Area Forecast Discussion
Issued by NWS Mt. Holly, NJ
FXUS61 KPHI 290738
Area Forecast Discussion
National Weather Service Mount Holly NJ
238 AM EST Tue Nov 29 2016
Strong low pressure is forecast to move across the Great Lakes
during the mid week period. The low will pull a couple of warm
fronts through our region Tuesday and Wednesday before a strong
cold front arrives from the west Wednesday night. A secondary low
is expected to develop over southern New England Wednesday night
before moving across Nova Scotia Thursday. High pressure follows
for Friday into the weekend, then low pressure or a cold front
may affect our weather Sunday night and Monday.
.NEAR TERM /THROUGH TODAY/...
A powerful upper-level trough will remain across Plains today,
however a strong short wave trough rotating northward up across the
eastern Great Lakes early this morning will help drive periods of
rain into our area. Today will be part one of much needed rainfall.
As the aforementioned system lifts into the Great Lakes, a southerly
low-level jet on the order of 50-65 knots overspreads the area this
morning. This will then shift east during the afternoon. This
feature will drive increasing warm air and moisture advections
across our area. The moisture advection is evident in the low-level
theta-e fields as well as PW values increasing to 1.5 inches by
later this morning. This will result in a corridor of enhanced lift
across much of the area, therefore periods of rain will occur from
southwest to northeast, some of which can be heavy at times. Given
the soaring PW values, rainfall amounts should average in the 0.50-
1.50 inch range. For more details on the rainfall, see the hydrology
section below. While some elevated instability may develop, the
profiles via forecast soundings are rather saturated therefore we
opted to not add in thunder at this time. We continued with a rapid
increase in the PoPs this morning, and the steadier/heavier rain may
be centered from the I-95 corridor on north and westward.
As a surface warm front lifts northward during the course of the
day, pushed by the robust low-level jet, boundary layer warming will
allow for turbulent mixing and therefore breezy conditions are
expected to develop for much of the area especially closer to the
coast. The gusty winds will then diminish toward evening. High
temperatures are mostly a blend of continuity and MOS. The very near
term hourly temperature and dew point grids were adjusted with the
latest obs, then the LAMP/LAV guidance was blended in.
.SHORT TERM /TONIGHT/...
A powerful upper-level trough remains anchored across the Plains
tonight. An initial impulse continues to track to our north pulling
a warm front with it. A robust low-level jet will be shifting
offshore early this evening, with much if not all of the lift going
with it. This should rapidly diminish any rain as it tapers to some
showers. There is some drying forecast to overspread our area,
especially in the mid levels, however the wind field is forecast to
be much lighter. This should result in lots of lower level moisture
remaining in place, and at least some forecast soundings indicate
that an inversion develops which helps to hold in the low-level
moisture. It is possible that enough drying occurs especially across
the western zones where the low clouds partially clear out for a
time. Given the light winds and moisture left around, some fog is
expected to develop. This could become more widespread especially if
the lower cloud cover thins or clears for a time. Given the
uncertainty regarding the extent of the low clouds, not anticipating
widespread dense fog at this time.
The next impulse rotating around the parent closed low will start to
approach from the southwest toward morning. This will increase some
warm air advection as another low-level jet begins to approach. The
ascent associated with this should result in some rain developing
once again. As of now, the bulk of this looks to hold off until
during the day Wednesday. Low temperatures were a MOS/continuity
blend and this results in mainly a milder night.
.LONG TERM /WEDNESDAY THROUGH MONDAY/...
A deep mid level feature is expected to pass across the Great
Lakes on Wednesday and Wednesday night before reaching southern
Quebec and Atlantic Canada late in the week. A mid level ridge
should begin building over the eastern states during the weekend.
The model guidance begins to diverge significantly for the early
part of next week.
Warm advection aloft along with a connection to the Gulf of
Mexico will result in the development of additional rain in our
region on Wednesday. As a cold front begins to approach from the
west late in the day, additional lift should result in increasing
rainfall rates. A moderate to heavy rainfall is anticipated into
Wednesday evening along with a chance of thunder. Additional
rainfall totals of 0.75 to 1.25 inches are possible across parts
of eastern Pennsylvania and northern New Jersey with lesser
amounts on the coastal plain. Locally higher amounts are possible
in the upslope regions.
The cold front is expected to pass though our region around
midnight on Wednesday night. Temperatures will be about 15 degrees
above normal from Tuesday night into Wednesday evening ahead of
Dry air and a return to near normal temperatures should follow
for the late week period into the weekend. A west northwest flow
may bring some cold advection stratocumulus at times, especially
to our northwestern counties.
