Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Raleigh NC
105 PM EST sun Feb 1 2015

Synopsis... a strong cold front will approachin from the west
tonight and swing across central NC on Monday. Highs pressure will
then build over the region through midweek.

&&

Near term /through tonight/...
as of 1025 am Sunday...

Middle/upper level ridging over the deep south is flattening this
morning as a vigorous shortwave digs toward the middle-Mississippi
Valley region. Middle-level moisture continues to stream across the
area from the southwest...with even a few enhanced radar returns
above 5-6k feet. The sub-cloud layer remains very dry with 15 to 25
degree dewpoint depressions at 850mb on most 12z radiosonde observations around the
southeast US. As the atmosphere slowly moistens from top down
today... expect we will continue to see some elevated radar echoes
associated with weak upper disturbances on the the eastern fringe of
the strengthening warm advection west of the Appalachians. An
isolated sprinkle is possible...but the afternoon and much of the
evening should remain dry cwa-wide. Statistical guidance tops out
in the 52-59 range...which should be attainable for the most part
given that visible imagery shows some breaks of thinner spots in the
otherwise mostly cloudy to overcast altostratus. -Bls



Tonight: aforementioned northern stream trough will advance eastward
into the Ohio/Tennessee valleys overnight. Associated deepening surface
low...located along the leading edge of intense 180-200 500 mb
falls...will track eastward along the Mason-Dixon line ... into the
Middle Atlantic States Monday morning. Strong deep moisture transport
in advance of the front...within the strengthening 60-65kt southerly
low level jet...will support a 4-6 hour window of categorical probability of precipitation as an
eastward band of showers move east across the area Sunday night-
Monday morning between 06z in the west to 15z in the east. Quantitative precipitation forecast
amounts look to be in the 0.25-0.50" range... limited by the
intrusion of a middle-level dry air racing in from the west. Models
continue to indicate modest middle-level lapse rates ~6.5 c/km with
MUCAPE of only 100-150 j/kg across southern/eastern portions of the
forecast area...and thus very low probability for some elevated
thunder. Expect a non-diurnal temperature trend...with lows occurring
before midnight before the arrival of the rain and strong warm air advection. Lows
Sunday night in the middle 40s northeast to lower 50s south...rising
into the 50s through the predawn hours.

&&

Short term /Monday through Monday night/...
as of 250 am Sunday...

Deep moisture plume embedded within the southerly low level jet will shift
east and out of the area at or shortly after 15z Monday...which
should mark the back edge of the widespread/categorical probability of precipitation...with
trailing surface cold front and lingering small chance of showers
lagging a few hours afterward.

Strong Post-frontal dry cold air advection regime will result in
west to east clearing and breezy/windy conditions during the
afternoon as bufr soundings indicate 35-40kts at the top of the
mixed layer and 25-30kts just off the surface. Highs Monday should
occur at or before solar noon...ranging from near 50 northwest to upper
50s/near 60 southeast. Temperatures will then fall into the 40s through
the afternoon as Continental polar high pressure builds into the
area Monday night and into the day on Tuesday. Winds look to
decouple overnight...especially in the west...which should help for
a wide range in temperatures. Lows Tuesday morning in the lower to
middle 20s...with upper teens possible in the typically colder
locations across the western Piedmont.

&&

Long term /Tuesday through Saturday/...
as of 250 am Sunday...

Tuesday and Tuesday night...the guidance has been consistent moving
a middle-level shortwave across central North Carolina during the
day...with the left exit region of a 140kt 300mb jet moving through
the area Tuesday morning with a 500mb jet just to our south. Ahead
of the shortwave...and in vicinity of the jets aloft both the NAM
and the GFS provide for some mostly middle-level cloudiness Tuesday
morning with column drying to follow for the afternoon. Central
North Carolina is in an 850mb Theta-E trough with k indices
negative...and the moisture aloft does not currently get below about
-20c and the air mass below that is quite dry. A period of possibly
broken clouds is only anticipated early Tuesday before late morning
and afternoon increasing sun. Subsidence Tuesday night under surface
high pressure moving southeast...with possibly a few clouds
overnight associated with the right entrance region of a 70kt 500mb
jet just to our north and just modest middle-level moisture. Highs
Tuesday and lows Tuesday night both about a category below
normal...45 to 50 for highs Tuesday and middle to upper 20s for lows
Tuesday night.

