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Area forecast discussion 
National Weather Service Mount Holly New Jersey
359 PM EDT sun Apr 19 2015

Synopsis...
high pressure will move farther off the New England coast through
tonight. Meanwhile, low pressure over the Midwest this afternoon
will strengthen as it tracks northward into the Great Lakes tonight.
The low will lift into Ontario Canada early in the week. A warm
front is forecast move northward into the middle Atlantic on Monday
before a cold front moves through Monday night. The low pressure
system over Ontario will slowly move eastward midweek before
stalling over eastern Canada and New England through next weekend.

&&

Near term /until 6 am Monday morning/...
high pressure will be moving to the east early tonight. Low pressure
and its associated front will be moving in from the Ohio Valley
overnight. Clouds will overspread the area and rain will arrive in
many areas after midnight...maybe a few hours earlier far S. The
rains will become steady overnight and patchy fog is possible. Low
temperatures will be in the low/middle 40s north and upper 40s to low
50s south. Winds will increase from the east at 10 to 15 miles per hour with
gusts 20 to 25 miles per hour overnight. Quantitative precipitation forecast will be around a tenth of an inch
North/East to a quarter inch short wave.

&&

Short term /6 am Monday morning through 6 PM Monday/...
a large batch of steady rains will be across the area during the
morning and a decent quantitative precipitation forecast up around an inch or so will occur in many
areas. Totals up to 1.5 inches are possible in a few localized
areas. While these totals are robust...the dry conditions across the
area presently should keep any flooding concerns at Bay. It is
possible that some localized poor drainage flooding may occur...but
the lack of any widespread threat will keep US from issuing any
flood flags at this time. The rain shield will pull to the north by late
morning and precipitation should be more showery for the afternoon. Clouds
will likely remain overcast north/west during the afternoon...but breaks will
develop across the Delaware-Maryland-Virginia and southern New Jersey.

The most recent Storm Prediction Center day2 outlook has pushed the slight risk area
across most of the short wave 2/3rds of the County Warning Area for tomorrow into tomorrow
night. A cold front will begin to approach late and this should
touch off scattered-numerous thunderstorms across the area. Cape values
should increase into the 1000-2000 j/kg range across the southern
areas...especially in areas where breaks in the clouds develop.

High temperatures will be tricky also...since a short break in the clouds
will have a profound impact on temperatures. We have a mav forecast
for the most part...with a few degrees lower in few areas.

&&

Long term /Monday night through Sunday/...

An upper low is forecast to deepen over the upper Midwest/Lake
Superior on Monday night. The surface reflection of the upper low is
forecast to meander over Ontario. There will also be a negatively-
tilted trough/occluded cold front extending southeastward from the
low while a warm front is expected to reside somewhere near central
portions of PA and New Jersey Monday evening. A Sample of model forecast
soundings across the area show modest instability being sustained in
the warm sector during the evening in response to a S-southwesterly low-level
jet helping to moisten the low levels and 700-500 mb lapse rates
near 7 c/km being conditionally unstable. Will mention a chance for
thunder south of the warm front from the Delaware-Maryland-Virginia to the greater
philly area where MUCAPE values between 750-1250 j/kg are forecast.
Farther north, limited thunder to slight chance with forecast MUCAPE
around 500 j/kg and with more cin for convection to overcome in the
cool sector. Even in our most southern zones that are deeper into
the warm sector, forecast soundings indicate instability being non-
surface based after sunset.

The upper low is forecast to cut off just north of the Great Lakes
midweek. The cold front is expected to move through the coastal middle-
Atlantic late Monday night into early Tuesday morning. Winds turn
westerly Post-frontal and colder/drier air advects into the area.
Trended slightly faster with an end to the precipitation. The forecast is
now dry for Tuesday except for some isolated showers/sprinkles in
the higher terrain of northeast PA/northwest New Jersey as the middle-level
shortwave trough pivots through. A zonal flow/neutral thermal advection
pattern will ensue Tuesday afternoon and night as the middle Atlantic
becomes situated between this low and high pressure over the
southeast coast. Seasonable temperatures are expected during this part of
the forecast.

Shortwave energy is forecast to dig southeastward toward the base of
the upper trough on Wednesday. Consensus from the 12z guidance show
precipitation expanding from the Midwest into the middle Atlantic Wednesday
afternoon and evening ahead of a cold front. The cold front is
currently forecast to move through the area late Wednesday night. The
12z GFS showed a more aggressive solution with respect to how
quickly the upper-level jet streak strengthens at the base of the
trough. In this scenario, the upper divergence in the left exit
region would enhance both lift over our area and coverage of precipitation.
However, there is still decent spread in the guidance with this
system, so probability of precipitation were only increased slightly from the previous
forecast.

The upper low should will eventually progress eastward into eastern
Canada/northern New England late this week and next weekend.
Cyclonic flow around the low will keep a cool airmass over the area
along with periods of stratocu. May even see snow showers in the
Poconos early Thursday. Temperatures will be below normal with highs each
day ranging from the upper 40s in the Poconos to near 60f in the
coastal plain Thursday through Sunday. Deep mixing will also yield
gusty west-northwest winds during the daytime hours. There is also a
potential for frost to develop at the some of cooler spots in
eastern PA and northwest New Jersey each night if winds in the boundary
layer are able to decouple and skies clear out. The growing season
starts for northeast PA (except Carbon and monroe) and northwest New Jersey
in the next couple of days, so frost advisories may be needed.

