Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
000
FXUS61 KPHI 182029
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
329 PM EST Wed Jan 18 2017

.SYNOPSIS...
Low pressure over the western Atlantic waters will continue to drift
out to sea tonight. Meanwhile, high pressure over the Midwest will
build east and then move offshore at the end of the work week. Weak
low pressure will impact the region this weekend, and then a
stronger and more complex area of low pressure will impact the Mid-
Atlantic and Northeast early in the new week.

&&

.NEAR TERM /UNTIL 6 AM THURSDAY MORNING/...
Shortwave axis has exited the coast as of late afternoon, with any
lingering sprinkles and rain showers diminishing, as subsidence
becomes dominant through the column. Yet a weak low-level inversion
remains across the region, with ample moisture below it, which is
maintaining considerable cloud cover. This cloud cover will persist,
with breaks developing after midnight, and partial clearing from
south to north beginning toward dawn.

With the inversion in place, northwest wind gusts will be limited,
in addition to the decoupling of the boundary layer this evening.
Wind gusts will range from 15 to 25 mph, on the lower end north of I-
78, and on the higher end over Delmarva. Winds will diminish rapidly
north of I-78 this evening, and more gradually toward midnight over
Delmarva.

In terms of low temperatures, sky cover will be the determining
factor, as winds become light and variable. With greater cloud cover
north of I-78, we adjusted temperatures slightly upward. Elsewhere,
with less cloud cover toward dawn, we went slightly colder.

With abundant low-level moisture in place, light and variable winds,
and clearing skies over portions of the area, there is a window of
opportunity for the development of patchy fog. Both model soundings
and indices also support this. Low temperatures are forecast at or
below freezing northwest of the NJ Turnpike/ I-95, as well as the
Pinelands of NJ. Thus, where fog develops across these areas, it may
freeze on contact. This could lead to slippery conditions,
especially on untreated surfaces.

&&

.SHORT TERM /6 AM THURSDAY MORNING THROUGH 6 PM THURSDAY/...
Patchy fog that does develop could linger into mid-morning, given
maintenance of the low-level inversion with a building ridge aloft,
and subsequent high pressure at the surface and light winds. The
deeper moisture will continue moving north of the area, with some
lingering clouds north of I-78 in the morning. Otherwise, a mostly
sunny day develops, with some scattered clouds below the low-level
inversion. Dry weather is expected, with no support for
precipitation. As for high temperatures, generally 10 to 15 degrees
above normal, from the low 40s in the Poconos to mid 50s in
Delmarva. Northwest winds 5 to 10 mph.

&&

.LONG TERM /THURSDAY NIGHT THROUGH WEDNESDAY/...
A prolonged period of warm and wet weather is on tap for the Long
Term period.

Broad area of high pressure along the Eastern Seaboard will move
offshore Thursday night as weakening low pressure approaches from
the west. Weak secondary low will form over the Mid-Atlantic, and
the primary low will dissipate as is tracks from the Great Lakes
into eastern Canada. This will send a quick shot of precip into the
Mid-Atlantic and Northeast, but not expecting much more than 0.25"
QPF or so. Best chances will be from midday Friday through Friday
evening. Conditions dry out after midnight Friday night. One concern
is timing of onset of precip for the Poconos and areas to the north
of I-80. If precip develops before noon on Friday, surface temps may
not yet have risen above freezing, and there could be a brief
initial period of rain/freezing rain. Otherwise, with onshore flow,
temperatures will be warm enough for precip to be all rain.

Weak high pressure then builds across the region Friday night and
Saturday. Patchy fog may develop Friday night and Saturday morning,
as with weak flow, there will not be much of a chance for the
moisture to scour out. Conditions will otherwise be dry and mild on
Saturday with highs in the upper 40s to mid 50s. High pressure moves
offshore Saturday night.

Meanwhile, an upper trough will move into the West Coast sometime
early in the weekend and will become a closed upper low over the
Southern Plains and Gulf Coast states during the day Sunday. As this
system organizes and develops, it will send waves of precip out
ahead of it and into much of the East Coast.

