Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000
FXUS62 KRAH 191144
AFDRAH

AREA FORECAST DISCUSSION
National Weather Service Raleigh NC
645 AM EST Wed Feb 19 2020

.SYNOPSIS...
A cold front will settle south of the area this morning.
An expansive area of Arctic high pressure will build slowly east
into the region tonight through Friday. Meanwhile, low pressure will
develop near the SC/GA coast on Thursday, then move quickly east and
out to sea late Thursday and Thursday evening.


&&

.NEAR TERM /THROUGH TONIGHT/...
As of 403 AM Wednesday...

Surface cold front, which is currently moving SE through the Triad,
will continue to progress SE, and should settle south of the area
~12z, slightly slower than previously forecasted. We will continue
to see widespread rain ahead of the front, followed by a period of
very light ana-frontal rain/drizzle until the 925-850mb cold fronts
and associated F-gen settles south of the area aoa 18 to 20z.
Additionally, a period of post-frontal NELY wind gusts of 20 to 25
kts can be expected later this morning, before diminishing north to
south during the afternoon.

CAA behind the cold front this morning and into the afternoon,
combined with the slow southward suppression of the ana-frontal rain
band and associated low clouds will make for a challenging, atypical
temp forecast. Across southern portions of the forecast area,
daytime highs in the mid to upper 50s will occur in the morning,
with temperatures then falling into the mid to upper 40s. Across
northern portions of the forecast area, after CAA and falling
temperatures through mid to late morning, partial clearing during
the afternoon will allow temperatures to rebound into the upr 40s to
lwr 50s.

Tonight: Next wave of shortwave impulses, embedded in the WSWLY
southern stream jet, will lead to renewed isentropic lift and
overrunning, along with a slight chance of light rain across southern
portions of the states late tonight/towards daybreak. Lows tonight
ranging from mid 30 north, to around 40 south.

&&

.SHORT TERM /THURSDAY AND THURSDAY NIGHT/...
As of 500 AM Wednesday...

...Rain is expected to change to snow Thursday night before tapering
off pre-dawn Friday...

What has changed: Reduced snowfall amounts a bit across the
southeastern zones and focused the peak accumulation amounts (1-2")
a bit further north and west to better coincide with latest model
trends. Our forecast remains much lower than most raw forecast
guidance due to numerous limiting factors, including; (1) Warm
surface temperatures, (2) Self-limiting CAA processes, and (3) Low
rain to snow liquid ratios. Impacts should remain rather minimal,
however, a few slick spots may form on area roadways during the
overnight hours Thursday into early Friday morning, primarily along
and east of US-1. If the forecast holds true, a Winter Storm
Advisory issuance may be needed in future forecast packages (Our
criteria is areas that are expecting to see 1" of snow or greater in
a 12 hour period).

Full Discussion: The surface front will settle in a nearly west to
east orientation in vicinity of the SC/GA border, with ongoing
surface CAA processes occurring north of the boundary. The front is
progged to buckle back north as a series of weak disturbances begin
to develop during the day Thursday, with strong coastal cyclogenesis
beginning to initialize off the SC coast as early as Thursday
evening. Around the same time, strong Canadian surface high pressure
will continue to settle southeast through the central Corn Belt
states, extending east through the Great Lakes and northeastern US
states before beginning to south along the lee side of the
Appalachians.

Location and strength of the developing coastal low/trough will be
key to the forecast going forward, as a stronger and closer to shore
area of low pressure (NAM) will help to promote more rapid cold air
entrainment from the north. Forecast models continue to trend
increasingly wetter during the daytime hours Thursday, primarily as
a result of strong convergence in the 850mb level where steepening
temperature gradients and FGEN banding sets up in vicinity of the US-
64 corridor. Around this same time, a 925mb weakly defined cold
front (marking the leading edge of the cold nose) will extend south
into the area. This will aid, at least initially, in the ability for
wet bulb/evaporative cooling processes to promote a light rain to
snow changeover to occur in vicinity of the VA/NC border mid to late
afternoon. From there, a gradual changeover to a rain/snow mix to
eventually all snow will occur. At first, accumulations will remain
little to none with more of a "white rain" type scenario unfolding.
Eventually, especially areas east of US-1 where rain rates will
remain higher for longer, the wet-bulb effect will help to cool
surface temperatures below freezing, helping to promote
accumulations primarily on grassy and elevated surfaces. Best timing
for this will be between 02z/9pm Thursday through 10z/5am Friday
morning.

