Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Raleigh NC
908 PM EDT Tuesday Jul 29 2014
Synopsis...dry high pressure will extend into the region from the
northwest through Wednesday...then weaken Wednesday night and
Thursday. Deep moisture will gradually return to the region Friday
through the weekend as an upper level trough approaches from the
west... and an attendant coastal front moves inland across the
Near term /through Wednesday night/...
as of 908 PM Tuesday...
..near record lows tonight...
The sunset was only 45 minutes ago and already the temperatures were
reported to be in the middle to upper 60s across our NE zones
including: the reports from Louisburg... Tarboro... Halifax... and
Henderson. There were some middle 70s reported from the southern
Piedmont and sandhills. The latest infrared satellite data indicated some
patchy middle level clouds with bases around 10k feet. Radiosonde observation data at
00z/30 July from Greensboro indicated the middle level inversion
continued to exist near 700 mb which continued to trap some moisture at
that level. Trends do suggest a decrease in areal coverage of the
patchy cloudiness in the past 3 hours. These trends are supported in
the latest model guidance which generally suggests mostly clear
skies overnight. With this in mind and considering the dry air near
the surface (dew points 50-60 degrees)... we will lower minimum
temperatures a few degrees from previous forecasts.
This will now yield lows of lower 50s north and east (rural areas)
into the lower 60s around the urban areas of Fayetteville. Current
projections indicate lows of 59 at Raleigh... 58 at Greensboro...
and 61 at Fayetteville. These forecasts will be fairly close (2-3
degrees) to the daily record lows for July 30. They include: Raleigh
56 degrees set in 1946... Greensboro 56 degrees set in 1946... and
Fayetteville 59 set in 1920.
Short term /Wednesday through Thursday night/...
as of 310 PM Tuesday...
Wednesday will have continued subsidence keeping dry conditions in
place with a slight warming trend but still in the low to middle 80s
for highs with upper 50s to low 60s once again for minimum
temperatures. Scattered to broken cloud cover in the 5-8 kft range
will have the heaviest concentration in eastern areas of the County Warning Area
which may help keep maximum temperatures down a degree or two from forecasted
highs.Deep upper troughing over the eastern half of the US will
continue Thursday...though with a brief relaxation and height rises
aloft between a shortwave lifting off to the northeast and energy
diving through the plains states. Thicknesses rise about 10m over
Wednesday...starting out at 1395m Thursday morning...which should
yield 83-87 under mainly partly cloudy skies. Dewpoints still in
the 50s and lower 60s will limit cape to a couple hundred j/kg or
less...though a little better moisture at the top of the boundary
layer may help enhance cloud cover across the southern portions of
the area...especially along the seabreeze. Any showers and storms
should remain south of the area. Clouds...especially middle and high
clouds will continue to increase Thursday night as a shortwave dives
through the deep south. Lows will be mostly in the middle 60s.
Long term /Friday through Tuesday/...
as of 310 PM Tuesday...
The Friday through Tuesday period is one of mixed confidence.
Models are in good agreement that the mean upper trough axis will
remain west of the Appalachians...placing NC in southwesterly flow
aloft and in the right entrance region of an upper jet along The
Spine of the mountains. An inverted coastal trough will slowly
meander westward over the weekend and then likely wash out by early
next week as the trough weakens and the western Atlantic ridge
builds westward. This will cause precipitable water to continue to increase to
upwards of two inches by Sunday. While this points toward a wet
weekend...confidence is low when it comes to best time frame for
precipitation. On Friday...models are showing a disturbance emanating from
convection over the High Plains that sweeps around the base of the
trough and then lifts across our region. Its hard to put much Faith
in such a feature three days out...but all models have shown an
increase in quantitative precipitation forecast for Friday...so have increased probability of precipitation into the 40-50
range. However...beyond Friday there is very little to key on for
forcing other than the aforementioned jet and weakening coastal
trough. Thus...will have chance probability of precipitation in each day be keep them below
50 percent until it becomes more apparent which day will be favored.
Thunder potential looks limited Friday and Saturday given limited
instability with weak lapse rates aloft and relatively cool daytime
temperatures in the upper 70s and lower 80s. A warming trend should endue
by Sunday and Monday as the upper ridge and Bermuda high strengthen
to our east.
Aviation /00z Wednesday through Sunday/...
as of 745 PM Tuesday...
24-hour taf period: VFR conditions and light/variable winds will
prevail through the taf period as dry high pressure extends into the
region from the northwest.
Looking ahead: VFR conditions will prevail Wednesday night through
middle-day Friday across central NC as the dry airmass currently in
place slowly modifies. Expect an increasing potential for adverse
aviation conditions associated with low ceilings and scattered to
widespread convection Friday night through the weekend as a coastal
front retrogrades inland into central portions of the state...in
advance of an upper level trough approaching from the West. -Vincent