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000 
FXUS61 KPHI 251817
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
217 PM EDT Thu May 25 2017

.SYNOPSIS...
Low pressure will move from the Ohio Valley to the eastern 
Great Lakes region today. Meanwhile, a warm front will lift 
northward into parts of our region today into tonight while new 
low pressure develops along it. This low pressure system then 
strengthens as it tracks away from our area tonight and Friday. 
Weak high pressure may briefly build in for Saturday, then low 
pressure and its associated fronts move through later Sunday. A 
secondary cold front moves through during Tuesday.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
100 pm update: Recent hi-res output has become more aggressive
with convection late this afternoon through this evening in the
Mid-Atlantic, with the last couple of HRRR simulations showing a
rather strong convective cluster moving SSW-NNE through Delmarva
and most of NJ in the 22Z-04Z time frame with some wraparound
showers continuing in E PA/NJ through 08Z. There is reasonable
support from the 12Z NAM Nest and WRF-ARW/NMM simulations as
well, though some discrepancies in timing/placement. Given
reasonably strong wind profiles and some decent surface heating
occurring near and either side of Chesapeake Bay, this evening
may be fairly active across the area. Though the CAPE-shear
combination is certainly not eye-opening, both ingredients
appear to be marginally supportive of isolated instances of
severe late this afternoon and evening.

945 am update: Made some modifications to the Wx/PoPs grids
today based on latest simulations/trends from convection-
allowing model guidance. Steadier precipitation associated with
isentropic lift poleward of a surface warm front located in the
southern Mid-Atlantic region was progressing northward past the
Mason-Dixon Line within the past half hour or so. This will
continue to lift northward, likely north of the I-78/I-195
corridor by late this morning. Near and south of the warm front,
precipitation will be showery/spotty, so felt that PoPs were too
high in these areas during the late morning and early afternoon.

There is a strong signal for low-topped convection to develop 
this afternoon in the vicinity of the surface trough as it lifts
slowly northward into the region, aided by the approach of a 
midlevel low/vorticity maximum, which should pivot northeastward
from the Lake Erie vicinity southeastward to the North Carolina
coast by 00Z Friday. The large-scale lift via differential 
cyclonic vorticity advection and substantial upper-level 
divergence on the northwest side of a meridionally-oriented 
140+ kt 250-mb jet will permit the development of sufficient 
midlevel instability (via midlevel cooling). Surface heating to
the south of the warm front should generate convection fairly 
quickly this afternoon, probably to the south and west of the 
CWA, before entering the region by late afternoon or early this 
evening.

Though instability will be generally weak, upper-level winds are
sufficiently strong for some storm organization. Marginally
severe hail is a possibility if storms can remain somewhat
discrete, given the cold midlevel temperatures and sufficient
deep-layer shear. Strong straightline winds are also possible
given the increasingly steep low- and midlevel lapse rates.
There is even a low risk of an isolated brief tornado for storms
taking advantage of the enhanced low-level helicity near the
warm front/surface low.

Previous discussion...

A pair of mid level lows were located over the Ohio and 
Tennessee River Valleys early this morning. The feature will 
progress eastward with the two centers reaching western 
Pennsylvania and Virginia early this evening.

There were two surface lows early this morning, as well. One 
was located in western Ohio and the other in southwestern 
Virginia. A warm front extended eastward from the Virginia low. 
The two low pressure systems are forecast to merge into a broad 
surface low over Pennsylvania and Maryland early this evening. 
The warm front is anticipated to lift slow northward into 
northeastern Maryland, Delaware, extreme southeastern 
Pennsylvania and southern New Jersey today.

The initial band of rain associated with the warm advection 
will continue to lift northward through our region this morning.
The back edge of the steady rain was into the far upper 
Delmarva and southern New Jersey around 6:00 AM. It is expected 
to work its way from south to north through the remainder of our
forecast area before 9:00 AM. Rainfall totals in the steady 
precipitation should range from about a third to three quarters 
of an inch.

A mostly cloudy sky and showers are anticipated for the balance
of the day to the north of the warm front. As the front reaches
our southern counties there should be some substantial breaks 
in the cloud cover as those location get into the warm sector.

We will continue to mention a chance of thunderstorms for this 
afternoon. Locations in the warm sector will become unstable 
with mixed layer CAPE values rising into the 600 to 1000 J/Kg 
range. The approach of the mid level low and some elevated 
instability will spread the potential for thunder to the north 
of the warm front. The Storm Prediction Center has southeastern 
Pennsylvania, southern New Jersey, Delaware and northeastern 
Maryland under a marginal risk for severe weather for today.

