Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 230731

Area Forecast Discussion
National Weather Service Mount Holly NJ
331 AM EDT Mon Apr 23 2018

Large high pressure over the Northeast United States will
continue to move east today and then move off the New England
coast tonight. Low pressure over the Gulf Coast states will lift
to the north and east and affect the Mid-Atlantic region during
the mid-week period. Another low may affect the region late in
the week, followed by a cold front next weekend.


A midlevel vort max will be moving offshore this morning while
an upper low over the Mid-South drifts eastward into the
Tennessee Valley today. In between, pronounced midlevel ridging
will seep eastward into the Mid-Atlantic. This will permit
mostly sunny skies and warmer conditions today (as midlevel
heights and 1000-500 mb thicknesses increase). GFS/NAM BUFKIT
soundings show about 2-3 degrees of warming today versus
yesterday with similar mixing heights. Surface temperatures were
about 3-6 degrees warmer than the simulated values as depicted
in these soundings on Sunday. A blend of MAV/MET guidance
represented these displaced values well for today (and are in
excellent agreement, in general). Combined this with a blend of
the previous forecast and 2-m HRRR-simulated temperatures for

Dew points tanked yesterday during the afternoon, which was
poorly simulated by virtually all models (a typical bias this
time of year). Model soundings indicate mixing that reduces dew
points about 2-4 degrees below a statistical/consensus blend. I
dropped dew points by this amount in the grids for today,
basically everywhere. This may not be enough in some spots and
will likely require some monitoring/editing during the day.

In addition, the sea (bay) breeze will again develop and move
onshore today. It appears high-resolution simulations were
somewhat too aggressive in moving the sea/bay breeze onshore on
Sunday, but there are a couple of differences in today`s
environment. First, surface temperatures will be warmer,
allowing for a stronger temperature gradient from the ocean to
land. Second, synoptic-scale flow will permit a return of
southerly/southeasterly flow, which should allow the sea/bay
breeze to move inland more readily. So, the forecast used a
blend of the more aggressive WRF-ARW/NMM to the somewhat slower-
to-materialize HRRR. This will again have impacts on temps/dew
points during the afternoon, particularly in southern/eastern NJ
and coastal Delaware, which was also considered in the forecast


The upper low in the Tennessee Valley will continue to trudge
toward the Appalachians tonight, with downstream ridging
progressing through the area tonight. A predecessor vort max
will progress into the central/southern Mid-Atlantic late
tonight, and attendant lift will allow mid and upper-level
clouds to increase late tonight in southern/western portions of
the CWA.

In advance of the increasing clouds, relatively clear skies and
light winds will allow temperatures to drop readily during the
evening hours. Therefore, even with the increasing cloudiness
allowing for the temperature drop to slow/stop after midnight, I
went a little below the consensus statistical blend for lows
tonight given its predilection for being too conservative with
the nocturnal cooling (especially of late). Think the potential
for patchy frost exists again in the usual trouble spots (Pine
Barrens; valley locations to the west/northwest of the Philly
metro), but this will certainly be more limited than the past
few nights. Forecast lows are about 3-5 degrees warmer than
those seen early this morning.

Models keep precipitation in association with the upper low and
the predecessor vort max to the south/west of the CWA through
the night. Slowed the onset of mentionable PoPs considerably
late tonight as a result. I also slowed the onset of increased
cloudiness to some degree, with the thinking that models are
probably on the aggressive side with the movement of the low
(typical bias with these setups).


Low pressure will be over central Kentucky and Tennessee
Tuesday morning, and a warm front will extend out ahead of it,
and that warm front will be just southwest of the Delmarva. That
warm front lifts to the north and east during the day Tuesday,
and developing rain ahead of that front will lift into the
Delmarva, southeast Pennsylvania, and southern New Jersey in the

Upper trough with closed low digs into the Tennessee Valley in
the evening, and a secondary low forms off the Southeast U.S.
coast Tuesday night. This system lifts along the coast Tuesday
night, and then it appears as if the 2 systems merge over the
Delmarva Peninsula Wednesday morning. With strong onshore flow,
PWATs will range from 1.25" to 1.50" after midnight Tuesday
night, and as a result, there may be a period of heavy rain.

