Scientific Forecaster Discussion
fxus61 kphi 231317
Area forecast discussion
National Weather Service Mount Holly New Jersey
917 am EDT Sat Jun 23 2018
a low pressure system in the Great Lakes region leads to a warm
frontal passage in our area today and a cold frontal passage
tomorrow. High pressure over Ontario builds into the region on
Monday and slowly moves off the eastern Seaboard to a position near
Bermuda by Thursday. A trough may move through our region Wednesday
night into Thursday.
Near term /until 6 PM this evening/...
previous discussion outlines today's forecast challenges as well as
the issues this warm front poses. Temps, winds, cloud cover and fog,
precip and a severe threat. These are all in play. What's standing
out this morning is the northeast winds across the region. The mav
stat guidance is more easterly, the met guidance is better as are
the near-term hi-res models. The question in my head this morning
is, is this warm front going to have more issues moving north today?
Soundings suggest southwest winds in the 5-15k foot range around 15
to 30 kt, not necessary the speeds one wants to see to accelerate a
front northward. Grant it, the ocean has warmed considerably, and
this isn't March, so the cool low-level pool on the north side of
the front isn't all that cool. The front is draped across the
southern Delmarva zones at this time. Wpc has it moving north to PA
Turnpike/I-195 in New Jersey by 18z and up to near i80 by 00z. This
progression simply might be too fast with this NE flow. That said, I
shaved off a degree or two across mid-zones in southeast PA and central New Jersey.
Confidence remains low.
The other thing to mention this morning is the vast difference in
pops comparing low-resolution and stat guidance vs. Near-term hi-res
models. The latter has a lot less coverage. Our current trends still
fit as we are highlighting PA and northern New Jersey with the best precip
chances today. I wouldn't be surprised if locations from southern
Ocean County southward into the Delaware and out Maryland zones see very little
until this evening. Although this area may not see a widespread
shower/thunder activity today, with the proximity of the front, low
clouds and fog will be prevalent making for a dreary day, at least
through this morning. We'll check back midday and amend were
Through 10 am...
Mid-level shortwave energy and the lf quadrant of the upper jet,
aided by orographic lift in the easterly low-level flow, within a
very moist airmass (pw values ~ 2 inches) will lead to heavy showers
across the Lehigh Valley and southern Poconos. Further southeast, a
wave of low pressure developing along the warm front just to the
south will lead to enhanced isentropic ascent across portions of
southern New Jersey and Delaware, where heavy showers are also possible.
Cannot rule out a rumble of thunder, with the greatest probability
over far southern New Jersey and Delmarva. In addition, some of the heaviest
rain showers could lead to localized rainfall amounts up to 1"/hour
but fast movement should preclude more than poor drainage flooding.
10 am thru 6 PM...
The warm front will begin to make northward progress, but there is
still uncertainty regarding its position by late afternoon. The GFS
and European model (ecmwf) are in better agreement with the NAM for the 00z/23 model
suite, and have the front located invof I-276 in PA northeastward to
I-95 in New Jersey by late afternoon. Any deviation in this position north
or south will have a profound impact on severe weather potential and
high temp forecast.
Severe weather potential...
Storm Prediction Center has outlooked a slight risk for extreme southeast PA and
southern NJ, with a marginal risk from portions of the Lehigh Valley
to include all of Ocean County New Jersey.
Across the slight risk area, favorable parameters continue, with ml
cape ~1,500 j/kg and bulk shear around 45 kt. Gusty winds are
possible, although modest 0-3 km lapse rates and meager d-cape
suggest damaging winds will be limited in coverage. Still very
concerning is the environment closer to the warm front, where 0-1 km
shear over 20 kts and low LCLs could favor a brief, weak tornado.
Hail parameters, including lapse rates in the hail growth zone,
remain weak so would expect mainly small hail. The exception would
be any rotating updrafts closer to the warm front, which would be
more favorable for large hail.
With precipitable water values ~2 inches, heavy downpours are
likely, especially if thunderstorms train near the warm front,
which may result in urban and poor drainage flooding.
A warm and humid airmass will advect northward to the aforementioned
area. South of the front, southerly flow, partly cloudy skies, and
mixing up to 850 hpa support highs in the mid to upper 80s. North of
the front, considerable cloudiness and an easterly flow will keep
temps in the 70s. Any oscillation in the frontal position could
raise/lower temperatures 10-15 degrees at a given location.
Short term /6 PM this evening through 6 am Sunday/...
the severe weather threat should diminish around 03z with the loss
of instability and shear. The shortwave axis and pre-frontal trough
also move offshore around midnight, with subsidence taking hold
through column and dryer air advecting in. There may be some partial
clearing, and with the moist ground, this would be conducive for the
development of fog. But near surface winds out of the west are
expected to remain elevated much of the night, so confined patchy
fog to the more sheltered locations of northern New Jersey and southeast PA.
