Area Forecast Discussion
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FXUS62 KRAH 201055
Area Forecast Discussion
National Weather Service Raleigh NC
655 AM EDT SAT AUG 20 2016
An upper level trough and attendant cold front will approach the
region from the west tonight, then progress east across the Mid-
Atlantic and Carolinas Sunday into Sunday night.
.NEAR TERM /TODAY AND TONIGHT/...
As of 330 AM Saturday...
Expect similar conditions to yesterday given little overall change
in the synoptic pattern, characterized by unidirectional flow aloft
in advance of an upper level low expected to progress from the upper
Midwest to the upper great lakes. Expect the location and coverage
of convection to depend upon several factors including diurnal
heating/destabilization, DPVA attendant small amplitude waves in
unidirectional flow aloft, a NW-SE oriented PWAT gradient
characterized by PWAT values ~1.80" in the Triad and 2.20" from the
Triangle south and east, and a much drier H85 airmass in
central/northern VA that could advect into portions of the N/NW
Piedmont this afternoon via weak N/NE H85 flow. Should this occur,
diurnal heating/mixing could result in a drier boundary layer,
weaker destabilization, and lower chances for convection in the N/NW
Piedmont. With the above in mind, expect the best potential for
convection to remain from the Triangle south and east. Highs near
90F, lows in the lower 70s. Flow aloft (well aloft at H3-H4) will be
strengthening Saturday aft/eve, however, with moderate
destabilization expected, updraft depth/intensity should be
insufficient to yield anything organization beyond that of
multicellular. Weak DCAPE and weak mid-level lapse rates suggest sub-
severe convection in general, with any isold/brief wet downburst
potential smaller than the potential for locally heavy rainfall and
isolated flash flooding, primarily from the Triangle south and east
into the coastal plain. -Vincent
.SHORT TERM /SUNDAY AND SUNDAY NIGHT/...
As of 345 AM Saturday...
Flow aloft will further strengthen Sun/Sun night as the
aforementioned upper low progresses ENE into southern Canada and
additional shortwave energy digging southeast through the central MS
river valley amplifies an upper level trough over the OH valley and
Great Lakes, and an attendant surface cold front approaches from the
NW Sunday afternoon, progressing through central NC sunday evening/
night. Expect the best overall potential for convection this weekend
to occur late Sunday afternoon through Sunday evening as the surface
cold front /low-level trough/ progresses into the region in vicinity
of peak heating in the presence of above normal moisture, marginal
to moderate destabilization, and small amplitude waves embedded
within relatively speedy westerly flow aloft. Deep moisture
/cloud-cover/ and weak mid-level lapse rates may temper instability
such that more favorable deep layer shear attendant stronger flow
aloft may not be fully realized/capitalized upon by updrafts at this
latitude. Weak DCAPE suggests weak cold pools, though ~20 kt
westerly flow in the lower-levels may support upscale growth
/downdraft consolidation/ and eastward propagation, perhaps evolving
into a multicells line along/east of hwy 1 with an isolated potential
for damaging winds via precip loading and downward momentum
transport. Regardless, expect the best potential for convection in
the late afternoon and evening, ending from west-east Sunday
evening/night as the surface front low-level trough progress east
toward the Carolina coast. Expect highs in the upper 80s to lower
90s and lows Sunday night driven by cold advection, ranging from the
lower/mid 60s far NW to lower 70s far SE. -Vincent
.LONG TERM /MONDAY THROUGH FRIDAY/...
As of 245 AM Saturday...
Surface high pressure will build into the area behind the cold front
early next week, resulting in much lower afternoon humidity values
and temps near to below normal along with dry weather. The surface
high will slowly shift off the Mid-Atlantic coast mid to late week,
allowing for moisture and temps to slowly increase. However, dry
weather is generally expected to continue through the end of the
workweek. High temps are expected to be in the 80s early to mid
week, before increasing into the upper 80s to lower 90s by late
week. Low temps will follow a similar trend, with lows generally in
the 60s throughout the week though (with possibly a few 50s in the
usual rural cool spots early to midweek).
.AVIATION /12Z SATURDAY THROUGH WEDNESDAY/...
-- Changed Discussion --As of 655 AM Saturday...
24-hr TAF period: Sub-VFR conditions associated with low ceilings
will affect eastern terminals, primarily in the form of IFR/LIFR
ceilings for several hours around sunrise. Otherwise, expect VFR
conditions to prevail through the remainder of the morning, with sub-
VFR conditions attendant scattered convection during the afternoon
and evening hours.
Looking ahead: Expect a continued potential for low ceilings Sunday
morning (at all terminals), with an excellent chance for convection
at all terminals Sunday aft/eve in association with a cold frontal
passage. VFR conditions will return in the wake of the front Sunday
night through mid next week. -Vincent
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