Tuesday, December 18, 2012

December 25-28 Potential Significant Winter Storm

I have upgraded the title of this potential winter storm from 'Winter Storm' to 'Significant Winter Storm'.

**This post is dedicated to Rachel Davino, age 29, a victim of the Connecticut shootings.**

We'll start with today's 12z GFS model and its forecast for the 500mb height layer. This is the forecast for the evening of December 27th. We see a deep depression in the 500mb height layer in the Midwest, signifying our storm system. It looks to be centered in central Illinois. If this looks familiar to you from the upcoming Dec. 19-22 blizzard forecasts, then you're correct- but the buck stops there. There's one thing that I think will make this system fairly big: The Rex Block.

Theweatherprediction.com
This is an image of a Rex Block from the site of theweatherprediction.com. The Rex Block involves a low pressure system in the Southwest US, and a high pressure system in the Pacific Northwest. In response to this highly meridional flow (north to south wind pattern), storm systems in the presence of a Rex Block are strengthened, as shown in this example of a storm system in a Rex Block-turned-Omega Block during May 2005.

While we're on the topic of this storm system in May, let's see if this storm system has any similarities with the GFS forecast above, and the ECMWF forecast below.

Are you surprised? It's almost like a carbon copy of the May 2005 storm is now in this ECMWF forecast for the morning of December 26th. Putting aside the (eventually crucial) time differences, we see the Rex Block has now transitioned to an Omega Block, as was pictured in the May 2005 picture above.

It should be noted that the ECMWF is showing an Omega Block while the GFS is showing a fair Rex Block, with a system in the Southwest and slight ridge in the Pac. NW. The slight difference in these two, as small as it may seem, is actually quite significant, and something we will need to watch later on.

What does this mean? It means that the models are in rare agreement of high pressure setting up on the West Coast, leading to an Omega Block (possibly Rex Block) over the area that then gives the storm system a much more amplified feel. Naturally, as the Omega block/Rex block produces a much more meridional flow, we see the vorticity centers in the low pressure system enhanced by the meridional flow, and suddenly a much stronger storm is born. Another thing interesting is the level of agreement from the ECMWF and GFS on the placement of the storm. Given, there are some time issues that need to be worked out, but it's reassuring to see this storm now showing up on more than one model.

For those wondering what precipitation forecasts are coming out of the models, here's a few images:

Evening of Dec. 27th precipitation forecast. All precip.
above blue line is snow. Multiply value on legend with 10 to get snow.

Yesterday's 12z forecast for precipitation from the ECMWF.

Today's 0z forecast for precipitation from the ECMWF.


Andrew

December 19-22 High-Impact Blizzard Event

**This post is dedicated to Rachel Davino, age 29, a victim of the Connecticut shootings.**

I am holding firm with yesterday's call as far as the track goes, but PLEASE DISREGARD THE SNOWFALL FORECAST. I have a new snowfall forecast below. I still expect the system to move from the Plains through the Midwest, most likely through north Indiana and southern Michigan. These areas will have a tougher time getting any precipitation, as dry slotting issues may occur. As the storm strengthens, it will wrap in air through its southern flank in response to very humid air (the precipitation) wrapping around its northern side. Wherever the low pressure system travels, dry slotting could cut down precipitation of any kind.

I'm still holding with my idea of a rain-to-snow situation in much of north Illinois, southern Michigan, Missouri and Kansas. The main snows for these areas will depend on the strength of the backside snow. Models were earlier indicating a heavy band of backside snow coming through the Midwest, and if such a solution happened, a few inches could fall.

The model consensus has aligned itself overnight with my track, leaving me with the logical option to hold this forecast in place.

Snowfall is expected to be heavy, and, combined with winds, will make travel dangerous (see Impact Scale below). I am anticipating amounts of 1 to 3 inches in the light blue colors. This is the tricky part, because this involves the areas right along the rain-snow line, something that troubles may forecasters and such rain-snow line forecasts are prone to 'busting', or failing. I am confident in a good 3-6 inch accumulation in the darker blue, and a 6-12 inch plus snowfall over southern Wisconsin, much of Iowa, extreme northwest Missouri, extreme southeast Nebraska and northeast Kansas. Amounts will increase as you move northeast through this forecast in response to the storm strengthening. Again, such a strengthening storm and the tricky rain snow line makes forecasts difficult to confirm. Watch for amounts over a foot in Wisconsin.

Green: Low Impact
Yellow: Moderate Impact
Red: Heavy Impact
Pink: Major Impact
Blue: Extreme Impact
We here at The Weather Centre want to put out forecasts that best interest you and your safety. For that reason, earlier this year, we developed our Exclusive Impact Scale, where I can identify areas where travel may be hazardous, among other things, depending on the season. My forecast for this system involves a Low Impact forecast for much of the region that will see some precipitation that is not wintry from this system. Low accumulation spots on the order of 1-3 inches then includes the Moderate Impact spot, where some people could see slippery roads and slightly hazardous travel weather. Heavy Impact status is expected in much of the 3-6 inch accumulation stripe, where there will be fair travel delays. Flights will have issues, and driving will be a 'challenge'. Unnecessary travel is not advised. Finally, I hesitantly put out a pink stripe of Major Impact status for the 6 inch+ areas. The only reason I do this is for the wind- very high winds will make whiteout conditions common in rural areas, and long-lasting travel delays will stem from those areas.


And now, our final graphic is for the kids. This is another tool recently developed here at The Weather Centre. I have made a forecast chance for a snow day based off of my previous snow day experiences and putting myself in the situation of a bus driver, administrator and parent, the big question being "Can kids safely get to school?" Although there are 10 categories, DO NOT use them as a 1/10, or 2/10 chance- that is not the intent.

I marked down generally low probabilities for much of the impacted region simply because of the lack of significant snow. I did put a 'Not Very Good' chance for the 3-6 inch range (you may have to adjust it west a tad, or clip off the southern edge of the yellow zone). The Medium forecast is again based off of blizzard-like conditions (or just plain blizzard conditions). I then went ahead and went up to 'Good' for Wisconsin (you should consider it likely). I did not put likely in there without loss of visuals of the counties.

You can also use a formula I made a year or two ago for snow day potential by clicking here.

Hunker down, folks, this one looks to be a nice introduction to winter. I will have more updates on the Facebook page, which you can find on the right-hand sidebar. If you haven't given it a 'Like', go ahead and do it- you can find my latest thoughts not seen here on the blog.

Andrew