A rapidly intensifying storm system along a cold front is approaching New England from the southwest. The center of this system will cross western New England (or eastern New York) later this afternoon and into the evening. This will result in widespread rainfall and high winds across New England. The entire region is under the gun for heavy rain while the high wind threat is primarily centered over eastern New England.
ALERTS (as of Wednesday morning)
TIMING
Rainfall will come in two main waves over the next 24 hours or so. The first is now ongoing. This first round of rainfall is coming as the storm's warm front lifts northward through the region. The warm air moving into New England rising over the cold air dam that has been set up over New England has allowed precipitation to break out well ahead of the front.
There will likely be a bit of a lull starting around midday in southern New England. This partial lull (it certainly won't be completely dry this afternoon everywhere) will lift northward during the afternoon. The lull will come as forcing for showers will weaken once the warm front lifts north. Forcing for rain will pick back up as the cold front approaches later in the day.
Below: HRRR showing potential weather around early to mid-afternoon today:
The main bout of rain will come in the second wave along the cold front. This round is expected to fire up later in the afternoon across western New England and sweep to the east through the evening and the first part of the overnight. This wave is when a bulk of the rain is expected. There may be some embedded thunderstorms within this wave, which could enhance both rainfall and wind in localized areas. The rain and wind threat from these thunderstorms will be discussed further in their respective sections below.
Below: HRRR showing potential weather this evening (1st image) and heading toward midnight (2nd image):
Once the cold front passes, the heaviest of the rains will begin to shut off, along with the highest winds. Snow showers will be possible across western New England and the White Mountains on the backside of the cold front, but these are expected to taper off before sunrise Thursday. Upslope snow showers may kick back up during the day Thursday.
Below: HRRR showing potential weather early Thursday morning:
RAIN
Deep moisture from the south will surge into New England along a strengthening low-level jet stream this evening. This will add more moisture to the already moisture-rich environment New England will be in throughout the day today. Add in strong forcing along a cold front and a strengthening storm system, and the recipe is there for bouts of heavy rainfall. A widespread 1-3 inches of rain remains likely across New England.
The highest of the rainfall is expected across interior portions of southern New England and into northern New Hampshire and interior Maine. Localized amounts of 4 inches will be possible, especially across interior southern New England. This is where the "fire hose" of moisture is most likely to set up. This fire hose of moisture is thanks to an atmospheric river sending a plume of high moisture streaming from the Gulf of Mexico up the entire east coast.
Below: Precipitable water values this evening. You can see a stream (or river) of moisture streaming up the east coast from the Gulf of Mexico:
While much of New England remains in a drought, rainfall rates could become heavy enough to support excessive rainfall across the region. There will be a risk of some urban/flash flooding with a partially or completely frozen ground in place, slowing absorption. All of New England except for Cape Cod, the Champlain Valley and the Connecticut panhandle are included in a "slight" risk (level 2 of 4) for excessive rainfall.
This storm will put a large dent in the ongoing drought across southern New England, though it won't completely erase it since the fall was so dry. The rain will be beneficial in the long-term, but the short-term impacts of heavy rainfall rates will likely lead to some flooding issues. This may be a case of a bit too much too quickly.
In all, melting snow across northern New England is expected to add an additional 1 to 2 inches of runoff to the 1.5-3 inches of rainfall. This will lead to a total runoff 2.5 to 4 inches across the foothills and northern woods. With the mountains holding onto some snow, it will limit the amount of runoff in those areas. With that said, the melting could dislodge river ice, potentially creating ice jam concerns.
Below: The amount of liquid expected to be added to runoff from melting snow:
At this point, major river flooding is unlikely. As of Tuesday morning, several rivers are forecast to reach minor flood stage across the region with additional rivers reaching action stage. Otter Creek in Rutland, Vermont and the Hoosic River in Williamstown, Massachusetts are currently forecast to reach moderate flood stage.
Below: Current river forecast as of Tuesday morning. Yellow indicates action stage, orange indicates minor stage and red indicates moderate stage:
THUNDERSTORMS?
Along the cold front, there is a chance for a fine line of thunderstorms to develop, thanks to the very high humidity (by December standards). Some instability is poised to build (albeit not much by summer thunderstorm standards) as the cold front and powerful low-level jet moves into the region.
Any thunderstorm activity will be difficult to forecast until they actually start forming, if they do form. Should thunderstorms develop, brief and localized bouts of very heavy rain will be possible along with lightning. The Storm Prediction Center has actually placed a portion of southern New England in the "slight" category for severe thunderstorms (level 2 of 5).
WIND
A very strong low-level jet will cross eastern New England later Wednesday into Wednesday night. This jet will have winds of 90-100mph at the 850mb level, which is about 4,700 feet above sea level. Naturally, not all of this wind will make it to the surface, but winds this strong at elevation can support gusts of 40-60+mph.
Below: Winds at the 850mb level (about 4,700 feet above sea level) Wednesday evening. This is the low-level jet helping drive this storm. It shows wind speeds of around 100mph at its core. You can also see the sharp western cutoff:
The strongest winds will be felt along the Maine coastline. All of eastern Maine will be under the gun for the highest winds. Here, gusts of 50+mph will likely extend well inland with gusts over 60mph possible near the coastline. The southern New England coastline will also be an area to watch for gusts of 50+mph. Widespread power outages are a major possibility with this one.
There will likely be a sharp western cutoff in the highest winds as it will take time for the low-level jet to organize as the storm quickly slides east. This means the best forcing for high winds may have passed for areas farther west before the jet ramps up. It may also take time for an inversion to weaken, which would help keep the strongest of the winds trapped aloft.
High wind warnings have been extended to include the entire Maine coastline along with the New Hampshire seacoast. In southern New England, all of eastern Massachusetts has been placed under the warning. The extension to cover all of eastern Massachusetts is primarily due to the threat of thunderstorms. Any convection/thunderstorm would allow a brief period of much stronger winds to make it to the surface as the thunderstorm passes (like a strong thunderstorm in the summer).
TEMPERATURES
Not to get lost in the storm is the fact that temperatures will be surging this evening as mild air races northward. 60+° temperatures will be possible and may come close to daily record highs in spots in southern New England. Northern areas won't get nearly as warm. Once the front passes, temperatures will crash back down to the point snow showers will be possible in western New England. Thursday will be cooler as cold air advection takes over. Friday and Saturday will be the peak of the cold that will wrap in behind the storm.
AFTER THE STORM
A strong area of high pressure will build into New England. This high pressure is currently modeled to have a central pressure of 1,044 to 1,048mb. For a comparison, Burlington, Vermont's December high pressure record is 1,051mb, so this will be a rather strong one. This will keep calmer and dry conditions around with cold temperatures through the weekend.
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