Snow amounts continue to remain highly uncertain in this event,
however, thinking that raw model output remains way too high in this
scenario. Several limiting factors remain at work, including: (1.)
Persistent above freezing surface temperatures that will help
temper/melt the majority of the snowfall. The exception to the rule
may be areas closer to the VA/NC border, however, these are the
areas that will likely see far less QPF throughout the event (2.)
10:1 snow to liquid ratios are not likely in an event like this,
expect closer to 3:1 at precipitation changeover onset, 5:1 toward
the middle of the event, and perhaps ending in vicinity of 10:1 if
we are lucky. (3.) While the CAA process will be key in the original
changeover, it will also become the ultimate limiting factor through
the frozen ptype portion of the event. Gradual drying is expected to
persistently take place in the lower-third of the boundary layer as
the cold air slides south into the region. This will work to greatly
reduce QPF from northwest to southeast in general. Higher amounts of
QPF in the southeast will remain, however, dewpoints in the middle
30s will keep widespread snowfall accumulations from occurring.

With these limiting factor in mind, have shifted the geographic
location of accumulations a bit further north and west. Amounts are
down a bit also with this run, topping out in the 1 - 2" range in
northeastern Coastal Plain. From there, if you follow the US-64
further west, expect accumulations in vicinity of an inch with
lesser amounts as you progress west of US-1 and south of US-64.
Alterations and fine tuning to the accumulation forecast is highly
likely to continue with sequential model runs and forecast packages,
but still finding it highly doubtful that significant snowfall
amounts will occur across central NC. For the snow lovers out there,
the canary in the mine that could indicate the potential for higher
amounts lies in surface dewpoints Thursday afternoon. If dewpoints
become significantly cooler, a possibility advertised by the
recently cold-initialized NAM, wet-bulb cooling processes may
promote an earlier changeover and perhaps higher amounts somewhere
across central NC assuming the moisture is available.

Precipitation should clear out of central NC rapidly pre-dawn Friday
leaving sub-freezing temperatures and a light northerly breeze in
its wake until sunrise. With sustained northerly winds in the 6 to
10mph range, thinking that drying processes in the coldest areas
should limit the black ice/flash freeze potential to only patchy
areas during the overnight hours. Sunshine and a breeze will help to
clear and dry any leftover frozen surface moisture rather quickly
Friday morning.

&&

.LONG TERM /FRIDAY THROUGH TUESDAY/...
As of 400 AM Wednesday...

Lingering flurries will come to an end quickly Friday morning across
the NC Coastal Plain as the departing mid-latitude coastal cyclone
pulls further east into the central Atlantic Ocean. In its wake,
cool Canadian high pressure will settle into place, keeping
conditions near normal through much of the weekend. High pressure
will slip east off the middle Atlantic coastline Sunday afternoon,
allowing southerly flow to return ahead of an approaching southern
stream disturbance, set to introduce showers back into the forecast
as early as pre-dawn Monday. Thicknesses ahead of the boundary
remain warm enough to keep the precipitation in liquid form early
next week with another round of dry/modified high pressure expected
to settle into the middle Atlantic by Tuesday afternoon and evening.
The quieter weather pattern should persist through mid-week with a
gradual warming trend underway.

&&

.AVIATION /12Z WEDNESDAY THROUGH SUNDAY/...
As of 645 AM Wednesday...

Behind the cold front that`s currently moving SE through KFAY,
ceilings will lift to MVFR and then VFR with a period of light
rain/drizzle before ending and conditions improving to VFR.
Additionally, expect a period of post-frontal NELY wind gusts into
the upper teens to lower 20s. Though we could see some very light
rain near KFAY by the predawn hours Thursday morning, VFR conditions
are expected to persist tonight-through early Thursday.

Outlook: Another storm system will take a srn track off the
Southeast coast and result in another round of precipitation and
associated sub-VFR conditions Thursday afternoon and Thursday
night. The rain could briefly mix with or change-over to all snow
Thursday evening and Thursday night before ending. VFR conditions
will return on Friday.


&&

.RAH WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$
SYNOPSIS...CBL
NEAR TERM...CBL
SHORT TERM...JJM
LONG TERM...JJM
AVIATION...CBL


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