Area Forecast Discussion
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FXUS61 KPHI 221858
Area Forecast Discussion
National Weather Service Mount Holly NJ
158 PM EST Sun Jan 22 2017
Low pressure over the Southern Plains will strengthen as it tracks
northeastward along a stationary front today and tonight. This low
will reach the central Appalachians by Monday morning before
redeveloping off the North Carolina coast Monday afternoon. The
coastal low is then forecast to move slowly northward up the Mid-
Atlantic coast Monday night before eventually turning northeastward
toward Nantucket late Tuesday. Several weak disturbances may impact
the region late in the week. An upper trough will amplify over the
eastern United States by next weekend.
.NEAR TERM /UNTIL 6 PM THIS EVENING/...
The Dense Fog Advisory has been allowed to expire as
visibilities have improved.
Another disturbance aloft is forecast to move across the area
this afternoon, which could help create additional showers
across the area. Otherwise, there remains a lot of moisture
trapped underneath the inversion above the surface, so low
clouds and areas of fog will continue into the afternoon.
.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM MONDAY/...
A complex low pres system over the southeastern states
continues its motion twd our region. Wind will increase and rain
will become heavier and steadier from s to n. A wind advisory
will go into effect toward the end for srn areas durg the
overnight hours. It is psbl that temps cud be cold enough in the
highest elevs of the Poconos for a wintry mix by daybreak Mon
but little accum is expected.
.LONG TERM /MONDAY THROUGH SATURDAY/...
Strong low pressure will reach the central Appalachians by
Monday morning before redeveloping off the North Carolina coast
Monday afternoon. The coastal low is then forecast to move
slowly northward up the Mid-Atlantic coast Monday night before
eventually turning northeastward toward Nantucket late Tuesday.
Below is a break down of impacts from this storm...
Heavy Rainfall... The window for heavy rain can now be narrowed
down to Monday: more specifically early morning-early afternoon
for Delmarva, late morning- afternoon for NJ and perhaps E PA
(possibly lingering into early evening north of I-195 and east
of I-287). Models have come into better agreement with the
location of the heaviest rainfall on Monday: over the coastal
plain from southern DE up to central NJ. The hi-res guidance is
picking up on a coastal front that sets up only a few miles
offshore on Monday. The S-N oriented axis of heaviest rain would
be favored along and to the west (i.e., on the cool side) of
the coastal front by about 25-50 miles. Low-level isentropic
lift and moisture flux convergence will be maximized on this
side of the boundary with the nose of the easterly LLJ directed
at the coastline. Additional light to perhaps moderate rainfall
amounts are forecast for Monday night and Tuesday. Forecast
storm-total rain amounts range from less than an inch across our
far western zones (from the eastern shore of MD up to Chester
and Berks Co. in PA) to 1-1.5 in the Delaware Valley to 1.5-
2.25 in the coastal plain. Locally higher amounts up to 3+
are still not out of the question toward the coast.
Wintry Precipitation... An overwhelming majority of models have
trended colder with boundary-layer temperatures on Monday and Monday
night. This looks to be in part due to the slower arrival of the
precipitation from the low to our southwest, which would allow extra
time for high pressure to build in from eastern Canada and thus for
the low-level cold air to drain southward into northeastern PA and
northwestern NJ. Accordingly, there is more of a concern for wintry
precip from I-80 northward. Forecast soundings and partial thickness
nomograms indicate the predominate ptype would be freezing rain
Monday- Monday night. For now, icing look to be primarily confined
to the higher ridges (elevations above 1000 ft) of Carbon, Monroe
and Sussex (NJ) counties. If thermal profiles trend even colder,
then sleet may cut down on icing from freezing rain across the far
north. Regardless, sleet will mix in at times when the precip
intensity picks up. A brief changeover to wet snow is also possible
in the Poconos Monday afternoon-evening if a band of heavier precip
makes it this far northwest and sufficiently dynamically cools the
column. Forecast snow/sleet accumulations are less than 1 at
elevations above 1000 ft along the I-80 corridor and 1-2 in the
southern Poconos for ridges above 1700 ft in elevation. These
amounts are conservative due to the high uncertainty of a changeover
to snow and near-freezing surface temperatures that are marginally
supportive of accumulations. Conversely, went higher with ice
accumulations at the higher elevations north of I-80. These icing
amounts were calculated using the FRAM model. With freezing rain
likely to not begin until around or just after sunrise Monday
morning, opted to hold off on any winter headlines at this point so
the day shift can re-evaluate the next cycle of guidance.
