Scientific Forecaster Discussion
fxus61 kphi 211020
Area forecast discussion
National Weather Service Mount Holly New Jersey
620 am EDT Mon Oct 21 2019
a weak area of high pressure slides across our area today then
shifts to our northeast tonight into Tuesday morning. A cold front
will cross our area late Tuesday, then high pressure builds in for
Wednesday and Thursday before shifting offshore Friday. The next
cold front moves through later Friday, with low pressure along it
sliding to our south and east Saturday into Sunday as high pressure
builds across New England.
Near term /until 6 PM this evening/...
no significant changes to the early morning forecast. Will make
minor adjustments to hourly grids based on latest surface obs.
Low pressure centered about 150 miles southeast of Atlantic
City will drift to the east today and out to sea. Meanwhile,
weak high pressure will pass through the region.
With abundant low level moisture still in place, fog and low
clouds will persist through a few hours after daybreak. Fog will
dissipate and clouds will lift and scatter out between 9am-
Skies should become mostly sunny for most of the region,
although mid and high clouds may linger along the New Jersey and
Delaware coasts as clouds wrap around the departing low.
Highs will top off in the mid and upper 60s.
Short term /6 PM this evening through 6 am Tuesday/...
high pressure moves offshore this evening, and then a cold front
approaches from the west. Clouds will increase and thicken after
midnight, but precip should hold off until after daybreak Tuesday.
Lows will be in the 40s and 50s, but if clouds are slower to arrive,
then there is the potential for radiational cooling prior to
midnight that could result in colder than forecast temps.
Long term /Tuesday through Sunday/...
summary...a cold front moves through late Tuesday, followed by
another cold front later Friday. Low pressure along the tailend of
the cold front tracks to our south and east Saturday into Sunday
while high pressure builds across New England.
Synoptic overview...a significant trough from the upper Great Lakes
to the lower Mississippi Valley Tuesday shifts northeast and lifts
into Canada and New England. The southeastern part of this large
trough may take on some negative tilt late Tuesday into Wednesday as
it crosses the northeast. The flow turns briefly more zonal during
the midweek time frame before another trough amplifies from the
Midwest through the plains late Wednesday into Friday. This feature
then shifts eastward through the weekend, however the southern
portion of the trough looks to close off across the southern states
and this introduces more uncertainty especially with timing into the
east along with sensible weather impacts.
For Tuesday...a strong area of low pressure is forecast to track
across the western Great Lakes and into adjacent Canada. The
associated upper-level trough may take on some negative tilt as it
arrives across the northeast late Tuesday night. An associated cold
front is expected to move across our region late Tuesday as a weak
surface low develops along it as it crosses our region. This may
tend to enhance the showers for a time, with the stronger forcing
sliding mostly across our northern areas. Given easterly low-level
flow ahead of the system, a few light showers cannot be ruled out
during the day with the main band of showers arriving later in the
afternoon and especially in the evening from west to east.
The guidance continues to keep lots of clouds in place given the low-
level warm air advection occurring with an easterly surface flow.
This will have an impact on the instability. The south to southwest
flow above the easterly low-level flow is forecast to strengthen
late in the day, and this will result in decent shear especially
from Delmarva southward. The forecast soundings however show limited
instability at best. Based on this and also collaborating with our
neighboring offices, opted to hold off on including a thunder
mention. It is possible that with enough frontal forcing, some low-
topped convection could occur however it is not clear if enough
instability will develop to result in lightning generation. The
showers rapidly end at night with some clearing occurring. There may
be a little bit of a wind surge in the wake of the cold frontal
passage with some initial cold air advection and tightened pressure
For Wednesday and Thursday...high pressure across the southern
states Wednesday is forecast to build across our region Wednesday
night and Thursday before gradually starting to shift offshore.
Given this and the flow aloft turning more zonal, dry conditions are
anticipated. The afternoon temperatures Thursday may near 70 degrees
across portions of our southern areas as some return flow and warm
air advection starts to get underway.
For Friday through Sunday...a complex setup looks to occur as the
next upper-level trough amplifies eastward from the Midwest and
plains. This feature may split with a closed low developing across
the southern states while the northern branch trough slides eastward
across the Great Lakes and the northeast. The northern trough will
drive a cold front across our area later Friday, however it may slow
as it clears the coast. The southern states energy looks to develop
a surface low along the tailend of the cold front and this system
may track south and east of our our region later Saturday and
Sunday. Given much more uncertainty regarding the timing and the
overall synoptic evolution, mostly blended in the 00z wpc
guidance with continuity and kept any pops no higher than the
Aviation /10z Monday through Friday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg, kilg,
kmiv, kacy and surrounding areas.
Today...lingering MVFR/IFR cigs/vsbys through 13-15z, then VFR.
North winds 10 kt or less will veer to the east, then southeast
later this afternoon.
Tonight...VFR initially, then clouds will lower and thicken with
MVFR cigs possible towards daybreak Tuesday. Light/vrb winds.
Tuesday...MVFR/IFR ceilings. A few light showers possible during
much of the day, them organized showers with visibility restrictions
move through later in the afternoon and evening from west to east.
The conditions improve to VFR during the night. East-northeast to
east-southeast winds near 10 knots, shifting to west and northwest
at night. Moderate to high confidence on conditions below VFR, but
low confidence on the timing of flight category changes.
Wednesday and Thursday...VFR. West-northwest to west winds 10-15
knots, becoming light and variable Wednesday night then
southwesterly around 5 knots Thursday. High confidence.
Friday...a period of sub-VFR conditions possible late day or at
night with possible showers. Southwest winds around 10 knots,
becoming west to northwest at night. Low to moderate confidence.
low pressure southeast of New Jersey moves out to sea today. A
cold front approaches late tonight.
Wind gusts have diminished below 25 kt on lower Delaware Bay,
so will cancel the Small Craft Advisory that was in effect. On the ocean, gusty
winds continue through this morning, and then Small Craft Advisory conditions,
mainly in elevated seas, will then persist on the ocean waters
this afternoon and into tonight.
Tuesday...as winds become westerly at night, wind gusts may
reach 25 knots for a time late. Seas however on the Atlantic
coastal waters may still be near 5 feet.
Wednesday...Small Craft Advisory conditions possible especially
in the morning with westerly wind gusts to 25 knots.
Thursday and Friday...the conditions are anticipated to be
below Small Craft Advisory criteria.
Marine...Small Craft Advisory until 7 am EDT Tuesday for anz450>455.