Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Mount Holly New Jersey
354 PM EDT sun Jul 13 2014
high pressure will remains parked offshore while a cold front
approaches from the west tonight and Monday. The front is expected
to gradually traverse the region Monday night through Tuesday night
before moving offshore Wednesday morning. High pressure builds into
the area later Wednesday through Thursday. The high will eventually
shift off the coast by the beginning of next weekend.
Near term /until 6 am Monday morning/...
variable amounts of cloud cover so far today has led to spotty
sunshine and varying amounts of cape across the area. The humid
airmass will likely be primed enough later today for some scattered shower
and thunderstorms. A severe watch has been issued across western New York and western
PA...but this activity will diminish as it crosses PA and it should
be sub-severe when it arrives here this evening. Still...decent
downpours and some gusty winds will still occur. Low temperatures tonight
will be mostly in the 70s and it will remain humid. Patchy fog
expected after midnight.
Short term /6 am Monday morning through 6 PM Monday/...
low pressure will move across Canada and a cold front will approach
from the N/W. Scattered showers and thunderstorms will become more numerous during
the afternoon and persist into the evening. The Storm Prediction Center has outlined our
area for severe potential tomorrow. Downpours will probably create some
local flash flooding concerns late...but the areas with the lowest
flash flood guidance (n/w) may not get the highest quantitative precipitation forecast until late Monday or Monday
night...so we will hold with the Flash Flood Watch...but still offer the
possibility for a watch to be raised tonight.
Hot and humid Monday with highs in the upper 80s and low 90s and
heat index values in the middle/upper 90s.
Long term /Monday night through Sunday/...
stormy weather pattern expected Monday night through Tuesday night as a
cold front moves slowly through the area. Multiple rounds of severe
weather and heavy rainfall are possible during this time. See Hydro
section for more information on the flash flooding potential.
The cold front will move southeastward from the Great Lakes toward
the Ohio Valley Monday night. Models show the development of a Lee
trough in central/eastern PA ahead of the cold front by the evening.
With dewpoints in the lower 70s and an anomalously strong wind
field, showers and thunderstorms will likely continue through the
evening and possibly overnight as the latest NCEP model soundings
indicate MUCAPE at or above 1000 j/kg for much of the night. Even if
convection become elevated, there should be enough lift with the
pre-frontal trough for parcels to reach their level of free convection and continue after
peak heating hours.
The cold front is then expected to jump the Appalachians toward the
leading surface trough over central PA Tuesday morning. Models have come
into much better agreement with the position and timing of the cold
front on Tuesday in recent runs. The trend has been to slow down the
movement of the front enough to keep the forecast area in the warm/
moist pre-frontal sector on Tuesday and even into Tuesday night.
This slower scenario would make sense given the track of the upper
low cutting well to our northwest and the amplifying S-SW steering
flow ahead of the trough becoming increasingly parallel to the
frontal boundary. While the best height falls and forcing for ascent
from the upper low will be reside over the eastern Great Lakes,
synoptic lift will increase over our region in the right
entrance-region of a 130+ knots ulvl jet streak and also with the cold
front itself. Forecast probability of precipitation of 60-70 percent for Tuesday-Tuesday night
are relatively high for convective setups but it seems warranted
given the pattern. The risk for flash flooding is enhanced in this
One of the big forecast uncertainties relates to cloud cover, which
will play a big role in determining the potential for severe
thunderstorms Tuesday afternoon-evening. There will likely be cloud
cover from overnight convection over our area Tuesday morning, but a
conditional risk for severe storms exists if appreciable breaks
occur that would allow for pocket of higher instability to develop
during the peak heating hours. The setup would favor a linear Mode
of convection with the front and the biggest threat would be
damaging winds. Uncertainty remains too high at this time to enhance wording
in the severe weather potential statement for a significant severe weather event, but the
possibility of it cannot be ruled out if full destabilization is
realized given the unseasonably strong 30-50 knots deep-layer shear
that is forecast to overspread the area by 00z Wednesday. Attm, the
southeastern half of the forecast area included by Storm Prediction Center in a 5
percent chance for severe storms on Tuesday.
The majority of 12z model guidance has the cold front moving off the
New Jersey and Delaware coast around daybreak Wednesday morning. The northwest winds
behind the front are forecast to be light in the boundary layer, so
drier and cooler air at the surface may be slow to arrive during the
day, especially along and east of I-95 if the front slows down
further or stalls just off the coast. In any event, enough
subsidence aloft should allow for skies to clear from west to east,
with some areas becoming mostly sunny during the afternoon.
