Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 181719
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
119 PM EDT Tue Apr 18 2017

.SYNOPSIS...
High pressure, centered over eastern Canada early this morning,
will build east and southward through today before shifting toward
the Canadian Maritimes tonight and Wednesday. A low pressure
system crossing the lower Great Lakes region on Thursday, will
move to southern New England Friday as it trails its cold front
through our area. Weak high pressure will spread to our north
on Saturday. Another low pressure system in the lower Ohio Valley
Sunday morning should move to the mid Atlantic coast by late
Monday.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
930 am update: Very nice day is on tap. Mostly sunny skies
prevail across the area with temperatures starting to warm
through the 50s. Forecast temperatures look reasonable today,
with max temperatures a few degrees above seasonal averages.

Main forecast issue today is the inland push of southeast winds
and modified air from the cool ocean waters. As the surface
ridge axis passes through later today, southeast winds will
become established near the coast, and a sea breeze will push
inland through the afternoon hours. High-resolution models are
reasonably consistent in pushing this boundary past the I-95
corridor by mid-to-late afternoon. With it will come a drop in
temperature of a few degrees and an increase in dew points.

Main changes to the forecast were stronger incorporation of the
hi-res guidance for temps/dews/winds today to depict the
evolution of this afternoon`s inland push of the sea breeze.
Otherwise, forecast is in good shape.

Previous discussion...

An upper-level trough is forecast to quickly exit the Northeast this
morning. Some ridging then quickly follows this afternoon, which
drives strong surface high pressure initially centered just north of
New England eastward by late in the day. This surface high extends
down the East Coast into the Carolinas.

The placement of the surface high will result in a low-level onshore
flow, initially northeast which then turns southeast this afternoon.
While the winds should be light overall especially farther inland,
some gustiness should occur through the day closer to the coast as
the boundary layer warming allows for increasing thermal
differential between the ocean and land. This will keep the coastal
communities much cooler and this cooling will spread inland at least
some this afternoon.

As the aforementioned upper-level trough shifts away from the
Northeast this morning, the accompanying upper-level jet will also
depart allowing any lingering higher level cloudiness to shift
offshore. There should be enough additional drying along with
increasing subsidence due to the incoming ridge to limit cloud
develop during the rest of the day. The airmass will be noticeably
drier today, with the exception closer to the coast due to an ocean
influence.

High temperatures are a MOS/continuity blend, then some of the high-
res guidance was blended into the hourly temperature grids to help
capture the influence of the ocean cooling for the eastern
zones.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM WEDNESDAY/...
A ridge axis quickly shifts to our east during the first half of
tonight, and the center of surface high pressure shifts more toward
the Canadian Maritimes. The surface high though still extends
southwestward into the Mid Atlantic region, allowing for a continued
onshore flow. As this all occurs, the mid level flow backs to
westerly while the low-level flow becomes more southwesterly just to
our west overnight. This allows for warm air advection to strengthen
to our west. This will also result in the increase of theta-e
advection. This is mainly focused to our south and west around the
southwestern periphery of the surface high. Some increase in clouds
are expected overnight from west to east, and while much of this
should be the mid to high level variety some guidance wants to bring
in areas of low clouds toward daybreak. This may be overdone given
the additional moistening that needs to take place, however warming
aloft beneath a cool boundary layer will result in a strengthening
inversion.

Much of the area should experience a quick cool down early in the
evening as the ocean influence expands inland. Low temperatures were
a MOS/continuity blend overall, however the hourly temperatures
included some high-res guidance to hopefully better capture the
cooling inland during the evening. Thereafter, radiational cooling
will occur before an increase in clouds arrive late.

&&

.LONG TERM /WEDNESDAY THROUGH MONDAY/...
500 mb:  Above normal heights over the mid Atlantic states Wednesday
will give way to a short wave passage across the northeast states
Friday. Ridging redevelops over the weekend ahead of a sharp
trough moving east from the lower Mississippi Valley. That trough
crosses the east coast early next week and it may be considerably
sharper and have a greater impact than the current progressive
suite of model solutions.

Temperatures: The first 17 days of April have averaged basically 6
to 7 degrees above normal. Calendar day averages Wednesday will
be near normal, about 8 to 12 degrees above normal Thursday and
Friday, slightly above Saturday. then normal or slightly below
Sunday and Monday.

Forecast basis: unless otherwise noted, a 50 50 blend of the 00z/18
GFS/NAM MOS Wednesday-Thursday except for temps where the warmer
GFS MAV guidance was applied and which was also a good match
for the GGEM; then GFS MEX MOS Thu night and Friday, and
finally, WPC 05z guidance Fri- next Monday.


The dailies...

Wednesday...small chance patchy IFR st/fog to start the day, otherwise
a partly sunny start gives way to overcast skies during mid or
late afternoon when scattered mid level sprinkles are possible.
Southeast wind.

Thursday...mild, possible patchy IFR st/fog to start the day.
Otherwise all day shower chances along and north of I-80 closer
to a stalled front north of our forecast area, while the
remainder of our forecast area to the south of I-80 could see a
period of showers and thunderstorms late at night ahead of the
approaching cold front. Southwest wind. Lows Thursday night 15
degrees above normal.

