Feb. 3 Technical Discussion

Synopsis:

 

A rather mild and calm weather will continue throughout the remainder of the week... A high pressure will build in the wake of the storm system that just passed south of the region yesterday. Expect a few clouds from time to time throughout Friday and Saturday. A strong system currently spinning up some severe weather in the Midwest will gradually weaken as it moves eastward. This system is expected to stay to our south, giving us only a few high clouds on Sunday. More clouds will pass over the Commonwealth Sunday night as a cold front drops down from Canada. The cold front will be relatively weak, so do not expect major temperature behind the cold front. A cutoff high will approach from the Midwest and keep the sunshine in store at least until the middle of next week.

 

Short-term (Friday to Sunday):

 

A 500-mb shortwave and its associate surface low will rapidly move away from the region on Friday. An upper level ridge will briefly build in behind the passage of the 500-mb trough. As result, the skies will become partly cloudy and the temperature will remain slightly above the season normal. However, a series of upper-level troughs will dip down from southeastern Canada starting Saturday afternoon, giving us a chance of a few passing clouds. Generally do not expect any precipitation to fall during the passages of the shortwaves, though extreme northeastern PA may see some isolated snow showers on Sunday due to stronger convergence. In addition, most of the precipitation should be orographically enhanced as the winds impinge on the western slopes of the Appalachians.

 

Mid-term (Monday to Tuesday)

 

Mostly sunny to partly cloudy skies will settle in the region on Monday as the upper level trough slides eastward. The temperatures should not change significantly as the cold air behind the cold front will be relatively weak. Some high passing clouds might remain over the region as a possible, shallower trough swings through, as indicated by the 00Z WRF. The GFS does not have this shallow 500-mb trough sweeping across the Northeast. Therefore, any cloud cover should remain sparse across the region on Monday and into Tuesday.

 

Long-term (Wednesday-Thursday)

 

A cut-off low currently spinning over the Rockies will slowly move eastward throughout the forecasting period. This surface low is associated with a broad 500-mb low cutoff over the Rockies. As result, expect slow movements and gradual weakening with this low. To the north of the low, a 'cut-off' high will weaken similarly as it shifts eastward… However, the high pressure system should help to keep the surface low far enough south that the region only sees a few high clouds flying over head on Wednesday and Thursday. Otherwise, expect partly cloudy skies for the middle of next week.

 

Changed discussion… a few short range models have trended toward a more significant interaction between the cut-off low and an upper-level trough dropping down from Canada. The trough may become negatively tilted enough that a low pressure system may form just offshore. Thus, we increased the probability of seeing some clouds or even light precipitation in southern portions of PA.

 

Special note on next weekend's storm potential:

 

This forecast discussion might be viewed as hype now because of the general spring-like weather pattern we have seen so far this winter season... However, the potential for a S.E.C.S (possibly a M.E.C.S) does exist for next weekend. For several days medium range GFS have hinted a marginally favorable 500-mb pattern to set up sometime next weekend. A vigorous 500-mb shortwave crashing into the Canadian Pacific coast will split into a northern stream and southern stream. This southern stream is what we should pay attention to as next week goes on. This southern packet of positive vorticity is forecast to swing southward into southern California; meanwhile a polar vortex will set up near Canadian maritime. Some model ensembles indicate this polar vortex may retrograde slightly westward toward next Thursday to allow for a series of shortwaves to spin around the vortex center. It will be interesting to see if any of the 500-mb shortwave dips far enough south to interact with the split 500-mb low entering southwestern U.S by the end of next week. A favorable synoptic set up for a coastal storm will depend on:

 

1) If a moderate ridge sets up over Greenland to induce a moderately negative NAO. This will help to push the polar vortex westward and keep it over eastern Canada instead of moving offshore. 500-mb shortwaves will be able to spin around this polar vortex to allow for multiple trough phasing opportunities.

 

2) The strength and speed of the southern stream trough entering southwestern U.S. The trough has to stay strong enough as it propagates over the mountainous terrains of southwestern U.S. The speed has to be fast enough so that it catches up with the retrograding motion of the polar vortex in eastern Canada.

 

3) The cold air must be in place. If the polar vortex phases with the southern stream, a high pressure might develop ahead of the trough to keep some cold air over the region. Though as of right now, it is too early to say whether this will happen. Warm air might completely take over the region if the troughs phases too far west.

 

4) Hardest of them all, the phasing. If no 500-mb low drops far enough south for phasing, then no storm occurs or storm forms offshore.

 

We have about more than a week to monitor the situation, so stay tuned!