Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000
FXUS61 KPHI 270708
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
308 AM EDT Wed May 27 2020

.SYNOPSIS...
High pressure will remain centered well off the East Coast through
the end of the week. Meanwhile, a weak wave of low pressure will
track up the coast over the next couple of days and move through our
region on Thursday. Low pressure tracking through southern Canada
will eventually drag a slow moving cold front through the region by
Friday night or Saturday. Behind the front, a dry northwest flow
develops with strong high pressure gradually building in from late
in the weekend through early next week.

&&

.NEAR TERM /THROUGH TONIGHT/...
Fog is slowly creeping inland across central/southern New
Jersey and Delmarva, but it is definitely having a harder time
than last night. Additionally, observations of a quarter mile or
less are fairly sparse/transient thus far. There are a few
reasons this appears to be the case. First, winds are a little
stronger within the fog bank (generally south to southeast up to
7 mph or so). Second, high clouds are moving across the area,
which tends to inhibit substantive nocturnal radiational cooling
slightly. Third, the prevailing wind direction has less of an
onshore component than last night. As these three factors will
persist for the rest of the overnight hours, I updated the
forecast to depict a slower spread inland of the dense fog. For
now, I made no changes to the areal extent of the dense fog
advisory, but I suspect that the northwest fringe of the
advisory will have a difficult time verifying. Should these
trends continue, a trimming of the advisory will be required. I
did move up the expiration time to 10 am, as dense fog should
dissipate after 8 am or so, based on consensus of high-
resolution guidance.

The main forecast concern once the fog dissipates this morning
is the potential for showers to creep into the southern and
western CWA this afternoon and especially tonight as a weak
system off the Southeast coast early this morning moves
northward into the Mid-Atlantic tonight and Thursday. There has
been a significant shift westward with the track of the
perturbation and the associated surface reflection, an
indication of a stronger western Atlantic ridge. Large-scale
lift associated with the perturbation is compact (as is typical
with these weak tropical disturbances). As a result, there is a
clear signal for a drier solution in our CWA, especially for New
Jersey. Nevertheless, there are indications that weak midlevel
vorticity maxima will emanate from the main perturbation,
providing sufficient lift for convection in our region by
tonight.

Think the prospects for precipitation in our region today are
quite low, in general, so kept PoPs confined to slight to low-
chance categories and mainly southwest of a Reading to Lewes
line. Gradually increased PoPs during the overnight hours, but
kept a clear gradient from east to west. QPF continues to look
fairly light, but I remain concerned that any stronger
convective cores could produce locally heavy downpours given the
high-PW air moving into the region (up to 1.4-1.6 inches in
Delmarva by late evening, increasing to 1.5-1.75+ inches by 12z
Thursday). However, given the westward trend regarding the
track of the system, think the better chances of impactful heavy
rainfall will be west of the area.

Other concern through tonight is another round of low clouds and
fog after dark. Surface flow will maintain an onshore
component, and low-level moisture will continue to increase.
There are questions regarding precipitation coverage in the area
overnight, which would affect the formation of fog, so
confidence is lower regarding widespread/dense fog.
Nevertheless, the fairly persistent pattern would suggest fog
will be present during the overnight hours, so have continued
mention across the CWA.

Statistical guidance is quite variable with highs today, but
general thinking is to go near or slightly higher than
consensus given that even muted insolation will aid the ambient
warm/moist advection regime in warming most of the area to
values near 80 degrees. It will be much cooler near the coast,
with onshore flow persisting. Lows tonight will be very warm
(60s basically everywhere), as considerable cloud cover and
warm/moist advection will mitigate nocturnal cooling
substantially.

&&

.SHORT TERM /THURSDAY THROUGH THURSDAY NIGHT/...
The farther west track of the surface low into the Mid-Atlantic
on Thursday continues to have implications on the forecast
Thursday and Thursday night for our area. The 00z GFS looks
quite dry across the CWA, with scattered convection primarily
confined to areas west of the Delaware River (and limited QPF,
at best). The NAM and ECMWF are slightly wetter, apparently
owing to an eastward-displaced vort max lifting northward on the
fringe of the stronger large-scale lift associated with the
system. The ambient environment will be moist, but thermal
profiles exhibit poor lapse rates and meager instability for
most of the day. Profiles will improve somewhat during the
overnight hours, and there are indications of subtle
perturbations upstream of the main system. As such, think at
least chances of showers (and possibly a storm by Thursday
afternoon) will exist for most of the period. However, lack of
stronger lift suggests coverage will be sporadic and areal-
average QPF will likely be low, even in a fairly moist
environment. Although locally heavy downpours would be possible,
if not probable, with the stronger convective cores, this
threat appears relatively low at the moment given the
aforementioned limiting factors.

