Apex, North Carolina 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

508
FXUS62 KRAH 211540
AFDRAH

Area Forecast Discussion
National Weather Service Raleigh NC
1040 AM EST Thu Nov 21 2024

.SYNOPSIS...
Seasonally cool high pressure will build across the Southeast today,
while a nor`easter tracks along the coast of the northern Middle
Atlantic and southern New England. A reinforcing, mostly dry cold
front will move across central NC this evening. Following high
pressure will build across the Southeast, while modifying, through
the weekend.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 1040 AM Thursday...

With the morning update, made some tweaks to slightly raise wind and
wind gusts with the cold front that will be approaching the region
around sunset. Minimal changes were made to today`s temperatures -
despite a sunny start, increasing clouds from northwest to southeast
will help to limit today`s high temperatures along with the colder
air mass behind last night`s cold front. Previous discussion follows.


As of 650 AM Thursday...

* Probably a frosty morning with temperatures in the lwr-mid 30s to
  start the day over the rural Piedmont and Sandhills
* Freeze Watch expanded to include all of cntl NC for tonight

At the base (equatorward) of a blocking anticyclone along the coast
of NL --and in west-based NAO position that typically favors cold
over the ern US and a storm track along the East Coast--  a strong
mid/upper-level cyclone will move slowly ewd across the Great Lakes
and nrn Middle Atlantic. A couple of significant shortwave
perturbations, one now over ern IA/nrn IL and another over the nrn
Lower Peninsula of MI, will pivot around the cyclone center and
across NC. Intense mid-level height falls (150-180 meters/12hr) and
frontogenesis will accompany the first of those shortwave troughs
across cntl NC late this afternoon-evening, while the second will be
accompanied by more-gradually falling heights and cooling mid-levels
late tonight-early Fri. Cyclonic, generally wly flow will otherwise
result across cntl NC, including downslope flow in the low-levels.

At the surface, high pressure will build across the Southeast, while
a coastal low now offshore the nrn Middle Atlantic coast will deepen
and occlude while moving generally nwd and inland across srn New
England through this evening, then wwd into cntl NY through 12Z Fri.
A secondary, reinforcing cold front now stretching from a cyclone
over the UP of MI swwd into the mid MS Valley, then wswwd across srn
KS/nrn OK, will move ewd and across cntl NC this evening. A
following, colder high will weaken while expanding sewd across the
Plains and MS Valley through Fri.

Earlier strong and gusty nwly surface winds have lessened, to calm
at many cntl NC reporting stations this morning; and this light flow
regime will remain the case through mid-late morning. Diurnal
heating/mixing will then cause wly surface winds to increase to
around 10-15 mph, with gusts mostly in the 20s mph from late morning
through the afternoon. After a sunny start to the day, altocumulus
will develop/spread ewd and become broken to overcast in coverage
across the Piedmont this afternoon. Given the expected intense
cooling aloft, the associated saturated/cloud layer, focused between
~7-10 thousand ft and with weak buoyancy of 25-75 J/kg, will likely
be a accompanied by an area of precipitation. A deep, and very dry
sub-cloud layer suggests most or all should sublimate/evaporate as
virga before reaching the ground. Temperatures today will be
seasonably cool and mostly in the 50s, to around 60 in the srn
Coastal Plain and ern Sandhills.

While skies should then briefly clear in the wake of that band of
lift and moisture early tonight, multi-layered clouds are apt to
redevelop overnight-Fri morning over the Piedmont, as the secondary
shortwave perturbation pivots east of the Appalachians. These
clouds may also be accompanied by virga, especially where cloud
bases will be lowest around 3500-4000 ft AGL over the nw Piedmont,
but any measurable precipitation should hold in upslope flow along
and west of the Appalachians. The presence of those clouds may also
impact temperatures tonight. Should they indeed develop and become
scattered to broken in coverage over the Piedmont, low temperatures
will likely hold in the lower 30s. If cloud coverage remains lower,
there will be a greater potential for upr 20s in rural Piedmont
locations. Regardless, mainly clear skies are likely to hold
downstream of the secondary shortwave trough and forcing for ascent,
from the Coastal Plain to the ern Sandhills, where lwr 30s and the
potential for freezing temperatures warranted an expansion of the
Freeze Watch.

