Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 240516
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
116 AM EDT Mon Apr 24 2017

.SYNOPSIS...
A low pressure system will organize over the Carolinas tonight and
Monday then move up the east coast through the middle of next week.
Another frontal boundary will approach our region from the north
later in the week before meandering over the region this
weekend.

&&

.NEAR TERM /UNTIL 6 AM THIS MORNING/...
1230 am update: Continued the trend set in the previous update
by delaying the onset of precipitation. The low-level air
remains fairly dry, especially just off the surface, in the
Delmarva Peninsula, so any precipitation falling in Delaware and
eastern Maryland is virga at the moment. High-resolution models
are not overly aggressive in bringing steadier precipitation
into the area overnight, with mainly sprinkles or light showers
generally along and south of a Stevensville, MD, to Dover, DE,
line. Reduced PoPs overnight, particularly north of the
aforementioned line.

Also made some adjustments to temperatures (generally too cool
in eastern Pennsylvania and northern/central New Jersey) and dew
points (generally too low everywhere). Slowed the onset of
increased cloud cover in the northern CWA as well per latest
satellite imagery.

Previous discussion...

High pressure that was over the area today will continue to shift
offshore overnight. Meanwhile, a cutoff low aloft and surface
low will drift out of the southern Appalachians and toward the
southeast coast. Well north of these features, moisture will be
streaming up the east coast overnight. Scattered showers may
creep into the central/southern portions of the Delmarva and
southern New Jersey by daybreak, but with dry air in the low-
mid levels, it will take some time to moisten the column and the
showers to advance north. The remainder of the area will remain
dry through daybreak with just an increase and thickening of
clouds.

Few changes were needed with the evening update, I did slow the
timing of the steady precip for Monday a little. Most of the op
models are showing the starting time to be later than what the
earlier models had.

&&

.SHORT TERM /6 AM THIS MORNING THROUGH 6 PM TUESDAY/...
1230 am update: Reduced PoPs for most areas north of the Mason-
Dixon Line on Monday, as precipitation will be moving northward
at a snail`s pace during the day. Also think it will take some
time for the lower portions of the atmospheric column to moisten
given substantial drying that occurred on Sunday. Most high-
resolution guidance is keeping precipitation very light and
fairly spotty during the morning with a stronger push of
steadier/more widespread rain moving in from the south/southeast
during the late afternoon. For this reason, lowered QPF during
the day as well.

Based on the latest trends in short-term model simulations,
think the northern CWA might be a little warmer than initially
forecast as well (with perhaps even some partial sunshine in
eastern PA/northern NJ during the morning). Increased hourly/max
temperatures in these areas, with considerable weighting given
to non-statistical output, which appears to have a better handle
on the slower storm-system motion.

Previous discussion...

The coastal storm continues to advance toward the southeast coast on
Monday, but remains well to our south. However, there will continue
to be plenty of moisture streaming up the east coast while several
short wave/vorticity impulses move northward as well. The dry air
will eventually be overtaken everywhere during the morning into the
afternoon, and rainfall is expected to spread across most of the
area by the late morning and early afternoon. With cloud cover and
precipitation across the area, temperatures are expected to be
several degrees below normal, especially the southern areas where
the clouds and rain will begin sooner.

&&

.LONG TERM /TUESDAY NIGHT THROUGH MONDAY/...
Monday night through Tuesday night:

A strong ensemble signal is present that low pressure over the
Carolina coastline will move up the east coast. A westward shift has
occurred with both the operational and ensemble modeling as they are
still ironing out details with tropical moisture influx and
convection along with how fast it closes off in the mid-levels.
However, operational and ensemble modeling has keyed in on two
areas of excessive rainfall the first being in the Carolinas and
Virgina where the greatest amount of lift is located Monday
night and the second over New England as the low moves up the
coast Tuesday into Wednesday in association with the deeper
tropical moisture. This leaves our region in between the two
areas of heavy rainfall. Still periods of mainly light rain are
expected and locations in Delmarva and along the coast should
still see over an inch of rainfall with lower amounts further
northwest. It should be noted the CMC and UKMET are still
maintaining a wetter solution which was also incorporated into
the QPF forecast along with the SREF. A dry airmass will
continue to be very slow to erode north and west of Philadelphia
as well, so while cloudy, it`s not hard to see an outcome where
the Southern Poconos do not see a half inch of rain by
Wednesday.

