December 19-22 Potential Blizzard Event- First Call

This is my first call on the potential December 20-22 blizzard event. Let's do a model rundown first, then review the teleconnections before we get into the forecast.

We start now with the American's GFS model on the evening of December 19th. Pictured above are 850mb temperatures in the dashed lines, sea level pressure values in solid black lines, and precipitation values as filled in colors. The rain/snow line is the farthest-south dashed blue line. Any precipitation above that southernmost dashed blue line is supposedly snow. The GFS predicts we see our storm system coming out of the Rockies on the evening of December 19th, with a snow event ongoing in the Plains. Accumulations could range from 1-3 inches at this time in the forecast. The presence of a ridge of high pressure in the Southeast tells me that this storm system will be forced more towards the north rather than along the Gulf Coast.

This forecast is for 12 hours later, and is valid on the morning of December 20th. We now see the storm system has fled northeast into Chicago, and the central minimum pressure is approaching 990mb. Precipitation shown above tells me that a light snow event would be ongoing in Wisconsin and possibly Iowa, probably accumulations of 1-4 inches. In the southeast quadrant of this storm system, heavy rain and thunderstorms are ongoing. The tight pressure gradient, tight temperature gradient and overall strength of this storm system tells me a linear severe weather threat certainly is possible for much of the southern Midwest, southern Ohio Valley and general Southeast US.

Snow accumulation forecasts favor Wisconsin as the big winner per the GFS:

So the GFS takes the system into Illinois and gives Wisconsin the big snows. Let's now turn to the European model.

The ECMWF model, regarded as a more trustworthy model than the GFS, shows the storm system ejecting from the Plains in Oklahoma with a central minimum pressure of 995 millibars. Again, we see that high pressure system in the Southeast which will support the prospect of this storm system staying into the Midwest and Ohio Valley. This forecast is valid for the evening of December 19th.

Moving into the evening of December 20th, we see that our storm system has now strengthened greatly since the 24 hours before, with the central minimum pressure now at 981 millibars. That is a change of 12 millibars in 24 hours- not a common sight away from the Northeast. The tight pressure gradient tells me that this will be a very windy event, and if cold air factors into the equation after the storm departs, lake effect snow could be on overdrive. Also, note that piece of lower pressure jutting out of the southern side of the storm system. This could be a push at another storm system developing, one which could go into the Northeast, but that remains to be seen.

A few images of the ECMWF's precipitation and temperature forecast centered over the Midwest:

Valid the morning of December 20th.

Valid the afternoon of December 20th.

Valid the evening of December 20th.

Now that we've covered the two big models, let's visit a couple other forecast models not as reliable, but still worth a glimpse.

This is the Canadian's GGEM model forecast for the afternoon of December 19th. Pictured above are sea level pressure (SLP) values for this timeframe, and the colored areas depict 10 meter wind speeds. We can see our storm system in north Texas, being squeezed towards the northeast as pressure from the ridge offshore the Southeast increases. The storm system has no choice but to shoot northeast- that's one thing we're confident in at the moment. The GGEM predicts the system will be much weaker when it exits the Rockies than other models are predicting.

Moving ahead to the morning of December 20th, we now see that the GGEM has taken the system into Indiana with a central minimum pressure of 994 millibars, indicating that some strengthening has occurred. Despite this, the system remains weaker than the GFS and ECMWF are predicting. Even though a strength difference may seem like nothing, I can assure you it can have significant implications. If the system is weaker than is being forecasted, the track can (and would) significantly change. Let's hope that doesn't happen, or this whole post would be moot. We still see that high pressure in the waters off the Southeast. Despite the presence of a tight pressure gradient, 10 meter winds are not showing up too strong, which I can understand, considering the GGEM is weaker than the GFS/ECMWF at this point in time. I am hesitant to disregard the GGEM, because it is in line with the GFS, meaning the two may be on to some sort of consensus.

