Area Forecast Discussion
Issued by NWS Mt. Holly, NJ-- Remove Highlighting --
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FXUS61 KPHI 220342
Area Forecast Discussion
National Weather Service Mount Holly NJ
1142 PM EDT Tue Mar 21 2017
A cold front will slide through the area tonight. A large area of
high pressure then builds in from the northwest and stays over our
region through Thursday before shifting off shore. A warm front is
forecast to lift through our region on Friday. Low pressure is
expected to approach from the west over the weekend.
.NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...
Cold front through the northern Great Lakes, and into western
NY/PA. Front will work its way through the region during the
early morning hours, generally starting around 3am, and working
its way through the rest of the CWA through daybreak Wednesday.
NW winds will increase behind the frontal boundary, and by
daybreak, will range from 10-20 MPH with gusts up to 30 MPH.
Will adjust hourly grids based on latest surface obs, but will
not make many significant changes, as forecast remains on track.
.SHORT TERM /6 AM WEDNESDAY MORNING THROUGH 6 PM WEDNESDAY/...
Strong high pressure will build towards the area Wednesday. A
strong and gusty NW wind with dry air will be across the area
through the day. Skies will be clear and temperatures will be
below normal. Highs will only be in the low/mid 40s over
Delmarva and the 30s elsewhere. Wind chill values will will only
be in the 20s in many areas for the afternoon. Winds will gust
30-40 mph at times.
.LONG TERM /WEDNESDAY NIGHT THROUGH TUESDAY/...
The long term will start off tranquil, but get complicated
Thursday...With the center of the high passing almost directly
over the region, Thursday should be a quiet and tranquil day
albeit colder than normal. Temperatures will generally be 10 to
20 degrees below normal, with some record lows possible Thursday
morning (see climate section below).
Thursday night into Friday...This is where the forecast starts
getting complicated. A warm front is expected to lift through
the region during the day on Friday (a bit slower than what most
models were depicting yesterday). At the same time, a broad
upper level ridge will be approaching from the west. So we could
potentially have mesoscale lift (associated with the front) in
the lower levels, while some subsidence is possible in the upper
levels. At this point, do not expect that as the ridge appears
to be trailing far enough behind the front that the two will not
be juxtaposed. Further complicating matters is the fact that
the boundary layer will be quite dry leading into this event,
with dew points likely starting in the teens and lower 20s
across much of the region early Friday morning. If we see enough
lift out ahead of the front, we could see a brief period of
freezing rain (and possibly some sleet) generally north and west
of the I95 corridor. Having said that, there have been a few
events this winter where models were too fast with warm air
advection across the Poconos and NW NJ, so they have the highest
risk for multiple hours of freezing/frozen precipitation, but
still considerable uncertainty.
Saturday...Warm air advection continues. The one possible
negating factor could be persistent cloud cover through the day.
However, will continue to go on the high side of guidance as
most guidance has had a consistent cold bias with southerly flow
Saturday night into Monday...Good agreement that a low pressure
system will be sliding northeast out of the southern Plains into
the Great Lakes region through this time. That is about the
extent of what I have high confidence in. Less confidence,
though there appears to be better model agreement today, that
the low will be weakening/filling as it propagates northeast.
What I have very low confidence in, a front is expected to slide
south as a cold front Saturday night, then stall, and lift north
as a warm front Sunday night. How far south this front will get
before stalling could have a big impact on temperatures
(especially with the highs on Sunday) and precip type Sunday
night into Monday morning. At this point, most model solutions
show the front stalling somewhere between Philly and the
Delaware Water Gap. For locations that stay south of the front
through the entire event, expect another very warm day on
Sunday, and all rain. Locations north of the front can expect
temperatures as much as 15 degrees below normal on Sunday, and
in the higher terrain, a wintry mix is possible Sunday night
into Monday morning. For now have stayed close to a mean of
model solutions given the high uncertainty.
Tuesday...Another low may develop along what is left of the
previously mentioned front. Models depict little upper level
support and it should fill and weaken as well as it propagates
.AVIATION /04Z WEDNESDAY THROUGH SUNDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.
For the 00Z TAFs...VFR through the period. BKN-OVC CIGs will
develop/lower to around 10 kft this evening through the early
overnight before a strong cold front pushes through the region
between 08Z and 12Z, after which skies will rapidly clear.
Winds will sharply increase and begin gusting after frontal
passage, with directions ranging from 300 to 350 most of the day
tomorrow. Speeds of 15-25 kts with gusts to 35 kts are possible
after frontal passage through late afternoon Wednesday.
Thursday...Mostly VFR conditions are expected.
Friday...MVFR or lower conditions are possible with
precipitation generally along and northwest of the Delaware
Valley (including PHL, ILG, PNE, TTN, ABE, and RDG). For ABE,
RDG, and TTN, there may be a brief period of a wintry mix Friday
Friday night through Saturday...mostly VFR conditions are
Saturday night and Sunday...MVFR or lower conditions are
possible with rain across the region.
No changes to the marine forecast this evening. Expecting
frontal passage in the 4-8 am time frame, with strong/gusty
northwest winds picking up immediately after passage. Gale-force
gusts expected most of the day Wednesday, especially during the
Nice conditions on the waters this afternoon will continue through
much of the overnight. A sharp cold front will cross the waters well
after midnight and cause winds to shift to NW and increase. Gale
warnings are in effect for the period after this frontal passage as
winds will probably gust 35 to 40 knots at times. We expanded the
Gale to cover Delaware Bay and the Delaware coastal waters. Cold air
and decent flow aloft should have no problem mixing down. No
precipitation is expected tonight or Wednesday.
Wednesday night...winds and seas will diminish through the
Thursday...Once wind gusts drop below 25 kt early in the day,
sub-SCA conditions are expected.
Friday...winds may approach 25 kt leading to SCA conditions
Saturday...sub-SCA conditions expected.
Sunday...there is a chance wind gusts could build above 25 kt
late in the day.
Relative humidity levels will drop behind the front Wednesday. Gusty
winds will cause near Red Flag conditions during the afternoon.
After calling the fire weather partners, it seems that soil moisture
is still rather high in many areas. The Delmarva is the only
exception where conditions may be rather marginal for RFW. We will
probably issue an SPS overnight to highlight these dry windy
More record events possible this week.
One or 2 of the following are vulnerable record equaling lows
Thursday morning March 23, primarily ABE. Others are listed for
PHL-14 1885, 1875
MPO- minus 8 1912
MARINE...Gale Warning from 6 AM to 11 PM EDT Wednesday for ANZ430-431-