Scientific Forecaster Discussion
fxus61 kphi 170844
Area forecast discussion
National Weather Service Mount Holly New Jersey
344 am EST Fri Jan 17 2020
strong high pressure will progress from the Great Lakes region
today to the northeast tonight before moving offshore on
Saturday. Meanwhile, developing low pressure in the Central
Plains will lift northeastward to New England by Sunday morning.
This will bring an attendant cold front through the mid-
Atlantic on Saturday night. A strong and broad surface high will
build into the eastern U.S. For most of next week.
Near term /until 6 PM this evening/...
strong high pressure located in the Great Lakes region early
this morning will build to the east today. The pressure gradient
between the high and the departing area of low pressure well
off Nova Scotia will maintain a brisk northwest wind in our
region today. However, wind speeds are forecast to begin
diminishing slowly this afternoon. Sustained wind speeds this
morning are expected to be 15 to 25 mph with gusts up to 30 or
The northwest flow will bring cold dry air into eastern
Pennsylvania, northeastern Maryland, Delaware and New Jersey.
Lingering clouds and light snow showers in the Poconos, parts of
the Lehigh Valley and northwestern New Jersey should dissipate
this morning. We are anticipating a sunny sky for much of the
day with perhaps some high clouds arriving from the west toward
Temperatures are forecast to be below normal for the first time
in over a week. Highs are expected to be mainly in the lower
and middle 30s, with readings not getting above the 20s in the
Pocono region and in far northern New Jersey. The high
temperatures are about 6 to 8 degrees below normal for January
Short term /6 PM this evening through 6 am Saturday/...
the center of the area of high pressure is expected to pass
across southern Quebec tonight. Our weather will continue to be
cold and dry. A northwest to north wind 5 to 10 mph is
anticipated to become light and variable overnight.
There should be an increase in high clouds from the west
tonight, out ahead of our next weather system. Some mid level
clouds may arrive toward daybreak.
The thin high clouds and the light wind is expected to allow
temperatures to drop into the teens and lower 20s in much of our
Long term /Saturday through Thursday/...
the medium-range period starts with a storm system moving
through the region Saturday and Saturday night and ends with a
long, mostly dry, and mostly cold spell.
A surface ridge will be straddling the East Coast Saturday
morning as progressive midlevel ridging moves through the
northeast. Upstream, a fairly potent trough will be migrating
through the central U.S., As an attendant surface low begins to
lift into the Midwest. Warm advection will begin in earnest
Saturday morning in advance of the trough, with clouds quickly
thickening and lowering. However, residual dry air will exist in
the low levels thanks to the predecessor surface ridge moving
through the region. I continue to suspect models are too
aggressive in saturating this very dry low- level air as
stronger isentropic lift reaches the area around midday. As
such, I continued the trend of lowering pops across the area
Saturday morning (especially before 15z), with a quick ramp-up
around 18z as the stronger lift reaches the area. The strongest
ascent reaches the northern mid-Atlantic during the mid-to-late
afternoon, and this could result in some fairly strong columnar
cooling via dynamic effects (in addition to initial wet-
bulbing). However, warm advection will be considerable in
advance of the trough, so expect gradual transition from snow
(at onset) to rain from south to north with time (though areas
north of I-78 may remain mostly snow before precipitation
cessation). Model soundings are suggestive that sufficient
warming aloft may occur to permit a period of sleet during the
transition, with some of the higher-res guidance suggesting more
of a waffling of precipitation types (owing to the competing
effects of warm advection and dynamic cooling). As such, did
include at least a chance of sleet, predominantly for eastern
PA, during the afternoon and early evening.
