastal plain, and thus forecast
soundings from FAY to RDU and east still show a solid 2000 MLCAPE on
Thursday afternoon.  GFS soundings show a similar profile.

In the wake of the MCS, dying or not, there should also be some
subsidence to subdue convection, but otherwise the trailing outflow
effective front may end up across the southern CWA and serves as a
focus for development, along with lee troughing ahead of the
approaching synoptic front and perhaps even another disturbance
approaching from the Deep South (which models tend to take south
into the Southeast state along with an MCS).

Deep layer shear will continue to be strong as the upper jet edges
eastward along with the upper trough and 500mb winds increase to 40-
50kt.

There is a big discrepancy between the CAMS and the coarser
NAM/GFS/ECMWF.  Most HREF members show every little redevelopment of
convection on Thursday and focus activity across SC and further
southwest, while the NAM/GFS/ECMWF suggest quite a bit of convection
in their QPF.  Tend to lean toward less coverage but with a
conditional severe threat where convection does develop.

Highs should range from the mid 80s nW to around 90 SE.  Lows in the
low to mid 80s.

Friday is another tricky forecast day, as multiple days of possible
convection should continue modify the local airmass, but larger
scale forcing will be better as the positively tilted trough over
the Midwest and Mid-Miss valley today swing east toward the Mid-
Atlantic states.  The limiting factor will be instability given that
the cold front is expected to have shifted across the southern CWA,
with highs dipping back into the mid 70s to lower 80s and lower
dewpoints infiltrating the Piedmont. Again, and conditional threat
for severe storms seems possible but confined to areas closer to the
SC border.

Cooler and drier air is expected behind the front as it finally
pushes through the area Friday, but favor the warmer side of low
temp guidance in the lower 50s for Friday night.


&&

.LONG TERM /SATURDAY THROUGH WEDNESDAY/...
As of 215 PM Wednesday...

Sat begins with broad troughing over generally the Northeast CONUS
characterized mainly by two embedded shortwaves, one of which is
shifting out over the western Atlantic while the second dives ESE
out of Upper Great Lakes and into the Ohio Valley by Sat afternoon.
The former will have shifted the axis of deep, rich moisture (PWAT
~1.6 inches) well offshore and usher in PWAT values around 0.5 of an
inch (near the 10th percentile for GSO, RNK, MHX RAOB) driven by
briefly strong subsidence and downsloping flow WNW flow. The latter
shortwave will pivot across Mid-Atlantic Sat afternoon and evening,
but central NC may only see glancing synoptic forcing from this
feature as the better DPVA remains north of the VA border. Point
soundings suggest shallow afternoon cumulus development to the base
of the subsidence inversion with better 700mb moisture (deepening
the moisture layer) moving into the area late Sat afternoon into the
evening hours. This may be enough to squeeze out some sprinkles over
central NC with better chance for very light rain closer to the
NC/VA border.

Surface high pressure and a secondary area of subsidence on the
backside of the departing trough will lead to dry conditions Sun
through Mon with temperatures in the 70s to low 80s. The next system
will move towards the area Mon night into Wed as a closed low near
the Four Corners Region Sun morning slowly drifts eastward. Closed
lows situated in between a split-flow regime often have timing
issues, and this one is no exception. Latest forecast gradually
brings in precipitation early Tues morning with chances lingering
into Wed with greatest chances diurnally favored during the
afternoon/evening. It would not be surprising to see the quicker
GEFS trend towards the slower EPS and precipitation onset becoming
12 hours later than is currently forecast. Temperatures will trend
back towards above normal mid-level height rises and the 850mb anti-
cyclone shifts offshore.

&&

.AVIATION /06Z THURSDAY THROUGH MONDAY/...
As of 135 AM Thursday...

TAF period: A line of showers and thunderstorms is expected to move
across the forecast area overnight into Thursday morning. There is
some uncertainty with high-resolution models as to how well the
thunderstorms will hold up considering the overnight timing. Was
confident enough to keep prevailing thunderstorms for some time at
all sites except RWI, as some models indicate the northern portion
of the line may significantly weaken before reaching RWI. Also have
gone with higher wind gusts in tempo groups for thunderstorms at all
sites before RWI. Do not think that ceilings will be as low as
previously forecast; think that INT/GSO could briefly have IFR cigs,
but otherwise INT/GSO/RDU should drop to MVFR with lesser potential
of restrictions at FAY/RWI. Think that the bulk of the rain should
move through by 15Z, then confidence in rain coverage is very low
through the afternoon. That potential will likely be dependent on
just how much convection holds together overnight, so have generally
gone with VCSH. The bulk of the rain should be over by sunset, and
have removed showers and dropped wind gusts that will occur during
the afternoon.

Outlook: Scattered showers/thunderstorms are possible everywhere
Friday with more isolated coverage on Saturday. Otherwise VFR
conditions are generally expected.

&&

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

&&

$$

SYNOPSIS...Green
NEAR TERM...Green
SHORT TERM...BLS
LONG TERM...Swiggett
AVIATION...Green