A Storm Friday? No Foolin'

Last week’s weather has not been kind to fans of warmth, of trees budding, of flowers blooming, and of sneezing and wheezing for allergy suffers.  

It may have been kind to those sufferers as temperatures were rather cool last week—and we were graced with the presence of snow not only twice (Wednesday and Thursday mornings, the former mixed with rain and the latter not) but also with the presence of thundersleet on Wednesday night as formerly severe thunderstorms marched through the region during the evening hours, dropping a quick hit of sleet, small hail and rain as they were marching east.  

Those thunderstorms were responsible for the tornado that caused about $4.5 million in damage as it dropped briefly.  Thankfully, no tornadoes dropped here and other than sleet, small hail, and some light snow, nothing of note really dropped here other than temperatures.

For those wondering about the difference between sleet and hail, the process comes down to temperature and dynamics within a thunderstorm. Sleet is merely rain that is frozen en route to the ground. A cold layer of air in the atmosphere between where precipitation begins to fall and the ground will freeze raindrops into sleet pellets.

Hail requires dynamics and lift in a thunderstorm. Hail is frozen raindrops that have cycled through a thunderstorm a number of times before falling to the ground. The updraft and uplift inside of a thunderstorm can pull hail stones back up into the cloud for multiple trips.

The more violent the updraft and stronger the thunderstorm, the greater the potential for larger hail. Both can occur with surface temperatures close to freezing, with sleet occurring even with temperatures in the 40s if the atmosphere has cold enough pockets of air to freeze raindrops. The easiest ways to tell the difference between the two is size (hail being larger) and that sleet is typically "clearer" in composition compared to hail.

Cool temperatures will continue to rule the roost over the next week as the calendar transitions from March into April. A weak system could scoot along to the south of Philadelphia on Wednesday or Thursday, much like the one that is passing to the south of us today, and could spread some light rain and/or snow across Delaware and New Jersey.

As of now, it looks cloudy and cool here as the system may just miss us. However, Friday shows the potential for a larger storm system that could develop near the east coast.  Computer guidance has been waffling between a coastal storm and no storm for a while for April Fools’ Day—with the waffles towards a storm showing anything from chilly rain to a mixed bag of everything. Uncertainty abounds with the pattern for the end of the week.

Regarding temperatures, expect below average temperatures for the most part, although we could nudge back into the 50s on Thursday. If the bigger storm doesn't materialize, perhaps we stay in the low 50s through week's end.

Last April featured mid- and upper-80-degree weather in the early part of the month (highs near 90 in Philadelphia on April 6 and 7), ushering in the warmest six-month stretch we’ve seen in the mid-Atlantic since data was first regularly collected in the 1870s. Unlike last April, we will not see such warmth in 2011 as the long-range pattern looks cool through the first 10 days of the month. There are hints of warming after April 10, however, so allergy sufferers can start stockpiling their pills and their tissues in anticipation of the sniffles.