Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 171547

Area Forecast Discussion
National Weather Service Mount Holly NJ
1147 AM EDT Mon May 17 2021

High pressure will remain near the Mid Atlantic region through
much of the upcoming week, before retreating westward Friday
into next weekend. Several weak surface troughs will also be
near the Mid Atlantic and northeast states during the week as
well. A stronger trough or backdoor cold front may impact the
area next weekend.


One last short wave trough embedded in the northwest flow will
cross our region later today as the larger, broad trough
progresses further away. Isolated showers are possible this
afternoon north and west of the I95 corridor as the shortwave
trough arrives. However moisture remains limited (precipitable
water values remain well below normal) and mid level lapse rates
are weak. Consequently, showers should be limited in coverage.
As mentioned by the previous shift, can`t rule out one or two
thunderstorms, but CAPE values should be mostly below 100 J/kg,
so will only mention showers.

As the said broad trough moves away, 1000-500 mb thicknesses
will increase. Consequently, expect afternoon high temperatures
to be slightly higher than yesterday, with highs mostly in the
70s except along the coast and in the higher terrain of the
southern Poconos and NW NJ.

As the mid level ridge begins to build in earnest tonight,
expect clearing skies. That, combined with the light winds means
efficient radiational cooling conditions. The main limitation
with lows tonight will be dew points which should increase ever
so slightly with the low level southerly flow. Consequently,
expect lows mostly in the lower 50s, though some locations in
the Pine Barrens and the southern Poconos/NW NJ could get into
the upper 40s. The other factor to consider with this
radiational cooling is the potential for fog as dew point
depressions will be quite low. For now, have mainly kept the
mention of fog across the Pine Barrens and northern Delmarva
which has a higher chance thanks to weak onshore flow during the


High pressure will remain in place across the Mid Atlantic
region for Tuesday through Wednesday. Meanwhile, ridging
strengthens to our west. This will allow thicknesses to begin to
build, and a warming trend will begin by Wednesday. While PW
values increase to 0.75-1.00 inches and several short
wave/vorticity impulses will move across the area within the
northwest flow aloft, there will likely be enough mid-level
capping and dry air to prevent showers from developing, keeping
a dry forecast.


High pressure across the southern portions of the Mid Atlantic
region Thursday will begin to drift westward Friday and into
next weekend. Meanwhile, ridging aloft continues to strengthen
to our west and build slightly into the Mid Atlantic region.
This will allow thicknesses to continue to build through the end
of the week into next weekend, continuing the warming trend.
The only wrench that could be thrown into the forecast and mess
up the warming trend is the potential for a stronger surface
trough or backdoor cold front to move across the area next
weekend. If this happens, it could keep the temperatures down,
especially if there is any precipitation around the area. We
stayed close to the NBM forecast, however, we do note there is
the possibility we could end up a few degrees lower than
currently forecast for next weekend. PW values continue to
increase through the end of the week into the weekend, reaching
1.00-1.50 inches. For Friday and through the weekend, there will
be several short wave/vorticity impulses moving across the area
as the ridge aloft shifts to our west. With the potential for a
couple of surface troughs and backdoor front to affect the area
Friday through the weekend, there will be a chance for showers
and thunderstorms across much of the area during this period.


The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Today...VFR conditions expected. A isolated showers possible in
the afternoon, but even if these showers move over a TAF site,
do not expect visibility or ceiling restrictions. Light and
variable winds becoming southwesterly 5 to 10 kt for most TAF
sites. However at KMIV and KACY, expect southeasterly winds 5 to
10 kt. High confidence.

Tonight...Mostly VFR conditions expected. Patchy fog may develop
after 06Z with visibility restrictions lowering to MVFR (brief
IFR conditions may be possible too). At this point, KMIV and
KACY have the highest risk, so it is only mentioned in those
TAFs for now. Winds starting southwesterly or southeasterly
around 5 kt, but may become light and variable after 06Z.
Moderate confidence on most aspects of the forecast, but low
confidence on the potential for fog.


Tuesday-Wednesday night...VFR conditions continue. West to
southwest winds 5-10 knots during the day, and mostly light and
variable overnight. High confidence.

Thursday-Thursday night...VFR conditions continue. Winds will
vary from southeast closer to the coast to southwest inland,
speeds 5-10 knots during the day, then light and variable
overnight. High confidence.

Friday...Mostly VFR. Slight chance of showers for portions of
eastern Pennsylvania and northern and central New Jersey. Winds
will vary from southeast closer to the coast to southwest
inland, speeds 5-10 knots during the day.


Winds and seas should stay below Small Craft Advisory criteria
through tonight. Patchy fog is possible late tonight, primarily
for near shore waters of Delaware and southern New Jersey.


Tuesday-Friday...Winds and waves will remain below advisory
levels through the period. However, winds may gust around 20
knots Tuesday and Tuesday night, while seas may linger near 4
feet Tuesday through Wednesday.


For the next several days, afternoon minimum relative humidity
values are expected to be near or below 30 percent. However winds
should be light through this period which should limit potential for
rapid fire growth.




Near Term...Johnson/Kruzdlo
Short Term...Johnson
Long Term...Robertson
Fire Weather...WFO PHI is the U.S. government's official web portal to all federal, state and local government web resources and services.