Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 220804
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
404 AM EDT Tue Aug 22 2017

.SYNOPSIS...
The Mid-Atlantic region will be located today between high pressure
that is parked offshore and a cold front that will be moving through
the Great Lakes and Midwest states. The cold front will move into
the region late tonight and slowly exit off the Delmarva coast on
Wednesday. Canadian high pressure builds into the region on Thursday
and should continue to be the primary influence on our weather into
early next week.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
Concerns last night regarding the low-amplitude vort max moving
through the region tonight seem to have been realized, as isolated
showers and even a couple of storms continue to develop in an arc
from northern to eastern Pennsylvania and adjacent portions of New
Jersey, in close proximity to the perturbation in the primarily
zonal midlevel flow. High-resolution guidance simply is of little
value with these perturbations, it seems, at least for this warm
season. With last night`s simulations basically precip-free as the
perturbation moves through, each successive run tonight either seeks
to diminish whatever convection develops after an hour or two with
little similarity to reality or depicts a completely alternative
precip evolution to the previous simulation, either of which
basically provides zero confidence in the subsequent evolution of
the convection.

Fortunately, the showers are sparse and weak, but unfortunately,
they are strong enough to tip the buckets at most sites they pass,
which means PoPs are not negligible, despite what the guidance would
have a forecaster think. In this manner, coarser guidance has been
more insightful in at least suggesting the occurrence of these
showers (though overdoing coverage/intensity, a common problem with
parameterizations and coarse grid spacing). With this in mind, the
latest NAM/GFS suggest the possibility of these sparse/weak showers
through daybreak, particularly north of the Mason-Dixon Line. Kept
slight chance PoPs across the region (given at least a couple of
simulations that generate a shower or two in Delmarva around
daybreak), with subtly higher values in eastern PA and NJ.

Patchy fog is also occurring this morning and should continue
through around daybreak. There appears to be a window of low clouds
developing around 12Z this morning as strong isentropic ascent
occurs downstream of the much stronger vort max moving through the
Great Lakes. As such, boosted cloud cover considerably in about a 4-
5 hour period, mostly north of the Mason-Dixon Line, as this ascent
peaks around daybreak.

Thereafter, boundary layer mixing should increase ceilings and
scatter the clouds out fairly rapidly mid-to-late morning as low
pressure intensifies well to the north. Southerly surface flow will
increase as the surface pressure gradient increases, and this may
allow for some gusts to around 20 mph or so, especially near the
coast. With strong mixing and (presumably) strong insolation late
this morning and this afternoon, temperatures will soar to well
above average values. Latest MOS guidance has highs in the mid 90s
in Philadelphia, but strong mixing suggests that dew points will be
considerably lower than those observed last Friday. Nevertheless,
heat indices should flirt with or exceed heat advisory criteria in
the urban corridor this afternoon. Did not change the current
advisory with this morning`s forecast package.

Aforementioned Great Lakes vort max marches east through the day. A
prefrontal trough will develop/progress into Upstate New York and
Pennsylvania this afternoon, with convection rapidly developing in a
region of focused ascent. However, this convection should remain
west of the area through the near-term period. Downstream of the
prefrontal trough, mechanisms for storm-scale ascent are limited to
orographic effects and differential heating. With stronger large-
scale ascent to the north and west of the CWA, suspect that
convective coverage in our region will be isolated/widely scattered
at best and primarily confined to areas northwest of the urban
corridor. Continued the trend of reducing PoPs this afternoon, with
chances primarily confined to north/west of I-95.

Regarding severe potential, CAPE-shear parameter space is more than
adequate for severe storms. MLCAPE could approach/exceed 2000 J/kg
this afternoon in the western CWA, with effective bulk shear of 35
to 45 kts. With a deep layer of positive buoyancy and unusually
strong veering wind profiles for this region, the potential for hail
in addition to strong/damaging winds is present. Main limiting
factor (aside from nebulous/weak ascent and a lack of near-surface
convergence) is weak midlevel lapse rates, which may prevent the
development of stronger negative buoyancy/cold pools in convective
downdrafts. Nevertheless, the ambient environment is quite
supportive of severe storms. The risk for hail and even an isolated
tornado is nonzero, especially with any storms that remain
discrete.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM WEDNESDAY/...
As the aforementioned pre-frontal trough approaches the region
tonight, a band or bands of loosely organized storms should
approach/enter the region. However, with nocturnal stabilization
commencing, the chances for severe storms are diminished in our CWA
versus points to the north/west earlier in the day. Additionally,
the vort max approaching the area will be weakening as it lifts
northeastward into New England. One more limiting factor is the
potential for fairly weak cold pools, given weak midlevel lapse
rates and a deep layer of substantial atmospheric moisture. All of
these factors suggest convection may struggle to survive as it
progresses eastward through the CWA overnight. This looks to be
especially true south of I-78, where large-scale ascent rapidly
weakens.

One potential modulating factor, however, is the nocturnal
increase of a low-level jet, which may enhance downward momentum
transport on the upstream side of linear convection. With residual
instability and strong shear profiles, high-resolution guidance may
be too quick to diminish convection after dark. One other factor to
consider: Winds will be slow to diminish after dark, which will keep
boundary layer mixing elevated. This would slow the process of
increasing convective inhibition, and models do tend to forecast
this process too quickly, in general.

With the above in mind, kept PoPs primarily slight chance south of I-
76, with much higher chances north of I-78 through the night. The
synoptic cold front should be on the doorstep late in the night, but
temperatures/dew points will be quite high in advance of it. Kept
forecast numbers close to MAV guidance, though confidence is low
given potential impacts from convection.

