Scientific Forecaster Discussion
fxus61 kphi 251342
Area forecast discussion
National Weather Service Mount Holly New Jersey
942 am EDT sun Aug 25 2019
high pressure located across New England and southeast Canada will
weaken through Tuesday. Low pressure will move into Ontario and
cause a cold front to cross our area during the middle of the
upcoming week. More high pressure will arrive for Thursday and
Friday. Another front may affect the weather next weekend.
Near term /until 6 PM this evening/...
930 am update: cloud cover is starting to fill in as the onshore
flow increases. Therefore, I increased cloud cover from the
Delaware valley and further east through the morning. Otherwise,
no significant changes to the forecast.
Models have really struggled with the evolution of a midlevel vortex
atop the northeast this weekend. The mid/upper low is quite a bit
farther south than yesterday's simulations were suggesting, and it
has been slower to move eastward as well. This has had subtle but
important effects on surface flow and may have noticeable impacts on
the weather today, especially near the coast. For starters, winds
have been slow to develop a more easterly component. Most surface
observations early this morning are between 340 and 30 degrees,
which is not very conducive for a surge inland of lower clouds and
drizzle/light showers. Radar indicates some sprinkles or light
showers off the New Jersey coast early this morning, but these have
shown little progress inland. As the upper low presumably shifts
eastward today, the surface high to our north/northeast should as
well. As this occurs, winds will veer to a more east-northeast
direction, which should begin to allow lower clouds and possibly
some light precipitation to move inland. Nevertheless, I am
suspicious that the models are overdoing the precipitation in
general, so I have lowered pops from the previous forecast.
Regarding cloud cover, the models have also struggled with the
scattered-broken midlevel clouds across the area early this morning,
so have bumped sky cover from the previous forecast accordingly.
These have had some effect on radiational cooling (at least in an
hour-by-hour sense), but have not had too much impact on the overall
low temperatures forecast early this morning, except in the far
northwest County Warning Area (where cloud cover has noticeably restricted a sharper
temperature drop). With these trends in mind, felt that a continuity
forecast for today was appropriate. Should see some gradient from
east to west this afternoon, especially with the onshore flow, but
the degree to which this occurs is uncertain given the above
discussion. Statistical consensus did a reasonable job yesterday but
may perform a little worse today owing to the projected increased
effects of sky cover and onshore flow. Nevertheless, confidence is
just not there to make too many deviations from these values owing
to the model errors observed so far.
All of the above said, it should be a sufficiently nice day to get
outside and enjoy the cooler conditions!
Short term /6 PM this evening through 6 am Monday/...
as discussed yesterday, models are still showing a shift southward
in the strongest low-level moisture fetch from the ocean during the
overnight hours. This should occur in conjunction with ageostrophic
southward acceleration of flow east of the higher terrain, inducing
higher pressure farther to the south overnight. Have continued to
depict this in the forecast, though my caveats regarding the overly
aggressive model generation of precipitation remain. As such,
lowered pops about 5-10 percent for the tonight period, with the
highest pops on the Atlantic coast with negligible pops northwest of
a Dover to Sandy Hook line. Additionally, there may be some
improvement in sky condition during the evening hours (especially
the farther north and west you go), with a resurgence of lower clouds
Effects on temperatures may be more pronounced than what has been
seen early this morning, with the north/West County warning area probably verifying
colder than model consensus and the south/east County Warning Area probably
verifying warmer. Have made some adjustments to the low-temperature
forecast with these effects in mind; however, the forecast is
reasonably close to continuity.
Long term /Monday through Saturday/...
the maritime airmass across the area, resulting from the onshore
flow behind the retreating high, will continue early this week.
This will produce some low clouds and some light precipitation
near the coastal areas. Variable cloud cover will be found
further inland across ern PA and nrn New Jersey. Temperatures both days
will be a little below normal for late August.
Temperatures will return to near normal Wednesday when a cold front
approaches the area. A batch of showers and tstms will accompany
the front with rainfall totals of 1/2 to 1 inch possible. We have
mostly chc pops in the fcst for now, since the models haven't fully
decided if the fropa will be during the day or overnight into Thu
morning. Some fine tuning will occur once the models come into
Late this week, high pressure from the west/SW will build back
across the area. A drier and slightly warmer airmass will return
to the middle Atlantic. Highs will become above normal with some
upper 80s arriving to the area.
Aviation /14z Sunday through Thursday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg, kilg,
kmiv, kacy and surrounding areas.
Through tonight...mainly VFR, with ceilings generally hovering
around 4000-8000 feet. Temporary MVFR ceilings are possible from
the Delaware valley and points east through mid day. Winds
becoming east- northeast 10 to 15 kt with gusts to 20 kt
Mon/Tue...mostly VFR, but some low clouds and light precipitation
possible near the shore. MVFR/IFR possible there.
Wed...MVFR possible with some showers/tstms.
Thu...mostly VFR expected.
advisory conditions are expected to persist through tonight on the
waters, with seas already around 5 feet at buoy 44009, and wind
gusts meeting criteria on the beaches. Expect a slow trend upward
with both through tonight, with gusts approaching 30 kt at times and
seas building to 5 to 7 feet on the Atlantic waters.
Models have been too slow in building the ocean wave heights to this
point, so have forecast conditions slightly higher than their
simulations depict. If trends continue, could see seas build to 8
feet late tonight.
Small Craft Advisory conditions will continue into Monday and perhaps maybe
extended into Monday night at some later time. Sub-Small Craft Advisory expected
after that. Clouds and a chance for light showers Mon/Tue. A
chance of showers/tstms Wed/Wed night. Mostly fair Thu.
Northeasterly winds in the 15 to 30 mph range and seas between 4 and
7 feet will result in a high risk of rip currents for the New Jersey
shore on Sunday with a moderate risk of rip currents expected for
the Delaware beaches.
This elevated rip current risk is expected to persist until at least
Tuesday as a prolonged period of onshore flow and elevated seas
a prolonged period of northeast to east flow will continue through
at least Tuesday. This persistent onshore fetch, combined with
increasing astronomical tides due to an approaching new moon on
Thursday, will likely lead to increasing coastal water levels.
Some spotty minor coastal flooding is possible with this
afternoon's/evening's high tide, but better chances for more
widespread minor-/advisory-level flooding appear to be with the
afternoon/evening tides on Monday and Tuesday.
the National Weather Service radar at Dover, Delaware (kdox), is out of service at this
time. The time when it will return is presently unknown.
New Jersey...high rip current risk until 8 PM EDT this evening for njz014-
Marine...Small Craft Advisory until 8 PM EDT Monday for anz430-431-