Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Raleigh NC
627 am EST Wednesday Dec 11 2013
Synopsis...cool and dry high pressure will build into the region
this morning and move across the area this afternoon. A dry cold
front will move across the region tonight... with cold high pressure
building into the area on Thursday behind the front.
Near term /through tonight/...
as of 215 am Wednesday...
Today: surface high pressure will move across the area today... with
a dry westerly flow aloft. This will yield sunny/mostly sunny skies
today with generally light winds and slightly below normal temperatures.
Afternoon low level thickness values are expected to be in the 1315
to 1325 meter range across the area. Keeping in mind that the
eastern/northern eastern part of the County Warning Area will not see any low level
return flow until late in the day... will go with high temperatures ranging
from the lower to middle 50s SW to upper 40s NE.
Tonight: a quick moving dry cold front will sweep across the region
tonight. Outside of a brief increase in middle to high clouds
accompanying the system... expect mostly clear skies tonight with
light southwesterly winds to start the night becoming northwesterly
behind the front an increasing to around 5 to 10 miles per hour. The
northwesterly breeze overnight may actually help to keep temperatures up a
bit. Thus.. will go with lows ranging from the middle to upper 20s
across the area.
Short term /Thursday through Thursday night/...
as of 240 am Wednesday...
Thursday and Thursday night: surface high pressure will build into
the area on Thursday and settle across the region on Thursday night.
This high has more of an Arctic origin and thus will result in cold
temperatures for Thursday and Thursday night. Any moisture aloft will
remain to the west of an north of central NC during this time.
Thus... expect mostly sunny/clear conditions... with a north-
northwesterly breeze on Thursday (8-12 mph)... before winds subside
with sunset/loss of mixing along with a weakening pressure gradient.
Afternoon low level thickness values are prognosticated to be around 20 to
25 meters lower than this afternoon. Thus... expect chilly well
below normal highs on Thursday despite full sun. Will go with high
temperatures ranging from the lower 40/around 40 north to the middle 40s
south. Lows Friday morning will be cold... given the surface high is
expected to be over the area during this time along with clear skies
and dewpoint temperatures at best in the teens. Thus... expect low temperatures to
range from around 20 to maybe even a few teens in the usual rural
cold sites to the middle 20s in some of the urban areas and far
Long term /Friday through Tuesday/...
as of 325 am Wednesday...
Friday...an area of high pressure over the Carolinas early will
scoot east and lie offshore by Friday evening. Return flow on the
back side of the high will advect a warm moist air mass up and over
the cool stable air near the surface. This should lead to an
increase in clouds late in the day from the SW. Thicknesses Friday
afternoon average about 15m below normal. This suggest maximum temperatures 3-5
degrees below normal. Favor maximum temperatures on the cool side of MOS
guidance (upper 40s north to lower 50s south).
Clouds will thicken and lower Friday night as moisture transport
increases especially after 06z. At that time...may also start to see
patches of drizzle and/or light rain develop over mainly the SW
third of the County Warning Area. This precipitation falling through the relatively dry sub
cloud layer will likely initiate a cold air damming event
(classified as insitu as not direct link to a parent high evident).
Thickening cloud coverage and patches of light precipitation should produce
a fairly uniform min temperature field across the region. Min temperatures middle-
Saturday into Saturday night... unsettle weather this period with
rain becoming widespread across central NC in the morning as 850mb
moisture transport increases through out the day and lift increases
through the column as region will be in the right entrance region of
a strong jet lifting across the central Appalachians into southern
New England. The rain will likely aid to maintain a cad air mass
over most of the Piedmont...keeping temperatures in the low-middle 40s for
most of the day. May see some recovery late in the day near Raleigh.
GFS suggest The Wedge boundary making inroads into the Piedmont by
00z sun while the 00z European model (ecmwf) holds the boundary in vicinity of our southeast
counties. This leads to a challenging temperature forecast for location
from Raleigh and points eastward with a potential for a big
temperature bust. Have raised temperatures up a few degrees in the southeast where
better chance for temperature recovery expected. Massage maximum temperatures up a
degree or two in the eastern Piedmont but this region may see warmer
temperatures if the GFS solution verifies. Maximum temperatures middle 40s northwest to low-middle
60s far southeast.
One thing to note is that the 00z GFS has backed off on its
depiction of a surface wave moving/strengthening over our region
late Saturday and Saturday night. This suggest that the threat for
strong/isolated severe T-storms has decreased compared to 14 hours
ago. However, this potential still needs to be monitored as wind
profile is of sufficient strength/orientation for strong low level
helicity values (gfs values 250-400 m2/s2 early Saturday
evening...mainly along and east of Highway 1). As is normally the
case this time of year...near surface instability lacking barely
exceeding 200 j/kg.
Rainfall Saturday into Saturday evening should be abundant given the
available moisture and lift. Precipitation amounts between 1 and 2 inches
probable across the region this period.
Saturday night into Sunday... moisture transport begins to weaken
after 00z and begins to shutdown by 06z as middle level trough and
attendant surface boundary crosses the region. Expect rain to diminish
from west-to-east late Saturday evening and into the overnight
hours. Any partial clearing that occurs will be restricted to the northwest
third of the forecast area. If surface winds light/near calm...may see
areas fo fog develop in this region by daybreak Sunday. Min upper
30s northwest to around 50 southeast.
Sunday through Tuesday will feature a drier and colder air mass as
high pressure builds into the region from the west behind the
exiting frontal system. A modified Canadian air mass will overspread
the region...leading to temperatures below to well below normal for middle
December on Monday and Tuesday.
Aviation /12z Wednesday through Sunday/...
as of 620 am Wednesday...
24 hour taf period: high confidence of VFR conditions continuing
through the 24 hour taf period.
High pressure will move across the area today... resulting in
generally light an variable winds and mostly clear skies. We may see
some winds approach 10 miles per hour out of the southwest at kgso/kint as the
high shifts eastward during the afternoon... before winds decrease
with sunset. A dry cold front will quickly move across the area
tonight/Thursday morning... resulting in continued VFR conditions.
Long term: VFR conditions are expected through Friday. A low
pressure system will approach the area near the end of the work week
and will likely bring adverse aviation conditions for the weekend.