000 
fxus61 kphi 230134 
afdphi 


Area forecast discussion 
National Weather Service Mount Holly New Jersey 
934 PM EDT Mon may 22 2017 


Synopsis... 
weak low pressure in southeast Virginia will move east of New 
Jersey later tonight while a weak cold front in central 
Pennsylvania crosses our area. 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 on Wednesday. 
Stronger low pressure organizes in the Ohio Valley Wednesday 
night sending its associated fronts through the mid Atlantic 
states early Friday. Then high pressure follows on Saturday. 
However another storm system should develop over the nations 
midsection Sunday, threatening to Mar a portion of Sunday or 
Memorial Day with a period or two of showers. 


&& 


Near term /until 6 am Tuesday morning/... 
showers have moved offshore. A few lingering pop up showers are 
possible going through tonight as a weak cold front over central 
Pennsylvania tracks east and passes through the region late 
tonight. Otherwise, cloudy and humid with abundant low level 
moisture due to the rainfall that fell today. 


Some fog has developed, mainly in the valleys, and in southern 
and eastern New Jersey where the heaviest rain fell. Despite 1/4sm visibility 
at Acy and mpo, as well as at blm earlier this evening, do not 
expect widespread dense fog to form. Therefore, will not issue a 
dense fog advisory. A Special Weather Statement may be issued, 
however, if more areas of fog develop. For now, will generally 
cap vsbys as low as 1sm. 


Depending on when that weak front passes through, ceilings and 
fog would lift prior to daybreak, as opposed to after daybreak 
with heating of the day. 


Overnight lows will generally range from the mid/upper 40s north 
of I-80 to the mid 50s for most of New Jersey and southeast PA, and near 60s 
for the Delmarva. 


&& 


Short term /6 am Tuesday morning through 6 PM Tuesday/... 
the frontal boundary that moves across the area overnight is 
expected to stall to our south. An area of low pressure is 
forecast to move along this boundary from the south, as the 
first low pressure lifts to the northeast. The low pressure, nor 
the frontal boundary, is expected to make its way back into our 
area during the day, however there will be a chance of showers 
to lift across portions of the area later during the day on the 
north side of the low. The greatest chance for showers will be 
across southern Delmarva and southern New Jersey. With no 
instability forecast, we do not expect any thunderstorms across 
our area. 


&& 


Long term /Tuesday night through Monday/... 
12z NAM was not applied for this discussion. It was not off to 
a good start today. 


Hazards: it seems almost inevitable that some sort of Hydro 
related products will eventually be needed by next Monday in 
this pattern, but whether its more than nuisance flooding flss 
is debatable. All we know is that widespread 1.5 to 2.5" 
occurred in less than 12 hours over parts of the Delmarva and 
southeast New Jersey today with a fairly large area of 3+ in southeast 
New Jersey. Suspect isolated Max amt of 4" today with small streams and 
rivers still rising late today. Legacy Dix stp was best with 
considerable low bias in dox sta/stp. Mrms was also low. 


500mb: a sharpening and unusually strong high amplitude trough 
in the nations midsection will close 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 somewhat Sunday and Monday, 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. We did modify guidance temps downward Thursday and 
Sunday in response to ggem and European model (ecmwf) cooler pattern. 


The dailies... 


Forecaster confidence is below average on details the entire 
period. 


Tuesday night through Wednesday...depending on the path of the 
next coastal...a pretty good chance of rain far southeast and 
probably dry north where a nice Wednesday is expected. Sref precipitable water 
is down. 


Wednesday night and Thursday...looks wet in warm air advection. 
Questionable how much warming can occur with what looks to be a 
negative tilt southern extension of the trough moving toward 
our area and causing a triple point low to approach through PA. 
That also may permit a better chance for isolated thunderstorms. 
Sref precipitable water back up to 1.75". 


Friday...partly sunny and a breezy west wind should develop 
with afternoon warming and only a small chance of a residual 
shower in the north. 


Memorial Day weekend...a good start is anticipated Saturday. 
However, Sunday and Monday should see a period or two of 
showers and possible thunderstorms. Still plenty of uncertainty 
but its not looking to be a picture perfect weekend. 


