Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Mount Holly New Jersey
926 PM EDT sun Aug 30 2015
a weakening cold front will arrive tonight, otherwise high pressure
will remain centered mainly near the middle Atlantic region through
much of this week. A weak cold front should slide south through
our region Thursday night or Friday. Thereafter, high pressure
will reside along the eastern New England coast through Sunday,
probably providing a little relief from the heat this coming
Near term /until 6 am Monday morning/...
a short wave trough will arrive into the northeast tonight and as it
does it should tend to weaken. A weak cold front at the surface is
tied to this feature, and with some increase in moisture an
isolated shower or storm could continue into the overnight. The
overall lift, shear and instability looks weak enough to keep any
convective cores from getting to robust. The initiation early on
appeared to be driven by terrain in northeastern Pennsylvania, and
visible satellite imagery this afternoon shows the main cumulus
development mainly over the higher terrain. The flow below about
10000 feet is 15 knots or less.
We therefore kept low probability of precipitation probability of precipitation going into the overnight, north
and west of Philadelphia. Clouds will alternate between partly and
mostly cloudy overnight.
It will be a warmer night across much of the area as the dew points
tend to hang near the muggy level. A MOS blend was used for the
low temperatures. However with cloud cover the temperatures were
slow to fall in spots and lows have adjusted upward a few degrees
in spots with the 9:30 PM update.
Short term /6 am Monday morning through 6 PM Monday/...
an upper-level trough is forecast to amplify some across New England
on Monday. The southwestern extent of this feature reaches into the
middle Atlantic region. This provides some cyclonic flow aloft, while
at the surface weak high pressure generally remains nearby. A weak
surface front or trough looks to be draped across our area, and
while a shower or thunderstorm cannot be ruled out this is of low
confidence. The model guidance overall does not have a strong signal
for convective development, and since the chance looks rather low we
opted to leave out any mention at this time. It is possible a few isolated
cells are around to start the day as some forecast soundings do show
some increase in middle level instability around daybreak.
Otherwise, a decent amount of middle to high level clouds should be
around which will combine with some cumulus development. A rather
warm to hot afternoon is expected given the southwesterly flow and
any notable cooling aloft is mostly across New England. As a result,
we leaned closer to the warmer GFS MOS for most places regarding
Long term /Monday night through Sunday/...
500 mb: in general, a fairly strong Summer ridge of high pressure
will prevail along the East Coast through next Sunday with only a
minor weakening of this ridge over our area late Wednesday into
Friday, as a strong short wave dives southeastward toward the Maritimes
(55n/55w). That permits a little more northerly component aloft
late this coming workweek and so an associated surface boundary
slips southward. Its cooling effectiveness felt primarily on the
Temperatures: August through the 29th was averaging 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 and will warm these last two days of the month.
Calendar day departures Tuesday through Thursday will be 6 to 12
degrees above normal with the largest positive departures north and northwest
of kphl over east PA and the northwest half of New Jersey. Thereafter, Friday
through Sunday should still average 5 degrees or more above
normal, possibly still 10 degrees above, especially northwest of I-95.
How much cooling occurs Friday and beyond is dependent on the
strength of onshore flow. It appears the greatest impact of
cooling onshore flow will be the southern New Jersey and Delaware coasts.
Sst's are well above average so the cooling effectiveness is less.
Forecast basis: unless otherwise noted, 12z/30 GFS MOS was the
primary guidance Monday night-Tuesday night, 12z/30 mex MOS
Wednesday through Thursday, then the 1521z/30 wpc gridded
elements of maximum/min temp, 12 hour pop, 6 hour dewpoint- sky-wind
from Thursday night Onward. The exception here...was mex and wpc
min temperatures were blended 50 50 Friday night and Sat night.
A heat wave will be in progress through Thursday for the I-95
corridor eastward to near the coasts, its duration should be at
least 4 days whether or not it begins this afternoon or Monday.
The question, does it break anytime Friday-Sunday or just go on
and on through early next week in the interior near and northwest of I-95.
It is possible the I-95 corridor could see an extended heat wave
beyond 6 days (today-Sunday through Thursday look solid for 90-95f
and Friday looks like a better than even chance of 90 plus). Maximum
heat index values in this heat wave so far are forecast to be in
the middle to upper 90s. There is a slim chance we will need a heat
advisory for the urban corridor either of any three afternoons
between this coming Wednesday and Friday.
Thunderstorm drenching rains? Near the cool front Friday but overall
as per wpc, the dry spell is going to last quite awhile, possibly
through the start of Labor Day week.
Monday night-Thursday...fair and hot afternoons with patchy fog
possible late at night over the Countryside, mainly northwest New Jersey and east
PA. Light winds tend to be west-northwest during each afternoon. Small chance
of convection east PA and nnj Wednesday afternoon and Thursday afternoon per instability
indicators including ki.
Thursday night-Friday...a southward moving cold front may spawn heavy southward
moving convection but only in a precipitable water of 1.6 inches. Slow movers-
Gully washers where they occur but little predictability at this time except
most of the activity should remain just inland. Potentially still
90f vicinity kphl southwestward and westward on Friday. Dewpoints higher,
around 70f and 1400j ml cape. 0-6km bulk shear is poor for any
severe. Stratus fog potential at night and in the morning, especially I-80
Next weekend...fair and very warm. Slight chance for afternoon/evening
convection Poconos to the Lehigh Valley and krdg per European model (ecmwf) ki and
a tendency for upslope flow-lift.
Overall confidence these next 6 days: above average.
Aviation /01z Monday through Friday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg,
kilg, kmiv, kacy and surrounding areas.
VFR conditions overnight through Monday. Southwest winds should
remain around 5 knots overnight. Any showers look to remain away
from the taf sites at this time. The main concern overnight will be for
some very patchy fog toward sunrise, it looks too patchy for taf
inclusion at this time. Periods of clouds around 20,000 feet will continue
to move northward through the region throughout the taf period and
may be complemented with some cumulus around 5,000 feet. Wind
gusts Monday afternoon may approach 20 knots in some spots as
well, moderate confidence with the gusts.
Outlook... Monday night through Wednesday...VFR conditions
expected though localized MVFR/IFR conditions with fog/St possible
each morning, mainly near kabe- krdg. Light wind, tends be northwest each
Thursday...mostly VFR conditions. Isolated thunderstorms possible
late. IFR ceilings/visibility possible north of I-80 late Thursday night.
Friday...mostly VFR with brief IFR conds possible in thunderstorms. MVFR
or IFR ceilings developing in northeasterly flow near and north of I-80 with
possible IFR visibility in fog at night.
Overall long term aviation forecast confidence: above average.
the conditions are anticipated to remain below Small Craft Advisory
criteria through Monday. High pressure generally remains near the
middle Atlantic which will keep a southwesterly flow across our area.
An increase into this evening is expected due to mixing and a sea
breeze circulation, and gusts to 20 knots are possible. The seas
overall are anticipated to be in the 1 to 3 foot range across our
Monday night through Thursday...winds and seas should stay well
below Small Craft Advisory criteria. Water temperatures are above
Friday...Atlantic seas may increase as an easterly flow develops but
attm, below Small Craft Advisory criteria.
a long period east to southeast swell will decrease to 9-10
seconds on Monday. This combined with southerly winds around 15
miles per hour may result in another day of moderate rip currents,
particularly for New Jersey.
In the meantime, swim safely within the watchful safety view of
our beach patrols.