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2011

January 2012 temperature forecast – Final

Blocking has failed to develop once again, leading to warm anomalies across most of the U.S. overall. There are some signals indicating we could see a prolonged turnaround in temperatures towards the second half of the month, but it looks like the warmth will control the pattern overall through January. Arctic air could start to intrude into western Canada around mid-January, and that would make it much easier to bring negative anomalies into the central and eastern U.S. IF we can get some blocking in the Atlantic.

The start of January will be cold in the East as an Arctic air mass moves through, but this should be transient in nature as the PNA ridge spikes in the western U.S. before fading into a -PNA regime. Aboves return by the end of week one and holds through at least week two. It’s going to be a tough balance between the aboves and belows in the central U.S. if the colder air can start to intrude into the U.S. from western Canada.

Here’s a look at the previous January forecast for comparison:

Christmas weekend snow thoughts and January update

UPDATE 12/22: Doesn’t look like it’s going to be a white Christmas unless you try to scrape out a little orographically-induced snow in the western Appalachians (which even then is a bit of a stretch unless the 500mb vort. from the northern stream intensifies like on the 12z GFS). I won’t put chances at 0% but it’s certainly very low.

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There is still a lot of disagreement between the models concerning the potential Christmas Day winter storm. Right now the 12z Euro Op. is bringing front-end snow into the Mid-Atlantic followed by rain that lasts until the end of the event. While the Euro has trended northwestward with this system in recent runs, it is not worth looking into the trend too much at this range considering the high amount of uncertainty with this system and its ability to jump wildly in both position and time in ANY direction. It is my personal opinion that true run-to-run trends aren’t that telling of specific winter storm systems until we get within the 72 hour window.

This isn’t even bringing the GFS Op. into account, which over the last two runs has failed to produce a system even remotely like the Euro. The lack of support by the GFS is another good reason to dismiss any trends on the Euro at this stage of the game since there’s obviously a lot of different possibilities still on the table.

Having said all that, I have come to the conclusion that this potential storm needs to get “punted” for at least another day (more likely two days). There’s no reason to call for snow, rain, mix or whatever at this point since the models can’t even come up with similar enough solutions. Given what usually happens when there’s a huge disagreement between the models, I’m going to say that the GFS picks back up on the storm by the 00z Thursday run while the Euro starts to lose the “trend” it has displayed over the last couple of runs and become more random with its shifts. Right now the Euro typically has an edge on the GFS in sniffing out the general storm track and timing, so I’ll give the Euro a heavy lean for now, but by no means am I going to take it verbatim.

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As far as January temperatures are concerned, I’m going to have to do a similar thing I did with December and basically flip my forecast around, resulting in a cool West and warm East. Aside from some blips in the stratosphere, there really isn’t anything to stop the eastern U.S. from “torching” in January. AO and NAO look to remain solidly positive going into the start of January as the PNA tries to shift negative (though I have a feeling it might stay neutral to slightly positive, which doesn’t have much an effect on the eastern U.S. given the much stronger AO/NAO signal). The forecast becomes a lot more uncertain heading into the second half of January, but confidence in a warm start to the month is higher than normal given what the medium range signals are forecast to be.

For the month as a whole, I currently have the East Coast around 2-3F above normal, but the first half of the month could easily be 3-5F above normal before any potential for a lasting period of cooler temperatures. I will post a U.S. forecast map for January at the end of the month.

Snowfall Verification for Dec 7-8

 
Quite a trying forecast, but overall it wasn’t a complete loss. Verified the mountain areas and highest totals fairly well, but the more heavily populated areas left much to be desired. Looking back, I could have been less generous with the 2-4 inch range in northern MD and southeastern PA, but other than that and bringing the 1 inch cut-off further north in MD I don’t see much that I could have done to change my overall thinking for this event.

The changeover from rain to snow was fairly messy east of the mountains, and most did not even transition to 100% snow. On top of that, the snowfall ended more abruptly and sooner than I had expected, which prevented the stronger cooling to make it to the surface in tandem with the back edge of the heavier precipitation. That slightly warmer boundary layer is what turned 1-4 inches into little to no accumulation.

