Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Raleigh NC
1250 PM EST Thursday Nov 27 2014
Synopsis...an upper level disturbance and associated low pressure
system will track east across the region through this afternoon.
High pressure will build into the region from the west late today
through tonight... then settle over the Carolinas on Friday and
Saturday... before pushing east off the coast Sunday.
Near term /through tonight/...
as of 1105 am Thursday...
Forecast required little change this morning. The leading shortwave
trough evident on water vapor imagery is now wobbling across north
central NC... generating numerous showers across the northern County Warning Area. Fast
on its heels is the second weaker wave generating snow showers on
the west slopes of the southern Appalachians... however much of this
appears to be drying up as the precipitation heads just east of the
mountains where the low level flow has a notable downslope
component. Nevertheless... both on the tail end of the leading wave
and with the second wave as it shifts east across the state... will
need to monitor for a few pockets of mixed rain and wet snow over
the extreme north-northwest County Warning Area. In fact... krax dual pol products indicate the
melting level at around 2200 feet... a bit lower than rap forecast
soundings suggest... so we could definitely see some wet flakes
where precipitation rates are higher. Temperatures range from the upper 30s to
lower 40s north to the upper 40s/near 50 south... and the northern
half is unlikely to see much more than a one to perhaps two category
climb for the rest of the day... and even then only if we can get
enough sunshine. These chances don't look too good given the
persistent moisture in the low levels noted on upper air analyses.
Will adjust northern highs down just a tad... to range from middle 40s
north (and again this may be optimistic) under mostly cloudy skies
to middle 50s south (which could be pessimistic given the fair skies
Tonight...skies will clear off fairly quickly west-to east late this
afternoon early evening as drier air overspreads the region. Cold
air advection will keep the near surface atmosphere well mixed until
just prior to daybreak. The cold air advection will send temperatures into the middle-upper
20s by early Friday.
Short term /Friday and Friday night/...
as of 305 am Thursday...
Friday...chilly high pressure will build into and settles overhead.
Low level thicknesses average 45m below normal...suggestive of maximum
temperatures 10-14 degrees below normal. This yields maximum temperatures in the low-
middle 40s. Skies will remain mostly clear aside from a few passing
cirrus in the afternoon. -Wss
Friday night: surface high pressure will settle over the region on
Friday night. Meanwhile... middle level flow will remain out of the west-northwest
with the potential for a few weak disturbances to move across the
middle Atlantic region. However... we will remain dry with only an
increase in some high clouds on Saturday morning. Still think the
clouds will be thin enough not to have a significant impact on low
temperatures. Given this... think we should see temperatures plummet into the 20s
across the entire area... with lows in the middle to upper 20s (maybe
even a few lower 20s in the usual rural cold spots depending on the
cloud cover). -Bsd
Long term /Saturday through Wednesday/...
as of 310 am Thursday...
High pressure will shift to the south and east of central NC this
weekend allowing for a warming trend. High temperatures are expected to be
in the lower to middle 50s for Saturday and lower to middle 60s for
Sunday. Middle level ridging is forecast to build northward during this
time frame and nudge into our area. However... as a northern steam
trough is forecast to track near the U.S./Canadian (east coast that
is) border during this time frame a trailing cold front will drop
southward and into central NC early next week. Model guidance has
trended towards the surface front moving into the area on Monday
night into early Tuesday morning... pushed by a 1035-1040 mb surface
high to the north/northwest. This should allow for one last warm day
on Monday... with highs in the 60s to lower 70s south.
Given the last several days medium range models have trended wetter
with the aforementioned front and the potential for at least some
hybrid cold air damming for Monday night into Tuesday. However...
wpc still prefers to keep probability of precipitation low with the arrival of the front
given the apparent spread in guidance/ensembles guidance over the past
few days and differences with the operational models vs their
ensembles. Thus... for now prefer to keep probability of precipitation at slight chance for
Monday night into Tuesday and trend temperatures downward. For Wednesday...
we may have some form of lingering cad across the area. However...
the parent surface high should be well off the northeast U.S. Coast
by this time. The next system will quickly approach from the
northwest. However... there is a large spread among the guidance in
timing of the front. Wpc holds the front to our north during this
time frame... going with a less amplified pattern. Given the lower
confidence will keep temperatures within a couple categories of normal at
worst... given the large possible spread in temperatures. This yields highs
on Tuesday in the upper 40s to 50s... with highs on Wednesday in the
50s to some 60s... and lows in the 30s and 40s.
Aviation /18z Thursday through Tuesday/...
as of 1250 PM Thursday...
Primary aviation concerns this cycle: low clouds at northern
terminals through 22z. A couple of upper level disturbances crossing
northern NC today have led to considerable sub-VFR clouds over the
northern terminals (int/gso/rdu/rwi) so far today. Ceilings have
improved a bit to MVFR as the showers have become a little more
patchy... and even in showers... visibilities have held mostly VFR. As the
disturbance shifts eastward late this afternoon... ceilings will
steadily improve to VFR... first at int (where the MVFR clouds have
already begun to break up to scattered) and gso by 21z... and across
to rdu/rwi by 00z-01z. Surface winds from the northwest sustained at
8-12 kts have had infrequent gusts to 15-20 kts... and these should
diminish by evening as the disturbance exits. Clearing this evening
will yield unlimited ceilings and visibilities at all sites after 03z. High
confidence in VFR conditions with mostly clear skies and light winds
through Friday... as high pressure builds in from the west.
Looking beyond 18z Friday... VFR conditions are expected to dominate
through Monday... although areas of MVFR or IFR fog are possible early
Sun morning and early Monday morning. High pressure will shift eastward
over the region through Sat... then move east and offshore Sat
night/sun. A cold front will move in from the north late Monday or Monday
night... bringing a good chance of MVFR/IFR conditions Monday night/Tuesday
and patchy light rain. -Gih