Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Mount Holly New Jersey
930 PM EDT Sat Aug 29 2015
high pressure will be centered near the middle Atlantic coast into
Thursday with a heat wave likely. A weak cold front should slide
south into our region late Thursday with a cooler onshore flow
developing next Friday and Saturday as high pressure becomes
dominant over coastal eastern New England.
Near term /until 6 am Sunday morning/...
High pressure at the surface across the area will continue to become
more centered offshore, and this will maintain a southerly flow
regime. Meanwhile, a short wave trough from the Midwest to the
upper Great Lakes will gradually shift eastward. Our weather
through tonight will be mainly dominated by the departing surface
high, therefore a dry night is anticipated.
Mostly some high level cloudiness will be around at times
particularly across the southern half of the region. Sky cover
was raised some over the next several hours based on current
satellite trends. With the southerly flow, dewpoints should edge
up a bit through the night as cooling occurs and the moisture
starts to pool closest to the surface. This could also lead to
ground fog in localized spots.
As for the low temperatures, we used mainly a NAM/GFS MOS blend and
these are a little higher overall with the expectation that the dew
points will be higher than previous nights.
Short term /6 am Sunday morning through 6 PM Sunday/...
a short wave trough is forecast to be arriving across the northeast
later Sunday, and as it does it should tend to weaken. A weak cold
front at the surface is tied to this feature, and with some moisture
on the increase there should be some showers and thunderstorms with
it to our west. The question is whether this activity can be
maintained as it approaches our western zones mainly later in the
afternoon. There is some instability during the day and into the
evening, with this probably better across the western zones.
However, the forecast soundings mostly show a skinny cape profile
which tends to keep updrafts on the weaker side. The bulk of the
lift associated with the trough axis is forecast to be to our west
during peak heating, however terrain influences and a weak surface
trough could come into play and therefore we included a slight chance
pop during the afternoon across the northwestern zones.
It will be rather warm to hot across much of the cwa, however dew
points may be higher in the morning due to heating of the boundary
layer allowing for some decrease in the afternoon. The pressure
gradient is not forecast to be all that tight, therefore light
enough flow should result in a sea breeze once again in the
As for the high temperatures, we used an even MOS blend then made
some local tweaks.
Long term /Sunday night through Saturday/...
500 mb: a ridge of warm high pressure centered near Bermuda develops
northwestward this week while a weak trough remains lodged
vicinity of the lower Mississippi Valley.
Temperatures: August through the 28th averaged near or above normal.
Kmpo and kged were the only two of the 8 climate sites below normal.
Kttn, kacy and kphl were averaging 1 to ~2f above normal. Calendar
day departures Sunday and Monday will warm the monthly averages
compared to normal. As it stands from the 12z/29 model guidance
temperatures should average 6 to 11 degrees above normal Monday-
Thursday. The only opportunity for significant cooling occurs Friday-
Saturday behind a southward drifting cold front and there is no
certainty that the cold front arrives Thursday. It could be delayed
or less effectively cooler than currently forecast for Friday and
Saturday. The outcome is in part dependent on the amplitude and
longitude of a trough east of the Maritimes late this coming week.
The greatest positive departures with respect to normal through
Thursday should reside near and north of I-78.
Forecast basis: unless otherwise noted, a 50 50 blend 12z/29 GFS/NAM
MOS guidance Sunday night-Monday night - favoring the warmer of the
available guidance since the 12z NCEP and European model (ecmwf) 2m temperatures are rather
warm and 850 temperatures 16-18c. This approach was extended through
Thursday except the 12z/29 mex MOS was applied Tuesday through
Thursday which was blended with the 1521z wpc guide. The 1521z/29
wpc gridded elements of maximum/min temp, 12 hour pop, 6 hour
dewpoint- sky-wind was used as is from Thursday night Onward.
Hazards through day 7: other than rip current moderate risk
today-then possibly again Sunday and the end of next week; its about
a probable heat wave and how long. For now, the maximum heat index is
forecast to be remain below advisory criteria...dewpoints still on
the lower side of what they could be. Heat wave begins Sunday or
Monday afternoon depending on location and probably extends through
at least Thursday. Our maximum heat index forecast right now is in the
Sunday night...variable cirrus, especially kphl south otherwise
fair. Light wind. Small chance of a shower or thunderstorm northwest of I-95, with
highest chance appearing to be the northern mountains in Monroe
and Sussex counties of NE PA and northwest New Jersey. Confidence: above average.
