Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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FXUS62 KRAH 211418

Area Forecast Discussion
National Weather Service Raleigh NC
1018 AM EDT Mon May 21 2018

A moist and slightly unstable air mass will remain entrenched across
central NC through the middle of the week.


As of 1018 AM Monday...

14Z sfc analysis shows the sfc boundary has shifted south to approx
the VA/NC border, or just south.  It is along this boundary that
we`re seeing a few showers, mainly across the NW Triad region and
farther west over the foothills.  The latest HRRR shows that with
additional daytime heating, convective development along this
boundary will become more numerous, particulary this afternoon, and
across the northern half of our CWA invof the aforementioned
boudnary which is progged to stall across the northern half of the
state.  Current forecast generally in good shape, so no signif
upates this morning.

Prev near term disc as of 210 AM Monday...Primary drivers of weather
today include a weak backdoor surface front moving in from the north
and minor mid level shear axes shifting in from the south. The
latest surface map shows a fairly uniform air mass across much of
the Carolinas and Southeast, albeit with lingering instability over
the eastern CWA, and a weak backdoor front stretches W-E across
central VA. This front is expected to drop southward into N NC this
morning before washing out and ultimately dissipating as it starts
to shift back N tonight. This front is more of a temporary wind
shift, with minimal change in low level thicknesses, so impacts
should be little more than weak low level mass convergence over the
N Piedmont this afternoon. But this resultant forcing in tandem with
afternoon heating and subsequent destabilization (tempered a bit by
considerable mid and high clouds across the region) should allow
scattered afternoon convection to be focused over the N Piedmont and
N/C Coastal Plain starting in the mid afternoon, perhaps dropping
into the Sandhills, S Coastal Plain and S Piedmont late afternoon
into early evening, although this will depend to a large degree on
cold pool propagation and a possible inland-moving sea breeze. While
moderate CAPE is expected over the N and E CWA, other forcing
mechanisms will be absent or very weak, including the aforementioned
surface frontal convergence and subtle mid level waves tracking
northward through the area between the offshore-centered mid level
anticyclone and low pressure over the FL panhandle. Latest model
runs are favoring pretty good convective coverage along the frontal
zone, and will retain the trend up to likely pops across the N/E
with lower coverage in the SW. Rather slow storm motion will raise
the risk of locally higher rainfall totals. Storm intensity will be
limited by the weak deep layer shear, although forecast soundings
suggest elevated D-CAPE with potential for strong wind gusts, and
high values of normalized CAPE favor high lightning activity. Expect
highs in the low-mid 80s, with some upper 80s SW. Storms should
decrease gradually in coverage and strength during the evening and
into the overnight hours, although residual surface heat flux and
high dewpoints necessitate keeping chance pops in through the night.
Lows in the upper 60s to near 70 under mostly cloudy skies. -GIH


.SHORT TERM /Tuesday through Tuesday night/...
As of 250 AM Monday...

Today`s weak backdoor front should be mostly gone by Tue morning,
leaving central NC within a light SW surface flow and light W mid
level flow, as deep offshore ridging re-asserts its influence over
the Southeast Coast. We will see passage of a mid level shortwave
trough to our north Tue/Tue night, in conjunction with a surface
trough crossing into the Mid Atlantic region Tue/Tue night NW to SE.
Models depict a minimum in PW translating NE through the area during
much of the day, and with a general lack in dynamic forcing for
ascent and slightly lower forecast MUCAPE on the 3 km NAM with poor
deep layer shear, storms may be fewer overall and focused across the
far N and NW CWA, just ahead of the surface trough where CAPE will
be greater (1500-2000 J/kg vs. the 500-1000 J/kg over much of the
remaining CWA). Will have chance pops, highest N and W and lower SE,
peaking in the afternoon. Pops should decrease in the evening and
overnight, although will hold onto scattered showers over the NW
through Tue night with an improving wind field resulting from
digging of the mid level shortwave trough over the Northeast. Expect
highs again in the low-mid 80s under partly to mostly cloudy skies.
Lows Tue night in the upper 60s. -GIH


As of 300 AM Monday...

A continuation of the wet weather pattern is expected during the
long term forecast with chances for rain each day of the forecast. A
couple of features continue to drive this pattern including a
frontal zone to the north of the area that is expected to remain
close to our northern border and a low pressure system coming out of
the Gulf of Mexico. While the frontal zone to the north with be a
bigger player in the weather in the short term, a cold front is
expected to pass through on Thursday. After this point attention
will switch to the low coming out of the Gulf of Mexico as this
feature is expected to bring large amounts of moisture transport to
the Carolinas, especially late Memorial Day weekend. While it is
unclear how this feature will ultimately evolve, a couple things
remain true. We will have chances for showers and thunderstorms
throughout the long term forecast. Temperatures will rise into the
80s each day with mid to upper 60s for lows.


As of 625 AM Monday...

VFR conditions will dominate at central NC terminals through the
next 24 hours, although MVFR to IFR conditions are expected through
13z this morning at FAY/RWI, with a lower chance at RDU.
Considerable mid level cloudiness based above 8 000 ft AGL is
expected today through tonight, with scattered to periodically
broken clouds based at 3 500 to 6 000 ft AGL this afternoon. Light
winds and high humidity early this morning will support
stratocumulus cigs based at 400-1200 ft agl across the eastern
sections in the hours around daybreak this morning. Scattered
showers are possible near RDU/RWI 15z-18z, then scattered to
numerous storms are possible 18z-00z later today at RDU/RWI, with a
lower chance near FAY/INT/GSO. Shower/storm chances will decrease
quickly after 00z. There is a chance of sub-VFR cigs once again 08z-
12z early Tue morning.

Looking beyond 12z Tue, another round of scattered showers/storms
with local sub-VFR conditions is expected 18z-00z Tue. This pattern
is expected to repeat through the week, with storm chances from mid
afternoon through mid evening, and low clouds or fog possible late
each night through dawn. -GIH





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