Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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FXUS62 KRAH 231054

Area Forecast Discussion
National Weather Service Raleigh NC
700 AM EDT Fri Mar 23 2018

Cold high pressure will extend across the region today, then move
offshore late tonight. An area of low pressure will approach from
the west Saturday and cross our region Saturday night and Sunday.


As of 315 AM Friday...

Mid level ridge axis has shifted offshore, with surface high
pressure over the western Great Lakes edging in to maintain cool air
advection today, suppressing high temperatures despite strong
sunshine. Pressure gradient relaxes during the afternoon, with
lighter northwest winds than past couple of days, 10-12 mph gusting
to 20 mph. Will thus be a little more pleasant given highs edging up
slightly to mostly mid 50s north, with potentially some low 60s in
the southern counties.

Initially clear skies with a cool airmass in place will allow for
decoupling early tonight and mins will fall to the mid 30s across
the north towards midnight, with temps bottoming out within a degree
or two of freezing across the northern tier by morning to mostly mid
30s across the south. Concurrently, a low pressure area will be
migrating east across the Plains, which will back the low level flow
and gradually increase moisture advection into the southwest
Appalachians after midnight. Precipitation will gradually spread
eastward as the isentropic lift strengthens and the airmass
saturates, but it appears that appreciable chances for rain or a mix
of rain/snow will hold off until after sunrise Saturday morning.


As of 350 AM Friday...

Still dealing with a complex weather pattern for the weekend, with a
wintry mix likely across the northern half of central NC Saturday
night into the very early Sunday morning.

A s/w in the nw flow aloft will dive sewd from the Upper Midwest
into the lower OH Valley Saturday. This system will encounter
increasing confluence aloft, so it should weaken with time. The
system will induce isentropic upglide across central NC Saturday as
a sfc warm front should lie to our south and a weak ridge of high
pressure lies overhead. There will likely be radar return early
Saturday morning when sfc temperatures are close to or just above
freezing in the Triad. However, most of the precipitation aloft will
likely evaporate as the sub cloud layer will remain quite dry
through mid morning. Thus, any snowflakes that reach the sfc will
melt on impact. The bulk of the precip Saturday will be in the form
of rain, overspreading central NC from the nw. With precipitation
onset expected to occur during the morning into the mid day hours,
temperatures will struggle to warm during the day. Thus have lowered
max temps Saturday, especially across the east and south from the
previous forecast.

The wintry mix will likely be first encountered across the far
northern counties in vicinity of the Virginia border late Saturday-
Saturday evening as precip rates increase. GFS sounding data suggest
a fairly deep isothermal layer extending from 950-700mb. This
suggest mostly snow during times of high precip rates, and rain/snow
during times of lighter rates. Currently appears the highest precip
rates will occur late Saturday evening into the first half of the
overnight. This places the greatest risk for accumulating snow
across the Virginia border counties and possibly the far northern
coastal plain. Expect impacts to be minimal or low as a good deal of
rain prior to wintry onset will warm/insulate the ground and sfc
temperatures expected to remain above freezing during most of the
episode. At worse, expect snow accumulation to be limited to grassy
areas and on the tops of decks, roofs, cars, etc., and no worse than
a slight slush build up on a few roads. Precip rates expected to
wane during the last half of the overnight through early Sunday
morning, and the atmosphere projected to dry out in the favored
dendrite growth area. This should lead to a mixture of
rain/sleet/snow, with a brief instance of freezing rain probable as
sfc temps get to or drop briefly below freezing at daybreak. Any
icing that occurs will be very spotty and light with no travel
problems expected. Do not plan to issue an advisory with this
package, though if the models remain consistent, an advisory will
likely be needed for portions of the northern Piedmont and northern
coastal plain.

Precipitation will end north to south Sunday morning as the wave
exits our region. Drier air will infiltrate the atmosphere, leading
to spotty light precip in the morning, and partial clearing Sunday


As of 325 AM Friday...

An improving weather pattern still appears in store for central NC
early next week with temperatures closer to seasonal norms by mid
week and beyond (finally!).

A deep upper level low off of the Eastern U.S. seaboard will serve
as a block to a digging l/w trough over the western U.S. In between
will be an amplifying upper ridge, initially extending from the Deep
South into the Great Lakes. This upper ridge progged to extend
overhead by mid week. Subsidence associated with this feature should
inhibit precip generation.

Temperatures to start the work week will remain well below normal,
generally in the upper 40s to lower 50s. Frosty mornings still
expected Monday and Tuesday with min temps in the upper 20s to lower
30s.  The moderating air mass will lead to afternoon temperatures by
Wednesday closer to normal, solidly in the 60s to near 70 south, and
mainly in the 70-75 degree range by Thursday.


As of 700 AM Friday...

24 Hour TAF Period: VFR conditions will continue through the 24 hour
TAF period as surface high pressure moves across the southeast U.S.
Skies will be mostly clear/mostly sunny skies today, with mid/high
clouds beginning to stream after 00Z from the northwest ahead of a
mid level short wave. This wave will increase moisture advection into
the west, with ceilings lowering progressively, although remaining
VFR, through 12Z Saturday.

Outlook:  The approaching storm system will result in a trend to
MVFR Saturday morning at INT/GSO, and Saturday afternoon at the
eastern sites. Meanwhile, precipitation will be spreading in from
the NW, with ceilings at INT/GSO trending to IFR Saturday afternoon.
The precipitation is expected to start off as a wintry mix in the
morning, changing to all or mostly rain by mid day. The rain could
mix with snow and/or sleet at GSO/INT Saturday night. IFR conditions
will be widespread Saturday night, with a trend to MVFR Sunday, and
further to VFR by Monday. VFR is expected for the remainder of the
mid week.





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