Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Mount Holly New Jersey
1018 am EDT Friday Jul 11 2014
high pressure will build across the northeast states today, then
shift offshore later tonight and Saturday. A warm front should
lift north of the region Sunday, then a cold front is forecast to
arrive late Monday night and especially Tuesday. This front should
be settling to our south and east Wednesday, as high pressure
gradually arrives from the west during Thursday.
Near term /until 6 PM this evening/...
the surface frontal boundary that stalled over the Delaware-Maryland-Virginia yesterday
has finally moved south and east of the region this morning. A broad
surface hipres over upstate New York will build eastward into New England today.
Light northeasterly flow around the high this morning has been associated
with marine stratus along the southern New Jersey coast/Delmarva. Stratus is
already mixing out quickly during the middle morning in southern New Jersey and northern
Delaware but farther south, where the 925 mb cold front has lagged
behind the surface boundary and low level moisture has pooled, low clouds
will take longer to erode from north to south.
Several hires models show pop up showers and storms developing in
extreme southeastern PA and over the Delaware-Maryland-Virginia this afternoon during the peak
heating hours. This appears to be where the higher moisture resides
with surface dewpoints in the upper 60s vs upper 50s/lower 60s farther
north). Lift would be attributed to an approaching shortwave trough
axis and even the RUC shows surface convergence developing near the Eastern
Shore of the chesepeake Bay around 18z. The forcing will be weak though,
favoring minimal convective coverage. Instability/shear profiles not
indicative of a severe threat, but cannot rule out an isolated strong
storm develop over the Delaware-Maryland-Virginia.
Maximum temperatures today in the low to middle 80s except 5-10f cooler along the
coast and in the Poconos. May need to lower temperatures a couple of
degrees in the central Delaware-Maryland-Virginia if low clouds hold on a bit longer.
Short term /6 PM this evening through 6 am Saturday/...
the middle level trough is forecast to move to our east tonight. Any
isolated precipitation early this evening should dissipate quickly
as the atmosphere continues to to stabilize and as high pressure
builds into our region from the north.
The sky is expected to be mostly clear for tonight and the wind
should become light and variable. The combination is anticipated to
allow temperatures to fall into the upper 50s to middle 60s in much
of our region.
Long term /Saturday through Thursday/...
the overall synoptic setup is comprised of a potent upper-level
trough that closes off as it digs across the Midwest and Great Lakes
region Sunday and Monday, then begins to shift into portions of the
northeast Tuesday and Wednesday. This feature will drive a cold
front across our area in the Tuesday into Wednesday time frame with
convective episodes ahead of it. There is the potential for strong
to severe thunderstorms to occur especially Tuesday as the main
features aloft move through. We used a model blend for Saturday into
Monday, then mostly blended the 00z wpc guidance into continuity
For Saturday and Sunday...as the center of high pressure shifts
farther offshore Saturday, the flow will turn from east-southeast to
more southerly. As this occurs, some increase in the warm air advection ahead of an
amplifying upper-level trough into the Great Lakes region should
organize a warm front to our southwest. This front should then start
migrating northward Sunday. Overall Saturday looks to be dry with
high pressure close enough to start and then overall a lack of
organized lift. If isolated convection where to form in the
afternoon, it should be to our west on the periphery of the surface
high and closer to the terrain influences. As the upper-level flow
starts to turn more cyclonic Sunday, some showers and thunderstorms
should develop mainly inland in the afternoon perhaps focused near a
Lee side trough. This would break our string of nice weather on the
weekends at least for a portion of our County Warning Area.
For Monday through Wednesday...a robust upper-level trough/closed
low for this time of the year is forecast to slide across the Great
Lakes region and toward the northeast. Low pressure tracking across
Canada will pull a cold front into the east later Tuesday into
Wednesday. The presence of strengthening cyclonic flow in
combination with an increasingly unstable airmass will initiate
showers and thunderstorms. This may be tied to an initial Lee side
trough Monday with more of a convective organization across our
western zones, then more focused Tuesday with the cold front /also a
pre-frontal trough/ along with some cooling aloft and stronger flow.
The instability should be enhanced some as deeper moisture advects
northeastward in advance of the surface front. It will be warm and
humid as well, and Monday looks to be the hottest day as the
low-level warm air advection should peak. As of now, we are not anticipating any
heat related hazards across the County Warning Area.
The large scale lift is forecast to increase substantially Tuesday
with the presence of a strong short wave sliding across the region.
This could result in some strong to severe convective episodes
Monday and particularly on Tuesday. While there is less certainty
with the details, the potential is there for severe thunderstorms.
Based on the forecast soundings, it appears that line segments with
embedded bowing structures would be the main storm Mode given a more
unidirectional flow through the column. The convection that occurs
on Monday may have an impact on what develops on Tuesday. The precipitable water
values are forecast to increase to around 2 inches Monday and
Tuesday, which will allow for locally heavy rain to occur. This
could also increase the threat for water loaded downdrafts. We will
maintain a mention in the hazardous weather outlook.
The cold front should be easing to our south and east during
Wednesday, with drying arriving from the northwest. There could be
some convection lingering to start across our eastern zones, and
therefore much of the region was kept dry.
For Thursday...a large upper-level trough is forecast to be in
place, however the center of it looks to be north of the Great
Lakes. This will maintain cyclonic flow across our area as surface
high pressure gradually builds in from the west. We are anticipating
a cooler airmass to be in place along with lower dew points. No
major changes were made to this time frame.
Aviation /14z Friday through Tuesday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg,
kilg, kmiv, kacy and surrounding areas.
Earlier low clouds at miv/Acy dissipated between 12-13z this morning.
VFR today, but an isolated pop up rain showers/thunderstorms and rain possible in southwestern area
terminals. The best chance to see this activity would be ilg, but
coverage too low to include in 12z tafs. NE winds 5-10 knots today will
become southeasterly this evening around 5 knots.
Saturday...perhaps local MVFR/IFR stratus or fog to start,
otherwise VFR. Southerly winds mainly 5-10 knots.
Sunday and Monday...times of sub-VFR with some showers and
thunderstorms around. The best chance appears to be for the farther
inland terminals each afternoon and evening.
Tuesday...a better chance for organized showers and thunderstorms as
a cold front arrives. Some storms may produce gusty surface winds.
the frontal boundary will continue to sink to our south today,
allowing high pressure from the north to build across the coastal
waters of New Jersey and Delaware for tonight. NE winds around
10-15 knots are occurring over the coastal Atlantic waters this morning.
These winds will decrease by late afternoon. Light and variable winds
Wind speeds and wave heights are anticipated to remain below the
Small Craft Advisory criteria for today and tonight.
Saturday...high pressure near the New England coast will shift
farther offshore. This will result in the flow turning more
southerly with time, however it is anticipated to be light.
Sunday through Tuesday...the flow becomes more southwesterly on
Sunday. These winds strengthen Monday and Tuesday ahead of a cold
front, however they should remain below Small Craft Advisory
criteria. If enough mixing occurs on Tuesday, some marginal advisory
gusts can occur especially nearshore. The southerly flow will build
the seas and an advisory may be needed Monday and Tuesday for the
coastal water zones. The wavewatch guidance however could be
overdone given the southerly flow within a warm air advection regime. Some showers
and thunderstorms are anticipated Monday and Tuesday, some with
potentially gusty winds.