Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Mount Holly New Jersey
519 PM EST sun Feb 7 2016
low pressure off the southeastern United States will rapidly
strengthen as it moves northeastward to the Canadian Maritimes
through Monday night. Another low pressure system is forecast to
develop in its wake near the middle-Atlantic coast Monday night into
Tuesday. Behind these two lows, Arctic high pressure will then
build into the central portion of the nation around midweek and
work its way toward the East Coast by the weekend.
Near term /until 6 am Monday morning/...
quiet weather will continue through the evening hours, before the
possibility of snowfall develops along coastal portions of Delaware
and New Jersey. Strong low pressure offshore of the southeast states
will lift northeastward, offshore of the East Coast tonight into
Monday morning. The precipitation shield is expected to expand
slightly on the west side overnight into Monday morning as the low
lifts to the northeast. As of now the question for our area is how
far west. Based on recent model diagnostics and collaboration, we
expect the majority of the precipitation to remain right near the
coast and offshore, then extend to our north and east. The NAM and
sref are currently thought to be high with their amounts, while a
blend of the GFS and European model (ecmwf) give a better expectation of amounts.
Another concern would be how much would accumulate based on
temperatures, as temperatures at the onset will likely be above
freezing. So for now we are expecting around an inch for much of the
central and southern coastal areas, with 1-2 inches across areas
further north into northern ocean and Monmouth counties and eastern
Middlesex. So we will not issue any winter advisories at this time
and let later shifts monitor future guidance and see if a short
fused advisory is needed.
Short term /6 am Monday morning through 6 PM Monday/...
on Monday, the strong low pressure system offshore will continue to
lift northeastward well offshore of the East Coast. Meanwhile, a
weakening backdoor front may sag down into our northern areas as
well. There may likely be some snowfall remaining on the backside of
the low, and as the backdoor front weakens as it moves southward
into our area, it could help enhance some light snowfall across the
northern areas. So there will remain a chance of snow across our
northeastern areas through much of the day Monday. Again, the
highest accumulations through the period are expected to be offshore
and to our northeast, so while there could be a light accumulating
snowfall, we do not expect advisory level snows to be met at this
time. Especially if snowfall mixes with or changes over to rain.
Long term /Monday night through Sunday/...
an area of low pressure is expected to develop near the middle Atlantic CST Monday
night. The guidance is in agreement on this part. The disagreement
lies in the details, which are important.
Yesterday, the ECMWF, went out on its own, with a stronger and
more eastward solution. It continued that trend on the overnight run and
again on today's run. It has a more consolidated low center,
brings that center further away from the CST, and has much less
impact on our area.
While the ecmwf's performance has been less than stellar this
winter, and it is a clear outlier in this case. However, its
overall statistics mean it's solution can not be totally ignored,
and stranger things have happened.
The rest of the other models do bring a generally light snowfall to the
area Monday night into Tuesday. Precipitation will begin later Monday and temperatures
will be warm enough to support rain or snow that has a hard time
accumulating in most areas and on many surfaces during the
After sunset, the snow would start to accumulate more. The guidance
also differs on the areas that would see the most quantitative precipitation forecast and chances of
heaviest snows. While most of the snow looks to fall would be on
the lighter side, even with the January blizzard, the models were
having trouble deciphering this even 12 hours beforehand.
So for now, will only make incremental changes to the snowfall
amounts. Attm, this looks like an advisory event or possibly low end warning
in some areas.
Another complicating factor is temperatures during the day on Tuesday. Temperatures
will be above freezing in many areas, so there could be a mix or
changeover to rain, especially in southern and eastern areas, and much
like we had the other day, even where it snows, it could end up
accumulating mainly on grassy sfcs and less traveled roads during
daylight hours, if it is not coming down hard.
Things should begin to taper off Tuesday night into Wednesday.
Then, a large area of Arctic high pressure will move into the central
Continental U.S.. this high will gradually move eastward through the end of the week.
Expect a generally dry and breezy northwesterly flow Wednesday through Sat. However
temperatures will not make it above freezing from Thursday on. With overnight
lows in the teens. Normal lows are in the lower 40s now. So we are
looking at high temperatures that will average 15 to 20 degrees below
nrml, and with the days getting increasingly longer, this is
increasingly difficult to accomplish, especially for an extendd
Aviation /22z Sunday through Friday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg, kilg,
kmiv, kacy and surrounding areas.
VFR conditions will remain across the taf sites for at least the
first half of the overnight. However, we expect clouds to lower on
the northwestern side of the strong low pressure moving to the
northeast well offshore of the East Coast. Looking at the soundings,
MVFR ceilings should begin affecting all areas by around 10-12z, and
could remain across the area through the day Monday. Another concern
with this system will be the possibility of snowfall. Based on
latest model diagnostics and collaboration, we expect snowfall to
remain across mostly coastal areas of New Jersey and Delaware and
offshore. The main taf sites that would be affected would be Acy,
which could experience IFR conditions with any snowfall, in addition
to miv which could also experience some lighter snowfall as well.
