Ag Weather Forum

The pattern is setting up for a big storm system to move through the United States next week. Upper-level troughs have been a regular visitor to the western U.S. during the last few weeks and that continues with one going into the weekend.

A small system may develop in the Northern Plains that moves into southern Canada on Saturday and Sunday, but without a lot of precipitation. The main lobe of that trough will move out into the Plains early next week with a storm system.

But it's the trough that will dive into the West behind it on Sunday and Monday, Nov. 17-18, that will draw the biggest attention.

That trough will get cut off from the jet stream as a ridge builds over the top of it in Canada. That means the cutoff low will only slowly push eastward through the U.S. next week, bringing the potential for a big and long-lasting storm system that brings down some much colder air than what we have seen so far this fall.

But the devil is in the details and the medium-range weather models really do not have a clue on how this will all play out. Every time you look at a new model run, it looks significantly different from the ones before.

Areas of rain and snow, which may be heavy, switch from the Plains to the Great Lakes and Southeast and back again. Meanwhile, the temperature forecast goes from 20 to 30 degrees Fahrenheit below normal to just 5 to 10 degrees below from run to run -- for a time of several days to just a couple.

With so much uncertainty in how the weather will shape up for next week, we could try looking at the ensembles for some clues.

An ensemble is just an average of a group of models. The American GFS and European ECMWF model ensembles, the GEFS and EPS, respectively, do shed some light on the most likely scenario for the weather situation for next week.

The GEFS likes to think a storm system will develop in the Southern Plains on Monday, heading to the Great Lakes on Wednesday, where it will wrap up and be in the region for the rest of the week.

The EPS shows the same general idea but lingers the low another day or two.

So why are forecasts all over the board?

Well, just because they have the same idea on how a system develops does not mean they have the same forecasts for what we care most about -- temperature and precipitation.

How this storm actually develops, how much moisture it pulls in from the Gulf of Mexico, and slight variations in temperature can all give rise to wild swings in how much precipitation occurs where and when, as well as its type.

The speed of the colder air moving down the Plains and the upper low itself also have a say on the eventual outcome. And, of course, the surrounding ridge keeping the trough cutoff will also play a role. Models have been changing the strength and duration of this ridge in each model run as well. The stronger it is, the longer the upper low will last in the U.S. And the EPS is stronger with that ridge overall.

Usually, with under a week to go, models start to make alignments and come together on a solution. But this doesn't seem to be the case this time around. This is a notorious situation for cutoff lows. Without access to the jet stream, the low meanders and drifts and this allows for more variables to be at play.

What the models do agree upon is that a big system will occur next week. It is likely to produce a lot of precipitation east of the Rockies, and some of that is likely to be in the form of snow. There is potential for severe thunderstorms, and a burst of colder air will follow behind a strong cold front. What they do not agree upon is exactly where, when, and how much. And forecasters may not have a solid idea until the weekend.

This gives rise to wild predictions on social media. It always seems to be that a large system draws the attention of armchair forecasters to put out some wild individual model runs. With the storm still many days out, these simulations are almost guaranteed to be wrong, and perhaps dramatically.

So, take note if you do see a post online that one individual model run is showing a major winter storm event, because it may turn out to be off wildly. The forecast just before that and the one after is likely to have been much different, giving low confidence to all forecasts put out there.

One other thing to consider is a developing tropical system in the Caribbean. It is likely to become a hurricane by Nov. 16 and move north into the Gulf of Mexico next week. How the upper-level low develops and moves will determine the track of the hurricane with forecasts currently directing it toward Florida around the middle of next week.

However, it could easily escape underneath the cutoff low and head west through Central America. Like the winter storm scenario, give little salt to a specific forecast as the storm has yet to form, and models will not have a great handle on it until it does, or when the upper low's features are more solidly in line with each other.

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John Baranick can be reached at john.baranick@dtn.com

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