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000
FXUS61 KPHI 191032
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
632 AM EDT Fri Apr 19 2019

.SYNOPSIS...
A strong low pressure system will continue to develop along a cold
front over the Ohio Valley with the potential for heavy rainfall and
strong to severe thunderstorms this evening and through the
overnight. This low will slowly move northeast, weakening as it
does so through early Monday. Weak high pressure will build into
the region Tuesday, before a backdoor cold front settles
southward Tuesday night into Wednesday. Slightly cooler and
unsettled weather look to affect the area for the latter half of
the week as another low pressure system moves across the area.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
For the 630 PM update, made some adjustments to the hourly temperature,
dew point and wind grids based on the latest observations and trends.
Trapped cool air at the surface across parts of the far north is
slowly eroding/mixing out. Some low clouds are developing mainly
near and west of the I-95 corridor. This looks to continue, however
with mixing these low cloud bases may rise at least some this
afternoon. Delayed the mentionable PoPs some given a lack of
shower activity thus far.

Warm air continues to surge northward across the area ahead of a
deep closed low. Low-level cool air though remains trapped across
the far northern areas, however this is expected to mix out through
the morning.

The strong closed low centered across the Gulf Coast states today
will slowly shift eastward. As this occurs, a deepening surface low
will track up the Tennessee and Ohio Valley`s as a frontal zone
remains to our north and west. Yesterday`s warm front continues to
clear our far northern zones early this morning. The scope of the
warm air across much of the area is evident by the ongoing southerly
breeze and gustiness as strong flow above the surface with a mild
boundary layer is maintaining mixing. This mixing will continue
through today, however one of the forecast challenges is how much
cloud cover will be around. This will have an impact on the
temperatures as any period of sunshine will result in temperatures
quickly rising. Much of the guidance continues to show stratus
developing prior to daybreak, however this may be overdone. We still
kept lots of clouds around (as there should be a decent amount of
higher level clouds moving through) however did go warmer given the
amount of mixing expected. It would not be surprising if more
sunshine occurs especially for a time this morning into the
afternoon (although perhaps some low clouds hug the coast as higher
dew points advect northward over the cooler ocean waters). The
amount of heating will have an impact on the how much instability is
realized. If some thunderstorms do occur, steepening low-level lapse
rates could result in locally strong wind gusts. Much of the hi-res
guidance (and some CAMS) show not a lot of coverage today.

A nearly 50-knot low-level jet shifts to mostly the coastal areas
then offshore this morning, however this is forecast to expand
inland toward evening. This combined with some surface convergence
due to the southerly wind into the coast may result in some showers
by later today for the coastal locales. Elsewhere, a few showers are
anticipated within the moist southerly flow however a short wave
rounding the upper-level ridge axis should organize showers and
perhaps some thunder across eastern Pennsylvania later this
afternoon. This may also be enhanced by southerly flow into the
higher terrain thus much of this activity may remain west of the
Fall Line, and also these areas may get within the right entrance
region of a departing mid level jet streak. The guidance looks to
continue with a slower trend, therefore PoPs were adjusted
accordingly. The bulk of the forcing and thus more widespread
showers look to occur tonight.

The area of eastern Pennsylvania in particular may be the spot to
watch this afternoon as showers/thunder may train for a time given
the storm motion fairly aligned with the deep southerly flow. This
setup within an increasingly high PW airmass can result in locally
heavy rain (flash flooding potential).

High temperatures are a blend of a model consensus and continuity
then these were adjusted up some especially south of the I-80
corridor. Deeper mixing with boundary layer warming (steepening low-
level lapse rates) should result in southerly wind gusts up to 35
mph especially this afternoon.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SATURDAY/...
The closed low is forecast to gradually turn more northward and up
the Tennessee Valley overnight. This will maintain deep southerly
flow across our region along with PW values surging to near 2
inches. Showers and some thunderstorms are expected at any time
tonight, however the main focus for an organized band of heavy
showers with embedded thunderstorms is late this evening into the
wee hours of Saturday morning. It is during this time frame when
enhanced lift moves through with an elongated short wave along with
a ramp up of the low-level jet. This is also when an intense mid
level jet starts to arrive from the south and southwest. The
strongest part of the low-level jet is forecast to be along the
coastal counties and offshore, however the flow aloft is still
strong everywhere through the overnight.

