Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Mount Holly New Jersey
401 am EST Friday Dec 20 2013
a high pressure system off of coastal North Carolina will move east
farther offshore today. A cold frontal boundary will slowly
approach the region from the northwest today. This frontal
boundary should stall just north of the forecast area as several
waves of low pressure travel northeast along it. The frontal
boundary should then move through the forecast area on Monday as
the last low pressure system moves into the Canadian Maritimes.
Another Canadian high pressure system should reach the Midwest on
Tuesday and our forecast area on Christmas day before moving
offshore on Thursday.
Near term /until 6 PM this evening/...
our middle-December warm up will continue today with a mild and moist
southwest flow into the region ahead of a very slow moving cold
front approaching from the Ohio Valley and lower Great Lakes.
Southwest winds early today will be light with limited mixing, and
any early morning ground fog and reduced visibilities that develop
over the snowpack across portions of eastern PA and northern New Jersey may
hang around until middle to late morning before dissipating. Otherwise,
more broken cirrus is expected overhead today, making for a partly
sunny day overall. Cloud cover may be a bit thicker, though, across
our northwest zones, especially late-day in advance of the slow
moving frontal boundary to the west. We have retained the idea for
some possible light rain reaching our far northwest locales for late
afternoon to evening with slight chance pops; although, only a trace
of precipitation may be realized as quantitative precipitation forecast looks rather limited.
With regards to temperatures, the warming airmass and model
850mb/925mb temperatures point to our daytime highs reaching about 10 to 15
degrees above middle-December normals. Overall, we took a blend of
met/mav guidance with some local adjustments and expect maximum
temperatures to reach into the 40s northwest of the I-95 corridor,
with 50s expected for areas to the south and east. Parts of our
lower Delaware-Maryland-Virginia zones may reach into the lower 60s.
Short term /6 PM this evening through 6 am Saturday/...
through tonight, the slow moving frontal boundary around the Great
Lakes will not make much progress eastward. Model soundings show the
column continuing to moisten, and we expect thickening and lowering
clouds. Forecast dewpoints also reflect increasing low-level
moisture. We have kept some low probability of precipitation in place for our northwest
zones for possible light rain/shwrs, but with minimal quantitative precipitation forecast. Of
perhaps greater impact may be the more widespread development of fog
across the region, especially over the areas of remaining snowpack
through eastern PA and northern New Jersey. We have added some fog to the
overnight forecast. For temperatures, a mild overnight looks to be
in store. We blended met/mav guidance with continuity, and expect
low temperatures in the 40s along the I-95 corridor and for areas to
the south and east, with lows in the 30s to the northwest.
Long term /Saturday through Thursday/...
the unseasonably and likely record breaking start to the long term
remains on track. The flooding threat appears to be diminishing
as the modeling is trending toward a larger separation between the
snow melt and arrival and location of the heavier rain. Christmas
evening/day still look dry and seasonably cold. Next precipitation threat
Neither the GFS nor the WRF-nmmb initialization looked spot on at
500mb with the GFS having more and larger errors. While within
initialization noise, lowered overall confidence about this
specific run. The WRF-nmmb looked slightly better at 925mb and
850mb. The gefs mean is slightly slower than the op GFS and the op
WRF-nmmb was even faster than the GFS. The latest European model (ecmwf) run
remained pretty stable from its 12z run and was used as the
foundation of the long term Sans late Thursday.
The GFS remains the southernmost modeling solution with the initial
push of the frontal boundary on Saturday. This looks similar to
today with a decent location north of the exit region of the 250mb
jet. We did a tightening of the pop gradient than lowering it toward
our northern border. Maximum temperatures over snow were sluggish yesterday.
This will be another day of melting. Problem for maximum temperatures
Saturday may be the extent of the low clouds and fog in the
morning as the low level jet is not here yet. So we remained conservative
there. Much more aggressive with maximum temperatures in snow-free regions.
Forecasting a record high for Ged.
Saturday night given the moist southerly flow, low clouds should
form. Not as confident about fog outside of potential left over
snow areas. Precipitation chances lean heavily toward the northwest part of
our County Warning Area.
We did not change probability of precipitation much for Sunday, but pcpnwise the day might
have many more shower free than shower occurring hours. There have
been successive sounding runs with this scenario where the last
surface low heading into New York state runs into an inhospitable
confluence zone as the initial closed low in the desert SW gets
sheared away. For our area, a strong DPVA lobe is forecast to
weaken as it moves across. Forecast soundings are taking dry air
above the boundary layer east of Florida and bringing it over our
County Warning Area during the second half of the day. If a perfect prog, most of
the showers would be in the morning and west (closer to the
weakening vorticity lobe) and then our County Warning Area would be in a precipitation lull
period. This would reduce the chances of a squall line forming,
energy too early, front itself too late. On the other hand, this
increases the chances of record high temperatures being reached. If this
Sunday air mass occurred in June based on forecast 925mb temps, phl would
be pushing 80f. Given clouds should muddle the picture. Plus relative humidity
forecasts at this temporal juncture get shakier than vorticity progs, precipitation
might verify more robust. Upshot we are forecasting record highs, but
did not sway from continuity or stat guidance much. The GFS forecast
low level jet has weakened. Although the wrf's still reaches 70 kts during
the morning. The forecast soundings better mixing is during the afternoon
when the low level jet is forecast to be farther east. Momentum Transfer wind
gusts for now are below Wind Advisory criteria.
