Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Mount Holly New Jersey
942 PM EDT Tuesday Mar 11 2014
low pressure in the middle Mississippi Valley will intensify rapidly
as it arrives in southern Ohio Wednesday morning, then across
northern New Jersey Wednesday evening. A very strong cold front
will whip across our area Wednesday evening, with the storm then
racing to Nova Scotia Thursday morning. Low pressure in eastern
Canada Friday and Saturday will send another cold front through
the Middle Atlantic States early Sunday. Low pressure then develops
along the southeast coast of the United States next Monday and
Tuesday with quite a bit of uncertainty on its eventual track.
Near term /until 6 am Wednesday morning/...
a frontal boundary that is stalled across the northern portions of
our area is expected to either remain nearly stationary or move
slightly northward during the night. This is in response to the
flow aloft gradually increasing some late as well as some warm air advection. We
do not have any strong short waves expected overnight, except
possibly a weak one across our northern areas. Therefore, we do
not expect much in the way of rain overnight. The exception would
be for the northwestern portion of the County Warning Area where some better
warm air advection/overrunning light rain could develop overnight. The radar
imagery is showing some returns approaching northeastern
Pennsylvania, however given only middle level clouds present and dry
air in the sub cloud layer, nothing is reaching the ground as of
yet. Overall, clouds will continue to increase especially across
the northern half of the County Warning Area.
The temperature drops will be most notable where less clouds remain
the longest as winds go light and variable for many areas. As the
clouds overnight increase some more, temperatures may increase a bit.
Temperatures are not anticipated to drop below freezing across the
area, but if they do it should be spotty/marginal and short-lived
as dew points are forecast to increase some and the temperatures
rise a bit through the early morning hours.
The hourly grids were adjusted based on the latest observations.
The lamp/lav guidance was blended in to assist with trends thus far.
Low temperatures were mostly a blend of the NAM/GFS MOS.
Short term /6 am Wednesday morning through 6 PM Wednesday/...
Wednesday looks to be an active day across the area, especially for
the afternoon hours. An area of low pressure will move through the
Ohio Valley and intensify rapidly as it tracks across our area
during the day. There is some timing differences with the models in
regards to the precipitation, with the NAM, Canadian, and European model (ecmwf)
being the slowest, and the GFS and sref faster. The faster solution
would bring in some warm advection/overrunning rain across the
northern half of the area through much of the morning, before the
rain spreads east through the afternoon. The slower solution would
not bring much of the precipitation in until after midday before
spreading east during the afternoon. For now, we cut back probability of precipitation a
little at the beginning and increase through the day, peaking during
the afternoon. The best large scale lift looks to arrive from about
midday on from west to east. Whatever overrunning precipitation we
have early in the day, will move east and most likely get replaced
by a line of showers with embedded thunderstorms.
Instability parameters are not that high with convective available potential energy at or below 500
j/kg and lifted indice's staying positive. However, k-index values approach
30-35, and total totals approach 50. These values, along with the
strength of the surface low and short wave, some embedded
thunderstorms are possible. With the strength of the low-level jet,
925-850 mb winds 50-60 knots, the Storm Prediction Center has put a portion of our area
in a slight risk for Wednesday. We have added gusty winds to the
weather grids. Some of the high resolution guidance suggests a
squall line of sorts develops to our west and arrives later in the
afternoon. This would be associated with the strong frontal forcing
enhanced by impressive height falls as the upper-level trough
arrives. In addition, robust cold air advection behind the front may enhance uplift
with the cold front as it starts to undercut the boundary layer
warmth farther eastward. This is shown in the rather impressive
thermal gradient along and just behind the cold front. Overall, our
best chance of strong to perhaps severe convective gusts may be tied to
this forced line of at least low-topped convection as it moves
through a strongly sheared environment. This appears to be centered
across the southern half of our County Warning Area during the middle to late afternoon
hours within the warm sector, but will ultimately depend on the exact
track of the deepening surface low.
High temperatures were generally a GFS/NAM MOS blend, and much of
the County Warning Area is expected to be rather mild ahead of the surface low and
strong cold front.
Long term /Wednesday night through Tuesday/...
