Scientific Forecaster Discussion
fxus61 kphi 140248
Area forecast discussion
National Weather Service Mount Holly New Jersey
948 PM EST Fri Dec 13 2019
low pressure will strengthen while moving across the mid-Atlantic
region late tonight and Saturday. The low will then move up across
New England and into eastern Canada through Sunday. High pressure
will build over the area later Sunday before moving away Monday.
Another low and its associated fronts will affect the area Monday
afternoon through Tuesday. High pressure will then follow for the
second half of next week.
Near term /through Saturday/...
the coastal front continues to shift inland this evening but very
slowly. The continuation of strong warm air advection with sharp
veering of the winds with height in the low-levels (captured by the
00z Wallops Island, Virginia raob) will gradually erode the rather shallow
cold air still stuck across parts of eastern Pennsylvania and
northwestern New Jersey (impressive sharp very low-level inversion
on the 00z Sterling, Virginia raob). Surface temperatures are responding
with a gradual rise in those areas. Otherwise, temperatures are in
the 50s east of the coastal front. The GFS lamp guidance is handling
the temperature and dew point trends well thus far, therefore
weighted the hourly grids more heavily with it through the
overnight. The rainfall amounts tonight were increased some across
especially southern Delaware, eastern New Jersey to northern New
Jersey given amounts so far and additional moderate to at times
heavy rain moving northward. The rainfall amounts should drop off
westward and especially across the far western part of the forecast
area (i.E. Berks County, Lehigh Valley into the poconos) as the
stronger moisture surge looks to be more eastward.
Higher dew points and warmer air surging up and over the lingering
shallow cold layer has resulted in fog, however as the warmer air
wins out the visibilities are improving. Therefore, will just
maintain some fog mention especially near and west of I-95 for
awhile tonight. Some locally dense fog may still occur. Overall, a
milder but wet night across the area overnight.
Otherwise, low pressure continues to develop to our south and it
will intensify as it moves towards the northeast and into our region
overnight. Strong positive vorticity advection and height falls ahead of the surface low will
continue to promote surface cyclogenesis ahead of the system as a
strongly negatively tilted mid-level trough lifts out of the lower
Mississippi River valley and into the mid-Atlantic. A strong 150-
knot 300 mb jet streak developing directly over the Delaware valley
will also provide forcing for ascent, and there is plenty of
moisture available with strong southerly flow off the Atlantic ahead
of this system.
Pwats from 1-1.5 inches advect into the region, thus, a widespread
moderate to heavy rain event is in store tonight. At this point, the
heaviest rainfall is likely to fall just ahead of the surface low
through 8 am Saturday. The progressive nature of this storm will be
the one saving Grace, as the current track of the surface low takes
it right across much of the garden state. As the low moves overhead,
rainfall rates will likely taper off a tad as we head into the later
morning hours. Some hi-res models are suggesting a line of heavier
rain or showers will develop into the early afternoon. There is some
instability associated with this, thus we cannot rule out a rumble
of thunder. After this, we will begin to dry out, save for a few
wrap around showers into the evening and early overnight hours. This
could mix with some wet snow for a brief time in the Poconos and
Sussex County, NJ, but no significant impacts are expected at this
Short term /Saturday night through Sunday night/...
quieter conditions Sunday and Sunday night as the low pulls off to
the northeast and high pressure continues to build into the region.
As low pressure lifts to our northeast, the pressure gradient behind
it will tighten quite significantly. Winds above the surface
increase as well and could mix down to the surface. This could
potentially lead to wind gusts of at least 25-35 mph, with
occasional higher gusts of near 40 mph late Saturday afternoon,
Saturday night into early Sunday. High temperatures will be cooler
in the low to mid 40s. Lows in the upper 30s Saturday night as
the low pulls out of the area.
Long term /Monday through Friday/...
Monday thru Tuesday night...
An area of low pressure will form across the lower Mississippi
Valley Monday and then move NE into the middle Atlantic area by
Tuesday. The system is not strong as it nears, but much like
the present system, it will deepen as it moves through. A shield
of precipitation will develop and run ahead of the system,
arriving across our area Monday. The precipitation will
continue into Tuesday before tapering off. Right now, much of
the precip will be rain, but the onset may contain some mixed or
frozen precip, especially if it begins early enough Monday.
Right now the rain/snow/rain-snow mix/ wording will be used, but
as we get closer to the event, some sleet or perhaps some
freezing rain may be added to the fcst.
Highs Monday will be a little below normal, but by Tuesday, readings
should climb above normal with low/mid 50s for the Delaware valley
(s/e) and mid/upper 40s for the Lehigh Valley, Berks County and nrn
New Jersey. A cold front will follow for Tue night, ending the milder
Wednesday thru Friday...
Colder and drier air arrive across the area Wednesday and lasts into
the end of the week. As high pressure noses in, fair weather is
expected, so we will keep the fcst dry for Wed thru Fri attm.
