FXUS62 KRAH 190845

Area Forecast Discussion
National Weather Service Raleigh NC
445 AM EDT Mon Mar 19 2018

Weak high pressure over the area this morning will migrate
offshore today. A warm front will retreat north into the area late
this afternoon and evening, and will briefly stall across the area
tonight, with a series of low pressure systems developing along
the front as it pushes slowly south and east in the vicinity of the
Carolinas through early Wednesday.


As of 430 AM Monday...

Surface high pressure will continue to migrate east and offshore
this morning. A warm front south of the area will lift north into
the area late this afternoon and evening, in advance of an area of
low pressure tracking east and into the Tn Valley.

Today: Initial surge of weak to modest low/mid-level warm moist air
advection, fueled by weak shortwave impulses embedded in the
westerly flow aloft, well downstream of the compact upper
low/vigorous shortwave trough moving east through the central Plains
and Mid MS Valley, will support patchy light rain across the area
today. There could potentially be a mid day/afternoon lull in precip
before the arrival of the stronger moisture advection and lift this
evening and overnight, owing to the approach of the aforementioned
shortwave trough and associated sfc low into the Tn Valley.

Forecast highs will be highly dependent on just how much precip we
see and if it will linger into the afternoon, especially across the
far northern Piedmont where sufficient precip falling into the
residual low-level dry air will help to lock in in-situ wedge
conditions. Highs ranging from lower to mid 50s north to lower 60s

Tonight: As the area of low pressure tracks east into the Tn
Valley/southern Appalachians, sfc warm front will lift north into
the area. Strong southerly/southwesterly warm moist air advection
rooted in a 40-50 kt LLJ overspreading the area from the SW, will
support widespread showers and possibly a few thunderstorms across
the southern zones as MUCAPE of 250 to 750 J/KG develops across the
the warm sector. While models currently show a lack of sfc based
instability across the area, the impressive deep layer shear
combined with enhanced low-level shear/helicity in the vicinity of
the secondary low pressure development, will need to be closely
watched for damaging wind and isolated tornado potential across the
far southern/southeastern zones overnight.

Overnight lows ranging from lower 40s north to mid 50s south.


As of 350 AM Monday...

Tuesday: A brief lull in the precip is generally expected on Tuesday
as the first in a series of disturbances shifts to the east of the
area. However, skies will continue to remain mostly cloudy to
overcast will low level moisture lingering. Highs on Tuesday are
expected to range from the lower 40s north to the lower to mid 60s

Very tricky forecast for Tuesday night through Wednesday as an upper
trough digs down into the area spurring surface cyclogenesis along
the NC/SC border. While this surface low will quickly move up the
Carolina coast, the upper low will linger behind and bring cold air
down from the north. This in turn with some leftover moisture could
lead to some deformation banding behind the system which could
result in a brief period of rain mixed with snow, or if
precipitation rates are high enough, a quick burst of snow resulting
in some spotty accumulations, particularly across the northern
counties. There are several mitigating factors however, that could
result in very little to no snow. These include warming temperatures
with the heating of the day. Thickness values in the wintry mix to
indeterminate areas of the nomogram for much of the event except the
very tail end where thicknesses drop to levels suggestive of all
snow for a couple of hours when moisture may or may not still be
available. Finally, lift drops off considerably after the surface
low pulls off to the northeast, making it very hard to produce the
rates needed for accumulating snow.

What we are expecting at this time is a period of a few hours of
rain/snow mix during the Wednesday morning hours resulting in very
little to no impacts. There could be some localized heavier snow
showers if precipitation rates are high enough. These should be brief
but could result in some minimal accumulations of less than a half
of an inch if they persist long enough. By the time a changeover to
all snow would occur Wednesday night, there will likely not be
enough lift or moisture left to produce more than a period of
flurries. Bottom line is a late March nuisance event will minimal


As of 350 AM Monday...

With the storm system moving up the New England coast and away from
the area, any remaining flurries will come to an end Thursday
morning and clearing will ensue as well as a return to dry weather.
Temperatures will struggle to get out of the 40s across the north
with mid 50s in the south. Dry weather continues on Friday as high
pressure remains in control and a warming trend begins as low to
upper 50s are expected north to south.

The next low pressure system will move out of the Great Plains and
into the Ohio Valley Saturday. Models then split on whether or not
the low takes a southward dive into NC but standing in its way will
be a moderately strong Bermuda high which may help to keep it north
of the area, similar to the EC solution. This will keep us in the
warm sector for the duration of the weekend with highs in the low
50s to low 60s Saturday and upper 50s to upper 60s Sunday.
Precipitation will be determined by the track of the low with the GFS
offering a much wetter solution than the EC. This low and any
associated frontal zone will try to clear the area by Monday morning
but will be quickly followed by several more shortwave disturbances
moving across the deep south and arriving by midweek next week.


As of 225 AM Monday...

24-Hour TAF Period: Ceilings are expected to lower overnight, though
should remain VFR at most TAF sites through 12z Monday. Shortly
thereafter, MVFR cigs are expected to spread into the area at KINT
and KGSO between 12 to 15z, and at KFAY between 15 to 18z as an area
of light rain spreads east through the area. It's possible that
these sub-VFR ceilings could hold off at KRDU and KRWI until the
late afternoon and evening when a much stronger surge of low-level
moisture advection, accompanying a warm front lifting north into the
area will allow ceilings to further lower to IFR/LIFR. Additionally,
widespread showers and possibly a few thunderstorms will spread east
across the area after 03z Tuesday, reducing visbys to IFR to MVFR
and potentially resulting in some wind gusts of 30 to 40kts.

Looking ahead:
The passage across the Carolinas of a series of low
pressure systems will result in sub-VFR conditions and periods of
rain, probably mixed with or briefly changing to snow at nrn TAF
sites late Tue night or Wed, through Wednesday. Drying and a return
to VFR conditions from west to east will occur as the trailing low
lifts away from the middle Atlantic coast Wednesday night. -MWS





Back to Main Forecast Discussion Page