11/8/17: Up & Down Pattern Giving way to Cold?

EPS 42

European ensemble forecast upper-level pattern for early Friday morning

Ohio saw 16 tornadoes on Sunday and damaging thunderstorm winds in excess of 100MPH, and now we’re looking forward to near-record cold for Friday!  It is certainly fall in the Great Lakes and Ohio Valley.  The late-week cold shot will be very brief, due to some flaws in the pattern if you want cold, but will also be quite intense.  A lack of blocking over the Atlantic (the North Atlantic Oscillation or NAO is near neutral) and Pacific jet hitting the West Coast fairly strong will cause moderation over the weekend and into next week, but not before we “endure” a day or two of very cold conditions for this time of year:

EPS 54 temps

European ensemble surface temperature departure forecast valid Friday afternoon

The above map is in Celsius, meaning that anything in the blues (-8C or greater) is a departure of over 14.4 degrees Fahrenheit colder than normal, with some of the darker blues (as cold as -12C to -13C) representing air of over 20 degrees F below normal! (yes, as a meteorologist I’m rather fluid with my Celsius to Fahrenheit conversion).  This is near-record cold and will be a rude awakening for a huge portion of the Midwest, Great Lakes, Ohio Valley, Northeast, and even into the Mid Atlantic.  Despite the warmth of the last two winters, it still can (and will) get much colder than average.

NAM3 snow

High-resolution forecast model snowfall forecast through Friday night

There will not be a ton of snow with this upcoming cold wave…outside of the lake effect Snowbelts and some of the higher terrain in northern New England where there will be a little bit of upslope snow, it will simply be “cold” but nothing more.  Parts of the Upper Peninsula and northern Lower Michigan along with parts of Ontario downwind of Lake Huron will see moderate to locally heavy lake effect, a sign of the season more than anything else.

EPS 144 500

As mentioned above, the big cold won’t last all that long due to a lack of “blocking” over the northern Atlantic (the NAO will turn positive) and due to the Pacific jet flooding much of the US with milder air (due to a –PNA).  This pattern won’t be extremely warm over the east, due to the low-level cold from the weekend being a bit slow to scour out, and due to a weak trough re-developing early next week along the East Coast, but it will modify to closer to normal (which outside of the far northern tier is still too warm for snow in early to mid-November).

EPS 264 500

European ensemble forecast upper-level pattern valid November 19th

Our mid-range forecast ensembles have been in persistent agreement in developing a –NAO over Greenland/the Davis Strait/northeastern Canada the weekend before Thanksgiving, which adds some intrigue over the eastern US as it favors a trough near the East Coast.  This pattern still won’t be very cold at this point given an expected –PNA continuing to blast the Pacific jet into the West Coast.  This will cause much of the central US to be mild, and this will butt up against the cooler weather over the east.  Note how the air feeding into the trough over the east will be coming from southern Canada initially due to the unfavorable pattern over the western US for eastern US cold, which means that although this –NAO development will likely force temperatures near or a bit colder than normal starting the weekend before Thanksgiving, especially over the Northeast, that it won’t be THAT cold initially.

GEFS 264

GFS ensemble forecast upper-level pattern valid November 19th

The GFS ensembles show a bit of a more favorable look with the trough centered a bit farther west, which allows for enough ridging over the western US for the eastern US trough to tap somewhat colder air.  The GFS ensembles would suggest a quicker change to farther below normal temperatures and potential snow in the interior Northeast and Great Lakes.

EPS 336 500

European ensemble forecast upper-level pattern valid November 22nd

The European ensembles do suggest the potential for a gradual change in the pattern by Thanksgiving that could allow colder weather into the eastern US, with the trough off the West Coast edging a bit farther northwest, allowing modest ridging to build over the western US that allows for a modestly deeper trough over the east.  The ensemble also generally maintains enough of a –NAO to help encourage the eastern US trough (though it’s not exactly a prolific block…but is a solid signal for 14 days out on an ensemble mean).

GEFS 336 500

GFS ensemble forecast upper-level weather pattern valid November 22nd

GFS ensemble forecast upper-level weather pattern valid November 22nd

The GFS ensembles remain more bullish on eastern US cold than the European ensembles, with a stronger –NAO, and with the West Coast trough backing off more, allowing for a taller ridge over the western US.  This look would be fairly cold over the eastern US near Thanksgiving and would allow for decent snow chances, especially in the lake effect Snowbelts and in the interior Northeast.


