Say it Ain't Snow

3/26 : This week could prove to be a perfect example of just how amazing weather can be here in New England. As temperatures climb into the lower 50s Wednesday and you step outside and sense spring in the air, forecasters will be zeroing in on the Thursday Night through Saturday morning time frame watching for the potential of a winter storm (or two). If things unfold as they appear they might, there could be quite a turn around over the next 2 days - as we head from springtime to mid-winter in a matter of hours. Sunshine and mild readings may be replaced by accumulating snow, even here on Cape Cod.

The culprit behind the potential late week storm is a strengthening battle ground between a wealth of cold air over Canada that is shifting south and eastward into the Northern United States and very warm air moving across the Southern U.S. Temperatures today over the Northern and Central Plains will hold in the 20s and 30s, as the deeper cold begins to sink southward. Meanwhile, temperatures in Texas will soar well into the 80s, with 60s and 70s building as far north as the Ohio Valley. This contrast will grow stronger over the coming days and where these two air masses collide (Midwest to Ohio Valley to Northern Mid-Atlantic), a series of frontal boundaries will take shape and help to fuel storm development. These storms will ride eastward, feeding on the contrasting temperatures and spreading shields of precipitation through the Plains, into the Ohio Valley and eventually to the East Coast by week's end.

What seems certain in the forecast is an extended period of unsettled, raw and nasty weather to close out the week. Temperatures are certain to average well below normal Friday and into the weekend, with gusty north winds to add to the misery. What remains up-in-the-air is what precipitation type will dominate through the storm event and just how much falls. This will be highly dependent upon where the frontal boundaries are placed and consequently just how much cool air is able to drain southward through New England prior to and during the evolution of the aforementioned storms. The concern here is for a surprise winter storm as cold air wins out - but that will hinge upon the exact placement of several systems and a very complicated evolution of the atmosphere.

Clouds and precipitation from the first wave of moisture riding eastward will quickly be spreading towards Southern New England Thursday, with clouds increasing through the day and showers likely breaking out around the region during the afternoon hours. This first wave of precipitation will be associated with a small storm center and the sagging southward of a surface front. This front will shift winds to the north and mark the arrival of a second surge of cooler air (the first being behind Wednesday's weather system). With time, these northerly winds will drain progressively cooler air over Southern New England, sending temperatures downward and possibly causing Thursday afternoon's light rain showers to mix with some sleet or snow Thursday night. But that may just be the beginning...

Another round of moisture associated with a storm center emerging off of the Mid-Atlantic will move into the region Friday and Friday night and will be far more substantial than its Thursday predecessor. This is when the forecast gets very tricky and quite interesting. The trend in computer guidance over the last 24 hours has been for a slightly cooler scenario, with those northerly winds helping to deepen and strengthen the amount of cold air over the area. This potentially sets the stage for frozen precipitation during the height of the Friday night storm.

Should the surface boundary set up shop south of New England as expected, the heaviest ribbon of precipitation would likely extend in an west to east line across northern PA, through southern NY across CT and RI right across Cape Cod, with over 1" of liquid equivalent certainly possible in this stripe. But what form does it take?

As mentioned, the atmosphere will be getting progressively colder as the event wears on as cooler and cooler air drains through New England in response to strengthening high pressure to our north. Earlier in the week, rain seemed like the safe bet, given a slightly further north placement of the surface boundaries. However, the trend is cooler. In fact, the most recent run of the NAM model cranks out over 2 inches of precipitation at Chatham - with just about all of that falling in a frozen form (snow)! While this over done, it does help show the potential of this late week storm. The off-hour run of the GFS deposited nearly 1" of QPF at Chatham, with almost all of that falling as snow as well, with the 00z run cranking out nearly .5" of QPF in the frozen form.

Now does this mean we will be shoveling by week's end? By no means is it a guarantee. The end placement of the surface boundaries will be critical to just how this plays out. It's going to be a fine line between just a rain storm ending as some sleet and snow and an all out snow storm. A few subtle shifts north or south will make a world of difference. The only real guarantee to the forecast at this point is for a return of unseasonably cold weather Friday into the weekend and a period of unsettled, raw weather to close out the week. The stakes are high but they've been that way several times in recent weeks and we've escaped without much harm. Check back for updates.