Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Mount Holly New Jersey
928 PM EST sun Nov 23 2014
intensifying low pressure in Missouri will arrive in the upper Great
Lakes Monday yanking a strong warm front north through our region
early Monday. It then sweeps a cold front off the East Coast Monday
night. High pressure follows late Tuesday. A coastal storm forming
off the North Carolina coast Wednesday morning moves east of New
England by Thanksgiving. A cold front passes through our area
Thursday evening and dry high pressure moves to the Carolinas next
Near term /until 6 am Monday morning/...
not much change needed with the update this evening, just slowed
the timing of the probability of precipitation a bit with plenty of dry air still in place.
Starting to see some over-running rain across southern Delaware-Maryland-Virginia
while the heaviest rain remains well to our south and west.
Otherwise, a potent synoptic setup will evolve through tonight.
This involves a large upper-level trough that will amplify from
the Midwest to the Gulf Coast states, which drives a deepening
surface low into the central Great Lakes overnight. Meanwhile, a
building ridge near the East Coast will shift offshore with its
center mainly east of Florida.
The strengthening of the upper-level trough to our west tonight is a
result of a potent 250 mb jet carving it out. As this occurs, the
height fields tighten quite a bit especially on the east and south
side of the trough. This will drive an increasingly robust wind
field across our area during the course of tonight. As this occurs,
short wave energy in combination with strengthening warm air advection will drive
increasing large scale ascent across our area. As a result, about a
4-6 hour period of rain is expected to surge from southwest to
northeast across the entire area during the night. As the Theta-E
advection really ramps up this evening, the low-level dew points
will be on the increase and this will lead to a jump in precipitable water values.
Therefore, some of the rain will be moderate to heavy at times which
could lead to localized poor drainage flooding. We therefore
continued to rapidly increase the probability of precipitation from south to north this
evening with categorical probability of precipitation everywhere overnight. There is some
elevated instability forecast to arrive across our southeastern
zones especially late, and this could produce a few isolated
As the flow increases above the surface and the injection of a warm
airmass works in across the area, we should start to mix better
overnight especially near and east of Interstate 95 and in the
higher terrain. This along with warm air advection will allow temperatures to warm
across the entire area as a warm front lifts northward across our
County Warning Area. As a result, we continued to indicate rising temperatures
through the overnight. By daybreak, some east and southern areas may
be into the lower 60s. The forecast soundings indicate better
momentum Transfer developing late especially the farther southeast
one GOES across the area, however given the warm air advection pattern there should
be a decent inversion present during the night even with the
boundary layer warming. As a result, the depth of the mixed layer
should not allow the Transfer of the rather robust winds with the
low-level jet down to the surface. There is a chance some fog develops
late especially as the warmer and more moist air surges in.
As for low temperatures, mostly a NAM/GFS MOS blend was used and
these were set to occur this evening. As mentioned above, a
temperature rise was then indicated through the night.
Short term /6 am Monday morning through 6 PM Monday/...
record warmth certainly possible Monday afternoon across the area.
A sharp upper-level trough is forecast to be to our west, and this
will drive a potent surface low across the Great Lakes and toward
Hudson Bay. This places our area within the warm sector and
southwesterly flow to its east. The main low-level jet and large
scale lift and associated warm frontal forcing is forecast to
quickly shift to our northeast during the morning. As this occurs, a
dry slot overspreads our area. This will result in rain tapering to
some showers and ending fairly quickly in the morning from southwest
to northeast. Therefore, the probability of precipitation decrease rather quickly during the
morning. There could be some low clouds and fog hanging on for a
little while across the northern areas and along the coast, however
confidence is low on this. After that, the forecast soundings
indicate that drying occurs aloft first associated with the dry
slot, however with our area being within the warm sector and still
strong winds above the surface we should mix out much of the lower
clouds. While a few showers could be around during the late morning
and afternoon, the soundings generally show a middle level cap
developing along with the drying. Therefore, this may limit any
shower development for awhile despite some instability present. This
brings US to the warmth and winds.
