Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 221607

Area Forecast Discussion
National Weather Service Mount Holly NJ
1207 PM EDT Mon May 22 2017

Weak low pressure is developing on a warm front from the
Delmarva Peninsula into southern New Jersey today. A cold front
approaching from the west will merge with the warm front just
off the mid Atlantic coast tonight. Low pressure in North
Carolina Tuesday will pass east of the mid Atlantic coast
Tuesday night. Weak high pressure is left behind over our area
Wednesday. Stronger low pressure organizes in the Ohio Valley
Wednesday night sending its associated fronts through the mid
Atlantic states late Thursday or early Friday as the low moves
into New England. High pressure should follow for the weekend.


High pressure offshore of Cape Cod will slowly drift eastward
through today, which will keep a steady onshore flow through the
afternoon. A warm front to our south will slowly drift
northward through the day as a cold front approaches from the
west. Meanwhile, southwest flow aloft will continue to keep
moisture over running the warm front. Periods of showers will
continue to move across the area through the day as a couple of
short wave/vorticity impulses traverse the area. Instability
still remains forecast as little to none, but we will maintain
the isolated chance of thunderstorms as there is some very weak
instability aloft, and it is possible for a few isolated
lightning strikes.

The main threat today will continue to be the possibility for
periods of moderate to heavy rainfall across the area. PW values
are forecast around 1.5-1.75 inches across the southwestern
two-thirds of the area, especially Delmarva and southern New
Jersey. So there is the potential for localized flooding,
especially in poor drainage areas and especially if any
thunderstorms develop. It still looks like there will be a sharp
drop off in rain totals north and west of I95.

Expect rain to taper off rather quickly late this afternoon as
the cold front approaches the region.


Low clouds are expected to persist through at least the first
half of the night until dry air advection dominates in the wake
of the cold front. Depending on how much rain areas see this
morning, some fog may develop as well, but the low clouds should
inhibit widespread fog development.

Models have trended slightly later with the arrival of the cold
front, as a result expect low clouds to linger a bit longer, and
thus there will be a shorter opportunity for efficient
radiational cooling, especially for the Coastal Plains.
Therefore, temperatures from the I95 corridor to the east may
not drop off that much overnight.


DRAFT DRAFT DRAFT for the 330 PM discussion:

**Flood potential may exist where its rained more than 2 inches
 today in SNJ and the Delmarva**

12Z NAM not applied for this discussion. Its not off to a good
start for today.

500mb: 500mb: A sharpening and unusually strong high amplitude
trough in the nations midsection will closed off and weaken as
it moves to new England Friday, with ridging to follow this
weekend. The next trough will be organizing in the Great Lakes
region early next week.

Temperatures: Calendar day averages Wednesday-Saturday near
normal warming Sunday and Monday to nearly 5 above normal,
ahead of the next trough.

Forecast basis: a 50 50 blend of the 12z/22 GFS/NAM MOS for
Tuesday night through Wednesday night,  then the 12z/22 GFS
MEX MOS for Thursday with the 15z WPC guidance for Thursday
night-next Monday. Any substantial deviations from the basis
above, will be noted:

The dailies...

Forecaster confidence is below average on details through Friday

Tuesday night through Wednesday...Depending on the path of
the next coastal...a risk of rain south and probably dry north.

Thursday...Looks wet in warm air advection.

Friday...After any early morning showery rains depart with the cold
frontal passage...a breezy west wind should develop with
afternoon warming.

Memorial Day weekend...Warming will continue in the lower levels
with a strengthening west to southwest flow as high pressure
starts to build in from the southwest. Showers may approach from
the west late Monday but for now...we`ll consider that possibility
as a low probability of occurrence prior to sundown Monday
evening the 29th.


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

IFR conditions will continue across the area through the day
today. KABE and KRDG remain MVFR, but they are also expected drop
to IFR during the next couple of hours. The heaviest showers
are expected through 18Z, but chances for rain exist through
00Z. There is a slight chance for thunderstorms but it is too
uncertain at this time to include in the TAFs.