The amount of instability is less certain given it is at night,
however strong southerly flow should maintain warmer temperatures
and while we expect some thunderstorms, a well developed squall line
with lightning all along it is less certain. Some of the hi-res/CAMS
as well as the HREF indicate a better chance of stronger convective
elements overnight near and south of Philadelphia. Given the amount
of shear despite lower instability, some convection even if lower
topped may pose a locally strong to damaging wind potential. This
risk will be fine tuned later today. A more organized band of heavy
showers will pose more of a flash flood threat, and given the latest
guidance we opted to not expand the flash flood watch at this time.

The closed low gets closer by late in the day Saturday, and after an
unsettled start to the day for much of the region the
showers/thunder should rapidly end and move offshore by late
morning. An intense mid level jet arrives during the course of the
day, and this should result in a dry slot moving across the region.
While some additional showers cannot be ruled out for the rest of
the day, the amount of drying aloft may lessen this potential. Some
sunshine is expected to return and it will be cooler although a
southerly breeze will continue.

&&

.LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/...
Overview...deep 500 mb low will completely cut itself off from the
main westerly flow, thus beginning to slowly fill and move off to
the northeast through Monday morning. A weak ridge attempts to build
across the mid-Atlantic Tuesday, but a quick-moving area of low
pressure will skirt across the Northern Tier of the country,
dragging a (backdoor) cold front through the area Tuesday night.
Though some discrepancy exists between the long range guidance,
cooler weather looks to dominate the latter half of the workweek as
an upper-level trough digs southward across the Northeast. On a
another note, it looks as if much warmer weather may make a return
as we close out the month.

By Sunday morning, the upper low looks to have reached its peak and
is fully in the occluded phase, thus, it looks to begin weakening.
Scattered showers will likely occur through at least Monday morning
as the low continues to migrate ever so slowly eastward across the
CWA. These showers will likely be more topographically favored over
the Poconos and NW NJ and also diurnally driven, tending to tapper
off Sunday night. Highs Sunday will be cooler in the mid 60s across
the area.

There is some disagreement with how long the upper low sits over the
region, with the ECMWF tending to drag the progression the system
out through the day Monday, while the GFS moves it to the east by
Monday night. Tending to side with the latter, as the ECMWF
sometimes stalls these systems a little too long. Thus, any showers
should end, with drier night expected Monday night. Highs will
be a little warmer Monday as the remnant upper low will have
weakened and shifted eastward. Therefore, highs will range in
the low to mid 70s.

A little warmer Tuesday as a ridge of high pressure builds behind
the upper low with highs sitting in the mid to upper 70s. This will
be short-lived as a shortwave trough zips along the Northern Tier,
driving a surface low and cold front into the region by Wednesday
morning. There does not look to be a lot of precip with this system
and dynamics are weak, however, a few showers look plausible Tuesday
night into Wednesday. Highs Wednesday will be noticeably cooler,
staying in the mid to upper 60s.

A trough of low pressure looks to dig into the Northeast for the
latter half of the workweek, keeping things fairly quiet, though
cooler. Some showers are possible Thursday night and Friday. The GFS
kicks the trough out a little faster than the ECMWF, thus am going
with a more blended approach given the previous dialogue on
this model tendency.

&&

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

Today...Areas of MVFR ceilings this morning, then these may
tend to improve to VFR for a time this afternoon before lowering
again toward evening. This is of low confidence. Some showers
around especially this afternoon and a few thunderstorms cannot
be ruled out. Southerly surface winds 12-18 knots with gusts
20-30 knots. Low-level jet of 40-50 knots at 2000 feet will
result in low-level wind shear mainly this morning at MIV and
ACY.