The primary surface low over the central Appalachians will
eventually transfer it`s energy to the coast Monday afternoon and
night. 00Z models have trended stronger with the coastal low (both
at the surface and aloft), resulting in a well-defined deformation
band on the backside of the low center on Tuesday. Precip may
transition to snow or sleet before ending Tuesday afternoon across
NE PA/NW NJ as winds turn out of the NW and colder air wraps around
the backside of low. This
Winds... High Wind Watches were upgraded to Warnings for eastern NJ.
Additionally, Wind Advisories were issued farther west along the I-
95 corridor and farther south into southern DE.
A tight pressure gradient is forecast to develop on Monday as low
pressure deepens near the central Appalachians and high pressure
noses southward from eastern Canada. There are still some timing
differences between models that need to be resolved, but generally
the strongest winds (50-60 mph gusts toward the Jersey shore and 40-
50 mph gusts along the I-95 corridor) are expected to develop from
SE to NW during the early morning hours Monday. Winds will diminish
from S to N during the afternoon and early evening as the easterly
LLJ lifts northward with time. There is still a potential that Wind
Advisories may need to be expanded farther to the west by a tier of
counties but confidence in reaching criteria drops off rather
precipitously farther inland.
Tuesday night through Saturday...
Conditions finally dry out Tuesday night and Wednesday as high
pressure builds east.
After that, temps remain above normal through at least
Thursday. Cold front comes through on Friday, and temps drop
off, but still could be a few degrees above normal.
Pattern changes comes late in the week and over the weekend as
a deep upper trough builds east, bringing with it a return to
near and possibly below, seasonal temperatures.
.AVIATION /19Z SUNDAY THROUGH FRIDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.
Another day of poor flying conds with IFR and LIFR continuing
with low clouds and fog. Then, an area of low pres will
approach from the sern CONUS and RA will pick up and get
steadier and heavier from s to n tonight. Wind will also
increase from the e later tonight.
Wind will start off the day fairly light but will become nely around
10 kts by aftn and 15 to 20 kts by eve. Wind will gust to at least
25 kts by mrng and gusts to 30 kts especially closer to the cst can
not be ruled out.
Monday and Monday night...IFR/LIFR with periods of rain. Also,
E winds continue to strengthen to 15-20 KT with 30-35 KT gusts
at ABE/RDG, to 20-25 KT with 35-40 KT gusts at I-95 terminals,
and 25-35 kt 45-50 KT gusts near ACY from early Monday morning
through Monday afternoon.
Tuesday...Lingering IFR conditions with rain continuing through
the day. Sleet or snow may briefly mix in at ABE before ending.
Gradually improving to MVFR and then VFR from S to N late in
Tuesday night and Wednesday...VFR. NW winds 10-15 kt with gusts
to 20 kt.
Thursday...Isolated showers, possibly briefly lowering conditions
The Dense Fog Advisory was extended through 6 pm. Otherwise,
conditions on the area waters are relative quiet today, but they
will begin to deteriorate as low pres approaches from the sern
states. Seas will increase and e to nely flow will strengthen by
tonight, storm and gale warnings have been posted.
Monday and Monday night...Easterly flow quickly ramps up early
in the morning Monday to gale force in the DE Bay and DE coastal
waters and to storm force in the NJ coastal waters. No changes
to the Storm and Gale Warnings from yesterday afternoon. Winds
quickly weaken across the southern waters during the afternoon
(but still at SCA levels). A more gradually drop off of the wind
speeds are expected farther north in the coastal waters off
central NJ late in the day but should eventually fall below gale
force during the mid evening hours.
Tuesday...Lingering SCA conditions possible.
Wednesday through Thursday...Brief lull in SCA conditions
possible early Wednesday, and then conditions ramp up late
Wednesday through Thursday.
There is a potential for limited hydro impacts from the
upcoming storm system. In short, we don`t think there will be
any river flooding unless basin-wide rainfall averages climb
above three inches. The most sensitive responding rivers still
appear to be the Millstone and the Rancocas.
The entire region has some positive things working in its
favor...precipitation has been below normal the last 30 days,
there`s really no consequential snow cover to speak of, there`s
no ice on area waterways, streamflows are either running at or
below normal as is soil moisture, and last but not least, the
ground isn`t frozen. We think all the above will help mitigate
the flood threat.
