textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

No significant changes.

KEY MESSAGES

1. Widespread showers and a few storms are expected today. There is a low risk that a few storms could be strong to severe.

2. 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 low risk that a few storms could be strong to severe.

Large scale troughing is in place across much of the eastern CONUS this morning, with a shortwave axis currently centered across the Ohio Valley region. This shortwave will pivot eastward today towards our region. As it does so, a diffuse warm front will lift through the area late this morning into this afternoon. As these features approach the region, a broad area of ascent will lead to widespread showers traversing much of the region from late morning into this afternoon. Much of this activity is expected to be confined to eastern PA and northern NJ where the best forcing will be located. Widespread clouds and showers will keep temperatures in this region in the upper 50s to lower 60s here.

Farther south where forcing will be a bit weaker, scattered showers and a few thunderstorms still appear likely to develop. Widespread cloud cover is expected to limit the degree of heating, with temperatures only climbing into the upper 60s to lower 70s. With this being said, cool temperatures aloft will allow some destabilization to occur. MLCAPE values are expected to generally be in the 250-500 J/kg range across far southeastern PA, southern NJ,and the Delmarva. Winds aloft will be fairly strong, leading to 0-6 km shear values around 40-45 kt. Given the meager instability, the severe threat does appear to be fairly limited. However, a few severe storms cannot be entirely ruled out with marginally severe hail and winds possible in the strongest storms. This appears most likely in the mid to late afternoon. Given this potential, the SPC has placed roughly the southern half of the forecast area in a Marginal Risk (1/5) for severe storms this afternoon.

All-told, QPF from this event looks fairly light, generally ranging from 0.1-0.25", though higher amounts will be possible in any thunderstorms. Showers and storms look to exit the area this evening. With light winds tonight and some residual ground moisture, the development of some patchy fog is possible. We will continue to monitor this potential. Lows tonight are expected to range from the upper 40s to mid 50s.

KEY MESSAGE 2...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 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. These troughs will drive a few cold fronts across our area, with the first one crossing our area Sunday night into Monday. The greatest forcing with the cold front looks to occur from the I-95 corridor and to the south and east Sunday afternoon and especially Sunday night through Monday morning. This may also overlap for a time when some sufficient instability is present for at least isolated thunderstorms. The front may be slow to settle south and east later Sunday night into Monday morning as the incoming main trough axis is displaced well to our northwest. A much warmer Sunday will give way to a much cooler Monday in the wake of the front.

The active 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. This initial arrival of warm air advection ahead of this system should drive an increase in shower activity during Wednesday. The chance for thunder is less certain at this time frame as it will depend on how much destabilization can occur. If the system closes off as it arrives Thursday into Friday, then surface low pressure may end up developing off the northern Mid-Atlantic coast. This would increase the intensity and coverage of showers/rain and result in a stronger onshore flow and therefore even cooler temperatures. This would also tend to result in a more stable air mass and thus little to no risk of thunder. The upper air pattern evolving during the second half of next week is less certain, as the amount and coverage of precipitation will depend on whether we end up with a closed low or just a sharper upper-level trough that quickly pivots through.

AVIATION /12Z SATURDAY THROUGH WEDNESDAY/

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

Today...Prevailing VFR. Widespread light showers are expected to begin moving into the terminals as early as 14-16Z from west to east. A few storms are possible after around 18Z, primarily for the I-95 terminals, MIV, and ACY. 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...VFR early as showers and a few storms clear all terminals. Winds quickly diminishing to light and variable by 01-03Z with calm periods likely overnight. Some low stratus or fog is possible at all terminals, which could lead to MVFR or IFR ceilings and visibilities. Have only included BR in the MIV and ACY TAFs where confidence in occurrence is the highest. Moderate confidence overall, low confidence in the extent of restrictions from low stratus or fog.

Outlook...

Sunday and Monday...A period of sub-VFR conditions probable later Sunday afternoon into Monday morning with showers. A few thunderstorms possible Sunday afternoon into early Sunday evening. Conditions should improve during Monday.

Tuesday...VFR.

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

MARINE

A Small Craft Advisory is currently in effect for all Atlantic coastal waters from Manasquan Inlet southward, as well as the lower Delaware Bay. Beginning at 10 AM, the advisory will be in effect for all NJ and DE Atlantic coastal waters and the lower Delaware Bay and continue through 12 AM Sunday. South winds 10-15 kt early this morning will increase to 15-20 kt with gusts around 25 kt late this morning into this afternoon. 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.

Outlook...

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

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.

PHI WATCHES/WARNINGS/ADVISORIES

PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory until midnight EDT tonight for ANZ431- 451>455. Small Craft Advisory from 10 AM this morning to midnight EDT tonight for ANZ450.