Turner Downs Forecast Discussion

000
FXUS62 KRAH 130742
AFDRAH

AREA FORECAST DISCUSSION
National Weather Service Raleigh NC
242 AM EST Fri Dec 13 2019

.SYNOPSIS...
High pressure will continue ridging into portions of the Piedmont
through tonight. A low over the Gulf of Mexico will track northeast
along the Southeast and mid-Atlantic coast through early Saturday.
A cold front will move east through the region late Saturday.

&&

.NEAR TERM /THROUGH SATURDAY/...
As of 1025 PM Thursday...

     Winter Weather Advisory for freezing rain is in effect for
portions of the Northwest Piedmont from midnight through noon
Friday...

The latest observational and radar data indicate banded
precipitation spreading/developing NE as expected into far SW North
Carolina and western and southern SC from GA. Models are in good
agreement in bringing this first round of precipitation into our
Piedmont between 100 am and 400 am, spreading into the northern tier
of the Piedmont (Triad) region around 400 to 500 am. Initially there
may be a little sleet and even a quick snowflake (in the initially
very dry and cold air mass) before quickly becoming rain as warming
aloft surges northward). Once rain begins, temps will
evaporationally cool into the lower 30s (or near freezing in the
advisory area creating the freezing rain potential). The WAA pattern
upstream is impressive aloft and this will drive the precipitation
development over much of western NC by 12z. With that will come
increasing temperatures aloft precluding any sleet or snow of
significance.

The initially favorable cold and dry air mass supplied by the 1040+
surface high will begin to exit aloft shortly. The parent
high has already moved off the NE coast, with an extension or ridge
axis south into NC/SC. The dry and cold advection has all but
ended. What has been deposited is still cold enough to allow
for at least some minimal icing in the Advisory area later tonight.
However, given the increasing warm nose aloft - this will be
self limiting and as any icing will end quickly by at least mid
morning Friday. The latest southern and eastern extent of the
forecast 32 degree wet bulb temperatures at or near onset of
precipitation suggests the current placement of the Advisory area
is in good shape, with most guidance indicating icing potential
should remain just NW of a line extending from High Rock and
Lexington to Graham to Roxboro. Further, the region on the north
and west side of the Winston-Salem area has the highest potential for
a few spots of 0.1 to 0.15 inch of ice on elevated surfaces before
rain overwhelms and melts any accrual by mid to late morning.

The critical time for freezing rain or light icing is expected to be
an hour or so after the rain develops - through around 8-10 am in
the morning. This is when the surface temperatures in and close to
the Winter Weather Advisory area will evaporatively cool to freezing
or slightly below. Therefore, in the Triad it appears that between
500 am and 900 am will be the most critical time in which some light
icing will likely occur. Rain should begin with temperatures around
33 or 34, then evaporative cooling will bring temperatures down
during this time frame to 31 or 32. At the same time this is going
on, it will be warming rapidly aloft as the H85 temperatures are
forecast to rise to near 45 by mid morning, then 50 or so by noon
Friday. This is a prominent warm nose and will eventually lead to
any icing to quickly become rain by 900 or 1000 am. Some colder low
lying areas may still have some lingering icing through the morning,
but the latest guidance suggests that this would be most likely
confined to the Foothill valleys. The Triad should be able to reach
the mid 30s by late morning, with a steady and locally heavy rain
expected to fall.

To the east, it appears that there is a possibility of some
light icing around daybreak a county or two to the east of
the current advisory area. This would include Randolph, Alamance,
and Person. The gradient between the 31-32 and 33 or so will make
all the difference between 500 and 900 am, and this region
has a chance at those readings - if only briefly before
the evaporative cooling is totally used up around that time - and
temperatures can rise slowly through the 30s. Confidence
is low in these counties, so we will not extend the Advisory
east at this time.

Elsewhere, only rain expected with lows in the lower to mid
30s, before rising later tonight from the SE (upper 30s
to near 40 by daybreak should be common around FAY to Goldsboro).

Previous AFD issued at 200 PM Thursday...

The coastal trough and Gulf low will strengthen on Friday, with the
low sliding northeast along the Southeast and mid-Atlantic coast
late Friday into Friday night. As the low approaches the Carolinas,
the wedge should largely erode and more convectively driven rainfall
moves into the area. Friday eve/night will have the more significant
rainfall amounts across central NC. Highs Friday will vary greatly
from NW to SE, from upper 30s to mid 50s. Lows will be similar to
highs, albeit a few degrees lower, upper 30s NW to mid 50s SE. When
all is said and done, total liquid precipitation for the period
Thursday night through Friday night could range from three-quarters
of an inch to almost two inches.

