Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000
FXUS61 KPHI 182203
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
603 PM EDT Wed Oct 18 2017

.SYNOPSIS...
High pressure remains in control of the weather for much of the
rest of the week into the weekend. A dry frontal boundary may
move into the area Thursday into Friday. A front and low pressure
moving along it will bring unsettled conditions for later Monday
into Wednesday.

&&

.NEAR TERM /UNTIL 6 AM THURSDAY MORNING/...
High pressure remains over the mid-Atlantic region. However, this
high will undergo gradual weakening tonight as an upstream shortwave
trough approaches from the Ohio Valley.

This upper shortwave trough will be associated with increasing high
clouds responsible prior to its arrival. Do not anticipate these
clouds to have a significant impact on radiational cooling given the
high base of the cirrus. Accordingly, temperatures should drop
rather quickly this evening once the boundary layer decouples and
winds become calm. Compared to recent nights, higher dewpoints
tonight will (1) raise the floor for how low temperatures can bottom
out, (2) inhibit frost formation and (3) increase the risk for
radiational fog. At this point, am skeptical to think fog will be
much more than patchy in coverage and shallow in depth (mainly
ground fog). Forecast lows range from the upper 30s/lower 40s in the
sheltered valleys of NE PA/NW NJ and the NJ Pine Barrens to the
lower 50s for the urban I-95 cities and along the coast.

&&

.SHORT TERM /6 AM THURSDAY MORNING THROUGH 6 PM THURSDAY/...
High pressure sinks southward toward the Southeast U.S. on Thursday.
Modest pressure (height) falls at the surface (aloft) occurs across
the northern mid-Atlantic region as a weak upper shortwave trough
and surface cold front progress eastward.

The aforementioned cold front will trail the upper trough enough to
remain upstream of the forecast area through the period. The setup
will allow us to remain situated in the pre-frontal warm sector and
for this warming trend to continue into tomorrow. Sided closer to
the warmer MAV guidance for temperatures, yielding forecast highs in
the lower to middle 70s (except cooler in the higher elevations of
NE PA-NW NJ).

Filtered sunshine with high clouds during the morning will give way
to sunny skies by the afternoon. Meanwhile, SW winds 5-10 mph in the
morning are forecast to increase to 10-15 mph by afternoon.

&&

.LONG TERM /THURSDAY NIGHT THROUGH WEDNESDAY/...
Thursday night thru Monday...High pressure at the surface and
aloft will be in control of the weather with fair weather
expected. Temperatures generally 5 to 10 degrees above normal.

Monday night thru Wednesday...A pattern change with a sharp h5
trough moving into the ern part of the country. Low pressure
and a slow moving front associated with the upper trough will
affect the area with occasional rains. Locally heavy rains are
possible with an upper trough beginning to take a negative tilt
as it crosses the area. The 12Z NA models are showing QPF
totals generally in the 1 to 3 inch range across the area for
the period Mon_Wed. The higher totals are across the wrn
counties. Much colder temperatures will be ushered in behind
the system with below normal temperatures for the middle part of
next week.

&&

.AVIATION /21Z WEDNESDAY THROUGH MONDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.

Generally VFR Thursday. However, patchy ground fog may develop at
the fog-prone, rural TAF sites between 05Z-11Z Thursday which could
result in a brief period of MVFR visibility. Confidence in fog
development was too low to mention in 18Z TAFs except at MIV.

SW winds 5-8 kt this afternoon will diminish to 5 KT or less after
sunset this evening. SW winds continue on Thursday, gradually
increasing to from 5-8 kt in the morning to 8-15 kt in the
afternoon.

OUTLOOK...
Thu night thru Monday...VFR expected thru the period.

&&

.MARINE...
Winds this afternoon are generally from the W-SW between 5 and 10
kt. However, winds are backed locally out of the S in the nearshore
NJ waters owing to the formation of a sea breeze front. Winds may
briefly increase to 10-15 kt late this afternoon and early evening a
few miles farther off the NJ coast in association with an Ambrose
jet.

SW winds 5-10 kt will continue into tonight and Thursday morning,
then increase 10-15 kt across the coastal Atlantic waters during the
afternoon. Wind field may become locally enhanced to 15-20 kt off
the coast of Ocean and Monmouth Counties via Ambrose jet late in the
day. Held off on a SCA at this juncture since poor mixing profiles
should inhibit higher winds aloft to reach the surface. Accordingly,
capped gusts to below 25 kt.

Seas will generally be around 3 ft in the coastal waters and 2 ft or
less in the Delaware Bay.

OUTLOOK...
Thu night...Near SCA winds across the nrn NJ coastal waters Thu
    evening, but not confid enough for a flag attm. Fair.
Friday thru Sunday...Sub- SCA conditions thru the period with
    fair weather.

&&

.CLIMATE...
**Top 5 warmest October on record for PHL and ABE appears likely
  and virtually certain top 10 for all of our forecast area**

Running todays max/min plus the Mount Holly 330 PM fcst for the
next 7 days, adding on day8-11 from FTPRHA GFS 2m max/min temps
and then normal the last 3 days of the month results in the
following.

PHL 63.5 degrees or 6 degrees above the October 30 year avg of
57.5. This again pegs PHL for a tie as #2 warmest October on
record.

1. 64.5 2007

2. 63.5 1971 (2017?)

3. 62.7 1947

To drop out of the top 10 warmest the PHL avg would have to lower
to 61.2 degrees (a 2.4 degree drop from the current projection).
Based on the 12z/18 ensembles both GEFS/NAEFS and ECEFS...this
will not happen unless todays ensembles completely miss the last
3 days of the month. So while a record is probably out of reach
top 5 is likely.

ABE 59.5 or 7 degrees above the 30 year avg of 52.5. This also
ties for #2.

1. 60.8 2007

2. 59.5 1984 and possibly 2017

3. 59.3 1947

4. 58.8 1971

For Allentown to drop out of the top 10, the projected avg
would have to lower to 57.2 or a drop of 2.3 degrees from the
current projection. Again, thats highly unlikely unless
something very unusual and not currently ensembled occurs the
last 3 days of the month.

It should be pointed out that the D8-14 fcst from the Climate
Prediction Center (CPC) .. is for above normal temperatures for
the last week of the month. We are on the edge of the transition
to cold that will be quite noticeable in the nations midsection
next week. Uphill (southwest flow at 500MB and the trough axis
constantly Appalachians or west) makes it difficult for cold
air to cross the mountains into our area.

Now a moment to share some information from a new tool recently
available to us...

The last two years in PHL 10/15-10/17/17 is the warmest in the
period of record...averaging 3 degrees above normal.

Ditto Allentown,,averaging 3.7 degrees above normal!

Ditto ILG...averaging 2.5 above normal.

We will look at all of this again either Saturday or Sunday
morning.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...None.

&&

$$
Synopsis...O`Hara
Near Term...Klein
Short Term...Klein
Long Term...O`Hara
Aviation...Klein/O`Hara
Marine...Klein/O`Hara
Climate...Drag 602



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