Wednesday 9 a.m.
Dry and moderately chilly weather will prevail for the Middle Atlantic region and southern New England for the rest of the week. A warming trend that spreads into the Great Lakes and Ohio Valley Friday and Saturday will be delayed in the Northeast by an onshore flow coming around a high pressure that will be over northern New England on Saturday. In fact, as that cold high pressure area approaches, the flow ahead of it will cause some limited lake-effect snow downwind from lakes Erie and Ontario. Temperatures will be hard pressed to get above freezing from east-central New York state through central and northern New England on Friday. This video has more:
Early next week looks to be much milder than usual for early December. This upper air flow forecast shows why, with the flow in the Northeast coming from the Southwest.
The pattern temporarily reverses later in the week, and a cold shot looks it will replace the warmth. However, the cold air mass may come then leave rather quickly.
Since yesterday, a high pressure area has expanded into New England. If you trace the flow using the isobars, you can see that the previous flow from the ocean has largely weakened. This means warmer afternoons for New England but causes cold nights.
The current surface pattern clearly shows the flow that is bringing ocean-cooled air in from the east. If the high pressure area to the north expands southward as expected by tomorrow, this flow will be weakened and perhaps cut off.
The cold front is embedded in a large area of clouds, showers and thunderstorms covering the middle of the country. It will advance past the Northeast coast by the end of the day Sunday. The frontal zone is shown on this picture from late morning Friday.
When weather systems are relatively weak, small scale variations cause forecast uncertainty because several different weather types (such as showers, sunshine, cloudiness, etc.) can coexist in the same region and change constantly. This map shows such a pattern:
Some of the thunderstorms can become severe, with damaging wind and brief cloudbursts of rain. The greatest chance for locally severe storms should be in the "S" areas highlighted on this map (based on the NWS Storm Prediction Center's guidance).
There is a slight risk for severe thunderstorms later today from north-central Tennessee up across Indiana and Ohio to Michigan and eastern Wisconsin (shown by the "S" area on the map below. Thunderstorms are not predicted for areas near the coast from Delaware to New England.