It's been awhile since my last post - Monday brought two and a half hours of live streaming video here on the site from 1 PM to 3:30 PM in advance of what turned out to be a significant severe weather outbreak for New York State and Western New England. Today's thunderstorms have been what was expected of them - pulse storms to severe wind in some spots, but heavy rain, large hail and frequent cloud to ground strikes as the biggest culprits. This is the result of the cold pool aloft to the tune of -17 to -18 C, meaning a steep low and mid level lapse rate is in place. This, combined with dry air in the mid-levels, creates a low altitude wet bulb zero (where saturated air would hit the freezing mark) which supports subfreezing air within a few thousand feet of the ground, meaning hailstones that develop through in-cloud turbulent processes have a much easier time reaching the ground before melting. This was the reasoning behind the 1.5" to 3" hail yesterday, and another round of hail today. With active ice nuclei in areas of saturation in air below -20 C, lightning production has been efficient in the storms.
Looking forward, the upper level low over Southern Ontario will gradually move east/southeast in the days to come, and this will keep the upper level cool pool over New England. This upper low stays nearly stationary thanks to a Rex Block well east of New England that holds the weather pattern in check, establishing a trough/ridge/trough pattern from the Eastern Pacific across the Western/Central Pacific and to the Northeast. In fact, the wobble of the upper low brings gradually cooling air aloft, and at the surface, in the days to come. The coldest surface air comes on Wednesday, with upstream daytime highs in the 50s and 60s, which should verify as the 60s in Northern/Western New England Wednesday afternoon and 70s farther south and east. Thereafter, this is rather elevated cold air due to no strong surface high to hold it in place, and the strong sun angle this time of the year furthers this trend for the cold air to remain at altitude. But several thousand feet in the sky, the upper low and trough axis will close in on New England toward the end of the week, and this means the chance for thunder increases as the trough axis makes its closest pass on Friday. All the while, the upper low will take on a less organized appearance with time, both because it will begin a gradual process of filling (weakening) but also because dry air will become wrapped into the system, and over time when an upper low sits in one place we tend to see weakening convection each day, and a gradual deterioration of the system each day, with the cold pool and trough axis housing the best chance for convection by the end of the period. The result actually could be a fairly bright and mostly dry Thursday before the enhanced convective threat on Friday.
All the while, this is an interesting time to build a ridge of the Central and Western United States, because the solar angle is unforgiving as we approach the summer solstice, which is why desert Southwest temperatures will come into the 110s in the coming days, with strong positive temperature anomalies over most of the Western United States in a dry pattern there. This sets up a dry and warm pattern that will continue to favor a ridge holding on in the west, with the Eastern ridge holding in place as a subtropical Atlantic ridge establishes over the Atlantic. This leaves a corridor of lower heights over the Eastern Seaboard and most areas east of the Mississippi River all the way down to the Gulf of Mexico, which...between the southwest and Atlantic highs...looks to be open for business as soon as this upcoming weekend. The presence of numerous northern stream shortwaves may keep heights suppressed enough to keep the subtropical jet aimed from the Gulf to the Mid-Atlantic coastline, which would keep the dose of deep moisture on Saturday across the Mid-Atlantic coastline. GFS Ensemble solutions suggest a big rain event in the central and southern Mid-Atlantic on Saturday, but this seems a bit too quick and perhaps a product of convective feedback, as a logical progression would take deep tropical moisture to the Atlantic seaboard late Saturday night/Sunday, then bring it north parallel to the coast. The evolution of any potential low pressure center amidst this moisture that would affect New England is still somewhat questionable given an uncertainty of the depth/amplitude of the Eastern US trough, and equally importantly, the timing of shortwaves rotating around the base of it. This will be the challenge for the end of the weekend and early next week.
Matt
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