Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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FXUS62 KRAH 210147

Area Forecast Discussion
National Weather Service Raleigh NC
940 PM EDT Tue Mar 20 2018

Cold surface high pressure will build into the region from the north
tonight as a storm system develops over southern Appalachians. This
storm will move off the coast early Wednesday, then track off the
eastern seaboard Wednesday and Wednesday night. Cold high pressure
will build into NC Wednesday night through Friday.


As of 940 PM Tuesday...

We will issue a Winter Weather Advisory for portions of the
northern Piedmont, including the Triad for the overnight and
early Wednesday period.

We are evaluating the observational and model data and it appears
that one band of elevated convective precipitation
develop/overspread our region in the next few hours. This band is
expected to lift north of the region later tonight into VA. Much of
this precipitation is expected to be rain, with the exception of a
change to a mixture of snow/rain, then wet snow over the far
northern Piedmont as the heavier precipitation rates arrive. Some
convection may produce some hail, but small enough and accumulations
should not affect roads with this initial band. A chance to snow may
occur with this band from Roxboro and Henderson northward.

Then later tonight, another band or two of precipitation is expected
to rotate into the region. Partial thicknesses forecast between 00z
and 06z, are expected to fall from 1315m and 1570m in the
lower and mid levels - to near 1300m/1555m by 06z/tonight. This is
an average of the model data. The current observational data
suggests that the lower levels are already colder than forecast over
the northern zones where wet bulbs are already 34 at Roxboro and
Henderson, forecast was for 39-40.

After 06z, all levels are forecast to cool into
the "mostly snow" on the Partial Thickness Universal Nomogram
throughout the NW and N Piedmont where thicknesses are forecast to
dive into the 1290s/1530s. This suggests that the late night
and early morning hours will have wet snow as far south and east as
much of the Piedmont including the NW Triangle area. Accumulations
will be dependent on snowfall rates late tonight, as wet bulb
temperatures should be able to fall to 31-32 with the snowfall
over the Piedmont (most all models show this). Therefore, due to the
snowfall occurring at night through mid-morning, a bit more in the
way of accumulation is certainly possible (1-2 inches). If the temps
do fall to 31 or 32, then some slick spots will certainly be
possible. We will have the advisory as noted below, running from 1
to 11 am. QFP of only 0.1 to 0.25 expected, with around or less than
9 or 8:1 snow/liquid ratios suggests very wet and slushy

To the south and east of the Advisory area, partials suggest a
mix with snow and rain later tonight, with a change to snow showers
on Wednesday morning. Essentially only a small, slushy snow
accumulation is expected on the grassy surfaces with temperatures in
the mid 30s from Albemarle to Raleigh and Halifax.

For FAY to GSB, only rain showers are expected at this time.


As of 410 PM Tuesday...

As an upper level low pressure system crosses the northern portions
of the state, chances for a precipitation mix or changeover to snow
will increase for areas mainly north of US-64. Looking at forecast
soundings, there is plenty of moisture in the dendritic growth zone
accompanied by sufficient lift to produce some snow as thickness
values drop across the Triad, northern Piedmont, and possibly the
northern coastal plain.

Timing: Expect precipitation to change over to snow in the Triad
beginning at 6z and all areas north of US-64 by 9z. Expect most
areas north of US-64 and east of I-95 to change over to snow by 12.

Accumulation: Much of the best lift with this system is expected to
come through prior to the changeover to snow, but there is a very
small window between 6z and 9z where there is some model indication
of potentially higher snowfall rates possible in the Triad that
maybe able to bring about a little more accumulation but we are
still expecting less than an inch (confident) and probably less than
half of an inch of accumulation (moderate confidence) in this area.
If we are going to bust the forecast on the high side, this is where
it will be. All points east of the Triad that are north of I-85
should see less than a half inch of accumulation. Areas south and
east of I-85 should see very little to no accumulation.

Impacts: At this time we do not expect many impacts across the CWA.
If there are to be travel impacts they would be in the area north of
I-85 and more likely north of I-85 in the Triad area. Otherwise, a
combination of warm ground temperatures, lackluster snowfall rates,
and limited timing should combine for no travel impacts across much
of the area south of the I-85 corridor.

Temperatures will top out in the low 40s across the north with low
50s in the south.


As of 230 PM Tuesday...

The upper trof will shift offshore, with ensuing deep northwest flow
settling in through the weekend to maintain unseasonably cool
temperatures. Initial surface high pressure building in on Thursday
and Friday will be dry, with increased sunshine allowing mins in the
morning to fall below freezing across the north to mid 30s south on
both Friday and Saturday mornings.

A lot of divergence in model solutions this weekend, as a short wave
will be racing southeast in the mid level flow, with deep cooling of
the airmass with potential precipitation breaking out on Saturday
and continuing through Sunday. This wave could produce some mixed
precip across the northern tier where CAD airmass will be deeper,
but will let this ride for now given low confidence. The cool
airmass will remain entrenched over the area on Monday, with modest
warming and perhaps some sun Tuesday as upper ridging amplifies up
the east coast. High temperatures all the way through the extended
will be hard-pressed to climb out of the mid 50s over most of the
area, with some 60s across the southern tier dependent on the extent
of the CAD airmass. Low temperatures will be mostly in the mid to
upper 30s, with perhaps some lower 40s in the south and east.


As of 749 PM Tuesday...

Widespread LIFR to IFR CIGS will continue overnight through 15z/Wed.
Occasional IFR to LIFR VSBYS with rain and fog will occur through
15z. Rain will mix with or change to snow at KINT/KGSO/ and likely
KRDU between 06z and 12z. Occasional MVFR VSBYS with light snow and
fog will prevail between 12z and 18z at all but KFAY and KRWI Wed,
where a mix of light snow and rain showers are expected.

VFR conditions are expected to return Wednesday night through
Friday, with unfavorable aviation conditions possible on Saturday
and into Sunday as a warm front approaches the area from the SW.


Winter Weather Advisory from 1 AM to 11 AM EDT Wednesday for



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