Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 211322

Area Forecast Discussion
National Weather Service Mount Holly NJ
922 AM EDT Tue Aug 21 2018

A warm front will gradually lift north across our area later today
into tonight, then a cold front moves through on Wednesday. High
pressure builds in Thursday and Friday, before shifting offshore and
also to our south Friday night into Saturday, then remaining there
Sunday and Monday.

915 am update: No major changes to the forecast...just some
minor refinements to POP/WX and T/Td grids based on latest obs
and trends. Latest HRRR still showing potential for localized
heavy rain amounts this evening over eastern PA but still don`t
have high enough confidence flash flooding will occur to issue
watch and also think if it does occur, it should be localized.

630 am update: Increased sky cover this morning slightly, as
overcast conditions are generally present at all sites now. This
should continue through the day (though a few breaks may occur
early this afternoon before the precipitation moves in).

Regarding convective evolution, latest HRRR/NAM Nest continue to
indicate showers/storms moving in this afternoon from southwest
to northeast, though noticeable timing differences exist among
the latest solutions. The HRRR is suggestive of multiple rounds
of convection in Delmarva, and if this trend continues, this
would suggest an increasing heavy rain/flooding threat in this

Previous discussion...

A midlevel shortwave trough will be advancing eastward through the
Midwest and Great Lakes region today, with an attendant surface low
lifting east-northeast from northern Illinois currently to far
southeastern Ontario by 00Z Wednesday. A baroclinic zone extends
eastward from the surface low this morning through the
central/southern Mid-Atlantic, and the big forecast question today
is how far north the associated warm front will lift in advance of
the Midwest/Great Lakes low.

Overall, models look a little too eager in returning the warm front
northward (as usual). As a result, I have kept the sky
cover/temperature forecast rather pessimistic today, with mostly
cloudy to overcast conditions expected across the area and
temperatures somewhat below consensus guidance.

In advance of the midlevel shortwave trough, subtle perturbations
will eject eastward through the Mid-Atlantic, though none of them
appear to be particularly potent, at least as far as the 00Z
operational model simulations suggest. Nevertheless, warm-air
advection atop the baroclinic zone should prove sufficient for the
development of (elevated) convection in much of the central Mid-
Atlantic today. The hi-res simulations initiate this convection to
the south and west of the CWA by early this afternoon before
progressing it northeastward into Delmarva and portions of south-
central/southeast Pennsylvania after 18Z. There is pretty good
agreement among the HRRR/NAM Nest/WRF-ARW/NMM simulations regarding
this evolution, and given the large-scale lift/phenomena involved,
followed this scenario rather closely in terms of PoPs/Wx grids and
their evolution over the course of the day.

There is some concern about the potential for locally heavy rainfall
with any of the convection that develops. Precipitable water values
will be increasing rapidly in the warm/moist advection regime. The
HRRR and WRF simulations indicate pockets of 1-2+ inch QPF where the
stronger convective cores track (despite reasonably progressive cell
motions). However, this threat area is generally south of where the
very wet past few weeks has occurred. Moreover, the localized nature
of the threat and remaining questions regarding available
instability suggest to me the threat is not high enough at this
point to justify issuance of a flash flood watch. Nevertheless, the
threat of localized flooding exists, and should confidence increase
in a more widespread threat (especially farther north), a watch may
be required later today.

Notably, there are indications the convection that develops
along/north of the retreating warm front may diminish with the loss
of daytime heating (i.e., farther north). This appears to be tied to
the associated perturbation shifting offshore by evening (with
downstream lift shunted eastward from the area). This further
justifies holding off on a flash flood watch at this time.


As the surface low entering southeast Canada continues its trek
along the Saint Lawrence Valley tonight, a cold front will approach
the coastal Mid-Atlantic through the period. A retreating warm front
is expected to progress through the region during this period as
well, allowing a brief time window in which the warm sector of the
large-scale cyclone advances into the area. Of course, this will
occur after peak heating, which will likely impede development of
more substantial instability, particularly given fairly widespread
antecedent cloud cover and meager midlevel lapse rates.