The temperatures at 925 mb are forecast to be around +15c during the
afternoon. The forecast soundings indicate that as we lose the lower
clouds and the boundary layer warms, mixing will deepen. This will
boost high temperatures into the 70s across much of the area. If we
clear out more, then temperatures could be a few degrees higher than
currently forecast. For now, used a MOS blend but did adjust up some
especially for the Interstate 95 area on south and east.
As for the surface winds Monday, while the intense low-level jet is
forecast to be shifting to our northeast during the day, the wind
field at 925 mb is forecast to be in the 40-50 knot range. The
forecast soundings show better mixing becoming established and if we
warm the boundary even more this depth would increase some. It still
appears though that we do not mix up fully to the strongest winds,
and therefore we carry gusts below advisory criteria. We did lean
closer to the GFS winds which are a little stronger at the surface
and boosted the gusts up to 35 miles per hour.
Long term /Monday night through Sunday/...
**winter storm-event vicinity I-95 corridor westward wednesday**
500mb: the strong short wave associated with the Great Lakes storm
lifts northeastward into Quebec Monday night. The next trough is on a more
southerly Route and crosses the East Coast late Wednesday night
with more coherence in ensemble solutions than presented 24 hours
ago. Heights rise toward next weekend but a general cyclonic flow
continues across North America with embedded short waves difficult
to discern in the ensemble mean.
Temperatures: temperatures for the month of November have so far
averaged 4 to 5 degrees below normal with yesterday the 10th
consecutive day of below normal temperatures. A brief hiatus from
the cold is at hand but even so...the month as a whole projects to
average well below normal. Did a quick extrapolation of maximum/min
for the rest of the month based our office forecast and we are
projecting 2.5 to 3.5 degrees below the monthly average of 47.6 degrees.
Calendar day averages Tuesday should be around 5 to 10 degrees above
normal (1201am Tuesday morning highs could be 60-63f phl eastward),
slightly below normal Wednesday with maximum temperatures on Wednesday 6
to 14 degrees below normal depending when the precipitation begins. Thursday
through Saturday calendar day average temperatures should average
below normal and next Sunday in may warm more than we have forecast.
Forecast basis: unless otherwise noted the long term period of this
forecast is generally based on a 50 50 blend of the 12z/23 NAM/GFS
MOS for the period Monday night-Tuesday night. Then the 12z/23 GFS
MOS was not applied Wednesday...it looks too warm by 3 to 5 degrees).
Thereafter (thursday onward) the 1522z/23 wpc gridded guidance
was used straight up. All this is eventually checked against the
12z/23 European model (ecmwf) through Thursday to see if we're in the ballpark
with European model (ecmwf) cyclically persistent 18z 2 meter temperatures.
Precipitation probability guidance is checked against the 09z/23
sref 3hrly probability of precipitation for .01 through 00z Thursday. The sref also has
significant probabilities for 1 and 4 inch snowfall amounts by 7pm
Wednesday i95 corridor westward but could be biased too cold! The
12z/23 gefs 6hr probability of precipitation for 0.05 are used after 00z/Thursday as a
check against other available guidance.
Monday night...small chance of a low top shower or sprinkle.
Otherwise clearing and maybe a bit breezy (west wind gusts 20 miles per hour
for a time) after the evening cold front passage when we see the boundary layer
wind forecast increases around 03-06z. Still above average temperatures for
this time of year...especially during the evening when temperatures will
be in the 60s. Guidance temps: used the warmer of the available temperature
guidance and blended with the even warmer 2m NAM temperatures at 00z and
03z/25 (prior to cfp).
Tuesday...partly to mostly sunny and breezy (first cirrus from Gulf
Coast short wave moving eastward vicinity tex/la and also rrq strong
250mb jet of 200 knots /230 miles per hour/ near l ontario). For the surface...am
expecting westerly wind gusts 25 miles per hour.