High pressure builds in from the Midwest late Wednesday into
Thursday. Maximum temperatures both Wednesday and Thursday will be around 80f
across most of the area (except cooler in the Poconos and along the
coast). The biggest difference between both day for locations along
and east of I-95 will likely be the humidity. While it won't feel
muggy on Wednesday (especially for July standards) with dewpoints in
the 60s on Wednesday, Thursday will be much drier with dewpoints in
The high starts to move offshore late in the week. Southerly return
flow developing on the backside of the high will be accompanied by
an increase in the heat and humidity. Depending on how quickly the
high moves offshore, shower/thunderstorm chances could return as
early as Friday. Both the 12z GFS/European model (ecmwf) and several of the ensemble
members indicate a warm front lifting northward toward the region
and waves of lopres developing along the boundary by the beginning
of next weekend. Precipitation chances increase Saturday accordingly.
Aviation /20z Sunday through Friday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg,
kilg, kmiv, kacy and surrounding areas.
VFR conditions have returned to the terminals as of the early
afternoon. Conditions should remain mostly VFR the rest of the
daylight hours with the only exception being the possibility of a
stray shower bringing a short periods of lower conditions. Towards
sunset...a line of showers and thunderstorms will probably move from north/west to
S/east across the area. Lower conditions will occur with this feature.
Winds today and into the evening will be mostly S or SW at 10 to 15
kts with some gusts around 20 kts. Winds near the shore will be more
S/serly. Overnight...mostly VFR early...but then some fog possible
overnight. Monday...VFR early then scattered showers/thunderstorms during the
Monday night through Tuesday night...multiple rounds of rain showers/thunderstorms and rain
during this period will lead to periods of sub-VFR conditions.
Southwesterly winds around 10-15 knots ahead of a cold front. Cold front
slowly moves through Tuesday night. Frontal passage will be marked by a wind
shift out of the northwest but speeds will be light.
Wednesday and Thursday...VFR with hipres moving overhead. Only
exception may be a brief period of low clouds near miv/Acy Wednesday
morning as the cold front is slower to move through. Drier air moves
into the region, so do not expect much in the way of fog at night.
Friday...hipres starts to shift offshore late. Maybe an isolated thunderstorms and rain in
vc of the far southern and western terminals late in the day.
we will continue with the Small Craft Advisory flag for the ocean waters into the
evening and leave the expiration time as is for now. Occasional gusts to
25 kts and a slowly building sea across the waters (mostly north) as
the pressure gradient remains enhanced with the approaching
low/front to the west. Scattered thunderstorms this evening and again Monday
afternoon will have local higher winds and seas.
Monday night through Tuesday...
gusty S-SW winds expected ahead of a cold front Monday night through
Tuesday. Small craft advisories may be needed during this time as
gusts to 25 knots and seas of 4-5 are forecast. Winds subside Tuesday
night with the pressure gradient weakening over the waters as the
cold front approaches from the west. Winds shift out of the northwest
Wednesday and west Thursday but remain light.
a moisture-laden air mass is forecast to be in place tonight
through Tuesday night ahead of a slow-moving cold front. This is
characterized by precipitable water values around 2.00 inches. The presence of a
Lee side trough Monday may help focus heavy rain producing
convection along and northwest of Interstate 95 corridor,
potentially resulting in an increased flash flood threat. The flow
is forecast to increase therefore the storm motions should be
decent, however convection should be efficient rainfall producers.
There are areas particularly in portions of southeastern and
northeastern Pennsylvania and also northern New Jersey where the
1-hour flash flood guidance is 1.50 inches or less. The highly
urban areas along the Interstate 95 corridor are also vulnerable
to quick runoff during torrential downpours. The setup for heavy
rainfall increases throughout the period with potentially the
highest risk for flash flooding on Tuesday afternoon and evening
when the cold front ever-so-slowly moves through.
The uncertainty revolves around where the axis of heavier rain
occurs as there is some model spread, and also the organization of
the convection. The latest quantitative precipitation forecast from wpc gives US 2-4 inches of
rainfall over the next three days. While some locations may see
less than this, there is a chance that other locations see this
amount of rainfall in only a few hours in heavier/slower moving
storms. No Flash Flood Watch at this time for Monday but will maintain a
mention in the hazardous weather outlook. This will be revisited
in later shifts.
Marine...Small Craft Advisory until 6 am EDT Monday for anz450>455.