NWS Philadelphia/Mount Holly wrote its last area forecast discussion before the first event at Jan 31, 9:27 PM EST, 2 hours 33 minutes earlier.
Area forecast discussion, Jan 31, 9:27 PM EST, 2 hours 33 minutes before
Area Forecast Discussion
National Weather Service Mount Holly NJ
927 PM EST Sun Jan 31 2021
.SYNOPSIS...
Low pressure in the upper Ohio Valley this afternoon will weaken
through tonight as a new low develops and strengthens off the Mid-
Atlantic coast and becomes a coastal storm. This low will slowly
move northeastward through Tuesday and into the Canadian Maritimes
Wednesday. High pressure builds in Thursday before shifting offshore
into Friday. A cold front crosses our region Friday night into
Saturday, then low pressure develops along it to our south into
Sunday as it tracks offshore.
&&
.NEAR TERM /THROUGH MONDAY NIGHT/...
...A long duration and complex winter storm will continue to
impact the Mid-Atlantic region through Tuesday...
9:30 Update...
Synoptically, the event is progressing as expected. Both the mixed
precipitation line and the dry slot are gradually moving northward,
and have both moved through most of Delmarva and far southern New
Jersey. The rest of the region continues to see steady light to
moderate snow. This will be the theme for much of the night, with
mainly steady but not especially heavy snow along and northwest of I-
95 and lighter precipitation with more mixing along and southeast of
that corridor. In the areas where it remains all snow, overnight
accumulations are forecast mainly in the 4 to 6 inch range.
A couple of changes with this update. First, updated the snowfall
forecast, which resulted in some minor tweaks but nothing at all
noteworthy. Where the heaviest banding sets up on Monday will be
critical in determining where the highest totals end up, and there
is still a bit of uncertainty with that. It is important to
mention that the storm total snowfall graphic on our website
currently represents the expected forecast for the entire storm,
including what has fallen so far. By tomorrow morning, this may
be changed to reflect expected remaining snowfall only.
Next, a couple of headline changes on Delmarva with this update.
Have cancelled all winter weather headlines for Sussex County, DE
and Caroline and Talbot Counties in MD. Wintry precipitation has
ended in those areas, and while some snow should eventually
redevelop late Monday and Monday night, impacts are not expected to
be sufficient to justify keeping the headlines up for so long in the
meantime. Have downgraded Queen Anne's County in MD to a Winter
Weather Advisory and may be able to drop headlines there later
tonight. Elsewhere, headlines remain unchanged and the going
forecast appears in good shape with a long duration and high impact
Nor'easter underway. Previous discussion follows...
A closed mid-level trough is located across the Ohio Valley today,
which will deepen and dig south through early Monday. At the
surface, low pressure located across the Ohio Valley will continue
weakening through tonight. A new coastal low, which is currently
taking shape across eastern North Carolina, will move offshore
tonight and deepen rapidly on Monday off the Delmarva coast then
push northeast. This is a classic Miller-B low pressure evolution
which will ultimately become a nor'easter into Monday. As the
surface low develops and quickly deepens offshore, a strong pressure
gradient will yield strong northeasterly winds, especially near the
coast. A modest arctic high located across eastern Canada will
provide sufficient cold (and dry) air throughout this event.
Precipitation has now overspread the entire region. The rather
cold and dry airmass in place at the onset of this event has
resulted in mainly snow falling area-wide so far. A surge of
warm air advection associated with the initial surface low
across the Ohio Valley is expected to result in a period of
mixing of precipitation and change over to rain tonight into
early Monday morning across much of Delmarva and
southern/coastal New Jersey generally south/east of I-95.
Guidance still varies in its depiction of the snow/mix/rain
lines, but rain is expected to stay south of I-95. Along the
I-95 urban corridor and south of Trenton, a mix of mainly sleet,
and perhaps some freezing rain, is expected around daybreak
Monday. This will significantly reduce snow totals near and
south of the Philly metro area compared to points farther north
and east.