Low pressure lifts to the north and east during the day. Dry
air will build along the coast in the afternoon, and rain tapers
off from south to north by Wednesday evening.

From there, unsettled weather looks to continue as an upper low
passes through the region. Will keep chance PoPs in the
forecast Thursday night and Friday.

For the weekend, the CMC-GDPS has low pressure affecting the
region while the ECMWF has a low forming as a cold front passes
through the region, but the low stays well offshore. The GFS is
similar to the ECMWF. Will carry chance PoPs in the forecast
through Saturday.

High pressure builds back into the region for the start of the
new week.


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

Today...VFR with winds becoming southerly or southeasterly
around 5 to 10 kts. Sea/bay breezes will develop and move inland
this afternoon, with somewhat stronger south to southeast winds
once the boundary passes. These boundaries will pass
MIV/ACY/ILG during the afternoon and may reach the Philly
terminals late in the day. High confidence in VFR; moderate
confidence with winds.

Tonight...VFR with increasing mid/upper cloudiness late. Winds
becoming light and variable. High confidence.


Tuesday through Wednesday...MVFR/IFR conditions, especially
Tuesday night/Wednesday morning as periods of rain will develop
across the region. East wind gusts up to 20 kt are possible
Tuesday night into Wednesday. Otherwise, generally light winds
(less than 10 kt) are expected. Moderate confidence on flight
categories, but low confidence on the timing of any flight
category changes.

Thursday and Friday...Generally VFR conditions, but scattered
showers could result in occasional MVFR conditions. West winds
on Thursday eventually shift to the south on Friday. Low
confidence on flight categories and wind.


Winds/seas will remain below advisory levels through tonight,
but southeast winds will be increasing through the period,
becoming 10 to 20 kts this afternoon and tonight with gusts
approaching advisory levels late tonight. Seas will slowly build
to 2-4 feet through the period.

There is some chance of patchy fog early this morning and again
late tonight, with some visibility restrictions possible but
expected to be fairly localized.


Tuesday and Wednesday...East to southeast winds will increase
to 15- 20 kt with 25-30 kt gusts, and seas build up to 10 feet
on the Atlantic coastal waters. Wind gusts to 25 kt are also
expected on the Delaware Bay. Winds shift to the west and
diminish to 10-15 kt late Wednesday. Seas remain elevated.

Wednesday night and Thursday...Winds shift to the northwest.
The main question will be how long seas remain above 5 feet on
the coastal waters. Current forecast is for elevated seas into
late Thursday, but there is still considerable uncertainty with

Friday...depending on if a coastal low develops, we may see
increasing southeasterly winds. If this happens, we could once
again reach SCA criteria.


Dew points were quite low on Sunday, allowing relative humidity
values to drop well below 30 percent across the area for a
lengthy period of time during the afternoon. Though winds were
generally light, the combination of low relative humidity and
fairly dry fuels allowed for the development of some wildfires.
Similar conditions will exist today. Winds will be below red
flag warning thresholds, but much like Sunday, sea and bay
breezes will develop and move inland during the day. As these
boundaries pass, winds may shift and increase rather suddenly.
For any preexisting fires, the passage of the sea/bay breezes
may cause erratic conditions affecting fire growth/spread.

Should winds be stronger than projected, a special weather
statement will likely be required indicating the increased
potential for wildfires.


MARINE...Small Craft Advisory from noon Tuesday to 6 AM EDT Wednesday
     for ANZ450>455.
     Small Craft Advisory from noon Tuesday to 2 AM EDT Wednesday
     for ANZ430-431.


Near Term...CMS
Short Term...CMS
Long Term...MPS
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