&&

.SHORT TERM /FRIDAY THROUGH FRIDAY NIGHT/...
As of 330 AM Thursday...

A potent mid/upper low (and associated occluded surface low) will
rotate from southern NY into eastern PA and NJ on Friday, then
offshore into the Atlantic on Friday night. A strong sea level
pressure gradient will be in place between this system and high
pressure over the Central US, resulting in gusty west winds up to 20-
30 mph during the day. It will be a brisk day with high temperatures
around 10 degrees below normal, in the upper-40s to lower-50s. Wind
chills will only be in the 30s during the morning and lower-to-mid-
40s even in the afternoon across the north. There may be some mid
and high clouds rotating around the upper low that reach mainly our
northern zones, but it should still be a partly to mostly sunny day.

It remains uncertain how much winds will die off on Friday night, as
point soundings from the GFS continue to indicate boundary layer
mixing and gustiness at times, while the NAM suggests decoupling
which would keep winds much lighter at the surface. This will affect
how low temperatures are able to get. For now forecast lows are
mostly upper-30s to lower-40s as the air mass will be starting to
modify by then, but some mid-30s are possible across the SW which
will be most displaced from the surface low to our NE.

&&

.LONG TERM /SATURDAY THROUGH WEDNESDAY/...
As of 400 AM Thursday...

Gusty W/NW winds will continue on Saturday, albeit not quite as
strong as Friday with the occluded low deepening but lifting NE into
the Canadian Maritimes. The air mass will continue to modify with
highs only slightly below normal, in the mid-50s to 60. Winds should
really drop off on Saturday night as high pressure approaches from
the Deep South. With clear skies, we should radiate quite well, so
low temperatures are expected to be colder than Friday night, mostly
mid-to-upper-30s with lower-30s possible in outlying areas (again
most likely in the SW). This will bring renewed frost and maybe
freeze concerns.

The surface high will begin to shift offshore on Sunday and Monday,
bringing a return to SW flow and temperatures back above normal.
Sunday`s highs will be lower-to-mid-60s with lows in the upper-30s
to lower-40s. Monday will be even warmer with highs in the mid-60s
to lower-70s and lows in the lower-to-mid-50s. Dry weather will
continue on these days with zonal flow aloft.

Meanwhile, the next mid/upper trough will move across the northern
tier of the US and drag a cold front that approaches from the west.
There are significant differences between the ECMWF and GFS on this
system, with the ECMWF indicating something much deeper that sweeps
the front through central NC on Monday night/early Tuesday, while
the GFS only depicts a weak surface low reflection and lags the
frontal passage until Wednesday/Thursday. The Canadian depicts a
solution closer to the ECMWF. This will of course have a major
impact on temperatures Tuesday and Wednesday, so forecast confidence
is low at this time. The front looks like it will be fairly moisture
starved by the time it gets here, and the best forcing will be
displaced well to the north. The GFS is wetter by Wednesday but is
even on the wet side compared to its own ensembles. So only have
slight chance POPs across the N/NW at this time.

&&

.AVIATION /12Z THURSDAY THROUGH MONDAY/...
As of 655 AM Thursday...

VFR conditions are expected through tonight. Gusty, wly surface
winds will develop with daytime heating/mixing by ~15-16Z, in a
modest MSL pressure gradient between a coastal low nearing the srn
New England coast and high pressure expanding across the Southeast
from TX. These surface winds may become locally enhanced to near 30
kts beneath 6-10 thousand ft ceilings and virga that will accompany
a vigorous trough aloft and reinforcing surface cold front late this
afternoon through this evening. A following, post-frontal surge of
nwly winds just above the surface may result in marginal low level
wind shear this evening, since surface winds will generally diminish
around sunset.

Outlook: Low VFR stratocumulus ceilings around 4 thousand ft will be
possible at INT/GSO Fri morning, as will more virga, as another
strong disturbance aloft pivots across the region. Nwly surface
winds will become gusty into the 20s kts again Fri, with mostly
teens kts expected on Sat.

&&

.RAH WATCHES/WARNINGS/ADVISORIES...
Freeze Watch from late tonight through Friday morning for NCZ007>011-
021>028-038>043-073>078-083>086-088-089.

&&

$$
SYNOPSIS...MWS
NEAR TERM...Green/MWS
SHORT TERM...Danco
LONG TERM...Danco
AVIATION...MWS