Friday...mild with any remaining showers moving off the coast during
midday.  Becoming Partly sunny during the afternoon. Southwest wind
shift to northwest, gusty to 20 mph. We have checked records for
Friday and with near 14C at 850 MB...its possible there will be
some upper 80s in s NJ. Our current fcst reflects lagging cloud
cover and sct showers ending...but should it clear sooner-
during the morning...our 330 am forecast temps for Friday could
be too cool by 6 degrees.


Saturday...For now we`re banking on high pressure dominating
but there is a chance that showers will be streaming eastward
from the Ohio Valley. Northwest to north wind.

Sunday-Monday...Showery rains and some of the rain could be
heavy. Looks like a wintry pressure pattern with an Ohio Valley
low redeveloping east of Virginia Monday and cold air damming
developing. For now, the system is modeled progressive so the
rain should tend to diminish or end later Monday. This may be
the beginning for a period of below normal temps that we`ll
need to keep the month out of the top 5 warmest Aprils on
record. It probably wont be enough to prevent a top 5 to 10
warmest. See climate section for more info on the month as a
whole.

&&

.AVIATION /17Z TUESDAY THROUGH SATURDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.

For the 18Z TAFs...General VFR conditions expected through
tomorrow morning. North/northeast winds should veer to southeast
and increase somewhat this afternoon as a sea breeze moves
northeastward from the coast. A few gusts 15-20 kts possible but
speeds primarily around 10 kts. General east-southeast winds
tonight below 10 kts with a gradual increase in mid-high clouds.
Mixed signals regarding patchy low stratus/CIG development late
tonight, but not confident enough to include in the TAFs at
this time. Southeast winds increase tomorrow with ceilings
gradually lowering to around 10k feet by afternoon.

OUTLOOK...
Wednesday...Patchy IFR st/fog possible to start, otherwise VFR
cigs develop midday. MVFR or IFR conds possible at night in
showers/st/fog. Southeast wind with max gusts 15-20 kt during
the day.

Thursday...Possible IFR st/fog to start, otherwise VFR cigs
variable brief IFR conditions in st/fog/a period or two of
showers and scattered thunderstorms. Southwest wind.

Friday...VFR cigs variable brief IFR conditions in st/fog/showers
ending during midday becoming VFR sct-bkn aoa 5000 ft during the
afternoon as a southwest wind in the morning shifts to
northwest...gusts 20 kt.

Friday night and Saturday...Mainly VFR conditions expected.
Northwest to north winds.

&&

.MARINE...
930 am update: Adjusted the wind forecast today to match buoy
observations this morning and to fine-tune timing of wind
directions changing to a more southeasterly direction by early
this afternoon. Still expecting sub-advisory winds through
tonight with fair weather.

Previous discussion...

Strong high pressure centered just north of the Great Lakes this
morning will shift into New England later today, then toward the
Canadian Maritimes tonight. This will result in a north to northeast
wind today, however shifting to the southeast this afternoon and
tonight. The winds will be gusty at times, however peak gusts look
to be about 20 knots. Seas will average 2-4 feet (1-3 feet on
Delaware Bay), and did raise wavewatch guidance a bit mainly across
the southern ocean waters given the initial northeast flow. As the
southeast flow occurs this afternoon and evening, some enhancement
may occur nearshore and up Delaware Bay due to the initial greater
temperature difference between the water and land. Overall, the
conditions are expected to be below Small Craft Advisory criteria
through tonight.

OUTLOOK...
44009 rts around 02z/17.

Wednesday...Mainly sub-advisory conditions on the area coastal
waters. East to southeast winds around 10 to 15 knots. Gusts may
be around 20 knots at times.

Thursday and Thursday night...Sub-advisory conditions expected. Seas
2 to 4 feet with south to southwest winds around 10 knots. Fog may
develop.

Friday...Seas will build on Friday and may be near 5 feet on
the ocean.

Saturday...Seas may be near 5 feet on the outer edges of the ocean
zones early but will be diminishing through the day.

&&

.CLIMATE...
We have checked records for Friday. Since it should be near 14C
at 850 MB...its possible there will be some upper 80s in s NJ.
Fridays record high for ACY is 84-1985. PHL 89-1976. Depending
on wind direction ACY may be within 2F of record.

April as a whole for PHL is averaging 6.2F above normal for the
first 17 days. Our 330 AM fcst plus the last 3 days of the GFS
2m temps and then normal temps on the 29th and 30th...yields a
positive departure of 4.2 degrees (58.2) or 4th warmest April
on record. To drop out of a top 10 warmest, this projection would
have to be 1.7 degrees too warm. Not likely to fall out of the
top 10 warmest Aprils on record.

For Allentown: April is projecting 5 degrees above normal or
54.9 degrees and that would be a #2 warmest in the POR dating
back to 1922. To drop out of the top 10, Allentown`s average
would have to be 2.3 degrees lower than our current fcst.

&&

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

&&

$$
Synopsis...Drag
Near Term...CMS/Gorse
Short Term...Gorse
Long Term...Drag
Aviation...CMS/Drag
Marine...CMS/Drag/Gorse
Climate...Drag



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