Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Raleigh NC
654 am EST sun Jan 25 2015
Synopsis... high pressure will extend northeastward from the Gulf
Coast into the Carolinas today. A strong upper level disturbance and
surface low pressure will cross the region Monday into Monday
evening. A cold front will drop through the area late Monday
night... then settle to our south through middle week... as high
pressure builds in from the west. Another low pressure area and cold
front will cross the region Thursday night into Friday.
Near term /through tonight/...
as of 255 am Sunday...
Satellite imagery shows our next system that will approach the area
trying to get organized over the Midwest this morning but this wont
approach until Monday morning...leaving US with settled weather for
Sunday. At this hour there are clear skies and light winds leading
to some fairly cool temperatures across the region. There are a
smattering of upper 20s starting to come into the observations at
the usually colder rural areas. This is to be expected with the
fairly good radiational cooling conditions. This pattern could start
to change however as forecast soundings show fairly good
environmental conditions for orographic cirrus and water vapor shows
increased moisture heading towards the Appalachians. That being
said..winds are fairly westerly and the trajectory of the moisture
could have cirrus...if it occurs...ending up primarily in Virginia as
opposed to NC. Regardless of orographic enhancement...high clouds
are expected to start to overspread the area later this evening.
Otherwise a fairly nice day ahead with highs in the low 50s northwest to
upper 50s southeast. Winds will be out of the west backing to southwesterly
by later in the day at 5-10 kts with some gusting to 15 kts possible
in The Triad late in the afternoon.
Short term /Monday through Monday night/...
as of 255 am Sunday...
A clipper system will move into the area with the surface low
tracking somewhere in the vicinity of NC. Models disagree slightly
with the southward extent of the track but in general the first wave
moves into NC Monday morning. Some upper level dynamics associated
with the left exit region of an upper jet will help provide some
lift above 500 mb but below that...the surface low will be just
starting to deepen. Therefore lack of strong dynamics and saturation
in the low levels will help keep precipitation amounts at Bay. In
general...the best chances for rain will be across the north.
Forecast soundings indicate that temperatures will be warm enough to
support a liquid rain event during the day on Monday. Rainfall
totals will generally be less than a tenth of an inch. Highs
temperatures will be tricky due to track of the low but expect upper
40s north to low to middle 50s south as a good starting point.
Later in the afternoon the low will eventually reach the coast and
really start to deepen and start to import a lot more moisture off
of the southwest Atlantic. By this time however...low will be far
enough east for US to have to worry about heavy rain threat here in
central NC. The attention at this time will then turn to a secondary
wave on the heels of the first wave diving down through the upper
level longwave trough. At this time a cooling and drying trend will
have initiated but there still could be enough saturation for some
precipiation. By 00z Tuesday wetbulb temperatures will start to
reach the freezing mark in The Triad...and down to Raleigh by 06z
Tuesday. If some lingering moisture is left...there could be enough
lift in the secondary wave to produce a quick rain or snow shower.
That being said...forecast soundings dry out rather quickly in the
snow growth zone...so the likelihood of this is not very high but it
is possible...especially further north and west. Otherwise expect
most precipitation to be light and all liquid.
Overnight Monday night into Tuesday...temperatures are expected to
drop to or below freezing in the northern half of the County Warning Area. With very
cold temperatures and rain ending earlier in the day...the question
of possible black ice is a logical one for early Tuesday morning. A
couple of things are stacking up against this at this point.
First...winds will remain near 10 knots after the precipitation
ends...which was already light amounts to begin with...so it is
entirely possible that roads could be dry before temperatures drop
below freezing. Even if they are not...very warm soil temperatures
and highs earlier in the day near 50 degrees would be hard to
support the development of any ice. Cant rule out a few icy patches
but overall this is not expected to be a problem. Lows Monday night
in the low 30s.
Long term /Tuesday through Saturday/...
as of 130 am Sunday...
