Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

Current Version | Previous Version | Graphics & Text | Print | Product List | Glossary Off
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

FXUS61 KPHI 180134

Area Forecast Discussion
National Weather Service Mount Holly NJ
934 PM EDT Wed May 17 2017

Broad high pressure in the western Atlantic will continue to
influence our area through tomorrow. A cold front will progress
southward through the Mid-Atlantic on Friday, with cooler
Canadian high pressure settling into the Northeast this weekend.
The boundary will return northward as a warm front Sunday
night. A cold front will sweep through the Eastern Seaboard on
Monday. Another system may affect the area by the middle of next


A large area of high pressure located over the western North
Atlantic will maintain a southwest flow of warm air in our
region for tonight.

We are anticipating high clouds for much of the night. Minimum
temperatures are forecast to be mostly in the 60s in eastern
Pennsylvania, New Jersey, Delaware and northeastern Maryland.

The potential for any precipitation in our region has come to
an end with the loss of daytime heating.


Hot conditions continue on Thursday as Bermuda high pressure
remains in place. Upper ridge flattens out and moves offshore
during the day as an H5 trough passes from the Great Lakes,
north of NY state, and into eastern Canada.

Most of the day should be dry. However, with the passage of
that trough to the north of the area late in the day, and with a
thermal trough developing just west of the Appalachians, this
may be enough to touch off isolated late day showers and
thunderstorms across the Lehigh Valley, Pocono Mountains, and NW
NJ. Will follow model guidance and keep PoPs capped at 20

Temperatures may end up being a degree or 2 higher than today,
topping off in the upper 80s to low 90s, and possibly into the
mid 90s across interior portions of the Delmarva Peninsula.
Surface dewpoints will be quite similar, generally in the low to
mid 60s. As a result, heat indices will not be far off from the
air temperature. Only parts of DE could have a heat index
possibly approaching the mid 90s, if dewpoints are a bit higher.


The subtropical high bringing unseasonably warm conditions to
the region this week will be dampened by a nearly stationary
cold longwave trough anchored near Hudson Bay and a shearing
predecessor vorticity maximum progressing into the Great Lakes
and southeast Canada on Friday. Attendant cold front will be
able to progress through the Northeast and Mid-Atlantic on
Friday. Main forecast question revolves around this front`s
timing, as a faster movement would keep max temperatures cooler
for more of the region, and a slower movement would give most of
the region another toasty May day. Operational models are
generally in good agreement with a frontal passage during the
afternoon and evening hours, allowing most of the region to warm
up to well above average values again (with the Poconos and
Lehigh Valley likely to be the areas with a more noticeable
cooldown on Friday). However, there remains at least some
uncertainty with timing, with the 12Z ECMWF the quickest and the
12Z CMC the slowest with frontal passage. Thus, forecast highs
on Friday have above average chance for large errors.

The other forecast challenge for Thursday night and Friday is
chances of convection. The large-scale environment is not
particularly favorable, with the strongest ascent well north of
the area in closer proximity to the vorticity maximum
progressing into the Northeast and adjacent eastern Canada
thanks to the remaining influence of the western Atlantic
subtropical ridge. Moreover, the presence of a capping inversion
may preclude convective initiation in much of the region as the
front moves through. Kept PoPs mainly confined to the northwest
areas Thursday night (closer proximity to the front as remnant
diurnal convection wanes with the loss of daytime heating) and
south of the Mason-Dixon Line on Friday (again, in closer
proximity to the front during peak heating). Models are
indicating a weak perturbation moving through the southern Mid-
Atlantic Friday night, which may provide sufficient large-scale
lift and midlevel cooling for more convection near the front
Friday afternoon/evening, as well. Bumped up PoPs in the
southern areas Friday afternoon/evening based on this signal.
Nevertheless, the dry look of the 12Z NAM/ECMWF is not
particularly promising. Note also that frontal timing is
critical here, as slower (faster) movement would imply PoPs
spreading farther north (sinking farther south).

