Area Forecast Discussion
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FXUS61 KPHI 231419
Area Forecast Discussion
National Weather Service Mount Holly NJ
1019 AM EDT Sun Apr 23 2017
Low pressure will deepen along an stationary front over the
Southeast U.S. while high pressure builds into the Mid Atlantic
today. Low pressure will reach the Southeast coast Monday and then
track slowly up the eastern seaboard Monday through Wednesday. A
cold front will approach from the west Thursday before moving
through the region Thursday night or Friday. This boundary may
return back northward as a warm front early next weekend while high
pressure builds to our north.
.NEAR TERM /UNTIL 6 PM THIS EVENING/...
Not a whole lot to change with the forecast. Weak high pressure
will briefly build across the area through today, then offshore
later today. Dry air in the low-mid levels is will continue to
move southward across much of the area today. Portions of
southern New Jersey and central/southern Delmarva may hold on to
the mid-level moisture through the day and keep more widespread
cloudiness. The rest of the area should only see high level
clouds through this afternoon.
Only made minor changes to the forecast highs today. Where more
sunshine occurs across the northern two thirds of the area,
temperatures are expected to warm more than the southern third
where more cloud cover will linger into the afternoon. Either
way, a nice day is expected.
.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM MONDAY/...
The upper low digs farther southeast into Georgia on Sunday night
while acquiring a slight negative tilt. An attendant surface low
will progress to near the Southeast Coast by 12Z Monday, with rapid
downstream ridge amplification continuing in highly difluent flow.
With a downstream jet streak off the coast of the Mid-Atlantic,
upper-level divergence will generate a region of broad, deep ascent
along and east of the Appalachians. Areas of rain/showers will
develop and slowly move northeastward, impinging upon the Delmarva
Peninsula during the night. The process will be slow given the
snail`s pace of the upper low, but current thinking is precipitation
should begin moving into the Eastern Shore of Maryland and Delaware
near or after midnight. However, there will be a pronounced dry
layer in the low and midlevels that will need to become saturated
before steadier precipitation begins, so measurable rainfall will
likely be slow to materialize during the night. For now, kept PoPs
fairly low or unmentionable north of the Mason-Dixon Line with a
sharp increase upward to the far southern portions of Delaware and
Maryland. QPF during this time should remain around or below a tenth
of an inch.
Low temperatures will generally be in the 40s, with the lowest
readings likely in the north where nocturnal radiational cooling
will be most pronounced.
.LONG TERM /MONDAY THROUGH SATURDAY/...
A cutoff low will drift eastward over the Southeast states on
Monday. A downstream ridge block over the western Atlantic Ocean
should steer the low northward up the eastern seaboard through
midweek. The predictability challenges that are commonplace with the
formation and decay of cutoff lows are at play with this upcoming
one. Models show varying solutions regarding how quickly the cutoff
low opens up, which will influence the track (both location and
timing) of the low as it turns northward up the coast. Although the
first half of the week is shaping up to be damp for much of the Mid
Atlantic, there are still timing details related to the onset,
duration and end of rain that cannot be predicted with very high
confidence at this juncture. A subtle trend has been seen in the
guidance over the past few days that corresponds to keeping the low
cutoff a bit longer on Monday along the Southeast coast. This trend
signals a slower northward expansion of the precipitation shield. NE
PA and N NJ could potentially remain dry with the surface ridge in
place during the day on Monday when locations in SE PA, S NJ and
Delmarva see light rain. Rain is currently forecast to be greatest
in coverage (60 POP in far NW zones to 90 POP in SE zones) and
intensity (0.5-1.0 inch) from overnight Monday night through Tuesday
evening, when our region becomes positioned near the nose of a
cyclonically-curved low-level jet around the northern side of the
low circulation. Assuming the low remains offshore, rainfall rates
will be modest since the region would be situated within the stable,
cool sector of the system. However, higher rainfall rates and
locally moderate to perhaps heavy rainfall would be a possibility if
the low winds up tracking inland, allowing a coastal front to
progress onshore. High temperatures will be below normal both Monday
and Tuesday owing to the persistent onshore flow and overcast skies.
However, temperatures north of I-78 may be able to get very close to
normal on Monday since these northern zones will likely see a later
arrival of the denser cloud shield.
We will continue to advertise conditions drying out during the day
Wednesday but this is subject to change depending on how quickly the
low moving up the eastern seaboard reaches our latitude. There is a
slight concern that the low continues to trend slower, extending the
rain into Wednesday. Since it doesn`t seem to be the most likely
scenario at this point, we will also continue to forecast clearing
from SW to NE during the afternoon and temperatures reaching the
lower 70s in Delmarva and mid to upper 60s in E PA and NJ.
Low pressure is forecast to cut well to our NW across the Great
Lakes, placing the Mid Atlantic in the warm sector ahead of a cold
front for the end of the week. Subsidence underneath the upper ridge
over the area will favor strong daytime heating that should allow
high temperatures to rise into the 80s across most of the area each
afternoon Thursday and Friday. An onshore component to the flow will
yield notably cooler conditions (60s-lower 70s) at the shore.
