Two developing storm systems – one off the East Coast and one in Plains, will both wreak havoc in the next few days, but for very different reasons.

Two low pressure areas are developing off the East Coast early this morning. The northern system is the stronger of the two, but the southern one is being monitored by the National Hurricane Center. It has the potential to develop into a tropical depression or subtropical storm later today. Whether it does or not, these two low pressure areas will eventually merge and become a rather strong non-tropical system over the next 24 hours. There is very little in the way of steering currents right now, so the system will just meander around off the coast until Saturday.

As the system drifts northward, it will produce gusty winds along the coast, especially in New England. Tides are astronomically low at the moment, but will be rising later this week, so coastal flooding, while not a major concern, will still be possible in some locations. A coastal flood watch has been issued for parts of Plymouth County, Cape Cod, and Martha’s Vineyard. Rough seas will also be a large concern for marine interests, with Storm Warnings now in effect offshore. However, the biggest concern and also the biggest question mark right now is heavy rainfall.

While the storm will likely spread some heavy rainfall into New England, there is still plenty of uncertainty as to how far north the heavy rain gets, as well as how much rain actually falls. Some of the models are producing extremely heavy rainfall, with totals in excess of 10″ in southeastern Massachusetts! While we aren’t buying the extreme totals, the fact that most of the models are showing this potential means that some very heavy rain is likely, especially south of Boston, where a Flood Watch has been issued.

Thanks to some dry weather over the past couple of months, we shouldn’t have to worry about any flash flooding, though downpours will result in ponding of water on roadways, and some locations normally prone to flooding in heavy rain will also have problems. However, since we’re expecting a prolonged period of heavy rain, flooding is still a possibility in some locations, other than the ones we just mentioned, especially some of the smaller streams and rivers. Strong winds will also start to take some of the leaves off of trees, which may clog up storm drains, resulting in some flooding as well.

While all this is taking place off the East Coast, some very cold air will settle into the Rockies and Plains states as low pressure starts to develop across the Central Plains. Record lows are likely to be set in numerous locations over the next few mornings across the region. This system will head north-northeastward while strengthening. The system isn’t expected to become that strong, but with a large high pressure area building in behind it, it will produce some strong winds. It will also draw warm and moist air northward from the Gulf of Mexico. As this air runs into the much colder air on the backside of the storm, the first significant snowstorm of the season is expected to develop across the Northern Plains.

Winter Storm Watches and warnings have been posted from parts of Idaho Montana, and Wyoming into parts of northern Nebraska and the Dakotas as well as northwestern Minnesota. across Idaho, Montana, and Wyoming, snowfall totals of 6-12 inches are possible, with some heavier amounts in the higher elevations. The biggest issues are expected across the Dakotas and northwestern Minnesota.

The precipitation will start as rain across this area later today, but strong northerly winds will usher much colder weather in, changing the rain to snow from west to east on Thursday. Like its East Coast counterpart, this storm won’t move at a rapid pace, so snow, possibly heavy at times will continue across this area into early Saturday, with snow showers lingering into Sunday.

This storm also has some questions with it’s precipitation shield. While heavy snow is likely across a large area, how much falls and where the heaviest snow will fall is still a question. Right now, it looks like the heaviest snow will fall from central South Dakota into central and eastern North Dakota, but that still could change. As for amounts, many areas could see more than a foot, with totals in excess of 2 feet possible in some areas. The snow will be accompanied by winds gusting to 40-50 mph, resulting in blizzard conditions, especially late Thursday into Friday.

While snow in October is not unusual across this region, snowfall amounts of this magnitude this early in the year are extraordinarily rare. In Grand Forks, ND, the largest October snowstorm on record occurred October 24-26, 2001, and it dropped 10.9″ on the city. In Fargo, ND, the largest October snowstorm on record is only 8.1″ on October 30-31, 1951. For Pierre, SD, the October snowstorm of record occurred October 30-31, 1943, and it only produce 7.2″ of snow. Current model forecasts are forecasting amounts that are 3-4 times the records. These model forecasts are likely too high, but it seems likely that many of these October records are going to be obliterated in the next few days. Hopefully, this is not a sign of what’s to come this winter.

In our forecast yesterday, we told you that some of the biggest questions were the track of the storm system and related to that, how far north the change to sleet/freezing rain would take place. Well, we’ve got some more clarity on that now.

Some of you are already seeing some snow falling this afternoon, but this is not related to the storm. Some ocean-effect snow has pushed into Cape Cod and Plymouth County, and will continue to push inland this afternoon. This will be light, with less than an inch in most spots. The steadier snow will move in late this afternoon and evening from the west, reaching southern NH and the Merrimack Valley first, then the rest of Eastern Massachusetts and Rhode Island. It’s after midnight where the changes start to take place.

The storm system looks like it will track a bit farther north, passing very close to Boston on Sunday. This will not only allow milder air to move into most of southeastern Massachusetts, but the milder air aloft will also move much farther north than we had anticipated yesterday. Snow will fall heavy at times overnight, especially north of the Mass Pike, but as the warmer air starts to move in, we’ll see a change to sleet and eventually freezing rain move into the South Coast towards Midnight. A change to plain rain is likely before daybreak across southern Rhode Island and southeastern Massachusetts, as temperatures may break into the 40s Sunday morning. Some of the rain could be heavy, which may wash away most of the snow that does fall. Farther north, snow will gradually change to sleet and freezing rain north and west of Boston towards daybreak, with the changeover likely getting into southern NH as well. This will significantly cut down on snow amounts, but could make travel quite hazardous if there’s enough freezing rain.

By midday, the storm passes by and moves offshore, with winds shifting into the north. This will bring much colder air back in, and rather quickly. Temperatures may get close to or a little above freezing during the morning from southern NH into the Merrimack Valley and the northern and western suburbs of Boston, but by afternoon they’ll quickly drop into the 20s during the afternoon and teens by evening. That will result in a flash freeze, and any untreated surfaces will quickly freeze up, resulting in very hazardous travel. It will also mean that anything you haven’t cleaned off of your car, driveway, sidewalk, etc. will quickly turn into cement. The bitterly cold air will continue to flow in, and temperatures on Monday may struggle to reach the lower teens, with wind chills below zero thanks to gusty northwest winds. These gusty winds may also produce some ocean effect snow across parts of Cape Cod, especially the Outer Cape.

So, how much snow are we expecting now? Here’s our current thinking:

Cape Cod/South Coast: 1-3 inches
Southeastern Massachusetts/I-95 corridor: 2-4 inches
MetroWest/North Shore: 3-5 inches
Merrimack Valley/NH Seacoast: 3-6 inches
Southern NH: 4-7 inches
Central NH/Southern Maine: 8-14 inches, with higher amounts the farther north you go.

Temperatures will slowly start to moderate on Tuesday, then another system may move in on Wednesday, but this one, believe it or not, may be mostly rain. We’ll see how that evolves over the next few days.