Another Tricky Forecast

TUE 4/29: A solid one to two inches of rain has been reported across Cape Cod over the last 24 hours, loudly slamming the door on a fairly long stretch (for the time of year) of dry weather. This healthy dose of water was just what we needed, as most spots on the Cape were running over 2 inches below normal for the month (nearly 4 inches of rain is "typical" for April). Anyhow, with this storm now shifting away, it's time to focus on the next forecast challenge - just how unsettled will the weather turn late this week and into the coming weekend?

However, before we get to the late week forecast, there is one important detail in the coming 36 hours to pay attention to. Behind Tuesday's storm an unseasonably chilly air mass will build into New England on gusty northwest winds. This air mass actually originated up over the Arctic, and while extremely modified, it's still packing a punch - with snows reported around some of the Great Lakes and even into the Appalachians. Thankfully, no snow is in the forecast locally, but cool temperatures certainly are. While frost advisories have been posted for much of the coastal plain in Southern New England for Tuesday night / Wednesday morning, it's actually the Wednesday night time frame that has me concerned. Winds will likely remain too gusty and temperatures a touch too mild for any frost on the Cape tonight as lows dip into the upper 30s to lower 40s. However, after a cool and dry Wednesday, northerly winds will tend to diminish Wednesday Night - allowing for good radiational cooling conditions. And with dew points forecast to be in the 20s, lows should be able to dip down close to freezing in some neighborhoods. As such, pockets of frost are certainly a possibility around the Cape Thursday morning - especially in sheltered locations set back from the water's edge.

Looking further ahead, we have another forecast challenge evolving for the Thursday through Sunday time period. A slow moving weather pattern, not at all unusual for the spring months, will transition to an Omega Block by the coming weekend. A storm lifting through the nation's midsection will become a cold upper level low centered over the Great Lakes, helping a narrow upper level ridge to build up the eastern seaboard, with a second upper level low spinning in place east of New England.

The big question for our forecast is whether a warm front and a wave of moisture extending eastward from the storm over the central part of the country can penetrate the ridge over the East and allow for rain to break out across southern New England Thursday Night and Friday. In a sense, this is not all that different from the set up this past weekend - when warmer, more moist air was running into the dry air from high pressure centered northeast of New England. Essentially, the dry air was the winner, with precipitation failing to make big inroads until Monday. In fact, Saturday turned out to be a beautiful day and we only managed a few sprinkles and light showers Sunday afternoon. But can we manage the same outcome this go around (essentially holding rain to our west longer) and stay dry late week and into the weekend?

The American guidance, the NAM and GFS models, once again say we can't - each model is emphatic on allowing moisture (clouds) to build into the region as early as Thursday evening with showers developing soon after. In fact, the most recent runs of each model suggest a prolonged spell of wet weather lasting from Thursday night right into Sunday. But the Canadian, UKMET, and ECMWF models want to keep the initial slug of rain *just* to our south and then eventually allow high pressure to muscle southward - splitting the moisture in two pieces - one to our east with the upper low over the western Atlantic and one to our west with the upper low over the Great Lakes.

So what forecast to follow? Well given the upper level set-up, at the current time, I think the way to lean is towards a glancing shot of precipitation Thursday Night/Friday morning, followed by improving conditions over the weekend. I could see a prolonged spell of wet weather over the Northern Mid-Atlantic and areas to our west, but a sharp gradient to the precipitation, with only some scattered light showers sneaking into the region for a brief time before high pressure forces it south. More updates later in the week.