Tuesday night...becoming cloudy with light wind. Seasonable temperatures.
Light rain spreads up onto the Delaware-Maryland-Virginia and possibly all the way
to phl by 12z Tuesday.
Wednesday-Wednesday night...our model of choice leans heavily on the
European model (ecmwf) with GFS wpc and sref mix. A coastal storm deepens a mb per
hour as it races northeastward east of the middle Atlantic coast. Low pressure
development aided by rrq of 200kt 250mb jet over the Gulf of St
One quarter to 1.25 inch amounts expected...heaviest probably in New Jersey.
Snow map posted along with briefing package to give our first
effort at snow accumulations Wednesday and Wednesday night.
There will be adjustments. Thinking that this forecast can
easily be in error 3 inches either side of what is posted on our
web site this Sunday afternoon.
Confidence on snow amounts reaching the posted values is highest
from reading PA through the Poconos into far northwest New Jersey. Confidence
No matter...there is a high probability of snow impacting travel
sometime Wednesday or Wednesday night...especially just west of
I-95. The 12z/23 European model (ecmwf) is the warmest model with the GFS colder
but also probably too far east. The ec has a definite above
freezing layer between 850 and 700mb. The GFS not.
Snow/rain: I-95 corridor or 30 miles northwest looks to be general
demarcation between mostly snow and mostly rain. Depending on your
model selected and there will be adjustments needed...it could be
rain-snow changing to all rain then ending as snow for i95; mostly
snow further west and north through eastern PA and northwest New Jersey.
Elevations will accumulate best where its colder.
Temperatures for the Wednesday forecast are the 330 am phi forecast blended 50 50
with the 00z-12z/23 European model (ecmwf) 2m temperatures and did not use the 12z/23 mexmos
temperatures which looked 3 to 6 degrees too warm. Maximum temperatures appear to be
occurring forenoon before precipitation wet bulbs colder readings during the
Temperatures for Wednesday night are at or just above the 12z/23 GFS mexmos
values and slightly warmer than the wpc temperatures.
Those looking at the models should be aware that guidance does not
handle snow cooling the surface temperatures and the column will be cold
enough for all snow not too far west of phl with only the lowest
1000 feet air temperature structure determining the surface ptype.
The larger accums for this event along I-95 may be reserved for the
evening as precipitation probably changes to all snow.
Southeast of i95...which includes most of Maryland East Shore and Delaware and
coastal New Jersey...mainly rain is expected. It could end as a period of
wet snow or flurries... all dependent on banding tail (500 mb vorticity
Thursday...a secondary trough passage...surface cold front with
instability and moisture (windex?) May be accompanied by snow
showers northwest of i95. For now only gridded in the Poconos.
Friday- Sunday...wpc guidance and high pressure slides to the
Carolinas. Lows temperatures Friday and Saturday morning might be around
8 degrees colder than forecast north of i78 if there is snow
cover (10 to 15f).
Aviation /02z Monday through Friday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg, kilg,
kmiv, kacy and surrounding areas.
Very little change in the current thinking/timing with the 00z
tafs, though a delayed onset of MVFR is possible by an hour or two
from south to north.
Tonight...VFR to start, then ceilings are expected to quickly
lower to MVFR and IFR /local LIFR/ as a warm front arrives. A
period of rain is expected to move through, some of which could be
heavy at times. This will also reduce the visibilities, with even
some fog possible late especially toward the coast and from near
kabe on northward. South to southeast winds at around 5 knots will
increase to near 15 knots overnight and could become gusty at
times. The strongest winds should be from near the kphl metropolitan on
south and east. The amount of gustiness that occurs late carries
lower confidence. An isolated thunderstorm cannot be ruled out
late mainly south and east of kphl. A potent low-level jet moves
across the area, and therefore low-level wind shear is included
for the overnight.