Today's temperature forecast is one of low confidence. The 
actual temperatures will depend upon where the warm front 
settles and how much clearing takes place over our southern 
counties. Generally, highs should range from around 60 in the 
Poconos to near 80 in southern Delaware.

The wind is forecast to remain from the east to the north of 
the warm front. It should veer to the southeast and south once 
the front passes. Speeds are expected to favor the 8 to 12 MPH 
range with some gusts of 15 to 20 MPH possible.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM FRIDAY/...
945 am update: Expecting storms to linger through the evening
hours as the midlevel low moves through the region, especially 
north of the Mason-Dixon Line. Kept storm chances longer, and 
extended them through much of the night in far east-central PA 
and northern NJ.

High-res guidance has the main window of convection in the 22Z
to 04Z time frame (6 pm to midnight), so this may end up being
more of an evening event for the area. With degree of cold air
aloft, suspect thunder will remain a threat through much of the
night where precipitation occurs.

Previous discussion...

The mid level low is forecast to pass over our region tonight, 
reaching southern New England toward daybreak on Friday. The 
surface low will likely follow a similar pattern.

We are expecting a mostly cloudy sky and showers for tonight. 
Thunderstorms may linger into the evening. The wind is 
anticipated to become light and variable tonight in much of 
eastern Pennsylvania and northern and central New Jersey before 
settling into the west around 5 to 10 MPH. A southwest wind 
around 10 MPH is anticipated to shift to the west on the upper 
Delmarva and in southern New Jersey.

Low temperatures are expected to favor the 50s for tonight.

&&

.LONG TERM /FRIDAY THROUGH WEDNESDAY/...
Summary...Unsettled at times, however warmer temperatures are 
expected. 

Synoptic Setup...A significant closed low is forecast to shift 
up into New England Friday then move out Friday night into 
Saturday. Some brief weak ridging should slide into the east 
Saturday, however the next trough amplifies from the Midwest and
Great Lakes later Saturday and Sunday. This trough looks to 
close off and be slow to shift eastward through Wednesday, 
sending a slow moving low pressure system well to our north 
while a series of cold fronts cross our area. Timing and 
therefore the details are a bit less certain given an amplified 
pattern with the idea of a closed low involved. We used a 
model/continuity blend for Friday through Saturday night, then 
blended in the 00z WPC Guidance thereafter. Some adjustments 
were then made following additional collaboration with our 
neighboring offices.

For Friday...Strengthening low pressure moves away during 
Friday, and therefore after some possible low clouds and a 
shower to start improving conditions are expected. However, 
cyclonic flow combined with another short wave and a surface 
trough reflection should result in some showers. The northern 
areas have a higher chance for some showers than farther south, 
but overall coverage looks to be isolated to scattered. A west- 
northwest wind will increase and become gusty for a time as the 
pressure gradient tightens and cooling occurs aloft allowing for
better mixing. The winds diminish at night along with any 
showers dissipating early, and lingering clouds should thin out.

For Saturday and Sunday...The flow turns generally more zonal 
with a weak but brief ridge in the east by later Saturday. The 
model guidance overall shows a short wave within this flow 
although the timing is a little uncertain, however this may 
ultimately track just to our south where a frontal zone will be 
located. A closed low centered north of the Great Lakes is 
forecast to amplify south and eastward gradually by late in the 
weekend. This will drive a cold front eastward and across our 
area later Sunday, however the model guidance overall suggests 
low pressure develops along a triple point as the main surface 
low to our west and north occludes. This may end up enhancing 
the showers and some thunder during Sunday, especially during 
the afternoon and evening. Backing up some, Saturday may end up 
being dry as enough ridging is in place, and short wave energy 
runs to our west and southwest. We will carry some more clouds 
Saturday but keep it basically dry, and therefore Saturday looks
like the better of the two weekend days. The flow looks to turn
more southeasterly in the lower levels Sunday, and therefore 
while still mild it will turn cooler along the coast.

For Monday and Tuesday...A closed low is forecast to expand 
across the Great Lakes, Midwest to the Ohio Valley and gradually
into the Northeast. This will drive surface low pressure 
eastward mainly north of the Great Lakes. An initial surface low
should be moving away from the Mid Atlantic coast early Monday 
as a frontal zone sits offshore. The main cold front is forecast
to move across our area Tuesday and shift offshore Tuesday 
night. The second half of Tuesday may be convectively active as 
a cold front arrives, however this will depend on short wave 
timing and potential convective development to our west. The 
timing overall is less certain given the scope of the closed low
to our north-northwest and plenty of short wave energy 
revolving around it. Kept chance PoPs for Monday then slight 
chance to chance PoPs on Tuesday. We are anticipating warmer 
temperatures ahead of the cold front.

