Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Mount Holly New Jersey
350 PM EDT sun Aug 2 2015
a high pressure system over the central Appalachians will gradually
move southeast through Monday. A cold front from the Great Lakes
is forecast to cross our region early Tuesday with a secondary
cold front following early Wednesday. This front will stall
just south of our area through the end of the week as waves
of low pressure move along it. The last and strongest wave of
low pressure should pass offshore on Saturday with a high pressure
system from eastern Canada building southward on Sunday.
Near term /until 6 am Monday morning/...
high pressure affecting the area will weaken this evening and
overnight. Quiet weather continues to be expected as there are no
short wave impulses expected to create any shower activity
overnight. Clearing skies overnight should allow for another decent
radiational cooling night with calm winds, although not likely as
cool as last night. Overnight lows are a blend of mav/met MOS and
Short term /6 am Monday morning through 6 PM Monday/...
on Monday, a cold front will be advancing through western and
central Pennsylvania as an area of low pressure lifts into eastern
Canada. The front will not make its way into the area during the
daytime period, but a pre-frontal/Lee-side trough will develop
across the middle Atlantic region. Most of the area is expected to be
dry through the day Monday, however, isolated showers/thunderstorms
may develop late in the day across the northwest portions of the
area as a short wave/vorticity impulse moves into the area.
Monday will be a few degrees warmer than Sunday, with everyone being
above normal once again. High temperatures are a blend of mixed
925mb European model (ecmwf) temps, mav/met MOS, and mosguide.
Long term /Monday night through Sunday/...
the WRF-nmmb/NAM initialization looked better at 500mb in the
east, but the GFS was better in the west. The Nevada energy
overall trended weaker with the WRF-nmmb still too strong with
it. Thermally the WRF/NAM was better at 850mb (gfs initialized
too warm) and a combination of the WRF (too cool) and GFS (too
warm) worked best at 925mb. Deep/dt, the West Coast trough was
not as strong, this may be why some more progressive
solution have occurred with the 12z runs. In addition, the
model runs that have less feedback vorts also offered faster
solutions. The op GFS was faster than the gefs mean and was
not used. On the opposite end of the spectrum, the Navy NOGAPS
has Saturday as the day with the heaviest rain. We have gone
closest with the 12z European model (ecmwf) as its closest to a model compromise
and wpc guidance.
Model differences start from the get go on Monday night with the
GFS remaining most bullish with carrying convection to the coast
and through the night. This includes the strongest middle level
qvec convergence of all the models. The predicted bulk shear
and eml supports potentially some hefty thunderstorms northwest of our
County Warning Area during the day on Monday. If anything, model trends are
slightly slower and there are still some models that have precipitation
dissipate before reaching the i95 corridor. We remain slightly
optimistic because organization is likely northwest of US,
our in situ air mass aloft should be able to maintain convection
and there will be a low level jet overnight. Given the evening, overnight
timing we did not carry any enhanced wording. The eastward
extension of Storm Prediction Center slight risk for severe storms remains right along
our northwest border. Because of decent southwest winds at night,
we went with relatively high stat guidance mins.
On Tuesday, we see the models edging toward each other. The GFS has
some mixed layer cape vs having none at all yesterday. The European model (ecmwf)
and NAM are more bullish, but then there bias is to be slow.
Overall there is a race between drier air moving into our County Warning Area and
convection being able to occur. Highest chances are in the
eastern/southeastern part of our County Warning Area with an added pop far
northwest late for proximity to the stronger short wave. Because
of low confidence of occurrence, coverage and storm relative
inflow being weak, no enhanced wording severe weather potential statement mention will be made.
We expect downsloping flow to compensate for cold air advection and kept maximum
temperatures close to continuity.
Tuesday night and Wednesday, a low chance early in Delmarva, otherwise
there is consensus of the convective temperature not being reached
Wednesday. This is the one dry day forecast during the work week.
With light winds and not many clouds expected, we radiate
outlying, more rural areas Tuesday night. Predicted 1000-850mb
thicknesses suggest a 1-2f drop off in maximum temperatures on Wednesday
from Tuesday levels.
While confidence is increasing for a heavy rain event for the Middle
Atlantic States, timing of when this will occur is still
uncertain. Naefs ensemble means are showing the late week surface
system is more than 2sd from normal early August mslp and the
ensemble mean over wva represent a once in 5 year return interval.
Much to be made as to the details and the geographical impact.
This has some similarities to the June 27th event. For now we ramp
up probability of precipitation starting Wednesday night, but the model consensus is
closest to a Thursday night to Friday day time frame as our
greatest potential for precipitation and heaviest rain. Because convection
is involved, the mesoscale convective vortex vorts are showing little run to run
continuity and have a tendency when there are more of them to slow
the works down and drag heavier precipitation farther north.
At least with the 12z runs, the relative good news is for improving
conditions over the weekend, with Sunday the more confident dry day
at this point. With a trough forming and staying over eastern noam,
temperatures will trend downward and should be at or below normal
through the second half of the long term.
Aviation /20z Sunday through Friday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg, kilg,
kmiv, kacy and surrounding areas.
VFR conditions are expected through the taf period.
Winds are generally west to southwest around 5-10 knots or less,
with a few locations continuing to be light and variable. This will
continue into the evening, before most locations become light and
variable/calm overnight. Winds on Monday will increase out of the
south to southwest by middle-late morning, and begin to gust 15-20
knots by late morning and into the afternoon
Monday night...a lowering to MVFR with ceiling/visibility restrictions in
Tuesday and Tuesday night...mostly VFR conditions. Isolated to
perhaps scattered showers/thunderstorms are possible eastern airports
Tuesday overnight through Wednesday night...mainly VFR conditions.
Thursday through Friday...MVFR conditions during showers and
thunderstorms with some IFR conditions possible. While confidence of
occurring is increasing, timing confidence remains low.
sub-Small Craft Advisory conditions will continue through tonight as
high pressure begins to weaken lose control over the area. A Small
Craft Advisory has been issued starting late Monday afternoon due to
an excepted increase in winds late in the day.
Monday night and Tuesday...we will continue the Small Craft
Advisory for the ocean waters. The low level jet does pass
over the ocean waters and while seas normally ramp up too
quickly, wave watch guidance (7 feet) leaves more than enough
wiggle room to still meet criteria for seas. We are not as
sure about Delaware Bay and will hold off with this package.
Small Craft Advisory conditions (seas last) should cease on
the ocean during the day on Tuesday.
Tuesday night through Wednesday night...sub Small Craft Advisory
conditions are expected.
Thursday through Friday...a stalled front and a wave of
low pressure moving along it could enhance seas and/or winds
for the ocean coastal waters and might bring Small Craft
Advisory conditions again.
Marine...Small Craft Advisory from 4 PM Monday to 10 am EDT Tuesday for