Generally have chance to likely PoPs for Thursday and Thursday
night (highest PoPs in western portions of the CWA). Despite the
broad-brush approach, not expecting a washout...much of the time
will be dry.

The temperature forecast remains tricky, owing to the unclear
evolution of precipitation across the area during this period.
Used a consensus raw/statistical model blend for this time
frame, with highs generally in the upper 70s to lower 80s
(cooler at the coast) and lows near 70 (given the continued
presence of considerable cloud cover and warm/moist advection).

&&

.LONG TERM /FRIDAY THROUGH TUESDAY/...
Overview...

A pattern change looms for the end of the week as a strong cold
front approaches the East Coast. A strong ridge over the East will
begin to weaken a little and shift further out to sea as the front
approaches. Meanwhile, a deepening trough behind the front will
settle over the Northeast for much of the weekend and into the start
of next week, bringing with it a cooler, drier air mass. Very much a
tale of two air masses for this period, with warm and humid
conditions Friday into Saturday, then a few days of cooler and much
drier air. In terms of sensible weather concerns, the main period to
watch will be Friday and Saturday ahead of and during the frontal
passage. Some potential will exist for strong storms and locally
heavy rain ahead of the front, but this will largely depend on
frontal timing, which is not yet well agreed upon by guidance.
Current indications are best potential for higher impact weather
will remain to our west, but still a couple days to keep an eye on
things.

Dailies...

Friday-Saturday night... At upper levels, a fairly common setup as
low pressure tracks well north of us across southern Canada,
trailing a cold front to its south. Strong offshore ridging will
help to slow the approach of the front and the troughing that lies
behind it. Eventually, the front will make it through the region as
ridging shifts further off to the east. Quite a bit of spread in the
guidance on when this front will come through, which will have a
significant impact on forecast details.

All in all, a little bit difficult to decipher how these days are
going to go. Friday seems the more obvious candidate for convection
given highs in the low 80s, dew points around 70, and PWAT values of
a rich 1.7 to 2.0 inches as moisture streams in ahead of the front.
And yet, am concerned there just won`t be much of a trigger with the
front so far west. Even a potential pre-frontal trough may be too
far west to do much good for us. Most guidance indicates some light
QPF over most of the area Friday, possibly trying to hint at some
low topped pop up showers in the tropical air mass. So kept PoPs
mainly high end chance to likely, but am not too enthused about
heavy rain or severe weather during the day Friday. Best chance for
that will probably be west of us with the better forcing. The 27.0z
NAM for example shows a rather favorable environment for severe
weather over interior PA late Friday in association with a pre-
frontal trough. Overnight, better dynamics shift closer to us but we
lose instability. So a shower/storm risk will continue but not sold
on much severe weather potential. Best chance for any stronger
storms should be in the western portion of the area during the first
half of Friday night. Given the high PWATs and wind profiles which
favor training storms, areas where the best convective dynamics set
up will have to be monitored for a heavy rain and flash flood threat
as well. Similarly, feel the greatest risk for this will also be to
our west, but will monitor.

Saturday is a challenge as well. If the front is fast enough, it may
be a mainly dry day or with just a few showers as the mid levels
start to dry out. Both the latest GFS and EC would favor this.
Meanwhile, the slower timing indicated by the latest GEM and UKMET
would favor a better chance for some showers and storms, especially
to the east. Kept the forecast as a compromise for now with PoPs
mainly in the chance range, highest to the east. By Saturday night,
should be post-frontal with a cooling and drying trend commencing as
northwest flow develops. Overall, an unsettled stretch Friday and
Saturday but still some work to do in terms of ironing out details.