Going to be a little generous and give myself a C- overall, though it wouldn’t take much convincing to push that to a D+ considering the bust in south-central/southeastern PA and MD and the western edge of the snowfall.

Winter Storm Threat: Dec 7-8 (final call)

Had to make some big changes in the northern areas… the boundary layer temperatures did not cool off as much as I thought it would on the models in addition to a quicker end to the snowfall.

 
I think that a couple of areas will be able to top out just above 8 inches in the mountains, so I left a small area over some of the WV/MD pandhandles.

Snowfall totals should not get hurt too much after the transition to snow finally occurs as a small period of sleet during the transition could/should provide a decent base layer for the snow to accumulate on. Keep eyes and ears peeled for the possible thundersleet/thundersnow during the transition from rain, especially from the northern VA/DC region up through central PA.

Winter Storm Threat: Dec 7-8 (first call)

Think 10/29 redux, only the temps and QPF line up a bit better for the changeover on the back-end:

 
The upper-level vort. max is potent enough to create an area of strong dynamics along the back edge of the system, which will pull cooler air down to the surface and make it cool enough within the boundary layer for some accumulation as the coastal low forms and tracks northeastward. Some banding will allow for locally higher totals along the axis that has the greatest snowfall through the WV/VA mountains, south-central and eastern PA and northern NJ.

I took the 12z Euro Op. solution and shifted the low center northwestward to account for for some model biases, which actually lines up fairly well with the 18z NAM after that adjustment. I believe the 12z and now 18z GFS solutions are too suppressed with this system given how strong the vort. max is.

The snowfall totals could go up as the storm’s track and QPF amounts get honed in on the models.

Snowfall is expected to start in the mountains of southern WV/VA/NC Wednesday evening, working into the DC region Thursday morning and departing the PA/NJ region Thursday afternoon.

Snow threat for Thursday, 8 Dec 2011

There is still a lot of uncertainty with a storm system that is expected to impact the Mid-Atlantic this Thursday, but there is now enough model support to think of the storm as a legitimate threat.

It is hard to determine where the rain/snow line is going to be at this point… I will try to get an initial snowfall forecast out tomorrow, but if forecast confidence is still very low at that point I might wait until Tuesday. Biggest area of concern is northern VA and areas north of DC, where both the Euro and GFS have snow potential.

The 06z GFS forecast for Thursday (108hr) shows potential for a heavy, wet snow just northwest of DC near IAD:

The upper-level vort. max and vertical velocities with this system support snow for some areas as well. Right now it looks like all rain in southern VA, but that could change. This threat has been borderline for the region for days now, but it’s looking good for snowfall potential SOMEWHERE in the region.

December 2011 temperature forecast – Final

The forecast is based off of weak signals provided by the La NiƱa, -PDO, -PNA, +AO and +NAO.

December is probably going to be messy temperature-wise, so confidence in the forecast is low. It has been a very frustrating forecast.

This is in STARK contrast to my forecast from mid-October:

(forecast anomalies based on the 1981-2010 normals)

So why the big difference? Blocking… or rather the lack thereof. A lot of forecasts called for the return of a -AO/-NAO blocking pattern in the Atlantic, which would pull cold air down from Canada into the U.S. This blocking was supposed to be set up by above normal October Siberian snowfall and stratospheric warming (with some persistence thrown in as well), but neither of these ended up occurring, resulting in a progressive +AO/+NAO pattern. This allows the Pacific to have more of an influence on the forecast as well, which with a -PDO and -PNA pattern puts a trough out in the western U.S. and ridging over the eastern U.S.

The big question for December is “how stable is the pattern?” and the answer is “not very.” Because of the uncertainty in the forecast, a lot of the CONUS is near normal (+/- 1F anomalies), with cooler temperatures in the western U.S. to reflect the -PDO/-PNA pattern and aboves along the East Coast to reflect the +AO/+NAO pattern.