Mild & Rainy Tuesday, Cold & Active Pattern Arrives Late Week

Happy Monday! Fair and chilly weather will continue for the rest of today, as high pressure remains in control. Some clouds will increase this afternoon, ahead of an approaching frontal system. But high temperatures will be in the upper 40s over the Interior to the lower 50s near the coast, which is near or slightly above normal for this time of year. But as we’ve been discussing before, some big pattern changes are coming for the rest this week, that will support more wintry weather for the mid-December — this pattern will be ushered in by the aforementioned cold front.

First, a strong cold front associated with a large upper-level trough will be approaching from the west tomorrow. Tonight will be mostly cloudy with temperatures not falling too much, with southerly flow and clouds increasing ahead of the front. Then the front will cause some showers to spread east during the day. Later in the afternoon and evening, dynamics will increase from a strong southerly jet, tight thermal gradient, and a moisture fetch from the Gulf of Mexico. Thus, rainfall may become heavy at times tomorrow night. There’s even a slight chance for isolated thunderstorms as some elevated instability increases.

Despite more cloud cover and rainfall, southwest winds ahead of the front will usher a warmer airmass into the Northeast. Temperatures will likely rise well into 50s and could approach or exceed 60 degrees in some spots, before the cold front moves through late tomorrow night. On Wednesday, the cold front will moving offshore. Any lingering showers early in the morning will give way to clearing skies and more sunshine by the late morning and afternoon hours. While colder air will be filtering into Northeast during the day, northwest downsloping winds should help temperatures rise into the upper 40s to lower 50s for highs.

However, it will gradually turn colder as the rest of week goes on, as a strong ridge builds over the West Coast, in response to a deep low near the Aleutian Islands. Temperatures will trend downward to several degrees below normal by Friday and the weekend in the Northeast. We are also monitoring a couple of disturbances that will be digging on the downside of the ridge and circling around base of the upper-level trough. These systems will bring a chance of some wintry precipitation to the parts of Northeast and Mid-Atlantic regions.

A southern stream shortwave will be running up the east coast Friday into Saturday. Model guidance has been showing various solutions regarding how much this disturbance will be able to amplify and interact with polar shortwave energy also digging down into the Midwest. Some model solutions show the polar shortwave energy becoming more amplified by itself, causing the southern stream shortwave to remain less amplified or sheared out. This leads to a weaker/flatter wave of low pressure moving farther offshore and little or no precipitation for parts of the Northeast.

However, a number of features stick out when examining the upper-level pattern and potential dynamics with this system. First, the presence of a strong coupled upper-level jet streak, that models show lying just west of the I-95 corridor and mid-level positive vorticity advection and q-vector convergence over Northeast and Mid-Atlantic. This could cause the low to deepen and track a little further west and also expand some precipitation further northwest. Then the polar shortwave could also still amplify and bring some light wintry precipitation Saturday night and Sunday.

At this time, though most model and ensemble guidance favors the bulk of moisture and dynamics with the southern stream low to remain just offshore, and then the clipper system staying weaker, while trying to reform a secondary low over the Northeast. This solution is plausible, as the flow over the Atlantic remains somewhat fast and zonal, without a more profound upper-level low feature underneath the Greenland ridge to help buckle the flow with more high-latitude blocking. So while details are still going to be ironed out this week, we don’t anticipate that this will be a major winter storm. However, many European Ensemble members were quite interesting with this storm, as the ECMWF guidance in general was deeper and more buckled with the trough — about a dozen members showed 6″+ of snow. Thus, this certainly bears watching. Regardless, there could be enough snow or wintry precipitation to cause some travel issues for parts of the Northeast or Mid-Atlantic on Friday.

Afterwards, the pattern will continue to support more colder than normal temperatures next week, as a new deeper low develops south of the Aleutian Islands and causes the ridge to amplify again over the West Coast. Temperatures may struggle to rise above the 20s or lower 30s for highs next week in the Northeast.

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