Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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FXUS62 KRAH 202007

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

A series of low pressure areas will move eastward along the surface
front through Wednesday. An upper level low will sweep through the
area late Wednesday through Wednesday night, finally allowing the
surface frontal system to move east and out of our region. Cool high
pressure will build into the area for Thursday and Friday.


As of 410 PM Tuesday...

Wedge front has made its way slowly southward this afternoon and
GOES-16 visible imagery giving a good interpretation of where it is
currently located, just crossing our southern tier of counties now.
Temperature observations are reflecting this as well as locations
such as Clinton have lost 13 degrees off of their earlier highs as
they have gone from 73 to 60 degrees. With the drop in temperature
comes a loss of instability as well and SPC mesoanalysis is showing
surface based and mixed layer CAPE values quickly vanishing in this
area and are expected to push further south over the next couple of
hours. Despite still being in the marginal risk for severe wx in
these southern counties, it is looking less and less likely that we
will be able to realize any thunderstorms there this
afternoon/evening. Shear values remain high near the front and
anything forming to the south could still track into the area as the
mean flow is from the southwest to northeast and there are still
good mid-level lapse rates to maintain anything elevated, but low
level lapse rates are diminishing substantially at this time. Upshot
is it will be very hard to get convection going at this time.

With that attention will turn to the west with the next wave of
precipitation that will form in response to low pressure developing
over eastern Tennessee. That is expected to start to blossom over
western portions of NC this afternoon and into our western Piedmont
around 00z. Precipitation will continue to overspread the forecast
area during the late evening and into the early morning hours but
will do so as rain as temperatures and thickness values do not drop
enough to support winter P-Types until after 6z Wednesday. Will
address that in the short term update.


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.
Expect snow to begin to change back over to rain after 15z.

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 225 PM Tuesday...

Residual cool airmass across the area will maintain predominantly
IFR conditions over the TAF sites through 00Z, with LIFR returning
along with widespread rain by 03Z. Visibility will remain mostly VFR
as winds remain 6-10 knots over night from the NW-NE. Expecting rain
to mix with, perhaps change over to snow in the Triad sites towards
09Z, concurrent with exit of upper short wave and arrival of deep
cooler air. Precip will be tapering off to more showery nature by
12Z, with ceiling improving slowly by late morning, perhaps
reaching MVFR by 18Z.

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.





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