Since the breakdown of stratospheric polar vortex more high-latitude blocking has become common the last few weeks. Model and ensemble guidance are again showing blocking returning again for next week. But will it give pattern change featuring just below average temperatures? Or will a parade of systems coming out the Pacific, cause the Southeast ridge to build more?
After much warmer than normal temperatures during March, April has averaged cooler than normal so far, with some unseasonable cold weather for some days. This weather might seem bit ironic since we’re moving further into the Spring season. Some major league baseball games have to been postponed due to more unseasonably cold weather and even wintry precipitation in Northern parts of the nation. There are signs of a warming trend going into this weekend and early next week. But there are signs that this may be interrupted again due largely to high-latitude blocking.
Later this week, a large, deep cut-off low over the Rockies will cause a strong ridge to build over the Great Lakes and Ohio Valley regions. Another cut-off low will form on the downstream side of the ridge, off the East Coast. Initially, model guidance showed this cut-off low backing towards the region and affecting our weather this weekend. But high-latitude blocking weakening this week allows this low to remain farther offshore.
While the calendar has pointed to Spring for several days, the weather pattern has not cooperated. This weekend will be no exception. Forecast models have come into better agreement over the past few days regarding the eventual development of a low pressure system off the Mid Atlantic coast, which will form as a result of a powerful mid level atmospheric disturbance dropping southward from Canada into the Great Lakes and Northeast.
A low pressure system is likely to develop from the Ohio Valley into the Mid Atlantic and off the coast of New Jersey. Precipitation is likely to develop on the northern end of the storm system, as moisture pushes northward from the low pressure area. This will allow for bands of precipitation to develop towards our area, likely falling as rain. The antecedent airmass is quite warm, and likely supports rain over any frozen precipitation, with melting of snowflakes occurring in the lower levels of the atmosphere. However, there is one main caveat in the setup: The potential for intense banding and dynamic cooling, producing heavy snow over some areas.
Over the past few days, we’ve been discussing on our new hazards dashboard the possibility for high winds for late Saturday night and Sunday. As we’ve gotten closer, model guidance continues to be consistent on this event. Therefore the National Weather Service has issued a High Wind Watch for the entire region late Saturday and Sunday morning. Details are still coming into focus, but confidence is growing that this could be a very strong if not damaging wind event for parts the area.
A vigorous mid-level shortwave will amplify and cause a surface low pressure coming east out the Great Lakes Saturday night to deepen rapidly over Southern New England Sunday morning. This will create extremely low mid-level heights and thus a frigid cold pool aloft — aiding in instability and dynamics, especially when combined with the off-the-charts vorticity being shown. An Arctic front accompanying this low will also move across the region late Saturday night with some rain showers, then perhaps a change to snow showers, during late Saturday night. Steep low-level and mid-level lapse rates and total totals in 50s show enough instability for snow squalls with gusty winds and possibly even thunder.