Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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439
FXUS61 KPHI 271647
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
1247 PM EDT Mon Mar 27 2017

.SYNOPSIS...
Northeastward moving low pressure from the Upper Great Lakes to
Missouri will pull a warm front northward through the Delmarva and
southern New Jersey today. The warm front should stall across
southeastern Pennsylvania and central New Jersey, awaiting the
passage of the Missouri low, through New Jersey, Tuesday evening.
Thereafter, Canadian high pressure will build into our area
Wednesday and Thursday. Low pressure in the Ohio Valley on Friday
will move east and off the mid Atlantic coast on Saturday. Weak
high pressure should follow on Sunday.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
1230 pm update: Quite a few changes to the forecast this
afternoon. Warm front is making very sluggish progress
northward, with 16Z placement roughly along a line from KBWI to
KMJX. Models continue to be way too aggressive moving this
northward, and am beginning to wonder if there will be much
progress north of the Mason-Dixon Line given close proximity of
an upstream shortwave trough and associated pressure falls that
may promote backing of flow to a more easterly component with
time. For now, only edged temperatures downward a little bit in
the northern CWA, but thinking the I-95 corridor northwestward
to the Lehigh Valley may be in "bust territory" with forecast
max temps today. Wind forecast needed considerable adjustment as
well, given the switchover to southwest winds south of the front
has not occurred as far north as forecast to this point. Will
continue to monitor temps/winds closely this afternoon.

Think most of the precipitation is done for the daytime hours,
with the band of rain having just pushed offshore. However, hi-
res guidance continues to hint at the developed of an isolated
shower or two, and there have been recent indications that
shower development may occur in the warm sector (i.e.,
Delmarva), where some instability may be present. If these
trends continue, I may be inclined to add isolated storms to the
grids this afternoon.

930 am update: Cancelled dense fog advisory as area webcams and
surface obs indicated visibilities generally over a quarter
mile, with only localized patches of lower visibilities
remaining (especially near the New Jersey coast). This can be
handled with nowcasts and special weather statements for the
rest of the morning.

Radar is showing a decent band of light to moderate rain from
the western suburbs of New York City to Philadelphia to Dover
moving east into New Jersey. This rain will quickly eradicate
any remaining patchy dense fog as it moves through. Hi-res
guidance generally has this precipitation moving out of the area
by noon, so adjusted PoPs/Wx grids for this timing.

Transient shortwave ridging will move into the region this
afternoon, but residual cold air aloft and mixing to the south
of the poleward-bound warm front may generate a few isolated
showers this afternoon. Forecast soundings look a little less
enthused about thunder chances this afternoon compared to how
they looked yesterday, so no mention for this afternoon.
Meanwhile, given weak subsidence aloft, not overly optimistic
about precipitation chances in general for the CWA after midday.
Lowered PoPs during this time frame.

Speaking of the warm front, it is beginning to make progress
northward through Delmarva this morning. Models, as usual, are
poorly forecasting its position/movement, generally way too
aggressive with its northward progress. Kept temps/dew points on
the low side of guidance, particularly in the north CWA, today.
Suspect further modifications will be required through the day
given the poor verification of guidance in general.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM TUESDAY/...
1230 pm update: Hi-res guidance has been consistently indicating
elevated convection moving through the CWA after 09Z, with
potential for isolated/scattered storms developing ahead of the
main band in portions of SE PA and northern/central NJ. Model
soundings show marginal buoyancy for parcels rooted in the
900-800 mb layer. If trends continue, will need to add thunder
chances to the grids tonight. Did raise PoPs north of the
Mason-Dixon Line during the overnight hours, and may need to
raise them more based on the latest simulations.

Also made considerable changes to the temp/wind forecast based
on latest guidance, particularly given the uncertainties in
frontal placement this afternoon having domino effects through
the evening hours. At least patchy fog looks possible across
most of the area, particularly if winds back to a more easterly
direction, as the latest guidance is keying in on.

Previous discussion...

Hopefully the warm front will have moved far enough away to have
a lull in the precip for the evening. More showers will arrive
overnight as the next short wave approaches from the W/SW. Pops
increase back to the chc range overnight, favoring slightly the
srn NJ and Delmarva areas. Mild with lows in the low 50s S/E and
mid/upper 40s N/W. Light winds expected.

&&

.LONG TERM /TUESDAY THROUGH SUNDAY/...
500 MB: A split flow regime will continue across the United
States during this period. A short wave crosses Pennsylvania
Tuesday and then phases with a northern stream system into a
closed low over the Maritimes Wednesday night. The next southern
stream short wave in the pipeline over the Mississippi Valley
Thursday night, weakens eastward off the mid Atlantic coast
Saturday. Yet another southern stream short wave will be moving
eastward from the lower Mississippi Valley early next week.

Temperatures: Calendar day averages should be about 5 degrees above
normal Wednesday, near normal Thursday and normal to 5 degrees
below on Friday, and then normal or several degrees above on
Saturday and Sunday.

Forecast basis: 50 50 blended 00z/27 GFS/NAM MOS Tuesday-
Wednesday, then 00z/27 MEXMOS Wednesday night-Thursday and
thereafter WPC guidance Thursday night-Sunday.

The dailies...

