Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Mount Holly New Jersey
925 PM EDT Wednesday Jul 30 2014
several weak surface troughs will move into the area through
Thursday, before high pressure over the western Atlantic pushes an
offshore warm front northwestward across the East Coast this
weekend. Several disturbances will move northeast along this
boundary through the weekend. Another cold front will then move
across the area late Tuesday into Wednesday.
Near term /until 6 am Thursday morning/...
an upper-level trough continues to influence our weather. A series
of disturbances rotating around the main upper-level trough are
producing convection to our west. The current activity is tied to a
well defined short wave from near Lake Ontario to western
Pennsylvania /via water vapor imagery/. Its forcing in combination
with some deeper moisture within an area of lingering instability is
maintaining some convection. This short wave will continue to lift
to the east and northeast through the night, however the convection
is weakening. Therefore, held onto some slight chance/low chance probability of precipitation
across the northwestern zones through about midnight for the
potential of a few showers. The instability is waning and based on
the downward trend in reflectivity and lightning to our west, the
thunder was removed. The model guidance overall does not show much
instability holding on through the night.
It is possible that an area of somewhat steeper middle level lapse
rates overspreads a portion of the region overnight to keep a few
showers going as the southern portion of the short wave slides by.
Overall, the precipitation chances were kept on the low side overnight
and mainly confined to the north and west of Philadelphia.
The hourly grids were tweaked based on the latest observations, then
the lamp/lav guidance was blended in. The cloud cover was adjusted
downward quite a bit to start based on satellite data, however some
increase was then maintained from west to east as at least some
cloud debris should arrive. No major changes were made to the low
temperatures at this time.
Short term /6 am Thursday morning through 6 PM Thursday/...
stronger shortwave interaction with increasing column moisture and
better, not great, instability should lead to widely scattered
showers and isolated thunderstorms. Maximum temperatures should be a degree
or 2 below normal with ample cloud cover and scattered or isolated
afternoon shower activity.
50 50 blend of the 12z/30 NAM/GFS MOS.
Confidence below average on where the greatest chance of a shower or
isolated thunderstorm should occur Thursday though am favoring i78 northward.
Even the timing is less than certain. There could be some dying
showers in east PA and nnj at 8am then newly developing
showers/isolated thunderstorms during middle afternoon in east PA.
Long term /Thursday night through Wednesday/...
a wet weekend ahead as periods of showers and thunderstorms are
likely for the end of the week, through the weekend, into early
The broad middle/upper trough will remain across the eastern half of
the country into the weekend. However, as the closed low begins to
lift into eastern Canada this weekend into early next week, the
trough will begin to flatten out. With the trough remaining in
place through this weekend, the East Coast will remain under
southwest flow aloft. Several short wave/vorticity impulses will
traverse the area within the southwest flow Friday, Saturday, and
Sunday. At the same time, high pressure over the western Atlantic
will begin to build westward, and push a frontal boundary back
toward the East Coast. While the front will be draped near the
East Coast, several weak surface waves will move along the
boundary at the same time as the short wave/vorticity maxes aloft. This
will help lead to several periods of enhanced precipitation
probabilities through the weekend.
By Monday, the middle/upper trough flattens out and the frontal
boundary may begin pushing back further offshore. However, there
will still be enough moisture around and we may be close enough to
the front that some shower/thunderstorm activity remains possible.
Another cold front is forecast to move across the area late Tuesday
into Tuesday night. There is not any strong short wave/vorticity
impulses currently forecast, so precipitation chances are small
at this time with the approach and passage of the frontal
boundary. Behind this front, high pressure should build in and
lead to drier weather for Wednesday.
With cloud cover and rainfall expected over the weekend, the highs
Saturday and Sunday are forecast to be several degrees above normal.
However, we return to above normal temperatures early next week as
we should begin to see more sunshine.
Aviation /02z Thursday through Monday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg, kilg,
kmiv, kacy and surrounding areas.
Rest of tonight...VFR, with some bases at or above 5,000 feet moving
in. There could be a few showers north and west of kphl.
South-southwest winds around 5 knots, becoming light and variable or
Thursday...mostly VFR ceilings developing with isolated to scattered
showers and thunderstorms possible. Local sub-VFR conditions in
shower/thunder activity, mainly across eastern Pennsylvania and
northern New Jersey. Southwesterly winds generally around 10 knots.
Thursday night-Friday...mostly VFR. Showers and thunderstorms
possible, especially Friday which could temporarily reduced ceilings and
Friday night-Monday...showers and thunderstorms possible
periodically through the weekend into early next week. Reduced ceilings
and/or visibilities possible over the weekend, especially Saturday through
announcement: we began a new experimental marine observation program
that any mariner can easily use to communicate with the National Weather Service here in
Mount Holly... provided you have internet access. This can help US
fine tune our forecasts...especially if the forecast is not reflecting
the reality at your marine location.
Forecast: winds and seas to remain below Small Craft Advisory
criteria through Thursday with winds staying below 15 knots and seas
between 1 and 3 feet. Southerly winds with a slight onshore component
each afternoon and a more westerly component late at night.
Thursday night-Monday...sub-Small Craft Advisory conditions are
expected through early next week. However, wind may periodically
gust around 20 knots.