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fxus61 kphi 300136 
afdphi

Area forecast discussion
National Weather Service Mount Holly New Jersey
936 PM EDT Wed Mar 29 2017

Synopsis...
high pressure builds in from the north tonight and Thursday before
moving offshore Thursday night. Low pressure in the Southern Plains
will move northeastward to the Ohio Valley by Thursday night, then
off the mid Atlantic coast by Saturday morning. High pressure will
return for the end of the weekend. Another low pressure system will
arrive later Monday night and Tuesday, then high pressure briefly
returns on Wednesday.

&&

Near term /until 6 am Thursday morning/...
an upper air analysis this evening shows a trough in northern New
England, a ridge from the Tennessee Valley northward to the Great
Lakes, then a robust trough in the central to Southern Plains. The
latter is helping to build the downstream ridge, placing strong
surface high pressure near Hudson Bay. There is also considerable
850 mb warm air advection indicated into portions of the southern
Midwest this evening.

High pressure centered near Hudson Bay this evening will continue to
build southward into the northern mid Atlantic through the
overnight. This along with a cooling boundary layer will allow for
less wind and cooler temperatures. Meanwhile, the aforementioned
warm air advection in combination with a 250 mb jet positioned to
our north will help pull some high level clouds across our area
especially late. This may thin eastward for awhile as the ridge axis
remains to our west. We still indicate some cloud cover arriving
through the overnight from west to east.

For the 930 PM update, the hourly temperature, dew point and wind
grids were adjusted based on the latest observations. Some locales
are experiencing a quicker drop in temperature thus far, therefore
some adjustments were needed. There is drier low-level air advecting
in from the north, therefore the dew points for many areas were
lowered a little faster.

&&

Short term /6 am Thursday morning through 6 PM Thursday/...
the center of the high tracks into eastern Canada on Thursday,
pulling an upper level ridge onto the eastern Seaboard late in the
day. Meanwhile, low pressure continues to organize over the Central
Plains and Midwest. The upper ridge should keep precip at Bay for
the daytime hours, but high level clouds initially will then lower
and thicken throughout the day.

Temperatures should be just shy of normal, topping off in the mid
40s up north to the mid and upper 50s elsewhere.

&&

Long term /Thursday night through Wednesday/...
long term looks wet, with potential for at least two systems
producing substantial precipitation accumulations for the
region.

The first system looks to affect the region beginning Thursday
night. Potent vort Max in the Ozarks region 00z Friday moves into
the Ohio Valley by 12z, acquiring a negative tilt. Impressive
difluence downstream and upper-level jet coupling from a retreating
anticyclonic jet streak in New England and a southern-stream
cyclonic jet streak nosing into the southeast will provide a
prolonged duration of strong/deep ascent along much of the eastern
Seaboard. Warm-air advection precipitation should break out Thursday
night across the mid-Atlantic. Residual northeast flow from the
departing surface high in southeast Canada and New England should
allow a cold wedge of near-surface air to seep well south into the
County Warning Area east of the Appalachians. Mesoscale model guidance is
consistently showing a freezing rain signature in the southern
Poconos Thursday night. A pronounced warm nose is present above the
near-surface cold(er) layer, with little change in the thermal
profile from Thursday night through Friday evening. Temperatures
will be flirting with the freezing mark much of this time, and
models often are too warm in such regimes, especially in the
southern Poconos (it seems). There is large uncertainty
remaining, however, given the marginal cold air near the
surface. Given the expected quantitative precipitation forecast (more on that below), there is
potential for quite a bit of ice accumulation, especially on
elevated surfaces, near Mount Pocono and in adjacent Sussex
County, New Jersey. Then again, very little may occur at all if the
boundary layer stays on the warmer side. Instinct is telling ME
that at least an advisory will be warranted for Carbon, Monroe,
and Sussex counties during this time frame. However, plenty of
time to figure out the details, as this remains 30-60 hours out.

Regarding the rest of the area, the dynamical nature of the vort Max
and the sustained southerly fetch downstream of it imply widespread
and substantial quantitative precipitation forecast are likely in the County Warning Area Friday. A south-southeast 50+ kt low-
level jet will advect a considerable amount of moisture (pwats well
in excess of 1 inch) into the mid-Atlantic during the day Friday.
With the vort Max approaching the region during the afternoon,
substantial differential cyclonic vorticity advection combined with
low-level isentropic lift along a pre-existing baroclinic zone
suggests widespread moderate to heavy rainfall for an 18-hour window
(generally 12z Friday to 06z saturday) across the entire region.
There remain some discrepancies among the model guidance,
particularly regarding the degree of moist advection (the NAM being
noticeably drier) and the locations with maximum quantitative precipitation forecast (consensus
being in a corridor from southeast PA to northern/central nj), but the
strength of the system and the associated lift give relatively high
confidence in 1+ inch quantitative precipitation forecast across the region, with potential for
localized 2-3 inch totals. To this point, instability looks
limited/negligible across the area, so kept thunder out of the
grids. However, some hydrologic issues may occur if the
stronger- seeming model simulations verify. Will allude to this
potential in the severe weather potential statement.

