Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Raleigh NC
1155 am EST sun Dec 4 2016

cold air damming surface high pressure will extend into the area
from the north into this evening as low pressure develops along
the Gulf Coast. This weak low pressure will track NE along a
coastal front tonight and offshore Monday. High pressure will build
in briefly from the north Monday afternoon, then a second stronger
low pressure system will cross the Gulf states and Carolinas Monday
night through Tuesday.


Near term /through tonight/...
as of 1155 am Sunday...

..cold air damming rapidly setting up over interior areas of NC...

As expected, surface high pressure (of sufficient strength - greater
than 1025 mb)and preferred location (extending from New York state south
into the carolina's) to deliver a cad event for our region, has
evolved. The center of the high pressure was located over central
PA into central Virginia (1027 mb) at noon. Cold air advection and even dry air
advection(daa)continued over our region. The dry air source over Virginia
reported dew points in the 25-30 range. This dry air was still
being pushed into our region. Meanwhile, areas of rain continued to
develop/spread NE and east from the Gulf Coast states through the Tennessee
Valley and across the southern Appalachians due to the approaching
disturbances in the mid/upper jet stream. While near saturation has
occurred over much of western and southern NC where current
readings ranged from 33-40 (a very chilly rain), areas in the north
and east yet to see the rain begin still have a way to go to reach
saturation. In other words, evaporative cooling will strongly ensue
this afternoon as the rain fills in over the region from west-southwest to east-northeast.
Expect highs to occur very early this afternoon, with falling
temperatures this afternoon as the rain increases. Highs have
probably already been achieved in The Triad and in the sandhills (40-
45) and they have the potential to fall into the upper 30s this
afternoon in the entire triad region. To the east, expect rain to
develop and overspread the region (very light at first, becoming
steady later in the afternoon). As this occurs, the temperatures
will fall from the 45-50 range early afternoon into the lower 40s
later. As for qfp, expect a general 0.10 to 0.25 in the northwest ranging
to 0.25 to 0.50 in the southeast for storm totals (just through this
afternoon and tonight) - the next system will affect US with
significant rain later Monday into Tuesday. See discussion below.
The latest radar and satellite data are in good agreement with the
overall rain system, with the steady rain expected to affect most
of our region mid to late afternoon through the overnight, tapering
off from the west overnight, leaving areas of mist and drizzle. The
cold air damming will be locked in place tonight. As temperatures
are expected to fall into the upper 30s northwest and lower to mid 40s southeast
through the afternoon. As the saturation is reached and the parent
high shifts offshore tonight, temperatures will steady out at those
upper 30s to lower 40s in the early evening, then remain overnight.
Some dense fog may develop later tonight as the main lifting
mechanism passes and the rain ends. This will leave low overcast
with areas of fog and drizzle for the morning commute.


Short term /Monday through Monday night/...
as of 305 am Sunday...

Much of Mon should actually be dry over central NC. Deep mid level
low pressure over northwest Mexico this morning will lift NE across Texas
through Mon, forcing a period of anticyclonic flow upstream over the
Gulf/southeast/Carolinas. The surface low will track NE away from
the Carolina coast Mon morning with subsidence in its wake, as high
pressure centered over the Ohio Valley into the central Appalachians
builds in from the north, suppressing the frontal zone down across
the central Gulf states. While there will still be a stable ridge
nosing into the area, the lack of overrunning and brief period of
subsidence aloft should generate a short-lived decrease in clouds
and a lull in The Wedge/damming regime. Expect highs mostly in the
mid 50s. But rain returns Mon night as the potent mid level wave
lifts NE through the lower miss valley, accompanied by a strong low
at the surface, inducing a northward lift of the frontal zone. Moist
upglide strengthens again rather quickly during the evening ahead of
the mid level wave/surface low, as a 40-50 kt 850 mb southerly jet
arrives from the west-southwest. Unlike late today/tonight when the best
forcing for ascent (and hence most steady rain) will be focused
across our southern half, on Mon night the entire County Warning Area will see
increasing precipitable water to around 1.5" and much stronger upper divergence, so
pops will increase SW to NE to categorical areawide by midnight.
Expect 0.5-0.75" of rain overnight. Lows in the mid-upper 40s. -Gih


Long term /Tuesday through Saturday/...
as of 335 am Sunday...

