Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Raleigh NC
1000 am EST Monday Dec 9 2013
Synopsis... a slow-moving frontal system west of the Appalachians
this morning will drift across our region tonight and Tuesday.
Chilly high pressure will build into our region Tuesday night and
Near term /through tonight/...
as of 1000 am Monday...
Not a lot of change needed to the forecast this morning. The cad
airmass is fairly well entrenched and will struggle to erode today.
While the depth of the residual cold pool has decreased on the 12z
kgso radiosonde observation...there is still a uniform 40kt flow at 850mb and enough
warm advection to keep static stability strong and the cad locked
in. Rap and hrr soundings keep the entire County Warning Area murky well on into
tonight. The current temperature forecast is at the lowest end of the
guidance spectrum...showing temperatures ranging from lower 40s to upper
50s. The biggest potential for error will be between the Triangle
and the coastal plain as the cad boundary tries to retreat north and
westward. Regarding probability of precipitation...the axis of deepest moisture is
currently over western NC...ahead of the slow moving cold front west
of the mountains. The narrow rain band that currently extends from
west of kclt to kgso and and into Virginia appears to be linked to
isentropic lift between 295k and 305k...which weakens this afternoon
as another wave of low pressure begins to form to our southwest
along the front. Even if a lull develops...drizzle will
continue...so high probability of precipitation will be maintained....particularly west of
Highway 1. -Smith
Tonight: short wave energy over the central rockies will push eastward
today into tonight... helping to push a cold front eastward towards
our region... with a surface low developing along The Wedge front
(ahead of the approaching cold front) somewhere across our
southeast/just to the south of our region late tonight. This surface
low and continued middle level short wave energy embedded in the southwesterly
middle level flow will help support a good chance of rain areawide.
Temperatures may actually rise or hold steady overnight where the cad holds
today. Will go with lows ranging from the low to middle 40s northwest to low
to middle 50s southeast. -Bsd
Short term /Tuesday through Tuesday night/...
as of 3 am Monday...
Tuesday...the surface low that develops along a front situated SW-NE
across central NC will aid to enhance low level convergence over our
region. Aloft...a strong jet streaking across the eastern lakes into
southern New England will increase upper level divergence over the
virginias and NC while the middle level short wave responsible for generating
the surface wave overhead will quickly shear northeastward across the Tennessee and
middle Ohio Valley. The combination of the available lift and moisture
suggest rain should be widespread across the region to start the day
Tuesday. Model guidance has varied as to where the axis of
"heaviest" precipitation (a quarter to a half inch) will set-up.
However...00z GFS and European model (ecmwf) have come into better
agreement...favoring the eastern Piedmont and coastal plain.
Still...better dynamics aloft will reside over the northwest Piedmont. At
this time plan to broadbrush the precipitation amounts across the region
with two to three tenths expected in the 12z-18z Tuesday time frame.
Middle level short wave will quickly shear out to the NE Tuesday
afternoon...propelling the surface front eastward to the coast by
Tuesday afternoon. Thus...should see precipitation taper off west-to-east
Tuesday afternoon across central NC. Developing northwest flow in the lower
levels will lead to decreasing cloudiness late in the day.
Developing downslope flow will initially offset cold air advection.
Residual cad air mass across the north-northwest Piedmont should hold temperatures
in the 40s. Expect warmer temperatures over the southern counties where
temperatures in the 50s (around 60 far se) probable.
Colder drier air will overspread central NC Tuesday night as a
modified Canadian high pressure system builds into the region.
Parent high expected to be centered over the central Appalachians by
early Wednesday with a surface ridge extending into NC. Thus...while
early Tuesday evening will be a bit breezy...looks plausible for surface
winds to decouple after midnight. This will allow temperatures to fall into
the 20s by daybreak Wednesday across the north...and near 30 across
Long term /Wednesday through Sunday/...
as of 330 am Monday...
Wednesday through Friday...relatively tranquil weather period
anticipated from middle week to the end of the week as broad upper
trough covers the eastern two-thirds of the nation. The northern
branch will be dominate in the beginning...keeping the storm track
mainly across the northern plains-Great Lakes and New England. The
surface high that covers our region Wednesday will quickly scoot eastward
by late Wednesday but is replaced by another chilly high that builds
into the middle Atlantic in the wake of a middle level short wave crosses the
Great Lakes into New England. Thus our current stretch of be low
normal temperatures will continue. High temperatures Wednesday and Thursday will be in the
40s while min temperatures will be in the 20s to around 30.
Saturday into Sunday...operational GFS and European model (ecmwf) offer a similar
solution for the upcoming weekend weather pattern...in sharp
contrast tot east 12z model runs which depicted a solution very similar
to our current weather pattern. Model guidance now advertising a
return flow regime becoming established Friday night into
Saturday...ahead of a frontal system dominated by a short wave in the
northern stream. This frontal system crosses our region Saturday
night with chilly high pressure building into our region Sunday. For
now will confine the best chance for precipitation to Saturday and keep it
all liquid. If precipitation comes into the region early enough Friday
night...may start out as a wintry mix before quickly transitioning
to all rain. Temperatures expected to remain below normal and highly
dependent upon precipitation coverage. Highs in the 40s to around 50. Lows
temperatures in the 30s Friday night and 30s to around 40 Saturday night.
Aviation /12z Monday through Friday/...
as of 730 am Monday...
24-hour taf period: adverse aviation conditions will prevail through
the taf period. Expect LIFR ceilings this morning to continue
through at least middle morning... with visbys possibly dropping to a
mile or less over the next couple of hours as well (possibly lasting
until middle morning too). Periods of rain and drizzle will be possible
at kgso/kint/krdu today... with more patchy rain and drizzle at krwi
and kfay. Expect ceilings will be very slow to improve today... and may
only improve to IFR ceilings given the cad airmass over central NC. In
addition to the poor ceilings/visbys and rain/drizzle... southwesterly
winds of around 40-50 kts are expected at around 2 kft above the
surface today... creating low level winds shear conditions. Thus...
have added low level wind shear to the tafs for today. Expect winds at around 2 kft
to weaken between 21-00z today.
LIFR conditions are again expected tonight... as ceilings lower and
visbys drop after 00-03z. Chances for rain/drizzle will continue as
Looking ahead: adverse conditions (ifr/MVFR and perhaps lifr)
associated/west the cold air damming wedge will most likely linger into the
day Tuesday as a cold front approaches from the west. VFR conditions
will return Tuesday night as dry high pressure builds into the region
from the west. VFR/dry conditions are expected to prevail through
the remainder of the work week.