Monday...partly sunny south and becoming sunny north (ci from
tropical system near fl). Hot, especially kphl north. Westerly
wind gusts 15 miles per hour during the afternoon, possibly 20 miles per hour during the
evening I-95 eastward. Small chance of an afternoon thunderstorm east PA or
the north 2/3rds New Jersey ahead a weak wind shift. Confidence: above
Tuesday...sunny and hot. Light north to northwest wind, gusts under 15 miles per hour.
Confidence: above average.
Wednesday...mostly sunny and hot. Could be a period of thick
cirrus. Light SW wind. Confidence: average.
Thursday...sunny and hot. Light northwest wind. Confidence: above average.
Friday and Saturday...maximum temperatures depend in part on the cold frontal
position and the strength of gefs onshore flow off the warmer than
normal Atlantic waters. Could still be hot in the interior? Looks
to be more humid with the ocean inflow off the SSTs in the 70s.
Eventually a chance of tstms? Probably most favored in the upslope
regions of east PA and northwest New Jersey but rather far in the future to be confident.
Confidence: below average (position of cold front).
Aviation /01z Sunday through Thursday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg,
kilg, kmiv, kacy and surrounding areas.
Tonight...VFR overall with remaining cumulus dissipating in the
evening. Scattered high level clouds focused across southern parts
of the area. Some patchy MVFR fog is possible with the best
chance at krdg and kmiv right around sunrise. Any sustained
southwesterly or southeasterly winds will decrease overnight.
Sunday... VFR, scattered cumulus around. A broken high cloud deck
as well from kphl to the northwest. Southwest winds around 10
knots, perhaps a few late day wind gusts under 20 knots. Kilg and
kacy once again could see another seabreeze in the afternoon
which may result in the winds being more southeasterly than in
the current 00z tafs.
Sunday night...VFR mainly cirrus. Localized MVFR possible with
Countryside fog Monday morning. An isolated shower or thunderstorm possible
vicinity kmpo/kabe during the evening?
Monday...VFR scattered-broken clouds at or above 5000 feet. West wind gust 15-20 knots
16z through 00z/1. Small chance of isolated thunderstorms kphl northward.
Monday night... VFR. Localized MVFR possible with Countryside fog
early Tuesday morning. Wind turning northwest-north late. Confidence: above
Tuesday...VFR scattered clouds at or above 5000 feet. Light north to northwest wind...gusts
under 15 knots. Confidence: above average.
Wednesday...VFR scattered clouds at or above 5000 feet. Light SW wind. Another
batch of cirrus possible. Confidence: above average
Thursday...VFR. Light west wind becoming north or northwest. Confidence:
high pressure becoming more centered offshore will allow for a
southerly flow through Sunday. While the wind speeds are expected to
be below Small Craft Advisory criteria, some increase is expected
through this evening mostly due to a sea breeze influence nearshore.
This may also occur again Sunday afternoon. Seas mainly 3 feet or
Sunday night- Thursday...a few gusts near or above 20 knots possible
Sunday night and/or Monday evening, otherwise winds and seas
should stay well below Small Craft Advisory criteria. Water
temperatures are above normal. Confidence: above average
a long period /around 11 seconds/ east-southeast swell continues
into tonight, with seas in the 2-3 foot range. A full moon
/higher tide levels/ and a southerly wind will allow for tricky
conditions at times in the surf zone especially for New Jersey. As a
result a moderate risk of dangerous rip currents along the New
Jersey coast continues through this evening, however a low risk is
maintained for the Delaware beaches.
Sunday...appears to be low enhanced with residual impact from todays
3 feet 11 second southeast swell but the swell should be subsiding somewhat
on Sunday. Still full moon impact so larger than normal tidal
Monday through Thursday...currently looking to be a daily low risk.
The risk may increase slightly next Friday-Saturday depending on
the strength of the onshore flow and size-period of the swell.
Concern: above normal sst's (upper 70s) and above normal temperatures
this last full week of Summer and swimmers-waders being too casual
in the surf zone since the rc risk is low. For greatest safety...
swim in the presence of life guards and during their coverage
hours. Swimming at jetties, piers, groins is ill advised since rip
currents repeatedly form there. Swimming at night or not in sight
of life guards is risky. Water trouble can quickly turn to
recovery instead of Rescue.
we have looked at records for next week (near 100f in 1953) and so
while confidence is very high that a heat wave occurs next week,
not enough heat is indicated to challenge the records. Record
highs next Friday, Saturday and Sunday are lower but not sure how
much onshore flow prevails, therefore keeping it cooler than