One other note: The progression of the convection and upstream cold
front appears to be slowing with the latest set of model guidance.
The updated forecast slows the progression of higher PoPs in
accordance with this trend. As a result, PoPs are generally
unmentionable in the far southern CWA until late in the night.

&&

.LONG TERM /WEDNESDAY THROUGH MONDAY/...
The cold front will most likely reside along or just south/east of I-
95 by daybreak Wednesday morning. The movement of the front will
then likely slow down as it progress southeastward through the
Delmarva during the day as it approaches downstream ridge blocking
over the western Atlantic Ocean. Accordingly, conditions should dry
out rather nicely on Wednesday along and especially north/west of
the I-95 corridor while locations in southeastern NJ and Delmarva
will continue to be under a chance for showers. If the front slows
down even more and some breaks in the clouds develop, there may be
enough instability to fuel thunderstorms toward the middle Delmarva
peninsula in the afternoon. Consensus from the latest suite of model
guidance keeps the more favorable environment for severe storms just
south of Delaware. An isolated strong to severe storm and heavy rain
producers is possible if the front gets hung up north.

High pressure centered over central Canada midweek shifts
southeastward into the Great Lakes Thursday through Saturday, then
eastward into southeastern Canada/Northeast U.S. Sunday and Monday.
This expansive high will control our regional weather pattern across
the Mid Atlantic during this time.

Barring any major changes to the synoptic pattern, the upcoming
Thursday-Monday period could potentially be only our second
(virtually) rain-free stretch that spans at least five days in
what has otherwise been a very wet summer. I say "virtually rain
free" because 1) cannot completely rule out an isolated shower
Sunday-Monday with the trough axis of the upper low expected to
move through at a time when onshore flow could act to moisten
the low-levels and 2) 8-13 June, which was the other 5+ day dry
stretch this summer, was technically not rain free as MPO
reported 0.01 inches of rain on both 10 June and 13 June. The
last time all eight of our climate sites have not recorded any
measurable rainfall for at least five consecutive days was 2-6
March 2017 (honorable mentions: 15-19 May except for 0.02 inches
at MPO on 15 May and the previously mentioned 8-13 June
period).

Below normal temperatures can also be expected Thursday-Monday
with highs in the mid 70s-lower 80s and lows in the 50s- lower
60s. This will provide very comfortable temperatures for outdoor
activities.

&&

.AVIATION /08Z TUESDAY THROUGH SATURDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.

Patchy fog has developed at KRDG, KABE, and KILG with VSBYs
occasionally sub-VFR. This should continue through 12Z, and
KMIV/KACY may see restrictions as well. Additionally, showers that
developed near the Philadelphia metro may encourage temporary lower
VSBYs the rest of the night in the urban corridor. Any fog should
dissipate rapidly after 12Z.

Meanwhile, guidance continues to suggest a brief period of sub-VFR
CIGs about an hour or two surrounding 12Z, but confidence is low.
Thereafter, VFR is likely through the day, with chances for storms
gradually increasing north/west of KPHL during the afternoon. Not at
all clear if storms will affect the urban corridor tonight, but the
chances are high enough for at least a PROB30 mention after 00Z.
Latest model guidance is coming in somewhat slower with the arrival
time of convection, so subsequent updates to the TAFs may require
some adjustments in this direction.

Outlook...
Wednesday...Predominately VFR. Any leftover showers should be
isolated and end by midday for I-95 terminals, N and W. Farther S/E
toward MIV and ACY, chances for showers and even a thunderstorm
could linger through the afternoon as a cold front slowly progresses
offshore. Localized/brief restrictions possible in thunderstorms.
Winds veer from W to NW during the morning with speeds 5-10 kt.

Wednesday night through Saturday...VFR. Generally light winds from
the N or NW, possibly becoming N-NE on Saturday.

&&

.MARINE...
No changes to the marine headlines this morning, as south to
southwest winds should continue to increase today and tonight on the
waters. Speeds should reach advisory criteria by mid-afternoon and
continue through tonight. Gusts may approach/exceed 30 kts during
the evening hours. Seas should range from 4-6 feet this afternoon
and tonight.

There is a chance of showers and thunderstorms tonight, with the
best chances in the New Jersey coastal waters. Winds and seas will
be locally higher near any storms that occur.

Outlook...
Wednesday...Did not extend the SCA into Wednesday as available wave
guidance shows seas in our coastal waters falling below 5 ft by
daybreak. However, will still leave the door open for an extension
into the morning if seas take a bit longer to subside. Winds change
direction out of the NW and decrease steadily during the day behind
cold fropa.

Wednesday night through Saturday...Winds and seas below SCA
criteria.

RIP CURRENTS...
The rip current risk is currently forecast to be moderate. Short-
period swells have developed with the increased southerly winds
on the waters early this morning. Guidance suggests that an
underlying longer-period swell will be present. However, the
short-period swell should be dominant, with winds generally
parallel to the coast. Thus, it is likely that the rip current
risk will remain in the moderate category today and tonight.
However, this will be reevaluated in the next couple of hours.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...Heat Advisory from 1 PM this afternoon to 8 PM EDT this
     evening for PAZ070-071-102-104-106.
NJ...Heat Advisory from 1 PM this afternoon to 8 PM EDT this
     evening for NJZ010-012-015-017>019.
DE...Heat Advisory from 1 PM this afternoon to 8 PM EDT this
     evening for DEZ001.
MD...None.
MARINE...Small Craft Advisory from 3 PM this afternoon to 6 AM EDT
     Wednesday for ANZ450>455.

&&

$$
Synopsis...Klein
Near Term...CMS
Short Term...CMS
Long Term...Klein
Aviation...CMS/Klein
Marine...CMS/Klein



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