Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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FXUS62 KRAH 281203

Area Forecast Discussion
National Weather Service Raleigh NC
703 AM EST Tue Feb 28 2017

A warm front lifting through the area this morning will result
in a mild southerly wind through Wednesday.  A strong a cold
front will cross the area Wednesday night.


As of 330 AM Tuesday...

The 00z/28th showed pretty decent warm advection in the low
levels, with a pronounced warm front just inland from the
Carolinas coast early this morning. A sheared disturbance aloft
is exiting the coastal plain, taking most of the high clouds
with it. A few showers have broken out south and east of FAY in
the past couple hours in response to the warm advection, though
hires guidance tends to keep the best isentropic lift focused
east of I-95, and any stratus looks to be fairly thin if it
develops in the next few hours.

The warm front will lift through the area this morning and yield
a southerly wind with 1000-850mb thicknesses reaching around
1385m, suggesting highs well into the 70s to around 80. Mid-
level lapse rates steep to 6.5-7 C/km this afternoon, but
strong warm near 850mb also creates a modest cap, per forecast
soundings, which will be hard to break given low-amplitude
southwest flow aloft and no good trigger for convection.
However, guidance is in pretty good agreement on the remnants of
convection on the Tenn Valley clipping the northwest Piedmont
after ~22Z, though generally remaining west of the better
destablization and limiting any thunder this evening. Chance
POPs will accordingly be highs in the northwest this evening.

Expect stratus overnight in a steady 5-10kt southerly flow,
resulting in very mile lows in the 60s.


As of 400 AM Tuesday...

Wed-Wed night: A complex mid-upper level trough compromised of
multiple embedded shortwave perturbations will move E from the
Plains states early Wed to the Northeast by Thu morning. At the same
time and at the surface, related waves of low pressure will migrate
from the mid MS Valley to New England, while the trailing cold front
will move from the Appalachians late Wed-Wed evening to the Atlantic
coast by Thu morning.

It will be warm and windy, but dry for most of the day Wed, owing to
a stout capping inversion/EML that will be only gradually lifted and
eroded Wed afternoon, in response to height falls accompanying the
aforementioned trough aloft. 925 mb wind progs and BUFR forecast
soundings indicate strong SWly flow will develop in the warm sector,
with sustained winds of 20-25 mph and gusts to around 40 mph. A Wind
Advisory may be issued tonight.

Morning stratus, trapped beneath the aforementioned inversion, will
disperse by midday or early afternoon, so temperatures should
consequently rise rapidly into the upper 70s to lower 80s. Pre-
frontal showers and storms will be possible over the far wrn
Piedmont, where the aforementioned removal of the preceding cap will
be most likely by around sunset. The better chance of showers and
storms will occur Wed evening and early Wed night, as frontal
forcing accompanying the surface front sweeps Ewd. Strong SWly deep
layer flow, oriented mainly parallel to the frontal zone, will favor
linear convection --amidst an environment characterized by generally
weak instability (around 250-750 J/kg)-- that would pose primarily a
damaging wind threat from both momentum transport and mesovortices
embedded within a probable broken band or two of convection. An
isolated tornado will also be possible, given the degree of low and
deep layer shear, though a relative lack of more robust instability
suggests damaging winds will prove a greater threat. Blustery post-
frontal NWly winds will follow in strong CAA late Wed night-early
Thu, with lows in the low-mid 40s west to middle 50s east.


As of 345 AM Tuesday...

A broad trough aloft will migrate across the eastern U.S. during the
end of the week, and reach maximum amplitude as a shortwave trough
in NW flow aloft amplifies into the Middle Atlantic states during
the day Fri.

Considerable high cloudiness, within the axis of a 150 kt WSWly
upper level jet, will stream across NC on Thu, before the S of our
region Thu night. Downslope NWly flow will help offset waning CAA.
MOS high temperatures typically perform well in such downslope
regimes, with upper 50s to lower-middle 60s anticipated. A
reinforcing nrn stream surface cold front will accompany the
aforementioned shortwave trough in NW flow aloft; and this front
will approach from the NW Thu night, then sweep across central NC
during the day Fri. Downslope flow will again develop behind the
reinforcing front on Fri, so MOS temperatures in the middle 50s to
lower 60s are favored. Cold and dry advection within a deeply mixed
(800-750 mb) boundary layer will support afternoon wind gusts around
30 mph.

Fri night through Mon: The trough will have moved offshore by this
weekend, with increasingly zonal flow aloft and/or broad/flat
ridging across the srn and sern U.S. Temperatures will consequently
moderate into the 70s by Mon, after a probable hard freeze with
excellent radiational cooling of a fresh 1030-1035 mb cP surface
high Sat morning, when temperatures are expected to fall into the
lower-middle 20s over the Piedmont, to upper 20s to around 30
degrees over the Sandhills, srn Coastal Plain, and urban areas.


As of 640 AM Tuesday...

Other than a small patch of MVFR ceilings around MEB and RCZ, VFR
conditions continue across central NC at 12Z/28th. MVFR ceilings are
still possible from INT/GSO to RDU as the aforementioned low clouds
spread north and lift through the morning hours. Confidence is
relatively low however. Winds will generally be 5-10kt out of the
south today, with s few gusts to 15-20kt possible but not mentioned
in the TAFs.

Some showers will be possible near INT and GSO late this afternoon
and evening as an impulse and associated convection move through the
Tenn Valley and across the mountains, which could result in a brief
period of sub-VFR conditions. more widespread IFR conditions will
develop after midnight. Low level wind shear is also a concern given
a 40-50kt low level jet and relatively light surface winds, but it
appears to come very late tonight and has not been added to the TAF.

Outlook:  Wind gusts around 30kt will develop on Wednesday ahead of
a strong cold front and eventual line of strong to severe storms
that will cross the area Wednesday evening/night. VFR will return on


As of 500 AM Tuesday...

Critical fire weather conditions are expected on Friday, after a day
Thu with low RH and drying fuels, and marginal NWly winds.

Behind a cold front that will cross central NC Wed night, surface
dewpoints will fall into upper teens over the NW Piedmont to 20s
elsewhere on Thu. When combined with afternoon temperatures in the
upper 50s to middle 60s, relative humidity values between 20 and 25
percent will result. However, NWly winds will be strongest between
10 AM and 1 PM - prior to the lowest humidity values being met
during the afternoon hours. In addition fuel moisture values will
probably be residually elevated after a round of showers and storms
accompanying the cold front Wed evening-night.

Nonetheless, the low RH will at least set the stage for Fri, when a
reinforcing cold front and following NWly winds and low RH will
result in critical fire weather conditions. Cold and dry air
advection behind the cold front, and associated deep mixing up to 7
or 8 thousand ft, will favor the development of breezy NWly
sustained winds of 14-19 mph, with 25-30 mph gusts. Afternoon
surface temperatures and dewpoints in the 50s to around 60 degrees,
and teens to lower 20s, respectively, will yield minimum RH values
between 20-25 percent. Rapid/explosive fire growth may result; and a
Fire Weather Watch may be coordinated with NCFS on Wed.




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