The main weather maker for the period will be on Tuesday as a couple
of low pressure systems affect the area in a Miller b type scenario
with the main low pressure system tracking west of the Appalachians
and a coastal low forming along the Carolina coast. This storm will
have the most moisture available in recent memory and significant
precipitation is expected with a widespread 1-2 inches of rain
possible beginning late Monday night through Tuesday night. While
the storm progresses to the northeast, a wedge will remain in place
for much of the day keeping temperatures very cool, particularly in
The Triad where highs will struggle to get into the mid 40s. There
will be a large temperature gradient across the County Warning Area as warmer air
filters in with the coastal front and highs on southern and eastern
fringes of the area could reach the upper 50s. The rain will end
Thursday night as the system heads to the northeast. Lows in the
upper 30s to low 40s.

Wednesday and Thursday are expected to be dry and warmer with highs
in the mid to upper 50s both days as the warm sector moves overhead
in front of a longwave trough that will move across the plains and
bring much colder temperatures to the area by Thursday night into
Friday. The timing of this feature is quicker in the GFS solution
and it is also drier as it moves through the area. The European model (ecmwf)
solution lags the front and allows for significant development of
the front with Gulf moisture. This scenario seems to be an outlier
when compared to ensemble means and so the thinking at this time is
for the drier/colder scenario seen in the GFS to prevail but I
wouldn't be surprised if the timing is a little fast, as can be the
case with the GFS. Lows Thursday night in the mid 20s to low 30s
with highs on Friday in the upper 30s to lower 40s.

The cold drier airmass persists with the coldest lows of the period
on Friday night with 20-25 degrees expected across the County Warning Area. Only
slight warming for Saturday with highs in the lower 40s.


Aviation /12z Sunday through Thursday/...
as of 635 am Sunday...

VFR conditions will hold through this morning across central NC,
although mid and high clouds will continue to overspread the area
from the west. Cigs will stay above 5,000 ft above ground level areawide through at
least 17z. But conditions will gradually deteriorate this afternoon
from SW to NE as surface low pressure and an upper level disturbance
track across the Gulf states toward the southeast. Cigs will drop to
MVFR at int/gso starting around 18z with light rain spreading in,
then at rdu/Fay starting around 21z, and at rwi starting around 02z
this evening. Cigs are then expected to drop to IFR at int/gso after
21z and at rdu/Fay after 02z this evening. Confidence is high in
poor aviation conditions (primarily IFR/LIFR cigs and MVFR/IFR
vsbys) dominating through daybreak Mon. The rain will initially be
light as it first arrives today with VFR vsbys holding, however as
the rain intensity picks up a bit later today, vsbys will drop to
MVFR in the late afternoon (int/gso) through early evening
(rdu/Fay/rwi). Winds will be light mainly from the NE initially
before shifting to blow from the east and southeast late today into tonight,
remaining light.

Looking beyond 12z Monday morning, rain will gradually taper off
from northwest to southeast Monday morning with conditions slowly rebounding to
VFR areawide from west to east during the morning. VFR conditions
will dominate from around 18z Mon until around 06z Tue. Then another
storm system will cause IFR conditions to return after 06z early Tue
morning, lasting through much of Tue with steady widespread rain
returning. Rain will end late Tue, although IFR clouds will hold
into Tue night. Improvement to VFR is expected Wed, then will drop
back to MVFR/IFR Wed night. Cold front passage may bring VFR
conditions once again Thu. -Gih


Rah watches/warnings/advisories...


near term...Badgett
short term...Hartfield
long term...Ellis

National Weather Service Glossary of Abbreviations