Scientific Forecaster Discussion

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Area forecast discussion 
National Weather Service Mount Holly New Jersey
1009 am EDT Friday Apr 18 2014

Synopsis...
high pressure will continue to extend across the eastern Seaboard
through today. The high will retreat as a weakening frontal boundary
moves across the area tonight into Saturday morning. High pressure
reestablishes itself across eastern Canada over the weekend and
builds southward along the eastern Seaboard Sunday into Monday.
Another frontal boundary is expected to affect the area next
Tuesday, followed briefly by high pressure Wednesday night into
Thursday.

&&

Near term /until 6 PM this evening/...
the latest estf update we did not make too many changes. The
latest rap which did well yesterday corroborates current thinking.
Maximum temperatures not changed much, if anything we back ended them
toward the end of afternoon.

Ely marine layer of high relative humidity saturating just below the inversion
around 1800-2400 feet. Not nearly as cold as Thursday morning when a
record low was set at Allentown PA.

Today...forecast basis is mostly NAM 3rly temperatures and maximum temperature during the
day and the higher of the NCEP 3 hrly MOS dewpoints. 50 50 blended
surface sustained winds.

Low clouds recede eastward as the bl warms but low clouds should dominate
most if not all of this Friday morning through midday. Then
considerable high cloud during middle and late afternoon. East wind gust near
15 miles per hour with the wind becoming S-southeast late in the day. Maximum temperatures near
5 PM this afternoon in the interior as the low clouds dissipate during
the afternoon and temperatures may rise 5 degrees between 2p and 5p vicinity
Philadelphia.

&&

Short term /6 PM this evening through 6 am Saturday/...
tonight...forecast basis is a 50 50 blend of the 00z/18 NCEP MOS guidance
with the colder of the low temperatures near 10z/Sat.

Considerable high cloud this evening with cold front passage scattered-broken SC near 4000
feet toward dawn Saturday. Light southeast-S wind during the evening turn west
or northwest late at night.

&&

Long term /Saturday through Thursday/...
we expect dry weather to build in across the area Saturday behind
the weakening frontal boundary that will cross the area early
Saturday morning as high pressure begins to build southward out of
Canada. The high should suppress the low to our south off the
southeast coast which should prevent any precipitation from reaching
our area.

This high will reestablish itself across eastern Canada and build
southward along the eastern Seaboard Sunday and Monday. This will
continue to provide dry weather over the weekend into Monday of next
week. With slight ridging aloft taking place, temperatures will
return closer to normal over the weekend into early next week.
Sunday will likely be the coolest day when thicknesses and 925 mb
temperatures are at their lowest.

The high will weaken and retreat Monday night ahead of an
approaching storm system. An area of low pressure is forecast to
move out of the Great Lakes region and strengthen as it moves into
New England Tuesday into Tuesday night. As this occurs, a cold
front will sweep through the area late Tuesday into Tuesday night,
which will bring the next chance of precipitation across the area.
Therefore we maintain chance probability of precipitation across the area during this time
period.

This low is expected to slowly pull to our northeast through
Wednesday and Thursday as high pressure begins to build southward
out of Canada once again. Wednesday and Thursday are expected to be
dry, but should be quite gusty with the tightening pressure
gradient. Winds could easily gust 25-30 mph, if not higher.

&&

Aviation /14z Friday through Tuesday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg, kilg,
kmiv, kacy and surrounding areas.

Through 12z...ceilings mostly 1600-2400 feet. Light east wind.

After 12z...the MVFR ceiling is expected to dissipate rapidly eastward
midday with a VFR cirrus deck above it during the afternoon.
Synoptic scale winds overall should be lighter (generally an eastward
component) than Thursday and a sea breeze front may form during
the afternoon as the wind tends to turn se-S.

Tonight...VFR cirrus with chance of a scattered-broken deck near 4000 feet toward
10z Saturday associated with a weak cold frontal passage (cfp).

Outlook...
Saturday-Monday night...mostly VFR. Gusty north-northwest winds 15-20 knots
Saturday.

Tuesday...MVFR conditions possible with showers with a frontal
passage.

&&

Marine...
seas 7 feet at 44009 at 5 am.

GFS ww4 guidance has been too low by a foot or 2 on the Atlantic
waters and so raised that guidance through this Friday evening.

Small Craft Advisory continues open Atlantic waters...mainly for seas and have converted
that Small Craft Advisory headline to Small Craft Advisory for hazardous seas here at the 3 am issuance.

Delaware Bay waters no headline through tonight. East-NE wind g 15 knots and seas
less than 3 feet today. Winds and seas diminish/subside tonight. The
wind turn S toward sunset then shifts to west or northwest toward dawn Saturday.

Outlook...
Saturday...sub-Small Craft Advisory criteria expected, though
winds may gust near 20 knots.

Saturday night-Sunday night...winds and seas increasing to Small Craft
Advisory criteria overnight Saturday and continue into Sunday. Seas
may remain elevated into Sunday night.

Monday-Monday night...a return to sub-Small Craft Advisory criteria.

Tuesday...frontal passage expected with near advisory conditions.

&&

Phi watches/warnings/advisories...
PA...none.
New Jersey...none.
Delaware...none.
Maryland...none.
Marine...Small Craft Advisory for hazardous seas until 6 am EDT
Saturday for anz450>455.

&&

$$
Synopsis...Robertson
near term...drag/gigi
short term...drag
long term...Robertson
aviation...drag/Robertson
marine...drag/Robertson