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.

Andrew

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.

PSU
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.

PSU
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.

Meteocentre
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.

Andrew


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.

Andrew

Sunday, November 16, 2014

November 22-25 Potentially Significant Winter Storm

A powerful storm looks to present itself to the US around the November 22-25 timeframe.

Tropical Tidbits
Since this post will primarily be a model analysis, we'll begin with the ECMWF-Ensembles. This image shows the forecasted 500mb geopotential height values in the color shadings, with sea level pressure contours and high/low pressure demarcations superimposed. In this graphic, valid for the evening of November 24th, we see a storm system placed on the border of northeast Illinois into northwest Indiana. The minimum pressure is about 1003 millibars, which is a rather weak storm system. Despite its weakness, the mere presence of a storm on this graphic tells us there is some confidence in a storm occurring within this timeframe. Confidence is low to begin with, but it is there.

Tropical Tidbits
We now move on to the ECMWF model, also showing 500mb geopotential height values and SLP contours. For future reference, those two parameters will show up on all model graphics we analyze here today. The ECMWF model favors a very strong 987 millibar storm striking central Illinois on the morning of November 24th, with windy conditions overtaking the Ohio Valley and East US within the storm's warm sector. The sub-540 geopotential height values in the Midwest tell us that a widespread accumulating snow event would likely occur, particularly in the central and northern Plains into the Upper Midwest.

Tropical Tidbits
Pushing ahead, we now analyze the GFS ensembles forecast, valid here for the evening of November 24th. We find a weak low pressure system over central Lake Michigan, with a minimum central pressure reading of about 1005 millibars. Ensemble systems typically tend to be weaker than their operational counterparts (as observed with the ECMWF-Ensemble and ECMWF graphics above), since the ensembles take into account double-digit forecasts and average them all out. Regardless, the mere presence of a storm system is re-assuring to confidence.

Tropical Tidbits
Continuing on, we arrive at the GEM model forecast, valid on the evening of November 24th. The GEM, made by the Canadian meteorological service, shows a deep low pressure system of about 995 millibars right over Chicago, Illinois. This would support some stormy activity from the Southern Plains to the East Coast, but due to the retracted 540 line into the far Northern Plains, I'm not confident that this model is supporting a snowstorm for the Upper Midwest and Central/Northern Plains.

Tropical Tidbits
We now arrive at the GFS Ensembles, which paints an interesting picture for this storm. We see a storm system of minimum central pressure 1001 millibars placed just south of Chicago. This graphic, valid on the evening of November 24th, also draws the 540 line south into the Midwest and Central Plains. Again, since this is an ensemble forecast, the storm is not as strong as individual model forecasts. However, since it is showing up in the first place, confidence continues to rise in a substantial storm in this timeframe.

Tropical Tidbits
We've saved the best for last: The above graphic depicts the GFS-Parallel model forecast for this storm. We see a 979 millibar storm striking the northeast Illinois-southeast Wisconsin border, while 500mb geopotential height color shadings tell this storm to be a closed low. With very windy and cold conditions extending across the Midwest and Great Lakes, an accumulating snow event would likely be expected in the Upper Midwest and Northern Plains.


To summarize:

- Model guidance is confirming the possibility of a substantial storm in the November 22-25 timeframe.
- Some guidance supports a powerful storm system, possibly bringing both accumulating snow to the Upper Midwest, and severe weather to the East/Southeast.
- Confidence in this solution remains low due to the extended timeframe of this storm.

Additional updates will be posted in the coming week as more information becomes available.

Andrew

Saturday, November 15, 2014

Thanksgiving Potentially Significant Storm System

Model guidance is now hinting at this Thanksgiving storm system to impact the Central and East US.

PSU
Beginning with the ECMWF model, with this forecast going out to 10 days, we see a deep trough pushing into the Southern Plains, neutrally-tilted, as the isobars pushing due south show. Note how pressure tendencies have rotated to the southeast corner of the vort max, likely telling us that this trough will start its maturing phase into a negatively-tilted trough in the next few days, if this model forecast were to go out further. A better description of tilted troughs is below from theweatherprediction.com .

The Weather Prediction
Given the freezing line / 540 thickness line in the top-right image being pulled all the way to the Southern Plains on that ECMWF graphic, as well as the expected negative tilt to this trough, the going ECMWF forecast would likey deliver severe weather to the Southeast, as well as some snow to those in the Great Lakes and southern Midwest, with exact areas to be determined.

Tropical Tidbits
Moving ahead to the GFS-Parallel forecast for the same November 24th timeframe as that ECMWF graphic, we see a very similar forecast. Once again, a strong trough is located in the Southern Plains, as shown by the deep negative height anomalies centered over Texas. If you guessed this trough was neutrally-tilted, you are correct! The height contours seem to be 'pushing' due south, which means it's neither positive or negatively-tilted. Unfortunately, I don't have access to the jet stream forecast for this timeframe from the GFS-Parallel model, which means we cannot tell if this trough is preparing to tilt negatively or not.

We don't have much to work with right now, since the storm's still about 10 days away, so here's a few graphics of precipitation forecasts from the GFS and GFS-Parallel model with this storm.

Tropical Tidbits

Tropical Tidbits
Andrew