Area Forecast Discussion
Issued by NWS Mt. Holly, NJ-- Highlight Changed Discussion --
-- Discussion containing changed information from previous version are highlighted. --
FXUS61 KPHI 251918
Area Forecast Discussion
National Weather Service Mount Holly NJ
218 PM EST Sat Feb 25 2017
-- Changed Discussion --In the wake of the strong cold front moving through our region
Saturday afternoon, cooler air will settle in over the region. High
pressure then builds south of the region on Sunday before moving
offshore on Monday. Low pressure approaching from the west will lift
a warm front through the region on Tuesday. The aforementioned low
will propagate from the Great Lakes Region into eastern Canada. The
associated cold front will move through Wednesday night or Thursday.
A quick moving low pressure system will slide over the Mid Atlantic
at the end of the week.
-- End Changed Discussion --
.NEAR TERM /UNTIL 6 PM THIS EVENING/...
130 pm update: Forecast updated to include severe thunderstorm
watch 39 for portions of southeast PA, southwest NJ, the Eastern
Shore of MD, and northern/central DE until 8 pm. Grids updated
to include isolated damaging wind gusts.
Environment continues to become more favorable for severe
storms, with storms rapidly intensifying from Harrisburg, PA,
southwestward to Charlottesville, VA. These storms are moving
into an increasingly unstable air mass, with MLCAPE 500-1000
J/kg and effective bulk shear 40-60 kts. With continued heating
in the low levels, near-surface lapse rates are continuing to
increase, with 7-7.5 C/km readings reaching the I-76 corridor.
Though thermodynamic profiles do not appear favorable for large
hail, some subsevere hail is possible with the strongest
1230 pm update: Environment is becoming more unstable this
afternoon as diabatic heating has increased with the partial
clearing observed in much of the area this morning. Model
analyses indicate MLCAPE 500-1000 J/kg generally south of I-76
(and away from the coast) with effective bulk shear 40-60 kts.
Though low-level lapse rates have been slow to improve this
morning, some 7+ C/km readings are now present in southeast PA.
As the cold front nears the region this afternoon, scattered
storms should continue to intensify in the increasingly unstable
air downstream. Already have showers developing in the southern
Poconos, with a clear uptick in convection in central PA during
the past hour. With fast unidirectional flow oriented parallel
to the front, one or more lines of storms should quickly
organize as it/they move east into the area along/ahead of the
front. Downward momentum transfer of this fast flow remains
possible with the stronger updraft cores, at least in isolated
instances, especially if surface heating continues this
afternoon (improving low-level lapse rates).
Made substantial modifications to the forecast this afternoon.
Increased temps across the area a bit more, changed PoPs to
account for latest trends, and kept mention of fog in coastal
NJ, where southeast winds will likely keep patches/areas of fog
around before the storms move in.
930 am update: Boundary layer mixing has successfully eroded
most of the lower clouds and fog in far southeast PA and much
of inland NJ; however, patchy dense fog continues across coastal
NJ. Coastal webcams showing Seaside Heights shrouded in fog,
with at least patchy fog also visible near Atlantic City,
Belmar, Tom`s River, and Cape May. Though improvement is
expected through the morning hours, nudged temperatures downward in
coastal areas just a touch through the day as the consistent
southeast fetch will likely advect cooler air over the ocean
waters inland and may keep low clouds in place through much of
Temperatures were running warmer than forecast in areas where
clouds have eroded, but suspect the warming trend will be brief
given the increasing clouds moving in from the west. Tweaked
hourly temperature grids to account for the current trends, but
only modified max temps a degree or two in most locations, as
the expected slowing trend of the warming will likely preclude
temperatures from exceeding guidance excessively, as has been
observed several times in the past week.
Regarding the convective potential this afternoon, not much has
changed with the forecast thinking. High-resolution guidance has
been quite consistent with the timing of the main line of
convection generally from 20Z to 01Z from west to east.
Instability continues to look marginal but sufficient for
updraft maintenance as the cold front approaches the region
this afternoon. Within an environment of favorable deep-layer
shear and unidirectional wind profiles with directions mostly
parallel to the front, a line of convection will likely progress
through the area. The line may contain little lightning given
the marginal/thin CAPE profile, but with steep low-level lapse
rates underneath fast midlevel flow, downward momentum transfer
of these strong winds should occur with relative ease during the
afternoon hours. With favorable low-level helicity in vicinity
of the front, bowing structures and even an embedded rotating
storm/mesovortex cannot be ruled out.
Previous discussion below...
Another rather mild early morning across the area. Several
items to discuss today as we have more warmth ahead of a strong
cold front. Some varying amounts of low clouds with patchy fog
this morning, with much of this (at least low clouds) across
portions of eastern Pennsylvania and northern New Jersey due to
more of an upslope component of the low-level flow. Some thicker
fog/stratus has been noted along portions of New Jersey coast
and near the far upper portion of Delaware Bay near Wilmington.
Some guidance keeps low clouds across much of the area all day,
however the thinking is that enough mixing and boundary layer
heating occurs to erode this by early afternoon. Some fog was
kept from northern Delaware eastward to the New Jersey coast.
We now switch to the convective potential with an incoming
strong cold front this afternoon.
A strong upper-level trough will continue to move from the
Midwest to across the Great Lakes region today. This trough is
forecast to start taking on a negative tilt as it reaches the
eastern Great Lakes later today. This will drive strong height
falls eastward this afternoon along with a strengthening wind
field. The forecast soundings indicate deep unidirectional flow
with some veering below about 900 mb. There should be enough
instability to produce some thunder this afternoon, however
strong forcing along the leading edge of the height falls along
with robust short wave energy should result in a convective
line. This line will probably be more low-topped, however
enough instability is expected inland to produce some lightning.