The first wave will develop over the TN Valley and lift towards the
area on Sunday. Both the GFS and the ECMWF bring measurable precip
into the region by Sunday afternoon, and then the bulk of the rain
approaches Sunday night and Monday as the main system lifts into the
Mid-Atlantic region. An area of concern: high pressure will nose its
way into eastern Canada and northern New England. Depending on the
placement of the high, winds could take on more of a NE/E flow as
opposed to an E/SE flow. This could then usher a shallow area of
cold air into mainly northern zones, and would result in a period of
wintry weather sometime Sunday night and Monday morning. Latest
track, based on both the GFS/ECMWF, keeps temperatures above
freezing, so will keep precip as all rain throughout the CWA.

There may be a brief break in the precip late Monday morning, but
the main low will lift through the region Monday afternoon and
evening, bringing another round of potentially moderate to locally
heavy rainfall. With deep onshore flow, PWATs will build to around
1.5". Pressure gradient tightens up, and with strong low level jet
passing through, can expect E winds to increase to 15-25 MPH with 25-
35 MPH gusts, mainly on Monday.

Rainfall ends fairly quickly from SW to NE Monday night through
Tuesday morning. Behind that departing system, a deep area of low
pressure will be entrenched over the northern Great Lakes region,
and several shortwaves will rotate around the low and into the local
area. This may touch off some showers during the mid-week period.

During this time, Arctic air remains bottled up to the north.
Temperatures will average above normal, with highs nearly 10 degrees
above normal.

&&

.AVIATION /21Z WEDNESDAY THROUGH MONDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Tonight...MVFR conditions across the region as of late afternoon.
Expect continued, gradual improvement into this evening, with all
TAF sites becoming VFR by 00Z. Northwest winds gusting up to 20
knots will subside this evening, becoming light and variable at
times overnight.

Thursday...Predominantly VFR conditions. There may be some SCT030
ceilings developing around noon. Northwest winds 5 to 10 knots.

OUTLOOK...
Thursday night...VFR. LGT/VRB winds less than 5 KT. High confidence.

Friday through Friday night...Sub-VFR conditions possible in -RA. SE
winds 5-10 KT on Friday become SW less than 5 KT Friday night.
Moderate confidence. Fog possible Friday night.

Saturday...Improving conditions Saturday morning. VFR. West winds 5
KT or less.

Saturday night...LGT/VRB winds. Mainly VFR, but cannot rule out
passing shower.

Sunday...E winds 10-15 KT. IFR conditions possible in RA.

Sunday night through Monday...IFR and lower conditions possible in
RA. E winds 15-20 KT with 20-30 KT gusts. LLWS possible with 60-65
KT LLJ Monday afternoon.

&&

.MARINE...
With northwest flow in place across the waters, areas of fog had
dissipated by the early afternoon.

Small Craft Advisory continues for the southern NJ ocean and lower
Delaware Bay waters, mainly for seas reaching 5 FT on the ocean, and
wind gusts up to 25 knots on the ocean and lower bay.

We canceled the Small Craft Advisory for the upper Delaware Bay
waters. The latest observation trends and model guidance favor wind
gusts remaining below 25 knots.

For the remainder of tonight into Thursday, high pressure moves
over the waters, with winds and seas below Small Craft Advisory.

OUTLOOK...
Thursday night through Sunday morning...A weak pressure gradient
will result in sub-SCA conditions during this time. VSBY
restrictions possible in rain on Friday, and in fog Friday night and
Saturday morning.

Sunday afternoon through Monday night...Easterly flow increases to
15-20 KT with 25-30 KT gusts starting Sunday afternoon, and then
gale force gusts of 35-40 KT likely Monday and Monday night.
Moderate to heavy rain with low VSBY expected. Conditions improve
late Monday night, and winds decrease to sub-SCA levels. However,
seas on the ocean should remain above SCA criteria.

Tuesday...Ocean seas remain above SCA criteria.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...Small Craft Advisory until 9 PM EST this evening for ANZ431-
     452>455.

&&

$$
Synopsis...MPS
Near Term...Franck
Short Term...Franck
Long Term...MPS
Aviation...Franck/MPS
Marine...Franck/MPS



USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.