Area Forecast Discussion
Issued by NWS Mt. Holly, NJ-- Remove Highlighting --
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FXUS61 KPHI 242038
Area Forecast Discussion
National Weather Service Mount Holly NJ
338 PM EST Fri Feb 24 2017
A strong cold front will move through the Mid-Atlantic on Saturday.
High pressure will build into the region on Sunday before moving
into the Atlantic on Monday. A surface low will develop in the
central U.S. on Tuesday and move northeast to the Great Lakes by
Wednesday and southeast Canada by Thursday. An associated warm front
is forecast to move through the Northeast on Wednesday before
another strong cold front sweeps across the East Coast on
.NEAR TERM /UNTIL 6 AM SATURDAY MORNING/...
Big question tonight will be when and where low clouds and fog
develop. Winds are expected to shift out of the southeast overnight.
This slight shift with more of a onshore component, could result in
low clouds advecting from the ocean on shore. Having said that,
guidance was fairly poor with the extent of low clouds this morning,
so not sure if it will be quite as widespread as what some guidance
With the onshore flow and potential cloud cover, temperatures should
stay quite mild, with low generally in the 50s across the region.
.SHORT TERM /6 AM SATURDAY MORNING THROUGH 6 PM SATURDAY/...
The first question will be how quickly early low clouds will
dissipate. Even if clouds persist into late morning though, models
have recently had a cold bias in warm air advection events, so have
gone slightly above guidance for highs during the day. If clouds
hang on longer or if the cold front moves in earlier, current
forecast would be too high.
The big story in the short term will be the cold front. Latest
guidance suggests it should be moving across our area from 18Z (for
far western Berks Co and the southern Poconos) to just approaching
the shore by evening. Forecast still looks on track for a line of
showers with embedded thunderstorms along and just behind the front.
The biggest question mark with thunderstorms will be surface based
or mixed layer instability. CAPE values will be marginal at best so
if we do end up being cooler than forecast, instability (especially
surface based instability) could be negligible. Given the model cold
bias though, I think we will have enough instability to support
Primary hazard continues to be gusty winds given the high shear
environment (0-6km bulk shear values could be above 50kt). Will
continue mention in the HWO and add mention to the forecast for this
Although there could be brief periods of heavy rain with the line,
the showers and storms should be moving through fast enough to
limit any flooding risk.
.LONG TERM /SATURDAY NIGHT THROUGH FRIDAY/...
New long term section will be issued by 430 PM. Previous long
Active pattern in the Sunday thru Thursday time frame.
Temperatures cool to seasonal levels on Sunday, with above
average temperatures returning for the Monday thru Thursday time
period, and additional chances for precipitation.
Overall, Sunday`s forecast is high confidence. Moving into next
week, there remains considerable uncertainty with regard to
timing and coverage of precipitation, as well as high
Sunday... We continue to expect a return to fair weather on
Sunday, along with near normal temperatures, and gusty northwest
Looking ahead to Monday thru Thursday: several shortwaves will
interact with a warm front that sets up near our region late
Monday into Tuesday, followed by a cold frontal passage
Wednesday into Thursday. Timing and coverage of precip continues
to remain uncertain, so the forecast reflects a general slight
chance of showers during this time frame.
.AVIATION /21Z FRIDAY THROUGH WEDNESDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.
VFR conditions are expected to continue through at least 03Z. After
03Z, as winds shift to more southeasterly (resulting in more of an
onshore component), could see low stratus move over the TAF sites.
As it does so, could also see some fog development underneath the
stratus. Timing is uncertain, so have mentioned the period of
greatest risk (08 to 14Z) in the the TAFs. After 14Z, any fog should
dissipate and ceilings should begin to slowly lift.
The next concern will be after 18Z as a cold front approaches from
the west. An abrupt shift to westerly winds, widespread showers (and
associated restrictions in visibility), and scattered thunderstorms
are possible with and just behind the front. KRDG and KABE may see
this between 18 and 21Z, while the rest of the TAF sites will be
impacted 21Z or later.
Saturday night and Sunday...Conditions quickly improving to VFR
behind the cold front, with showers/storms moving off the coast.
West or northwest winds 10-20 knots with gusts to 30 knots likely.
Above average confidence.
Sunday night and Monday...VFR with west winds around 10 kts becoming
southwest by Monday afternoon. Average confidence.
Monday night through Wednesday...Periods of showers with associated
sub-VFR conditions possible, especially during the day Tuesday and
again Wednesday afternoon. Primarily light southerly or
southwesterly winds, but a more easterly component may develop on
Tuesday north of a developing warm front, and gustier southwest
winds may occur Tuesday night or Wednesday as the warm front moves
through the region. Very low confidence on timing and
Seas will slowly build overnight and should be above 5 feet on the
ocean waters near or shortly after sunrise. Winds will also increase
after sunrise with gusts above 20kt possible. The strongest winds
(and an abrupt shift to westerly winds) are expected to come with
and behind a cold front which will move over the Delaware Bay late
tomorrow afternoon. The front is not expected to arrive on the ocean
waters before tomorrow evening.
Saturday night...Cold front will move through the coastal waters
with showers/storms likely. Strong/gusty winds near storms. Small
craft advisory (SCA) conditions likely as winds become west or
northwest behind the front. Gusts may approach gale force. Average
Sunday...Continued SCA conditions likely with winds slowly
diminishing during the afternoon. Above average confidence.
Sunday night and Monday...Sub-SCA conditions likely. Above average
Monday night through Wednesday...Generally sub-SCA expected, though
winds and seas may approach criteria after potential warm frontal
passage Tuesday night or Wednesday. Periods of rain/showers likely,
especially during the day Tuesday and again Wednesday afternoon.
Very low confidence.
MARINE...Small Craft Advisory from 7 AM Saturday to 6 PM EST Sunday for
Small Craft Advisory from 6 PM Saturday to 6 PM EST Sunday for