Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

fxus61 kphi 101157 
afdphi

Area forecast discussion
National Weather Service Mount Holly New Jersey
657 am EST Tue Dec 10 2019

Synopsis...
low pressure will pass north of the area today with its cold front
crossing through the region late this afternoon into this evening.
Strong high pressure will build in from the west Wednesday night
into Thursday and moving offshore Thursday night. A low pressure
system will move through the southeastern states and into the mid-
Atlantic late Friday into Saturday. The low will quickly move off to
the northeast allowing high pressure to build into the region for
the start of the new week.

&&

Near term /until 6 PM this evening/...temps remain well above
normal today and i've had to keep boosting temps through the
overnight hours. Expect temps to peak between 10-1pm before rain
showers and a cold front quickly drop temps back into the 40s.

Previous discussion...lingering light rain is slowly coming to
an end as warm air advection pushes into the region behind a
warm front. Temperatures across Delmarva are warming into the
60s which is more than 10 degrees f above our normal climo Max
temp for the daylight hours. This in turn is warming the soil
temps quite a bit more than guidance likely is handling and
we'll return to this later in the short term discussion.

Synoptically there's a deep upper level trough with a strong 250mb
jet nosing into northern New Jersey. The jet eventually pushes
towards the north this morning and then we start to lose the
synoptic lift as well as the higher precipitable water values. The 00z sounding
from okx had 1.28" of precipitable water which based on the Storm Prediction Center precipitable water climo would
be a daily record! That all certainly helped contribute to the 1-
1.5" of rain across the i95 corridor early today however I expect
the 12z soundings to be closer to 0.9-1.0".

The mid-Atlantic will sit squarely in the middle of the warm sector
today as southerly winds continue to pump warm air into the region
through the morning. Because of this I went on the slightly
higher side of guidance with highs in the upper 50s to low 60s
across the region even under overcast skies. 925mb temps from
the notoriously cold GFS warm to +9c to +12c. If this were under
clear skies those 925mb temps support temps near 70! Which
would be record territory.

By late in the afternoon we'll see an abrupt end to the warm air as
a cold front will start to push through. The front looks pretty
certain to be anafrontal which means that the precip lags behind the
front rather than ahead of it.

&&

Short term /6 PM this evening through 6 PM Wednesday/...
as mentioned in the near term the anafrontal nature of this system
presents an interesting forecast challenge. The cold front will
track through the region and setup up offshore overnight. This will
usher in strong cold air advection at the same time that the final
round of precip starts to move in. So this is where the forecast
gets tricky.

Models are showing the potential for a strong meso band
of snow to setup as the precip tracks through. There a potent 700mb
fgen band that tracks through at roughly the same time as the we sit
in the right rear entrance of the upper level jet axis. This means
we should see some pretty potent lift across the region and with the
dgz becoming fully saturated we should see a pretty good thump of
snow especially across northern New Jersey and eastern PA. The
challenge will be can the rate of snow dynamically cool both the
column and the surface such that we'll see the snow accumulate.

Here's where i'm coming back to the warmer than normal skin temps
highlighted in the near term discussion. Skin temps are forecast to
warm towards the mid 50s from the i95 corridor east. If we get that
materializes, its going to be a challenge for ME to see pavement
temps across the region cool that fast even as snow falls. Grassy
and elevated surfaces will certainly cool faster than the pavement
temps so I anticipate we'll see some moderate to even at times
heavy snow falling in the hours leading up to rush hour, however
accumulations on the Road surfaces should be generally 1 to 2 inches
north and west of the i95 corridor with a dusting to perhaps a half
and inch on grassy surfaces through Wednesday morning.

At this point, the Wednesday morning commute looks like it will be
impacted by snow, but I dont quite yet have a great handle on the
degree of impact. The morning commute on Wednesday could become
messy, but I generally feel like Road conditions will be on the
wet to perhaps slushy side rather than seeing significant snow
accumulations. We'll certainly need to continue to pay attention
to how the evolution of today affects the skin temps tomorrow
and how that affects snow accumulation.

Precip rapidly comes to an end in the afternoon and by late
Wednesday afternoon we should start to see clearing skies with temps
getting close to the mid 30s for afternoon highs on westerly
flow.

&&

Long term /Wednesday night through Monday/...
high pressure will build across the region on Thursday, moving
offshore Thursday night. While there will be plenty of sunshine
across the area, the air accompanying the high will be quite cold,
and dry, resulting in highs in the 30s across the majority of the
forecast area with the southern Poconos struggling to break out of
the 20s. With relatively clear skies to start the night, the
temperatures should radiate efficiently and lows Thursday night will
drop down into the teens to lower 20s. Areas right along the coast
may be more moderate and remain in the upper 20s to lower 30s.