Rain will continue Fri night into Sat morning as we remain under the
influence of the incoming deep negatively tilted trough, focused on
two distinct waves, one approaching early Sat morning and the other
late Sat afternoon according to the ECMWF/Canadian. Surface low
pressure over the eastern Carolinas Fri evening will deepen as it
tracks through E VA to New England through Sat, sweeping the cold
front through the area and offshore. Will retain high pops Fri night
into Sat morning, tapering them down S to N in the afternoon as
drier air filters in with lowering PWs to near to below normal,
along with rising heights aloft as the trough swings to our NE late
Sat. The initial air mass right behind the front is not that
cool, with a southern-stream high building in from the WSW and
the true Arctic front holding well to our NW. So after somewhat
mild lows in the upper 30s NW to around 50 E, expect highs Sat
from the mid 50s NW to lower 60s SE.

&&

.SHORT TERM /SATURDAY NIGHT THROUGH MONDAY/...
As of 240 AM Friday...

The chance of a stray shower along Interstate 95 Saturday
evening is too low to include in the forecast at this time, and
skies should clear quickly during the evening. While
temperatures tonight will range from the upper 30s in the
northwest to the lower 50s in the southeast, overnight lows
should be more uniform Saturday night, with temperatures falling
into the upper 30s everywhere. Weak high pressure across the
Northern Plains will slide east over New York Sunday and Sunday
night, which should allow for mostly sunny skies and
temperatures to remain a couple degrees above normal. Clouds
will be on the increase as low pressure develops over Arkansas
by Monday morning and continues to move northeast. Yesterday the
ECMWF showed a track slightly farther to the southeast than the
GFS did, clipping northwestern counties with some precipitation
from the warm front. However, the latest run of the ECMWF has
come more in line with the GFS, and this track keeps all
precipitation out of North Carolina on Monday. With the models
coming into better agreement on the dry solution, have removed
pops from the forecast for Monday. Southerly winds and upper
level ridging will help temperatures to rise slightly above
normal, with most locations rising into the 60s.

&&

.LONG TERM /MONDAY NIGHT THROUGH THURSDAY/...
As of 240 AM Friday...

While the 00Z GFS moves low pressure into the Northeast faster
than the 00Z ECMWF, both models are still in agreement that the
cold front will sweep across the region late Monday night and
into Tuesday. Have increased pops to categorical in all
locations Tuesday. While the deterministic GFS shows drying
after 00Z Wed, several GFS ensemble members and the
deterministic ECMWF keep precipitation in the area a little
longer, so have held onto pops a little longer, especially east
of I-95. Once conditions dry out, surface high pressure should
move in across the southeastern United States. With an upper
level trough across much of the eastern United States,
temperatures will fall below normal, with highs in the 40s
everywhere on Wednesday and some slight improvement on Thursday.

&&

.AVIATION /08Z FRIDAY THROUGH TUESDAY/...
As of 1243 AM Friday...

Forecast remains on track with precipitation just now entering the
forecast area as of 0540Z. Precipitation in the Triad is likely to
begin as a period of very light rain, changing over to freezing rain
as evaporative cooling drops the temperature just below freezing.
Primary period of concern for light ice accumulations at INT and GSO
is between 08Z and 12Z, after that temps should rise just above
freezing and remain as such through the end of the TAF period.
Elsewhere at RDU, FAY, and RWI, freezing rain will not be a concern.
At all sites, expect a rapid thickening and lowering of clouds as
widespread IFR cigs anticipated area-wide by mid morning. Periods of
light rain will also bring in IFR vsbys by late morning which will
persist well into this evening. Guidance trying to suggest
widespread 1/2SM to 1/4SM vsbys late tonight into Saturday AM and
that may need to be introduced into the next set of TAFs later this
morning.

Saturday should see a gradual improvement in cigs and vsbys, MVFR
possible early Saturday with further improvement to VFR Saturday
afternoon/evening. Looking ahead, VFR likely Sunday and Monday but
another round of (liquid) precipitation looks possible on Tuesday
with a return of MVFR cigs/vsbys at all sites.

&&

.RAH WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until noon EST today for NCZ021-022-
038.

&&

$$
SYNOPSIS...Badgett
NEAR TERM...Badgett/Hartfield/KCP
SHORT TERM...TG
LONG TERM...TG
AVIATION...DL

NWS RAH Office Area Forecast Discussion