Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Raleigh NC
145 PM EDT sun Mar 16 2014
Synopsis... an area of low pressure will develop over the Southern
Plains today and then track eastward along the northern Gulf Coast
states and coastal Carolinas through tonight.
Near term /today/...
as of 1030 am Sunday...
A weak cold front has drifted south of the area and essentially
washed out this morning...with a secondary Arctic front draped
across south-central Virginia. Meanwhile...an elongated and
complex...positively tiled upper trough is evolving over the Southern
Plains...with a lead shortwave and associated 1002mb surface low
kicking northeast across the Lower/Middle MS River Valley region. And
expansive precipitation shield is spread across Georgia/SC and into southwestern
NC...with measurable precipitation now ont he doorsteps of the southern
Piedmont. While large scale forcing...weak height falls and warm
advection....will drive precipitation across the area today...there are two
main concerns evolving in the upstream radar/satellite obs: 1) a dry
slot surging across northern Alabama and Tennessee and 2) the
orientation/track of convection across the Gulf Coast states.
Isentropic lift will be maximized across central NC between 18z and
00z...which is when confidence in highest probability of precipitation/quantitative precipitation forecast occurs. The dry
slot may help to cut down on precipitation rates across western portion
of the area by as early as 21z...while the Gulf Coast convection
may act to direct the best low-level jetting and warm advection
toward the Carolina coastal areas. The latest rap quantitative precipitation forecast and hrrr
simulated reflectivity hint at this...with the axis of heaviest
precipitation across central SC and eastern NC. Have adjust probability of precipitation prior to
noon to reflect current radar trends...with categorical still after
noon. Minor adjustments to quantitative precipitation forecast lead to near and inch in the far
south and one half inch across the north.
Temperatures are likely reaching their highs for today...mostly in the 50s
areawide. Evaporative cooling and slowly strengthening cold
advection from the north...as a surface wave begins to develop
offshore...will cause temperatures to hold steady or fall into the 40s
(mainly north) this afternoon.
Short term /tonight through Tuesday/...
as of 320 am Sunday...
..Winter Weather Advisory for mainly minor accumulations of
freezing rain will be issued for midnight to noon Monday for
locations near and north of Interstate 85...
Difficult forecast during this period as there continues to be some
significant differences in the nwp guidance...especially for the
Monday into Tuesday time frame. The end result is a complex and busy
forecast with differing impacts...most of which will be minor to
Widespread rain will be ongoing this evening at the start of the
short term period as a surge in low level warm advection
precipitation enhanced by the right entrance region of a 125kt upper
level jet across the middle Atlantic. At the same time a back door cold
front will push south into central NC with a push of considerably
colder and drier air at the surface. Precipitation coverage and
intensity will wane considerably late in the evening and overnight
as the best forcing lifts northeast of the area and as the middle/upper
levels dry out. Light quantitative precipitation forecast amounts on the order of .10 inches west to
15 inches east are expected after midnight. Guidance differs with
the strength of the surface based cold air over central NC. The NAM
is considerably colder...likely as a result of the slightly stronger
and further south placement of the moderately strong surface high
which provides a closer cold air source region and which develops a
slightly stronger pressure gradient to push the cold/dry air further
south. Used a blend of GFS/European model (ecmwf) guidance to produce surface
temperatures which fall to freezing at around 3 to 4 am near the Virginia
border. The NAM gives a surface temperature of 26 at kgso at 8am while the GFS
gives a cold but more reasonable 29. The 32 degree surface temperature line
reaches as far south as Louisburg....Durham and High Point Monday
morning with subfreezing temperatures further north. Most of the
precipitation should fall as freezing rain or freezing drizzle with
a brief period of sleet possible when moisture is still sufficiently
deep enough for ice aloft. With this in mind...will issue a Winter
Weather Advisory from midnight through noon Monday for our northern
tier of counties...mainly north of Interstate 95. Surface
temperatures recover a bit on Monday as precipitation ebbs and the surge of
cold/dry air subsides. In addition...it is difficult to get
meaningful ice accrual with freezing rain during the daytime hours
in middle March at our latitude. So expect most of the freezing rain
and drizzle to fall between 4am and 9am with accumulations generally
confined to elevated surfaces such as Tree Tops in the colder air
aloft and removed from the ground which should be radiating some
heat. Lows Monday morning will range from near 30 at the Virginia border
to the upper 30s toward the SC border.
Expect precipitation to be rather light and intermittent late Monday
and Monday afternoon as drying aloft moves in and central NC is in a
region of weak ridging between systems. Sensible weather is best
characterized as cloudy skies with periods of light rain or drizzle
with probability of precipitation in the 30-40 percent range. Temperatures should recover
slightly with the absence of widespread rain and some insolation
through the cloud cover. Highs will range in the 34 near the Virginia
border to the lower 40s near the SC border.
