026 
FXUS61 KPHI 151714
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
114 PM EDT Sat May 15 2021

.SYNOPSIS...
A large area of surface high pressure will remain near the region 
into late next week. A trough in the upper levels of the atmosphere 
will cross over the region Sunday into Monday.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 100 PM, cumulus development continues as the convective 
temperatures have been reached away from the coast. So far, some 
showers have been focused mostly on terrain features with even a 
hint at some development now in interior southern New Jersey near a 
weak surface trough. The air mass is rather dry, especially on the 
12z Sterling/Dulles, with more moisture on the Upton RAOB in the mid 
levels. Temperatures are now in the 70s, with the exception of the 
coast and parts of the Poconos where it is cooler. The overall flow 
is light and the vertical growth of the showers may be such to not 
result in charge separation, and showers should tend to be slow 
movers (pulsing). Made some tweaks to the hourly grids to keep them 
current based on observational trends, and also made some 
adjustments to the PoPs. Opted to remove the southern extent of the 
thunder mention given less moisture the farther south one goes.

A couple of additional short waves will move across the area 
overnight, however instability will wane after sunset with loss of 
daytime heating. Therefore, any shower activity is not expected to 
last much longer after sunset for most of the area with some mid 
level clouds persisting across parts of the area for awhile.

&&

.SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/...
A pair of mid level short wave troughs embedded in the larger 
northwesterly flow will cross our region during this period. The 
first, which is expected to cross Sunday afternoon into Sunday 
evening will likely be the deeper of the two, but even that trough 
should be filling and weakening by the time it reaches our region. 
None the less, this trough will probably provide the best chance for 
showers in our region through the next week. In addition to broader 
lift associated with the trough, some low level lift will be 
possible along a sea breeze which is likely to develop during the 
afternoon. One factor with how widespread rain will be will likely 
be moisture, as models depict precipitable water values below 
average for this time of year.

On Monday, a weaker mid level short wave trough will cross the 
region. Lift with this trough is expected to be even weaker than the 
Sunday trough, so expect more limited coverage of rain on Monday.

&&

.LONG TERM /TUESDAY THROUGH FRIDAY/...
Broad mid level ridge should slowly build over the eastern U.S. 
through this period. Consequently the surface high will likely build 
west closer to the mid level ridge axis. There is still some 
question as to where the ridge axis will develop as well as if the 
late week trough northeast of our region and the surface high over 
the Hudson Bay will have much impact on the ridge over our region. 
With the 00Z runs, models were further east with the late week 
trough (previous depicting it over far eastern Canada, now more over 
the northern Atlantic), and depicted the Hudson Bay high further 
north. Consequently, going just by the operational models, a late 
week backdoor cold front in our region looks less likely. That being 
said, I'm hesitant to jump on a single model run as there is going 
to be some uncertainty any time we have such a significant pattern 
shift. Therefore, stayed close to a blend of guidance with a blend 
of the previous forecast. In general, we should see mostly dry 
conditions, though the closer we are to the periphery of the ridge, 
the more likely we are to see rounds of rain, with a warming trend 
through at least Thursday. If the cold front stays north of our area 
on Friday, then the warming trend could continue through the
week.

&&

.AVIATION /17Z SATURDAY THROUGH WEDNESDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, 
KMIV, KACY and surrounding areas.

This afternoon...VFR overall. A couple of showers around this 
afternoon (very isolated thunder cannot be ruled out). Variable 
winds around 5 knots, becoming southeast to south-southwest up to 10 
knots. A few gusts to around 15 knots possible mainly at MIV and ACY 
due to a sea breeze. Low confidence on a shower or thunderstorm 
impacting any terminal.

Tonight...An early shower around, otherwise VFR ceilings at times. 
Southeast winds 5-10 knots early, becoming light and variable to 
locally calm. Moderate confidence.

Sunday...VFR ceilings around. A few showers or a thunderstorm are 
possible especially from midday on. Light and variable winds, 
becoming southwesterly 5-10 knots. An afternoon sea breeze should 
lead to south or southeast winds 10-15 knots mainly at ACY and MIV. 
Low confidence if any showers/storms move over a terminal, and the 
timing and progression of the sea breeze.

Outlook... 

Monday...Mostly VFR conditions are expected. There is a slight 
chance for showers Monday afternoon which could result in temporary 
ceiling and visibility restrictions if any directly impact a TAF 
site, but coverage is expected to be quite limited. Winds will be 
close to southerly, with south southwesterly more likely for the 
Delaware Valley and SE PA TAF sites (KPHL, KILG, KPNE, KTTN, KABE, 
and KRDG), while the NJ coastal TAF sites (KMIV and KACY) are more 
likely to be south southeasterly. Wind speeds should be 5 to 15 kt 
during the day, decreasing below 10 kt overnight. Moderate 
confidence on most aspects of the forecast, but low confidence if 
any showers will directly affect a TAF site.

Tuesday and Wednesday...VFR conditions are expected. Winds starting 
light and variable, eventually settling out of the southwest with 
wind speeds less than 10 kt. Moderate confidence.

&&

.MARINE...
Winds and waves are expected to remain below advisory levels today 
and tonight. Our friends at the USCG like to remind everyone 
enjoying the water in the Spring that water temperatures are 
still in 50s.

Outlook...

Sunday through Wednesday...winds and seas should stay below SCA 
criteria through this period.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...None.

&&

$$
Synopsis...Johnson
Near Term...Gorse/Robertson
Short Term...Johnson
Long Term...Johnson
Aviation...Gorse/Johnson/Robertson
Marine...Johnson/Robertson