For Wednesday...As the center of the closed low gradually 
shifts across eastern Canada, the overall trough becomes more 
established across the Northeast and into the Mid Atlantic. The 
main surface low is forecast to be east of James Bay Canada with
its trailing cold front east of our coast to start Wednesday. A
cyclonic flow will be in place and a surface trough should be 
sliding across the Great Lakes region. While less likely at this
time, a few showers still cannot be ruled out given the 
synoptic setup. For now, kept some slight chance PoPs across the
northern and western areas. Given the core of the closed low 
well to our north, keeping the colder air aloft there, daytime 
temperatures look to be on the warmer side.

&&

.AVIATION /18Z THURSDAY THROUGH TUESDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, 
KILG, KMIV, KACY and surrounding areas.

Tough day for the terminals so far with IFR sticking around
along/north of warm front positioned just south of ILG and KACY.
Expect IFR CIGs and transient sub-VFR VSBYs to continue through
late this afternoon from KPHL northward. There may be some
improvement 20Z-22Z from KPHL southward as the warm front
approaches. Scattered storms likely in much of the area this
evening, generally from 22Z-04Z, with lingering showers possible
through 09Z, especially in NJ. Slow improvement in CIGs this
evening/overnight, with generally VFR conditions by daybreak
Friday. Winds 10 kts or less through tonight, but becoming
west/gusty tomorrow.

Confidence in TAFs very low overall, especially for the first 12
hours.

OUTLOOK... 
Friday...Possible MVFR/IFR ceilings early, otherwise improving 
to VFR. Isolated to scattered showers should develop during the 
day before dissipating in the evening. West-northwest winds 
increase with gusts up to 25 knots, then diminishing in the 
evening.

Saturday...VFR overall. Light northwest winds turning southerly
toward evening. Clouds increase at night and possibly lower.

Sunday...MVFR/IFR conditions possible with showers and some 
possible thunderstorms, especially in the afternoon and evening.

Monday...Sub-VFR conditions possible at times with a chance of 
showers and afternoon/evening thunderstorms.

&&

.MARINE...
A warm front is expected to lift slowly over the waters of 
Delaware and New Jersey today. Low pressure from the west and 
southwest should pass across the waters tonight.

Wave heights on our ocean waters are expected to remain in the 
4 to 7 foot range for today and tonight. As a result, the Small 
Craft Advisory will stay in effect. The wind is anticipated to 
work its way gradually around from the east to the south to the 
west during the period. Speeds should favor the 10 to 20 knot 
range with some gusts around 25 knots on our ocean waters.

RIP CURRENTS...
The flow is expected to veer from the east to the south today 
along the coasts of Delaware and New Jersey with wind speeds 
mainly in the 10 to 20 knot range. As a result, we will keep the
moderate risk for the development of dangerous rip currents.

The wind flow is forecast to become westerly for Friday. As a 
result, the rip current risk is anticipated to transition to low
at that time.

OUTLOOK... 
Friday...Small Craft Advisory extended through 22z for the 
ocean zones as elevated seas gradually subside with an offshore 
flow. Some wind gusts may reach 25 knots during the day 
especially nearshore, however confidence is not high enough to 
issue an advisory for Delaware Bay.

Saturday through Monday...Conditions are forecast to be below 
advisory criteria, however winds may gust to around 20 knots at 
times especially on Sunday.

&&

.TIDES/COASTAL FLOODING...
A Coastal Flood Advisory was issued for portions of the Eastern
Shore of Maryland. Levels on the Chesapeake Bay continue to rise
at Cambridge and departures are already 2 feet and rising. They
should continue to rise, before slowly lowering through high
tide. But with the expected departure of near 2 feet around high
tide, levels are expected to reach advisory levels. 

The astronomical tides with the new moon are particularly high 
with the evening high tide cycle. Positive tidal departures near
1 foot are anticipated again for today. As a result, we expect
to see widespread coastal flooding expected with this evening's
high tide. We will continue the Coastal Flood Advisory for this
evening in the coastal counties of New Jersey and Delaware, as 
well as for the counties along Delaware Bay and Raritan Bay.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...Coastal Flood Advisory from 7 PM this evening to 1 AM EDT 
     Friday for NJZ012>014-020>027.
DE...Coastal Flood Advisory from 7 PM this evening to 1 AM EDT 
     Friday for DEZ002>004.
MD...Coastal Flood Advisory until 8 PM EDT this evening for MDZ015-
     019-020.
MARINE...Small Craft Advisory until 6 PM EDT Friday for ANZ450>455.

&&

$$
Synopsis...Gorse
Near Term...CMS/Iovino

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