textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Minor adjustments to the timing of the storms today - highest risk is now through early this evening.

There is an increasing risk for patchy fog overnight into Sunday morning.

KEY MESSAGES

1. Widespread showers and a few storms are expected today. There is a marginal (1 out of 5) risk that a few storms could be strong to severe.

2. Fog is possible across the region overnight into Sunday Morning.

3. Several opportunities for much needed rainfall from late Sunday through next week.

DISCUSSION

KEY MESSAGE 1...Widespread showers and a few storms are expected today. There is a marginal (1 out of 5) risk that a few storms could be strong to severe.

The weak and diffuse warm front has made it through much of our region as of mid day Saturday. That being said, the highest risk for severe storms still appears to be primarily over the coastal plains of South Jersey and Delmarva that saw some clearing earlier today and thus has had temperatures increase as high as the mid 70s.

As of 2 PM, we are starting to see scattered showers develop. So far, only one storm has had any lightning activity with it (in central Jersey), but with still a few more hours of diurnal heating, would expect some more showers to develop into thunderstorms.

Interestingly, much of the shower and storm activity appears to be initiating off a weak boundary (I suspect it is an outflow boundary from the morning rain showers), that lies to the southeast of the I-95 corridor. North and west of this boundary, winds have shifted in some areas out of the northwest, which could slightly limit the instability in these areas.

No significant changes in the overall nature of the severe threat. Even in the coastal plains, CAPE values are likely to be near or just above 500 J/Kg. So overall not a lot of instability. However, winds aloft will be fairly strong, leading to 0-6 km shear values around 40-45 kt. The primary hazard of concern is strong winds, but small hail is also possible.

All-told, QPF from this event looks fairly light, generally less than one quarter inch, though higher amounts will be possible in any thunderstorms. Showers and storms look to exit the area by late evening.

Key message 2...Fog is possible across the region overnight into Sunday Morning.

With light winds tonight and some residual ground moisture from rain today, the development of fog is likely. Many of the model soundings depict fog, especially northwest of the I-95 corridor. Have included a mention of patchy fog across the area.

Right now, the risk for dense fog or widespread fog is relatively low. However, if clouds clear out earlier than currently forecast, or if winds drop off earlier, that would increase the risk for those threats.

KEY MESSAGE 3...Several opportunities for much needed rainfall from late Sunday through next week.

An upper-level trough is forecast to dominate across much of the East to end this weekend and continue right through next week. Some guidance is much more potent with the trough amplification later next week with a closed low developing in the vicinity of the Mid- Atlantic and then moving off to the northeast. This upper-level pattern will keep the extended portion of the forecast unsettled.

Our first system brings a cold front through the area tomorrow night into Monday morning. Rain showers start to develop tomorrow afternoon into the evening with some isolated thunderstorms possible given some instability present. Overnight Sunday into Monday morning, some guidance shows more a stratiform band of rain developing ahead of the front with the best coverage being near and southeast of the I-95 corridor. An isolated thunderstorm will remain possible into early tomorrow night as instability decreases into the overnight. Behind the front, we trend drier into Monday afternoon.

After a brief dry period late Monday into Tuesday, the active weather pattern looks to reload as another upper-level trough quickly replaces the first one across much of the East. This feature may become closed off in the vicinity of the Mid-Atlantic Wednesday through Friday. Beyond Friday, some guidance moves the closed off upper low off to the northeast into Saturday.

As the pattern turns unsettled again, rain showers develop and increase in coverage late Tuesday night into Wednesday with a surface low advancing into the region. These showers continue through Wednesday night. In terms of any isolated thunderstorms, this will be dependent on how much instability is present. For Thursday into Friday, if the upper trough closes off and we get a surface low off the coast of the Mid-Atlantic, then this would lead to more showers across the area and lower temperatures due to the onshore flow. Uncertainty remains for the end of the week as we get towards the end of the extended forecast period.

AVIATION /18Z SATURDAY THROUGH THURSDAY/

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

Through 00Z...Prevailing VFR. Isolated TSRA is possible. The highest risk for TSRA directly affecting a TAF site is at KMIV and KACY. For KPHL, KPNE, KTTN, and KILG, there is a risk for TSRA in the vicinity, primarily to the south and east, but the risk directly over the TAF site is relatively low. MVFR or lower visibilities are possible in any heavier showers or storms. South-southwest winds 10-15 kt, with some gusts around 20 kt. Moderate confidence.

Tonight...Storms should clear out by or just after 00Z, leaving VFR conditions to start the overnight period. There is an increasing risk for fog overnight, particularly after 06Z. There is some risk across the region, but the highest risk for IFR and even LIFR conditions is at KRDG and KABE at this point. Winds quickly diminishing to light and variable by 01-03Z with calm periods likely overnight. Moderate confidence overall, low confidence in the extent of restrictions from low stratus or fog.

Sunday...Once any morning fog clears out by 15Z, prevailing VFR conditions likely. 20 to 40% chance for SHRA and TSRA after 18Z. If any SHRA or TSRA move over a TAF site, ceilings and visibilities could briefly drop to MVFR. Winds out of the west near or below 10kt through much of the day. Moderate confidence in the overall pattern, low confidence in any one TAF site being directly affected by SHRA or TSRA.

Outlook...

Sunday night through Monday...A period of sub-VFR conditions probable Sunday night into Monday morning with showers. A few thunderstorms possible into early Sunday night. Conditions should improve through the day Monday.

Tuesday...VFR.

Wednesday through Thursday...Sub-VFR conditions possible along with showers.

MARINE

A Small Craft Advisory is remains in effect for all NJ and DE Atlantic coastal waters and the lower Delaware Bay and continue through 12 AM Sunday. South winds 15-20 kt with gusts around 25 kt are expected through this evening. Seas are expected to be 3-5 feet. In addition, showers and a few storms could contribute to hazardous marine conditions. Overnight, winds and seas are expected to subside below SCA levels and continue through the day on Sunday.

Outlook...

Monday...A brief period of Small Craft Advisory conditions possible.

Tuesday...The conditions are anticipated to be below Small Craft Advisory criteria.

Wednesday...A period of Small Craft Advisory conditions possible.

Thursday...The conditions are anticipated to be below Small Craft Advisory criteria.

PHI WATCHES/WARNINGS/ADVISORIES

PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory until midnight EDT tonight for ANZ431- 450>455.