By early Monday, the surface low intensifying and pivoting off the
coast will result in increasing winds and increasing cold air
advection as the mean flow turns northeasterly. Impressive wind
fields (925 mb winds 40-50+ kts along and south/east of I-95) will
develop as a result of the gradient leading to strong winds at the
surface. The strongest winds will be along and near the coasts with
gusts possibly in excess of 50 mph. Farther inland, wind gusts of 35-
45 mph will likely be common along and south/east of I-95. A Wind
Advisory remains in effect for coastal Sussex and coastal Cape May
Counties for Monday where gusts up to 50 mph are possible. A High
Wind Warning has been issued for coastal Atlantic and coastal Ocean
Counties for sustained winds of 35-45 mph with gusts up to 60 mph.
The same wind speeds and gusts are expected for coastal Monmouth,
but this hazard is covered under the Winter Storm Warning in effect
for this zone.
In response to the the deepening surface low and increasing cold air
advection, the wintry mix along I-95 will quickly change back over
to snow Monday morning, and the rain/snow line will push back toward
the coast throughout the day. The strongest winds and heaviest snow
rates are not forecast to align (strongest winds likely occurring in
rain and mixing), therefore precluding the need for a Blizzard
Warning. A change over to all snow is expected on the back side of
the storm, even all the way to the coast, as the system begins
departing into Monday night.
The exact placement and strength of the low will dictate exactly
where the heaviest snow falls. The complexity of this system lies in
details of the system transitioning from an overruning precip regime
to a mesoscale banding (frontogenetic forcing) and wrap-around
precip regime early Monday. The latest 12Z guidance suite has
continued trending the f-gen banding potential a bit northward,
which suggests the greatest potential for this to occur across
portions of northern New Jersey (north of I-195) and eastern
Pennsylvania (Lehigh Valley and east/north). West of the f-gen
forcing, some guidance suggests that a dry-slot will advect in
around daybreak west of the Philly metro. The storm total snowfall
forecast was updated to reflect these features and the mixing
farther south/west. Generally 6-12" is forecast for around the
Philly metro with less south/east and more to the north. The axis of
f-gen can certainly change over the coming forecast updates. Snow
totals in the banding may exceed the broadbrush ~18-24" storm total
snowfall locally that is currently forecast in these areas.
Localized areas of the highest terrain, including the Poconos and NW
NJ, are most likely to see totals closer to 2 feet. We've also added
mention of blowing snow for the higher terrain given the gusty windy
and potential high snowfall rates. Closer to the coast, snow totals
are likely to diminish significantly given much more mixing and rain.
Mesoscale banding and forcing for widespread precip will exit the
forecast area into Monday night. However, lingering light wrap-
around precipitation is forecast to continue through much of the
night. Additional light accumulations of a couple inches or so are
possible north/west of I-95 overnight.
It is important to note that this will be a abnormally long duration
storm (36+ hours of snow and wintry precipitation), and that the
highest snow totals and highest snowfall rates will likely not be
realized until later on in the event late Monday. The storm total
snowfall amounts we are forecasting encompass the entire event as a
whole, including what has already fallen and all the way through
Tuesday.
&&
.SHORT TERM /TUESDAY THROUGH TUESDAY NIGHT/...
The synoptic system will be departing the Mid-Atlantic by early
Tuesday. Much of the guidance still depicts some light QPF amounts
with thermal columns supporting all snow (possibly mixed with rain
near the coast). Therefore, light snow is expected to linger into
Tuesday morning, but this will depend on exactly how progressive the
storm is. Some light snow accumulations (less than 2") is forecast
through Tuesday, especially north and east of Philadelphia as the
storm departs. Otherwise, conditions will be generally improving by
late Tuesday and winds will diminish. Highs will be in the low to
mid 30s with lows Tuesday night in the mid to upper 20s. Conditions
will remain a bit breezy as well with northwesterly winds gusting up
to 30 mph.
&&
.LONG TERM /WEDNESDAY THROUGH SUNDAY/...
Summary...A cold and breezy start then some warming to end the week
especially for the I-95 corridor and the coastal plain. Turning
colder again Sunday, and potentially turning unsettled late in the
week and over the weekend.
