Scientific Forecaster Discussion
fxus61 kphi 221453
Area forecast discussion
National Weather Service Mount Holly New Jersey
1053 am EDT Tue Oct 22 2019
a cold front approaches the region later today and passes
through the region tonight. High pressure returns for the
midweek period before moving offshore on Friday. Another cold
front passes through Friday night. Low pressure may affect the
area late this weekend and into the start of the new week.
Near term /until 6 PM this evening/...
The forecast is mainly on track this morning. The main change with
the latest update was to slow down the arrival of higher pops by a
couple of hours in most areas, based on radar trends. Main frontal
rain band definitely has more of a northward motion component than
an eastward one. So steadier rain will probably not reach most
places until late this afternoon or early this evening, a little
sooner out west. Otherwise, overcast conditions continue across the
area with a few patchy light showers or areas of drizzle. Have also
seen visibilities drop to 2 to 4 miles in many areas, locally lower,
but not expecting much if any dense fog, mainly just a low stratus
layer. Previous discussion follows.
A strong upper-level trough in the Midwest to the lower
Mississippi Valley will gradually shift eastward today. However,
the center of the trough (closed low) will shift into Canada
later today. Surface low pressure tied to the trough aloft will
lift into Canada later today. Meanwhile, an associated cold
front will approach our area this afternoon and then cross our
The aforementioned surface system is occluded, and an east to
southeast low-level flow will be across our area. There is a
sharp increase in the moisture advection forecast by later today
as the 925 mb to 500 mb flow increases. This results in a
narrow corridor of 1.5 to near 2.0 inches of precipitable water
advecting northward and into our area. Due to this moisture
advection, surface dew points will be on the rise and given the
time of the year it will feel more humid this afternoon.
The forecast challenge for today is the timing of the rain
along and just ahead of the occluded front and any showers prior
to. While the guidance is in pretty good agreement in bringing
this into our western counties (pa) between 2-4 PM, some
guidance wants to produce some light showers or drizzle starting
this morning. The latter is likely due to a low cloud deck that
is in place for many areas, and while some drizzle cannot be
ruled out the bulk of the rain arrives late this afternoon from
the west. As a result, our forecast pops are more focused on
this organized area of rain arriving from the west.
Despite the wealth of cloud cover expected today, low-level
warm air advection along with some increase in the flow just
above the surface will take afternoon temperatures into the 60s
for many places (even close to 70f for some southern areas). It
should be cooler along the coast though given a breeze off the
water. The forecast soundings continue to show a pronounced
inversion just below 850 mb today given warm air advection above
an east to southeast lower level flow. This results in a lack
of instability through the day overall. The main rain band then
arrives with the front this evening (more on this in the short
term section below).
Short term /6 PM this evening through 6 am Wednesday/...
a strong upper-level trough will continue to shift eastward
from the Great Lakes region with the associated occluded surface
low moving into Canada. The cold front will swing across our
area tonight, especially by late evening. The front will be
accompanied by mid and upper level jet streaks. A low-level jet
on the order of about 40 knots will cross mostly our southern
zones this evening in conjuction with a weak surface low
tracking along the front. These will enhance some of the rain
however the entire system looks progressive enough and therefore
heavier rainfall duration should be limited. Given an axis of
nearly 2-inch precipitable water values ahead of the front, some
heavier rain cannot be ruled out with the highest rainfall
amounts still expected across mainly northeastern Pennsylvania
and northern New Jersey.
The forecast soundings continue to show some elevated
instability developing this evening primarily across Delmarva
into far southern New Jersey. These soundings also show weak mid
level lapse rates which results in weaker buoyancy. Given the
continuation of this in the model soundings, opted to maintain
no thunder in the forecast. While the main trough remains to our
west during the much of the night, robust short wave energy
lifting up the east side of it along with a 500 mb jet near 85
knots allows the trough to take on some negative tilt. This
looks to happen later at night when the frontal rain is moving
off the coast therefore additional enhancement looks low at this
point. It should be noted that during the evening especially
across the southern areas where the low-level jet is stronger
(higher shear), some heavier rain could result in brief stronger
wind gusts although the soundings overall do not look all that
favorable regarding the Transfer of stronger winds down to the
As the front moves offshore overnight, a wind shift to west and
northwest occurs allowing cold air advection to take over and
also some drying to work in. We are expecting clearing to take
place overnight from west to east as a result, and there could
be a brief period when the winds turn a bit gusty behind the
cold front given the rapid increase in cold air advection.
