Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Mount Holly New Jersey
334 am EDT Thursday Aug 21 2014
the region will see an active weather pattern for the next two days
as one area of high pressure remains over the deep south, while
another is situated over the Hudson Bay. The Canadian high will
eventually build south over the region by late this weekend and
remain the the main weather feature for the middle Atlantic through middle
week next week.
Near term /until 6 PM this evening/...
there are a couple of areas of isolated/scattered showers around the
area early this morning, all associated with a few individual
vorticity impulses moving through the cyclonic flow aloft. The first
is exiting our area to the northeast, and will bring with it it's
area of showers. The others are located across central and western
Pennsylvania. The hrrr and RUC have these drifting eastward and
mostly drying out or lifting to our north before they get here.
After these showers early this morning, we should get a period from
middle/late morning into the early afternoon where we should be mostly
shower free. However, as we move into the middle to late afternoon, we
expect showers to begin to increase to our west and move into our
area. These will be associated with another short wave/vorticity
impulse that will be moving into our area as the middle/upper low
slowly drifts eastward. At the surface, an area of low pressure
remains across the eastern Great Lakes, while a weakening frontal
boundary extends south of the low. Another frontal boundary remains
snaked from western Pennsylvania southward through Virginia and
North Carolina. None of these features are expected to move into the
area today as the low is expected to remain to our north and the
frontal boundary to our south is expected to remain stationary
through the day.
There remains some instability forecast today, although there is
not much shear or much of a wind field, so we do not expect severe
weather at this time. If there are any thunderstorms later today,
and if they grow tall enough, some gusty winds could be possible and
with the wetbulb as low as it is, some small hail cannot be ruled
out. Precipitable water values are generally forecast around 1.5-1.75 inches, so
some showers/storms could produce a period moderate to heavy
For highs, used a blend of mav/met MOS, mosguide, and mixed 925 mb
Short term /6 PM this evening through 6 am Friday/...
we expect showers and possibly thunderstorms to be ongoing at the
start of the short term period as the vorticity impulse will be
moving through the area as the middle/upper low continues to drift
eastward into New England. Also, a second area of surface low
pressure is forecast to develop near the Chesapeake Bay along the
stationary boundary to our south as it tries to lift northward into
our southern areas. This may keep a focus for precipitation
overnight across our southern areas where showers may last into the
early morning hours of Friday. As the energy from the vorticity
impulse moves away from the area, showers should mostly end by
For lows, used a blend of mav/met MOS and mosguide.
Long term /Friday through Wednesday/...
Friday through Saturday...the region remains under northwest flow as
the Stout Ridge remains over the Mississippi Valley, and the other
high remains over eastern Canada. With the northwest flow, expect a
few embedded short wave troughs to bring chances for thunderstorms
through this period. However, the timing of the individual short
wave troughs remains uncertain.
Sunday through Thursday...the high over eastern Canada builds south.
With the associated cold air advection, could see highs especially
early in the weekend 5 to 10 degrees below normal. By Tuesday
however, the high starts to shift off shore, leaving the region
under southerly flow and leading to a modest warming trend. However,
also towards the end of this forecast period the GFS and European model (ecmwf)
solutions differ greatly on the timing of an upper level trough and
associated surface cold front that will be propagating east on the
heals of the high. Still, both solutions show these features just
beyond the seven day forecast.
Aviation /07z Thursday through Monday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg, kilg,
kmiv, kacy and surrounding areas.
Overall, we expect mostly VFR conditions for the taf sites today,
although there is the potential for lower conditions later today
into this evening. A period of MVFR ceiling and/or visibility is possible
early this morning mainly for Abe/rdg/miv, but this should return to
VFR by mid-morning.
We expect scattered showers, and possibly thunderstorms, to develop
to our west trough the day and move into our area later this
afternoon into this evening as a disturbance aloft moves through the
area. Confidence in thunderstorm activity occurrence and timing is
low, so they were not included in the tafs at this time.
There is the potential for some moderate to heavy showers, so we
have included moderate showers with MVFR conditions later today into
this evening. One model gives US VFR through the period, while the
other brings everyone to IFR. Not confident on IFR, but think we
could get some lower conditions temporarily with the heavier showers.
Winds today should be mostly south to southeast around 5 to 10 knots.
Friday and Saturday...mostly VFR. Isolated to scattered thunderstorms and rain are
possible across the region. Under any thunderstorms and rain MVFR ceilings and
brief IFR visibilities will be possible.
Sunday through Monday...generally VFR.
sub-Small Craft Advisory conditions expected today with seas 2-3
feet or less and southeast winds 5 to 10 knots.
Friday...conditions expected to stay below Small Craft Advisory criteria.
Saturday...occasional gusts near 25 knots and wave heights approaching
5 feet are possible especially Saturday afternoon through Saturday
Sunday and Monday...wave heights and winds slowly falling below
Small Craft Advisory criteria Sunday morning and should remain
below criteria through Monday.