Raleigh Weather Discussion
The coldest week in awhile is upon us as arctic air has descended into the area. We will struggle today to climb out of the mid 30s as we have constant cold air advection. Tonight will be a very cold night with temperatures in the mid to upper teens across the state. We were last in the teens on 1/4/12 when RDU airport hit 19. Before that we were 16 on 1/23/11 and 17 on 1/22/11. So as you can see this is the coldest air mass in over a year and could be the coldest in a couple of years.
A weak upper level disturbance will move through tomorrow night with the chance for some snow showers in the NC Mountains but likely little east of the mountains. It will remain cold though through the week and I am watching the Friday afternoon/evening period for the chance of some light wintry precipitation.
The setup is this; cold arctic air will be in place over the eastern US late this week while an upper level disturbance moves into the Ohio Valley. The arctic air is going to be tough to displace and we will likely see a hybrid or in-situ cold air damming setup. The models have been trending southward with this disturbance and suggest the main low pressure system will move into Kentucky and then reform off the Va Capes. In this setup we would likely see snow for the mountains of TN/NC, KY, and much of Va. While across western and northern NC we would probably see a brief snow to a wintry mix event. Of course it depends on the track of the low pressure system. Some prior runs and some other model data suggest a further north track with the parent low move into Ohio/WV and reformation off the mid-Atlantic coast. In this setup it is possible for a brief period of wintry precip at the onset but we would likely see mostly rain for much of the area. However, some ensemble members are further south with the track taking the parent low into TN and reformation off the Carolina coast. This would result in a colder solution and likely more snow. So this could go either way. The operational ECMWF model has been consistent the last few runs showing a track into KY and reformation off the Capes and painting several inches of snow in the mountains and a snow to wintry mix scenario for the foothills and much of the northern piedmont. This will be an event where the further north you are the better chance you will have. Many of the ECMWF ensemble members are also on board, so it is enough for me with this now being 4 days and in to have some confidence. I would like to see the NAM come aboard and the operational GFS become a bit more consistent. The 6z GFS was cold enough for wintry precipitation but was rather tepid with the precip, with most areas getting less than 0.1 of an inch of liquid QPF.
Unlike the last event, cold air will have been entrenched well before this system arrives and will be in place as it arrives. The question is whether or not the system will take a track to knock the cold air out before precip begins to fall. Right now, I favor a moderate snow for the mountains and a wintry mix east of the mountains roughly along the I-40/85 corridor north. The time frame for the RDU area as of now will likely be mid to late afternoon through evening. Right now, I doubt this would trend to a major event, as this is not a gulf low with copious moisture, but it could become a nuisance event for sure. Especially with cold air likely to move in behind it.
In fact, if the ECMWF is right, temperatures would struggle to get above freezing Saturday and possible crash into the low to mid-teens Sunday morning. For now, I will not go that cold, but this weekend looks to cold and if there is a snow/ice fall on Friday it will likely stick around through the weekend.
We will see a warm-up next week though as a trough moves into the western US and a ridge pops back up over the Southeast. This will likely lead to a week of highs in the 50s/60s next week with rain likely by mid to late week. It does appear though that this rain will be on another strong cold front which will likely lead to another cold period by late next week and into early February.
National Extended Weather Discussion/Forecast
It does look like this system on Friday I discussed above from a southeast US perspective, has the potential to produce a nice 4-8 inch type snow across the Ohio Valley/mid-Atlantic and Northeast coastal sections including the big cities.
The ensembles are in pretty good agreement that a fairly deep cold trough will re-establish itself over the Midwest/East by late next week and into early February. The main culprit appears to be the PNA turning positive as a ridge forms along the west coast of the US into British Columbia and we maintain some blocking in the AO and NAO regions, although the NAO index is neutralish to weakly negative literally. The MJO wave has stalled in phase 7 with model projections show in slowly progressing into phase 8 over the coming couple of weeks. This is normally a good thing for cold lovers in the east as phases 7, 8, and 1 are typically cold phases for the East.
So in general it looks like the 11-15 day period is going to be cold in the East/Lakes and warmer in the West/Rockies. I think there is support to think that at least the first half of February is going to be cold for the East.