006 
FXUS61 KPHI 180513
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
113 AM EDT Wed Mar 18 2020

.SYNOPSIS...
High pressure slides across our region tonight into Wednesday before 
shifting offshore. A weak area of low pressure tracks across our 
area Wednesday night into Thursday morning along a developing warm 
front. Stronger low pressure tracks into the Great Lakes Thursday 
night which pulls the warm front across our region. A strong cold 
front follows for later Friday, then strong high pressure builds in 
from Canada for the weekend before shifting offshore early next 
week.

&&

.NEAR TERM /THROUGH TODAY/...
High pressure is moving into the region early this morning, with
winds beginning to slacken and directions becoming more variable
with time. Models are running too warm so far overnight, but
temperatures will likely begin to drop precipitously in the next
few hours as decoupling continues. I lowered temperatures a
little bit between 2 am and 8 am, especially northwest of
Philadelphia, in anticipation of this.

High pressure will move through the area today, with winds
acquiring a southeast direction on the upstream side this
afternoon. Large-scale descent will dominate, allowing for a
period of mostly sunny skies. Temperatures should warm close to
Tuesday's values.

Clouds will begin to increase late in the day as a midlevel
perturbation approaches the northern Mid-Atlantic this
afternoon. Elevated warm advection will begin in earnest after
18z, so the cloud cover increase should be fairly rapid.
Depending on timing, this may curb more substantial warming
during the late afternoon, but suspect the timing is late enough
that a statistical model blend will suffice for today's highs.

&&

.SHORT TERM /TONIGHT/...
A soaking rain event will occur tonight as a potent perturbation
moves through the region.

During the evening, strong isentropic ascent will occur in the
region as a low-level jet noses into the area. With projected
850-mb flow of 30-45+ kt by 06z Thursday, widespread rain should
develop in the central Appalachians and move into the region
late in the evening into the overnight hours. The timing of the
heaviest precipitation looks to be in the 06z-12z window, with
the CWA on the northern fringe of the low-level jet in a 
favorable region of midlevel differential cyclonic vorticity 
advection. With winds nearly parallel to the temperature 
gradient, lift should be quite pronounced during this period, 
with general model QPF in the 0.5-1.5 inch range across the area
thanks to PWs in excess of an inch by this time (highest 
northwest of the I-95 corridor). Though these rain amounts are 
by no means anomalously high and should occur for a 
sufficiently long duration in general, will need to watch the 
more susceptible locations for some poor-drainage and small-
stream flooding.

Temperatures will fall little during the overnight hours as
clouds/precipitation will keep radiational cooling in check. As
alluded to above, expecting thermal profiles to be warm enough
for all rain across the entire area.

&&

.LONG TERM /THURSDAY THROUGH TUESDAY/...
Summary...Temperature roller coaster into the weekend with a period 
of rain associated with a strong warm front Wednesday night and 
Thursday; Very warm air surges in for Friday followed by significant 
cooling for the weekend; Turning milder again early next week.

Synoptic Overview...A ridge from the Gulf of Mexico to the Southeast 
U.S. is forecast to build northward some into Friday. As this occurs 
a strong warm front advances northward across the Mid-Atlantic and 
Northeast. An upper-level trough then amplifies from the Midwest to 
the Great Lakes and eventually across the Northeast by late Friday. 
This will drive surface low pressure up across the Great Lakes and 
adjacent Canada, with a strong cold front sweeping off the East 
Coast by later Friday. The trough prevails in the East for much of 
the weekend before it lifts out and the flow turns more zonal into 
early next week.

For Wednesday night and Thursday...A potentially challenging 
forecast especially regarding temperatures and dew points. A 
weakening wave will slide across our region later Wednesday night 
and Thursday morning. A surge in warm air advection will result in 
an area of enhanced forcing for ascent aiding by a ribbon of modest 
frontogenetic forcing. As moisture increases northward (precipitable 
water values surge to around an inch), some of the rain should be 
heavy at times especially early Thursday morning. Rainfall amounts 
look to be on the order of 0.50-1.25 inches with the lower amounts 
across portions of Delmarva. The passage of this wave of energy 
looks to slow the northward advancement of a developing warm front. 
As a result, we slowed down the northward increase of temperatures 
and dew points Wednesday night and Thursday morning. There should be 
a lull for a time Thursday, however lingering northeast low-level 
flow and cloud cover should keep temperatures from warming as 
quickly especially for much of eastern Pennsylvania and northern New 
Jersey. The main surface warm front looks to make some better 
progress northward Thursday night as the flow better aligns from a 
southwesterly direction and starts to increase. Another round of at 
least showers may accompany the warm fronts arrival with this 
especially occurring across our western to northern zones. Some fog 
may become an issue as dew points rise.

