Forecast Discussion From Mount Holly

130
FXUS61 KPHI 221403
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
903 AM EST Mon Jan 22 2018

.SYNOPSIS...
High pressure off the Southeast United States coast continues
to drift offshore today as a warm front ahead of low pressure
over the Central Plains lifts north through the region. That low
moves through the region on Tuesday, and then a cold front
passes through the region Tuesday night. High pressure builds
into the region through the end of the week before moving
offshore on Saturday. Another low pressure system looks to
affect the region next weekend.
&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
9 am: Made some minor temperature adjustments for today based
on latest obs and trends. Otherwise, forecast on track with no
changes.

630 am update: Models are still struggling with temperatures
across the area this morning, with a negative bias of around 3-7
degrees in Delmarva and far southeast New Jersey. Continued to
make adjustments to hourly temperatures for the next couple of
hours, but confidence is rather low in nudging temperatures too
much beyond that point.

Some sprinkles moved through southeast Pennsylvania and most of
New Jersey the last couple of hours. Mostly trace amounts were
reported (including here at the office), so did not feel the
need to include PoPs as these sprinkles moved through. The next
few hours look fairly dry but will watch upstream precip closely
to determine if PoPs are too low later today, especially in the
northern CWA. Still looks like fog/drizzle are the main sensible
weather impacts, and did not make changes to these references in
the grids/public forecasts with this update.

Previous discussion...

Tricky forecast today, as a cyclone in the central plains
(currently in northwest Missouri) and its eastward-extending
baroclinic zone are expected to move little through the daylight
hours. Isentropic lift atop the boundary in the northern Mid-
Atlantic has generated patchy drizzle and fog in the northern
CWA and even some light showers north of I-78, though these
should shift east of the area within the next few hours. With
very little movement expected with the surface low during the
next 12 hours, I am very skeptical that the warm front in our
region will be moving much today. Surface obs do not provide a
very obvious position of the boundary (with associated gradients
in potential temperature, mixing ratio, and winds rather
diffuse), but there is reason to suspect that the I-
76/I-276/I-195 corridors are a good starting point with most of
the accumulating precipitation to the north, and this may be
"north-generous" given the sharper temperature gradient and
more noticeable wind shift/increase in the Delmarva Peninsula.

Some of the hi-res models are beginning to clue in on this
slower retreat of the warm front northward today, and even
statistical guidance is hinting at east or northeast winds north
of I-78 through most if not all of the day. Should such flow
become established, topographic effects (i.e., cold air damming)
could prove too much for the warm front to overcome today in
the northern CWA. With collaboration from OKX, decided to lower
hourly temperatures in the northern third to half of the CWA
quite a bit today (in some cases, by 3-7 degrees). There is real
potential for large errors in max temperature forecasts across
the area. Note that current (3 am) temperatures in southern
Delaware are in the mid 50s, with upper 30s in the northern CWA.
It is quite possible northern New Jersey and eastern PA
struggle to reach the middle 40s while Georgetown, DE, is
flirting with the middle 60s. For the Philadelphia metro, the
current forecast is for highs in the middle 50s, but anything
between 50 and 60 is certainly plausible.

Regarding precipitation, think very little measurable
precipitation will occur in our CWA today. North of the warm
front, patchy fog and drizzle should occur and have modified the
grids considerably to indicate this. A vort max will approach
the Northeast late today, but the strongest lift looks to be to
our northwest, with little QPF generated in most of the models
(hi- and lo-res) in our CWA. Retained occasional slight chances
in the Poconos and vicinity, but confidence in occurrence,
coverage, and timing is not high.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM TUESDAY/...
The surface cyclone in the central plains gets a boost from a
digging shortwave trough in south-central Canada tonight, and it
will begin to lift east-northeast to the Great Lakes by 12Z
Tuesday. This should allow the warm front across the area to
lift gradually northward overnight (though I suspect not as
aggressively as most of the models are depicting). Residual
fog/drizzle was extended in the forecast through the evening
hours approximately north of I-80 to account for the suspected
sluggishness of this northward retreat.

Meanwhile, a strong vort max will approach the Mid-Atlantic
tonight, and large-scale lift via the combination of
differential cyclonic vorticity advection, rather pronounced
warm advection (especially near/north of the warm front), and
improved positioning of an approaching upper-level jet streak
will generate widespread showers and maybe even a lightning
strike or two to our west overnight. This precipitation should
spread into our area with time, most likely after midnight.

The synoptically-induced low-level jet stream will be quite
strong (925-mb flow of 50+ kts). Though mixing of these winds to
the surface will be mitigated by nocturnal stabilization, even
weak downdrafts in convective showers may provide the means to
do so. Think this threat is more pronounced after the short-term
period, but we will need to monitor this threat as the
precipitation approaches the region.

The good news is that cell motions will be fast, so heavy
rainfall is not a particularly major threat. Nevertheless, model
QPF is certainly decent with the precipitation reaching the
area by daybreak, with widespread totals of 0.25-0.50 inches
generally along/west of the Fall Line (with likely more to come
on Tuesday).

Regarding temperatures, do not see considerable drop-off from
daytime temps given overcast conditions. It is quite possible
forecast lows are too cool south of the boundary, especially in
Delmarva (particularly given temperatures observed early this
morning).

&&

.LONG TERM /TUESDAY THROUGH SUNDAY/...
By Tuesday morning, deep low pressure will be over the Great
Lakes and will continue to lift to the north and east throughout
the day. A strong cold front will lie across western NY and
western PA and will extend through the Ohio Valley, and that
front will move across the region Tuesday afternoon, and then
strong CAA develops behind the departing cold front Tuesday
night and Wednesday.

