Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Mount Holly New Jersey
404 PM EDT Monday Aug 3 2015
a cold front from the Great Lakes will slowly approach from the
northwest tonight, eventually tracking through our region on
Tuesday. This front is forecast to stall just south of our area
through the end of the week with waves of low pressure moving along
it. The final wave of low pressure looks to pass offshore
southeast of the region on Saturday. A high pressure system from
eastern Canada should then build southward on Sunday and Monday.
Near term /until 6 am Tuesday morning/...
not a bad August afternoon where heat index temperatures are pretty much
the same as the ambient temperatures. The exception is across our southern
zones where dewpoints are higher. North of the Delmarva, dewpoints
are in the upper 50s to lower 60s.
Tonight's lows will bottom out in the lower 60s across the far north
and in the lower to middle 70s across the middle and southern zones. This
is warmer than last night.
A prefrontal trough or remnant outflow boundary has been working
across central and eastern PA this afternoon. It's not running into
a highly buoyant or moist atmosphere, so it's having a tough time
getting going. The actual cold front will pass late tonight into
We'll slowly bump up probability of precipitation from west to east this evening into the
overnight, but keep the extreme far southern and eastern zones dry.
Highest probability of precipitation (50%) will be placed across the Poconos. 30% probability of precipitation have
painted across the Delaware valley and portions of the coastal
The gusty southwest winds we're experiencing this afternoon will
Short term /6 am Tuesday morning through 6 PM Tuesday/...
front gets hung up along the coast tomorrow. With some additional
moisture (vs. Today) pooling near the front and a shortwave in the
area, the thinking is that showers and thunderstorms could be more
numerous tomorrow across the eastern zones. There's a significant
difference between the NAM and GFS with the NAM much further west
with precipitation. I've chosen to go closer to the GFS with support
from stat guidance.
Temperatures tomorrow, overall, should be a couple degrees cooler than
today's high. Forecasting 90 for philly.
Winds will be a bit more westerly tomorrow afternoon with similar
gusts vs. Today.
Long term /Tuesday night through Monday/...
in a model initialization flip from the weekend, the WRF/NAM
initialized the 500mb pattern better than the GFS or European model (ecmwf) in the
western conus, while in eastern noam the GFS looked better. The GFS
looked closer at 850 mb and 925mb. Deep/dting the initialization
over the past 24 hours, the ridging in Montana and Idaho is
stronger and down the Road this is leading to a more bifurcated
solution to how the energy rounds this ridge, thus there is no
decrease in uncertainty with this sounding run. The GFS was not
used because of its weaker western Continental U.S. Initialization and poor
run to run continuity. Gefs mean with the wave of low pressure is
farther south and then a faster exit on Saturday. Our forecast is
closest to a European model (ecmwf)/can ggem/mf-arpege blend. It should be noted
that the UKMET has trended southward with a much weaker low while
the NOGAPS and jma have gone back to not having any precipitation reach our
County Warning Area until Saturday.
So the active middle stretch of the long term continues with
temperatures closer to the normal. The potential remains for a
soaking rain centered around Friday, although there remains
uncertainties as to the impact within our County Warning Area. Last night's naefs
ensemble mean continue to show an anomalously strong surface low
pressure system for early August with a five year return period and
South Dakota of between 2 and 3 as it crosses the central Appalachians.
The lack of the run to run continuity remains. The way the models
resolve convective systems that will develop across the Midwest
and upper Mississippi Valley as the lows moves east is mainly
why. The more robust "feedback" lends to stronger and farther
north lows, while models with less mesoscale convective vortex bullseyes have weaker and
more suppressed solutions. It remains hard to separate the true
mcs(s) from prolonged feedback hangovers. Likely this will be the
case into Wednesday as the convection that day is the cause of
most of the modeling heartburn.
Tuesday night, the model timing has slowed in moving the cold front
offshore and we maintain low probability of precipitation southeast part of our County Warning Area for
thunderstorms. Given how drier air is moving, the more robust storms
should be done by this time period. We are either at or slightly
above stat guidance for mins given the slower modeling trend.
