Scientific Forecaster Discussion

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Area forecast discussion 
National Weather Service Raleigh NC
1000 PM EDT Wednesday Apr 16 2014

Synopsis... unseasonably strong...polar high pressure will build
east across the middle Atlantic and northeast states through
Thursday. A low pressure system is then expected to track near the
southeast coast over the weekend.


Near term /through Thursday/...
as of 1000 PM Wednesday...

Pressure readings continue to rise across central NC this evening as
a strong 1036mb surface high over New England reaches down the East
Coast. The main story for the overnight period is the potential for
freeze and frost. Winds have decoupled in some spots...but the
building surface ridge is keeping a light wind stirring over most of
the area. Should see more areas decouple by after
midnight...especially across western areas. However...the pressure
gradient is forecast to tighten a bit over eastern NC by 09z in
response to a westward migrating inverted trough offshore...and the
rate at which temperatures fall over the coastal plain will slow as the
ridge axis settles toward the Piedmont. Could issue a frost
advisory for the coastal plain...but rap soundings show the boundary
layer staying at least partially mixed throughout the night...and
the frost should remain patchy in nature. The freeze warning will
continue as forecast. Infrared satellite imagery shows quite a bit of
strato cumulus offshore...which will begin to spread inland late tonight
and toward the I-95 corridor during the middle-morning hours on
Thursday. -Smith

Thursday...circulation around the low level ridge will result in a
long NE fetch off the Atlantic into eastern NC. This flow will
transport a shallow thin layer of moisture into the coastal
plain...resulting in a scattered to briefly broken layer of stratocu
over the coastal plain later Thursday morning into Thursday
afternoon. Layer too shallow to support anything more than sprinkles
if even that.

Temperatures will remain well below normal for middle April though some
recovery compared to today anticipated. Afternoon thicknesses
projected to be about 20m warmt than this afternoon which suggest
afternoon temperatures Thursday about 5 degrees warmer than this afternoon.
This is more similar to the cooler GFS MOS guidance. Maximum temperatures upper
50s to lower 60s.


Short term /Thursday night and Friday /...
as of 330 PM Wednesday...

Thursday night and Friday....sfc ridge axis will likely maintain its
influence over the Piedmont of central NC through this period as
parent high becomes anchored over northern New England/Canadian
maritime due to confluent flow aloft. While the air mass underneath
this ridge will gradually modify...presence of dry air under this
weather system will maintain a basically cloud free atmosphere in
the lowest 10k feet. Meanwhile...a short wave is projected to move east-southeast
across the central NC plains and into the Tennessee Valley where it is
projected to begin to interact with a system lifting east-NE across the
northern Gulf. Upper level flow over the Carolinas will back in
response to the approach of these systems...transporting high level
moisture into our region. This should result in an increasing veil
of high clouds Thursday night into Friday. The thickness of these
high clouds will play a role in whether scattered frost will develop
over the Piedmont Thursday night/early Friday. In addition...low
level Ely flow will advect moisture off the Atlantic into the
coastal plain by Friday morning. Min temperatures middle 30s west to upper
30s/around 40 east.

Should see at a minimum a mixture of sun and clouds Friday with the
character of the sky becoming mostly cloudy or overcast by Friday
afternoon. Isentropic upglide does not appear strong enough to
support precipitation Friday afternoon...aside from a stray shower across
the far southeast.

Air mass will continue to gradually modify but afternoon temperatures will
remain well below normal. Maximum temperatures 60-65.


Long term /Friday night through Wednesday/...
as of 3 PM Wednesday...

Friday night through Sunday night: the weekend forecast continues to
have some uncertainty with regard to the evolution of a possible
closed low lingering across the southeastern U.S/offshore. While the
northern stream energy is progressive and pushes eastward Friday
night into Saturday. In advance of the northern stream trough both
the 12z GFS and European model (ecmwf) pulls moisture northward... and produced some
light rain across our far southeast/east on Friday night into early
Saturday morning... with the bulk of the precipitation expected to be to
our east and south. However... model guidances is struggling with
what will happen with the lingering southern steam energy that
possibly closes off into a closed low and lingers near Florida and off
the southeast U.S. Coast. The 12z European model (ecmwf) has come a more in the line
with the 12z GFS in showing lingering middle level energy slowly
shifting eastward and far enough away from the coast to keep US dry
Saturday afternoon through the rest of the weekend. However...
guidances has shown poor run to run continuity. Thus... confidence
is not high that late Saturday into Sunday that we will be
completely dry at this time across our entire area (if precipitation
linger... locations across the east/southeast would see the best
chance/most). For now will go with a dry forecast as mentioned above
from Saturday afternoon Onward.

With surface high pressure building into the area from the north and
a cool east to northeasterly low level flow expect high temperatures will
remain slightly below normal with at least some cloud cover (more
east vs west). Thus... will go with high temperatures this weekend in the
upper 60s to lower 70s... with low temperatures in the 40s to lower 50s.

Monday through wednesday: next middle level disturbance is expected to
cross the region on Tuesday... along with a weak surface trough.
However... with lack of any significant moisture return will keep
probability of precipitation low at this time. Temperatures early to middle next week are expect to
warm again... with highs in the 70s and lows in the 50s.


Aviation /00z Thursday through Monday/...
as of 720 PM Wednesday...

High pressure building into central NC from the north will maintain
VFR conditions through Thursday morning. VFR conditions should
largely continue into the day on Thursday...with the exception of a
period of marginal MVFR ceilings with bases between 2500-3000kft
across eastern and possibly central terminals Thursday afternoon
owing to northeasterly maritime flow which could support a broken diurnally
enhanced flat stratocu deck. Otherwise northeasterly winds will prevail with
wind gusts of 17 to 20kts...strongest across eastern terminals.

Outlook...a developing low pressure system in vicinity of the NE
Gulf and the Florida Peninsula may spread enough moisture into eastern NC
by late Friday through Saturday to cause a period of MVFR/IFR
ceilings. At this time...appears the best chance for rain will be
confined to the coastal areas. Improving weather conditions and
associated VFR parameters anticipated Sunday and Monday.



The following are the record low temperatures for Thursday morning...and
the record low maximum temperatures for today:

Wednesday 04/16 Thursday 04/17

Low maximum/year low/year
fay: 56/1950 31/1953
gso: 48/1929 29/1953
rdu: 46/1890 29/1962


Rah watches/warnings/advisories...
freeze warning from midnight tonight to 9 am EDT Thursday for



near term...wss/Smith
short term...wss
long term...bsd