Strong WAA out ahead of the cold front will usher an
unseasonably warm and humid airmass into the region with highs
generally in the 50s and 60s, and surface dewpoints in the 50s
as well. PWATs will range from 1 to 1.3". Meanwhile, a 130 KT
jet streak will dig into the region, and then will strengthen a
LLJ from 55-65 KT to 65-75 KT across eastern NJ and into
southern DE. This allows for a period of elevated instability
with MU CAPE of up to 500 J/kg. Will continue to carry isolated
thunder, and some of the stronger showers and possible
thunderstorms could mix down some of those stronger winds. Do
not think there will be full mixing of those winds to the
surface, but cannot rule out a marginally severe storm or two
Tuesday morning, especially over southern NJ and the Delmarva.
Locally heavy rain is possible as well, but given that 6 hour
flash flood guidance values are 1-2 inches over northern NJ and
3-4 inches over southern NJ, and storm total QPF Monday night
and Tuesday looks to be around 1 inch north and west of the Fall
Line, and between 1/2 and 3/4 inch south and east of the Fall
Line, not expecting flash flooding, but poor drainage flooding
is possible. Have to watch how rivers respond as well.

Showers taper off from west to east Tuesday afternoon.

Temperatures fall into the mid 20s in the Poconos, and into the
low to mid 30s elsewhere. Behind the cold front Tuesday
afternoon, west to northwest winds increase to 10-20 MPH with
gusts up to 25 MPH. Think there will be a long enough period
between the end of the rain and onset of the sub-freezing
temperatures to where a flash freeze should not be much of an
issue, especially with the gusty winds Tuesday afternoon and
evening.

High pressure builds in from the north. With strong CAA, and
with falling heights and thicknesses, highs on Wednesday will be
in the 20s in the Poconos, and in the 30s and 40s for the rest
of the region.

Dry weather through the end of the week. The coldest day looks
to be Thursday with highs in the 30s. The high moves offshore on
Saturday, and return flow sets up and allows for strong WAA and
highs climbing up into the 40s and 50s.

The next low may impact the region Saturday night and Sunday,
and based on latest model guidance, precip should be rain.

&&

.AVIATION /14Z MONDAY THROUGH FRIDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Today...Some patchy fog/drizzle will continue at RDG/ABE through
around 15Z before improvement to consistent VFR, while VFR
should prevail elsewhere today. Winds generally light/variable,
but may pick up a predominantly southeast component later this
afternoon (possibly more east or even east-northeast at
RDG/ABE). Overall confidence is average.

Tonight...Conditions will deteriorate during this period on
multiple fronts. Low-level (southwesterly) wind shear is a
concern after 00Z as a strong low-level jet develops overnight.
CIGs will lower through the evening hours, becoming MVFR/IFR at
most terminals between 06Z and 12Z. VSBYs may also become sub-
VFR with fog development possible, at least locally. Finally,
showers will move in from the west after 06Z, likely affecting
the Philly terminals around/after 09Z. Lightning cannot be
completely ruled out, though confidence in this is too low for
TAF mention at this point. Overall confidence is average.

Outlook...

Tuesday...IFR or lower in rain and isolated TSRA. Locally heavy
rain possible. Gusty winds possible in heavier showers and
isolated TSRA. Showers taper off from west to east Tuesday
afternoon as cold front moves through the region. South winds
15-20 KT with gusts up to 25 KT Tuesday morning become W-NW in
the afternoon. VFR conditions likely in the afternoon. LLWS
issues possible. Moderate confidence.

Tuesday night...VFR. West winds 10-15 KT. Moderate confidence.

Wednesday through Friday...VFR conditions expected. West to NW
winds 10-15 KT with gusts up to 20 KT during the day Wednesday
and Thursday. Light and variable winds Friday. High confidence.

&&

.MARINE...
Sub-advisory conditions are expected today across the waters.
However, there may be some patchy fog/drizzle off the
northern/central NJ coast, with occasional/localized visibility
restrictions.

Tonight, south winds should begin to increase, especially after
midnight, with speeds reaching small-craft advisory criteria a
couple of hours before daybreak. No changes to the current
marine headlines were made, with more discussion in the
"Outlook" section below. A chance for showers increases by
daybreak Tuesday.

Outlook...

Tuesday...Gale Watch remains up on the ocean waters Tuesday
morning and afternoon, but confidence is low that widespread
gales will develop. Although there will be a strong LLJ, there
will be an inversion over the ocean that should inhibit those
winds from mixing down. The exception will be in heavier showers
and isolated thunderstorms. Marine Weather Statements and
possibly a Special Marine Warning may be needed for these
locally stronger winds. Otherwise, Small Craft Advisory
conditions will be in place for the ocean. SCA conditions
expected for DE Bay Tuesday. South winds in the morning abruptly
shift to the W-NW in the afternoon behind the passage of a cold
front.

Tuesday night through Thursday...SCA conditions likely on at
least the ocean waters during this time.

Friday...Winds and seas are expected to stay below SCA
criteria.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...Gale Watch from Tuesday morning through Tuesday afternoon for
     ANZ450>455.
     Small Craft Advisory from 4 AM to 6 PM EST Tuesday for ANZ430-
     431.

&&

$$
Synopsis...MPS
Near Term...CMS/Fitzsimmons
Short Term...CMS
Long Term...MPS
Aviation...CMS/MPS
Marine...CMS/MPS

NWS PHI Office Area Forecast Discussion