Synoptic Overview...A closed low across the Northeast and Mid-
Atlantic regions Wednesday will gradually lift out through Thursday
with some ridging arriving in its wake. This results in an axis of
surface high pressure sliding over our area Thursday before shifting
east into Friday. An upper-level trough across the Plains amplifies
northeastward across the Midwest and Great Lakes through Friday, and
combined with high pressure offshore will result in strengthening
southerly flow across our area. Given the surface low tracking well
to our west and northwest, milder air will move into our area ahead
of a cold front. A southern wave of low pressure looks to develop on
the front into Saturday which will assist in pulling the entire
front offshore into Sunday.
For Wednesday and Thursday...A closed low across the Northeast and
Mid-Atlantic region Wednesday will gradually lift out through
Thursday. This will allow surface high pressure to slide across our
region Thursday before shifting offshore into Friday. A tightened
pressure gradient will be in place Wednesday and with efficient
mixing, wind gusts to around 30 mph are expected. The winds subside
Wednesday night as mixing wanes and the overall pressure gradient
starts to relax. It will be on the cold side Wednesday especially
with the added wind (plus areas where snow cover exits), then
temperatures start to moderate on Thursday as warming aloft gets
underway with a warm front approaching from the southwest. If the
surface dew points increase much faster than currently forecast, fog
may become an issue Thursday night especially in areas where snow
cover remains. This potential looks to highly depend on the
increasing dew points, and as of now a surge in these values is not
anticipated.
For Friday through Sunday...An upper-level trough lifts into Canada
taking surface low pressure with it Friday, and an associated cold
front works across our area Friday night and Saturday. The overall
trough is then forecast to amplify eastward across the East over the
weekend, however there is quite the variability in the model
guidance regarding the amplification of it. Some guidance wants to
hang back short wave energy which carves out another trough in the
wake of the initial one. In this scenario, a surface low develops
across the southern states then tracks across the Mid-Atlantic
turning into a coastal storm over the weekend. The short wave energy
delayed to amplified the trough later on resulting in a coastal
storm while possible, given the uncertainty at this time frame
leaned toward continuity which shows the front moving offshore
Saturday with a wave of low pressure along it tracking to our south
and then offshore into Sunday. It will turn milder Friday ahead of
the cold front especially along the I-95 corridor and the coastal
plain, then a little cooling Saturday with more notable cooling
occurring on Sunday.
&&
.AVIATION /03Z MONDAY THROUGH FRIDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.
Tonight...MVFR to IFR ceilings. IFR/LIFR visibilities this
evening should improve some from RDG to TTN south and east
overnight as the snow mixes with or changes to sleet and
precipitation may lessen, with even a change to rain at MIV and
ACY. Some heavier snow this evening may result in hourly rates
near 1 inch. Northeasterly winds 15-20 knots with gusts 20-30
knots, with the strongest winds overnight. Low confidence
regarding timing of mixed precipitation/change to rain and how
far inland this gets. ABE expected to remain all snow however.
Monday...IFR ceilings with varying visibilities as precipitation
continues, some of which will be on the heavier side (potential for
1+ inches per hour snowfall rates from around the PHL area northward
at times). The visibilities however should improve some near PHL to
the south and east where a mix occurs in the morning before changing
back to snow. Rain at MIV and ACY should result in MVFR to IFR
visibilities with even a lull in the precipitation during the
morning, then the rain may change to snow in the afternoon.
Northeast winds 15-25 knots with gusts 30-35 knots with the
strongest winds closer to the coast, then the winds should turn more
from the north at night. Low confidence on timing of precipitation
type changes and intensity.
Outlook...
Tuesday...IFR ceilings should improve some through the day as snow
comes to an end. North to north-northwest winds 10-15 knots with
gusts to 25 knots. Moderate confidence.
Wednesday...VFR. Northwest winds 15-20 knots with gusts up to 30
knots, dropping off to around 10 knots at night. Moderate confidence.
Thursday...VFR. West winds 5-10 knots, becoming light southwest.
Moderate confidence.
Friday...Sub-VFR conditions possible with a chance of rain.
Southwest winds near 10 knots. Moderate confidence.
&&
.MARINE...
Conditions will deteriorate quickly this evening with gales
developing across all waters and seas building to 5-10 feet.
