Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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FXUS62 KRAH 171734

Area Forecast Discussion
National Weather Service Raleigh NC
1230 PM EST Thu Jan 17 2019

A weak front stalled across southern NC this morning will lift back
north as a warm front through the early afternoon. A fast moving
upper level trough will cross the region Thursday evening, producing
unsettled weather. Weak high pressure will bring dry weather Friday,
then a strong storm system will move through Saturday through early
Sunday. Sharply colder air will arrive Sunday into Monday.


As of 1050 AM Thursday...

Only minor changes for the mid morning update to match forecast with
in-situ measurements. High temps look to be on track, limited
greatly by BKN/OVC skies and precipitation arrival has remained
persistent with earlier thinking.

Previous valid discussion...

A low-amplitude shortwave trough, comprised of several pieces of
shortwave energy, will progress quickly east from the Mid MS Valley
this morning to exiting off the southern Mid-Atlantic Coast shortly
after midnight Thursday night. In advance of the shortwave trough,
the weak front stalled across southern NC, will retreat northward as
a warm front through midday. While associated rain showers are not
expected to arrive into the western Piedmont until the late
afternoon/early evening(~21z), a thickening/lowering AC/AS cloud
prior to the arrival of the rain will greatly tempered insolation.
Highs ranging from lower to mid 40s north to lower/mid 50s south.

Area of spotty light rain showers will quickly traverse the area
this evening, with the best shower coverage expected across the
northern half of the forecast, where deeper moisture profiles are
projected. Due to the fast movement of this system, rain amounts
will be light, generally less a tenth of an inch or less, with the
highest amounts near the Va border. Lows ranging from mid/upper 30s
north to lower 40s south.


As of 320 AM Thursday...

Synoptic scale subsidence and W-NWLY down slope flow in the wake of
the exiting shortwave trough and trailing weak surface front is
expected to scoured out the bulk of any left-over low-level
moisture/clouds between 12 to 18z Friday. Surface high pressure will
briefly build south into the area from the north late Friday
afternoon/evening, pushing the front just south of the area. Highs
Friday ranging from mid/upper 50s NW to lower 60s SE.

The emergence of the amplifying shortwave trough into the
central/southern US Plains late Friday night/Saturday morning will
lead to a marked  increase in clouds overnight via weak southerly
upglide. However, with the exception of some very light rain across
the far western/southwestern zones, mostly dry conditions are
expected to persist through daybreak Saturday. Lows in the upper
30s/near 40 NE to lower/mid 40s south.


As of 400 AM Thursday...

The main story in the long term will be the well below normal
temperatures for Sunday night into Tuesday morning as Canadian high
pressure moves into central North Carolina. Wind chills in the
single digits will be possible Monday morning. After Tuesday
temperatures will begin to rise again as mid level heights are
forecast to once again rise above normal.

Starting Saturday a mid level shortwave will be centered across
Southeast Texas with a potent upper level low located over the
Hudson Bay. Towards central North Carolina a remnant back door
cold front will begin to wash out and retreat north. The ECMWF
continues to wash out the front near the SC/ NC border with the
CMC and NAM following suite. The GFS is further north and
appears to be a bit of the outlier here. As this occurs, some
form of cold air damming will occur as isentropic upglide
commences. Isentropic surfaces on the GFS show limited moisture,
but have kept temperatures trended more towards the ECMWF or
the cooler solution (due to the potential clouds and rain).
Saturday night into Sunday a potent upper level low will dive
south out of Ontario pushing an arctic cold front south while
surface low pressure is pulling east across TN. The GFS, ECMWF
and CMC have the general same idea this weekend, but differences
remain in what happens next. The ECMWF amplifies the upper
level trough axis more Sunday into Monday due to a slightly
stronger low over the Hudson Bay, while the CMC and GFS are
slightly weaker with the amplification. These trends are even
supported on the GEFS, GEPS, and EPS. This means the ECMWF keeps
the surface low slightly further south and is a tad slower with
the progression of the wave (compared to the GFS and CMC).
Precipitation wise, either solution looks to produce a soaking
rain as by Saturday evening the split jet moves overhead
providing broad upper level diffluence. The area also looks to
fall under a RRQ and LFQ. PWATs ahead of the front surge up to
around 1.00" (above the 90th percentile for this time of year).
The potential for a shallow convective line also exists here as
organization will likely occur along the leading line of PVA (or
500 mb height falls) and leading cold front. This initial cold
front will be ahead of the arctic cold front, or the main surge
of cold air. Instability out ahead of the system looks limited,
but is non-zero. The NAM actually shows up to 500 J/kg of MU
CAPE. For now have left the mention of thunder out of the
forecast, but this will have to be watched. Either way, some
strong winds could end up mixing down in the convective line
Saturday night/ Sunday morning.

The arctic cold front will then cross the zones late Sunday morning
into Sunday afternoon with temperatures falling through the day.
Behind the front, thermal profiles quickly collapse with 850 mb
temperatures falling from 9 degrees C to 9 degrees C below
zero. Looking at GFS forecast soundings, momentum transfer
techniques are supporting wind gusts nearing 35 MPH. Given the
strong CAA and isallobaric wind this seems plausible. Strong
Arctic high pressure will surge southeast Monday morning with
temperatures falling into the teens. Wind chills will be in the
single digits Monday morning. Temperatures during the day Monday
will be cold with highs only in the 30s as 850 mb temperatures
remain below zero. Differences in the strength of the Arctic
high exist here though due to the mentioned strength of the
trough axis. The ECMWF is much stronger with the upper level jet
streak and therefore allows for stronger ageostrophic
convergence and surface high pressure. This in turn also keeps
the Arctic high around for slightly longer. The GFS/ CMC
solution has temperatures warming into the 50s by Tuesday while
the ECMWF only has highs in the lower 40s. For now have kept the
forecast trended towards the ECMWF/ colder solution. This
solution is also slower with the next system and holds rain
chances off to Wednesday. Have kept the forecast trended this


As of 1230 PM Thursday...

VFR conditions remain dominant across central NC early Thursday
afternoon, with the exception of the northern Piedmont terminals
(primarily KGSO) which continues to see MVFR CIGS (~1200 to 1700 ft)
associated with a now stalled backdoor cold front. This front is
expected to quickly retreat northward as a warm front early this
afternoon as a low-amplitude shortwave trough over the Mid MS Valley
quickly approaches the southern Appalachians from the west late this
afternoon. In advance of this system, top-down moistening will
result in lowering ceilings through the day, but CIGs should remain
predominately VFR through 00z Friday before dipping into the
MVFR/IFR ranges during the overnight hours. The precipitation
related to this feature will likely impact KINT/KGSO between 00 -
03z, KRDU between 01 - 04z, and KRWI/KFAY between 02 - 06z or so.
Behind the system, low CIGS and varying VSBYs will likely continue
into the mid morning hours on Friday, although, the intensity and
duration may be linked to if/how much precipitation falls in the
terminal vicinities. Improvement likely on Friday, with most
terminals expected to return to VFR by 18z (1pm).

Outlook: In the wake of the exiting system, VFR conditions are
expected to return on Friday. The risk for sub-VFR conditions
increases starting late Fri night as a strong storm system
approaches from the west, with rain likely to arrive mid day Sat
through Sat night, along with strong/shifting winds. VFR conditions
should return Sun into Mon with brisk winds and sharply colder air.




LONG TERM...Haines
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