&&

Aviation /20z Sunday through Friday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg,
kilg, kmiv, kacy, and surrounding areas.

VFR conditions for the rest of the afternoon and into the early
evening. The deep Erly flow and the approaching warm front will
combine to produce a quick increase in moisture across the area this
evening and conditions will quickly deteriorate overnight. We expect
that conditions will go IFR after midnight in most areas and a 6 to
8 hour period of low conditions, rain and low level wind shear will be across the
terminals until 12z-14z Monday. This will have big impacts on travel
across the area Monday...especially phl Monday morning. A warm front
will cross the area Monday and conditions may improve behind it Monday
afternoon. Rather low confid in the forecast later Monday morning. Winds
will be Erly tonight and Monday morning and then veer southeast to S during
the afternoon.

Outlook...

Monday night...another round of rain showers and possibly thunderstorms and rain ahead of a
cold front. IFR conditions in heavier precipitation but we may also see
ceilings/visibilities try to improve to MVFR/VFR in stretches of no precipitation.

Tuesday and Tuesday night...VFR. West-northwest winds gust between 20 and 30
knots during the day and subside at night.

Wednesday and Wednesday night...chances for rain showers increase during the
afternoon and night ahead of a cold front. Predominately VFR though
localized MVFR restrictions possible in steadier precipitation.

Thursday...VFR. West-northwest will be gusty again.

&&

Marine...
winds and seas will continue to build on the waters tonight as low
pressure and a warm front approach. We have decided to raise the
rest of the Small Craft Advisory flag with the 330 PM coastal waters forecast issuance. Winds gusts will
begin to reach 20 to 25 knots this evening and increase overnight.
The GFS guidance was quite bullish with the winds with close to
gales overnight...but we will not completely buy this at this time and keep
the numbers a bit lower. The GFS seems to have a mesolow crossing
the area overnight. Rain/fog overnight with low visibilities possible. A
continuation of poor conditions Monday with winds and seas beginning
to subside late. The precipitation should become more showers in the
afternoon with thunderstorms possible. Areas of fog may linger with the
water temperatures remaining very cold and dew points over the waters quite
high.

Outlook...

Monday night...SW winds 15-20 knots in the evening will shift out from the
west-northwest overnight behind a cold front. Given SST in the upper 40s and
lower 50s and a warm the airmass above, the setup does not favor
much mixing of higher gusts aloft down to the surface. Even if gusts
to 25 knots will be infrequent, seas will still be 5-7 feet off the New Jersey
and Delaware coast. Therefore, a Small Craft Advisory is in effect.

Tuesday...Small Craft Advisory may be needed for Tuesday as deeper mixing on land
will yield gusty westerly winds of 20-25 knots near the coast. Meanwhile,
winds farther offshore will likely not see these gusts. Seas will
be around 4-6 feet in the morning, diminishing to 2-4 feet late in the
day and at night. Will hold off on a fourth period Small Craft Advisory in this
marginal setup.

Wednesday...SW winds increase to near Small Craft Advisory levels late Wednesday.

Thursday and Friday...Small Craft Advisory may be needed due to a gusty W-NW. Airmass
looks cold enough for gusty winds to be sustained even out to 20 nm
miles offshore.

&&

Fire weather...
relative humidity has increased gradually thruout the day has increased as a
result of onshore flow. The only locations with relative humidity below 30
percent in far northwest New Jersey and the Poconos but winds are light there.
Not expecting conditions to meet red flag warning criteria through
the remainder of the day.

&&

Tides/coastal flooding...
a strengthening southeast flow tonight combined with influence from
the recent new moon, may cause some minor coastal flooding to occur
with tonights high tide cycle in coastal Delaware and southern New
Jersey, mainly from Cape May County southward and up the along the
Delaware Bay and into the Delaware River. The guidance may still be
running a bit on the low side and we will continue to monitor the
situation and any need for a coastal Flood Advisory.

As a warm front lifts northward into the region Monday, the winds
will shift to more of a southerly direction. The winds across the
northern areas may remain more east or southeast ahead of the front
and this may increase the potential for some minor tidal flooding
from Atlantic City northward to Sandy Hook Monday morning.

&&

Phi watches/warnings/advisories...
PA...none.
New Jersey...none.
Delaware...none.
Maryland...none.
Marine...Small Craft Advisory until 6 am EDT Tuesday for anz452>455.
Small Craft Advisory until 6 am EDT Tuesday for anz450-451.
Small Craft Advisory until 10 am EDT Monday for anz430.
Small Craft Advisory until 10 am EDT Monday for anz431.

&&

$$
Synopsis...Klein
near term...O'Hara
short term...O'Hara
long term...Klein
aviation...Klein/O'Hara
marine...Klein/O'Hara
fire weather...Klein
tides/coastal flooding...meola

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