Tuesday...The position of the warm front Tuesday will probably
result in a wide range of temperatures across our area with
large error potential I-78 region northward where global models
are warm but the ever improving 12km NAM and its downscaled 3KM
temperatures, are much cooler. DREARY north of that warm front
with patchy dense fog possible and a likelihood of heavy showers
during the afternoon and evening as the Missouri low cuts east
along the Mason Dixon line, heading for a NJ seaward exit
Tuesday evening. There could be afternoon and evening
thunderstorms with this short wave. Within the warm sector in
advance of the cold front, we expect sunshine to break out south
of the warm front with temperatures reaching the upper 70s. A
low- level moist tongue will also be in place across this area,
ML Cape (400-800J), along with negative Lifted and Showalter
Indices, within an environment characterized by poor lapse rates
and weak shear. Thunder is expected but despite the warm
front,right now SVR is not. Do follow later PHI and SPC
discussions. Precipitable water values around 1.25 inches,
should yield some heavy showers near the path of the low.
Movement should be east northeast and there could be some
training and backbuilding along the warm front so that this may
need to be monitored for narrow bands of poor drainage street
flooding rain Tuesday afternoon/evening. Winds light northeast,
north of the warm front (near I-78) and southerly in the warm
sector Delmarva and s NJ. Confidence: above average except the
location of the warm front and the associated temps near the
warm front.


Tuesday night...Showers still may be heavy in the evening (still
thunder possible?), then end from west to east late and the
wind turns north everywhere. Confidence: above average.

Wednesday...a gusty northerly flow to 25 mph of cold air
advection. Dry. Confidence: above average.

Thursday...A sunny start, then increasing clouds and cool ahead
of the next storm system. Light wind. Confidence: above average.

Friday...Increasing model consensus that this actually looks
like a decent little mid Atlantic coastal storm with one half to
1.5" of qpf, and currently, only a slight chance of a little
wet snow near and north of I-80 since models have trended
slightly northward.

Friday evening tides may exceed the minor advisory threshold
along the NJ Atlantic coast provided the sfc low passes south
of ACY.

overall Friday confidence: above average.,


Saturday...Precipitation(mainly rain) ends, but clouds may
linger since there doesn`t seem to be much in the way of high
pressure following the low. Confidence: average

Sunday...Considerable cloudiness. Low confidence on what might
transpire this day.

&&

.AVIATION /18Z MONDAY THROUGH SATURDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.

Several modifications to the TAFs this morning to account for
faster timing of the precipitation moving through the region.
General IFR conditions continue this morning north of the
Mason-Dixon Line with rapid improvement to MVFR/VFR in Delmarva
and far southern New Jersey (including KMIV/KACY). Expect
general improvement to VSBYs after rain passes the I-95
corridor by 15Z, but much more gradual improvement to CIGs.
Expecting MVFR conditions to occur by mid-afternoon, but very
low confidence in timing. VFR conditions likely most of the day
at KMIV/KACY. Any improvements observed this afternoon will be
short-lived, as sub-VFR conditions will return by late this
evening across the region.

OUTLOOK...

Confidence: Above average all days.  Exception being the details of
the warm frontal position Tuesday and timing of poorest conditions
where there is less than average confidence.

Tuesday...IFR or MVFR conditions Tuesday morning at all TAF sites,
with improvement to VFR by afternoon at all but ABE, RDG, and TTN
where IFR may may linger all day.  Showers become widespread in the
afternoon/evening with light northeast flow at ABE, RDG, TTN and
light south or southeast flow elsewhere.  Isolated Tstms expected.

Tuesday night...MVFR possible early in showers, otherwise becoming
VFR late.  Winds becoming north everywhere late.

Wednesday...Mostly VFR. north wind gusts 20-25 kt.

Thursday...VFR.  Light north wind.

Friday...IFR or MVFR conds in periods of rain. Mostly east wind
ahead of the low.

&&

.MARINE...
1230 pm update: Dense fog advisory has expired, as visibilities
have improved on the waters. Rain is currently moving through,
and the warm front has pushed north through most of the marine
zones. South winds will increase to around 10 kts this
afternoon, but should remain well below advisory thresholds.
Seas are slowly dropping, and look to be below advisory
threshold by mid-afternoon. Small craft advisory may be
cancelled early.

Previous discussion below...

Seas remain elevated off the New Jersey coast, so the small
craft advisory remains in effect until 7 pm.

Beach webcams and satellite continue to indicate poor
visibilities on the coastal waters, so the dense fog advisory
continues through noon for all marine zones. There is a chance
that fog may persist all day off the northern/central New Jersey
coast, but improvement is expected by early this afternoon on
Delaware Bay and off the Delaware coast. Another night of fog
potential exists tonight.

OUTLOOK...

Tuesday...Fog may be marine hazard? otherwise no SCA expected at
this time. Thunderstorms could occur, especially late in the
day. Confidence: average.

Tuesday night...SCA northerly flow may develop late. Confidence:
average.

Wednesday...Northerly flow SCA. Confidence: above average.

Thursday...No headlines. Confidence: above average.

Friday...SCA potential as low pressure heads for the mid
Atlantic coast. Confidence: above average.

&&

.CLIMATE...
Georgetown 3/28 RER max is 80 set in 1960.

PHL temperatures continue to project near 1 degree F below
normal for the month as a whole.

&&

.EQUIPMENT...
KNEL appears to be reading 10F too warm the past couple of
days.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...Small Craft Advisory until 7 PM EDT this evening for
     ANZ450>453.

&&

$$
Synopsis...Drag
Near Term...CMS
Short Term...CMS/O`Hara
Long Term...Drag
Aviation...CMS/Drag
Marine...CMS/Drag
Climate...Drag
Equipment...Drag



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