Monday...MVFR/IFR /local LIFR/ ceilings for a time in the morning,
then ceilings should rise to VFR in the afternoon as a much warmer
airmass is in place. It is possible though that some low clouds
linger longer especially closer to the coast and north of kttn and
kabe. Rain departs early, however a few showers through the day will
be possible. Southwesterly winds increase to 15-20 knots with gusts
up to 30 knots. The core of low-level jet shifts farther offshore
during the day, therefore low-level wind shear should decrease
during the morning.
Monday night-Tuesday night...mainly VFR. Westerly winds may briefly
gust 20-25 knots Monday night subsequent to a cold front passage that could be
accompanied by a sprinkle. West winds should gust 25 knots on Tuesday
then diminish considerably late Tuesday night as they turn north
to northwest. High confidence on this scenario.
Wednesday...a coastal storm will bring MVFR or IFR conditions in
rain coast (kacy/kmiv), rain snow kttn kpne ktphl and kilg and
mostly snow kabe and krdg. In addition, expect persistent northerly
or northeasterly winds for generally the latter half of the day
with gusts 12-25 knots from west to east. Precipitation should end as
snow most taf sites at night except possibly not at kmiv/kacy.
Significant pavement accumulations possible krdg/kabe especially
Thanksgiving...conditions slowly return to VFR and winds shift to
westerly or northwest. There could be brief MVFR conds in scattered
snow showers later Thursday mainly just north of krdg and kabe. Winds may
gust to 25 knots.
a potent storm system will track well to our west later tonight and
Monday. This will drive a warm front north of the area tonight along
with an increasingly robust wind field. The 925 mb winds are
forecast to increase to 50-70 knots through the night, then decrease
to about 40-50 knots on Monday. The forecast soundings indicate that
better mixing occurs for awhile tonight before a much warmer airmass
arrives late and during Monday. On Monday, the forecast soundings
indicate that a very strong low-level inversion is present which
would keep the stronger winds above the surface. Therefore, a Gale
Warning remains in effect starting at midnight tonight for all our
zones /lower confidence for upper Delaware Bay/ and continues
through Monday morning. As we go through Monday, the best mixing
should shift nearshore and over land. Therefore, we did not extend
the Gale Warning at this time. The seas will build quite a bit during the
night into Monday with strong southerly winds, although just how
much wind actually makes it to the surface will influence the seas.
Monday night...SW wind shifting to west with the cold front passage. Small Craft Advisory likely.
Tuesday...Small Craft Advisory expected during cold air advection on westerly flow.
Tuesday night...probably below Small Craft Advisory conditions.
Wednesday...strong northeast shifting to northwest Small Craft Advisory with a
possible gale. Seas forecast on the Atlantic waters may be 2
to 4 feet too low Wednesday afternoon and evening.
Thanksgiving...winds are expected to eventually fall below Small Craft Advisory
criteria but Atlantic seas may not drop below 5 feet until at night.
Friday...possible Small Craft Advisory cold air advection west to northwest flow.
for our eight climate sites, here are the record high temperatures
for November 24th and the most recent year of occurrence.
Record highs 11/24:
Acy: 72 set in 1999
phl: 71 set in 1979
ilg: 73 set in 1979
abe: 69 set in 1931
ttn: 71 set in 1979
ged: 75 set in 1992
rdg: 69 set in 1979
mpo: 64 set in 1931
With the possibility of a midweek coastal storm bringing snow to the
region, we have included daily snowfall records below for our four
primary climate sites that have a long-standing unbroken period of
record (por) with respect to historical snowfall data.
Best chance of coming close to a "daily" record as of this Sunday
afternoons collaboration appears to be kabe.
Daily snowfall records:
November 26th: November 27th: period of record:
Acy: trace 1977,1957,1955 1.2 inches 1978 1874
Phl: 6.0 inches 1898 6.9 inches 1949,1938 1872
Ilg: 0.1 inches 1950 4.5 inches 1978 1894
Abe: 1.7 inches 1925 7.0 inches 1938 1922
Marine...Gale Warning until 11 am EST Monday for anz430-431-450>455.