The heaviest rain is expected to fall on Monday. Storm-total
rainfall amounts will generally between 1-2 with locally
higher amounts to 3" possible in eastern NJ. It appears poor
drainage or nuisance flooding is more likely, not flash flooding
or river flooding, especially if the heavier rain falls in the
coastal plain as currently predicted.
A Coastal Flood Advisory (CFY) was issued for coastal
communities along the DE and NJ coasts as well along the DE Bay.
The CFY currently only covers the early Monday morning high
tide. The CFY will likely need to be extended into Monday
afternoon/evening to cover the following high tide cycle.
Although local regression and ETSS/NOS model guidance does not
indicate moderate coastal flooding, it is too early to rule it
An onshore flow is forecast to develop along the coasts of
Delaware and New Jersey today as an initial low passes off the
Mid-Atlantic coast. Tide levels should begin to respond to the
onshore flow this afternoon but they will likely remain below
the coastal flooding thresholds at that time.
The onshore flow is expected to strengthen tonight as another
low begins to approach from the southwest and the pressure
gradient tightens. A storm surge along the coasts of Delaware
and New Jersey is anticipated to build to 2 to 3 feet by the
early morning high tide on Monday. Increasing wave action and
the resulting beach erosion will likely compound the effects of
the surge. Waves breaking along the shore may build to 5 to 8
feet. Widespread minor flooding is forecast to develop along the
coasts of Delaware and New Jersey, on the back bays, along
Delaware Bay and along Raritan Bay. Thankfully, the astronomical
tides will be relatively low on Monday since we are coming off
the last quarter phase of the moon. Nevertheless, a number of
roads will likely flood and become impassable for a time around
The strongest of the onshore flow is forecast to occur on
Monday morning. The maximum surge of 3.5 to 4.0 feet is
anticipated to occur around the low tide late on Monday morning.
However, water will likely remain trapped in the back bays of
Delaware and New Jersey at that time.
The onshore flow is expected to begin weakening from south to
north on Monday afternoon as low pressure begins to arrive from
the southwest and the tight pressure gradient ahead of the
system lifts into New England. However, the storm surge is
forecast to remain in the 2.5 to 3.5 foot range along our
coastline around the high tide late on Monday afternoon. As a
result, another round of widespread minor coastal flooding is
expected at that time. There could be pockets of moderate
flooding from Long Beach Island up to Raritan Bay depending upon
the rate that the onshore flow actually begins to lessen.
Lingering minor coastal flooding is again possible for the high
tide early on Tuesday morning being that the flow is not
expected to become fully offshore until the daylight hours on
Based on the forecast guidance and the expected pattern we are
not anticipating any coastal flooding on the upper eastern shore
of Maryland. Also, the potential for anything more than spotty
minor tidal flooding appears low for the tidal Delaware River
above the Commodore Barry Bridge. There may be some areas of
minor flooding on the portion of the tidal Delaware River that
runs between New Castle County, Delaware and Salem County, New
Jersey especially around Monday morning`s high tide.
PA...Wind Advisory from 6 AM to 7 PM EST Monday for PAZ106.
Wind Advisory from 6 AM to 4 PM EST Monday for PAZ070-071.
NJ...Wind Advisory from 6 AM to 7 PM EST Monday for NJZ008-010-015-
High Wind Warning from 3 AM to 9 PM EST Monday for NJZ012>014-
Coastal Flood Advisory from 2 AM to 11 AM EST Monday for
Wind Advisory from 3 AM to 2 PM EST Monday for NJZ016-021.
High Wind Warning from 3 AM to 4 PM EST Monday for NJZ022>025-
Wind Advisory from 6 AM to 4 PM EST Monday for NJZ017-018.
DE...Coastal Flood Advisory from 2 AM to 11 AM EST Monday for
Wind Advisory from 3 AM to 2 PM EST Monday for DEZ001>004.
MARINE...Dense Fog Advisory until 6 PM EST this evening for ANZ430-431-
Storm Warning from 1 AM to 4 PM EST Monday for ANZ452-453.
Storm Warning from 1 AM to 10 PM EST Monday for ANZ450-451.
Gale Warning from 1 AM to 4 PM EST Monday for ANZ430-431-454-