Tuesday, November 25, 2014

December 2-6 Potential Winter Storm

Another winter storm threat is on the horizon.

The image above shows forecasted 500mb vorticity values with superimposed 500mb height contours over Japan for November 26th, off the GFS model. In this graphic, we see a depression in the contours over Japan, as well as a swath of red positive vorticity values,  indicating the presence of a storm system. Using the Typhoon Rule, which states weather phenomena in Japan is reciprocated in the United States about 6-10 days later, we might expect a storm system to trek across the US in a December 2-6 timeframe.

The ECMWF model is already hinting at this potential storm, as the above frame shows a positively-tilted trough entering the Central Midwest on December 4th and 5th. The extent and intensity of snowfall remains in question, but for now, the possibility is certainly present.


Monday, November 24, 2014

Thanksgiving Potential Coastal Snowstorm

It looks like the Northeast will see a pretty white Thanksgiving this year, as a coastal storm will lay down plowable snowfall.

Instant Weather Maps
The image above shows accumulated snowfall from the GFS model, valid over the next five days. We see a swath of snowfall across the Northeast region, primarily coastal areas, as the storm will be located relatively offshore, restricting snowfall. This forecast lays down a general 3-5" of snow for many on the coast, with spots of 6" and slightly above for a handful of areas. Given that this is November, warm air immediately near the coast will be acting to put a damper on any intense snow activities, something that will have to wait until later in the season.

Instant Weather Maps
The NAM model, a shorter-range forecast, puts down a similar 3-5" swath of snow, but enhances the coastal areas with 6-8" of snow, as the pink area shows. Far southeastern parts of Maine even look to see a shot at near 10" of snowfall, if not a bit more.

Peak storm time looks to be on November 26th, intensifying in New England that evening. Regardless of how this storm ends up, millions in the East US will have to contend with a very messy Thanksgiving commute.


Sunday, November 23, 2014

November 24th Potential Accumulating Snow

This is a regional discussion for accumulating snow potential in the Great Lakes. Please note these types of regional discussions will become more prominent this winter.

Model guidance supports a swath of potentially plowable snowfall in the Great Lakes on November 24th, into the 25th.

Instant Weather Maps
The image above shows total snowfall from this morning (Sunday) to Tuesday morning. In this forecast, from the NAM model, we can define a swath of 3-6" of snow from Iowa to Michigan in the light blue to dark blue colors, with amounts over 6" (and even approaching the 10" mark) depicted in the pink colors.

This snowfall potential comes from a storm system currently placed in the southern Plains, forecasted to move north in the next day or so. As this happens, it is expected that a band of precipitation on the backside of this storm will form, laying down accumulating snow in parts of the country. The question is, where could this occur? Right now, the NAM model shows the brunt of the snowfall hitting the Wisconsin/Illinois border, but as of its most recent 18z run, snowfall has been shunted to eastern Wisconsin and Michigan. Let's see what the GFS model says.

Instant Weather Maps
The GFS model supports a swath of snowfall hitting primarily Northern Illinois, as well as southern Wisconsin and eastern Minnesota. Amounts would be maximized near Chicago, where the legend tells us 4-5" would be predicted to fall. Lesser totals nearing 4" would then be forecasted in the Northwoods region. We are awaiting word from the 18z GFS model run as I type this to see if we can build any bit of consistency. Why do we need consistency? If you haven't noticed yet, we currently have three completely different solutions:

• Solution 1 (12z NAM in top image): Snowfall strikes the Lower Great Lakes. Rockford, IL into south-central Wisconsin sees the heaviest totals (over 6").

• Solution 2 (18z NAM, not shown): Snowfall fails to fall anywhere, save for Michigan and eastern Wisconsin. Amounts totaling 3-5" in most places.

• Solution 3 (12z GFS, image above): Snowfall impacts North Illinois primarily, laying down just under half a foot of snow.

You can track the latest updates on this event on our Facebook Page, which you can find on the 'Social Media & Contact Info' tab on the right sidebar.


Friday, November 21, 2014

Thanksgiving Potentially Strong Storm System

I'm still watching for the threat of a Thanksgiving winter storm, this time with renewed caution concerning its evolution. Please read here for the discussion on how this potential came about in the first place.

