Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000
FXUS61 KPHI 211436
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
936 AM EST Tue Nov 21 2017

.SYNOPSIS...
A cold front will cross the area Tuesday night and early Wednesday,
followed by high pressure into Saturday. Another cold front is
expected to cross the area Saturday night, with high pressure
anticipated for the first part of next week.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
Morning forecast mainly on track. Will adjust hourly grids based
on latest surface obs and satellite trends, but no significant
changes.

Otherwise, digging vort max in the Upper Midwest will continue
southeastward through the Great Lakes region today. Downstream,
midlevel ridging will amplify in response to low-level warm-air
advection expanding the atmospheric column. Predecessor
perturbations in the flow will progress through the East today,
though these should have generally minimal impacts on sensible
weather other than increasing high clouds, especially this
afternoon. As the surface ridge to our south continues offshore
today, southwesterly flow will continue, getting a boost from
diabatic heating and a strong surface low moving through Ontario
(aiding in an increased surface pressure gradient). As such,
expect breezy conditions again today, especially along/southeast
of the urban corridor, as areas to the northwest will be in
somewhat of a surface col by afternoon.

Statistical guidance shows good agreement, but I suspect there
is a bit of a cool bias with temperatures today given the
warming profiles and the amplified ridging. Hard to go against
good agreement among the guidance, but I did nudge up
temperatures a degree or two in spots from the inherited
forecast. It is quite possible that Philly reaches 60 today,
with overall max temperatures about 2-7 degrees above seasonal
averages.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM WEDNESDAY/...
A perturbation over the Southeast will lift northeastward
during the day, likely in vicinity of the Carolinas by 21Z.
Precipitation should break out near this vort max, with a
surface wave likely developing during the day near or off the
coast of Florida. This southern-stream system will be affected
strongly by the northern-stream vort max, with some phasing
likely to occur during the overnight period, and rapid
development/northeastward progression of the surface low
near/off the Carolina coast. However, despite the overall
excellent agreement in the large-scale pattern, the NAM/GFS
depictions of the southern-stream low leave some question marks.
The GFS looks a little on the fast/progressive side, which is a
typical bias it exhibits in the short term. Comparison to the
RGEM suggests it might be on the order of 3 hours too fast, and
this difference may be enough to keep the GFS too dry tonight.
It will be critical to watch the evolution of the southern-
stream system today (and the precipitation/upscale feedback
processes) to determine if the GFS is keeping us too dry
tonight.

Meanwhile, the NAM looks very similar to the RGEM in the
overall evolution of the system tonight, but the NAM is simply
drier than the RGEM (another bias observed on multiple occasions
this season). Based on this (and the GFS bias), I weighted
guidance that was slightly slower/wetter (e.g., the NAM Nest and
the WRF- ARW, along with the RGEM) somewhat higher. Also
included some of the 00Z ECMWF with the QPF grids tonight,
despite its coarser resolution, as it seems to be doing a good
job this fall depicting the cutoff of the poleward side of the
precipitation somewhat better than the NAM/GFS (likely because
of its general slower/deeper depictions of these phasing
systems).

The northern-stream upper-level jet streak will be in a favorable
position by night`s end for rather deep ascent, but by this point,
the stronger lift will probably be offshore. Nevertheless, mesoscale
banding of the precipitation is clearly evident in most of the model
simulations tonight, and this does occur as far west as I-95 in most
of these simulations. This makes the QPF a challenge, since these
setups can be underdone in the models (both with precipitation rates
and the slow progression of these phenomena compared to the speed of
deep-layer flow). This was another reason I upped the QPF from the
previous forecast, but tempered to some degree versus the wettest
guidance (SREF/ECMWF).

With the above in mind, brought PoPs up to chance generally urban
corridor southeastward and to likely southeast of a Dover, DE, to
Sandy Hook, NJ, line. The timing of the precipitation looks to be
mainly after midnight, but there is some uncertainty here given the
discrepancies mentioned earlier.  Amounts generally in the tenth to
quarter inch range along/southeast of I-95 to potentially a half
inch near the coast, but confidence in these values is low because
of the uncertainty with the exact track of the precipitation. Also,
the cutoff of the precipitation will likely be sharper than I have
depicted in the grids (an inevitable result of precipitation
forecast uncertainty). Bottom line: Large errors in the PoPs/QPF are
possible tonight given the remaining model discrepancies.

Temperatures were generally a blend of MET/MAV/ECS MOS, though I did
nudge temperatures up a little, in general given the increased
clouds and remaining low-level south/southwest fetch. Looks like
temperatures will be too warm for non-liquid, non-freezing
precipitation anywhere in our CWA through daybreak.

One final note: Given the strengthening dynamics aloft, strong lift
will aid in midlevel cooling possibly enough to generate some
marginal instability. Not totally out of the question to see a
couple lightning strikes, though the best chances are offshore.

