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October 2011

Snowfall Verification for Oct 29

I’d give that a B+ …probably the best verification yet!

The WV/VA mountains and western PA contouring was off, especially along the southern edge of the mountains, but from central VA and south-central PA and points northeast verification was excellent. Virtually nailed the rain/snow line and sharp gradient along the eastern edge.

GFS actually did better than the NAM regarding the contouring along the western edge (not sure about the actual totals).

Winter Storm Threat: Oct 29 (final call)

Here’s the final snow map… it’s crazy for the northern folks. Waited until the 12z Euro came out to make sure it agreed with what my latest thoughts were with respect to this storm. Widespread 4-8″ of wet snow to bring down lots of branches and such that still have plenty of leaves on them. This is going to be messy for the areas receiving several inches of snow or more.

Localized areas of greater than 12 inches are possible in eastern PA and northern NJ. The northern VA/MD/WV mountains could squeeze out 8-12 inches in higher elevations.

I went with a solution similar to the 12z Euro, which is kind of a blend of the 12z GFS and NAM. This is perfect for me, since once we get this close to the event I find that a near-even blend of the GFS and NAM solutions works pretty well most of the time.

Winter Storm Threat: Oct 29 (first call)

The storm will start to develop over NC/VA Friday night as a strong 500mb vort. max dives down from the northern Plains and phases with a short-wave traveling across the southern U.S. As this energy moves closer to the East Coast, a surface low will form off the NC coastline and rapidly deepen as it heads NNE along the East Coast through early Sunday. It will draw in some cool air from the north as it develops, but there is not much cold air available, so the surface temperatures will be marginal for snowfall accumulation in many areas. In the southern edge of the snowfall accumulation, surface temperatures are expected to remain too warm for accumulation of an inch or more. Some higher elevations (above 800 feet) just south of the one inch cut off could scrape out a dusting to an inch of accumulation. Weak warm air advection N/NW of the surface low is expected to keep the lower levels warm enough to prevent most of the accumulation in southern and central NJ and northeastern Delmarva.

What we’re looking at here is a fairly rare event… a solid coastal storm in the Mid-Atlantic in October during a La Niña. Statistically speaking, this storm shouldn’t happen, but it is and here’s the forecast:

I decided to go with a 50/50 split between the latest (12z) GFS and ECMWF operational models. The NAM seems to be out to lunch with this system, as most other models agree with what the GFS and ECMWF put on the table.

I’ll probably do an update tomorrow with some minor adjustments… still trying to figure out the southern extent of the one inch accumulation area, with some uncertainty in the total accumulation in central PA and in the WV/VA mountains.

Models try to lock in on a coastal “winter” storm

For all of the non-mountain Mid-Altantic people out there, I’ll go ahead and say right off the bat that it’s unlikely we’ll see any snow from this event. If this system does develop, it will likely be cold rain throughout, with a slight chance of seeing some snow in the air (NOTE: not accumulating) early Saturday before the storm moves off to the northeast. The real target of this system is northern NJ, Upstate NY and New England.

So what are we looking at? The last couple of model runs from both the GFS and ECMWF are showing a coastal storm… one that is expected to form off the Carolina coastline Friday night into Saturday morning. Exact timing/placement of the surface low will be crucial in trying to get snow anywhere in the Mid-Atlantic. Should the development be too slow (as is often the case in thread-the-needle type events in the Mid-Atlantic), the low will form too far off the coast and will not deepen enough to produce much precipitation in the region. There is a small break between an earlier system and this system to allow for a weak area of high pressure to develop over the Canadian Maritimes, but it would be weak at best. This high pressure would help slow the storm’s eastward movement and allow it to develop closer to the coast. While not critical, it would be a nice feature to have in place if we want to get snow.

Furthermore, we are relying on upper-level energy from the northern stream (i.e. the energy behind that develops this system digs down into the region from the north as opposed to originating to the south). The system could be modeled too far south at this range, and a further north solution would not yield snow for us. Also, there really isn’t that much cold air available… we’ll be chilly, but it will likely stay above freezing in the lowest levels of the atmosphere, which would melt any snow before it gets the chance to hit the ground.

Should the low develop close to land, the eastern slopes of the Appalachians could see some accumulating snow in the higher elevations (at this point we’re looking at places above 1500 feet) if the precipitation can actually make it that far west.

Snow is very hard to get in October in the lower elevations this time of the year, so don’t expect to see much. If the Saturday morning snow threat is still viable in a couple of days, I’ll try to post an initial snowfall forecast Wednesday afternoon/evening.

Winter 2011-2012 (DJF) forecast – Update

Here’s the updated forecast for this winter. I used the same six analog years, with different weights (more below).

IMPORTANT: The updated outlooks use the NEW 30-year normals (1981-2010), whereas the original forecast used the OLD normals (1971-2000). Overall, there were no drastic changes to the forecast.

Winter 2011-2012:

The monthly breakdown:

in [year (weight)] format

2010-11 (more)
1995-96 (more)
2008-09 (normal)
2000-01 (normal)
1970-71 (less)
1962-63 (less)

So what changed? Overall there was slight warming in northern California, cooler Gulf Coast and a warmer northern New England. Most, if not all, of the changes can be attributed to subtle strengthening in the blocking pattern (more negative AO/NAO) that will allow the cold to work further into the south-central and southeastern U.S. At the same time, this blocking pattern will pull slightly warmer air in off of the Atlantic into northern New England. The ridging in the western U.S. has shifted slightly further west, which is why northern California turned warmer. It is also the other contributing factor that caused Texas to turn cooler. Warm risks are in place in Texas and the Southeast as ridging could be more prevalent in these areas.

Precip/snowfall forecasts that I made previously are fairly similar to the update, so there isn’t any need to go into detail about these. There is a wet risk in the Northeast, which I decided to leave as near normal while the region is borderline normal to above normal (especially in southern New England).

From the analogs, 2010-11 was given more weight (equal to the weight of 1995), while 2008-09 and 1970-71 were given slightly less weight. I was going back and forth on whether or not to add 1955-56, but I decided that the La Niña in 1955-56 was too strong from the start of the summer through most of the winter compared to what is forecast (which will likely be a borderline weak/moderate La Niña).

I may issue an update once we’re into the season for the end of winter, but we’ll see if it’s needed.

Warmer temps are on the way… but how warm?

The tyranny of cold, damp and miserable weather is over for the time being in the Mid-Atlantic as a giant ridge of high pressure starts to build into the eastern U.S. The next few days will feature near normal to slightly below normal temperatures, but we’re looking at a warmer period ahead once we head into the weekend.

The placement of this giant ridge will not put the Mid-Atlantic in the warmest temperature anomalies, but it should set us up to be above normal (highs in the mid to upper 70s in general) this weekend through around mid-week next week. Temperatures are expected to stay above average going into mid-month, but possible tropical/sub-tropical activity could hinder our highs as lows stay on the warmer end during the back half of next week.

For now, enjoy the ample amount of sunshine and return of warmer weather. Normals are starting to drop quickly as we head into the latter half of meteorological fall, and it may be awhile before we see lasting warmth like this again.