Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Mount Holly New Jersey
946 am EST Monday Dec 29 2014
a weak area of low pressure over North Carolina will move east and
offshore later today and tonight. A Canadian high pressure system
will enter the northern plains on Tuesday and slowly build to the
southeast reaching the southern middle Atlantic coastal waters on
Friday. A low pressure system and warm front should advance
toward our area on Saturday. A new wave of low pressure along its
cold front might develop and pass southeast of the region on
Near term /until 6 PM this evening/...
light rain continues across our southern zones this morning but is
starting to weaken and push more to the south as some middle level
energy rotates through the region. The chance for some light rain
will gradually taper off toward this afternoon with the low clouds
drifting to the east as well.
The west to southwest flow in the middle levels will usher in some
clouds across the region today and we should mainly see some middle
and high level clouds building through this afternoon.
Temperatures look to be on track with highs reaching into the 40s
across much of the area, with 30s across the northern most zones.
The north to northwest flow at the surface will allow for some
cooler air to trickle into the region and this will keep
temperatures lower than what we saw on Sunday.
Short term /6 PM this evening through 6 am Tuesday/...
as a weak short wave trough arrives late tonight, a stronger short
wave is forecast to arrive mainly across the central and southern
zones. The vorticity associated with this short wave though is more
channelized and the moisture is not all that deep. It was generally
this feature days ago that some models were showing a decent
overrunning type event, however it appears to be farther south and
weaker now. While there is some middle level lift associated with it,
much of this may end up in the production of a middle level cloud deck.
If some light precipitation were to develop late, it would be across
the Delaware-Maryland-Virginia and possibly into far southern New Jersey. The forecast
soundings in general have a dry look to them with perhaps some
virga/snirga moving through. Since there is not much to latch onto
with this short wave moving through late, we maintained some slight
chance probability of precipitation across the far south. Given the cold air advection continuing, it does
become cold enough for the precipitation type to be snow.
Otherwise, a fair amount of clouds across the County Warning Area although the far
north may end up having less clouds for awhile being north of the
channelized short wave. If there is less cloud cover longer, then
temperatures will tend to be colder given light enough winds and
lowering dew points.
As for low temperatures, we used a NAM/GFS MOS blend then made a few
tweaks for local affects.
Long term /Tuesday through Sunday/...
a cold start and unsettled end to the long term in more ways than
one. A high pressure system originating in Siberia will enter the
Continental U.S. On Tuesday. This will solidify colder weather entering the
New Year. As this moves offshore, a storm system from the
southwest is predicted to affect our area over the weekend. The
gefs mean is predicting a dual teleconnection switch which is a
fairly confident signal a precipitation event.
As for the details, the models have had a difficult time on
keying on the correct 500mb pattern much more than 4 to 5 days in
advance since autumn. Getting down to the surface, these differences
have not been solidified much before 72 hours. As a recent example
the non-European model (ecmwf) models did not lose their snowier Tuesday solution for
our County Warning Area until the 12z run Saturday and in the nam's case not until
yesterday. So in one corner we have the GFS, NOGAPS and (inferring)
UKMET take a very strong low through the Saint Lawrence valley.
There is a plethora of cold air in eastern Canada and have low
confidence that this will be easily done. In the other corner we
have the ECMWF, pgfs and can ggem that take a much weaker low
into the Saint Lawrence valley, hang back energy in the desert SW
and then do another shearing version of our present scenario over
the weekend. While we like the weaker Saint Lawrence valley track
forecast, the hanging back of energy, especially with the ECMWF,
opens up a whole new bowl of uncertainty. So given the divergent
solution with this package, we leaned heavily on the latest wpc
solution for any adjustments, which for right now paints a very
climatological picture of snow to rain to snow for the weekend.
The model initialization looked pretty good and a WRF-nmmb and GFS
blend was used for the start of the long term.
As for some day to day details...
On Tuesday a channelized vorticity maximum is still expected to pass across
the far southern part of our County Warning Area. Middle level moisture is still
sufficient as is predicted middle level qvec convergence to generate
precipitation. The lower level soundings are dry and not sure how much of
that precipitation would be able to make it to the ground. Probability of precipitation were kept as
chance or lower. Because of the light precipitation intensity, ptype macros
suggest rain or snow could occur because of the warm boundary layer.
