* Pattern Evolution Begins Next Week *

I continue to see indications that the mild pattern of the past three weeks will soon begin to fade. Some high-latitude ridging looks to develop across the Northwest Territories of Canada by early next week and that will direct an initial blast of Arctic air into the Great Lakes and Northeast for the Thanksgiving holiday. Until then, we'll remain in a pattern dominated by Pacific and maritime tropical air masses, and thus temperatures will continue 4 to 8 degrees above normal through next Wednesday. Precipitation-wise, this morning's drizzle will gradually develop into a steadier rain late today into tonight. Sunny and mild conditions will then return Friday into Saturday before a southern storm brushes by with a chance of rain later Sunday into Monday.

The first step in the pattern evolution will come the middle of next week as a strong trough digs across the Great Lakes on Wednesday and to the East coast by Thanksgiving morning. A brief blast of unseasonably cold air will drop in Thursday into Friday, so I expect high temps to pull back into the 40s for the end of next week (upper 30s in the mountains) with wind chills in the 20s and 30s. Some lake effect snow showers will likely fly across the mountains of northwestern PA. Still, I don't see the cold locking in for the long haul just yet--for that there has to be some downstream blocking over Greenland (which I see no sign of yet). Consequently, I expect early December to feature several progressive blasts of cold air passing through with modest warm-ups between cold spells. This up-and-down pattern can produce snow given just the right storm track; although I don't see anything in the pipeline for southeastern PA just yet.


Monday's discussion:

I see no changes this morning to my story line from last week: Arctic air is locked up in Alaska and Siberia, while jet stream ridging continues over the southeast US; which means temperatures here will remain at or above normal into late month. Weak passing cold fronts can bring temporary "cool downs", however. For example, Sunday's high of 71 degrees--which is pretty amazing for mid- November--will be cut down a high near 60 degrees today and a high in the mid 50s on Tuesday, due to a cold front that dropped through last night. Still, average high/low temperatures for this week are 53/32, respectively, and I don't see us dropping below those levels at all this week. In fact, another warming trend may develop by Friday and Saturday with highs rising back to the upper 50s or low 60s.

Precipitation-wise, we'll continue with the relatively dry pattern established this month. The first two weeks of November have yielded less than a half-inch of rainfall, a dramatic change from the October pattern that produced nearly 6 inches of rainfall. The only threat of showers this week will come Thursday into Friday as an upper-level system lifts slowly from the MS Valley into the Great Lakes. High pressure should lead to improving conditions later Friday into Saturday, before a more-moist southern storm perhaps threatens late in the weekend.

Beyond that, the first half of next week looks to continue with the mild pattern. However I do see the potential for a jet stream rearrangement beginning the second half of next week. This could lead to a turn to colder conditions in the day following Thanksgiving. So enjoy the mild weather while it lasts!

Issued: (Horst) 11/19/09