High pressure to our south slides eastward as a fast-moving yet strengthening Mid-Latitude cyclone now over the Rockies threatens on Tuesday. Arriving in western PA around daybreak and leaving eastern PA around midnight, precipitation type is a tough question since the rain-snow-and-mix lines converge over the state. Behind the front, the winds will pick up and lake effect snows will bombard the western parts of the Commonwealth on Wednesday. For Thursday, the sun will come out, but the high pressure system that will provide the sinking air is in west-central Canada. Another system will clip the state from the south, heading east/ east-northeast for the weekend with wintry precipitation possible, heaviest near the PA-MD border.
Models Used: 00z & 18Z NAM, GFS, & Hi-RES NAM, 12Z ECMWF, 21Z SREF
Early Week
Models over the weekend have been consistent between each other and between runs. The question is where exactly to place the precipitation type. Forecast soundings showed that the layer between the surface and 900 mb are isothermal very close to the 0°C line with colder air above the 850-mb level. Southern PA and the northeast zone will see rain and wet snow with some freezing drizzle. Central PA will see snow, sleet and freezing drizzle and the rest of the state sees all snow. The Hi-Res model has been the most helpful, but the hour-by-hour guidance for the entire storm won't be available on the eWall until Monday's 12Z or 18Z runs. The winds pick up overnight.
Mid-Week
Wednesday – The Hi-Res NAM & 12-km NAM have been the most helpful with both wind and lake-effect snows. As for wind shear, it won't become favorable for lake-effect snows until the morning hours. The fetch will be long (some energy from Lake Michigan) but short-lived. The fetch off Lake Erie will be enough to sustain long bands of heavy snow in the northwest and north-central zones. With the winds, getting snow to accumulate to a dusting in State College and Altoona (two cities past the Allegheny Ridge) is not too tough a task. With the fetch off of Lake Michigan, accumulations near Johnstown are not out of the question. The instability is enhanced by the long-wave trough expected to pass over the Lakes around the same time.
Thursday – The high pressure over Central Canada I alluded to in my synopsis will become elongated.
End of Week
The 850-mb 0°C isotherm will be just north of the Mason-Dixon Line. The GFS places the heavier precipitation in extreme southern PA.
-Jaron Breen
Addendum to Thursday's Forecast:
ReplyDeleteModel guidance suggested that while the high is far away, some of the energy/ ridge will make it into PA, allowing for clearing skies. Guidance will need to be monitored for any hint of position changes of the high & elongated ridge.
-Jaron