Given the deep unidirectional flow contributing to
strengthening shear, some bowing segments are possible within
the line. Momentum transfer of stronger winds from aloft down to
the surface can result in locally damaging gusts, especially
with any bowing segments along with stronger cores due to water
loading of the downdrafts. While some smaller hail cannot be
ruled out, instability should be limited as soundings show more
of a thin CAPE profile. We will continue to carry gusty winds in
the forecast with thunder (may need to add damaging winds
later), with the convective line generally moving from west to
east across the area from about 20-00z. The intensity of the
convection should lessen as it reaches the coast due to the
cooler ocean influence.
Southerly flow ahead of the cold front will result in another
very warm day, with several areas already starting rather warm.
Highs will be able to over achieve expectations if more clearing
occurs ahead of the cold front during peak heating. High
temperatures were a MOS/continuity blend but then raised some
away from the coast given a warmer start.
.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM SUNDAY/...
A negative tilt upper-level trough will be lifting across New
England tonight, with low pressure tracking across eastern
Canada. This will push a strong cold front offshore early in the
evening with and showers and thunder quickly coming to an end.
Strong cold air advection occurs immediately behind the cold
front through about the first half of the night. This combined
with stronger flow should create a gusty west to northwest for a
time. As the colder air advects into the area, it will also be
drying out however some forecast soundings indicate that as a
subsidence inversion develops, there will be some lingering
moisture trapped for a time. This may result in bands or areas
of stratocumulus for awhile especially across the northern
It will be noticeably colder tonight with an added wind chill
to the mix, reminding us that it is still winter. Low
temperatures were mostly a MOS/continuity blend.
.LONG TERM /SUNDAY THROUGH SATURDAY/...
-- Changed Discussion --Few changes made in the long term so as to focus on current hazards
Unsettled weather pattern is on tap through the week.
Monday and Tuesday are very low confidence especially with respect
to precip chances. As long as the high isn`t more progressive in
shifting off shore, during the day Monday should stay dry. Monday
night into Tuesday there is low confidence as to the extent of rain
ahead of the arrival of the warm front and with the timing of the
warm front. Models have continued their run-to-run jumpiness with
the timing of this front.
Wednesday and Wednesday night looks to be the highest chance of
precip with the region solidly in the warm sector. Thunderstorms may
be possible as well if we can warm up enough to realize some modest
surface based instability.
Cold front still on track to move through late Wedensday night or
Thursday. Once it moves through, we should see a lull in the precip,
albeit a brief one.
The next question will be the timing and track of a quick moving low
which could move across the region Friday or Friday night. Moisture
may be limited as there will be strong dry air advection behind the
cold front, but there is some chance for precip around the area.
-- End Changed Discussion --
.AVIATION /19Z SATURDAY THROUGH THURSDAY/...
-- Changed Discussion --The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.
For the 18Z TAFs...Mostly VFR before storms move in this
afternoon. Timing looks to be 19-23Z for KRDG/KABE, 20-00Z for
KPHL/KPNE/KTTN/KILG, and 22-02Z for KMIV and KACY. Sub-VFR
VSBYs/CIGs expected as the precipitation moves through, with
gusty/erratic winds 30-45 kts possible in proximity to the stronger
storms. Winds will quickly veer to westerly or northwesterly
after shower/storm passage late this afternoon/evening, with
winds gusting 20-30 kts through tomorrow afternoon before
Sunday night and Monday...VFR conditions expected.
Monday night through Thursday...periods of rain and showers are
possible. When/if this does occur, MVFR ceilings or lower are
possible. Highest chance is Wednesday and Wednesday night. There is
considerable forecast uncertainty in this period.
-- End Changed Discussion --
-- Changed Discussion --1230 pm update: Dense fog continues to plague the waters off the
coast of New Jersey. Extended the dense fog advisory through 6
pm. Winds are slowly increasing over the waters as well, so
current SCA looks good. Rest of forecast unchanged.
930 am update: Webcams continue to show dense fog over the
coastal waters of New Jersey. Expanded the dense fog advisory to
cover all marine zones adjacent to the coast through noon.
Otherwise, forecast is in good shape.
A narrow area of stratus and/or fog continues right near the
coast of New Jersey with this mostly confined from portions of
Ocean County northward. Farther south, the low-level trajectory
suggests less development however some fog has been reported.
The dense fog advisory this morning remains in effect for just the
northern two Atlantic coastal waters zones. Otherwise, a
southerly flow will continue today and the increase may be
slowed due to much warmer air over cooler water. This will
increase especially later today as a strong cold front arrives,
with colder air quickly arriving tonight in its wake. It is
tonight when stronger winds are expected as mixing becomes much
more efficient. There could be some low-end gale force gusts
tonight, however it looks marginal and potentially short in
duration therefore we will maintain the Small Craft Advisory.
Seas will also respond to the increasing wind, however this was
slowed a bit for this morning.
A line of showers and embedded thunderstorms will move across
the area late this afternoon and especially early this evening.
These may weaken some as they encounter the marine influence,
however some local gusts of 30-40 knots are possible along
portions of the line.
Sunday night and Monday...wind gusts to 25 kt may linger for a few
hours on Sunday evening, but will be below SCA criteria for the
remainder of this period.
Tuesday through Wednesday...winds and seas should generally stay
below SCA criteria, though wind gusts above 20kt will be possible on
Wednesday. Periods of rain/showers possible, especially on Wednesday.
Wednesday night and Thursday...Northwesterly wind gusts around 25 kt
-- End Changed Discussion --
MARINE...Dense Fog Advisory until 6 PM EST this evening for ANZ450>453.
Small Craft Advisory until 6 PM EST Sunday for ANZ450>455.
Small Craft Advisory from 6 PM this evening to 6 PM EST Sunday