Area Forecast Discussion
Issued by NWS Mt. Holly, NJ
FXUS61 KPHI 291912
Area Forecast Discussion
National Weather Service Mount Holly NJ
212 PM EST Tue Nov 29 2016
Strong low pressure in the upper Midwest will send a warm front
north through our area this afternoon. The low moves through the
Great Lakes region Wednesday pushing a cold front eastward across
the mid Atlantic states Wednesday night. The low redevelops to the
Canadian maritimes Thursday and Friday. High pressure moves to the
mid Atlantic states this weekend. Low pressure could affect our
weather late Sunday or more likely early next week.
.NEAR TERM /UNTIL 6 PM THIS EVENING/...
Mid morning surface analysis shows a 985 mb low over the Upper
Midwest. A cold front extends southeastward from the low across the
Great Lakes and then southward near the spine of the Appalachians. A
warm front was becoming better organized across the Delmarva and
southern NJ. Temperatures in the warm sector have jumped into the
60s under a breezy southerly wind. North of the frontal boundary,
the winds are light and temperatures are still in the low 50s.
The position of the warm front is also evident on radar. Overrunning
rain has become steadiest and heaviest along and northwest of the
warm front as a 60 kt southerly low-level jet enhances lift of warm,
moisture air from the Gulf of Mexico over the boundary. The previous
forecast was in good shape but some minor adjustments were made to
the PoP, Weather and QPF grids to pinpoint where the band of
heaviest rainfall is expected to today. With the warm front slowly
lifting northward today, the heaviest precipitation amounts are
expected from I-95 northwestward into the Poconos and northwest NJ.
Rainfall amounts in the axis will range between 1-2 inches today.
There may be isolated amounts higher than 2 especially toward
the I-78 and I-80 corridors, which could lead to ponding of water
on roadways and other poor drainage flooding. Precipitation will
gradually taper off from west to east late in the day and into the
Minor updates were made to the temperature and wind grid for today.
Breezy winds will occur today in warm sector today with the highest
gusts to 30 mph likely in the coastal plain. High temperatures will
range from the mid 50s in northeast PA and northwest NJ to mid 60s
in Delmarva, Philadelphia, and southern NJ.
.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM WEDNESDAY/...
A powerful upper-level trough remains anchored across the Plains
tonight. An initial impulse continues to track to our north pulling
a warm front with it. A robust low-level jet will be shifting
offshore early this evening, with much if not all of the lift going
with it. This should rapidly diminish any rain as it tapers to some
showers. There is some drying forecast to overspread our area,
especially in the mid levels, however the wind field is forecast to
be much lighter. This should result in lots of lower level moisture
remaining in place, and at least some forecast soundings indicate
that an inversion develops which helps to hold in the low-level
moisture. It is possible that enough drying occurs especially across
the western zones where the low clouds partially clear out for a
time. Given the light winds and moisture left around, some fog is
expected to develop. This could become more widespread especially if
the lower cloud cover thins or clears for a time. Given the
uncertainty regarding the extent of the low clouds, not anticipating
widespread dense fog at this time.
The next impulse rotating around the parent closed low will start to
approach from the southwest toward morning. This will increase some
warm air advection as another low-level jet begins to approach. The
ascent associated with this should result in some rain developing
once again. As of now, the bulk of this looks to hold off until
during the day Wednesday. Low temperatures were a MOS/continuity
blend and this results in mainly a milder night.
.LONG TERM /WEDNESDAY THROUGH MONDAY/...
500MB: A ridge in the northeast USA Wednesday gives way briefly to
a trough Thursday. Thereafter, there are major model discrepancies
regarding the evolution early next week. The 00z/29 GEFS develops
a ridge in the east central USA while the operational ECMWF and
yesterdays 12z/ecefs lifted a significant southern stream short
wave northeastward from Mexico into the northeast states.
Temperatures: November as whole will average 1 to 3 degrees above
normal (see climate section at 5 am for more details). Calendar day
averages Wednesday will range from about 13 degrees above normal
far north FWN-MPO) to as much as 23 degrees above normal far
south (GED), cooling to near 10 degrees above normal Thursday,
still around 5 degrees above normal Friday and 1 to 4 degrees
above normal Saturday, Sunday and Monday.
Forecast basis: Unless otherwise noted, a 50 50 blend of the 00z/29
GFS/NAM MOS guidance Wednesday-Thursday, thereafter the 00z/29 GFS
MEXMOS Thursday night, and then the 04z/29 WPC guidance Friday-Monday,
at times blended with the 00z/29 GFS MEXMOS guidance or the long
term temps were adjusted because of the 00z/29 ECMWF 18z 2m temp
fields. The WPC POP Sunday into Monday was cut 30% each period to
indicate a slight delay in the southern stream qpf threat and hold
onto the drier GEFS and GGEM trends which reflects a slower
arrival in the 00z/29 NAEFS mean qpf fields.