Residual wraparound showers may occur in PA/New Jersey through Saturday
morning, but the surface low should be well offshore by this point.
Winds will switch to northerly, but the southern origins of the
system suggest temperatures will fall little after system passage.
Additionally, there may be some downsloping impeding any cold air
advection that may be present.

High pressure builds in Sunday and Monday, and this period should
generally be dry. Temperatures will warm to seasonal or slightly
above seasonal values.

Clouds will once again be on the increase Monday as the next
southern-stream system advances to the East Coast. The track of this
system continues to look more southwest to northeast (from the mid-
south to the Ohio valley), which suggests a warmer scenario compared
to the Friday/Saturday low. Another substantial fetch of moist air
will precede the system, and the closer proximity of the warm sector
suggests a higher probability for convection. Timing/track
uncertainties remain, with the 12z GFS a noticeable flat/north
outlier compared to the CMC/ECMWF. Both of these latter models
produce high amounts of quantitative precipitation forecast across the southern County Warning Area (given the
southward displacement of the surface low track), but there remain
discrepancies between these two models regarding the nature of the
precipitation (with the CMC providing a prolonged period of
isentropic lift along a zonally-oriented warm front, whereas the
European model (ecmwf) indicating more influence from pre-cold frontal
convection, at least in delmarva). Meanwhile, the GFS definitely
has more of a warm-sector precipitation scenario. My suspicion
is that the southern solutions make more sense, but too far out
and too much run-to-run variability to feel very confident. Did
raise pops across the region given the strong signal with
precip/timing among the model suite. As with the end-of-week
system, substantial quantitative precipitation forecast looks possible with this next low.

After a brief dry period Wednesday, models suggest another system
affecting the region by the end of next week.

&&

Aviation /02z Thursday through Monday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg,
kilg, kmiv, kacy and surrounding areas.

Tonight...VFR. Northerly winds near 10 knots, becoming light and
variable later this evening and overnight at most terminals.

Thursday...VFR. A ceiling at or above 10000 feet will develop mainly
in the afternoon. Variable winds near 5 knots becoming northeast in
the morning, then turning southeasterly.

Outlook...
Thursday night...VFR ceilings lowering, potential to MVFR toward
daybreak Friday as some rain arrives. East or southeast winds less
than 10 knots.

Friday and Friday night...sub-VFR cigs/vsbys and rain likely. East
to southeast winds 10 to 20 kts with higher gusts near the coast
becoming more northerly late Friday night. Confidence average.

Saturday through Sunday night...mainly VFR with mostly north or
northwest winds at or below 10 kts. Exception will be Saturday, with
speeds up to 20 kts (gusts up to 25 kts) possible, especially near
the coast. Confidence above average.

Monday...generally VFR with winds around 10 kts becoming more
easterly. Increasing cloudiness likely. Confidence above
average.

&&

Marine...
high pressure builds across the waters tonight through
Thursday. North winds may gust to 20 knots on the ocean waters late
tonight through Thursday morning, but small craft advisories
should not be needed.

Outlook...
Friday and Friday night...advisory-level winds/seas likely.
Gale- force gusts possible, especially off the New Jersey coast.
Rain and visibility restrictions likely.

Saturday and Saturday night...advisory winds/seas likely. A chance
of rain early on Saturday.

Sunday and Monday...sub-sca winds expected. Seas may remain somewhat
elevated early in the day Sunday.

&&

Tides/coastal flooding...
though astronomical tides will be gradually diminishing through the
week now that we are past the new moon, the threat of minor tidal
flooding along the New Jersey and Delaware Atlantic coasts increases by Friday. We
currently have high tides about 0.5 feet above the astronomical
tide. We expect this to continue through Thursday. Beginning
Thursday night, a low pressure system will bring a prolonged period
of onshore flow, further increasing the surge. The tide of most
concern is still the high tide Friday evening and Friday night,
particularly along the northern and central New Jersey shore. By
this tide cycle, it will take a surge of 0.8 to 1.0 feet to reach
minor flooding thresholds, which is likely. However, not sure yet if
we will have another 0.3 feet surge to reach advisory thresholds.
The exact magnitude of the surge will be dependent on how quickly
the onshore flow develops and how strong it will be by Friday. Etss
shows water levels at Lewes and Sandy Hook touching minor flooding
thresholds with the high tide cycles tonight and Thursday night.
This is unlikely though as with the expected wind direction, we
should not see a surge any higher than what we currently have at
least through Thursday night.

&&

Phi watches/warnings/advisories...
PA...none.
New Jersey...none.
Delaware...none.
Maryland...none.
Marine...none.

&&

$$
Synopsis...cms/gorse
near term...gorse

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