Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Raleigh NC
920 am EST Thursday Dec 18 2014
Synopsis...high pressure over the Mississippi River valley will
extend eastward into the middle Atlantic and Carolinas on through
Friday. A low pressure system will develop along the Texas coast on
Friday and track east along the Gulf Coast on Saturday...then move
offshore the southeast coast Saturday night into Sunday.
Near term /through tonight/...
as of 920 am Thursday...
An already low amplitude trough aloft over the middle-Mississippi River
valley this morning will continue to deamplify as it skirts across
the Tennessee Valley and Virginia through tonight. An area of light
rain moving across eastern Tennessee...focused near the h925-850 mb
frontal zone where isentropic lift is greatest...continues to
gradually weaken with eastward extent. Expect this trend to the
continue as the upper disturbance moves off to the east-northeast
and the forcing decouples. So no precipitation is expected over central
NC. East of the mountains...the airmass remains very dry with all
moisture above 12k feet. While there is a fair amount of high level
moisture upstream...guidance suggests there will periods of sun
mixed in with the high clouds (potentially orographically enhanced
at times this morning. Temperatures should get to within a couple degrees
of the current forecast highs of 49-55. If anything...they will end
up on the cooler side.
Skies will begin to clear off by early Friday morning with the
passage of the aforementioned middle level trough. Given a rather weak
mslp gradient across the area with surface high pressure extending
into the area from the west... we should see light and variable to
calm winds overnight. This combined with clear skies will allow US
to cool quite nicely tonight. Expect low temperatures will range from the
upper 20s to around 30 (with maybe even a few middle 20s) in the usual
rural cold locations to the lower 30s elsewhere.
Short term /Friday and Friday night/...
as of 310 am Thursday...
Friday and Friday night: another southern stream disturbance is
expected to approach the area on Friday. Short wave energy is expected to
lift northeastward out of the Southern Plains on Friday. Meanwhile
central NC will experience brief short wave ridging again on Friday in
between the aforementioned system tonight and the approaching system
on Friday. This will yield mostly sunny skies... possibly becoming
partly sunny by late in the day as high level cloud cover begins to
stream north and eastward in advance of the next system. Afternoon
low level thickness values are expected to be a bit lower on
Friday... around 5 meters or so. However... with more sun expected
on Friday think we will see similar high temperatures compared to today.
Thus... expect high to range from 50 to 55.
As the middle level disturbance shifts northeastward towards our region
from the lower MS River Valley on Friday evening/early Saturday
morning surface low pressure will develop across the northern Gulf
of Mexico and begin tracking eastward. As the system approaches
moisture will stream northward across central NC... with shallow
isentropic lift beginning to increase across the western and
southwestern portion of the County Warning Area during the early morning hours of
Saturday. At the onset of light precipitation spreading into northwest
Piedmont we could see a brief period of some sleet mixed in with the
light rain. However... temperatures are expected to remain above freezing
across the western/northwestern Piedmont on Saturday morning no
impacts are expected. In fact will keep any mention of sleet out of
the forecast for now.... as any occurrence would be brief and of no
impact. Low temperatures Saturday morning will range from the upper 30s
south/southwest to the lower 30s northeast.
Long term /Saturday through Wednesday/...
as of 347 am Wednesday...
Saturday...while a surface dry Air Ridge and parent high are still in a
favorable location to support a cold air damming event
Saturday...the latest model guidance has taken the middle level short wave on
a more southerly trajectory than previous model runs and parent high a few
millibars weaker than previous model runs. This still yields a
hybrid damming event though precipitation amounts appear to be lighter than
earlier forecasts. Short wave has recently entered the upper air rawinsonde
network so the 12z model run should give an indication of eventual
strength/track of this system regardless...overcast skies and
areas/widespread light rain will result in a cold damp day. MOS
guidance appears too warm and am reluctant to raise maximum temperatures
Saturday from current forecast (near 40 northwest to upper 40s far se). If
rain coverage ends up being spotty versus widespread...then could
see an adjustment in maximum temperatures to values comparable to the NAM MOS
(mid-upper 40s, near 50 se).
Saturday night...subsidence behind the exiting short wave should aid to
diminish light precipitation with greatest threat for spotty light precipitation
over the sandhills/southern coastal plain (again due to the more southerly
track of the S/w). Otherwise continued considerable cloudiness with
patchy drizzle and/or light fog. Min temperatures middle-upper 30s.
Sunday through Wednesday...a major storm may affect the southeast U.S. Late
Tuesday into Wednesday (including central nc) with potential impact
on travel due to heavy rain and strong gusty winds.
Model guidance flip-flopped a bit for Sunday into Monday with
guidance now indicating a narrow dry air surface ridge across the region
Sunday with another short wave in the southern stream initiating another
area of rain late Sunday into Monday. This scenario has also been
picked-up by the latest European model (ecmwf). Partial thicknesses support all
liquid across the Piedmont at this time. Due to the shift...plan to
confine probability of precipitation to just chance at this time late Sunday into Monday. If
later model runs maintain some consistency...likely probability of precipitation warranted
for overnight Sunday into Monday morning. Will continue below normal
high temperatures Sunday into Monday...with maximum temperatures Monday as much as 7-8
degrees below normal if rain materializes.
Tuesday into Wednesday...amplifying l/west trough in the northern
stream will tap Gulf moisture and stream it northward into the
Carolinas Tuesday into Tuesday night. Model guidance indicate a
strong jet rounding base of trough late Tuesday night into Wednesday
morning causing the trough to become negatively tilted across the southeast
U.S. If this holds together...could see a band of heavy showers with
a few storms cross central NC overnight Tuesday night into early
Wednesday. The parent low deepens significantly Wednesday and
Wednesday night with the potential for strong gusty winds across
central NC. A brief bump up in temperatures Tuesday ahead of the storm
system Tuesday followed by another round of below normal maximum temperatures
though truly frigid air not seen in our region through Christmas.
Aviation /12z Thursday through Monday/...
as of 635 am Thursday...
24-hour taf period: high confidence that VFR conditions and light
and variable winds will dominate the 24 hour taf period as high
pressure extends into the area from the west. Scattered to broken
middle and high clouds will continue to traverse the area as a middle
level disturbance approaches the region from the west and moves
across the area tonight. Skies are expected to clear tonight...
generally in the early morning hours of Friday.
Looking ahead: VFR conditions are expected to continue into Saturday
morning. Aviation conditions are expected to deteriorate on Saturday
middle to late morning into the afternoon as a storm system is expected
to form over the Texas Gulf Coast late Friday...with low pressure
tracking east across the southern Gulf states Saturday...to along or
just off the southeast coast Sat night and sun. Areas of rain with
sub-VFR conditions appear likely from Saturday middle Saturday morning
through Saturday night. VFR conditions are expected to briefly
return for Sunday... before another disturbance lifts northeastward
out of the Gulf... spreading rain into central NC Sunday night into
Monday... with sub-VFR conditions.