Saturday, July 07, 2012

Forecast for Sunday, July 8, 2012

Philadelphia reached at least 100 degrees this afternoon, along with many other observation sites. Some high clouds spread over the region as an earlier Mesoscale Convective System weakened across southern portions of New York State, spreading the remaining outflow across New Jersey. This system weakened as the cap was not broken yet across an increasingly hot region. However, soundings and surface observations are indicating that the cap is now potentially starting to burst and should it burst completely, the thunderstorms will have sufficient axis to very strong instability and ample moisture in place for a potential severe thunderstorm event into the Philadelphia Region. The trigger for thunderstorms is a cold front slowly sliding southward. This cold front has helped to initiate a new Mesoscale Convective System off another remnant MCS. Looking at the mesoscale analysis, there is still some surface based CIN across our area and this makes the forecast so challenging in that it may try to weaken a very damaging complex of storms rapidly moving southeast from Lock Haven and Williamsport in Central Pennsylvania. However, should this CIN not be present or diminish, there is going to be sufficient energy in place for damaging winds from any thunderstorm in this complex that maintains severe limits. Meanwhile, an isolated thunderstorm may develop on the old outflow prior to these thunderstorms moving in from Central Pennsylvania. Large hail is not as big of a threat (although some is possible and has occurred in this line) as there are very hot air temperatures in the boundary layer and a higher freezing level as a result. We remain under a slight risk of severe thunderstorms through the overnight hours. The current thunderstorms near Lock Haven and Williamsport would track into the Philadelphia region between 7 and 10 p.m. tonight. Again, this is a wait and see situation. I should also note that there has been some weak rotation noted.
Throughout Sunday, this cold front ever so slowly sinks southward. It may be tough to sink it south of the Pennsylvania Turnpike and Intestate 195. Waves of energy will move along the front on Sunday and any location still located within the hot and humid sector has the potential to see the development of additional showers and thunderstorms. Any thunderstorm could produce damaging wind gusts and some hail on Sunday. Temperatures will be in the nineties south of the boundary.
Assuming Philadelphia remains on the hot side, we may have the 11th consecutive and final 90+ degree. The model guidance has been performing very poorly on the thunderstorm development so far this weekend, but has hinted at times at a more organized event closer to the Delmarva. The entire region from Philadelphia South and East is in a slight risk designation for severe thunderstorms on Sunday.
This front meanders to our south for much of next week. It may at some point try to lift back north as a warm front. With continued waves of energy riding along the front, this keeps the chance of showers and perhaps a thunderstorm, especially from Philadelphia and points south. If the front moves northward, it could make for some tricky temperatures up near Philadelphia. It appears northern areas will be drier than southern areas this upcoming week.