European ensemble (left) and GFS ensemble (right) 5-day surface temperature departures from November 19th-Novemebr 23rd

Both ensembles do have a cool/cold period over the east/northeast heading into Thanksgiving, but the differences in the GFS and European patterns result in a few to several degree difference in surface temperatures over the 5-day period.  The European backs the trough off the West Coast to the west slower, causing the Pacific jet to blast into the west for a longer period of time.  It also has less of a –NAO.  This results in milder air protruding farther east, and also prevents the eastern US trough from tapping as much of the cold air over the northwest portion of the continent.  Either outcome would potentially result in some snow chances over the Great Lakes and interior Northeast (especially New England), but the GFS ensemble pattern would be more conducive for snow, especially farther south.

EPS 240 500

European ensemble upper-level pattern over the Pacific valid November 18th

The big key will be when the above northern Pacific pattern breaks down or changes some; the above forecast is for 10 days out, and shows a persistent/deep trough over eastern Asia, that continuously pumps up the north-central Pacific ridge, which in turn keeps forcing a trough near the western coast of North America.  The Euro does not really break this pattern down by Thanksgiving (though does show some potential changes right at the end of the run).  The GFS ensemble does begin breaking this pattern down, allowing for more of a ridge over the west.

MJO ensembles

GFS and European ensemble MJO forecasts for the next two weeks

One potential cause for the difference in the handling of the Pacific pattern among the ensembles is different handling of convection in the deep tropics.  The GFS ensembles have persistently shown the MJO (Madden Julian Oscillation; a “pulse” of clouds and storms in the tropics that affects the jet stream pattern in the mid–latitudes) getting into phases 8 or 1 (both cold over the eastern US) by Thanksgiving week, while the Euro ensembles have been consistently less bullish.  The GFS ensembles have been trending towards the Euro on the MJO for over a week, so at this point I’m inclined to believe the Euro over the GFS…suggesting that while a –NAO will allow for somewhat cooler normal conditions to develop over the eastern US (especially Northeast) next weekend and through Thanksgiving, that a less than hospitable Pacific pattern will keep it from getting very cold.  With that said, we’re far enough out for some changes from both models…the idea of cooler weather is legitimate, but how cold is certainly in question (as is the amount of snow it brings).

Moving into December, the forecast pattern on the European ensemble in particular for later this month has some key similarities to my “analog” years for December.  Analog years are years that featured similar patterns in global ocean temperatures and large-scale weather patterns to this year and may give clues to how the upcoming pattern may evolve.  Here is a look at the upper-level pattern in my analog years for December:

analogs dec

Analog years upper-level pattern for December

I circled a few key areas: the higher heights from the Davis Strait east across the northern Atlantic, the higher heights over the Bering Sea points north, and the lower heights over northeastern Asia.  Here’s a look at the European ensemble mean forecast pattern valid at the end of the run (November 23rd):

EPS 360 500 circles

European ensemble forecast upper-level pattern valid November 23rd

Some differences, but the three circled areas are similar.  The difference lies in how the Euro ensemble is blasting the Pacific jet in the western US, which floods the western 2/3rds of the US with milder air.  The analogs in December were cold overall for much of the central and eastern US:

Dec analogs

Analog years mean temperature departure for December

Does this mean we’ll have a cold December this year?  Not necessarily.  BUT, some key similarities to my analog years may be developing in the high-latitudes late this month, which if they were to hold could mean colder weather is more likely than warmth in December.  This will be something I watch closely over the next couple of weeks.

So, to summarize:

-BIG cold shot late this week in the Northeast is short lived and gives way to more seasonable conditions next week

-A negative NAO will likely lead to “cooler” weather returning to the east the weekend before Thanksgiving and lasting through that week, but uncertainty over the Pacific pattern, which may remain somewhat hostile towards bigger cold, makes it difficult to say exactly how cold it will get

-Signs are starting to point towards a cold December in the central and eastern US (especially farther north), but there are some issues to work out that I’ll be watching over the coming weeks











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