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fxus61 kphi 201320 
afdphi

Area forecast discussion
National Weather Service Mount Holly New Jersey
920 am EDT Thu Jul 20 2017

Synopsis...
high pressure will remain offshore through early next week. A
weak front will cross the area tonight and Friday. This front
will remain nearby over the weekend and keep conditions
unsettled through the period. Cooler temperatures will arrive
for the first part of next week.

&&

Near term /until 6 PM this evening/...
main concerns today are heat and storm chances, with heat
taking much of the forecast priority.

Guidance has come in considerably warmer (i.E., About 2-5
degrees) for highs today versus what they were calling for at
this time yesterday...yet another case of statistical models
underforecasting temperatures within strong ridging until the
near-term period. Using yesterday's temperatures and dew points
as a guide and making adjustments for expected mixed-layer
evolution (in areas not affected by diurnal convection
yesterday), temperatures will likely reach the mid to locally
upper 90s with dew points near or just above 70 during the
afternoon. This places much of the region in heat advisory
criteria. Excessive heat warning continues for the I-95 urban
corridor, with heat indices generally well above 100 this
afternoon for several hours. Heat advisory was issued for the
urban corridor of the Lehigh Valley (reading and allentown)
eastward to Middlesex and Monmouth counties and southward to the
Delmarva Peninsula, generally surrounding the excessive heat
warning.

There are some uncertainties with advisory/warning criteria
being met, including cloud cover this morning associated with
some showers that developed near Chesapeake Bay. Moreover, the
hrrr is showing the potential for more convection late this
morning into this afternoon generally along and south of I-76.
There is not much support from other high-resolution guidance,
but the rest of the guidance did fairly poorly with the showers
that popped up in northeast Maryland overnight (at least, more
so than the hrrr).

Another complication is mixing this afternoon, with higher-
resolution guidance showing a well-mixed boundary layer this
afternoon in much of the area, particularly along and west of a
surface trough that will set up in vicinity of I-95 today.
Similarly, there are indications of increased mixing in southern
Delaware and adjacent eastern Maryland, which may keep the
Georgetown and Easton areas generally below advisory criteria.
Despite these concerns, the intensity of this particular period
of hot temperatures (which will also be fairly long duration,
given the past couple of days and continuing through at least
friday) warrants advisory issuance.

Regarding convective potential, the presence of a surface
trough with very strong diabatic heating today suggests
potential for an isolated storm or two in much of the area.
Thought it wise to include slight chances of convection across
the County Warning Area late this morning and this afternoon. Given a well-mixed
boundary layer, strong wind gusts may occur with any storm that
develops.

&&

Short term /6 PM this evening through 6 am Friday/...
main concern tonight is storm chances.

Latest high-resolution guidance is suggestive of a potential
cluster or clusters of storms approaching/entering the County Warning Area this
evening and/or overnight. Perturbations in the flow are
convectively driven and poorly simulated, even at such short
time ranges, so confidence is not particularly high. The
strongest signal is for reasonably organized convection to move
through the northern County Warning Area at some point this evening/overnight.
The 06z NAM is quite aggressive in producing a swath of fairly
high quantitative precipitation forecast generally along/north of I-78 in the 00z to 06z time
period. The 00z GFS provides some support, though with somewhat
lighter/broader quantitative precipitation forecast (per its coarser grid spacing, at least
partially). The 06z hrrr also provides a similar depiction,
though perhaps shifted somewhat northward (toward the New York
border) in this general time frame. The 00z NAM nest, meanwhile,
suggests weaker/weakening convection, but perhaps in multiple
rounds. This solution seems like an outlier, especially compared
to the WRF-arw/nmm simulations. Analysis of subsequent
simulations will be critically important to hone the timing and
evolution of convection during this period.

With a decent, though small-scale, perturbation moving through
the region during this period, large-scale ascent will be
available for increased organization of convection. Given the
hot/humid air along and south of the stronger midlevel flow,
cold-pool development/maintenance may not be problematic,
despite nocturnal stabilization processes. As such, strong to
severe storms with straightline winds the primary threat cannot
be ruled out, especially this evening.

Capped pops at slight to low-chance categories for now, given
uncertainties with upstream convection and model variability
with timing and location of the more organized storms, with
decreasing chances with southward extent.

Temperatures tonight will be warm, and conditions will be
muggy. Not much relief from the heat expected today.

&&

Long term /Friday through Wednesday/...
hot and humid conditions will continue through the weekend with
some relief beginning Tuesday. An excessive heat warning for
the Philadelphia Metro areas extends into Friday with temps and
dew points near what they will be today. Heat index values of
Friday may be a degree or two lower than Thursday, but still in
the upper 90s to low 100s across most areas. Clouds will be more
frequent Saturday and Sunday, so while it will still be hot, a
bit less so than Thu and Fri. By Tuesday of next week, cooler
air from north/west will arrive over the area behind a cold front.

Showers will be frequent during the long term but it's
difficult to find a period when then chances are too low to put
in the grids, or to pinpoint which locations are more favored
than others to get them. Operational models are not great at
predicting convection in general, so a broad brush with help
from the ensembles usually works well in these kind of patterns.
Thunderstorm activity associated with a flattening short wave
will begin to diminish as it arrives over our area Thursday
evening. The associated surface front returns northward for the
weekend and chc or low likely pops for tstms are in for these
periods and also Monday. It will not rain all of the time, but
certainly consider showers when making weekend plans.

Another cold (or cool) front passes the area late Monday. It
appears for now, that this will finally bring drier and cooler
air to the region. We should experience a few rain-free days
early next week as high pressure builds in from the west.

&&

Aviation /13z Thursday through Monday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg,
kilg, kmiv, kacy and surrounding areas.

Generally VFR through the period. A cumulus deck around 5000
feet is anticipated through much of the day. Storms may reach
areas north/west of kphl near/after sunset tonight, with
restrictions possible, especially at kabe. Winds generally light
west or southwest through the period.

Outlook...
Fri thru Monday...mostly VFR. Patchy haze and sct tstms. Areas
of fog possible in the pre-dawn hours.

&&

Marine...
sub-advisory conditions are expected the next 24 hours, though
winds are expected to be southwesterly at 10 to almost 20 kts
this afternoon and evening. There is a slight chance of storms
today and tonight, especially after sunset in the northern New
Jersey coastal waters.

Outlook...
Friday thru Monday...sub-sca conditions overall. The main
hazard will be sct tstms Fri night thru Monday. Locally higher
winds and seas with tstms.

Rip currents...
the underlying 12 to 16 second southeasterly swell is forecast
to persist today. As a result, we will continue the moderate
risk for the development of dangerous rip currents.

&&

Equipment...
the kdov dew point readings are unrepresentative of the area
and should not be used when diagnosing humidity/heat indices.

&&

Phi watches/warnings/advisories...
PA...heat advisory until 7 PM EDT this evening for paz060>062-101-
103-105.
Excessive heat warning until 8 PM EDT Friday for paz070-071-
102-104-106.
New Jersey...heat advisory until 7 PM EDT this evening for njz009-010-012-
013-016-020>023-027.
Excessive heat warning until 8 PM EDT Friday for njz015-
017>019.
Delaware...heat advisory until 7 PM EDT this evening for dez002-003.
Excessive heat warning until 8 PM EDT Friday for dez001.
Maryland...heat advisory until 7 PM EDT this evening for mdz008-012-015-
019-020.
Marine...none.

&&

$$

Synopsis...miketta
near term...cms/iovino

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