RAH just wrote a book!
.LONG TERM /SATURDAY THROUGH THURSDAY/...
As of 330 PM Thursday...
...A significant winter storm is expected Sat night through Mon;
a winter storm watch is being issued for the NW Piedmont...
Overview for Sat-Mon: A significant wintertime storm for NC is
growing more likely with every model run. Differences among the
models regarding timing, track, precip amounts, and the vertical
thermal structure and advection patterns persist, which makes it
challenging to pinpoint the details, but the models are coming into
better agreement now. It's very likely that parts of central NC,
primarily N and W of the Triangle, will see several inches of snow,
at least, along with some icing. Lesser (but still impactful)
accumulations of ice and snow are expected at some point of the
event over all but the far SE.
-Synopsis: A potent mid-upper trough/low will cross the S Plains,
Gulf States, and Southeast during this period, as a surface low
tracks along the Gulf Coast/S GA/FL Panhandle, evolving into a
Miller B configuration as this energy transfers to a deepening low
just off GA/SC on Sun. This low should then intensify as it tracks
NE just off the NC coast through Mon. A second, strong polar stream
wave will dive through the Midwest and lower Miss Valley Sat night
through Mon, shearing across the Southeast states and prolonging the
lift and moisture over central NC as yet another deepening low
pressure center holds just off NC.
-Timing: Model agreement has improved since yesterday, with the
ECMWF/Canadian still the preferred solution, while the NAM is quite
close and the GFS still a fast solution. Leaning toward the
ECMWF/Canadian/NAM, have brought chance pops gradually into the SW
CWA on Sat, expanding slowly to the N and NE late day and overnight.
The heaviest precip is expected to fall from early Sun morning to
mid afternoon Sun, in conjunction with the most intense upper
divergence/mid level DPVA and most vigorous and deepest moist
upglide. After this time, drying aloft (in the -12C to -18C layer)
and waning forcing for ascent should mean a trend to lower precip
rates, although high pops will persist.
-Precip types: Increased confidence in a cold column has prompted
expansion of the areas likely to be impacted by wintry weather. The
far SE is still likely to see mostly rain through Sun, albeit with a
little wintry precip early in the event and again on Mon. Roughly
along the Highway 1 corridor including the Triangle, a longer
duration of a wintry mix is anticipated, with a light glazing
possible along with some snow and/or sleet accumulation ranging from
one half inch to a few inches (this is the area that could see a
tight gradient of lighter to heavier accumulations over a short
distance). We could see a trend to a cold rain sometime on Sun in
this zone, if low level warm advection on the N side of the coastal
low becomes more prominent. For the NW Piedmont including the Triad,
snow and sleet will be the predominant precip type for most of the
event, with accumulations from several inches to just under a foot
not out of the question. Some low level drying is expected by Mon,
but continued lift and moisture in the -12C to -18C depth should
result in a secondary chance of some light snow or a wintry mix
associated with the polar wave and some additional light
-Temps: Expect highs in the upper 30s to mid 40s Sat and Mon, with
around 30 to the mid 40s for highs on Sun. Lows will be mainly in
the upper 20s to mid 30s. The diurnal range in most areas will be
-What to watch out for/what could change: First, the inland
penetration of warm air on the N side of coastal lows is always
problematic. This could cause a significant warm nose to develop
aloft, shifting snow over to more sleet or freezing rain in some
areas. Second, if surface temperatures don't warm up as advertised
on Sun, especially along and E of the Highway 1 corridor, wintry
precip could last longer, increasing accumulations of both snow and
ice. Third, there are indications in the models that a band of
heavier convective precip is possible over the Southeast, just
inland to just off the Southeast coast, and this could serve to
interrupt transport of high moisture into central NC; if this were
to occur, liquid equivalent precip amounts would be lower. Finally,
if we can achieve banded snow at any point in this event, we could
potentially see localized zones of very heavy snowfall over portions
of the Piedmont.