&& 


Aviation /01z Tuesday through Saturday/... 
the following discussion is for kphl, kpne, kttn, kabe, krdg, kilg, 
kmiv, kacy and surrounding areas. 


Below average forecaster confidence for tonight through Tuesday 
morning. 


Generally speaking, an IFR forecast for low cigs on tap for at 
least most of tonight. Some terminals are already reporting LIFR 
conditions, and kacy has 1/4sm visibility in fog. 


Fog will continue to develop, but despite IFR/LIFR cigs, do not 
expect much in the way of IFR or lower vsbys. Vsbys should stay 
at MVFR, for the most part. 


Depending on when that front comes through, conditions may begin 
to lift prior to daybreak. The lav guidance has the low cigs, so 
will trend with that forecast. But there may be amds needed 
should conditions lift earlier than expected. 


Light and variable winds through the overnight. As the occluded 
frontal boundary moves across the area, winds will become 
north-northwest to north-northeast. Speeds through the next 24 
hours are expected to be 5-10 knots or less. 


Outlook... 


Forecaster confidence on details is below average through Friday 
morning. 


Tuesday night...VFR conditions north and possibly the entire area 
but a risk of MVFR/IFR conditions in showers vicinity 
kmiv/kacy. East to 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 ceilings with probable periods of MVFR/IFR conditions 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 25 kt possible 
in the afternoon. 


Saturday...VFR. Westerly wind. 


&& 


Marine... 
winds and waves will remain below Small Craft Advisory levels 
tonight through Tuesday. Wind directions will begin out of the 
southeast this evening and overnight, before becoming north- 
northwest to north-northeast late tonight into Tuesday as an 
occluded front moves across the area. 


Areas of fog expected on the waters, but think for the most 
part, vsbys should be greater than 1nm. There is the potential 
for fog to reduce vsbys to locally less than 1 nm through 
Tuesday morning, but for now, confidence remains low, and will 
not issue a marine dense fog advisory. 


Outlook... 


Forecaster confidence on this outlook section is below average 
Tuesday night and Wednesday then above average thereafter. 


Tuesday night and Wednesday...may need an Small Craft Advisory for hazardous seas 
for the Delaware Atlantic coastal waters. Otherwise northeast to east 
wind with gusts generally under 20 kt. 


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


Friday...Small Craft Advisory may continue for leftover hazardous seas on the 
Atlantic portion of our coastal waters. Westerly wind develops 
with nearshore gusts to 20-25 kt. 


Saturday...marine headlines not likely. 


&& 


Hydrology... 
hazards: it seems almost inevitable that some sort of Hydro 
related products will eventually be needed by next Monday in 
this pattern, but whether its more than nuisance flooding flss 
is debatable. All we know is that widespread 1.5 to 2.5" 
occurred in less than 12 hours over parts of the Delmarva and 
southeast New Jersey today with a fairly large area of 3+ in southeast 
New Jersey. Suspect isolated Max amt of 4" today with small streams and 
rivers still rising late today. Legacy Dix stp was best with 
considerable low bias in dox sta/stp. Mrms was also low. 


&& 


Tides/coastal flooding... 
astronomical tides are at some of their highest of the year 
this Wednesday and Thursday evenings. Some onshore flow will 
contribute positive surge departures and its probable that a 
few locations along the Atlantic coasts of Delaware and New Jersey will exceed 
the minor coastal flood advisory threshold. It only takes about 
6 to .7 feet surge above high tide to reach the threshold. 
It's all in the details, which are yet to be confidently 
determined. The sit and etss modeling, forecasts minor with a 
small chance of a moderate episode if, prior to high tide, 
onshore flow is strong enough and for enough duration. Right 
now...no action needed. 


&& 


Climate... 
as of 5:25 PM EDT, a record rainfall of 1.85" was recorded at 
Acy. This breaks the old record of 1.19" set in 1909. 


&& 


Phi watches/warnings/advisories... 
PA...none. 
New Jersey...none. 
Delaware...none. 
Maryland...none. 
Marine...none. 


&& 


$$ 
Synopsis...drag 
near term...Robertson/mps 
short term...Robertson 
long term...drag 



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