After 15 days without measurable rainfall in many New England communities, rain is back in the forecast - and has the potential to be back in a big way.
Flood Watches are in effect for much of New England through Thursday morning. Coastal Flood Watches are also in effect for some parts of New Hampshire and Maine.
The MBTA has already cancelled some Orange Line buses due to the weather, and the Massachusetts Emergency Management Agency is warning that rainfall rates could exceed an inch per hour at times, and gusting winds could lead to downed trees and power outages. The greatest flooding threat is in urban, poor drainage and low lying areas.
And all this could just be a precursor to the arrival of Tropical Storm Joaquin this weekend.
The recent pattern has featured frequent intrusion of dry, cool high pressure systems - fair weather makers - from Eastern Canada into New England. This has assured, time and again, that significant moisture capable of cranking out rainfall has remained south of New England.
As of the end of last week, the 10-day forecast indicated more of the same was likely, but not inevitable - rain showed up for midweek this week due to a 30-percent chance a feed of moisture could move into New England. In the days that followed, this past weekend, it became clear that chance was increasing. Now, with an upcoming change in the jet stream winds aloft, it appears our dry pattern is destined to change.
This map depicts the jet stream configuration as of Monday - note the upper level energy digging in across the South-Central United States, and the active jet stream flow near the Canadian border: the first represents abundant moisture (and with a counter-clockwise flow around the deep atmospheric storm, a southerly wind to transport moisture from the Gulf of Mexico, Bahamas and Tropics northward), and the second represents ample energy and some available cool air in Canada, north of the jet stream winds:
By midweek, New England feels the squeeze of tropical moisture streaming north between a Western Atlantic ridge of high pressure (fair weather dome) and the incoming energy and moisture from the west. This combination, especially when coupled with an increasing clash of southern warm air with northern cooling, should result in an efficient rain-maker for our drought-stricken region, quenching our parched ecosystem with 1.5 to 3 inches of rain between Tuesday and Thursday.
Tuesday starts with showers blossoming from time to time Central and South, then consolidating to an area of downpours and thunder in Northern New England by late day and evening. By late Tuesday night and Wednesday, the heaviest rain sweeps through New England and the wind shifts from a relatively warm east or southeast wind, to a colder north and northeast wind. Thursday is likely to bring a classic, moist, cool New England wind flow from the northeast.
The end of this week holds special interest for New England, as tropical depression #11 is forecast by the National Hurricane Center to become Tropical Storm Joaquin while turning northward and paralleling the Eastern Seaboard.
There are some inherent uncertainties here: storm integrity and strength (tropical systems have had trouble surviving in the Atlantic this season due to wind shear aloft), storm track, and just how many tropical characteristics the storm will have when it arrives. Regardless, the bottom line is the same - this entity represents another slug of tropical moisture that will be available, while Wednesday's cool front will have stalled just southeast of New England, and will help to usher this moisture and energy northward. The result should be another shot of at least some rain later Friday into part of Saturday - even if the bulk of the tropical creature misses, it still seems likely we maintain a moist east wind all the while, locking in clouds and cool air.
There are some opportunities for change to this forecast - namely, the potential for Wednesday cold front to jog just a little farther offshore, steering potential Joaquin farther offshore and allowing for a bit more dry air to move in. Another important part of the forecast not only is the track of Joaquin, but moreover the difference in barometric pressure between the storm and a high pressure cell over Canada - where the pressure changes greatest, strongest wind will be found. In New England, a persistent moderate-strength northeast wind can churn the seas, resulting in pockets of coastal flooding. Obviously, a closer pass of the storm could increase the chance of coastal flooding further.
In short, there's plenty to watch in the coming days, and when the unsettled weather breaks is dependent upon several factors. At this point, we're thinking improvement begins in earnest Sunday, but will keep you posted.