Wednesday...becoming breezy and rather warm along and southeast
of I-95 while its mild northwest of I-95. Periods of showery rains
with PWAT increasing to 1.6 inches by Wednesday evening near the
I-95 corridor. SWI -2 , TT 52 and KI near 35 should result in
isolated strong tstms southeast of I-95 (please see SWODY2).
Thunder was trimmed a little to the southeast based on the more
positively tilted EC and its projected instability fields. Heavy
rain potential continues and probably near and nw of I-95. FLS
issuance for poor drainage street flooding rains is anticipated.
Northwest of I-95...rains are probably a little more persistent
through the day and fog may be a problem, especially high terrain.
Additional rainfall totals of 0.5 to 1.5 inches are expected across
parts of eastern Pennsylvania and northern New Jersey with lesser
amounts southern Delaware. See climate section for daily rain records.
South to southwest winds will probably briefly gust 30-40 mph in
southern Delaware during Wednesday afternoon.
Confidence: Above average.
Wednesday night...heavy showery rains end from west to east with a
wind shift from south to west toward midnight. Still, Thursday morning
min temps at least 20 degrees above normal (Calendar day mins for
December 1 will be lower due to cold air advection Thursday
evening). Confidence: Above average.
Thursday: Dry and breezy, but still relatively mild. Gusty west
northwest wind to 30 mph. Confidence: Above average.
Friday-Saturday...Dry air and a return to near normal temperatures.
A west northwest flow may bring some cold advection stratocumulus
at times, especially to our northwestern counties. Winds will gust
20 to 25 mph at times, especially afternoons. small chance of a
flurry or sprinkle in the Poconos Saturday. Confidence: Above
Sunday-Monday...chance of precipitation, including snow in the
northwest portion of our forecast area. Large spread in the
guidance makes this a low confidence forecast. The GFS is
suggesting dry high pressure while the ECMWF is continuing to
indicate that low pressure will influence our weather at that
time. WPC POPS were cut Sunday into Monday due to the uncertainty
seen in the GEFS 6 hr pops for .05 and the possibility that the
short wave ejection from Mexico is delayed a day.
.AVIATION /19Z TUESDAY THROUGH SUNDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.
Rain has overspread the terminals this morning. The heaviest rain
resides over our western terminals (ABE-RDG), where VSBYs have
lowered to IFR and CIGS to low end of MVFR. Expect predominately
IFR conditions today for these western terminals. The I-95
terminals, including PHL, are seeing MVFR conditions under steady
light to moderate rain. These I-95 terminals may bounce between
IFR and MVFR throughout the day depending on the intensity of the
rain (which will be variable). The southeastern terminals (MIV-
ACY) are still VFR in light rain but conditions should gradually
lower to MVFR later this morning and continue into the afternoon.
The steady rain will taper off from west to east between 21-00Z
though there could be scattered showers through the evening across
eastern terminals. Low clouds will hang around after the rain ends
and should result in most terminals seeing IFR CIGS. There is a
potential for breaks in the clouds to develop late this evening
and overnight. If this happens, the setup for fog will be
favorable. VSBYs would lower to IFR and possibly LIFR if clearing
is more prominent. There is still uncertainty in tonight`s
Low-level wind shear is included this morning as a southerly low-
level jet (at 2,000 feet) increases to 45-60 knots. This occurs
initially when surface winds are southeasterly at 10 knots or less.
The surface winds increase after a warm front lifts north later in
the morning and the low-level jet gradually shifts east, therefore
low-level wind shear concerns generally decrease. Southerly surface
winds will increase to 12-18 knots with gusts to around 25 knots
across much of the area from later this morning through mid/late
afternoon. Less confidence regarding gusts at RDG and ABE.
Wednesday...KPHL KPNE KILG KMIV KACY: IFR conditions with low
ceilings and fog should improve to MVFR during the day where a gusty
south to southwest wind of 20-30 kt develops during the afternoon.
Periods of showery heavy rains will produce brief IFR conditions
during the afternoon and evening and there could be a gusty tstm
(please see SWODY2).
KABE KTTN KRDG: Generally IFR conditions in st/fog/periods of
showery heavy rains. Small chc of an afternoon tstm. LLWS possible
in the afternoon with a 45 to 50 knot low level southwesterly jet at
Wednesday night...MVFR and IFR conditions in moderate to heavy rain
during the evening with a chance of thunder. A cold front should
arrive around midnight followed by clearing and improvement to VFR.
