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forecast

Snow Threat: Jan 10-12 (first call)

Here is my preliminary call for the 10-12 January 2011 event:

 
The operational models have not necessarily been agreeing with each other as to the storm’s track, but each model has had their own consistencies in producing snowfall along the East Coast, so I decided to venture a first guess. A final map (and maybe an intermediate map) will be issued as the event gets closer.

It will be interesting to see if the GFS (and NAM) snow-enhanced zone north and west of Richmond, VA will verify as it seems to like having the secondary low over the southern Midwest a little better than the ECMWF does. Mixing will be a problem in NC, and that will likely need to be adjusted a bit. I haven’t thrown in anything above 8″ yet, but that may become a concern when making the update.

Snow Threat: Jan 7-8 (final call)

After the dusting to a quarter-inch of snow that we saw just before sunrise today in the DC area, another round of snow will be on tap tonight and into the afternoon tomorrow.

DC could see 0.5-1.0 inches of snow by the end of tomorrow.

Areas to the east near Baltimore could see 1-2 inches, with isolated areas of 2-4 inches in the Delmarva area.

 

January analogs using Niña winters and strong blocking

Januarys with a moderate/strong Niña in NDJ (-1.0 or lower) and the corresponding January AO and NAO values 1950-present:

Year (AO, NAO)

1951 (-0.085, +0.08)
1955 (-1.163, -1.84)
1956 (-1.204, -0.22)
1965* (-1.046, -0.12)
1971* (-0.163, -1.13)
1974** (+0.232, +1.34)
1976 (+0.034, -0.25)
1985 (-2.806, -1.61)
1989* (+3.106, +1.17)
1999** (+0.110, +0.77)
2000 (+1.270, +0.60)
2008** (+0.819, +0.89)

* Years where the previous winter had a weak El Niño
** Years where the previous winter had a mod/strong El Niño

Using the four highlighted years, which indicate a mod/strong -AO during January (no weak -AO cases), we get this composite for January, using 1971-2000 normals:

 
1965, the ONLY analog from 1950-present with a mod/strong -AO coming out of any sort of Niño to mod/strong Niña transition year:

 

And here is more along the lines of what people were thinking, with years coming off of a mod/strong El Niño (much like this year):

 

A much colder January than you might expect!

As the first half of January comes into better focus, it appears that the blocking pattern over the Atlantic which has allowed the eastern half of the U.S. to be so cold in December will be making a comeback for a good part of January, which means another round of cold weather (but perhaps not as cold as December in the eastern half of the U.S.).

The forecast made by me (and made similarly by many others before December) argues for a more typical La Niña January, which calls for above normal temperatures through most of the country. However, typical La Niñas do not feature a large blocking pattern over the Atlantic, which prevents storms from progressing eastwards over the Atlantic Ocean. Instead, these storm systems retrograde, or move westwards, across Canada. The result is a greater northwesterly flow across North America, which, coupled with cold air from Siberia and western Canada, brings deep cold all the way down into the Southeast U.S. Conversely, the flow of maritime air from the Atlantic into eastern Canada results in above normal temperatures in that region.

What will be different this time around is the weather pattern in the West, which favors more storm systems digging southward along the West Coast as opposed to diving in west of the Rockies. The result will draw the below normal temperatures into most of the West as the upper-level energy travels across the southern U.S. The flow coming out of the mid-continent will suppress these storms and keep them further south than what would normally happen if there was no blocking pattern. Should the blocking pattern relax, these storm systems would be able to run northeast in the eastern U.S., which would bring the return of near normal to above normal temperatures to the East.

The result of the January weather pattern would be below normal temperatures for nearly the whole U.S., but the cold will at least be less dramatic in the eastern U.S. as was seen in December, which had a much more meridional flow with above normal temperatures in the western U.S.

If we were to keep this blocking pattern for most of January, this is what we could be looking at with respect to temperature:


Risks:
– Warmer West
– Warmer Southeast/Mid-Atlantic
– Colder mid-country/Southeast
– Warmer Great Lakes/Northeast

Did I just say the Southeast had warmer AND colder risks? I most certainly did. Why? Because with more storm systems digging into the West, we could see them lift north more quickly as they work into the Southeast. This goes for the Mid-Atlantic as well. Conversely, if the blocking pattern we get is stronger than expected, the deep cold would be able to work farther into the Southeast.

Snowfall forecast for DCA and Montgomery County

Very In-My-Back-Yard (IMBY) forecast, but since I’ve been out of town and not keeping up with the weather I can’t really make a nice snowfall forecast.

With so much uncertainty with the cut-off for the big snowfall totals, I will go with 3-6″ of snow for DCA and Montgomery County (65% confidence), with higher totals (including 10″+) in eastern MD and 2-4″ further west in places like Leesburg, VA. There is about a 25% chance that DCA and MoCo could see higher than 6 in., and about a 10% chance that they could see less than 3 in.

This will be an event that will have to be closely followed on radar to see where the significant snowfall line sets up. The difference of a few tens of miles could be all it takes to go from “meh” to “wow this is a lot of snow!”

Either way, it looks like this snow storm will put DCA in the above normal snowfall category for December (we’re only ~1.2″ short of normal at this point), which would be a great start to a La Niña winter in the Mid-Atlantic as they tend to have below normal snowfall totals for most of the region.

Mid-Atlantic – The Week Ahead (Nov 14-21)

Skies will be mostly cloudy tonight through tomorrow, with a few showers over the Appalachians as a weakening cold front tries to push through the region. This front will stall-out over the region as a new low pressure system develops in the Southeast. This new system will draw a good amount of Gulf moisture northward as a broad area of rain moves over the region Tuesday and Wednesday. Some areas will see 0.5-1.0+ inches of rain. The models are having a tough time with regards to the development of this system, but both agree that this will be a good rain event for the region.

There will be a weak disturbance coming out of the Midwest soon after the Tue/Wed storm passes through, which could bring some light snow over the northern parts of the region to go along with the scattered rain showers, depending on the timing of the disturbance. The NAM has a slower spacial progression of both systems, which would be more favorable for snow at this point since the NAM would put rain/snow showers over the region Thursday night. In contrast, the GFS has rain showers over the region during the daytime hours on Thursday. It looks like there won’t be enough cold air available for D.C., Baltimore and points south to get cool enough for snow.

Temperatures should quickly rebound to near normal to slightly above normal for the weekend as a subtropical ridge over the Southeast provides our region with winds from the southwest. All-in-all, we should have another beautiful weekend with mostly sunny skies.