500 mb: a sharp cold trough crosses the region Wednesday night-
Thursday morning with ridging to follow late Thursday and Friday.
Thereafter... a large scale cyclonic flow dominates the pattern
across the northeastern United States with spokes of short wave
energy transiting through the overall westerly flow over our area.
Temperatures: calendar day average well below normal Thursday and
Friday with daytime maximum temperatures at least 20 degrees below normal
Thursday. Temperatures warming to near normal this weekend but cooling to
maybe 10 degrees below normal early next week.
Forecast basis...12z/11 50-50 blended GFS/NAM MOS guidance Wednesday night-
Thursday night, GFS mex MOS guidance Friday and Friday night then 1522z/11
wpc guidance Sat-Tue. The 09z/11 sref 3 hour probability of precipitation and 12z/11 gefs 6 hour
probability of precipitation for .05 were reviewed.
The dailies below...
Wednesday night...Wind Advisory potential with about 10 mb 3 hour pressure
rises combining with strong cold air advection and a deepening 980mb surface low moving
east-northeast from coastal southern New England should permit isolated power
outages caused by downed tree branches. Strongest wind (bl in excess
of 30 kt) is currently modeled for the Delaware-Maryland-Virginia in the 01z-08z/13
time frame. Am expecting widespread gusts 35-45 miles per hour later Wednesday night
with the strongest wind on the Delaware-Maryland-Virginia where gusts to 50 miles per hour are
possible. With surface pressure well below 1000 mb...the 1000mb wind might
verify straight up for a time Wednesday evening in the cold air advection. Am concerned
about wind. This is not an Ordinary storm.
Any leftover thunder-severe risk in the Delaware valley through the state
of Delaware should exit the coast around 02z.
The question of leftover moisture yielding snow or snow showers
portions of east PA and nnj. At this time since this isnt a middle level
closed low at our latitude... not thinking much new snow cover
except maybe i80 north. Confidence below average on accurately
predicting snow accumulate but at this time...unlikely to have more
than a dusting anywhere except the Poconos and high terrain Sussex
County where an inch or 2 may occur. Due to flash freeze north of
i80...can see the need for a Winter Weather Advisory or a Special Weather
Statement for a short period of time highlighting the issue.
Wind chill...near 15 below in the Poconos for a time Thursday morning.
No Wind Chill Advisory since there is still uncertainty regarding low
temperatures Wednesday night. If the temperatures are a couple degrees warmer than now
posted or the wind slightly less...wind chill would not reach
Temperatures....the forecast was generated using a 25 percent equal
weighting of the 12z NAM/GFS MOS, the 12z 2m temperatures of the NAM and
rgem. The forecast temperatures could be 2 degrees too cold across the
entire area (ie a cold bias). Watch the NAM bl temperatures in its fous
t1 output... if it continues on track as posted in the 12z/11
output... the numbers will verify within 2f. Not sure if this was
noted... the 12z NAM lost 23c of bl temperature in 6 hours Wednesday evening and the
18z vsn 25c. That combined with a bl wind of 29 knots highlights
Stark changes ahead from comfortably warm in the afternoon to winter
jackets by midnight.
Thursday...blustery and very cold for the end of winter down here.
Many northwest gusts near 40 miles per hour with potential for isolated gusts
near 50 miles per hour. Maximum temperatures generally at or below 30f except Delaware-Maryland-Virginia with 850
temperatures around -18c near phl. Max's at least 20f below
normal....possibly 25 below normal in the Poconos.
Friday...not as cold nor as windy with wind shifting to southwest
and only gusty to 20-25 miles per hour during the afternoon. Warm advection
cirrus during the afternoon. 12z/11 NAM slightly more ominous with clouds.
Saturday...variable clouds ahead of a cold front passage with a small chance of a
light shower. Relatively mild!
Sunday...dry and colder.
Monday-Tuesday of next week...uncertainty in the pattern and not
much energy devoted to this part of the forecast. European model (ecmwf) has been back
and forth on this and so have little confidence on what will happen.
Aviation /02z Wednesday through Sunday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg, kilg,
kmiv, kacy and surrounding areas.
Overnight...overall VFR, with ceilings at or above 8,000 feet.