Long term /Wednesday through Monday/...
cold front will be offshore on Wednesday, and high pressure
centered over southeast begins to build in from the west. The
center of the high will pass over Delmarva on Thursday before
moving out to sea on Friday. Tranquil conditions on tap with
sunny skies and near normal temperatures with highs in the mid
and upper 60s.
A cold front approaches on Friday and passes through the region
Friday night. Some showers are possible, mainly across the
southern Poconos, northern New Jersey, and the Lehigh Valley
Friday afternoon and Friday evening.
From there, the forecast is highly uncertain, as there is
little consistency among the models as to what comes next. The
only consensus is that low pressure looks to affect the region
sometime this weekend. However, the timing is off.
The 00z GFS has the front hanging over the area, and low
pressure developing along the front as a deep 500 mb trough digs
through the Great Lakes and Ohio Valley. This keeps rain in the
forecast for pretty much all day Saturday.
However, the 00z European model (ecmwf) and the 00z CMC have high pressure
building in from the west and passing overhead Saturday night
before moving offshore on Sunday. This results in sunny skies
and dry conditions for Saturday.
On Sunday, however, the 00z GFS is dry, whereas the the 00z
European model (ecmwf) and 00z CMC have the low approaching during the day, and
rain moves into the region in the afternoon. Some lingering
showers are possible on Monday.
Aviation /15z Tuesday through Saturday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg, kilg,
kmiv, kacy and surrounding areas.
Today... mainly IFR conditions expected through the remainder of the
day. There has also been a good deal of LIFR conditions across the
area with ceilings down in the 200 to 400 ft range. While LIFR may
remain prevalent especially towards ilg, miv, and Acy, believe most
areas should remain IFR as ceilings may even try to lift a bit early
this afternoon due to the heating of the day. Scattered showers
possible through early afternoon before steadier rain moves in after
that time. Steady rain should reach near ridge and Abe around 18z and
should approach phl towards 20z. Continued southeast flow of around
10 kt is expected.
Tonight...IFR to MVFR conditions with rain, then the conditions
improve to VFR mostly after midnight as the rain ends.
Southeasterly winds 5-10 knots, becoming west to northwest and
could gust for a brief time up to 20 knots. Moderate confidence.
Wednesday through Thursday...VFR with mostly sky clear conditions.
West winds 10-15 kt with gusts to 20 kt on Wednesday, becoming
light/vrb Wednesday night, and then SW less than 10 kt Thursday.
Friday through Friday night...VFR initially, then MVFR
possible, mainly at krdg/kabe in -shra. South winds less than 10
kt, becoming northwest Friday night.
Saturday...generally VFR, but low confidence as there is little
consistency among model guidance. Northwest winds less than 10 kt, but
confidence is low on that as well.
Small Craft Advisory conditions through at least tonight. On
the New Jersey and Delaware Atlantic coastal waters, seas remain
elevated early this morning (6-7 feet) and while these should
subside some today they may not drop completely below 5 feet. In
addition, an increase in southeasterly winds later today and
then a surge of west to northwest winds tonight should result in
gusts to around 25 knots. As a result, the advisory has been
extended through tonight.
For Delaware Bay, there is the potential for a southeasterly
wind surge this afternoon and evening followed by a west to
northwest wind surge later tonight. Given a high enough
potential for some periods of wind gusts to about 25 knots, a
Small Craft Advisory has been issued for this afternoon and
Wednesday...Small Craft Advisory conditions during the day with northwest winds 15-20
kt with 25-30 kt gusts. Winds diminish late.
Wednesday night through Friday night...winds and seas should
stay below Small Craft Advisory criteria.
Saturday...generally sub-Small Craft Advisory conditions, however gusts to 20 kt
are possible on the ocean waters.
Marine...Small Craft Advisory until 6 am EDT Wednesday for anz450>455.
Small Craft Advisory from 2 PM this afternoon to 6 am EDT
Wednesday for anz430-431.