Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Raleigh NC
333 PM EST Sat Jan 31 2015
Synopsis...Arctic high pressure over the Carolinas today will move
offshore through early Sunday...ahead of a low pressure system that
will move east across the Tennessee Valley and Middle Atlantic States
late Sunday through early Monday.
Near term /through tonight/...
as of 230 PM Saturday...
High pressure encompassing the southeast US will shift offshore
tonight as the middle-level ridge over the deep south flattens in
response to a shortwave digging through the Central Plains. The
upper level flow will become more southwesterly and allow the
expansive cirrus over the central US to spread east. Models are in
pretty good agreement that the entire area will be mostly cloudy
after midnight...though the cloud deck should be thicker over the
west. Thus...temperatures will likely drop rapidly after
sunset...especially in the east...before the cooling levels off with
the increasing clouds. Guidance seems too warm in the east...so
will take a few degrees off mav/met values...with some middle 20s east
to around 30 in the west.
Short term /Sunday through Monday/...
as of 330 PM Saturday...
With high pressure offshore Sunday morning...southerly flow will
allow additional moderation as thicknesses climb back toward 1320m
which is 5-10 above normal. Middle and high clouds will continue to
blanket the region...but expect the southerly wind to overcome some
of the loss of heating...so will keep temperatures in line with statistical
guidance in the middle to upper 50s and maybe touching 60 in the south.
Meanwhile...a surface low will develop in response to the upper
trough digging through the Central Plains and middle-Mississippi
Valley...moving across the central Appalachians Sunday night.
Models are in pretty good agreement on precipitation arriving in the
western Piedmont after 00z...with the bulk of the precipitation after
midnight as a 50kt low level jet shifts across the area. Precipitable water values
stay below 1... with additional support from upper divergence
in the left exit region of a speed maximum strengthening over the deep
south...would lean a little more toward the NAM/GFS quantitative precipitation forecast totals which
are higher and give between a quarter to half inch in total. Weak
lapse rates and poor diurnal timing will limit instability...though
the GFS produces 200-300 j/kg MLCAPE at kfay as it steepens middle-
level lapse rates to around 6 c/km. This seems overdone given
upstream soundings...but if there is a chance of thunder it should
be in the southern coastal plain Monday morning.
Strong 150+ meter height falls and a cold front will sweep across
the area by early Monday afternoon. Forecast soundings show 25-35kt
in the deepening Post-frontal mixed later Monday afternoon...but
models can often overdo these gradient winds. Will cap wind gusts
at 30kt for now.
Temperatures may cool into the upper 40s across the north Sunday evening
but will likely rise back into the 50s Sunday night ahead of the
front. Guidance suggests the coastal plain may reach the 60s by
early Monday...but most areas will see temperatures falling through the day
Long term /Monday night through Saturday/...
as of 332 PM Saturday...
Deepening area of low pressure will continue moving northeastward
away from the middle Atlantic coast... and winds will diminish
overnight as high pressure over the Ohio Valley moves eastward over
the area on Monday night into Tuesday. This high will move off the
coast on Wednesday... providing comparatively warmer temperatures in
southwesterly flow. Meanwhile... upper level low over the southwest
will gradually begin to open into a wave as it moves eastward to the
western Gulf of Mexico... where surface low pressure forms over the
northern Gulf. Progression of this system still remains quite
uncertain... with considerable variation amongst model solutions in
handling the primary southern stream energy... and the trailing
northern stream wave. European model (ecmwf) remains suppressed with the southern
stream upper level wave and much weaker with the second northern
stream one... and pushes the low eastward out to sea with no local
impacts... while the GFS brings the low along the southeast coast as
stronger northern stream energy dives into the area... resulting in
widespread rain Thursday. With no strong consensus to drive a
deviation from the current forecast... will continue with slight to
chance probability of precipitation on Thursday... with dry conditions Friday and Saturday.
Temperatures will remain below normal Thursday through Sat.
Aviation /18z Saturday through Thursday/...
as of 100 PM Saturday...
Skies are clear across central NC this afternoon with high pressure
across the southeast US. Winds will remain light and mostly
variable or westerly this afternoon. Increasing middle/high clouds over
the central US will spread east tonight but ceilings will remain
above 10k feet. High pressure will then shift southeast by early
Sunday and winds will become southwesterly during the
day...increasing to 8-12kt during the afternoon with a few gusts
toward 20kt possible at mainly kgso and kint.
Outlook...ceilings will lower to MVFR by late Sunday afternoon and
evening ahead of a low pressure system moving through the
Tennessee/Ohio Valley. Rain will overspread the area Sunday
night...likely causing ceilings to drop to IFR levels...as a strong
low-level jet surges across the region. A cold front will then swing
across central NC on Monday...with gusty northwest winds in its wake
Monday afternoon. VFR conditions are then expected through midweek