Scientific Forecaster Discussion
fxus61 kphi 190206
Area forecast discussion
National Weather Service Mount Holly New Jersey
906 PM EST Thu Jan 18 2018
a broad surface high will build into much of the eastern U.S. Late
this week before moving offshore this weekend. A strong surface low
will develop in the Central Plains this weekend, lifting northeast
into southeast Canada early next week. This will bring a strong cold
front through the area Monday night and Tuesday. High pressure will
build back into the region during the middle of next week.
Near term /until 6 am Friday morning/...
9 PM update...a quiet evening with the forecast on track as
surface ridging is building into the area with diminishing winds
and clear skies. The only real change was some adjustments to
evening temperatures based on latest obs and trends.
Previous discussion...the only thing of note to talk about in
the near term period is an upper level short wave trough which
is expected to pass well north of our region, but could bring in
some high clouds to the area. Otherwise, rather tranquil
pattern will continue. Light northwesterly winds should back to
westerly as the high continues to build closer to the area. With
few clouds, light winds, and dew points in the single digits
and teens across the area, expect prime radiational cooling
conditions. Lows are expected to range from the teens to low 20s
across the area.
Short term /6 am Friday morning through 6 PM Friday/...
a modest warming trend is expected tomorrow as the 1000 to 500 mb
thickness increase. Pressure gradient will be rather weak tomorrow
with the high to our south. Consequently winds should be light (10
mph or less) through the day.
Long term /Friday night through Thursday/...
a warm and dry weekend followed by an inclement start to the new
Model solutions remain rather stable through Tuesday of next
week, with the midlevels featuring generally zonal flow through
the weekend followed by some ridging downstream of a negatively
tilted trough moving through the Midwest/Great Lakes late this
weekend into early next week. The trough passes the region
Tuesday and Tuesday night, with reinforcing perturbations in
transient northwest flow Wednesday. Model discrepancies become
large by this time, as phase differences among the ECMWF, GFS,
and CMC become pronounced. However, in terms of sensible
weather, think the Tuesday night through Wednesday night period
should be dry before uncertainty becomes high enough at the end
of the long term for at least mentionable (slight chance) pops.
Not much to discuss sensible-weather wise this weekend, as a
strong surface high in the southeast moves offshore. Statistical
guidance remains consistent in bringing temperatures near or
slightly above 50 for the Philadelphia metropolitan area this
weekend but remain muted in warming it up much more than this.
Given the stubborn zonal flow (i.E., Lack of increased
amplification downstream of the developing trough in the central
U.S.) Depicted, the plateau of temperatures around 10 to 15
degrees above average Saturday through Monday makes sense,
though I do wonder if models are underdoing the amplification
somewhat. As such, I kept things close to MOS guidance, but did
bump maximum temperatures a degree or two during this period.
Isentropic ascent downstream of the negatively-tilted trough on
Monday may lead to warm-advection precipitation in the northern
mid-Atlantic and northeast, which required keeping/increasing
pops Sunday night and Monday, especially north of the Mason-
Dixon line. Such regimes tend to be favorable for freezing rain
or sleet (i.E., A low-level warm nose), and did retain mention
of this in the far northern County Warning Area Sunday night and Monday morning.
As the strong cyclone in the plains moves northeast into the
Great Lakes and southeast Canada Monday night and Tuesday, a
strong cold front will sweep through the mid-Atlantic during
this period. Models continue to show decent quantitative precipitation forecast in vicinity of
the front, with some disagreement on timing (gfs fast; European model (ecmwf)
slow; CMC in between). Timing continues to favor Monday night,
though it has slowed in the past couple of days to include
Tuesday morning (and perhaps most of the day in the
northern/eastern cwa). Given the strong/consistent agreement
among models Monday night, introduced categorical pops during
this period and increased pops a little bit through the day
Tuesday. Precipitation may be showery, especially close to the
front, but large-scale lift will be pronounced, suggesting a
more widespread rain may occur. Should showers/convection
develop, quantitative precipitation forecast may be locally enhanced, and this will need to be
watched as the event approaches. A rumble of thunder cannot be
ruled out either, given cold midlevels with the approaching
trough, though this threat looks too low for mention at this
There are some indications of upstream perturbations moving
through the transient northwest flow Tuesday night and
Wednesday, which may lead to flurries or snow showers in the
Poconos, but the signal is not agreed upon amongst the
operational models. Kept Tuesday night through Wednesday night
dry as a result, but temperatures will fall back to seasonal
averages as colder high pressure builds into the area.
The 12z European model (ecmwf) keeps the high entrenched in the mid-Atlantic
through Thursday, but the 12z CMC is quick to bring a developing
low through the area late in the long term (with some indication
of this in the GFS as well, though pushed off to our southeast).
Will add slight chance pops for Thursday at this time, though
this may be on the fast/aggressive side.
Aviation /02z Friday through Tuesday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg,
kilg, kmiv, kacy and surrounding areas.
Tonight...VFR conditions are expected with few high clouds. Light
northwesterly winds this evening are expected to back to westerly.
Wind speeds through the night are expected to be less than 10 kt.
Tomorrow...VFR conditions with few, if any, clouds. Westerly winds
near or below 10 kt.
Thursday night through Sunday...primarily VFR, though increasing
clouds should occur Sunday. Winds generally west around or below
10 kts. High confidence.
Sunday night and Monday...sub-VFR conditions possible with
increasing chances of rain or showers during the period.
Generally light and variable winds becoming south or southeast
around or below 10 kts. Low confidence.
Monday night and Tuesday...periods of sub-VFR likely with a good
chance of rain/showers, especially through Tuesday morning.
Southeast winds becoming west or southwest after frontal passage
on Tuesday (probably by the afternoon). Winds may become gusty
after frontal passage and in vicinity of showers. Medium
winds and seas on the coastal waters have diminished below
Small Craft Advisory levels as of early this evening and should
remain below these levels through the overnight. Gusts above 20
kt will again be possible, primarily for the New Jersey coastal waters,
during the day tomorrow.
On the Delaware Bay, winds and seas should stay below Small Craft Advisory criteria
tonight and tomorrow.
Friday night through Saturday night: marginal advisory
conditions may occur off the northern/central New Jersey coast;
otherwise, sub-advisory conditions and fair weather expected.
Sunday through monday: sub-advisory conditions, with an
increasing chance of rain or showers by Monday.
Monday night and tuesday: advisory-level winds possible (south
or southeast before frontal passage; west after frontal passage),
with a good chance of rain or showers.