As the story goes this summer, tropospheric moisture content will be
anomalously high in the pre-frontal environment. Thus, locally heavy
rain with any convection remains the primary threat tonight. Have
continued the mention of this threat in the forecast. However, a lot
of question marks remain regarding the overall threat. One of the
biggest involves the degree of large-scale ascent available in the
CWA versus surrounding areas.

Specifically, there appear to be two portions of the large-scale
system that will aid in the development of fairly widespread convection.
The first will be shifting north-northeastward, coincident with
the Canada surface low and a retreating 250-mb jet streak exiting
the Northeast. The second is a southern-stream perturbation surging
eastward to the south of the area during the night. As convection
nocturnally weakens along the cold front progressing through the
Appalachians, operational models clearly favor two regions with
a greater convective threat. The first is in portions of New York,
New England, and adjacent Quebec (in closer proximity to the surface
low), and the second is to the south/southeast of the CWA as the
southern-stream perturbation rounds the base of the larger-scale
trough late in the night. This leaves the CWA in between the
two more favorable regions. As such, questions remain regarding
the extent of the heavy rain threat, especially given the unfavorable
timing of the cold front. Therefore, no flash flood watch was issued
at this time.

PoPs reach their maximum during the late evening to early overnight
period (high-chance to likely), with general lowering late in the
night as the strongest synoptic-scale lift begins to advance east of
the area.

Forecast lows are in the mid 60s in the Poconos to the low-to-mid
70s in the urban corridor, Delmarva, and the coastal areas.


Summary...A change in the pattern during midweek allows for a much
drier airmass to settle in for the second half of this week and into
the weekend. However, turning warmer to hot with increasing humidity
to end the weekend and especially early next week.

Synoptic Setup...A sharpening upper-level trough is forecast to swing
across the Great Lakes, eastern Canada and the Northeast Wednesday
into Thursday. This trough looks progressive therefore it lifts
out Thursday night, with some ridging arriving from the west. This
ridge should build some late this week and to start the weekend
as a new trough slides across the Midwest. This trough then may
weaken as it shifts across the Great Lakes and the Northeast this
weekend, while an east to west ridge strengthens some across the
southern part of the U.S.

For Wednesday...An upper-level trough is forecast to amplify from
the Great Lakes to the Northeast by late Wednesday. This drives
surface low pressure well to our north during the day, with a cold
front shifting offshore by evening. Our region will be in the warm
sector prior to the cold fronts arrival and some showers and maybe a
few thunderstorms should be ongoing to start the day. Some
additional showers should occur during much of the day even behind
the cold front as cold air advection creates a bit more instability
before substantial drying moves in from the northwest. The strongest
cold air advection however is forecast to arrive during the night.
We placed the highest PoPs through about midday then gradually
lowered them into the evening hours. Due to the expectation of
ongoing showers and cloudiness which will act to hold down the
overall instability, the intensity of any thunderstorms should be
sub-severe. Given the cold air advection along with cyclonic flow,
areas of cloudiness may hang on for much of the night before
increasing subsidence arrives. A substantial dew point drop is
expected to occur during the course of Wednesday night under a north-
northwest breeze.

For Thursday and Friday...A strong upper-level trough moves across
our region during Thursday, however the flow is progressive and
therefore this feature should be offshore by Friday morning. This
will result in a cooler day Thursday with much drier low-level air.
Some guidance suggests that parts of our region may experience dew
points dropping into the mid and upper 40s during peak heating
Thursday. A narrow ridge is forecast to start overspreading our area
Friday, allowing surface high pressure to be over our area although
this feature starts to shift offshore Friday night.

