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

Persistent Cold Pattern Expected to Succeed Current Warmth

Another bout of cold weather is expected to replace the nice weather that most of the United States is currently feeling.

NAEFS

The first image we'll analyze above is the North American Ensemble Forecast System, or NAEFS 500mb height forecast. The contour lines show isobars at the 500mb level, with colored regions showing the degree of uncertainty, or "spread" among the ensemble members.

The graphic above, valid October 3rd, shows a deep trough pushing south into the Central US. We can see this well by the NAEFS projected depression of 500mb height contours. A glance around North America tells us this troughing pattern looks to be supported by two areas of ridging. One body of high pressure looks to be positioned in the West US, in a more suppressed version of last winter's ridging episodes, while the other ridge appears to be positioned along the Southeast and East Coast, evident by the arcing motion of the 500mb height contours.

Something we will have to monitor in the days leading up to this outlook of October 3rd is the high uncertainty over the Plains with this trough. The NAEFS outlook shows greens and yellows encompassing the expected placement of the trough, which means ensemble members aren't so sure this will actually end up happening. However, as the trough is actually showing up on this image, it means more members support it than not, a positive sign for this trough being able to form.

CMC

This next ensemble projection, from the Canadian ensemble system, shows 500mb height contours valid on October 6th. In agreement with the NAEFS ensemble forecast, we still see that trough centered over the Plains and Midwest, now trying to push a bit east into the East Coast. Sustained ridging is still highlighted over the West US, and a bit still present in the Southeast. A look at the individual members of this ensemble mean image shows only two of the twenty members do not show a large trough in the Central US on this date. Therefore, confidence is rather high that this cooler pattern will play out.

ECMWF

Lastly, we'll take a look at the ECMWF ensembles' outlook for October 3rd, the same time period as the NAEFS graphical outlook. In this forecast, we see a large trough placed over the Central US, echoing the projections made by the NAEFS and CMC ensembles. Adding to the similarities is the rather large spread / degree of uncertainty among ensemble members, as the deeper purple colors on the right-hand panel above show over the Plains. Because it's still showing up in the ensemble mean images here, I would keep confidence in this cold outlook for the start of October, but still be wary for changes in the outlook in the near future.

To summarize, model guidance is agreeing on the potential for a cold blast to kick off October, but individual projections are exhibiting varying degrees of uncertainty as to just how intense the cold may be.

Andrew