Low temperatures are expected to be a few degrees above normal.
Long term /Sunday through Friday/...
summary: thunderstorms are possible on Sunday as a cold front
arrives in the region. High pressure will bring tranquil weather
Monday and Tuesday. Chances for showers and thunderstorms again
increase Wednesday through Thursday as a trough approaches from
Sunday...in the mid and upper levels, winds become more westerly
early in the day on Sunday resulting in dry air advection. A mid
and upper level trough digging southeast will arrive just behind
the surface cold front late in the day. Despite still being in
the warm sector that will be responsible for thunderstorms
today, the dry air advection and a mid level inversion should
limit instability on Sunday. Thunderstorms are possible, but
strong or severe thunderstorms are not expected at this time.
Monday and Tuesday...Canadian high pressure builds south. The
cold front will only drop temperatures to near normal. Forecast
temperatures, especially on Tuesday are higher than most
guidance as the pattern favors very large dew point depressions
in the boundary layer. Models typically underestimate the depth
of the mixing layer in these cases which could result in higher
than forecast Max temperatures and lower than forecast dew
Wednesday and Thursday...another large low pressure system,
taking a track further north than the one today, will bring a
trough into the region either Wednesday night or Thursday. Ahead
of this trough, southerly low level return flow develops,
resulting in moisture and warm air advection. There are still
some timing differences between the models on the trough, which
has an impact both on rain chances and temperatures. Have gone
with a middle of the Road solution, bringing the trough through
late Wednesday night/Thursday morning, resulting in modest
warming trend on Wednesday, but significantly warmer on
Thursday. In this scenario, the prime period for showers and
thunderstorms across our region would be Wednesday night and
Thursday, though some may develop primarily west of the fall
line on Wednesday afternoon thanks to enhanced orographic lift.
Friday...a high should start building in from the southwest. How
quickly it does so will determine if we see another round of
showers and thunderstorms during the day Friday and how quickly
really muggy conditions return to the area. Given the source
region of this air mass, would not be surprised if heat index
values once again get above 100, if not on Friday, then likely
Aviation /13z Saturday through Wednesday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg,
kilg, kmiv, kacy and surrounding areas.
Widespread IFR cigs this morning will gradually improve late this
morning with VFR at most locations by late afternoon. The exception
will be Abe and rdg, where cigs will only improve to mfvr. Medium
Winds easterly this morning, gradually shifting to south-southwest
late this morning into the afternoon, except Abe and ridge where winds
remain easterly. South-southwest winds could gust up to 25 mph. Low
During the afternoon into the evening, rain showers and thunderstorms and rain are expected at
most terminals, accompanied by brief cig/vsby restrictions and gusty
winds. High confidence.
Saturday night...thunderstorms and rain possible until around 03z. Thereafter, MVFR in
low clouds and patchy fog, mainly Abe, rdg, miv, and Acy. West winds
less than 10 kt. Low confidence.
Sunday...MVFR conditions will be possible with any showers and
thunderstorms that develop. Otherwise, VFR conditions are
expected. Winds will be shifting to northwesterly behind a cold
front, but wind speeds should remain near or below 10 kt.
Monday and Tuesday...VFR conditions are expected. Northwesterly
winds on Monday become light and variable Monday night into
Tuesday. High confidence.
Wednesday...mostly VFR conditions are expected. There is a
chance for showers and thunderstorms to move in through the day,
especially at kabe and krdg. If this happens, MVFR conditions
will be possible. Moderate confidence.
updated the winds. They're more from the NE this morning.
Confidence is low with the winds today. Shifting from the south
depends on how far north this warm front gets.
Small Craft Advisory continues for the Atlantic Delaware/New Jersey and lower Delaware Bay waters
through this evening. East-southeast winds this morning 15-20
kt, shifting to south-southwest this afternoon 10-20 kt,
becoming west-southwest around 10 kt tonight. Gusts around 25 kt
are possible today. Seas remaining elevated around 5 ft today
and tonight. Higher winds and seas possible near thunderstorms
this afternoon and evening.
Sunday through Tuesday...winds and seas should stay below Small Craft Advisory
criteria. Thunderstorms are possible on Sunday, but then high
pressure builds in leading to dry conditions.
Wednesday...Small Craft Advisory conditions, for both winds and seas, are
possible. Southerly winds increase ahead of the next low
pressure system. Seas should subsequently increase as well.
Easterly winds this morning into the early afternoon shifting to the
south-southwest. Winds may gust up to 25 mph at times. Seas in the
surf zone 3 to 4 ft. A low risk for the formation of dangerous today
for the Delaware beaches. With more of an onshore component of flow and
swell, there is a moderate risk for the New Jersey beaches.
Marine...Small Craft Advisory until 9 PM EDT this evening for