Tonight through Saturday...MVFR to IFR ceilings tonight and
visibilities as low as IFR due to heavier showers and some
thunderstorms, then conditions improve to VFR by early Saturday
afternoon. The extent of the thunder tonight and early Saturday
morning is less certain. Southerly winds 10-15 knots with gusts up
to 25 knots. The gusts may drop off for a time tonight.

Outlook...
Saturday night...MVFR to IFR cigs in the morning. Southerly winds 10
to 15 knots.

Sunday...VFR cigs may transition to MVFR at times. Showers may lower
restrictions at times. South to southwest winds 10 to 15 knots,
dropping to 5 to 10 knots at night.

Monday...generally VFR, though a few widely scattered showers are
possible in the morning. Light and variable winds with a prevailing
westerly direction.

Tuesday...For the most part, VFR. Showers arriving in the afternoon
and into the overnight from northwest to southeast. Westerly winds
turning northwest 5 to 10 knots.

&&

.MARINE...
Small Craft Advisory for Delaware Bay through early Saturday
afternoon, and elsewhere through early Saturday evening. Strong
southerly flow will be maintained, however the overall mixing is
expected to be limited some due to much warmer air moving over the
cooler waters. It is possible some low-end gale force gusts occur
for a time late today and especially tonight, however confidence is
low due to a strong low-level inversion expected over the waters.
Seas will remain elevated and actually build today into tonight.

Outlook...
Sunday...Sub-SCA winds from the south at 10 to 15 knots. Seas remain
on the higher side with wave heights lingering from 5 to 7 feet.
They subside somewhat by Sunday night, however.

Monday...Sub SCA winds. Seas 3 to 5 feet.

Tuesday...southerly winds 10 to 15 knots. Seas 2 to 4 feet.

&&

.HYDROLOGY...
A Flash Flood Watch remains in effect for roughly along and north/
west of I-95.

Areas of 1-2 inches of rain is expected with local amounts up to 3
inches. Hourly rainfall rates potentially nearing 2 inches
later today and especially tonight will result in the locally
higher amounts, and where this occurs is where the flash
flooding potential is the greatest.

River/Stream/Creek rises will begin late today and tonight and
persist through the weekend. Mainstems won`t crest until at least
Sunday. Smaller creeks and streams will crest on Saturday. Poor
drainage and low-lying flooding will quickly occur during times of
enhanced rainfall rates.

Keep in mind that the hydrologic forecasts are based on predicted
rainfall. Actual response will be different if the rainfall
increases/decreases across a specific basin. Keep in touch with
future forecasts into the weekend.

&&

.TIDES/COASTAL FLOODING...
Subsequent high tides also have potential for minor coastal
flooding, especially if heavy rain materializes tonight.
Additionally, with southerly flow continuing little evacuation
of water will occur between high tides making it increasingly
likely flooding thresholds will be reached, especially with the
higher astronomical tide tonight and especially in the back
bays. Another coastal flood advisory will be needed, however
this will be re-evaluated later today.

Response on the eastern shores of Chesapeake Bay will be
slower, but prolonged southerly fetch is favorable for some
minor tidal flooding. Models continue to suggest this
possibility by Saturday morning`s high tide. We will continue to
monitor this potential.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...Flash Flood Watch from 2 PM EDT this afternoon through
     Saturday morning for PAZ054-055-060>062-070-071-101>106.
NJ...Flash Flood Watch from late tonight through Saturday afternoon
     for NJZ001-008-010-012-015.
     Flash Flood Watch from 2 PM EDT this afternoon through
     Saturday morning for NJZ007-009.
DE...None.
MD...None.
MARINE...Small Craft Advisory until 6 PM EDT Saturday for ANZ450>455.
     Small Craft Advisory until 2 PM EDT Saturday for ANZ430-431.

&&

$$
Synopsis...Davis
Near Term...Gorse
Short Term...Gorse
Long Term...Davis
Aviation...Davis/Gorse
Marine...Davis/Gorse
Hydrology...
Tides/Coastal Flooding...