Scientific Forecaster Discussion
fxus61 kphi 170339
Area forecast discussion
National Weather Service Mount Holly New Jersey
1039 PM EST Sat Dec 16 2017
a cold front will move southeast across our region tonight,
then it will return northeast of the region as a warm front late
Sunday night. Another cold front will then move through the
mid- Atlantic states Tuesday night. High pressure follows
Wednesday night. Thereafter, to end the week, an area of low
pressure will track northeast from the Great Lakes and advance a
warm front into our region.
Near term /until 6 am Sunday morning/...
as of 900 PM, a cold front was draped west to east across about
the middle of our region. This front will continue to settle
south-southeastward through the night. Strong northwesterly flow
above the surface continues to pull plenty of cloud cover
across much of our area, although far southern New Jersey,
Delaware and northeast Maryland have pretty much cleared out.
These clouds may be stubborn for much of the night as an
inversion develops in the wake of the cold front. Some drier air
may entrain though and assist in eroding at least the southern
portion of the cloud field overnight. We increased the clouds
some especially across the northern areas. The presence of the
clouds is keeping temperatures from dropping as fast, and
farther south there is just enough of a southwest wind ahead of
the front keeping the temperatures from dropping faster. As a
result, temperatures needed some adjusting. Low temperatures
especially up north might be to low if the cloud cover hangs on
The earlier area of light snow across central New York to just
northwest of the Poconos has dissipated, although radar shows
some weak returns moving toward the Poconos. We carried a
mention of flurries through the rest of the evening across the
far northwest zones.
Short term /6 am Sunday morning through 6 PM Sunday/...
weak high pressure will be across the region on Sunday, with the
cold front just to our south. This front will begin to lift north as
a warm front, and cloudiness will increase during the day. For now,
precip is expected to hold off until later in the evening. (See the
long term section for more details on this system). Max temps will
be a few degrees below normal with light winds.
Long term /Sunday night through Saturday/...
500 mb: a warm ridge centered over the southern Appalachians will
weaken Wednesday with the passage of a substantial short wave through
se Canada. Thereafter...warmer that normal heights prevail over
the se USA with a resurgence of the ridge over the se coast next
weekend. Cooler than normal heights are near Hudson Bay, resulting
in a fairly strong WSW jet from the Southern Plains to the mid
and North Atlantic coast.
Temperatures: the first 15 days have averaged below normal, ranging
from 3.5f below normal at kmpo to near 1 degree below normal elsewhere.
(Monthly melted W.E. Pcpn has been below normal but snowfall,
at least in our area of the i95 corridor is already well above
the monthly normal at both phl and NYC!)
In any case calendar day average temps are modeled near normal
around 5f above Monday, around 10 above Tuesday, maybe as much
as 5 above Wednesday, near normal Thursday then probably
warming again next Thursday and Friday to at least several
degrees above normal (the ec suggests 15f above normal average
temps next Saturday-we used the cooler wpc/mex blend).
Forecast basis: unless otherwise noted Sunday night-Monday
night is a 50 50 blend of the 12z/16 GFS-NAM MOS, Tuesday the
12z/16 GFS mexmos and next Wednesday-Saturday (d4-8) is the
15z/16 wpc 12 hr elements of the Max-min temps/pops and 6 hrly
Sunday night...our grids have a 25 to 40 pct chance for a
period(s) of light snow or mixed freezing-frozen pcpn in east PA
and the n2/3rds of New Jersey in weak waa. There is considerable
uncertainty regarding the extent and timing of any light pcpn so
snow and ice amounts are not officially posted in storm total
grids (less than 55% chance). Also, the forecast wording is also
therefore fraught with uncertainty. Near 1/2" snow is possible
near and north of i80 and spotty trace to .03 radial freezing
rain icing is possible in the i78-i276 region. Subfreezing
ground temps will need to be considered for any possible
advisory. Otherwise mostly cloudy and milder. Light south to
southwest wind. Precipitable water 0.55" confidence: average.
Monday and Monday night...variable cloudiness or p/S and milder.
Precipitable water dries out slightly from NW to se to 0.45". West wind.
Tuesday and Tuesday night...a very very nice mid December day-
warmup...southwest wind may gust 20-25 miles per hour during the afternoon,
then shifts northwest by midnight. Above normal temps by 10 to
14 degrees. A cold front passage during the evening may generate a few rain
showers-sprinkles far north (end as snow showers or flurries
poconos?) But precipitable water is further drying out from NW to se so
confidence: average or below average on whether it precipitates.
Wednesday...high pressure building in from the west. P/S cooler
with a NW wind gust 20-25 mph during the morning diminishing mid
Thursday...variable, mostly high clouds. Seasonable colder.
Saturday...conservative and much below average confidence on
what may happen. Pretty much wpc grids.
Aviation /04z Sunday through Thursday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg,
kilg, kmiv, kacy and surrounding areas.
Tonight...VFR, with a ceiling mainly between 4000-8000 feet at most
terminals. Southwest winds 5 knots or less, becoming west and
then northerly overnight (locally light and variable) as a dry
cold front moves through.
Sunday...VFR overall. Some cloud bases around 2500 feet may develop
especially north and west of kphl. Northeast winds around 5 knots or
light and variable.
Sunday night: brief sub-VFR possible with patchy light rain,
mixed precipitation possible at kabe, kttn and krdg. South to
southwest winds under 10 knots. Confidence: average or below.
Monday and Monday night: mainly VFR. Westerly sustained winds
generally under 10 knots. Confidence: above average.
Tuesday through wednesday: VFR. An isolated shower cannot be
ruled out Tuesday evening north. Southwest wind gusts around
15-20 knots then shifting to northwest and increasing to around
20-27 knots Wednesday morning. Confidence: above average.
Thursday...VFR. Light wind. Confidence: above average.
the cold front has moved south of the northern and central New
Jersey shore. As a result, winds and seas have diminished.
Therefore, the Small Craft Advisory has been cancelled.
As the front continues south overnight, winds will veer from
southwest to north- northwest, then to the northeast on Sunday
(expect southeast on Delaware bay), but winds are not expected to
return to Small Craft Advisory levels late tonight and Sunday.
Sunday night-Thursday...no headlines anticipated except for
Wednesday morning when a northwest flow Small Craft Advisory of 25-30 kt wind gusts is
anticipated. Confidence: above average.