Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Mount Holly New Jersey
406 am EDT Wednesday Aug 20 2014
an area of low pressure and associated frontal boundary will move
through the Great Lakes region today and weaken as they move into
upstate New York Thursday. A second area of low pressure will
develop near the Chesapeake Bay Thursday night along the stationary
boundary to our south. High pressure will build down along the East
Coast out of eastern Canada later this weekend into next week.
Near term /until 6 PM this evening/...
surface low pressure, which was located about 250 Michigan off the
southeast Virginia coast early this morning, will continue to pull away
from the eastern Seaboard. Another area of low pressure that is
currently located near Lake Michigan will move across the Great
Lakes region today. Both of these lows developed along a stalled
front. High pressure ridging down the New England and middle-Atlantic
Seaboard will keep this frontal boundary south and west of the
forecast area today.
Winds will be light/variable to start the day with the surface ridge
axis overhead. Winds then veer out of the southeast along the coast
and south farther inland this afternoon as the ridge shifts
offshore. Middle clouds streaming in from the west will result in a
mostly cloudy start to the day for NJ/PA. There will be opportunity
for clouds to scatter out a bit later this morning, especially along
and east of I-95. The warmest temperatures this afternoon are thus forecast
for the urban I-95 corridor and eastern Maryland (highs in the middle 80s).
Despite more sunshine farther east, slightly cooler conditions can
be expected with an onshore flow.
A complex of showers and storms associated with a middle-level shortwave
trough out ahead of the Great Lakes low should weaken this morning
as it moves eastward into an increasingly stable airmass that
resides east of the Ohio Valley. This impulse is forecast to reach
central PA by midday and eastern PA between 18z-00z. There is a
potential for convection to redevelop ahead of the shortwave trough
during the middle to late afternoon over the higher terrain. With
models continuing to show stronger static stability farther east
across the cwa, confined precipitation to west of I-95 for this afternoon.
Short term /6 PM this evening through 6 am Thursday/...
high pressure will retreat off the eastern Seaboard tonight as the
abovementioned shortwave trough moves into the area from the west.
Expect showers and perhaps a few thunderstorms west of The Divide.
Forecast is conservative in regards to how far east showers and
storms were mentioned as models generally show convection decaying
as it moves eastward into a more stable airmass over NJ/de. Clouds
cover, which will increases across the entire area with the approach
of the trough, will limit radiational cooling tonight. Official min
temperature forecast slightly warmer than MOS guidance.
Long term /Thursday through Tuesday/...
the long term forecast starts off with rain chances, before a drying
out period sets in across the area.
On Thursday, the area of low pressure and associated frontal
boundary to our northwest will weaken as they move out of the Great
Lakes and into upstate New York. However, the middle/upper-level low
will swing through the northeast and middle-Atlantic through Thursday
and Thursday night. This will allow for showers, and with a fair
amount of instability, scattered thunderstorms to develop and move
into the area. There is not much shear or much of a wind field, so
we do not expect severe weather at this time. Precipitable water values are
generally forecast around 1.5-1.75 inches, so some showers/storms
could produce a period of moderate to heavy rainfall.
Thursday night, a second area of surface low pressure is forecast to
develop near the Chesapeake Bay along the stationary boundary to our
south. It is possible that this may help keep a focus for
precipitation Thursday night, before it sags to our south.
For Friday into the weekend, the middle/upper low will remain to our
northeast, keeping a northwest flow aloft across our area.
Meanwhile, high pressure will begin building down across the East
Coast from eastern Canada. Before the high takes full control of the
weather, there will remain a chance of scattered showers Friday and
Saturday as there may be a few short wave/vorticity maxes that affect the
area in the northwest flow aloft.
By Sunday into early next week, the surface high builds further into
the middle-Atlantic region, which should bring dry weather to the area.
Highs are expected to start out close to or slightly above normal
Thursday into Friday, but as the high builds down out of eastern
Canada and the northwest flow aloft remains across the area over the
weekend, 925/850 mb temperatures and thicknesses lower. This will allow for
slightly below normal temperatures over the weekend. As we move into
early next week, we should start to get more of a return surface
flow and ridging aloft begins to take place once again, allowing
for rebounding temperatures.
Aviation /08z Wednesday through Sunday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg, kilg,
kmiv, kacy and surrounding areas.
VFR conditions expected today and tonight. Broken middle-level ceilings
impacting most of the terminals early this morning. Expect these
clouds to scattered out toward midday before clouds increase across the
western terminals this afternoon and the rest of the area terminals
tonight. Prob30 group for rain showers added for ridge/Abe late this afternoon and
early this evening when a disturbance moves through. While thunderstorms and rain cannot be
ruled out, coverage/confidence too low to mention in tafs at this time. Same
GOES for rain showers chances farther downstream into phl and to the east.
Thursday-Thursday night...generally VFR. Chance of showers and
thunderstorms which may lower cigs/vsbys.
Friday-Sunday...periods of low end VFR or MVFR ceilings possible with a
chance of showers, especially Friday into Saturday.
easterly winds between 5-10 knots this morning will become
southeasterly this afternoon as hipres moves offshore. Locally higher
winds to 15 knots possible this afternoon and evening.
Thursday-Friday night...sub-Small Craft Advisory conditions expected.
Saturday-Sunday...seas may begin to approach Small Craft Advisory
levels as a long fetch develops around high pressure building in
from the northeast out of eastern Canada.