After frontal passage Friday, the weekend looks much cooler.
Canadian high pressure should build into the Northeast, with
onshore flow keeping temps near to slightly below seasonal
averages on Saturday and Sunday. Meanwhile, a deep large-scale
trough in the central U.S. will amplify the downstream eastern
U.S. ridge, which will gradually impede the cold front`s
progress further into the Southeast and instead instigate a
retreat northward as a warm front late this weekend. However,
the strength of the Canadian high and the attendant easterly
(onshore) surface flow should develop a stout wedge of near-
surface cold air. This will be difficult for the warm front to
overcome, and will likely delay its passage through our area
until Sunday night or Monday. As always, I suspect models are
overdoing the speed of this warm frontal passage, which has
implications on temperatures and precipitation chances Sunday
night and Monday.

The general consensus seems to be a relatively warm day Monday
with cold-frontal convection sweeping through the region during
the day. However, if a slower northward progression occurs,
parts or most of the region may be stuck on the cold side of the
boundary, with warm-advection/elevated convection Sunday night
and Monday. For now, kept PoPs highest Monday with the timing of
the cold front, but I did spread/increase PoPs into the Sunday
night time frame to indicate the potential for the
aforementioned warm frontal (warm-advection) showers. I kept
temps slightly below guidance on Monday given fears of a slower
northward surge of the warm front as well, though not very
confident of this at all.

Model timing of the cold front on Monday has sped up a tad,
which has implications on surface-based instability that builds
during the day. If the warm sector reaches the region and the
cold front`s timing is more toward afternoon/evening, there is
some chance for rather strong convection given the glancing
influence of a potent vort max moving through New England.
However, large uncertainty remains at this point.

Tuesday looks dry on the upstream side of the cold front, but a
stronger system looks to be quick on its heels, which will dig
farther southward and have a more pronounced impact on our
region midweek. For now, kept/raised PoPs on Wednesday in
anticipation of this stronger system, with the usual caveats
regarding 7+ day forecasts.


The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

VFR conditions are expected through the TAF period with high
clouds overnight and few to scattered cumulus on Thursday.

A light southwest wind is forecast overnight. The wind may
become light and variable in spots. A southwest wind around 8 to
10 knots with gusts into the upper teens is anticipated for

We have removed the mention of low level wind shear from the
TAFs for overnight. The potential appears to be marginal and
short-lived. The southwest wind may increase to around 35 knots
around and above the 2000 foot level for a couple hours around
2:00 AM.


Thursday night and Friday: Slight chances of convection,
especially KRDG/ABE Thursday night and KILG, KMIV, and KACY
Friday/Friday night. VFR outside of convection. Southwest winds
becoming northwest after frontal passage, with speeds 5-15 kts
and gusts to 20 kts, especially after frontal passage.
Confidence slightly below average.

Friday night through Sunday: Generally VFR with east or
northeast winds 10-20 kts during the day (lighter at night).
Confidence above average.

Sunday night: Sub-VFR conditions possible, with east/southeast
winds around 10 kts. A slight chance of showers. Confidence well
below average.

Monday: Sub-VFR conditions possible with scattered
showers/storms. South-southeast winds veering to west after cold
frontal passage. Speeds 10-20 kts with higher gusts near/after
frontal passage. Confidence well below average.


S-SW winds 10-15 kt. The development of an Ambrose jet locally
enhanced the winds off the NJ coast to near 20 kt this evening.
Winds weaken tonight. Conditions on Thursday should be similar
to those of today.

Thursday night: Seas approaching/exceeding 5 feet with winds
remaining just below advisory criteria. Fair weather expected.

Friday through Sunday: Sub-advisory conditions expected, with
mainly fair weather.

Sunday night and Monday: Increasing chances for showers and
storms, with seas likely exceeding advisory criteria and winds
approaching it.


This table below contains the daily temperature records for
tomorrow (5/18).


ACY    93-1987

PHL    94-1962

ILG    94-1962

ABE    96-1962

TTN    95-1911

GED    91-1974

RDG    96-1962

MPO    87-1962

Also of note for Philadelphia:
-The last 90-degree day (as reported by the ASOS at KPHL
 airport) was 9/14/2016.

-Last year, there were three consecutive 90-degree days in May

-Today was the earliest 90-degree day occurrence in the
 calendar year since 2009 (there were three consecutive days in
 the 90s that year in April from the 25th-27th).





Near Term...Iovino
Short Term...MPS
Long Term...CMS
Climate...Staff is the U.S. government's official web portal to all federal, state and local government web resources and services.