However as moisture increases and with a frontal boundary across
northern New Jersey and northeastern Pennsylvania, ceilings may
lower to MVFR very late tonight at krdg and kabe. The winds in
general from the southwest and west at around 5 knots will become
light and variable at most terminals.
Wednesday...VFR to MVFR ceilings from west to east as a rapidly
intensifying storm arrives. The timing of the MVFR to even at times
IFR conditions is of lower confidence, as it may take much of the
day to occur closer to the coast. Any rain mainly from near kttn to
kabe northward should be light, then showers will move in from west
to east during the course of the day and become more widespread and
heavier. This is when IFR conditions should have a higher chance of
occurrence at times. There should be a line of heavier showers and
some thunderstorms arriving in the afternoon with the cold front. As
this unfolds, timing and impacts will be refined. Since there is
lower confidence in the timing and organization of the thunder, it
was not included at this time.
The surface winds will increase to around 10 knots from the east and
southeast, then shift to the south and southwest during the late
morning and afternoon. There should be some gustiness developing by
late in the day especially from about the kphl metropolitan on south and
east. The winds around 2,000 feet are forecast to increase to around
50 knots by late in the day mainly from kttn to kphl and kilg on
eastward, however since some surface gusts are anticipated low-level
wind shear was not included.
Wednesday night...MVFR and even brief IFR conditions will be
possible during the evening in showers. A strong cold front will
produce an abrupt change with gusty westerly and northwesterly
winds in the evening. A line of showers and some thunderstorms
with gusty winds is possible with this front near 00z from kttn-
kphl-kesn and then off the coast by 02z. A small chance a severe
storm within this line. However, conditions should improve to VFR
within a few hours of the frontal passage. Northwest wind gusts to
30-40 knots. Confidence above average.
Thursday...VFR scattered-broken at or above 3500 feet. Northwest wind gust 30-40kt. Wind
diminish at night. Confidence above average.
Friday...VFR cirrus with SW wind gust 15-20 knots during the
afternoon. Confidence average.
Saturday...MVFR conditions possible in rain showers primarily at kabe and
krdg. Otherwise VFR conditions expected. Wshift from SW to northwest.
Sunday...VFR. Northwest wind gust 20 knots. Confidence average.
sub-advisory conditions expected across the waters tonight. Winds
and seas will increase ahead of an approaching storm system during
the day Wednesday and likely reach Small Craft Advisory conditions
by midday. However, have forgone the Small Craft Advisory in favor
of the Gale Warning.
Wednesday night and Thursday...gale conditions expected behind a
cold front that will push through Wednesday evening. Winds not
expected to subside below gale until Thursday evening. Light
freezing spray Thursday...especially near sunrise. Small chance we
will need to eventually upgrade to a storm for the lower part of Delaware
Bay out through the Delaware Bay coastal waters.
Friday...Small Craft Advisory likely.
Saturday...Small Craft Advisory possible.
Sunday...probably sub Small Craft Advisory conditions.
operational quantitative precipitation forecast amounts, on the high side, average around 0.50-0.75
inches across our northwestern half of the area, and about 0.10-0.25
across the southern half of the area. While this is not expected to
make any of our rivers reach flood levels, there will be rises due
to the combined rainfall and melt off of the snow across the northern
half of the area. With any thunderstorms late in the day, there could
also be some heavy rainfall that could produce urban and poor drainage
Something to keep an eye...per the Hydro ensembles, which are running
with more rainfall than operational solutions, 1.00 to 1.50 inches
of basin-wide rainfall would increase the flood threat across the
northern half of the hsa.
Luckily, we do have some wiggle room with 0.50 to 0.75 inches in the
today has been a nice refreshing sign that Spring is on the way.
For all of our climate sites, today has been the warmest day since
December 22, 2013. The high temperatures on December 22, 2013
For Wednesday March 12th, high temperatures will likely be rather
warm again. This looks to be the warmest two day period since
December 21st and 22nd of 2013 across the region.
Marine...Gale Warning from 7 PM Wednesday to 1 am EDT Friday for
Gale Warning from 7 PM Wednesday to 8 PM EDT Thursday for