Area Forecast Discussion
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FXUS62 KRAH 131734
Area Forecast Discussion
National Weather Service Raleigh NC
1235 PM 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 1040 AM Friday...
Main adjustments to the near term forecasts was to adjust max temps
this afternoon up couple of degrees, and to mention a few sprinkles
across the north late this morning.
Strong sfc cold front orientated west/nw-east/se across southern VA
poised to enter our northern coastal plain. Due to a fairly deep wly
flow, the sfc front will initially have difficulty moving southward.
However, expect a more substantial surge southward by early evening
as an arctic high pressure over the Great Lakes moves east into
southern New England and builds southward along the East Coast.
With temperatures already close to forecast max temps across the far
north-northeast, have raised max temps in this region into the mid
60s. These high temps will occur by early afternoon with falling
temps anticipated during the mid-later afternoon as low level ne
flow advects in the the much cooler air. Elsewhere, expect another
afternoon with much above normal temperatures (on the order of 20-25
degrees above normal) as temps should range between 70-75, warmest
across the southern Piedmont-Sandhills. Temps across the Triad
region may need some adjustments downward due to the presence of a
fairly solid deck of stratocu, inhibiting insolation. High temps
more in the 62-65 degree range may occur here rather than upper 60s.
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 into the mid 30s to lower 40s. -SMITH
.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 /18Z FRIDAY THROUGH WEDNESDAY/...
-- Changed Discussion --AS OF 1235 PM Friday...
VFR conditions across central NC will gradually deteriorate
northeast to southwest this afternoon and this evening as a strong
cold front drops southward. Low level northeast flow will usher in a
moist air mass, resulting in the formation of a stratus layer and
associated MVFR/IFR ceilings. These adverse ceilings should be
areawide by 03Z. An intrusion of drier air in the 1000-2500 ft layer
may allow ceilings to lift temporarily into the VFR category at KRWI
and possibly KRDU during the overnight hours before ceilings lower
back to the IFR/MVFR categories by daybreak Saturday.
Pockets of very light rain should develop over sections of the
Piedmont this evening and persist into the overnight hours.
This probability appears greatest in vicinity of the Triad
terminals, and less so toward KFAY. In addition, a stout ne sfc wind
will occasionally gusts between 18-23kts tonight.
The patchy light rain will gradually diminish Saturday morning
though widespread IFR/MVFR ceilings will linger Saturday and
An unsettled aviation weather period is expected Sunday through
Wednesday as abundant moisture in the lower half of the atmosphere
and a series of perturbation aloft will maintain extensive
cloudiness and instances of light rain. This suggest prolonged
periods of MVFR/IFR ceilings are likely Sunday through Wednesday.
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