Winds will remain mostly east-northeasterly around 5-10 knots for
the rest of today into this evening. Winds will begin increasing
overnight as the low lifts northward, and likely become gusty 20-25
knots by Monday morning and continue gusty through the first half of
the day, before the gusts drop off by afternoon.
Monday night and Tuesday...precipitation will be ongoing during this time and for
for kphl, kpne, kmiv, kacy will transition from rain or mix to
snow Monday night. Elsewhere will be snow Monday night. Precipitation should be
light for the most part but there could be some MDT or heavy bursts.
Expect MVFR/IFR conds. On Tue, temperatures should warm enough that for
kphl, kpne, kacy, kmiv should again see a mix or changeover to rain,
depending on how warm temperatures get. Low to MDT confidence.
Wednesday...some lingering shsn, then improving to VFR. Breezy with west
to northwest wind gusts around 20 knots. High confidence.
Thursday...VFR and breezy with northwest wind gusting 25 to 30 knots. High
Gale Warning remains in place overnight into Monday as a strong
coastal low pressure system will be lifting east of the waters
tonight into Monday and the pressure gradient strengthens through
the night as the low lifts northward. As we progress through Monday
and the low lifts away from the area, the pressure gradient weakens
and winds should begin to subside into Small Craft Advisory levels.
Monday night through Wednesday...another low pressure system will develop off
the middle-Atlantic coast. An extended period of Small Craft Advisory conditions
likely during this time. It is possible that there could be some gales,
especially on Wed, but confidence remains low on the track and
strength of the low.
Thursday...solid Small Craft Advisory conditions. Northwest winds behind a cfp, with the best
chance for gales.
***minor coastal flooding is likely near the Monday morning
high tide. There is increasing potential for moderate coastal
flooding near the Tuesday morning high tide***
High pressure will anchor over eastern Quebec/Labrador while low
pressure passing offshore well to the southeast undergoes explosive
deepening overnight and through the day Monday. With the
aforementioned high remaining to our north, a broad area of low
pressure will develop off the Outer Banks Monday night and pass
offshore southeast of our region during the day Tuesday.
These two systems will maintain an onshore flow along the coast through
Tuesday, with only a brief pause Monday afternoon and very early
Monday night, as the pressure gradient relaxes between systems.
Setting the stage, astronomical tides will be high early this week,
to due to the new moon on Monday.
For the Monday morning high tide, we think the NAM backs the winds
too much /more northerly/ as we approach high tide, possibly due to
an isallobaric effect with the deepening offshore system, and it was
deemed an outlier. With high astronomical tides and northeast wind
gusts quickly ramping up to gale force just offshore overnight,
we expect a surge of 1 to 1.5 feet. We have high confidence in
widespread minor coastal flooding, especially along the northern
New Jersey coast where pockets of moderate flooding are possible,
due to the presence of a greater component of onshore flow. We
have also tempered down the high-biased et and estofs guidance due
to a residual of nearly 1 foot in spots. A coastal Flood Advisory
has been issued for the Monday morning high tide along the New Jersey and
Delaware oceanfront, as well as Delaware Bay. There is a bit more
uncertainty on whether we reach the minor threshold at Reedy
Point, which is largely dependent on ekman transport in a
northeast flow, but we expect it to be close nonetheless.
A coastal Flood Advisory continues for the New Jersey and Delaware oceanfront
for the Monday evening high tide, as minor coastal flooding is
expected to linger.
For the Tuesday morning high tide, a more easterly component/longer
fetch to the wind will have a greater impact along the entire coast,
including Delaware Bay, and portions of the tidal Delaware. We agree
with the stronger GFS winds, especially given the synoptic setup.
The main uncertainty is regarding a broad low pressure/trough moving
northeastward along the coast, which may allow winds to quickly
relax near the time of high tide, particularly Delaware-Maryland-Virginia and far
southern New Jersey. Nevertheless, aside from a brief lull in the flow
Monday afternoon and very early Monday night, we'll have had
nearly 48 hours duration of onshore flow, which will make for
higher tidal departures compared to Monday morning, in addition to
the astronomically higher tides. In terms of guidance, we sided
more with the higher etss surge of 2 to 2.5 feet, even considering
the aforementioned residual. A coastal Flood Watch has been issued
for the Tuesday morning high tide for the New Jersey and Delaware oceanfront
with the highest confidence in widespread moderate coastal
flooding for central and northern New Jersey. Elsewhere, widespread minor
coastal flooding is anticipated, perhaps up to Philadelphia, given
the more easterly flow up Delaware Bay. For these areas, a
coastal Flood Advisory will likely be needed.
New Jersey...coastal Flood Watch from late Monday night through Tuesday
afternoon for njz012>014-020>027.
Coastal Flood Advisory from 5 am Monday to 4 am EST Tuesday
Coastal Flood Advisory from 9 am to 1 PM EST Monday for njz016.
Delaware...coastal Flood Watch from late Monday night through Tuesday
afternoon for dez002>004.
Coastal Flood Advisory from 5 am Monday to 4 am EST Tuesday
Coastal Flood Advisory from 9 am to 1 PM EST Monday for dez001.
Marine...Gale Warning until noon EST Monday for anz450>455.
Small Craft Advisory until 6 am EST Monday for anz430-431.