863 
FXUS61 KPHI 222023
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
423 PM EDT Wed Apr 22 2020

.SYNOPSIS...
A warm front is anticipated to lift into our region tomorrow. Low 
pressure is expected to move from the Ohio River Valley tomorrow 
evening to the waters off Nova Scotia on Friday evening. Weak high 
pressure should follow for Friday night into early Saturday. Another 
low is forecast to progress from the Ohio River Valley to the waters 
off Nova Scotia during the period from Saturday night through 
Monday. High pressure approaching from the west is expected to build 
across our region on Monday and Tuesday.

&&

.NEAR TERM /UNTIL 6 AM THURSDAY MORNING/...
Amplified troughing in the Canadian Maritimes will progress slowly 
northeastward overnight, with northwest midlevel flow continuing 
across the Mid-Atlantic. A residual surface pressure gradient exists 
across the area today, leading to breezy conditions this afternoon. 
However, as the associated surface low in eastern Canada continues 
to weaken and lift northeastward, the pressure gradient will relax 
as high pressure to our northwest approaches. The region will reside 
in a local col (or mathematical saddle point) during the overnight 
hours, which will promote a rapid calming of winds during the late 
evening. It is at this time when temperatures are expected to drop 
rapidly, with conditions likely approaching freezing in the northern 
third to half of the area (despite increasing cloudiness and 
associated warm advection late). Though raw model guidance provides 
a fairly large range of solutions temperature-wise tonight, think 
the colder guidance is more likely since (1) higher-level clouds 
tend not to preclude stronger cooling, at least as much as modeled, 
in this region, (2) fewer clouds during the late evening should be 
timed with the weakening winds, and (3) there is good consensus 
regarding the timing of the nocturnal decoupling. As a result, 
decided to issue a freeze warning generally from Berks County to 
Ocean County northward and a frost advisory for southeast PA and 
southern NJ outside of Philadelphia and the beaches. Confidence is 
rather low on the specific delineation of these products, and 
evening trends will be monitored closely to determine if some fine-
tuning will be required.

The only other concern is with potential for precipitation in the 
Poconos late tonight. Generally suspect that the precipitation will 
be well to the north/west of the area, but there are indications 
that a weak perturbation will straddle the New York/Pennsylvania 
border between 06z and 12z. Cannot rule out some light rain/snow in 
the southern Poconos, so kept in some slight-chance PoPs during this 
period. Any precipitation is expected to be light with minimal 
impacts.

&&

.SHORT TERM /6 AM THURSDAY MORNING THROUGH FRIDAY/...
A strong midlevel vortex will eject from the central Plains on 
Thursday, with difluent flow downstream in the Appalachians and Mid-
Atlantic. Warm-advection precipitation will develop in the 
Ohio/Tennessee Valleys during the day, spreading northeastward 
toward the East Coast by late afternoon. GFS/ECMWF timing is 
decently similar (generally between 18z Thursday and 00z Friday), so 
confidence was high enough to lower PoPs early in the day and 
increase them by 00z. Precipitation will ramp up considerably 
Thursday night, with a lengthy period of occasionally moderate to 
heavy rain expected as several ingredients come together: (1) an 
increasing low-level jet in advance of a surface low moving through 
the Ohio Valley, supplying anomalous moisture into the northern Mid-
Atlantic (1.25-1.5 in. PWs 06z-12z Friday), (2) considerable warm 
advection (aided by the low-level jet) as a zonally-oriented 
baroclinic zone becomes established east of the low, (3) an upper-
level jet streak departing the East Coast, leaving the region in the 
favorable right-entrance region (allowing for considerable upper-
level divergence), and (4) differential cyclonic vorticity advection 
increasing in advance of the approaching midlevel perturbation 
(and coinciding with the nocturnal maximum in the low-level jet).

The result is QPF on the order of 1-2 inches by midday Friday. 
Precipitation rates will not be excessive given the overall setup 
(i.e., the region generally being on the cold side of the attendant 
surface low); however, would not be surprised to see a few instances 
of nuisance-type flooding in the usual spots (especially if the QPF 
maximum overlies the urban corridor, a distinct possibility given 
the 12z model output).

Precipitation may linger through the afternoon via wraparound 
trajectories on the northwest side of the surface low (likely 
tracking through the Delmarva region during the day). Decided to 
increase PoPs a bit for Friday afternoon as a result. However, the 
heaviest precipitation should be over by this time.

Regarding temperatures, stuck pretty close to statistical consensus, 
though confidence is a little lower than average with highs on 
Thursday given uncertainty with the increasing cloudiness and 
approaching precipitation. This results in highs in the upper 40s to 
low 50s in the southern Poconos and northwest New Jersey and mid 50s 
to low 60s elsewhere. Temperatures should drop little Thursday night 
as clouds and precipitation will limit nocturnal cooling. Friday's 
temperatures will be tricky, as clouds/precipitation will likely 
limit heating north of the low, but temperatures may soar south of 
its track. Given the low's track will likely be near or over 
southern portions of the CWA, this suggests a large temperature 
gradient may exist across the area. For now, I cooled temps up north 
slightly and warmed them in Delmarva to increase the general 
gradient, but the result will likely be much sharper than the 
current forecast suggests.

&&

.LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/...
Overview: The active weather pattern continues into early next week. 
The low pressure system Sunday and Monday is expected to take a 
similar path as the Thursday/Friday low pressure system, bringing 
another chance for widespread rain across the region. Another low 
pressure system is expected to approach the region mid week next 
week.