There is better model consensus of precipitation turning anafrontal as the
positive tilt trough sharpens Sunday night into Monday. Sunday night
is also the forecast of best instability aloft. (Moisture returns). Forecast
precipitable waters have lowered a bit and are around 1.50 inches. While we are
not ready to sound the all clear, the delay in arrival of heavier
pcpn, plus the current predicted location (more southeastern half of our
County Warning Area where snow cover is currently none or scanty) of the heavier
precipitation is lowering concerns. We are seeing this in marfc's mmefs
guidance as the number of gaged streams expected to reach flood
stage is lowering.
Operative word is slow improvement in the weather on Monday given
how far west the long wave trough still will be. Highest probability of precipitation are
east and also during the morning. We kept low chances in east on
Monday evening. While The Gap between the arrival of colder air
for snow and the precipitation shield are narrowing, they are not
coincidental yet. Because of the slower gefs timing of the cold front
easing east, we leaned toward the European model (ecmwf) solution. Also its MOS for
No big changes then for the rest of the long term. A blustery
Christmas evening followed by a dry and seasonably cold Christmas day.
It could be a cold radiating Christmas morning. A bit of an air
mass recovery should occur on Thursday. Next precipitation shot its in our
Thursday night grid. For now we are forecasting it more like a
cold front passage than what the op European model (ecmwf) has been depicting. The southern
stream system is literally born from convection which lessens the
chances of it being correct. The ec's 12z and 00z ensemble mean
have not agreed with the op run on this.
Aviation /09z Friday through Tuesday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg, kilg,
kmiv, kacy and surrounding areas.
Today...overall, mainly VFR conditions will be in store across the
region through today. The main exception, however, will be for
krdg/kabe where some MVFR or possible IFR visibilities in ground fog may be
an issue early today before improving by middle to late morning.
Otherwise, some broken cirrus is expected overhead with light
southwest winds or light/variable winds until middle morning.
Thereafter, southwest winds pick up some into the 8 to 12 knots range
across the region.
Tonight...southwest winds are forecast to diminish to 5 knots or less.
Increasing low-level moisture, especially in areas of snowpack to
the northwest out toward krdg/kabe, will lead to a more widespread
coverage of possible MVFR/IFR conditions in developing fog and low
clouds. Isolated showers may also be possible for the northwest taf
sites, but as coverage and duration would be minimal, this was not
included in the ridge/Abe tafs.
Saturday...some MVFR or IFR conditions in northwest airports,
combination of snow pack fog and greater chance for showers.
Saturday night...MVFR to IFR because of ceilings and also visibilities
northwest. Low level wind shear potential.
Sunday through Sunday night...predominately MVFR. IFR at times in
low clouds, fog and precipitation. Strong gusty southwest winds potentially
exceeding 30 kts on Sunday.
Monday and Monday night...predominately MVFR becoming VFR during the
day northwest airports and by evening southeast airports.
Tuesday...VFR conditions with continued gusty northwest winds.
an increasing southwest flow through today and tonight across our
area waters will lead to a steady increase in seas for our coastal
waters. Seas in the 3 to 5 foot range are expected off the coasts of
New Jersey and Delaware later this morning and through the remainder of the day.
Some wind gusts between 20 to 25 knots can also be expected,
especially this afternoon. We therefore raised a Small Craft
Advisory beginning 12z this morning and lasting through the day
Saturday, as similar conditions will continue. For Delaware Bay,
sub-advisory conditions are expected today as wind gusts and seas
will not be as high, but conditions will continue to be monitored
into this weekend.
Saturday...expecting to remain just below Small Craft Advisory
wind conditions on Delaware Bay. Confidence is moderate. Small
Craft Advisory continuing on the ocean through the day with likely
Saturday night and Sunday...high confidence about Small Craft
Advisory conditions expected on Delaware Bay and the ocean. Chances
for reaching gale force wind gusts decreasing.
Sunday night and Monday...high confidence of Small Craft Advisory
for swells on the ocean. Winds might diminish below criteria. On
Delaware Bay, winds and seas are expected to become sub Small Craft
Advisory Sunday night and continue on Monday.
Monday night and Tuesday...another ramp up of potential Small Craft
Advisory criteria winds in the northwest flow over all of the area
waters. Slow improvement late.
daily record maximum temperatures that are vulnerable to be exceeded.
12/21 kged 65-1956 por back to 1949
Kacy 65-1998 por back to 1874
kphl 64-1998 por back to 1872
kilg 64-1998, 1984 por back to 1894
kabe 61-1998 por back to 1922
kttn 64-1998 por back to 1865
kged 68-1949 por back to 1949
krdg 61-1998, 1949 por back to 1869
kmpo 58-1990 por back to 1901
For the month we've taken a look at the records and for the time
being we will only list the vulnerable short period of record
kged 77 set 12/1991. The monthly records for all our County Warning Area long term
climate stations are in the 70s and in most cases featured the
period of 12/4-7 1998. The mpo all time December record is 66.
Marine...Small Craft Advisory from 7 am this morning to 6 PM EST
Saturday for anz450>455.