We'll begin with the GFS model forecast for November 27th. The top left panel shows 500mb vorticity values in shaded colors, as well as 500mb height contours superimposed. The top-left image depicts 1000-500mb thickness values (people north of the red dashed '540' line generally can expect snow) as well as sea level pressure. The bottom-left panel gives an indication of low-level relative humidity, basically a measure of moisture in the air, while forecasted precipitation is on the bottom right in conjunction with 850mb temperature values. Glancing over this image, we see a significant storm evolving in the East US, with the combined energy of a trough in the southern jet stream, as well as a potent clipper system dropping down from Canada. These energies combine into one strong trough, traveling northeast and dropping intense snowfall along the East Coast. I'll provide analysis of this scenario after the GEM model, what I'm doing now is explaining what it shows.

The GEM model is different. Using the same definitions for each panel in this graphic as the GFS model, we see the storm is separated into two pieces of energy. We see our primary southern-stream storm in the Southeast US with the expansive precipitation shield, but that clipper is now located to the northwest, not conjoined with the Southeast-US storm. As a result, we see accumulating snow strike the Midwest and Plains, putting down precipitation amounts as follows.

This image shows snowfall accumulation in its liquid-equivalent from right now to 10 days from today, but the clipper's snowfall is shown by the swath of greens and light yellows stretching from the Dakota into Indiana and Ohio. Doing a quick conversion tells us 10 millimeters is equal to 0.39 inches, 15mm is 0.59 inches, and so forth. At face value, the GEM would be kicking out a good 4-9" snowstorm from this clipper, with the highest amounts in those yellow shadings. With higher ratios taken into account, we would probably be facing a 5-10"+ snowstorm. Since the GEM is notorious for exaggerating snowfall amounts, this isn't something to hang your hat on. However, it does give breadth to the idea of two solutions to this storm.

For our model analysis, I want to focus on the problems with the GFS model. I've mentioned a handful of times that the GFS model (and most models in general) are prone to a progressive bias. This means that forecasts will move storms along quicker than they should, which might be leading to that merging of the clipper and southern-stream storm on the GFS. Dropping/correcting this bias might lead to a forecast not unlike the GEM model.

For now, purely due to uncertainty, I'm not willing to side with one model over the other. However, keeping in mind this bias which does appear to be showing itself in the GFS model (for now), a solution similar to the GEM forecast might be expected, something that could easily change in coming updates.


Wednesday, November 19, 2014

November 23-25 Significant Storm System

A strong storm system looks to impact the US in the November 23-25 period.

Tropical Tidbits
The image above shows the temperature forecast for 5,000 feet off the ground, as forecasted by the ECMWF model, valid on November 24th. In this image, we see a storm system of 973 millibar strength shooting northward into Wisconsin, surrounded by above-freezing air temperatures on all sides. Pure observation of this chart tells us there won't be that much of a snowy side, but given the potential for model guidance to cool down as the forecast grows colder, or more likely yet, the precipitation shield does extend into the cold sector, snowfall may still occur. This snow would not be significant, at least according to this forecast, but the storm itself would be.

Tropical Tidbits
The GFS model gives a very similar story to the ECMWF. We see a 973 millibar low pressure system over the Upper Peninsula of Michigan on November 24th, but now with significantly more cold air on the western fringe of the storm. This appears to have happened as the storm wrapped itself up and occluded, pulling all that Arctic air to the south (this will have significant implications for the Thanksgiving storm, which we will discuss tomorrow (Thursday) afternoon).

Tropical Tidbits
That GFS forecast does lay down some hefty snows in the Upper Midwest, where amounts of 6-12" may be seen. The heaviest snow appears in northern Minnesota into Canada, where amounts closer to the 2' mark may be anticipated. However, it remains to be seen if this solution will win out against the ECMWF, or vice versa.

To summarize:

- Model guidance favors a very strong storm system moving into the Upper Midwest by the start of next workweek.
- Some model guidance favors heavy snow in the far northern US, while other guidance keeps this a rain/ possible severe storm event.