Wednesday, remains popless as high pressure builds into the region.
The GFS is standing alone with stronger cold air advection and overall stronger surface
winds. It also stands alone with channelized vorticity maximum precipitation getting
into the northern part of our County Warning Area. Given the deep/dt trend, going
faster does not seem to have much corroboration. We are closer to
a non-GFS solution and NAM MOS for high temperatures. Also no probability of precipitation. But,
convective temperatures should be reached north and some popcorn to maybe
even ceiling cumulus likely. While it will be drier, not out of the
question of a couple of more 90s in the eastern/coastal part of
our County Warning Area.
Wednesday night, dry, as this is one forecast period all of the
models agree upon. With high pressure over the area, comfortable
stat guidance mins look reasonable.
Thursday, probability of precipitation were moved back approximately 12 hours. Too late
in its forecast cycle to have confidence in the WRF/NAM timing, but
many of the models do not have any measurable at all Thursday day
based on the more roundabout approach for the short waves.
Because of the drier trending, we went above stat guidance for maximum
Thursday night through Friday night there unfortunately remains
uncertainty as to the evolution of the system, but the synoptic scale
set-up still points to heavy rain, even if it does not necessarily
occur or affect our County Warning Area the most. The heavy rain will be tied to
convection and thus how unstable aloft it becomes within our County Warning Area
will determine the final outcome. Highest probability of precipitation are Thursday night
and Friday with thunder emphasized more for the southern part of our
County Warning Area. Come Friday night the predicted instability (really lack
thereof) aloft does not substantiate the ecmwf's precipitation/quantitative precipitation forecast
solution and was not used. This does not mean heavier rain could not
happen earlier. Maximum temperatures may very well be overstated on Friday if
this timing holds.
Because of timing discrepancies we kept in a low chance for
precipitation on Saturday. After above, the end of the long term
appears quiet as high pressure noses its way southward from Canada
and the frontal boundary and low pressure systems move offshore.
Aviation /20z Monday through Saturday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg, kilg,
kmiv, kacy and surrounding areas.
VFR conditions are expected through the taf period, but confidence
isn't at all time high due to high res solutions. The hrrr continues
to develop a solid pre-frontal line out in central PA and moves said
line into the region. Sat pictures nor radar supports this. So as of now,
the tafs are precipitation free. The exception is a probability 30 for kphl (30
hour taf) for late tomorrow as the cold front stalls along the coast.
Winds will gust out of the southwest to 20 knots for the remainder of
Tuesday night...mostly VFR conditions. Isolated to perhaps scattered
showers/thunderstorms are possible very early southeastern terminals.
Wednesday and Wednesday night...mainly VFR conditions.
Thursday through Friday night...MVFR conditions during
showers/tstorms, with some IFR conditions possible. While confidence
of occurrence is increasing, timing confidence remains low.
Saturday...predominately VFR conditions expected to return as
shower frequency diminishes.
will keep the Small Craft Advisory headlines in place tonight for
the ocean waters and extend them through tomorrow. Gusts to 25 kts
can be expected along with seas in the 4 to 6 feet range. Wavewatch
numbers were higher and were trimmed.
In Delaware Bay, we'll top the wind gusts out at 20 kts (sub
Tuesday night...Small Craft Advisory conditions for seas on the
ocean might last into the evening. Confidence too low to extend.
Wednesday through Thursday...sub-advisory conditions are expected.
Thursday night through Friday night...mostly sub-Small Craft Advisory conditions;
however, a stalled front and a wave of low pressure moving along it
could enhance seas and/or winds for the coastal waters, making Small Craft Advisory
Saturday...sub Small Craft Advisory conditions expected to return.
This is based on timing of waves of low pressure close, confidence
at this juncture is less than average.
there is a moderate risk for the formation of dangerous rip
currents for the New Jersey shore and the Delaware beaches through
tonight. This moderate risk might extend into Tuesday.
Marine...Small Craft Advisory until 8 PM EDT Tuesday for anz450>455.