For Saturday and Sunday...The pattern looks to undergo some changes
once again, with a short wave trough weakening as it tracks across
the Great Lakes to the Northeast Saturday and Sunday. A ridge across
the southern states expands some and with high pressure anchored
offshore and to our south will result in a more southerly return
flow. This results in warm air advection and also with time the
moisture returns allowing for dew points to rise once again to the
humid levels. There looks to be a surface trough developing in our
vicinity Sunday, while the aforementioned short wave trough is
mostly glancing the area. It is less certain if this will be enough
to develop some convection given the moisture return and warmer
temperatures. For now, opted to keep the forecast dry given the

For Monday...A ridge centered to our southwest is forecast to expand
a bit more to the north, with our area near the northern periphery
of it. This results in a more zonal flow regime with some energy
sliding eastward across the Great Lakes region. A cold front is
forecast to be draped well to our north, however a surface trough
should be in place given the warm to hot conditions and a weakness
in the surface pressure field. There is some potential for
convection to develop during the afternoon and evening especially,
however the overall focus for any development is less certain and
may be mostly tied to the surface trough. As of now, carried some
slight chance PoPs in the gridded database for the northwestern
zones. It looks rather warm/hot with afternoon temperatures around
90 degrees for many places, and afternoon heat indices especially in
the I-95 urban corridor may be well into the 90s.


The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Today...CIGs will vary between VFR and MVFR today (with bases
generally 1500 to 5000 feet), with increasing chances of
showers/storms this afternoon, moving in from the southwest.
General timing looks to be after 18Z at the terminals. Once
precipitation moves in, CIGs are expected to become more solidly
MVFR, with VSBYs locally/temporarily lowering as well. Winds
will be slowly shifting to a more southeasterly direction with
time during the day, with speeds ranging from 5 to 15 kts.
Moderate confidence.

Tuesday night...Prolonged sub-VFR conditions expected with a chance
of showers/storms, especially between 00Z and 06Z. Winds
becoming more southerly or southwesterly around 10 kts or so.
Moderate confidence, but lower with timing of precipitation and
associated worse VSBY/CIG restrictions.

Wednesday...MVFR/IFR conditions possible to start, otherwise VFR.
Some showers and thunderstorms though should be around before a cold
front shifts offshore, with local restrictions possible. Southwest
winds becoming west-northwest 10-15 knots.

Thursday through Saturday...VFR. Northwest winds 10-15 knots
Thursday, then becoming light and variable at night and continuing
Friday into Saturday.


Made some changes to the forecast today and tonight. Models are
trending stronger with winds as they become
southerly/southwesterly tonight, and have hoisted a small craft
advisory for the Atlantic waters (except the far northern coast
of New Jersey) beginning at 8 pm.

Chances of showers/storms increase this afternoon, with local
visibility restrictions and higher waves/winds expected in their

Wednesday and Thursday...Southwest winds may gust to near 25 knots
to start Wednesday, the northwest winds may gust to near 25 knots at
times Thursday. Seas on the Atlantic coastal waters should build to
near 5 feet Wednesday before subsiding some Thursday.

Friday and Saturday...The conditions are expected to be below Small
Craft Advisory criteria as high pressure is mainly in place.

Increased the risk of rip currents for the New Jersey coast to
moderate given the trend of increasing winds today and
especially this evening. With onshore winds of 10 to 20 kts and
seas increasing late this afternoon and this evening to 3-5
feet, the risk of dangerous rip currents will likely be
increasing through the day. Thus, elevated the risk to moderate
for the NJ coast. The risk looks a little lower for the Delaware
beaches, owing to the less favorable angle between the surface
winds and the coastline.


MARINE...Small Craft Advisory from 8 PM this evening to noon EDT
     Wednesday for ANZ451>455.


Near Term...CMS/Fitzsimmons
Short Term...CMS
Long Term...Gorse
Marine...CMS/Gorse is the U.S. government's official web portal to all federal, state and local government web resources and services.