Even as the precipitation begins to taper off after 18Z, expect
the low clouds to linger through the rest of the day time hours.
If there are breaks in the clouds late this afternoon into the
evening hours, patchy fog may develop.

Conditions should begin to improve after 06Z behind a cold front
which will bring in drier air.

DRAFT for the 330 PM issuance.

Forecaster confidence on details is below average Tuesday night
through Friday morning.

Tuesday night...VFR conditions north and possibly the entire area
but a risk of MVFR/IFR in rain vicinity KMIV/KACY. Light
northeast wind.

Wednesday...VFR except possible MVFR/IFR conditions during the
morning KMIV and KACY. East to northeast wind gusts to 15 mph.

Thursday...VFR CIGS with periods of MVFR/IFR in showers and isolated
thunderstorms. easterly winds probably becoming southeast to
south at night.

Friday...Improving conditions after any early morning showers end
with mainly VFR expected. Westerly wind gusts to 20 kt possible
in the afternoon.

Saturday...VFR.  Westerly wind gusts to 20 kt.


Although some gusts above 20 kt are possible this evening, we
are expecting conditions to remain below SCA criteria today and
tonight. By this evening, fog may develop on both the Delaware
Bay and the coastal waters. However, it is uncertain at this
time how widespread or dense any fog will be. Any fog that does
develop is expected to dissipate in the pre-dawn hours as
northerly winds increase.

Draft for the 330 PM issuance.

Forecaster confidence on this outlook section is below average
Tuesday night through Friday morning.

Tuesday night and Wednesday...Leftover SCA for hazardous seas
possible...mainly the DE Atlantic coastal waters. Otherwise
northeast to east wind with gusts generally under 20 kt.

Thursday...SCA probable for the well organized Ohio Valley low.
East to southeast winds build the seas to b between 5 and 8
feet on the Atlantic waters.

Friday...SCA may continue for leftover Hazardous seas on the
Atlantic portion of our coastal waters.  Westerly wind develops
with gusts to 20 kt.

Saturday...Small chance an SCA for westerly nearshore gusts of
20 to 25 kt.


1137AM: DOX and DIX STA (DP)/STP (LEGACY) are underestimating
rainfall. Ditto MRMS. We are using FFMP HPE. PNS for 1.75 inch
or greater amts will post soon as we are seeing numerous reports
of 1.5 to 2.5 inch rainfall since 230 this morning. At this
time...DIX STP (Legacy is performing best vs its DP). DOX
STP/STA are almost identical and far too low.

The Flood Advisory updated statement (FLS) will post soon. At
1PM we will consider extending the NJ portion and cancel the MD
DE portion.

The Flood Advisory for generally nuisance potentially travel
slowing/altering poor drainage street flooding continues. This
advisory is because of overrunning along and just north of the
warm front that shows light southerly flow in the warm sector
and light northeast flow just north of the boundary. PWAT is
near 1.75 inches. 12z NAM is off to a poor start.


DRAFT for 330 PM discussion:

Astronomical tides are at some of their highest of the year this
Wednesday and Thursday evenings. Some onshore flow will contribute
positive departures and its probable that a few locations along
the Atlantic coasts of DE and NJ will exceed the minor coastal
flood advisory threshold. It only takes about .6 to .7 feet
above high tide to reach the threshold. It`s all in the details,
which are yet to be confidently determined. This mornings
12z/22 NAM does not loom to be off to a good start. We will
eight its guidance less than other traditionally used models.
for now the SIT and ETSS modeling forecasts minor with a small
chance of a moderate episode if prior to high tide onshore is
strong enough for enough duration.


Record daily rainfall at ACY has a good chance of exceedance.
1.19 inches is the daily record set in 1909. as of 1205 PM 1.13
inches...appears well on its way to a record for this date.




Near Term...Johnson/Robertson
Short Term...Johnson
Long Term...Drag/Meola
Tides/Coastal Flooding...
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