Low clouds, rain and an easterly onshore flow will keep temperature
ranges small and rather chilly with highs in most spots only in the
50`s. This forecast update is still a bit cooler than MET/MAV
for highs and slightly warmer with lows. Winds should pick up
as well from the east with some gusts around 20 mph at times
based on bufkit analysis. It is not a favorable environment for
mixing but with the low pressure nearby a tighter pressure
gradient will be present leading to the wind gusts.


Wednesday and Wednesday Night:

Fairly good agreement is present to linger scattered showers
into Wednesday, the westward jog in the track means a slower
clearing trend from what we were looking at with previous model
runs. With some breaks in the rain and maybe even the clouds we
should be several degrees warmer than Tuesday.


Thursday through Friday night:

A busy late week and weekend in Philadelphia. A good piece of
news is that high pressure will become more dominant near the
region resulting more sunshine and a break from the gloom most
of this week will bring. Current thinking is that a cold front
may provide enough lift for a thunderstorm or two ( only slight
chance and out of the point and click for now) as it moves into
the region late Thursday and Friday. Confidence is higher today
that we will see abundant southerly flow ahead of the front
pushing highs into 80`s along with muggier conditions leading
to warmer lows. Bufkit soundings indicate the potential for some
20 mph gusts Thursday afternoon as well.


Next weekend:

The modeling continues to show this front may evolve into a
backdoor or warm front for the weekend somewhere near our
region. A very wide temperature range was present both days
with operational model and ensemble data. Overall the
temperature spread has actually increased a standard deviation
to 5 from 24 hours ago. An example is the high temperatures both
days produced over 20 degrees of a difference between the 06z
and 12z GFS runs. Needless to say the forecast stuck close to
the previous forecast blending some of the the latest ensemble
guidance. With the front nearby, enough lift still may be
present for a shower or thunderstorm, slight chance and out of
the point and click for now.

&&

.AVIATION /05Z MONDAY THROUGH FRIDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.

For the 06Z TAFs...VFR conditions are expected at the terminals
through at least early afternoon, but CIGs will gradually
approach MVFR by late afternoon or early evening from south to
north. Rain is expected to move in to KILG/KMIV/KACY around or
after 21Z and at KPHL, KPNE, and KTTN between 00Z and 06Z.
Conditions will continue to deteriorate through the evening,
likely becoming IFR near or after 06Z Tuesday. Light and
variable winds will become primarily E or ENE by late morning,
slowly increasing to 10 to 15 kts after 15Z, with potential for
occasional gusts to 20 kts or so, especially at KMIV/KACY.
Overall confidence is around average for winds but below average
for CIGs/VSBYs.

OUTLOOK...

Monday night: MVFR and IFR conditions spreading from south to north
with light rain moving in. East northeast winds 10-15 with gusts
up to 20 knots.

Tuesday through Wednesday: MVFR and IFR with several periods of
rain. Perhaps some intervals of VFR for KABE and KRDG where
more breaks in the rain are likely. Rain will end on Wednesday
from west to east with a slow improvement to VFR. Easterly winds
becoming northerly, gusts up to 25 knots on Tuesday from the
east decreasing to under 10 knots from the north on Wednesday.

Wednesday night through Friday: Mainly VFR. Winds under 10
knots but southwesterly gusts up to 20 knots Thursday afternoon.

&&

.MARINE...
Conditions remain just below advisory levels most of tonight.
However, winds and waves are expected to begin increasing across
the southern waters tonight, then advance northward during the
day Monday. The current timing of the Small Craft Advisory
remains unchanged at this time.

OUTLOOK...

Seas will remain above five feet through Wednesday and
potentially through Friday. Wind gusts will increase Tuesday
from the east. Mixing continues to look poor but 925 mb winds
are modeled around 40 knots for a short period Tuesday afternoon
and evening moving from south to north. So gale gusts are still
possible in this timeframe. Winds will gradually decrease and
turn more northerly Tuesday night and Wednesday. Southwest wind
gusts Thursday may approach SCA criteria as well.

&&

.TIDES/COASTAL FLOODING...
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.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...Small Craft Advisory from 6 AM this morning to 6 PM EDT
     Tuesday for ANZ452>455.
     Small Craft Advisory from noon today to 6 PM EDT Tuesday for
     ANZ430-431-450-451.

&&

$$
Synopsis...Gaines
Near Term...CMS/Robertson/O`Hara
Short Term...CMS/Robertson
Long Term...Gaines
Aviation...CMS/Gaines
Marine...Gaines/Robertson
Tides/Coastal Flooding...Staff



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