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000 
FXUS61 KPHI 261038
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
638 AM EDT Tue Sep 26 2017

.SYNOPSIS...
Hurricane Maria will move slowly northward off the coast of the 
Outer Banks through Wednesday before turning sharply east-
northeastward into the open Atlantic through the end of the week. A 
cold front will move through the Mid-Atlantic on Thursday, and a 
reinforcing front will progress through the area Friday and Friday 
night. Cool high pressure will settle into the Northeast this 
weekend through at least early next week.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
A stratus deck, along with patchy fog, continues over most of
the area. There may be an hour or so after sunrise where the fog
briefly increases and becomes thicker, but should otherwise
dissipate shortly thereafter for most along and west of the 
I-95 corridor. Closer to the coasts, however, due to the 
onshore influence, it may take until the late morning hours for 
the fog and stratus near the coasts and over the coastal plain 
to dissipate. 

High pressure remains over the area, and slowly lifts to the 
north and east today. Meanwhile, Hurricane Maria is about 210 
miles SE of Cape Hatteras, and will track to the north parallel 
to the Carolina coast today. As it does, NE winds will increase 
to 15-20 MPH with gusts up to 25 MPH along the NJ and DE coasts.
Inland, NE winds will generally range from 5-10 MPH. As such, a
cooler airmass spreads into the region, and highs today will be
about 5-7 degrees cooler than Monday. The coolest temperatures 
will be across the coastal plain of NJ and into most of DE and 
NE MD, where highs will range from the mid 70s to around 80. 
West of the I-95 corridor, though, highs will climb into the mid
and upper 80s. Exception will be the Poconos, which will be in 
the upper 70s to around 80.

Some outer rain bands associated with Maria will lift to the 
north, and some light showers may make it into parts of DE/NE 
MD/extreme southern NJ, but the high should be strong enough to 
keep the bulk of the showers suppressed to the south. Any rain 
that does fall will only result in light QPF amounts.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM WEDNESDAY/...
Low clouds and patchy fog will develop once again across the 
coastal plain and the Delmarva late tonight. NE winds will 
diminish to 5-10 MPH along the coasts and to 5 MPH or less 
inland. Meanwhile, Maria will begin to weaken to a Tropical 
Storm as it begins to sharply curve out to sea. A few outer rain
bands may move into southern portions of the forecast area, but
not expecting much, if any, QPF. Lows tonight generally in the 
low 60s to the north and in the upper 60s to around 70 to the 
south.

&&

.LONG TERM /WEDNESDAY THROUGH MONDAY/...
Main forecast concerns for the long-term period are
precipitation chances Wednesday and again Friday night and
Saturday and a sharp downward trend with temperatures versus
what has been observed recently.

Analysis of the 00Z operational model suite suggests that the
overall large-scale pattern is decently agreed upon, but rather
large discrepancies exist with the smaller-scale vorticity
maxima that will be progressing through and/or affecting our
region through the period. Additionally, poor run-to-run model
continuity continues, owing largely to uncertainty with the flow
pattern in the northern Pacific and the apparent substantial
influences downstream.

At 12Z Wednesday, Maria will be off the coast of the Outer
Banks. A potent vort max will be progressing through Ontario
with southwesterly midlevel flow between the two. Although the
main impacts from Maria will be well south/east of the area, the
pressure gradient will be tightening in the southern CWA, so
expect breezy conditions to develop in southern Delaware and far
southeast New Jersey. Northeasterly surface flow should keep the
area socked in clouds much of the day, with only gradual
improvement from northwest to southeast. Some showers are
possible, especially near the coast, as Maria makes its closest
approach to the area. However, a considerable portion of the 
model output is dry through Wednesday night. The exceptions are 
the 00Z CMC/ECMWF, which indicate some QPF close to the coast. 
The totals from the CMC look overdone, but the geographic 
agreement with the ECMWF suggests that precipitation chances 
should remain. I did lower them considerably northwest of I-95,
where there is little remaining evidence of sufficient lift for
showers. One noteworthy development: the 00Z ECMWF is 
considerably slower and continues on the west side of the 
guidance with the track of Maria. By Thursday, it becomes an 
outlier with the position of Maria, so I neglected to give the 
new simulation strong weighting in the forecast.

As the Ontario trough moves rapidly east-northeast into the
Canadian Maritimes on Thursday, Maria should begin the eastward
turn. The ECMWF is weakest with this trough and considerably
more sluggish in advancing Maria farther offshore - this does
lower confidence in Thursday's forecast a little bit. A surface
low will develop and progress rapidly eastward through Quebec 
and New Brunswick, with a cold front sweeping across the Eastern
Seaboard Thursday. Cooler and noticeably drier air will seep 
into the Mid-Atlantic upstream of the front, with forecast highs
Thursday around 10 degrees colder in the urban corridor and up 
to 20 degrees colder in the southern Poconos. The change should 
be less severe near the coast (which will be cooler on Wednesday
given the onshore flow). Kept most of Thursday dry given the 
subsidence upstream of the front.

