Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Raleigh NC
300 am EST Sat Nov 22 2014
Synopsis...high pressure remains over the region today into
tonight. A strong low pressure moving from the Southern Plains into
the Great Lakes Sunday will spread moisture over the region during
the day on Sunday. Another cold front will move east across much of
the area Monday...with low pressure moving along the southeast coast
Tuesday into Wednesday.
Near term /through tonight/...
as of 300 am Saturday...
The latest surface analysis showed surface high pressure over
central North Carolina...and the 00z upper-air analysis noted very
dry air at 850mb with dew point depressions well above 40c. The
temperature had fallen into the upper teens at least at ktdf...and
watching the krdu overnight low so far the low was 25 as of this
writing. For today...bufr soundings are continued dry and stable.
The hrrr WRF forecasts noticeable high clouds today...but other
guidance such as the Canadian cloud cover model...and what one would
consider looking at the upper-level relative humidity forecasts and
bufr soundings...suggest less. Satellite data upstream suggest less
as well...along with the jet lifting north. Under expected sunshine
highs should be 50 to 55.
Overnight...clouds should gradually increase...but it may be a slow
increase as the air mass remains dry in the mean at least through
06z Monday...and moisture aloft does not show a strong increase on
bufr soundings until late tonight. The Canadian cloud cover model
also shows a slow and gradual increase...and even a few areas of
mostly clear to partly cloudy conditions especially northeast even
late tonight. Overnight lows will be forecast on the cold side of
guidance and a few of these lows may even be too high if less clouds
occur for longer periods in the overnight. Under the westward
extension of the ridge...winds should be light for much of the night.
Will keep a dry forecast as both the GFS and NAM seem to be a little
slower with the approach of deeper moisture. By 12z Sunday...the NAM
is fairly moist aloft...but the sub-cloud layer remains very dry.
The 850mb Theta-E gradient is just beginning to approach central
North Carolina on both the NAM and GFS at 12z Sunday...and those
models are also subsident at that layer at that time. Despite the
dry sub-cloud layer...if any precipitation would occur over the far
southwest Piedmont...for example...sufficient warming aloft pushes
thicknesses well into the liquid category...and surface wet bulb
temperatures are above freezing. If anything does occur...it would
be very close to 12z Sunday and then almost certainly in the form of
Short term /Sunday through Monday night/...
as of 300 am Saturday...
The NAM and the GFS are slow to bring in good lift to central North
Carolina Sunday. However...numerous parameters...including 850mb
Theta-E...k indices...general 850mb lift...700mb vorticity...among
others suggest that precipitation will move in from the southwest
with the greatest likelihood of rain in central North Carolina from
about 18z to 05z. Have confined the highest chances in the gridded
forecast from 20z to 04z...and some of the WRF guidance is even
slower than that with the onset particularly from the Triangle north
and east. Future forecasts may end up trending a little slower...but
for planning purposes anticipate high probabilities of rain during
the aforementioned window.
Clouds should increase...though the uncertainty with the onset of
rain makes the temperature forecast challenging. Except in The
Triad...guidance highs are relatively close...and will plan to go
between there. If rain would move in earlier...would almost
certainly lean toward the cooler NAM but with some WRF guidance void
of much precipitation even to 18z Sunday think at least a blend is
wise. In terms of thunder...bufr soundings provide for greatest
lapse rates very late Sunday afternoon through the evening in
advance of a dry slot which both the NAM and GFS bufr soundings
forecast moving in from the southwest during the night with
increasing 850mb subsidence. 1000-500mb MUCAPE on the GFS is less
than 200j/kg and the sherbs3 is less than one...but the NAM is
noticeably more unstable...although it is difficult to see surface-
based concerns on the NAM bufr soundings with a slight inversion
just off of the surface. 0-3km shear and helicity...as expected...
are very high as winds just off of the surface increase greatly.