Tuesday/Tuesday night: the polar front should have settled south of NC... as
the surface low bombs off the midatlantic coast. Sheared vorticity
will continue to stream southeast and east across the Carolinas into the
base of the mean polar trough. We maintain a decent amount of
moisture from 925 mb up through 600 mb (with some variability among
the models regarding shallow dry layers through this depth)... so
expect to retain a good amount of cloud cover through the day... but
any precipitation is apt to be quite patchy with only trace amounts
expected... as low level flow takes on a northwesterly downslope
component. Will leave probability of precipitation under 15%. Despite some low level
mixing... thicknesses drop to values nearly 20 M below normal...
suggesting highs from near 40 north to the upper 40s south when
factoring in cloud cover. The column dries out Tuesday night as high
pressure shifting over the Ohio Valley attempts to push over the
mountains. With a slackening mslp gradient and lowering wind speeds
under clearing skies... expect chilly lows well down into the 20s.
Wednesday-Thu: the middle level polar low off New England lifts northeast
with the mean trough finally pushing eastward off the East Coast
early Wednesday... leaving NC beneath weak middle level shortwave ridging.
This will generate mainly quiet weather and a gradual modification
of temperatures through middle week as surface high pressure builds east-southeast over
the Carolinas Wednesday before pushing southward to over Florida and off the southeast
coast Thursday. Another clipper shortwave trough moving through the
Dakotas and upper Midwest Wednesday night will cross the southern Great Lakes
and Ohio Valley region late Thursday... and a shot of weak middle level DPVA
across NC along with increasing upper level divergence (in the left
exit region of a strengthening upper jet across the middle/lower miss
valley into the midsouth) and accelerating cyclonic low level flow
into the forecast area will lead to increasing and thickening clouds
with lowering bases Thursday. Expect highs Wednesday mostly in the middle-upper
40s with plenty of sunshine. Lows in the middle-upper 20s Wednesday night and
highs Thursday in the upper 40s to lower 50s... just a couple of degrees
Thursday night-Sat: the aforementioned shortwave trough coming out of the
Ohio Valley swings through the eastern states while amplifying Thursday
night through Friday night. There is not much moisture return ahead of
the associated front with projected precipitable water values just a bit above
normal... so despite decent middle-level DPVA and upper divergence Thursday
night/Friday morning... expect any precipitation to be brief and light... and
will keep probability of precipitation low. Dry weather follows for the latter half of Friday
into Friday night... with initially blustery northwest winds Post-front
weakening Friday night. Pleasant weather Sat with brief flat or
shortwave ridging behind the exiting trough with high pressure again
nosing into NC from the Gulf states. Model solutions really start to
diverge here but we may see yet another clipper wave diving through
the upper Midwest and Great Lakes region Sat with some additional
energy emanating from the closed low off Baja California streaking eastward
into NC... meaning increasing clouds Sat. -Gih
Aviation /12z Sunday through Thursday/...
as of 655 am Sunday...
24 hour taf period: much of the taf period will be VFR as only high
clouds threaten the area for most of the day. There could be a brief
period of wind gusts at kint and kgso between 21z and 00z but
otherwise 5-10 kts out of the west or southwest. Conditions will
begin to deteriorate near the end of the taf period with the onset
of MVFR ceilings and visibilities in The Triad around 8z or so and
then down to IFR or even possibly LIFR just before 12z. Further
east... might only see MVFR conditions by the end of the period for
krdu and krwi and it is very possible that kfay remains VFR through
the entire taf period. Deteriorating conditions are in association
with an approaching clipper system that is expected to bring rain to
the area on Monday.
Long term: the next clipper system will brush northern locales on
Monday with precipiation expected to fall as rain. This may cause
some periods of sub-VFR conditions Monday into Tuesday morning. VFR
conditions will return for midweek with a weak system for Thursday
night possibly presenting some adverse conditions but confidence in
that is low at this time.