Aviation /03z Saturday through Wednesday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg, kilg,
kmiv, kacy and surrounding areas.
Overnight...IFR ceilings will improve to MVFR for a time especially
from phl south and eastward, otherwise ceilings should become
widespread LIFR overnight along with periods of rain some heavy
at times. Northeast winds 4-8 knots, becoming mostly light and
variable. Strong southerly winds around 2000 feet late tonight
and early Saturday morning along the coast will result in a
period of low-level wind shear mainly at Acy. High confidence
on IFR occuring; lower confidence on how low conditions will get
and the extent/duration of improving ceilings to MVFR.
Saturday...IFR conditions, especially ceilings, through much of the
day (lifr ceilings should be around to start the day). Rain at times
will result in lowered visibility, however this should become more
showery by afternoon. Light and variable winds will increase out of
the east to southeast 5-10 knots, then become south to southwest 10-
15 knots with gusts to 20-25 knots toward mid to late afternoon.
Saturday night...an improvement to MVFR is possible during the
evening hours and may improve to VFR overnight. Winds become
westerly around 10-15 knots with gusts of 20-30 knots possible
at times. Moderate confidence.
Sunday...MVFR ceilings may develop after sunrise for a few hours,
before lifting to VFR by late morning into the afternoon. West winds
15-20 knots, with gusts 20-30 knots. Moderate confidence.
Sunday night...VFR conditions. West winds early, become light and
variable overnight. Moderate confidence
Monday thru Tuesday night...MVFR or IFR expected with low ceilings.
Rain/snow and fog expected. Low confidence.
Wednesday...VFR returning to the area. Moderate confidence.
Small Craft Advisory tonight and Saturday as seas are above 5
feet on the ocean waters. Winds will remain 10 to 15 knots out
of the east with gusts around 20 knots.
Winds will turn more to the south and then southwest on Saturday
increasing to 15 to 20 knots with gusts around 25 knots
starting late afternoon/early evening. Seas will remain elevated
Saturday night...gale watch in effect for all waters, including
Delaware Bay as 35-40 knot wind gusts could develop behind a
departing low pressure system.
Sunday...gale watch continues for the Atlantic coastal waters as
wind gusts could continue to reach 35 knots through midday before
diminishing to advisory levels during the afternoon.
Sunday night...winds lower below advisory levels overnight and seas
subside below 5 feet as well.
Monday thru Tuesday night...Small Craft Advisory expected. Rain and fog expected.
rainfall amounts will range from 0.50-1.00 inches for portions of
eastern Pennsylvania, northwestern New Jersey, and northern
Delmarva. This rainfall amounts will not cause more than some
ponding on roadways and nuisance type flooding. However, farther
east, rainfall amounts are forecast to range from 1.00-1.50 inches,
with some spots potential reaching 2 inches, especially across
northeastern New Jersey, portions of coastal New Jersey, and
southern Delaware. Again, ponding on roadways and poor drainage type
flooding will be likely in these areas. However, for portions of
northern and central New Jersey, as well as Bucks County have the
potential for more widespread flooding impacts, including some river
and creek/stream flooding. Therefore a Flood Watch has been issued
for Bucks County in Pennsylvania, as well as portions of northeast
and central New Jersey. The majority of the rainfall will occur
tonight and the faster responding creeks and roadway flooding would
occur tonight into early Saturday morning. However, some larger
rivers, like the Passaic and Millstone, would not flood until later
Saturday into Saturday night.
confidence has increased that we will see minor tidal flooding on
the Atlantic oceanfront, south shores of the Raritan Bay and
adjacent back bays.
Astronomical tides remain high thanks to the full moon which just
occurred on Thursday. We will have persistent onshore flow through
much of the day Saturday. A coastal flood advisory is in effect
for the Atlantic oceanfront and the south shores of the Raritan
Bay. For the Delaware Bay and tidal portions of the Delaware
River, spotty minor flooding is likely, but there is less
confidence there will be widespread minor flooding, so have held
off on any advisory in these areas for now.
We will see an abrupt shift to offshore flow by Saturday night.
Therefore, not expecting the coastal flooding to continue into
subsequent high tide cycles.
With only a brief period of southerly flow, tidal flooding is not
expected on the northeastern shores of the Chesapeake Bay.
PA...Flood Watch through Saturday morning for paz105-106.
New Jersey...Flood Watch through Saturday morning for njz008>010-015.
Flood Watch until 6 am EST Saturday for njz012>014-019-020-026-
Coastal flood advisory from 7 am to 1 PM EST Saturday for
Delaware...coastal flood advisory from 7 am to 1 PM EST Saturday for
Marine...gale watch from Saturday evening through late Saturday night
Small Craft Advisory until 6 PM EST Saturday for anz450>455.
Gale watch from Saturday evening through Sunday morning for
Small Craft Advisory from 1 PM to 6 PM EST Saturday for anz430-