An unsettled atmospheric pattern has taken hold of the area early this week, and will continue to be in place through around Wednesday evening. Although the main storm system won't arrive until Tuesday Night into Wednesday morning, numerous small disturbances in the atmosphere will provide a chance of showers on Monday, and will likely keep clouds around as well. Temperatures on Monday and Tuesday are only expected to reach into the 50's for highs, as the surface winds turn east off the Atlantic waters. In addition to keeping temperatures down, this flow of air combined with the unsettled pattern in the atmosphere will keep plenty of clouds around through the middle of the week as well. The steady rain will likely wait, as we mentioned, until later Tuesday Night through Wednesday, when a strong low pressure area approaches from our south and west. This low pressure will traverse northeast through the Ohio Valley and into the Northeast, before redeveloping somewhere off the coast. The presence of strong lift for precipitation as the storm passes will set the stage for moderate to heavy rain at times overnight Tuesday into Wednesday. The storm is expected to wrap up by Wednesday afternoon, with the potential for an isolated rain or snow shower thereafter. In the image to our right, we can see the NAM model showing the weak atmospheric disturbance over our area today, and the strong disturbance over the Plains heading in our direction (both circled in yellow).

Today (Monday): Increasing clouds, with a 30% chance of rain. High near 54. New rainfall amounts of less than 0.10" possible.

Monday Night: Cloudy, with a chance of rain. Chance of precipitation is 30%. Overnight low near 45. New rainfall amounts of near 0.10" possible.

Tuesday: Partly cloudy, with a 30% chance of rain. High near 53. New rainfall amounts of 0.10" to 0.25" possible. 


Over time, you've probably heard us mention the term "South-west flow event" or "SWFE" as a way of defining a winter storm or type of system. This has become somewhat loosely defined, but essentially describes the type of storm system we're dealing with this coming Tuesday into Wednesday. On the image above, at the top of this post, you'll see the strong atmospheric disturbance in the mid levels of the atmosphere, which at the time of the image is situated over the Plains states. This energy is ejecting northeastward, towards the Ohio Valley. As it does so, you'll also notice a ridge building ahead of this feature (black lines buckling ahead of it). This ridge is developing as a result of the southwest flow in the mid levels of the atmosphere, which is driving the system northeastward.

This southwest flow can develop through the mid, and even low levels of the atmosphere. During the winter, these systems can cause forecast nightmares. Part of the problem develops when there is a strong surface high near our area at the time of a south-west flow events approach. Imagine the strong surface high, keeping cold winter-like air at the very surface of the atmosphere, while a south-west flow event brings warm air into the levels above it. This often results in sleet, which forms as snowflakes form in the upper atmosphere, melt in the middle atmosphere (as a result of warm temperatures from the south-west flow) and then re-freeze at the cold surface.

Luckily for us, this south-west flow event is occurring just a few weeks before we typically have a winter-like cold airmass in place. This time, we don't have to worry about precipitation-type and changeovers. We'll leave that problem to our neighbors to the north over Central New England. Something tells me, though, we'll be revisiting this problem sooner than later.


Article written by JH. Published November 21st, 2011 at 11:34am. Looking for a forecast? Use our new Forecast Beta tool to see the latest forecast in your specific area for the upcoming week and holiday, or view our Technical Forecast Discussion for the more serious weather enthusiasts. Also, check out our new Forecast Overview tab. For up to the minute details on forecasts, watches, warnings, and alerts, follow us on Twitter and Facebook.