As Tuesday day continues, moisture is predicted to become more
limited and we ended the chances during the afternoon. Stat guidance
looked decent and was supported by model soundings and was followed.
The downward progression of temperatures will continue on Tuesday
night. The predicted gradient is borderline for decoupling so in
outlying areas we met min temperature potentials halfway.
On Wednesday, a secondary surge of Arctic air will occur. Forecast
1000-850mb thicknesses are in the 1250m range. There is another vorticity
maximum predicted to pass through and convective temperatures predicted to be
reached northwest, model differences central and not there
Delmarva/southeast. Granted that the short wave timing could be
off, but Arctic thicknesses have a way of generating flurries and
maybe even snow showers. We added the chance of flurries as a
starter. Looks like less decoupling chances on new years evening. No
precipitation related issues, but it will be overall colder.
A sunny, but brisk and cold day for New Years Day. The lake
effect trajectory is predicted to remain north of our County Warning Area. A
better radiating night is possible on Thursday night with a slight
air mass recovery on Friday.
Because of the overall slower timing, we removed probability of precipitation from Friday
night. We are keeping the forecast of a one-two punch of the
initial low and cold front precipitation on Saturday with the secondary
shearing low precipitation possible on Sunday. The gefs members are pretty
much all clustered similarly to the op GFS. The can geps has about
three quarters of its members similar to the gefs and a quarter
similar to its operational and European model (ecmwf) solution. The European model (ecmwf) ensemble
mean does not have much of a secondary low Sunday as it opens up
the closed SW Continental U.S. Low much faster than the op. There is a swath
of deep purple uncertainty about surface pressure over New England
sun eve, not exactly smoking on the water. Operationally seven
days away, this has been more the rule than the exception this
cold season for all of the models.
Aviation /15z Monday through Friday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg, kilg,
kmiv, kacy and surrounding areas.
Today...VFR ceilings. North-northwest winds becoming north-northeast
at 5-10 knots. Some light rain early this morning south of kilg.
Tonight...VFR ceilings mainly around 10000 feet. North-northeast
winds generally 4-8 knots.
Tuesday...VFR conditions expected. Gusty north winds 20 to 25
knots. Airports in Delaware-Maryland-Virginia south of kilg and in New Jersey south
of kacy and kmiv there is a risk of MVFR ceilings at times.
Tuesday night through Friday...mostly VFR conditions expected.
Gusty west winds exceeding 20 knots possible during the day on
both Wednesday and Thursday.
our area will be north of a stalled frontal boundary today and
tonight. A weak system is tracking along it early this morning and
will shift off the Delaware-Maryland-Virginia coast through this morning. There is cold air advection
occurring today and that will strengthen some by late today and
tonight. As another weak system slides by to our south later tonight
and high pressure builds into the Great Lakes, the pressure gradient
will tighten across our area. This combined with cold air advection will allow for
better mixing. This will result in a north-northeast wind increasing
tonight especially across the southern waters. It does appear to
generally be marginal, however wind gusts look to near advisory
criteria across the southern waters toward 12z Tuesday. Therefore, a
Small Craft Advisory has been issued for the lower Delaware Bay and
the coastal waters south of Little Egg Inlet starting at 10z Tuesday.
Tuesday...we have the Small Craft Advisory ongoing for lower
Delaware Bay and the southern Atlantic waters for a surge of
around 25 knots gusts occurring as the low pressure system moves
farther offshore and intensifies. Peak gusts should occur mainly
during Tuesday morning. Upper Delaware Bay and northern coastal
waters conditions are expected to stay below Small Craft Advisory thresholds.
Tuesday night and Wednesday...sub Small Craft Advisory conditions expected. May
come close late in the afternoon Wednesday.
Wednesday night through Thursday night...westerly winds with gusts
near Small Craft Advisory criteria are possible, primarily on the
Atlantic coastal waters.
Friday...sub Small Craft Advisory conditions expected.
Marine...Small Craft Advisory from 5 am to 4 PM EST Tuesday for anz431-