000 
fxus61 kphi 240148 
afdphi 


Area forecast discussion 
National Weather Service Mount Holly New Jersey 
948 PM EDT Mon Apr 23 2018 


Synopsis... 
a large area of high pressure will shift slowly east of the 
region overnight. A low in the lower Mississippi Valley will 
lift into the mid Atlantic states by Wednesday, with widespread 
rain arriving across the region in advance of the system. 
Another low pressure system passing to the south of our region, 
could bring another chance of rain late this week. By late in 
the weekend, high pressure begins to build south towards the 
region. 


&& 


Near term /until 6 am Tuesday morning/... 
quiet conditions will continue tonight across the area as high 
pressure remains offshore of the New England coast. Our area 
will remain under the influence of southeast return flow, which 
will allow moisture to increase some overnight. Overnight lows 
will drop through the night as winds diminish and mostly clear 
skies will begin the night. However, with the flow being more 
from the southeast, and the fact some clouds may begin moving in 
late in the night, overnight lows are not expected to be as 
cool as Sunday night. Most places should drop close to normal, 
except portions of The Pine Barrens which might drop a few 
degrees lower and have some patchy frost develop. We will 
include patchy frost in the forecast, but not issue a frost 
advisory yet as there is still uncertainty as to how low the 
temperatures will get. 


&& 


Short term /6 am Tuesday morning through 6 PM Tuesday/... 
dry conditions will begin the day Tuesday, before rain chances 
increase through the day. An occluding low pressure system will 
continue moving across the southern Appalachians early in the day, 
before slowing moving into the Carolinas by the afternoon. Our area 
will be well north of the low pressure system and the slowly lifting 
warm front through the daytime hours. However, south-southwest flow 
aloft will lead to increasing moisture through the day. A couple of 
short wave/vorticity impulses will lift across the area through the 
day as well. Rain chances increase, especially during the afternoon 
as the second of the short waves moves through and the best 
moisture/lift combination takes place. There is little to no 
instability forecast, so no thunderstorms are expected at this time. 


Winds will become gusty through the morning and into the afternoon 
with gusts reaching 20 to 25 mph at times. 


&& 


Long term /Tuesday night through Monday/... 
overview: the main focus through this period remains on the 
Tuesday night and Wednesday time frame and the potential for 
heavy rain (though this threat appears to be decreasing). 
After that, the next chance for rain looks to be primarily on 
Saturday as our region will be in between a low off the coast 
and a cold front approaching from the west. 


Details: the low continues to very slowly progress northeast, 
reaching the mid Atlantic by Wednesday. The window for heavy 
rain has narrowed a bit, now looks to be confined mainly late 
Tuesday night, though lingering light rain is possible into 
Wednesday evening. 


As for the heavy rain threat, precipitable water values on 
Tuesday night are still expected to be well above normal, 
possibly in the 90th percentile for this time of year. However, 
there are a few factors working against the heavy rain threat. 
The warm cloud layer is relatively shallow (generally 8000 to 
9000 ft), thanks to continued below normal temperatures. 
Additionally, storm motions aren't expected to be particularly 
slow, around 20 mph, although training storms could make this 
point moot. In general, it looks like the highest threat for 
heavy rain will be over the coastal plains of Delmarva and far 
southern New Jersey. 


There is some very limited elevated instability Tuesday night, 
but it is very elevated (generally for parcels at or above 850 
mb). Thus, have not mentioned thunderstorms in the forecast for 
now, but will continue to monitor this potential as we get 
closer. 


For the late week period, there remains some uncertainty in how 
two low pressure systems, one lifting out of the southeast and 
becoming a coastal low and one digging out of the north central 
U.S., Will interact and what the impact will be for our region. 
However, the models today are in better agreement than what we 
have seen the last few days, with a brief chance for rain on the 
back side of the coastal low on Friday night or Saturday, and a 
limited impact from the cold front moving in from the west. 


&& 


Aviation /02z Tuesday through Saturday/... 
the following discussion is for kphl, kpne, kttn, kabe, krdg, kilg, 
kmiv, kacy and surrounding areas. 


Tonight...VFR conditions will continue overnight. South- 
southeast winds around 5-10 knots will become light and 
variable most places overnight. 


Tuesday...VFR conditions will begin the day across the taf sites 
through at least midday Tuesday. However, clouds will thicken and 
lower through the afternoon, and could eventually reach MVFR by late 
in the afternoon or early evening. Rain will also be moving from 
the south during the afternoon hours. 


Outlook... 


Tuesday night and Wednesday...MVFR and even localized IFR 
conditions will be possible through Wednesday morning as 
periods of rain are likely across the region. Easterly wind 
gusts up to 20 kt are possible. Moderate confidence on flight 
categories, but low confidence on the timing of any flight 
category changes. 


Thursday...VFR conditions expected with light westerly winds. 
High confidence. 


Friday and Saturday...mostly VFR conditions are likely, though 
there is a chance that rain and low clouds could lower 
conditions to MVFR at times. Wind is forecast to be westerly or 
southwesterly near or below 10 kt. Moderate confidence on 
flight categories and wind. 


&& 


Marine... 
conditions will remain below advisory levels tonight and into the 
first half of Tuesday. However, winds could begin gusting around 25 
knots by the afternoon, while seas are expected to build to 5 feet 
or higher by the afternoon as well. Therefore the Small Craft 
Advisory remain in place starting noon Tuesday. 


Outlook... 


Tuesday night and Wednesday...easterly and southeasterly winds 
increase to 15-20 kt with gusts up to 30 kt, and seas build up 
to 10 feet on the Atlantic coastal waters. Wind gusts near 25 kt 
are also expected on the Delaware Bay primarily Tuesday night. 


Wednesday night and Thursday...winds shift to northwesterly and 
should decrease. 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 this. 


Friday and Saturday...winds and seas are currently forecast to 
stay below Small Craft Advisory criteria on all waters. However, there remains 
some uncertainty with if a coastal low develops and intensifies. 
If this happens, there is a slight chance for Small Craft Advisory criteria, 
primarily on Saturday. 


&& 


Phi watches/warnings/advisories... 
PA...none. 
New Jersey...none. 
Delaware...none. 
Maryland...none. 
Marine...Small Craft Advisory from noon Tuesday to 6 PM EDT Wednesday 
for anz450>455. 
Small Craft Advisory from noon Tuesday to 2 am EDT Wednesday 
for anz430-431. 


&& 


$$ 
Synopsis...Johnson/99 
near term...99 



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