Scientific Forecaster Discussion
fxus61 kphi 221947
Area forecast discussion
National Weather Service Mount Holly New Jersey
347 PM EDT sun Apr 22 2018
large area of high pressure over the northeastern United States will
continue to slowly build east, building off shore by tomorrow.
Meanwhile, a low in the deep south will slowly lift into the mid
Atlantic states by mid week. Another low pressure system and its
associated cold front could affect our region late in the week.
Near term /until 6 am Monday morning/...
an upper-level trough axis will shift east of our region this
evening along with a stronger embedded short wave. As this occurs, a
250 mb jet streak will continue to shift to our east as well. Given
the positioning of the upper-level jet streak, the high level clouds
have remained much more thinned this afternoon even across the
southern zones. However, a small area of mid level cloud has
developed across parts of southern Delaware where there appears to
be some local convergence occurring with a sea breeze boundary and
there are higher dew points just to the south. In addition, there
continues to be areas of thicker high level clouds across West
Virginia to Virginia and if these hold together then they may skirt
parts of our Delmarva zones through this evening. Otherwise, a
mainly clear sky is anticipated. A sea breeze gradually working its
way inland is allowing for much cooler air to its east along with a
bit more of a southeasterly breeze.
The presence of a ridge gradually approaching from the west is
resulting in expansive surface high pressure across the mid-Atlantic
and northeast. This will continue to provide ample dry air and
enough low-level subsidence, along with winds going light and
variable to calm. Any wind should drop off quickly this evening.
This will set the stage for good radiational cooling conditions,
however the airmass looks to be moderating some. We are still
expecting it to be a chilly night (mostly closer to daybreak) given
the lack of clouds, light winds and dry air. Speaking of dry air,
the surface dew points dropped considerably today for much of the
area with ample boundary layer warmth. These are expected to recover
some through the evening as the boundary layer warmth wanes and a
residual sea breeze front marches inland more before dissipating.
There very well can be some frost toward daybreak, especially in The
Pine Barrens of New Jersey and portions of southeastern Pennsylvania
however this may be more patchy. For this reason, we opted not to
issue a frost advisory at this time.
Low temperatures are mostly a multi-model blend with continuity,
then some local adjustments were applied to account for local better
Short term /6 am Monday morning through 6 PM Monday/...
the dry conditions continue as an upper-level ridge gradually slides
over our area during the afternoon. This will place the center of
surface high pressure closer to coastal New England, resulting in
south to southeast low-level flow across our area. This is all ahead
of a closed low that is forecast to be traversing the Gulf Coast
states and the Tennessee Valley region.
The presence of the ridge aloft should limit any clouds to the high
level variety and this is mainly for portions of Delmarva. The low-
level flow looks a little stronger during the afternoon, and the
gradient flow becomes more from the southeast therefore this will
result in some cooling across eastern New Jersey and parts of
Delaware. Quick warming through the morning though may set up a sea
breeze front which then should just be enhanced some by the
southeasterly gradient low-level flow. High temperatures are mostly
a MOS/continuity blend, which ends up being warmer from
previously for inland areas given a bit of warming aloft.
Long term /Monday night through Sunday/...
overview: the first focus of the long term period is the
expected rain (potentially heavy especially along the coastal
plains) Tuesday into Wednesday. Late this week, the forecast is
low confidence as there will be two lows interacting, one from
the southeast U.S., And one digging in from the northwest. Depending on if or
which one of these lows becomes dominant, we could see a period
of unsettled weather either Friday or Saturday.
Details: the next low currently over the deep south, will
continue to very slowly progress northeast, reaching the mid
Atlantic by mid week. Timing has changed a bit as models are
coming into better agreement of both the rain moving in earlier
(during the day tuesday), and a dry slot developing and bringing
an end to the rain quicker (as early as Wednesday morning for
locations near and south of philly).
Due to the slow progression of the low and low level onshore
flow, there is some concern of heavy rain, primarily Tuesday
night. Precipitable water values are expected to be well above
normal, possibly in the 90th percentile for this time of year.
However, there are a few factors working against the heavy rain
threat. The warm cloud layer is relatively shallow (generally
8000 to 9000 ft), thanks to continued below normal temperatures.
Additionally, storm motions aren't expected to be particularly
slow, on the order of 20 mph, although training storms could
make this point moot. In general, it looks like the highest
threat for heavy rain will be over the coastal plains.
Once that low lifts northeast away from our region by Thursday,
the forecast is a bit more muddled. Yesterday, it looked as if
we would have a brief period of high pressure, followed by a
cold front sweeping through on Saturday. Now however, models are
split on the overall pattern after Thursday. The biggest
challenge is figuring out how a short wave trough in the
southeast on Friday will interact with the main upper level
trough over the north central U.S., And the implications with
the surface pattern. Some models with the latest runs are
depicting this trough, and the associated surface low, becoming
the dominant low, resulting in a coastal low system for US on
Friday. While other models continue to depict the northern tier
trough and surface low being the dominant feature resulting in
only a brief period of rain with the cold front on Saturday. At
this point I stayed close to the previous forecast as there
isn't a clear signal one way or the other how this will develop.
Aviation /19z Sunday through Friday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg, kilg,
kmiv, kacy and surrounding areas.
Rest of this afternoon...VFR with some high clouds. Light and
variable winds or locally west or northwest less than 10 knots. A
sea/Bay breeze resulting in a southeasterly wind at Acy and ilg, and
this should reach miv into early this evening. For phl, light enough
flow is resulting in a more south-southeast (light) surface wind
direction however this may turn more south-southwest for a time.
Tonight...VFR. Winds quickly becoming light and variable to locally
Monday...VFR. Light and variable or calms winds, becoming south-
southeast 5-10 knots mainly from late morning through the
Monday night...mostly VFR conditions are expected. There is a chance
that MVFR ceilings could move in from the south very late. Winds
will be light and variable for much of the period. High confidence
on most of the forecast except the timing of the MVFR ceilings, if
they come at all during this period.
Tuesday through Wednesday...MVFR and even localized IFR conditions
will be possible especially on Tuesday night into Wednesday morning
as periods of rain are likely across the region. Easterly wind gusts
up to 20 kt are possible Tuesday night into Wednesday. Otherwise,
generally light winds (less than 10 kt) are expected. Moderate
confidence on flight categories, but low confidence on the timing of
any flight category changes.
Thursday and Friday...expect mostly conditions improving to VFR,
though there remains a slight chance that rain and low clouds could
lower conditions to MVFR at times. Wind could be starting out
westerly, but shifting to southeasterly. Low confidence on flight
categories and wind.
the conditions are expected to remain below Small Craft Advisory
criteria through Monday. A south to southeast flow may turn west to
northwest (light) later tonight, then a more dominant southeasterly
flow occurs on Monday. There will be some mainly nearshore
enhancement to the winds (gusts to about 15 knots) into this evening
due to a sea/Bay breeze, then again Monday afternoon.
Monday night...winds and seas are expected to remain below Small Craft Advisory
Tuesday and Wednesday...easterly and southeasterly winds increase to
15-20 kt with gusts up to 25 kt, and seas build up to 10 feet on
the Atlantic coastal waters. Wind gusts near 25 kt are also expected
on the Delaware Bay.
Wednesday night and Thursday...winds shift to northwesterly and
subsequently decrease. The main question will be how long seas
remain above 5 feet on the coastal waters. Current forecast is for
elevated seas into late Thursday, but there is still considerable
uncertainty with this.
Friday...depending on if a coastal low develops, we may see
increasing southeasterly winds. If this happens, we could once again
reach Small Craft Advisory criteria.