Quincy

I've been casually watching Sunday across the north-central states. A lot can change and there has been some model adjusting, but keeping an eye on a vigorous shortwave forecast to eject across the northern Plains Sunday into early Monday. There may be somewhat of a disconnect between the best forcing, strongest wind fields and the warm sector, but details can be ironed out in the coming days. Based on model indications now, I'd peg the focus on the eastern Dakotas into the Upper Midwest as an area to keep an eye on. Nothing significant and not outbreak-material, but the way this year has gone, at least it's something on the radar. I wonder how this fall will shape up, if it may try to pull a 2013 or keep the trend of generally lackluster severe threats. I'd tend to lean toward the latter.

A series of supercell thunderstorms moved through central and eastern Massachusetts this afternoon. Some of the storms prompted tornado warnings and there were multiple significant severe weather reports as a result. Some of the thunderstorms originated as far west as eastern New York and later reached peak intensity as they moved into portions of central and southern New England. Even the coastal community of Boston was hit with some regionally impressive severe weather. The prelude to these afternoon storms actually came in the form of early-day severe storms this morning. A line of severe wind-producing thunderstorms affected portions of southeastern Connecticut, Rhode Island and Cape Cod. An area that does not often see severe weather, especially during the morning hours. Once those storms moved out, there was some clearing, which led to plenty of daytime heating to fuel another round of thunderstorms. By early afternoon, there was an area of moderate to strong instability developing across the eastern half of Massachusetts, coinciding with seasonably strong wind shear. The mesoanalysis showed a corridor of moderate buoyancy coinciding with more than adequate wind shear for supercell thunderstorms. Bulk wind shear was in excess of 50 knots across the region. With clusters of thunderstorms ongoing across portions of the area, the storm mode was going to be critical in the evolution of the event. If storms merged, lines would form, suggesting more of a damaging wind threat. If the storm mode was messy, the inflow region and overall environment could be disrupted, resulting in mainly sub-severe storms. However, if any storm could remain discrete and from on the southeastern fringe of the activity, in order to take better advantage of stronger instability, then things would become a bit more interesting. Two thunderstorms began to split in northern Worcester County shortly before 2 p.m. The northern most storm tracked east-northeast into far southern New Hampshire. This storm merged with surrounding storms and did not strengthen much. The southern storm took a bit of a right turn and continued in a generally eastward direction. This cell moved through northern Worcester County, organizing quickly into a robust supercell. Velocity and hail signatures increased, indicating the likelihood of 1-2″ diameter hail. This storm further organized with rotation becoming more focused. A hook was noted on radar and a Tornado Warning was issued. The storm continued into northern Middlesex County, dropping more large to significant hail, but the velocity signature was marginal at best for tornadic development. The main story was hail. Speaking of hail, a second supercell developed on in west-central Worcester County a short time later. This storm was semi-discrete, but remained that way for quite some time. It also dropped large hail and continued for two more hours, moving right through downtown Boston. Large hail was reported throughout portions of the city and surrounding areas. Overall, there were numerous reports of golf ball or larger hail across Massachusetts, with at least three confirmed reports of hail at least two inches in diameter. A few additional discrete storms developed later in the afternoon, but earlier storms had overturned the atmosphere a bit. These storms were generally strong to only marginally severe in nature. No tornadoes have been confirmed as of 10:00 p.m. Although there was some localized backing of low-level wind fields, the lack of favorable low level helicity was a limiting factor in the tornado potential. Nonetheless, a fairly uncommon significant hail event affected the eastern half Massachusetts, an area that based on a 1980-2006 average, only reports significant hail once every 6.75 years. This is based on four reports across all of eastern Massachusetts, during the time. While there were four significant hail reports in that 27 year span, there were three such reports this afternoon alone. Keep in mind that the time of record is relatively short, as reports from 2007 to this year were also not included. A look at the 2 p.m. mesoanalysis showing 1000-1500+ J/kg MLCAPE and locally backed near-surface winds in the vicinity of the most robust supercells:

There is the potential for a regional severe weather event on June 22nd. The threat zone extends from portions of the middle/upper Mississippi Valley into the western Great Lakes. All severe weather hazards are possible and given the nature of the setup, forecast trends are being closely monitored. As at least two pieces of shortwave energy rotate from the upper Midwest into the Great Lakes on Monday, a surface low is forecast to deepen from Iowa/Minnesota into Wisconsin. Impressive wind fields for this time of the year will combine with strong instability to create an environment favorable for severe weather. As it stands right now, two rounds of storms are expected. The first round would most likely be in the form of a mesoscale convective system (MCS) Monday morning, moving into the Great Lakes by midday and early afternoon. The second round develops in the wake of this activity, at and shortly after peak heating, later in the afternoon. The setup is a bit complicated and there are reasons to believe that two rounds of mixed intensity are favored over one robust atmospheric punch. The MCS during the first half of the day could pose a threat for primarily damaging winds and isolated hail. Once that system moves out, there should be modest air-mass recovery. The issue is that wind fields in the lower levels begin to veer, causing a more unidirectional shear pattern. This will likely dampen the tornado potential somewhat. Nonetheless, even forecasts on the more conservative end of the spectrum indicate a setup favorable for at least a few tornadoes. The afternoon to early evening threat may feature fairly widespread damaging winds, if storms were to merge into a linear system. Supercells still appear probable and they may extend into the early evening. However, for the reasons listed above, this does not look like an ideal setup for a major severe weather outbreak. Trends for this event tend to move the surface low a bit quicker, which also speeds up the veering of winds. The best wind fields may also become somewhat displaced to the northeast of stronger instability to the south. The Euro has joined the GFS/NAM in developing strong instability, as earlier it was showing a less unstable setup. The RGEM is also on-board and all of the models are in general agreement in terms of timing and geographic placement. Wisconsin is the target for the initial MCS on Monday. That system should weaken as it moves into portions of upper and lower Michigan. Later in the day, the target becomes southern Wisconsin and northern Illinois. Portions of lower Michigan and northwestern Indiana may also get in on some action. Damaging winds should be the predominant threat. The stronger cores can produce large hail and a few tornadoes seem probable. At least one of the following scenarios would need to occur to more fully maximize the tornado threat: First, outflow from the morning MCS could work to set up one or more boundaries to locally enhance a tornado threat. If the MCS decays faster than projected, that could lead to less disruption of the wind field and even stronger instability. A mesoscale low and/or a main low tracking further south into southern Wisconsin (instead of northern sections) may maintain more backing in the lower level wind fields through the threat zone. It should be noted that even with the projected wind fields, at least a few tornadoes remain likely. Given both a strong low level jet and 0-6km shear in excess of 50 knots, there is at least some potential for strong tornadoes. All of the factors currently forecast likely place Monday into more of a mid-range (SPC moderate risk) severe weather threat, as opposed to a higher-end (SPC high risk) outbreak. Stay tuned to later forecasts to see how the forecast setup evolves. The NAM forecast below for 21z Monday shows winds veering substantially across Wisconsin and Illinois: