Scientific Forecaster Discussion
fxus62 krah 211044
Area forecast discussion
National Weather Service Raleigh NC
644 am EDT Sat Oct 21 2017
high pressure will extend across the eastern United States through
Near term /through tonight/...
as of 345 am Saturday...
This discussion could be summarized in one word: persistence
The models indicate the mid-upper ridge now over the sern United
States will add about 20 meters as it builds to the coast of the
Carolinas by tonight.
At the surface, 1026 mb high pressure analyzed over the Middle
Atlantic States will likewise build ewd and offshore the middle
Atlantic and northeast coast, but continue to extend swwd across
the sern US through tonight, and the remainder of the weekend.
The result, as noted above, will be a generally persistence
forecast, though with an increase in cirrus in wswly upper level
flow that may yield a period of partly cloudy conditions this
afternoon and evening. High temperatures are expected to again reach
the upper 70s to lower 80s, while strong radiational cooling,
perhaps delayed by cirrus early tonight, will favor low temperatures
mostly in the mid-upper 40s to lower 50s. Persistence radiation fog
centered over the coastal plain and far nern Piedmont will also
again likely result.
Short term /Sunday and Sunday night/...
as of 355 am Saturday...
The mid-upper ridge in the vicinity of the coast of the Carolinas,
and the associated surface ridge axis extending swwd into the sern
US, will move little through the weekend. Persistence will still
largely rule, although another day's Worth of Ely low level flow
around the ridge will have allowed surface dewpoints to climb more
solidly into the 50s, to around 60 degrees or so in the sandhills
and srn coastal plain, by sun evening. This will favor both slightly
higher relative humidity values than recent days, and also (mostly) few-scattered
stratocumulus midday Onward. Bufr forecast soundings from both the
NAM and GFS suggest this stratocumulus may become broken in
coverage/mostly cloudy by late afternoon-evening over srn portions
of the rah forecast area. Cloud cover, including some low clouds and
fog centered again over ern portions of the forecast area, will
further increase into the mostly cloudy range overnight, but the
continued presence of a capping inversion aloft and lack of deep
moisture, suggest conditions will remain dry.
Long term /Monday through Friday/...
as of 355 am Saturday...
Monday through tuesday: increasing southerly to south-southwesterly
flow in advance of a deepening mid/upper level trough and potential
closed mid level low over the lower MS valley region and deep south
along with an approaching associated cold front will lead to a warm
but cloudy weather on Monday. Chances for showers will increase
across the western portion of the area by the afternoon. However,
the main band of convection is expected to move across the area
Monday night into Tuesday morning. Low level winds profiles will
strengthen as the band of convection moves into and across the area,
while the lead mid level impulse may attempt to take on a negative
tilt. However, overall surface instability remains in question given
the non-preferred diurnal timing (high shear, but low cape). With
that said, instability is expected to remain rather weak and may be
the limiting factor. Given the strong low and deep layer shear any
surface based instability at all could prove very problematic, with
the potential for damaging wind gusts and even a tornado or two.
Stay tuned to this part of the forecast as a severe weather threat
may be possible Monday night into Tuesday morning. Expect high temps
on Monday will be warm in the 70s to around 80 se, with lows on
Tuesday morning generally in the 60s.
The lead cold front and mid level dry punch is expected push the
main band of convection to the east of the area by Tuesday afternoon
(given the GFS and European model (ecmwf) current timing). This should end any severe
threat, though temps will remain rather mild with any good cold air advection
expected after sunset. Highs temps are expect to generally range
from the upper 60s northwest (which may come in the late morning or early
afternoon) to the mid 70s se/E.
Tuesday night through friday: drier and cool weather is expected mid
to late week as the deep/cold mid/upper level trough shifts across
the area. This will result in quite weather during this period, with
temps generally in the upper 50s to 60s for Wednesday and Thursday,
with temps rebounding some into the 60s and 70s for Friday as
surface high pressure shifts offshore. Low temps Wednesday morning
are expected to be in the 40s northwest to the 50s southeast. Lows on Thursday and
Friday morning are expected to generally range fro the upper 30s
into the 40s, with some of the usual rural cold spots dropping into
the mid 30s on Thursday morning if the mslp gradient is able to
Aviation /11z Saturday through Wednesday/...
as of 640 am Saturday...
Under the influence of high pressure over the Middle Atlantic
States, persistence VFR conditions are expected to prevail, aside
from late night-early morning radiation fog at rwi. In clockwise
flow around the high, light nely to sely surface winds, are expected
through the taf period.
Outlook: the threat of fog, and some associated low ceilings, will
increase late Sun night-early Mon, owing to increasingly moist east
to sely low level flow off the Atlantic Ocean. An approaching
frontal system will then result in the ewd progression of a band of
IFR-MVFR ceilings, showers and isolated storms, and strong/shifting
winds with height late Mon night through early Tue. Breezy swly
winds will follow behind the associated cold front Tue afternoon.