We have mentioned a chance of precipitation for Sunday night and
Monday. However, the spread in the guidance makes this a low
confidence forecast. The GFS is suggesting a cold frontal passage
while the ECMWF is indicating that a strong area of low pressure
will influence our weather at that time.
.AVIATION /08Z TUESDAY THROUGH SATURDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.
Today...Clouds lower through this morning. Rain overspreading the
area from southwest to northeast mainly between 10-15Z, then periods
of rain continues for much of the day. The ceilings quickly lower to
MVFR after the rain begins, then periods of IFR for much of the day.
The visibilities will lower to MVFR/IFR during heavier rain.
Low-level wind shear is included this morning as a southerly low-
level jet (at 2,000 feet) increases to 45-60 knots. This occurs
initially when surface winds are southeasterly at 10 knots or less.
The surface winds increase after a warm front lifts north later in
the morning and the low-level jet gradually shifts east, therefore
low-level wind shear concerns generally decrease. Southerly surface
winds will increase to 12-18 knots with gusts to around 25 knots
across much of the area from later this morning through mid/late
afternoon. Less confidence regarding gusts at RDG and ABE.
Tonight...Any lingering showers end early with MVFR/IFR ceilings
continuing, however areas of fog should develop as clouds may thin
some in combination with light winds and lots of leftover moisture.
Confidence regarding low ceilings is lower than average as they may
partially clear for a time especially in the RDG to ABE corridor.
Wednesday...IFR conditions with low ceilings and fog should
improve to MVFR and possibly VFR during the day. However, rain is
expected to build into the region and it will become moderate to
heavy at times with the potential for IFR conditions returning.
Wednesday night...MVFR and IFR conditions in moderate to heavy
rain during the evening with a chance of thunder. A strong cold
front should arrive around midnight followed by clearing and
improvement to VFR. Low level wind shear is possible in the
evening due the the expectation of a 45 to 50 knot low level
southwesterly jet at that time.
Thursday through Saturday...mainly VFR.
A Small Craft Advisory is in effect for all zones today and the
Atlantic Coastal water zones tonight.
A southerly low-level jet on the order of 50-65 knots will move
across the region today. This will allow warmer air to stream
northward along with a surface warm front. Periods of rain will also
occur and this should keep the low-levels from mixing as efficiently
(poor lapse rates). As a result, kept a strong advisory in place and
held off on upgrading to a Gale Warning however there still is a
possibility for a few local gale force gusts by later in the day
especially with any heavier downpours. The winds diminish tonight as
the low-level jet shifts offshore, and with higher dew points
remaining for awhile this could allow for some fog to develop. The
seas will build into the 5-8 foot range today, then subside tonight.
Wednesday...A Small Craft Advisory is in effect for our ocean
waters for wave heights of 5 to 7 feet. Also, wind gusts of 25 to
30 knots are possible in the afternoon.
Wednesday night...Wind gusts of 25 to 30 knots are possible
initially from the southwest then from the west following the
passage of a cold front after midnight. There could be a brief
surge of gale force gusts on either side of the frontal passage.
Thursday through Saturday...West to northwest wind gusts of 25 to
30 knots are possible through the period.
Much needed rain across the region the next couple of days.
For today and tonight...A robust low-level jet will drive increasing
moisture advection during the day today. The PW values are forecast
to increase rapidly to 1.5 inches by Midday. As enhanced ascent
occurs from the low-level jet periods of rain will result, some of
which can be heavy at times. It appears that 0.50-1.50 inches of
rain should be common with the highest amounts along and north/west
of the I-95 corridor. The placement of this though will be
determined primarily by the advancement of a warm front today.
It has been rather dry, therefore just some ponding of water on
roads during heavier rainfall rates is anticipated along with some
possible poor drainage flooding. Some poor drainage flooding though
could be locally enhanced where fallen leaves clog storm drains. The
rain/showers taper later this afternoon and early this evening, with
much of tonight tending to feature a lull before the next batch of
rain arrives Wednesday.
For Wednesday...Storm total rainfall (even including Tuesday`s
amounts) will average 1-2 inches but isolated higher amounts of
3+ inches are certainly possible if the heaviest rain from both
waves of rain falls over the same areas. Conversely, some areas
on the coastal plain may receive less than an inch of rain.
Through late Wednesday, we are not expecting main stem flooding
or even smaller basin flooding since these rainfall amounts will
be spread out over a 48 hour period (with a lull in between) and
the antecedent conditions are rather dry. Poor drainage flooding
is always a possibility especially during heavier rainfall rates
but impacts from this type of flooding are generally minor.
MARINE...Small Craft Advisory from 6 AM this morning to 6 PM EST
Wednesday for ANZ450>455.
Small Craft Advisory from 6 AM this morning to 6 PM EST this
evening for ANZ430-431.