The high pushes off towards the Canadian Maritimes on Friday leaving
our area in east to southeast flow. In general, Friday looks to be
relatively dry to start but precipitation will start to push into
the area ahead of the next approaching system. Temperatures at the
onset may be quite cold but it looks like the precipitation holds
off until the afternoon and when temperatures have risen well above
freezing in most areas so expect just plain rain to fall.

Low pressure will track from the Gulf Coast states through the
Carolinas and up into the mid-Atlantic Friday night into Saturday
before continuing up in the New England by Sunday. Ahead of the
system we will be able to warm up quite a bit with highs on Friday
rising into the 40s to lower 50s and we should see plain rain fall
across the region. There is some concern that some wintry mix may
develop across the northern areas Friday night into early Saturday.
Temperatures may remain in the mid to upper 30s across portions of
the southern Poconos and northwest New Jersey, and with warm air
moving in aloft, there may potential for some patchy freezing rain
to occur. However, not overly confident that we will see temps drop
enough before the warm air and precipitation arrive so will remove
any mention of freezing rain from the forecast.

The low will track through the region on Saturday and then move
northward towards New England, deepening as it tracks north. With
warm air in place, it looks to be another rain event for the region
with the potential for some areas of heavy rain to occur as pwats
rise to around 1.0-1.3". As the storm pulls away, the bulk of the
precipitation should go with it but we may continue to see some
showers occur through part of Sunday with some snow possibly mixing
in on the back side of the storm across the higher elevations.

Conditions start to dry out, especially later Sunday, as high
pressure starts to edge its way into the area. Monday looks to be
dry at this time as the high builds across the region. However, the
next low pressure starts to develop across the southern US and will
start to move towards our area in the new week.

&&

Aviation /12z Tuesday through Saturday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg, kilg,
kmiv, kacy and surrounding areas.

Tuesday...MVFR with periods of IFR ceilings possible. Expect
scattered rain showers through the afternoon today with local IFR in
reduced visibilities. Southwesterly winds around 10 kts turning
westerly after 18z. Moderate confidence.

Tuesday night...MVFR/IFR in rain changing to snow overnight.
Winds west/northwest 5-10 knots. Moderate confidence.

Wednesday...widespread MVFR/IFR in rain becoming snow in the
morning. By mid afternoon conditions will rapidly improve to VFR as
the precipitation comes to an end. West winds near 10 kt.

Outlook...

Thursday through Thursday night...VFR conditions expected. Light
northerly winds during the day becoming light and variable
overnight. High confidence.

Friday..mainly VFR conditions expected. MVFR or lower conditions
possible as rain develops through the afternoon and evening. Light
northeast to east winds. Moderate confidence

Friday night through Saturday...MVFR or lower conditions expected in
rain. Conditions may start to improve late Saturday. East winds
around 8 to 12 knots Thursday night veering to the south Saturday
morning and then to the southwest Saturday afternoon. Moderate
confidence

&&

Marine...
sca's continue for the ocean waters today as waves are well in
excess of 5 feet across the region. Strong winds aloft aren't mixing
to the surface and so while there could be an occasional gust up to
30 knots the winds will generally be westerly 10 to 20 knots through
the day today. Waves stay elevated until late overnight Tuesday
night and so the Small Craft Advisory was extended for the ocean water while being
cancelled for the Delaware Bay.

Outlook...

Wednesday through Wednesday night...conditions are expected to
stay below Small Craft Advisory criteria. However, wind gusts
around 25 knots are possible, mainly late Wednesday into early
Thursday.

Thursday...sub Small Craft Advisory conditions are expected on
the area waters with north to northeast winds gusting around 15
to 20 knots and seas around 2 to 4 feet.

Friday...increasing winds and building seas expected, especially
later in the day, but expected to remain below Small Craft Advisory
conditions through the day. East winds around 10 to 15 knots with
gusts around 20 knots and seas around 2 to 4 feet.

Friday night through Saturday...east winds will continue to increase
Friday evening and gusts are expected to exceed 25 knots Friday
night. Winds will turn to south and then west on Saturday. Seas will
build from 2 to 4 feet to 3 to 5 feet by early Saturday morning, and
continuing to build to 5 to 8 feet through the day. A Small Craft
Advisory will likely be needed.

&&

Phi watches/warnings/advisories...
PA...none.
New Jersey...none.
Delaware...none.
Maryland...none.
Marine...Small Craft Advisory until 4 am EST Wednesday for anz450>455.

&&

$$
Synopsis...meola
near term...deal

National Weather Service Glossary of Abbreviations