Sunday-Tuesday... Behind the front, an upper trough settles overhead
as it continues to amplify. At the surface, high pressure gradually
builds in out of the Midwest. Certainly a much different air mass
than the one that preceded it as PWAT values plummet in the
northwest flow regime. A model consensus has 850mb temperatures
bottoming out near 0C by Monday and generally 5C or less most of
this period. A cool and dry Canadian air mass, though given the
time of year hardly anything troublesome in terms of the "cool".
All three days look pretty similar with highs mainly in the low
70s, perhaps slightly cooler Monday. Overnight lows mostly 40s
and low 50s. Cannot rule out a sprinkle to the north on Monday
as a shortwave moves through, but lack of moisture should
largely preclude this and this looks like a dry set of days. So
this should be a nice stretch overall especially for those
looking to keep summer`s heat and humidity on pause. Rain
chances may go back up towards midweek as a warm front sets up
near or south of the region.

&&

.AVIATION /07Z WEDNESDAY THROUGH SUNDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.

Rest of tonight...Low clouds/fog will gradually creep westward
toward the Philly terminals, with quick deterioration to LIFR
likely. Main question will be VSBYs, as there are some
indications that low CIGs will be the primary impact (especially
at PHL). LIFR CIGs/VSBYs should persist the rest of the night
at MIV/ACY. For RDG/ABE, MVFR/IFR CIGs are expected to move in
after 09z, but VSBYs may remain VFR or possibly MVFR. Winds
light south or southeast. High confidence for MIV/ACY/ILG; low
confidence for PHL/PNE/TTN; moderate confidence for RDG/ABE.

Wednesday...Residual sub-VFR conditions are expected to improve
to VFR/MVFR by late morning and predominantly VFR by afternoon.
Light southeast winds. Moderate confidence.

Wednesday night...Deteriorating conditions are again expected,
with CIGs/VSBYs becoming sub-VFR during the evening and likely
continuing through the rest of the night. Showers possible,
especially west of PHL. Light southeast winds. Moderate
confidence.

Outlook...

Thursday and Thursday night...Restrictions possible at times,
with the highest likelihood during the early morning hours and
the overnight hours. However, a period of VFR is probable
Thursday afternoon. Chances for showers and even a storm, though
coverage and timing are big question marks. South to southeast
winds 5 to 15 kt. Low confidence.

Friday-Saturday night... Periods of sub-VFR conditions likely due to
low clouds and areas of showers or thunderstorms. Winds mainly from
the south or southwest at 5 to 15 kt from Friday into Saturday
morning. A shift to westerly then northwesterly is expected Saturday
with speeds mainly less than 10 kt. Low confidence on the timing and
extent of restrictions, as there may be a wide variety of conditions
both by time and location through this period. Better confidence of
conditions returning to VFR by Saturday night.

Sunday-Monday... VFR. Winds mainly from the northwest at 10 to 15
kt, afternoon sea breezes possible towards the coast. High
confidence.

&&

.MARINE...
Sub-advisory winds/seas are expected through tonight.

Dense fog will continue through this morning, with an advisory
in effect until 11 am. Conditions may only improve slightly
during the late morning and afternoon, with VSBYs perhaps
staying between 1 and 3 nautical miles most of the day offshore.
Another round of denser fog is probable Wednesday night.

Outlook...

Thursday and Thursday night...Marginal advisory conditions are
possible as seas build to around 5 feet by late Thursday
morning. Southeast to south winds of 10 to 20 kt are probable,
but not expecting gusts frequently exceeding advisory criteria.
Owing to the marginal nature of the threat, have not issued a
small craft advisory yet. Fog is once again possible, especially
Thursday morning.

Friday-Saturday... SCA conditions possible mainly on the Atlantic
coastal waters due to seas running 4 to 6 ft. South-southwesterly
winds may gust to near 25 kt on Friday and Friday night before
decreasing on Saturday and shifting to northwest Saturday night.
Conditions should subside below SCA levels by Saturday night.

Sunday-Monday... Sub-SCA conditions are expected.

Rip Currents...
There is a low risk for the development of dangerous rip
currents on Wednesday. For Thursday, the risk may increase to
moderate.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...Dense Fog Advisory until 10 AM EDT this morning for PAZ070-071-
     101>106.
NJ...Dense Fog Advisory until 10 AM EDT this morning for NJZ009-010-
     012>027.
DE...Dense Fog Advisory until 10 AM EDT this morning for DEZ001>004.
MD...Dense Fog Advisory until 10 AM EDT this morning for MDZ008-012-
     015-019-020.
MARINE...Dense Fog Advisory until 11 AM EDT this morning for ANZ430-431-
     450>455.

&&

$$
Synopsis...O`Brien
Near Term...CMS
Short Term...CMS
Long Term...O`Brien
Aviation...CMS/O`Brien
Marine...CMS/O`Brien


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