Details:

Saturday...A surface ridge is expected to briefly build in on 
Saturday, bringing a lull in between the two low pressure systems. 
Temperatures will remain slightly below normal, with highs in the 
50s and lower 60s. 

Saturday night through Monday...The next low pressure system will be 
crossing from the Ohio Valley to the Canadian Maritimes. We should 
stay mostly in the cold sector, so we expect most of the 
precipitation to be a steady stratiform rain. While there remains 
quite a bit of uncertainty with how much rain we will see. However, 
there is a concern that depending on how much rain we see with the 
Thursday/Friday system, that some small creeks and streams could 
already be elevated. We will continue to monitor this especially as 
we see how the Thursday/Friday system evolves.

Monday night and Tuesday...High pressure is expected to once again 
briefly build in, bringing another period of tranquil weather, 
albeit cool conditions. Temperatures are expected to be 10 to 15 
degrees below normal, with highs in the 40s and 50s.

Tuesday night and Wednesday...Another low pressure system may 
approach the region. This one though appears to be taking a 
different path than the two earlier systems, with a mid and upper 
level trough digging southeast out of south central Canada. However, 
there is considerable difference between the models in how far south 
this trough will dig. Thus, for now, have stayed close to a blend of 
guidance through this period.

&&

.AVIATION /21Z WEDNESDAY THROUGH MONDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, 
KILG, KMIV, KACY and surrounding areas.

Tonight...VFR with west to northwest winds 10 to 20 kt with gusts to 
30 kt through early evening rapidly diminishing and becoming 
light/variable overnight. High confidence.

Thursday...VFR through most of the day, but clouds will be 
increasing and lowering with time, and precipitation will likely 
move in near/after 21z with sub-VFR conditions developing fairly 
quickly. Light/variable winds becoming southerly around 10 kt late. 
Medium confidence.

Thursday night...Sub-VFR conditions and rain probable with winds 
generally northeast (north of PHL) to southeast in Delmarva and 
southern New Jersey (speeds generally 5 to 15 kt). Some LLWS 
possible. Medium confidence.

Friday...Sub-VFR persisting for much of the day, though some 
improvement may occur late. Rain likely, especially in the morning. 
Winds generally northeast to north 5 to 15 kt becoming northwest 
with time. Medium confidence. 

Outlook...

Friday night and Saturday...Mostly VFR conditions are expected.
Winds are expected to shift from northerly to southeasterly by 
mid day Saturday. Wind speeds of 5 to 15 kt. Moderate 
confidence.

Sunday...Conditions could lower to MVFR or even IFR as rain moves 
into the region. Southeasterly winds may shift to northwesterly 
through the day with wind speeds of 5 to 10 kt. Low confidence.

Monday...Conditions are expected to improve to VFR as the rain 
leaves the region. Northwesterly winds of 10 to 15 kt. Moderate 
confidence.

&&

.MARINE...
Small-craft advisory conditions continue on the waters this 
afternoon (and occasionally, a gust to around 35 kt has been 
observed on the central/southern New Jersey coast). However, winds 
are trending downward during the past couple of hours, and continue 
to expect sub-advisory conditions to become the norm on Delaware Bay 
by mid-evening. Conditions on the Atlantic should improve by 
midnight. As such, made no changes to the small craft advisory 
through the evening hours.

Thursday through Friday...After a period of light/variable winds in 
the morning, an east to south wind will become established during 
the afternoon. Advisory-level winds may develop on the ocean by 
evening, especially on the Delaware/southern New Jersey waters. 
During this period, a steady rain should develop and continue 
through most of Friday. By midday, winds will initially weaken and 
become somewhat variable in direction, then switch to north and 
become a little stronger (speeds 10 to 20 kt with higher gusts), 
with another period of advisory-level winds possible late in the 
day. Seas will build to 5 feet by Thursday evening and likely remain 
near/above advisory criteria through Friday.

Outlook...

Friday night and Saturday...Elevated seas may linger on the 
coastal waters through this period. Otherwise, winds should stay
below SCA criteria.

Sunday and Monday...Winds and seas are expected to increase on all 
waters leading to a prolonged period of SCA conditions.

&&

.FIRE WEATHER...
The combination of continuing gusty winds for today along with 
relative humidities dropping below 30 percent may lead to an 
increased risk of wild fires. The limiting factor however will 
be fuels due to recent rainfall. In coordination with our 
partners, we've issued an SPS highlighting this threat for 
central and southern NJ (Mercer to Monmouth Counties southward),
Delaware, Southeastern PA, and northeastern MD as this area 
generally saw less than a quarter of an inch of rain Tuesday.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...Freeze Warning from midnight tonight to 8 AM EDT Thursday for 
     PAZ060>062-103-105-106.
     Frost Advisory from midnight tonight to 8 AM EDT Thursday for 
     PAZ101-102-104.
NJ...Freeze Warning from midnight tonight to 8 AM EDT Thursday for 
     NJZ001-007>010-012-013-015-020.
     Frost Advisory from midnight tonight to 8 AM EDT Thursday for 
     NJZ016>019-021-022-027.
DE...None.
MD...None.
MARINE...Small Craft Advisory until 11 PM EDT this evening for 
     ANZ450>455.
     Small Craft Advisory until 8 PM EDT this evening for ANZ430-
     431.

&&

$$
Synopsis...Johnson
Near Term...CMS
Short Term...CMS
Long Term...Johnson
Aviation...CMS/Johnson
Marine...CMS/Johnson
Fire Weather...Johnson