Area Forecast Discussion
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FXUS62 KRAH 131155
Area Forecast Discussion
National Weather Service Raleigh NC
655 AM EST Fri Jan 13 2017
A cold front will push south through the area this evening and
overnight. Cool high pressure will briefly build in from the
northeast on Saturday, followed by a secondary cold front on
Sunday, and then another reinforcing high pressure Sunday night.
.NEAR TERM /THROUGH TONIGHT/...
As of 350 AM Friday...
Light southerly flow continues this morning on the western
periphery of a high pressure over the western Atlantic, and in
advance of a cold front that stretches from New England to
Arkansas. The cold front has been fairly slow moving but will get
a push from an arctic high over the Great Lakes, making an
aggressive surge into the northern piedmont late this afternoon, and
then south through the area overnight. Ahead of the front, a strong
590dm H5 ridge over the Southeast US will again support highs well
above normal today as H10-H85 thicknesses approach 1370m, some 60m
above normal. This supports highs in the lower 70s in the south,
though some statistical guidance suggest mid 70s in the Sandhills.
Increasing moisture between 5-10k ft will cut down on heating a
bit across the north as the front approaches, where highs look to
be more in the mid 60s, especially with the faster NAM showing the
front crossing the VA border by early afternoon. This pattern
tends to result in a stronger than normal gradient from north to
south, so will nudge temps up/down across the south/north, which
By all accounts the front and it`s low-level moisture will be
through the area by 03Z, followed by a fairly substantial surge of
dry air from the northeast below 3k ft, though moisture between
3k and 10k ft will keep skies overcast. The flow quickly veers to
southeasterly in the low-level, and the induced isentropic lift
should result some areas of light rain after midnight, focused
mostly west of US 1 per hi-res WRFs. The combination of cold
advection and evaporation cooling (across the Piedmont) will
drive lows int he mid 30s to lower 40s.
.SHORT TERM /SATURDAY THROUGH SATURDAY NIGHT/...
AS OF 350 AM Friday...
High the parent high supporting the onset of CAD tonight will
quickly shift offshore on Saturday, southerly low-level flow and
light rain atop the cold dome should maintain the wedge airmass,
especially across the north as the best isentropic lift shifts
north throughout the day. Temperature recovery will be weak, and
guidance is likely too warm with mid 40s to lower 50s, so will
undercut statistical guidance and flavor a NAM-like temperature
range of 40-49.
The H9 flow will swing all the way around to westerly by Saturday
night ahead of a secondary front moving into VA. Models indicate
some drying above the boundary layer and even hint at some
clearing of low clouds, but this seems overdone given that it`s
overnight. Some low clouds should remain, and lows will be in the
low to mid 40s.
.LONG TERM /SUNDAY THROUGH THURSDAY/...
As of 400 AM Friday...
Little change in overall forecast rationale during the medium range,
in a pattern that local expertise can demonstrate significant value
--and better accuracy-- versus national blended data sets.
A shortwave trough will migrate through the base of a broad trough
over central and Ern Canada, and off the NErn U.S., coast early Sun.
Following height rises and subsidence will maintain strong, 1030-
1035 mb arctic surface high pressure over the Northeast and Middle
Atlantic, whose leading edge will be marked by a cold front, now
draped from the Great Lakes to the Central-Srn Plains, that will
move SWwd through central NC during the Sun. Both low and mid level
cloudiness, the former of which will become a widespread OVC in post-
frontal NEly flow, will limit the degree of heating prior to frontal
passage, which is forecast to occur during the morning in the NE
Piedmont/Nrn Coastal Plain; midday to early aft in the Triad and
Triangle; and late afternoon in the Sandhills and Srn Piedmont.
Highs in the mid 40s NE to around 60 SW.
Low level CAA will continue in earnest through the night Sun and day
Mon, with a pronounced 1030 mb ridge over NC "overrun"
--isentropically-- by warmer air just above the surface - a pattern
that will promote stability and associated CAD that will be enhanced
by widespread low OVC, and light rain/drizzle over the Wrn Piedmont.
Some afternoon erosion of the low OVC may occur on the SE flank,
typical in these patterns, but mid level ceilings even there will
keep temperature from climbing much. As such, it will be a cool day,
with highs in the 40s, to perhaps low to mid 50s in the S if low
clouds indeed break. If not, 40s areawide. Guidance temperatures,
and associated blended national data sets, have (not surprisingly)
been steadily trending toward our cooler official forecast.
A subtropical ridge will become increasingly amplified/dominant over
the SErn U.S. during the middle of the week. While associated deep
layer SWly flow will favor warming and increasingly humid conditions
for January (surface Td`s in the 50s), the warming will be somewhat
muted by both multi-layered cloudiness, and lingering in-situ CAD
over the Wrn Piedmont through at least Tue.
After a chance of drizzle or light rain along and behind the
aforementioned backdoor cold front on Sun, which will continue
within the heart of the wedge /low OVC/ over the Wrn Piedmont
through Tue, the best chance of any appreciable precip --in the form
of rain showers-- will be with the approach and eventual passage of
a slow-moving cold front mid to late week.
.AVIATION /12Z FRIDAY THROUGH TUESDAY/...
-- Changed Discussion --AS OF 655 AM Friday...
VFR is expected through early afternoon with a light westerly wind
ahead of a cold front moving into VA. A cold front will surge
south into NC this afternoon, resulting in 10-15kt northeasterly
winds and MVFR cigs this evening. Models have trended a little
quicker with the arrival of a cold front at RDU and RWI this
afternoon, as early as 18-19Z. The lower cig may lag the wind
shift to northeasterly by a couple hours, and confidence is MVFR cigs
increases after 21Z. After 03z, drier air below 3k ft is forecast to
advect in behind the front, which may actually allow cigs to lift
above 3k ft for a period overnight. This is most likely at RWI and
least likely at INT/GSO. However, winds above the surface will
quickly swing around to southerly, causing the low-levels to
remoisten and light rain to break out between RDU and INT/GSO. Cigs
should accordingly lower back to MVFR/IFR around 12Z Saturday.
Long term: A prolonged period of sub-VFR is expected into early next
week as low clouds become entrenched over the area and northeasterly
flow is reinforced by high pressure over New England.
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