Models have trended later with the arrival of the cold front
compared to runs 24 hours ago. Accordingly, a mention of showers and
thunderstorms with the front was delayed until Thursday night but
PoPs were kept low for Thursday night since timing of convection
does not coincide favorably with diurnal cycle. Slight chance PoPs
were extended into Friday with the possibility of the cold front
still in the process of moving thru.
Yet another storm system is forecast to organize over the Great
Plains heading into next weekend. Southerly flow ahead of this
system may allow the cold front that previously moved through the
region to return back northward as a warm front. However, the front
may struggle to advance northward with high pressure anchored to our
north. Temperatures and our sensible weather for Saturday will
greatly depend on the position of the warm front. Highs in the 80s
would be possible on Saturday if the front lifts north of the region
but temperatures could wind up being 20 degrees or more lower
if the warm front owing to onshore flow on the cool side of the
boundary. Blended our previous forecast with 00Z MEX, which
hedged between both extremes.
.AVIATION /14Z SUNDAY THROUGH THURSDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.
General VFR conditions through the period. Partial to almost full
clearing will occur north of the Mason-Dixon Line with more
considerable mid and high cloudiness to the south through the day.
CIGs will lower gradually by late afternoon/early evening to the
south of KPHL but should remain VFR at the terminals through the
night. Winds will veer from northerly to easterly or southeasterly
during the day, remaining generally below 10 kts. Rain is expected
to progress slowly northward across the Delmarva Peninsula late
Sunday night and may begin to affect KILG, KMIV, and KACY by
daybreak Monday with conditions deteriorating to sub-VFR once the
rain moves in.
Monday...Rain expands northward across Delmarva, SE PA and S NJ.
CIGs likely to lower to MVFR from PHL southward and possibly IFR at
ACY/MIV later in the day. E-NE winds 10-15 kt Phila terminals, east
and 5-10 kt farther N/W.
Monday night through Tuesday night...Periods of rain. Restrictions
may start out in MVFR earlier Monday night, but will eventually
lower to IFR from S to N later Monday night. IFR will be commonplace
Tuesday and Tuesday night. E-NE winds 10-15 kt with higher gusts of
20-25 kt in the coastal plain.
Wednesday...IFR at the start will improve to MVFR and then VFR as
rain ends from SW to NE.
Wednesday night and Thursday...VFR and light winds.
Winds are expected to stay below advisory thresholds through at
least this evening and likely for much of the night across the
coastal waters of New Jersey and Delaware, with directions switching
from northerly to more easterly during the day. However, wind speeds
should begin increasing late tonight off the Delaware and southern
New Jersey coasts, with advisory-level conditions expected by around
daybreak Monday. Seas will generally remain just below advisory
criteria through tonight, but are expected to begin rising to 5 feet
or higher near daybreak Monday. Seas especially will need to be
monitored closely tonight to determine if small craft advisory
conditions will be met earlier than expected, as with east/northeast
wind regimes, seas are usually higher than model simulations
Monday through Tuesday night...Confidence in both winds and seas
(just winds for the DE Bay) meeting criteria high enough to issue a
third and fourth period SCA. The SCA will need to be extended into
Tuesday and Tuesday night as we get closer. E-NE winds will
strengthen over our waters as an area of low pressure slowly moves
up the Southeast coast. Expect winds and seas to ramp up to SCA
levels rather quickly in our southern coastal waters Monday morning,
then farther north along the coast and in the DE Bay Monday
afternoon. Both the 00Z NAM and GFS indicate a brief surge in winds
during the late afternoon and evening with the potential for gusts
near gale force in the coastal waters adjacent to S NJ and DE. Winds
may briefly increase close to gale force again 24 hours Tuesday
afternoon/early evening. There is currently much higher confidence
in SCA than GLW but will still warrants a mention of "gales
possible" in the HWO. Seas look to peak Tuesday afternoon and night
to 7-9 ft in our coastal zones.
Wednesday and Thursday...Winds will diminish but seas may still
remain elevated at or above 5 ft, necessitating a SCA.
Positive tidal anomalies are expected with persistent onshore
flow occuring through Tuesday night. Ensembles included in the
Stevens Flood Advisory System indicate a high probability for
minor coastal flooding at our tidal points along the oceanfront
and DE Bay with the Tuesday afternoon-evening high tide. Coastal
Flood Advisories may eventually be needed as tidal departures
of around a foot above astronomical tide would be sufficient to
produce minor coastal flooding with the new moon on Wednesday.
A few of the outlier ensemble members in the Stevens tidal
guidance predict moderate coastal flooding, particularly at
Reedy Point and Lewes. However, there is a much higher
likelihood that the magnitude of flooding would be limited to
minor since onshore flow will not be particularly strong
(predominately be below gale force) and the low pressure center
will be relatively weak (MSLP above 1000 mb) at the time it
moves in close proximity to our region.
MARINE...Small Craft Advisory from 6 AM Monday to 6 AM EDT Tuesday for
Small Craft Advisory from noon Monday to 6 AM EDT Tuesday for