Of course, snow amounts remain a tricky forecast owing to the
precipitation type transition (and increasingly marginal surface
temperatures). As mentioned in yesterday's discussion, a
combination of pattern recognition, prior model performance, and
reasonable model agreement all suggest a sub-advisory event for
the I-95 corridor and southeastward. Amounts around or under an
inch seem to be the most likely outcome in these areas (with
mostly a rain event southeast of a Stevensville to Atlantic City
to Asbury Park line). Current forecast has 1-2 inches generally
for the western suburbs of Philadelphia toward the Trenton and
New Brunswick areas, with 2-5 inch totals from the reading to
Morristown areas northwestward (highest amounts being in the
poconos). Confidence in snow amounts is always on the low side
when precipitation type issues combine with marginal and warming
low-level temperature profiles. However, additional inhibiting
factors are in play: residual dry air near the surface (which
may end up being stronger than progged), a fairly quick-hitting
system (which will likely shut off rapidly after dark given the
timing of the attendant midlevel vort max), and somewhat
unfavorable timing of the maximum lift (late afternoon, plus or
minus). All of this compels ME to keep amounts near or below
consensus (i.E., Within the constraints of advisory amounts County Warning Area-
wide). Given the above (and per collaboration with surrounding
offices) will go ahead and issue a Winter Weather Advisory for
the aforementioned reading to Morristown corridor northwestward
from late Saturday morning through Saturday evening, with fairly
high confidence most locations in this outlined area will see
advisory- level winter-weather impacts. There is still plenty of
time to amend these products as needed in subsequent forecasts.
As the associated surface low lifts through New England and
offshore on Sunday, northwest flow will become established with
an increasing surface pressure gradient. Another breezy day
should be expected, with some snow showers possible in the
Poconos and vicinity. Model soundings are a little more
impressive with mixing for Sunday (compared to this time
yesterday), so did bump up wind gusts a little bit from the
previous forecast. Anticipate gusts of 30 to 40 mph will be
possible. There is also some indication of an increased chance
of snow showers or even squalls as a reinforcing front moves
into the area late in the day or in the early evening. Will
continue to monitor this potential.
Much of next week looks dry and chilly, as a strong, broad, and
cold surface high becomes established in the eastern half of
the U.S. The colder look of the GFS/CMC still looks preferable
to the faster moderation of the ECMWF, as larger-scale troughing
in eastern North America tends to be eroded too quickly. The
surface ridge will keep any meaningful precipitation chances at
Bay Monday through Friday.
Aviation /09z Friday through Tuesday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg,
kilg, kmiv, kacy and surrounding areas.
Today...VFR under a mostly clear sky. A northwest wind 15 to 20
knots with gusts around 30 knots this morning should begin to
slowly diminish this afternoon. High confidence.
Tonight...VFR with increasing high clouds. Northwest to north
wind 5 to 10 knots becoming variable 6 knots or less. High
Saturday and Saturday night...rapidly deteriorating conditions
expected as precipitation moves in from the west by afternoon.
Precipitation will likely start as snow at the philly terminals
northwestward, with gradual transition to rain by evening at the
philly terminals. Precipitation may mix with rain at ridge/Abe
before tapering off during the evening. Meanwhile, miv/Acy may
see mostly (or completely) rain. South winds increasing to 5 to
15 kt by afternoon, transitioning to southwest or west late
Saturday night. Low level wind shear probable through Saturday evening. Overall
confidence is moderate (but with winds is high).
Sunday...improvement to VFR expected as winds become west to
northwest 10 to 20 kt with gusts 30+ kt. Cannot rule out snow
showers (generally north/west of phl) in the afternoon/evening.
Sunday night...VFR with northwest winds 10 to 15 kt with gusts
in the evening to 25+ kt. High confidence.
Monday through Tuesday...VFR with northwest winds 5 to 15 kt
(possibly with a few gusts to 20 kt or so during the daytime
hours). High confidence.
a brisk northwest wind should continue over the coastal waters
of New Jersey and Delaware for today. Gale force wind gusts are
expected until mid to late morning, followed by a period of
Small Craft Advisory level conditions into the early evening.
Wind speeds and waves heights will likely fall below Small Craft
Advisory criteria for tonight.
Saturday and Saturday night...at least advisory-level south to
southwest winds are expected, with seas quickly building to 5 to
8 ft during the afternoon and evening hours. A chance of rain
and visibility restrictions late in the day through the
Sunday...winds will become northwest, with gusts approaching
gale force. Seas should slowly subside to around 4 to 6 ft by
Sunday night...lingering advisory conditions expected as
northwest winds slowly diminish.
Monday...mainly sub-advisory winds/seas expected (especially by
late morning onward).
Monday night and Tuesday...generally sub-advisory winds/seas
expected, though northwest winds will approach 25 kt at times
during this period.
PA...Winter Weather Advisory from 10 am Saturday to 1 am EST Sunday
New Jersey...Winter Weather Advisory from 10 am Saturday to 1 am EST Sunday
Marine...Gale Warning until 10 am EST this morning for anz430-431-