Low level wind shear is possible in the evening due the expectation
of a 45 to 50 knot low level southwesterly jet at that time.
Thursday through Saturday...Mainly VFR sct-bkn aoa 4000 ft. West to
northwest wind gusts 25-30 kt Thursday afternoon and 20-25 kt
both Friday and Saturday afternoons.
Upgraded the Small Craft Advisory to a Gale Warning for the
coastal waters of DE and NJ through 8 PM this evening. Upstream
buoys to our south (near the SE VA coast) have reported southerly
winds gusting 35-40 kt. Expect intermittent gale force gusts to
develop farther up the coast late this afternoon and early this
evening as the low-level jet moves directly overhead. The Small
Craft Advisory for the DE Bay was extended through 8 PM.
A southerly low-level jet on the order of 50-65 knots will move
across the region today. This will allow warmer air to stream
northward along with a surface warm front. Periods of rain will also
occur and this should keep the low-levels from mixing as efficiently
(poor lapse rates). As a result, kept a strong advisory in place and
held off on upgrading to a Gale Warning however there still is a
possibility for a few local gale force gusts by later in the day
especially with any heavier downpours. The winds diminish tonight as
the low-level jet shifts offshore, and with higher dew points
remaining for awhile this could allow for some fog to develop. The
seas will build into the 5-8 foot range today, then subside tonight.
Wednesday...A Small Craft Advisory is in effect for our ocean waters
for wave heights of 5 to 8 feet. Also, wind gusts of 30-35 knots are
possible on Wednesday afternoon along with a possible strong tstm
over the DE and s NJ waters. A Special Marine Warning may need to
be issued to highlight gale force gusty showers/tstms, especially
if convection can become organized.
Wednesday night...Wind gusts of 25 to 30 knots are expected initially
from the southwest then from the west following the passage of a
cold front after midnight. There could be a brief surge of gale
force gusts on either side of the frontal passage.
Thursday through Saturday...West to northwest wind gusts of 20 to 30
knots are expected through the period with an SCA probable for
Thursday and a lesser chance of an SCA Friday and Saturday.
Much needed rain across the region the next couple of days.
For today and tonight...A robust low-level jet will drive increasing
moisture advection during the day today. The PW values are forecast
to increase rapidly to 1.5 inches by Midday. As enhanced ascent
occurs from the low-level jet periods of rain will result, some of
which can be heavy at times. It appears that 0.50-1.50 inches of
rain should be common with the highest amounts along and
north/west of the I-95 corridor. Locally higher rainfall amounts
exceeding two inches are possible in this band. The placement of
this though will be determined primarily by the advancement of a
warm front today.
It has been rather dry, therefore just some ponding of water on
roads during heavier rainfall rates is anticipated along with some
possible poor drainage flooding. Some poor drainage flooding
though could be locally enhanced where fallen leaves clog storm
drains. The rain/showers taper later this afternoon and early this
evening, with much of tonight tending to feature a lull before the
next batch of rain arrives Wednesday.
For Wednesday...Storm total rainfall (even including Tuesday`s
amounts) will average 1-3 inches but isolated higher amounts of
3+ inches are certainly possible if the heaviest rain from both
waves of rain falls over the same areas. Conversely, some areas on
the coastal plain may receive less than an inch of rain,
particularly southern Delaware.
Through late Wednesday, we are not expecting main stem flooding
or even smaller basin flooding since these rainfall amounts will
be spread out over a 48 hour period (with a lull in between) and
the antecedent conditions are rather dry. Poor drainage flooding
is always a possibility especially during heavier rainfall rates
but impacts from this type of flooding are generally minor, except
for travel during the morning and evening commutes.
PHL should average 50 degrees for the month or 2.4 above the average
of 47.6. It is estimated that the large positive departures (warmth)
of today and Wednesday will warm the monthly departure by an estimated
0.3 to 0.9 degrees from the values calculated through the 28th.
Daily record High temperatures for Wednesday 11/30 where they appear
vulnerable to being within 2 degrees of record.
GED 74 - 2006
Daily record rainfall values for Nov 29 and 30 (less confidence on
approaching records today, whereas on Wednesday, several sites may
establish a new record rainfall for the date.)
NOV 29 NOV 30
ACY 1.93-1963 1.25 1923
PHL 1.68-1971 0.96-1972
ILG 2.25-1945 1.28-1934
ABE 1.70-1935 0.86-1923
TTN 1.40-1987 0.76-1898
GED 1.47-1982 1.71-1987
RDG 1.69-1984 1.52-1987
MPO 2.71-2005 3.00-1923
MARINE...Gale Warning until 8 PM EST this evening for ANZ450>455.
Small Craft Advisory until 8 PM EST this evening for ANZ430-