Another surge of cold air is expected on Monday night with surface
temperatures getting even colder than Monday morning. Both the
GFS/European model (ecmwf) drive the wet bulb freezing line even further south across
the western Piedmont with the orientation of the wbz line shifting
from west-east on Monday morning to more southwest-northeast by
Tuesday morning. The NAM remains an outlier during this period as it
appears to go bonkers by developing an intense surface low near
Savannah and drives the low northeast along the Carolina coast.
There is another period of freezing rain and even some sleet
possible Monday night with the ultimate impact driven not only by
The Mount of cold air but more importantly by the quantitative precipitation forecast. Guidance
differs with the amount and timing of precipitation during the
period but there will likely be around 0.10 to possibly 0.20 inches
of liquid equivalent that falls across the colder and potentially
wintry weather area. Lows Tuesday morning should range from near 27
across the far northern parts of The Triad to middle 30s across the
Continued cloudy and unsettled on Tuesday as both the GFS/European model (ecmwf) push
a surface wave away from the middle Atlantic while the NAM appears lost
by driving an intensifying surface wave into the Chesapeake Bay.
Temperatures should recover a bit and climb above freezing
everywhere by middle morning before highs eventually reach the 40-45
degree range. The greatest chance of precipitation will occur
across the southeast ahead of the southern stream short wave and
closer to the surface wave.
Three-day quantitative precipitation forecast values range from around an inch near the Virginia border to
1.5 inches near the SC border although some guidance...especially
the NAM/sref are considerably higher. Much of that precipitation
arrives Sunday afternoon and evening with generally lighter amounts
expected for Monday and Tuesday. These amounts are likely
insufficient for a flash flood or aerial Flood Watch. See the
hydrology section below for more details. -Blaes
Long term /Tuesday night through Saturday/...
as of 300 am Sunday...
A lot of uncertainty still exists in the long term. A series of
shortwaves are expected to move through the area and the GFS and
European model (ecmwf) handle them differently with respect to timing...amplitude...
and precipitation. In general...the European model (ecmwf) is slower...wetter...and
cooler than the GFS. Although there is a descent chance that
precipitation will linger through the day Wednesday...it should be
all liquid and fairly light. Given the model discrepancies and
resultant uncertainty...will maintain a mainly dry forecast
Wednesday night through Sunday...with the exception of a slight
chance for rain with the frontal passage on Saturday. In the low levels...the
cold air wedge...present in both models over the northwest...should
weaken as a frontal boundary approaches from the west Thursday. A
weak surface high will move through the area behind the front on
Friday...ahead of the next frontal system to the west. Both models
indicate the cold front with this system will progress through
central NC Saturday eve/night...although the speed and orientation
differ between the models. High pressure is again expected to build
in behind the front. High temperatures will generally increase
through Saturday...lowest in the northwest and highest in the
Aviation /18z Sunday through Friday/...
as of 145 PM Sunday...
Medium to low confidence in the current taf period. Precipitation and low
ceilings have been slow to spread across central NC from the
southwest...with rain just recently reaching The Triad terminals and
rdu. Ceilings are still at VFR levels...but are expected to fall to
MVFR by around 21z and then to IFR by 03-06z. Visibilities will also drop
to mostly around 3sm... possibly lower in heavier rain...which is
more likely at krwi and kfay. A coastal will develop overnight and
cause northeasterly winds to increase into the 12-15kt range. Later
tonight...colder air will push into the area from the north...and a
period of freezing rain or drizzle will be possible at kgso and
kint...mostly between 09z and 15z. Precipitation rates will remain light and
impacts are not expected to be significant. Ceilings will remain
low...likely in the IFR to MVFR range through the end of the taf
Outlook... low ceilings will continue to plague area terminals
Monday into Tuesday...with another period of freezing rain possible
north and west of krdu on Monday night and early Tuesday.
Improvement will likely not come until Wednesday.
as of 1115 am Sunday...
With flash flood guidance still 2.5 inches or greater for three
hours or greater...will continue to refrain from any flood watches
given the expected quantitative precipitation forecast. Should note that deep convection over the
far southeast Continental U.S. May impact some of the expected higher rainfall
totals over the sandhills and coastal plain of central North
Carolina. Consistently it appears the probability of minor flood
stage being reached is highest at Smithfield where a river Flood
Warning is in effect...less so at Clayton...and noticeably lower at
other mainstem river sites even based on the highest quantitative precipitation forecast available
on the ensemble river level output of the mmefs.
the record low maximum temperatures for March 17 (monday) are...
Kgso...38 degrees from 2005.
Krdu...32 degrees from 1892.
Kfay...39 degrees from 2005.
The record low maximum temperatures for March 18 (tuesday) are...
Kgso...36 degrees from 1961.
Krdu...36 degrees from 1892.
Kfay...41 degrees from 1998.
Winter Weather Advisory from midnight tonight to noon EDT
Monday for ncz007>009-021>023.