An upstream shortwave trough digs southeastward into the Great
Lakes Thursday night, and this is where much of the forecast
uncertainty lies. The GFS is much more progressive with this
trough than the CMC and especially the ECMWF. The GFS has
trended stronger with the first trough in eastern Canada and
weaker with the second, which does not agree with the recent
trends of the NAM or ECMWF. Additionally, there is very large
variation in the upstream flow run-to-run with the GFS. The
stronger agreement between the CMC/ECMWF made me lower the
weighting of GFS-based guidance Friday onward, and I may not
have done so enough.

The main effects on the forecast are with precipitation chances
Friday night and Saturday, with the GFS about 6-12 hours faster
in general (more Friday night versus early Saturday for the 
CMC/ECMWF). For now, kept the highest PoPs late Friday night
(a compromise of the two camps), but my suspicion is that PoPs
need to linger into a portion of Saturday, especially for the
eastern zones. Trended this direction with the forecast, but
expect further (perhaps substantial) changes with PoPs during
this period.

Cool/dry high pressure moves in thereafter, with a prolonged 
period of near to slightly below seasonal temperatures Saturday 
onward. Models have large phase/amplitude variations with the 
midlevel ridge developing in the East during this period, so the
duration of the cool/dry spell is a little uncertain at this 
point.

&&

.AVIATION /12Z TUESDAY THROUGH SATURDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, 
KILG, KMIV, KACY and surrounding areas.

Fog and stratus producing LIFR conditions at KACY/KMIV will be 
slow to dissipate, and conditions will probably not lift to VFR 
until midday or so. Otherwise, IFR conditions in patchy fog and 
stratus along and east of I-95 will improve to VFR shortly after
sunrise.

VFR west of I-95 through the TAF period.

Fog/stratus looks to redevelop late tonight with sub-VFR conditions 
likely once again.

E-NE winds will increase to 5-10 KT, except at KACY, where E-NE 
winds will increase to 10-20 KT.

LGT/VRB winds tonight.

Outlook... 

Wednesday: MVFR/IFR conditions possible, especially from the
urban corridor eastward, with gradual improvement to VFR from
northwest to southeast during the day. Some showers possible at
KMIV/KACY. Northeast winds 5 to 15 kts with stronger gusts near
the coast (up to 25 kts or so). Confidence below average.

Wednesday night and Thursday: Winds should become 
north/northwest as a cold front sweeps through the region. May
see some lingering showers/low clouds at KMIV/KACY Wednesday
night. Speeds should reach 10 to 20 kts on Thursday, with higher
gusts possible. Confidence average.

Thursday night and Friday: Predominantly VFR. Northwest winds 5
to 15 kts. Confidence above average.

Friday night and Saturday: Showers and localized sub-VFR
CIGs/VSBYs possible. Winds generally north or northwest around
10 kts. Confidence well below average.

&&

.MARINE...
Small Craft Advisory continues today and tonight for the ocean 
waters. Seas on the ocean will build to 9-11 feet. NE winds will
range from 15-20 KT with gusts up to 25 KT for southern NJ and 
DE ocean waters. Otherwise, NE winds range from 10-15 KT.

With these increasing offshore seas, can expect breaking waves at 
the shores of around 7 feet at southern NJ and DE beaches. A high 
surf advisory has been issued for tonight through Wednesday for 
southern NJ and DE coastal areas.

Outlook...

Wednesday: Northeast winds with gusts above 25 kts likely in the
southern NJ/DE coastal Atlantic, trending lighter in Delaware
Bay and the northern/central marine zones. Seas near 10 feet in
the Atlantic, with rough surf likely. A high surf advisory
continues for the southern NJ/Delaware beaches. Showers
possible.

Wednesday night: Gradual decrease in winds, though advisory-
level gusts still possible south of Atlantic City. Seas will be
slow to decline. A chance of showers.

Thursday and Thursday night: Cold frontal passage will increase
winds to advisory level in the Atlantic waters and potentially 
Delaware Bay. Seas well above advisory criteria.

Friday: Seas nearing but likely still above 5 feet, but winds
should diminish below advisory criteria.

Friday night and Saturday: Sub-advisory conditions expected, but
a chance of showers.

Rip Currents...

Due to swells emanating from Hurricane Maria, there continues to be 
a HIGH risk for the development of dangerous and life threatening 
rip currents for the ocean beaches today and tonight.

The rip current risk will likely be in the high category Wednesday 
and Thursday as long-period southeasterly swells from Maria 
continue. Swell heights around or above 10 feet on Wednesday will 
likely generate high surf/beach erosion issues for much of the surf 
zone but especially for southern New Jersey and Delaware.
Overall, not a good time to spend at the beach the next few
days.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...High Rip Current Risk through Wednesday afternoon for NJZ014-
     024>026.
     High Surf Advisory from 6 PM this evening to 6 PM EDT 
     Wednesday for NJZ024-025.
DE...High Rip Current Risk through Wednesday afternoon for DEZ004.
     High Surf Advisory from 6 PM this evening to 6 PM EDT 
     Wednesday for DEZ004.
MD...None.
MARINE...Small Craft Advisory until 6 AM EDT Thursday for ANZ452>455.
     Small Craft Advisory for hazardous seas until 6 AM EDT 
     Thursday for ANZ450-451.

&&

$$
Synopsis...CMS
Near Term...MPS
Short Term...MPS

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