Turner Downs Forecast Discussion

FXUS62 KRAH 260548

National Weather Service Raleigh NC
1245 AM EST Wed Feb 26 2020

A cold front will move east across NC late today and this evening.
This will be followed by cold high pressure late week.


As of 1040 PM Tuesday...

A frontal boundary separating dew points in the low 50s to the NW
and dew points in the low 60s to the SE is currently draped across
central NC, and dew point depressions are only a few degrees or less
across the entire region. Areas of lowering visibilities and even
dense fog (with visibilities as low as a quarter mile) are beginning
to develop, especially over the NE Piedmont. This is expected to
continue through the overnight hours, with a threat of dense fog
existing over the entire region, and a Dense Fog Advisory may need
to be issued. Across the Triad, cloud ceilings around 2-3 kft still
currently exist, and those would need to scour out for fog to
develop there.

Forcing for any precipitation will remain very weak through the
overnight hours, with the only slight chance for any spotty drizzle
or light showers across far southeastern parts of the area where
some low level moisture transport exists. All the cloud cover will
prevent temperatures from getting much lower than they currently
are, with forecast lows generally ranging from the upper 40s in the
NW to the upper 50s in the SE.


As of 330 PM Tuesday...

Wednesday and Wednesday night:

The amplifying upper trough over the Mid MS valley Wednesday morning
will take on a more negative tilt as it ejects northeastward across
the Appalachians late Wednesday afternoon through Wednesday night. A
secondary surface low will develop along the eastern slopes of the
southern Appalachians and will steadily deepen over the northern Mid-
Atlantic States, along the eastward moving cold front that will exit
off the Mid-Atlantic coast shortly after midnight.

The weak/occluded surface boundary bisecting the area Wednesday
morning will likely dissipate in advance of the secondary frontal
zone. The morning stratus/fog will gradually lift through late
morning, but skies are expected to remain mostly cloudy, which
should temper heating. Highs ranging from mid 60s NW to lower/mid
70s SE.

After a continued lull, the arrival of the upper trough and
attendant cold front into the region during the late afternoon and
evening will lead to high chance/likely PoPs, as a narrow band of
convection crosses the area between 23 to 05z. Very weak
destablization of 200-400 J/Kg MLCAPE could result in some isolated
embedded thunder, mainly across the northern/northwestward Piedmont
in proximity to the better upper forcing and sfc low spin up. Given
the strong shear parameters, if more in the way of  organized
convection forms or moves into the area, there is a conditional
threat for an isolated strong wind gust or two, mainly across the
northern zones.

Post-frontal CAA and clearing will drive overnight lows in the lower
30s NW to lower 40s SE.


As of 215 PM Tuesday...

Longwave troughing will prevail across most of the Atlantic states
Thursday into the weekend. Initially, dry northwesterly flow will
prevail across the area on Thursday resulting in sunny skies and
below normal temperatures but this will change quickly as the upper
low deepens and moves into Ontario and Quebec Friday into the
weekend. A series of fast moving shortwaves will move through the
base of the trough and into NC Friday and Saturday. Timing the
specifics of these waves is a bit of a challenge but it looks like
the initial trough will move through Friday afternoon with another
wave mid-day Saturday. While the BL should be somewhat moisture
starved, today`s 12Z guidance is showing a narrow plume of elevated
BL moisture ahead of these troughs - perhaps enough to squeeze out a
few hundredths of an inch of liquid Friday and again on Saturday.
The deterministic runs of the EC and GFS aren`t terribly enthused
about precip chances with these waves but a handful of the 12Z GEFS
members are showing precip and nearly a third of the EC ENS members
are also indicating precip. NBM PoPs are in the 15-20 range which is
a slight nudge upward from previous forecasts and this seems
reasonable at this lead time. Any precip that falls should be in the
form of liquid although wet bulb temps are going to hover right
around freezing. Not going to forecast any accums given the warm
ground temperatures at this stage of the game.

BL dries out considerably on Sunday which will result in another day
of mostly sunny skies and below normal temperatures. Attention then
turns to an upstream upper low over the mid Mississippi Valley, with
a surface warm front extending southeastward into GA and SC. The
warm front should move through NC late Monday but the strongest
forcing remains well to the west of the forecast area. Best precip
chances will be near the Triad with PoPs tapering off toward the
Coastal Plain into Tuesday. The trailing cold front looks to stall
west of the mountains and not sweep through central NC until
Wednesday or later resulting in an unsettled (but warmer) pattern
for at least the first half of next week.


As of 1245 AM Wednesday...

Very moist low levels characterized by surface dewpoints in the 50s
to around 60 degrees will then favor the development of LIFR
conditions in fog and stratus overnight. That moist layer and
associated sub-VFR conditions will lift and scatter to VFR by late
morning or midday Wed. A band of low-topped showers and possibly
isolated thunderstorms will develop along a cold front from PA swd
across VA and nrn NC Wed evening. Showers and isolated storms with
wind gusts up to 35-40 kts and pea hail may affect the KINT and KGSO
areas between 23z/26 and 02z/27.

Outlook: Cntl NC will be on the srn-most extent of the
aforementioned band of convection, so it is unclear if it will
affect KRDU, KRWI, and KFAY as the front crosses cntl NC through 06Z
Thu. Otherwise, post-frontal west winds may gust at times up to 25-
35 KT Wed. A return to VFR conditions is expected early Thursday
likely lingering through Sunday in the NW flow aloft.





NWS RAH Office Area Forecast Discussion