https://easternmassweather.blogspot.com/2019/12/first-call-tuesday-mess.html
First Call: Tuesday Mess
During mid week, it became evident that a mixed bag of precipitation would be likely across the area in association with the system poised to impact the region next Tuesday12/17. Eastern Mass Weather posited that it would be difficult for the majority of the area to remain as snowfall since the surface high pressure was escaping seaward, and the upper level energy was tracking to the north. While this still appears to be the case
Some subtle, but important morphologies have taken place over the past 84 hours. Note that the receding high pressure area in the atlantic is now arching more back towards Maine. The impact of this on sensible weather will be that low level cold will now be more difficult to scour out even closer to the shore, thus increasing the risk of more frozen precipitation.
The difference in guidance from this past Wednesday is quite evident, as that appendage of cold +pressure pattern was not apparent.
The reason that we have seen this trend is very apparent, and one need not take more than a cursory glance at the modeled hemispheric 500mb pattern to understand why this has taken place.
Impact of Negative NAO
The aforementioned alterations to the pressure pattern along the northeast coast can be traced back to what will verify as a decidedly more negative NAO than was modeled over the course of this past week.
Here is the forecast NAO index from Wednesday:
While it was technically forecast to be negative, it was only marginally so. However here is the forecast from Friday:
The NAO is going to verify significantly more negative than anticipated mid week. While the upper level energy is still going to be track to the north, the more deliberate departure of the cold high pressure area in the vicinity of Maine due to the more blocked downstream heights will force the surface reflection to the south of southern New England, effectively shearing and weakening the system.
The fractured, weaker disturbance in conjunction with the surface system being displaced to the south will allow for a colder solution than otherwise may have transpired, but the mid levels of the atmosphere will still warm considerably across at least the southern half of southern New England. Warmth overrunning cold air entrenched at the surface is a recipe for ice.
Storm Evolution and precipitation Types
Snowfall should begin to overspread the area from southeast to northeast on Monday evening.
Precipitation falls as snow when the entire atmosphere is subfreezing. Snowfall should reach the Mass pike region after midnight, as mixed precipitation already begins working onto the south coast.
Local climatology dictates that the progression of the sleet should slow considerably as it encounters more resistance in northern Connecticut and Rhode Island from the retreating high pressure area. It may creep to the vicinity of the Mass pike by the Tuesday morning commute. Dangerous icing should be ensuing across much of interior Connecticut and Rhode Island at this point, where the cold layer will be very thin near the surface. North of this region, where the colder layer is thicker, the snowflakes will still pass through a warm layer in the mid levels of the atmosphere, thus melting into rain drops. However the rain will have had more time to refreeze into pellets of ice referred to as sleet, by the time that they reach the surface.
The heaviest period of precipitation should arrive mid to late morning on Tuesday, as the mid level warm air advection reaches its zenith, potentially promoting a period of sleet to the north of the Mass pike, with freezing rain potentially nearing the pike. Precipitation type will be crucial during this particular time, since it will be steadiest.
The precipitation should taper off throughout the afternoon, as the cold air collapses back to the south and east.
First Call:
Relatively signifiant amounts of freezing rain accretion are possible throughout interior Connecticut, and Rhode Island, with less across interior southeastern Mass and south of the Mass pike.
Stay tuned for Final Call on Monday-