Wednesday through Saturday...primary concern continues to be focused
on the system for Thursday...as a cold front approaches Wednesday
and moves through much if not all of central North Carolina
Wednesday night with a surface low moving over some part of the
southeast Thursday. The GFS has trended toward the previous few
versions of the European model (ecmwf)...with a deeper middle-level trough moving
southeast across Canada and into the Great Lakes which suppresses
the surface and middle-level low tracks such that central North
Carolina would receive little if any precipitation. It is still
close...though...and for what they are Worth the Canadian and the
dgex are still farther north and wet for central North Carolina at
least for Thursday. The higher quantitative precipitation forecast models have partial thicknesses
that would be liquid on Thursday for the area...while the drier
guidance is colder. The GFS has certainly trended the colder
way...with highs by Friday on the GFS mex MOS guidance barely above
freezing in The Triad.

Complicating things...the latest European model (ecmwf) is slower with the timing of
the middle-level trough moving out of Canada...with a correspondingly
slower cold frontal passage and a now slightly farther north surface
low track. Like the Canadian and the dgex...thicknesses are warm for
much of Thursday before colder air settles in. While the probability
of precipitation may...overall...have leaned slightly lower...with
the operational GFS trending progressively drier in each of its last
four runs...GFS ensembles on Thursday still have some members with a
more moist air mass and precipitation. The prudent course for
Thursday is to maintain some chances with slight chances west of
about U.S. 1 and chances east...keeping precipitation liquid and
confined to Thursday until overall features resolve themselves more
clearly. The warmest day should be Wednesday ahead of the cold front
with thicknesses and the currently expected degree of any middle- and
high-level moisture allowing for highs in the 50s. There have been
huge swings in MOS guidance highs for Thursday...40s to 50s and back
again...and 40s to 50s to 30s in The Triad on previous versions of
the European model (ecmwf) MOS. By Friday some confidence that most highs should be
in the 30s moderating some for Saturday...but still below normal and
mostly in the 40s then...as 1000-850mb thicknesses rise 40m to 50m
versus values of around 12z Friday.

&&

Aviation /18z Sunday through Friday/...
as of 105 PM Sunday...

Conditions are currently VFR across central central NC with cloud
bases at or above 10k feet and a light south-southwesterly wind. The
pressure gradient will continue to strengthen this afternoon ahead of
a low pressure system moving into the Ohio Valley...and as surface
winds increase to 8-12kt...a few wind gusts to 15-20kt may
occur...mainly from krdu west to kint/kgso.

Precipitation associated with the aforementioned low pressure system looks
to be ahead of the forecast by an hour or two...so will introduce
MVFR ceilings and a chance of rain in the tafs just a little
earlier. Expect ceilings to be near MVFR (3-4k ft) by 03z at
kgso/kint and more toward 06z from krdu east. The main rain band
with this system will cross the area between 06z and 15z...with
predominately IFR ceilings possible during this period. Southerly
winds will increase to 15-20kt and gust to near 30kt overnight as
the precipitation and cold front approach....then shift to west-
northwesterly and gust into the 30-35kt range behind the front on
Monday.

Outlook... northwest breeze at 10-15 knots will persist Monday night as high
pressure builds toward the region from the west. VFR conditions will
rule through middle-week as dry high pressure builds over the Carolinas.

&&

Rah watches/warnings/advisories...
none.

&&

$$
Synopsis...cbl
near term...cbl/bls
short term...cbl
long term...djf
aviation...cbl/bls

National Weather Service Glossary of Abbreviations