Added a slight chance of thunder during Sunday evening to the
forecast for the Interstate 95 corridor as at least there is a
slight chance of at least elevated thunder...and for the severe weather potential statement will
likely add a mention of at least wind gusts to 40mph or a little
higher as even showers in a rapidly tightening surface gradient...
with 925mb winds approaching 50kt...should result in a few quite
noticeable wind gusts simply with precipitation. Storm Prediction Center has noted a
slight risk of severe weather where thunder will be added to the
forecast and will certainly need to be watchful of instability
trends in later forecasts...as other parameters are plenty high
enough for isolated severe weather. Overnight lows Sunday night not
much different from the highs Sunday...steady to rising in The Triad
and maybe a difference of a degree or two elsewhere in general. -Djf
Monday and Monday night...mid-level dry air intrusion and subsidence
in the wake of the exiting shortwave lifting off to the north will
result in mostly dry conditions during the day on Monday. The
trailing cold front will traverse the area Monday evening/night but
airmass is just too dry with little to no forcing/lift aloft to
support probability of precipitation. Temperatures Monday afternoon in the pre-frontal warm
sector will warm into the upper 60s/near 70 northwest to middle 70s southeast. Lows
Monday night will range from lower 40s northwest to lower 50s southeast as cold air advection
spreads west to east. -Cbl
Long term /Tuesday through Friday/...
as of 300 am Saturday...
Tuesday through wednesday: southern stream trough embedded at the
base of the amplifying long wave trough will traverse the Gulf Coast
region on Tuesday and then across the southeast on Wednesday. After
a dry start to the day on Tuesday...rain chances will return Tuesday
evening/night and into the day on Wednesday as western edge of
deeper moisture axis attendant to offshore frontal zone retreats
inland in response to strong DPVA and upper level divergence within
the right entrance region of a 140-150kt jetstreak. Both the GFS and
ec continue to trend wetter during this period with the potential
for central and eastern portions of the state to receive significant
rainfall. As such... will raise probability of precipitation to chance in the west to likely
in the east. No p-type issues here with just a cold rain for central
NC. Lows Tuesday night in the middle/upper 30 northwest to lower 40s southeast. Highs
Wednesday in the 40s. Precipitation and clouds pull out Wednesday
evening/night with lows back down into the 30s.
Thursday and friday: uncertainty remains high regarding the approach
of the next batch of shortwave energy rotating eastward within the
base of the longwave trough...with some guidance suggesting a middle-
level short wave may try to close off over the middle-Atlantic or
Carolinas. Its Worth noting that northern stream systems like this
are often moisture-starved... especially east of the mountains.
Given there has been little continuity among the deterministic
guidance along with large spread among the ensemble members...will
play the conservative approach and maintain a dry forecast for
Thursday and Friday. It looks almost certain however that this
system will deliver another shot of cold Arctic air into the
southeast by the late week and into next weekend.
Aviation /06z Saturday through Wednesday/...
as of 1250 am Saturday...
For the 06z valid taf period...VFR conditions continue. Light winds
this morning gradually veer with a slight increase in wind speeds
after sunrise...with sustained values expected to be under 10kt.
Surface winds under high pressure become light or calm again this
evening backing slightly.
Beyond the 06z valid taf period...conditions deteriorate Sunday and
as areas of rain move in to central North Carolina numerous areas of
sub-VFR...likely IFR...conditions develop during the day continuing
at least into Sunday evening. Winds off of the surface will also
increase Sunday and reach or even exceed 50kt by 2000ft which could
make low-level wind shear a problem by late in the day into the
overnight hours. Improving aviation conditions are possible Monday
and while southwest winds off the surface will not be as strong...a
gusty character to the surface winds continues. As low pressure
moves along the southeast coast Tuesday into Wednesday...uncertainty
increases in terms of the potential for a return to a period of sub-
VFR conditions and precipitation...especially east of The Triad. The
NAM model returns these conditions much faster Tuesday...while the
GFS and European model (ecmwf) are slower...more overnight Tuesday into Wednesday.
The current thinking is toward the slower solutions.