For Friday...An amplifying upper-level trough shifting eastward from 
the Midwest and Great Lakes will drive strengthening low pressure up 
across the Great Lakes to adjacent Canada. Increase southwesterly 
flow is expected to allow the surface warm front to be to our north, 
placing our entire area into the warm sector. The 850 mb 
temperatures are forecast to be +10C to +16C across our region 
during Friday ahead of a strong cold front. It looks like the 
boundary layer becomes decently mixing given the substantial heating 
forecast, however the warm sector may be characterized with quite a 
bit of cloud cover. If the cloud cover ends up being quite 
extensive, then the warming will be held back some. At this time, we 
are still anticipating nearly the entire area to get into the 70's 
and some highs reaching 80F especially across Delmarva and perhaps 
into interior southern New Jersey. The rather warm air over land may 
allow the surface winds to back a bit more to southerly during peak 
heating, keeping some of the beaches cooler.

We will need to watch for the potential of some thunderstorms Friday 
afternoon into early evening as the cold front arrives. While there 
is stronger flow, it is not as clear with the amount of instability 
that will be present to support thunder and especially deeper 
convection. The model soundings show little to as much as 500 J/KG 
of instability. Much of the energy looks to peel off to our north as 
the cold front arrives and moves through, and therefore the 
precipitation with the front in the afternoon may be limited. 
However given the warmth and dew points around 60F, we will have to 
watch for some convective development. The showers should contain 
downpours given the precipitable water values are forecast to be 
near 1.5 inches ahead of the cold front. Everything clears out 
Friday night as cold air advection increases as well as significant 
dry air advection.

For Saturday and Sunday...Strong surface high pressure builds across 
the region later Saturday into Sunday before shifting offshore. This 
Canadian surface high will result in a much colder air mass compared 
to Friday, although highs should be closer to normal Saturday before 
going below normal Sunday. A northerly breeze Saturday will diminish 
as the surface high builds over our area.

For Monday and Tuesday...Surface high pressure is forecast to be 
positioned offshore which then weakens and moves farther away. 
Meanwhile, some southern stream energy should result in a weak 
surface low off the Southeast U.S. coast. This low tracks 
northeastward, although there may be an inverted surface trough 
extending to near our area for a time. This may bring some showers 
to the area especially Monday night and Tuesday, however the overall 
extent of coverage and timing is of lowered confidence. Temperatures 
however are forecast to be warming as we go through early next
week.

&&

.AVIATION /06Z WEDNESDAY THROUGH SUNDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, 
KILG, KMIV, KACY and surrounding areas.

Rest of tonight...VFR with lingering northwest winds becoming
light and variable. High confidence.

Wednesday...VFR with light and variable winds in the morning
acquiring a general southeast direction during the afternoon.
Speeds should generally remain below 10 kt. High confidence.

Wednesday night...Rapidly deteriorating conditions during the
evening as precipitation moves into the region between 02z and
06z. Conditions should quickly become IFR after onset of rain,
with the rain expected to last the rest of the night. Surface
winds will generally be east or southeast around/below 10 kt.
May see some LLWS after 06z as a south-to-southwest low-level
jet moves through the region. Moderate confidence.

Outlook...
Thursday...Widespread MVFR to IFR conditions continue Thursday 
morning, then some improvement later Thursday afternoon. A 
period of rain and some fog, with most of the rain overnight 
Wednesday night and Thursday morning. Southeast to east winds 
near 10 knots. Moderate confidence.

Friday...Areas of MVFR/IFR conditions probable early, then 
improvement to VFR for awhile should occur. Southwesterly winds 
increase to 10-20 knots with gusts up to 30 knots especially in the 
afternoon. Showers result in sub-VFR conditions in the morning 
especially in the RDG to ABE area, then potentially everywhere for a 
time in the afternoon and early evening. A few afternoon and early 
evening thunderstorms are possible. Low confidence on the overall 
timing and thunderstorm threat. 

Saturday and Sunday...VFR overall. Northwest to north winds 10-15 
knots with gusts to 25 knots Saturday, diminishing Saturday night 
and Sunday. Moderate confidence.

&&

.MARINE...
Extended the small craft advisory for the Atlantic waters
through 7 am, as seas remain near/above 5 feet. Should see seas
fall below 5 feet thereafter, as winds become easterly 10 to 15
kt during the afternoon.

Winds will become southeast and increase to 15 to 20 kt (lighter
on Delaware Bay) tonight, with seas gradually building. May see
advisory conditions develop late in the night.

Rain will overspread the waters tonight, with visibility
restrictions expected.

Outlook...
Thursday...A period of wind gusts up to 25 knots possible with 
seas on the ocean building to 5 feet. 

Friday...Small Craft Advisory conditions probable, however a very 
warm air mass will be moving over the cooler waters which should 
limit the mixing some especially away from the coasts.

Saturday and Sunday...Northwest wind gusts to 25 knots Saturday 
should diminish through the day and especially at night and Sunday 
as the winds turn north to northeast.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...Small Craft Advisory until 7 AM EDT this morning for 
     ANZ450>455.

&&

$$
Synopsis...Gorse
Near Term...CMS
Short Term...CMS
Long Term...Gorse
Aviation...CMS/Gorse
Marine...CMS/Gorse