Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC-- Highlight Changed Discussion --
-- Discussion containing changed information from previous version are highlighted. --
FXUS62 KRAH 211349
Area Forecast Discussion
National Weather Service Raleigh NC
950 AM EDT THU JUL 21 2016
Surface high pressure will build across the mid-Atlantic states
today, then move offshore tonight. A strong high pressure ridge will
expand from the central U.S. acoss the Carolinas through the
weekend, bringing hot and humid conditions.
.NEAR TERM /THROUGH TONIGHT/...
As of 950 AM Thursday...
Convection chances still appear quite low and confined to the SW
CWA. Latest surface map shows the weak surface ridge nosing in from
the NNE, poised to build overhead today as it weakens further.
Humidity will remain elevated today as surface dewpoints drop
minimally from their current readings in the upper 60s to lower 70s,
and the lack of clouds currently overhead and upstream combined with
dry air aloft as noted on the water vapor imagery (and likely to dry
further with acceleration of the NNE-erly upper jetlet over the
Delmarva/SE VA/NE NC today) should lead to no worse than partly
cloudy skies today, boosting isolation and causing surface temps to
peak in the upper 80s to lower 90s. Latest CAM runs including the
HRRR/RAP and SPC`s SSEO favor a storm-free day for much of Central
NC, with the exception of the far W and SW near the old boundary
where low level mass convergence will take place, and perhaps just
off the higher terrain which will be subject to differential
heating. Convection over Central NC should be further smothered by
slow steady warming aloft, with gentle but persistent mid level
height rises continuing into tonight, leading to low CAPE values.
Will limit pops to isolated thunder in the extreme west / SW. -GIH
Previous discussion as of 355 AM Thursday... WV imagery and model
analysis PV fields depict a couple of perturbations in NW flow aloft
over the DelMarVA and Lower OH Valley. The models indicate the
former will amplify off the VA/NC coast through 18Z, while the
trailing one will dive across west-central NC, to NERN SC, through
the same time. The tail end of the lead perturbation over the
DelmarVA has already aided in the development of a few showers over
the Shenandoah Valley, amidst an area of 6-9 thousand ft strato/alto-
cumulus centered over central and SRN VA; and the HRRR suggests a
few additional showers may develop over the NC Piedmont during the
morning to midday hours, presumably as the strongest QG-forcing for
ascent accompanying the trailing Lower OH valley wave moves SEWD.
Otherwise, 1025 mb surface high pressure now over WV will build
east across and offshore VA today, in the subsident wake of the
aforementioned perturbations aloft. The surface ridge axis and
associated NE low level flow around it, over the ERN half of NC,
will result in temperatures AOB those of Wed, with generally
upper 80s to around 90 degrees anticipated. To the west of the
ridge axis, a light SLY return flow will develop across WRN NC,
including the SRN and WRN Piedmont; and an associated 925-850 mb
theta-e ridge there warrants a continued slight chance of an
afternoon shower or storm.
Light return flow will envelop all of central NC tonight, once the
aforementioned surface ridge migrates offshore, with dry and
mainly clear conditions. Lows in the upper 60s to lower 70s.
.SHORT TERM /FRIDAY THROUGH FRIDAY NIGHT/...
As of 410 AM Thursday...
A strong sub-tropical ridge centered over OK at 500 MB and AR at
700 mb, will gradually expand EWD this period, and more so this
weekend. Warmth beneath the ridge will likewise expand EWD, with
highs in central NC mainly in the lower 90s. Lows 70 t0 75. Associated
warm and dry mid levels will limit the chance of showers and
storms to differential heating boundaries in the mountains and
along the coast.
.LONG TERM /SATURDAY THROUGH WEDNESDAY/...
As of 320 AM Thursday...
Period of hot and humid and conditions with heat index
values in excess of 100 expected for the weekend into Monday...
Central NC will be located in along the eastern periphery of an
eastward extending central Plains ridge at the beginning of the
long term period resulting in a weak, diffluent northwest flow aloft.
Over the weekend into early next week, the ridge will shift east
with rising heights across the Carolinas. At the surface, a Piedmont
trough will be present on Saturday and Sunday while a cold front
across the Mid Atlantic washes out as it stalls near the Mason-Dixon
line. A hot and humid air mass will be in place across the region
with surface dewpoints in the lower 70s only modestly abated to the
west of the Piedmont trough during day time mixing. PW values will
average between 1.75 and 2.0 inches throughout the period. Deep mixing
will likely generate some high-based afternoon cumulus most days but
warming mid levels with 500 hPa temps warming to -5 to -4C on Sun
and Monday in a generally subsident pattern should limit convective
coverage, only warranting slight chance PoPs during the afternoon
and evening. Morning thickness values in the 1420s will increase to
1435m on Sunday and Monday supporting highs in the mid to upper 90s
during the period. The hottest days appear to be Sunday and Monday
with heat index values climbing above 100 all three days and
exceeding 105 on Sunday and Monday.
The ridge relaxes a bit across the Carolinas on Tuesday into
Wednesday allowing the westerlies to drop southward. A cold front
will settle south across the Mid Atlantic on Tuesday, potentially
moving into NC on Wednesday. Not real confident the front will be
able to push that far south but its proximity, a more cyclonic flow
aloft and increasing shear along with weak to moderate instability
will result in an increase in the threat of convection. Increased
cloud cover and the retreat of the ridge will result in high
temperatures dropping back into the lower 90s with lows in the lower
to mid 70s. -blaes
.AVIATION /12Z THURSDAY THROUGH MONDAY/...
As of 745 AM Thursday...
A disturbance in NNW flow aloft will dive SEWD across the Carolinas
early today. The passage of this feature and associated lift
combined with diurnal heating may cause a showers to develop and
spread across the Piedmont, mainly over the SRN half (from KVUJ to
KFAY), through midday. After early morning fog at KRWI and KFAY
dissipates through 13Z, VFR conditions and a light and variable
surface wind will follow, and continue through tonight.
Outlook: Sub-tropical high pressure will expand across the SRN and
Central Appalachians, and Carolinas, and result in mostly dry/VFR,
and increasingly hot conditions, through the weekend. However, a
small chance of early morning fog/stratus, and isolated to widely
scattered afternoon and evening storms, will be possible each day,
but the probability of occurrence at any given TAF site will be
Record high temperatures and the year in which the record was most
recently set at Raleigh, Greensboro, and Fayetteville.
Sat Jul 23 Sun Jul 24 Sat Jul 23
RDU 105/1952 101/2011 102/2010
GSO 99/1952 99/1914 101/1914
FAY 103/2011 105/1952 103/1914