Wednesday, October 10, 2007

NOAA releases winter outlook

NOAA has just released their official winter outlook for the US. Their forecast calls for generally above normal temperatures over the US south and southeast and generally wetter than normal conditions over the Ohio Valley and Pacific Northwest. This is in response to the developing La Nina which would tend to produce below normal heights (trough) off the West Coast and above normal heights (ridge) over the southeast US with a storm track riding from the US west coast into the Great Lakes.

For the northern Plains/southern Prairies.. there is essentially an equal chance of either below/near or above normal conditions this winter.. i.e. a non-forecast. In other words, there is no clear signal how this year's La Nina will affect the southern Prairies one way or the other. If indeed the general synoptic pattern sets up as described above, then there would be a greater likelihood of below normal temperatures this winter over the Prairies, especially the western Pariries, with perhaps above normal snowfall over the eastern Prairies closer to the storm track.

Here in Winnipeg, our last extended La Nina occurred from mid 1998 to early 2001. During those winters,

1998-99… generally warmer than normal.. especially February and March.
1999-2000.. Generally much above normal..with near normal January. (snowcover was gone here by end of February)
2000-01... A real see-saw winter. Third coldest December on record, followed by a mild January then cold February
2001-02.. Generally warmer than normal, cold March

So as you can see, La Nina doesn't necessarily equate to a cold winter here, at least in the eastern Prairies. One also has to consider the remarkably low Arctic sea ice cover recorded this year (an all time low), which presumably would tend to delay the onset of deep and persistent Arctic airmasses in the far north. That could mean a delayed start to winter over the Prairies, before colder than normal conditions set up for the latter half of the winter, mainly over the western Prairies. If that storm track is further north, then Alberta could see a snowier than normal winter with storms off the Pacific, with above normal temperatures more likely over the eastern Prairies.

A TV meteorologist in the US has produced an analog map showing temperature/precip anomalies based on previous La Nina years back to 1950. It supports above normal temperatures in the southeast US, and colder than normal temperatures over the north/northwest US including the southern Prairies.

So there are hints that perhaps we'll see colder than normal conditions at some point this winter, but there are also conflicting signals that reduce confidence in this outlook, especially in light of warming trends in our winters over the past decade and this year's record low Arctic ice cover.

NOAA will update their winter outlook on October 18th and Nov 15th.


  1. I would like to see a winter with above normal snowfall. This would more likely happen with a warmer winter (correct me if I am wrong). But a colder winter with less snowfall doesn't appeal to me. Lots of snow and cold would be o.k., better than cold and no snow.

  2. During La NiƱa winters, Southern Manitoba and the Dakotas seem to get more Colorado Lows than normal. Likely because of the strong temperature contrast and resulting strong jet stream (as opposed to a split flow pattern where the gulf moisture and bitter arctic air rarely interact), and also because of the ridge in the east/trough in the west configuration to the upper flow. This is especially true if the ridge in the east is sharp enough and builds into the great lakes deflecting colorado lows into the upper midwest/southern Manitoba. So it is possible to have lots of snow and below normal temperatures here.

    Also a heavy snowpack turns the red river valley into a vast tundra. A very large percentage of the sun's energy is reflected back into space. Snow covered sufaces also radiate heat very efficiently allowing for very sharp temperature drops at nite. As a result, heavy snow cover over a large area has a strong cooling effect especially in the absence of other strong forcing mechanisms (ex. zonal jetstream from Pacific)