Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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FXUS62 KRAH 241356

Area Forecast Discussion
National Weather Service Raleigh NC
955 AM EDT Sat Mar 24 2018

An area of low pressure and upper level disturbance will approach
from the northwest today, and cross central NC through tonight.
Chilly high pressure will build into the Carolinas from the north
Sunday through Monday.


As of 955 AM Saturday...

Primary change this morning was to raise temps, especially across
the far S and E CWA, where readings have already risen to the low-
mid 40s. Light precip associated with a weak leading perturbation
continues to spread into western NC, although high cloud bases and
high surface dewpoint depressions further east into the Piedmont are
resulting in mostly virga over the RAH CWA so far. This virga or
very light precip will continue spreading east primarily over far
western and northern sections through mid afternoon, with overall
light amounts. This will change as moist upglide ramps up late this
afternoon, with precip filling in and increasing in intensity by
early evening as 850 mb southwesterly flow strengthens up and over
the surface based cold pool ahead of the approaching low level
circulation. Have made minor tweaks to pops, slightly downward over
the SW CWA, slightly slower, and slightly lower amounts through the
mid afternoon. It still appears that based on the surface wet bulb
temps and the low level thermal and moisture structure, this precip
through mid to late afternoon will fall as just a chilly rain,
although this too will change as rates pick up this evening,
suggesting potential bursts of mostly wet snow across the far north.
More on this later. Highs 43-54 NW to SE. -GIH

Previous discussion from 345 am: A narrow ridge of high pressure at
the sfc aligned nw-se across central NC has aided to maintain clear
skies over most of the region overnight, excluding the southern
Piedmont counties. This sfc feature is expected to maintain a
presence over central NC through 00Z Sunday. A s/w and attendant low
level wave was noted on satellite and water vapor imagery diving
sewd from the upper Midwest into the Mid MS Valley. The leading
precip shield associated with this feature currently crossing
sections of the TN Valley, and verifying well by the near term

This morning: The presence of the sfc ridge will maintain a cool dry
air mass in the lower levels of the atmosphere through much of the
morning, especially across the ne half of the forecast area. In
addition, best moisture advection aloft will remain immediately west-
sw of our area. Thus, potential for partial sunshine to occur over
the east half of central NC through 15Z, with clouds eventually
thickening/lowering by late morning. This brief period of partial
sun should aid in the recovery of temperatures, which are expected
to start out within a degree or two of freezing. Now appears that
the better lift/moisture depth will hold off across the western
Piedmont until late morning, so have delayed the onset of higher
PoPs until late morning/mid day. Still a small threat for a brief
period of rain/snow over the NW piedmont prior to 14Z as evaporative
cooling will cool the lower atmosphere enough to support some frozen
precip. After 14Z, the lower atmosphere should warm enough to cause
all precip to be liquid.

This afternoon: A notable change in the near term models compared to
previous runs was an increase in the boundary layer temperature
profile. Not sure if this the start of a new model trend or just a
hiccup with later model runs reverting back to a cooler boundary
layer. If the current model runs verify, bulk of precipitation this
afternoon across the northern Piedmont will be all liquid with a few
snowflakes notable during times of heavier precip rates. Based on
latest CAMs and HRRR, this threat for wintry mix will not occur
until late afternoon-early evening. Potential for the widespread
rain to hold off until early-mid afternoon in the west, and late
afternoon in the east. If this verifies, high temperatures may end
up a couple of degrees warmer than forecast, especially in the east-


As of 345 AM Saturday...

Tonight: Widespread precipitation anticipated as the s/w and
attendant low level wave passes overhead. The s/w will be
encountering a confluent flow aloft, so it will be shearing out with
time. This, in turn, will result in a baggy/weakening low level
wave. The translation of the energy aloft sewd still expected to
generate the development of a secondary wave in the vicinity of Cape
Fear, suggesting a Miller B scenario, after 06Z. Thus, may see the
precip bands concentrate more across the southeast half late
tonight, versus the north-northwest where the atmosphere will be
colder. With the expectation of the precipitation to be spotty
across the north-northwest as the air mass becomes cooler, appears
that the lighter precip rates will support more of a rain/snow mix
and less all snow/sleet, cutting back on expected frozen
accumulation. Based on this scenario, appears snow accumulation of
less than an inch along and north of I-85 with an outside chance of
close to an inch in vicinity of Roxboro. With the relatively warm
ground, any snow that does accumulate will melt within an hour or
two after the snow ceases. Thus expect little if any impacts. prior
to 12Z, model sounding suggest the atmosphere drying out in the
favored dendrite growth region. this would support the precipitation
turning back to mostly rain or rain mixed at times with sleet.

Sunday: Precipitation expected to decrease north-to-south in the
morning as the newly developed sfc wave pulls away from the coast as
the s/w exits offshore. Precipitation Sunday morning expected to be
all liquid as the favored dendrite region will continue to dry out.
Cyclonic circulation will maintain overcast skies through early
afternoon with gradually clearing expected in the mid-late
afternoon, commencing int he north-northeast, gradually eroding to
the south. High temperatures will remain well below normal for late
March ranging from the low-mid 40s north to near 50 south.


As of 330 AM Saturday...

Cold air advection will be underway as high pressure ridges down the
Atlantic coast Sunday night and Monday. Lingering cloudiness is
expected in the west overnight and into Monday due to moisture
pinned up against the terrain by low level easterly flow, otherwise
dry and cool with lows Monday morning from 30 to 35 followed by
highs only from 50 to 55 despite mostly sunny skies. The cool ridge
holds through Tuesday, though gradually weakening and lifting north
into New England. Highs Tuesday will stall in the low to mid 50s
after morning lows again in the low to mid 30s.

The surface high lifting out combined with a mid level ridge axis
over the area edging east and offshore Tuesday night will result in
deep southwestest return flow with strong warm air advection setting
up early Wednesday. Highs will warm significantly, reaching the 60s
after a final cool morning in the mid to upper 30s.

While all of this is going on locally, a long wave trof will be
deepening over the Plains, with strong frontogenesis stretching out
of the Gulf northeastward into the Ohio Valley during the mid week.
The mid level ridge just east of the area will block and slow down
the eastward progression of this surface front, allowing deeper
layer moisture advection into the area, with precipitable waters
edging above 1.5 inches prior to the front`s passage early Friday.
A lot of favorable ingredients will be also be present which would
support stronger shower and thunderstorm activity, which would most
likely manifest as a line of strong convection immediately ahead of
the front. Timing will be key, as just a little delay of frontal
passage from Thursday night into Friday would add increased
instability to the mix. Highs Thursday and Friday will be in the mid
60s north to mid 70s south after mild mins near 50 each morning.


As of 645 AM Saturday...

Cloud coverage will continue to overspread and gradually lower
across central NC today. In the Triad (GSO/INT) TAF sites, VFR
conditions will linger through late morning, 16Z-ish, lowering to
MVFR as precipitation spreads across the area. Precipitation could
be mixed at the onset, but predominantly rain through the daylight
hours. Conditions will lower to IFR, even MVFR after 00Z and
continue overnight with potential for a rain/snow mix. The east and
south, the same ceiling/visibility trends are expected, albeit
delayed a bit and with a lesser chance of precip mixing with snow,
with MVFR by mid to late afternoon, and IFR/MVFR overnight.

Outlook: Precipitation is expected to end from west to east through
Sunday morning, periods of MVFR to low VFR ceilings will linger into
the afternoon when drier works its way south into cntl NC. IFR
ceilings may redevelop late Sun night-early Mon, with VFR conditions
otherwise expected through the middle of the week.





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