Visibility restrictions in snow early will change to rain throughout
the evening. The strongest winds and highest seas are forecast to
occur during the day Monday, with storm force winds and seas 15-20
feet most likely Monday afternoon and evening across the New Jersey
offshore waters. Periods of moderate to heavy rain throughout the
day. Winds and seas will begin to improve slowly into Monday night
with gales likely lingering into the overnight with seas around 10
feet. Light rain or snow expected.
Outlook...
Tuesday and Tuesday night...Small Craft Advisory conditions are
likely with winds gusting up to 30 kts and seas 5 feet or
higher. Some lingering rain or snow showers possible early
Tuesday.
Wednesday...Small Craft Advisory conditions expected, with Gales
possible especially on the southern waters.
Wednesday night and Thursday...Small Craft Advisory conditions
Wednesday night subside for Thursday.
Friday...Increasing southwesterly flow may result in a period of
Small Craft Advisory conditions.
&&
.TIDES/COASTAL FLOODING...
The onshore winds will continue to increase overnight as the complex
and strong storm nears the region. Much of the guidance agrees that
several upcoming high tides will have water levels that will require
CFW flags to continue. The flooding will not affect the upcoming
high tide at the shore this evening, since the surge will not be
excessive by then. Tomorrow morning, the combination of the strong
winds and the higher of the two daily tides occurring will bring
widespread minor to moderate coastal flooding. The following two
high tides will have elevated water levels as well, since the storm
will be slow to abate.
The only change that we have made with the CFW flags with the Sun
afternoon update was to put Kent DE in a Coastal Warning with Bowers
beach expected to flood. Across the Bay at Bivalve, water levels
should be reduced since the flow will be mostly offshore, but some
uncertainty remains, so we have issued a Coastal flood Advisory for
now with levels expected to go well into the Minor flood range. We
are not expected any strong impacts further up Delaware Bay or for
the tidal parts of the Delaware river at this time. Additionally,
Chesapeake Bay areas near NE Maryland will probably have only
minimal impacts from tidal flooding with this event.
&&
.PHI WATCHES/WARNINGS/ADVISORIES...
PA...Winter Storm Warning until 1 PM EST Tuesday for PAZ054-055-061-
062-105-106.
Winter Storm Warning until 10 AM EST Tuesday for PAZ060-070-
071-101>104.
NJ...Winter Storm Warning until 1 PM EST Tuesday for NJZ001-007>010-
012>015-020-027.
Winter Weather Advisory until 10 AM EST Tuesday for NJZ021>023-
025-026.
Coastal Flood Advisory from 10 AM Monday to 5 PM EST Tuesday
for NJZ021.
Coastal Flood Warning from 7 AM Monday to 5 PM EST Tuesday for
NJZ012>014-020-022>027.
High Wind Warning from 7 AM to 10 PM EST Monday for NJZ025-026.
Wind Advisory from 7 AM to 7 PM EST Monday for NJZ024.
Winter Storm Warning until 10 AM EST Tuesday for NJZ016>019.
DE...Winter Weather Advisory until 10 AM EST Tuesday for DEZ002.
Coastal Flood Warning from 7 AM Monday to 5 PM EST Tuesday for
DEZ003-004.
Wind Advisory from 7 AM to 7 PM EST Monday for DEZ004.
Winter Storm Warning until 10 AM EST Tuesday for DEZ001.
Coastal Flood Warning from 10 AM Monday to 5 PM EST Tuesday
for DEZ002.
MD...Winter Weather Advisory until 10 AM EST Tuesday for MDZ015.
Winter Storm Warning until 10 AM EST Tuesday for MDZ012.
MARINE...Storm Warning until 10 PM EST Monday for ANZ450>453.
Gale Warning until 6 AM EST Tuesday for ANZ454-455.
Gale Warning until 10 PM EST Monday for ANZ430-431.
&&
Synopsis...Gorse
Near Term...O'Brien/Staarmann
Short Term...Staarmann
Long Term...Gorse
Aviation...Gorse
Marine...Gorse/Staarmann
Tides/Coastal Flooding...WFO PHI