&&

.LONG TERM /WEDNESDAY THROUGH MONDAY/...
The extended period starts off with a cold front near or along the
coast Wednesday morning, moving east. Chillier air, normal for this
time of year, will move in behind this front. With ridging aloft
over the western U.S. and troughiness in the east, a seasonably cool
but mostly dry pattern is expected through most of the long term
period, with a chance of precipitation later Saturday and Sunday as
another cold front and associate deep upper level trough crosses the
mid-Atlantic region.

The fropa early Wednesday will trigger a few showers N/W of PHL. A
sfc low pressure is also forecast to develop off the NC/SC coasts
with moisture spreading north ahead of it along the DE/NJ coastal
areas. The highest PoPs continue over southern NJ and Delmarva,
counter to climo. Overnight temps should remain warm enough so that
all precip will be in liquid form. However a few showers may linger
past sunrise, and with cold advection behind the front a few snow
showers or flurries are possible in the Poconos Wednesday morning.

The weather for Wed afternoon through Friday night will be dominated
by high pressure moving across the mid-Atlantic region. Temperatures
will be a few degrees below normal on Thursday but should return to
near normal by Friday as the high shifts offshore and a more swly
flow develops. A cold front may push south in to NY state and New
England on Friday but probably will not affect our area to the south,

Another fast-moving frontal system is forecast to push through our
area on Saturday. In part because of its fast movement it will not
be able to pick up much moisture. The current forecast continues
chance PoPs, higher N/W of PHL, but even this may be overdone.
Again, temps on Saturday appear to be warm enough to avoid any
frozen precip. However, the airmass behind this front seems to be
colder than the one mid-week, with strong low level cold advection
beginning Saturday night and continuing through the rest of the
weekend. This pattern of NW flow will favor lake effect snow with
some streamers possibly reaching the Poconos and vicinity.

&&

.AVIATION /15Z TUESDAY THROUGH SATURDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.

Today...VFR with southwest winds 5 to 15 kts with occasional
gusts to 20 kts or so, especially from the I-95 corridor
southeastward. High confidence.

Tonight...Rapidly increasing clouds, possibly to sub-VFR,
especially closer to the coast. Showers should spread from
southwest to northeast through the area after 06Z Wednesday,
mainly confined to the urban corridor southeastward. For now,
confidence only high enough for mention in the MIV/ACY TAFs as a
prevailing condition. Sub-VFR VSBYs/CIGs are possible with the
heavier showers, but confidence on timing and how far inland the
precipitation reaches is very low, so kept TAFs generally VFR
through the period for now. Winds generally southwest around or
below 10 kts, with some veering to westerly expected by
daybreak. Medium confidence with winds; low confidence with
CIGs/VSBYs.

Outlook...

Wednesday...Rain possible south and east of I-95 early Wednesday
morning with MVFR or lower conditions...then becoming VFR. SW
winds 5-10 KT become NW Wednesday afternoon and increase to
10-20 KT.

Wednesday night and Thursday...VFR. NW winds gradually diminishing.
A storm system is currently expected to remain offshore. Forecast
confidence: Medium to High.

Friday...VFR. NW winds 5-10 KT.

Saturday and Saturday night...VFR early...then MVFR or lower
conditions possible in rain showers later in the day and at night.
SW winds 10-15 KT.

&&

.MARINE...
Small craft advisory in effect for Delaware Bay and the
Delaware Atlantic coastal waters through today and for the New
Jersey coastal waters through this evening as strong southwest
winds continue/increase today. Winds should diminish overnight,
especially over Delaware Bay and the adjacent Atlantic waters,
though confidence is not particularly high given the potentially
brief duration of these lighter winds.

Showers should spread northeastward overnight (generally after
midnight), with localized restrictions possible. Expect erratic
gusts and locally very choppy seas in vicinity to showers. A
lightning strike or two is not out of the question, though the
probability is too low for inclusion in the forecast at this
point.

Outlook...

Wednesday...Lull in SCA conditions possible Wednesday morning.
Otherwise, winds shift to the NW and increase to 15-20 KT with 25-30
KT gusts.

Thursday through Friday...Sub-SCA conditions expected.

Saturday and Sunday...SCA conditions are possible with the passage
of a strong cold front.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...Small Craft Advisory until 4 AM EST Wednesday for ANZ450>453.
     Small Craft Advisory until 10 PM EST this evening for ANZ454-
     455.
     Small Craft Advisory until 7 PM EST this evening for ANZ430-
     431.

&&

$$
Synopsis...Miketta
Near Term...CMS/MPS
Short Term...CMS
Long Term...Miketta
Aviation...CMS/Miketta/MPS
Marine...CMS/Miketta/MPS


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