Now, for the last model we look at, we see SLP values in solid black lines, 850mb temperatures in dashed lines (the same rain/snow rule as the GFS applies here), and precipitation values in colors. This is the NOGAPS forecast, made by the Navy but not regarded as a solid model that you can trust. I'm including it moreso to give some spread to the forecast, not so much to find a solution from the NOGAPS. This model starts out with the system ejecting from the Rockies at a minimum pressure of 993 millibars in the early morning hours of December 19th. Again, watch that timing difference! An already-tight pressure gradient in OK/AR/TX/MO tells me a severe weather situation may be trying to form in that area.

Moving ahead to the early morning hours of December 20th, we find the NOGAPS projecting this system to possibly have split in two. We see one storm system with a 1005mb minimum pressure centered in Arkansas, and another storm system centered in Wisconsin with a central minimum pressure of 1000 millibars. I will refer to them as System 1 and System 2, respectively. System 1 shows a more linear alignment of precipitation, telling me that if this solution verified, we would probably see a mesoscale convective system (MCS) or small squall line in the area as a result of tight temperature and pressure gradients in the region. As for System 2, this looks to be the main piece of energy. The precipitation looks to be a modest snowstorm, with accumulations around the 3-6 inch mark based solely on this image. If the two systems had stuck together, a solution similar to the other models may have evolved. Needless to say, I am discounting the NOGAPS model.

Teleconnections are also a big deal, in terms of the North Atlantic Oscillation (NAO). When the NAO is in its negative phase, you'll see a tendency for storm systems to move up the Northeast coast and strengthen rapidly into Nor'easters. This comes in response to the highly meridional flow set up by the high pressure system in Greenland and the storm system in the Northeast. There are two sub-categories of the NAO, and that is west-based and east-based. In a west-based negative NAO, you'll see the high pressure to the west of Greenland, hence the term west-based. Likewise, in an east-based negative NAO, you have the high pressure system to the east of Greenland. The difference is which one affects which areas. In a west-based negative NAO, you'll see the East US getting in on the most cold and storms, while the East-based negative NAO keeps the cold and storms in the Plains and Midwest. The forecasts for the west-based negative NAO are stronger than the east-based, so it is plausible to think that the East Coast has an advantage. However, because of that Southeast ridge, we'll see the track take a detour north and then go northeast through the Great Lakes. The whole situation is very complicated, but in summary, the NAO does favor a track being shown by the ECMWF or even the GFS, maybe a bit farther south if the negative PNA holds in place.

This image shows what the meteorologists at the Hydrometeorological Prediction Center (HPC) think in terms of best model choice for the next 7 days. Looking at the D4 and D5 areas, we see that the HPC generally leaned towards the ECMWF model, the ECMWF Ensembles (ECMWF EPS) and the Canadian model. Surprisingly, no mention of the GFS model is recorded.

Going off of all the models printed above, the teleconnection forecasts and the HPC forecast preferences, I have made my first call as printed below.

Forecast Preference: ECMWF/ECMWF EPS
Confidence: 55%

Andrew

An Early Look At Weather For Christmas 2012

The holidays are right around the corner, so let's start to take a look at who could be seeing good holiday travel weather, who may see some stormy conditions, and even who could see a White Christmas.

This is the GFS model forecast for Christmas Day. Pictured above are precipitation values in colors, high and low pressure systems denoted by H and L signals, respectively, and 850mb temperature lines in light reds and light blues. The dark blue line is the rain/snow line. The GFS forecast calls for some light snow in the Great Lakes, with some flakes flying in the immediate coastal areas in the Northeast. 850mb temperatures are as low as -18 (Celsius) in the North Plains, so a chilly Christmas is not out of the question. An emerging storm system in the Plains could cause New Year's troubles, something I'll be addressing in coming days. Also, a rain event could be ongoing for the West US. But before you take this and run with it, the GFS is typically unreliable beyond the 5 day mark, and this forecast is more than 10 days out.

The two images above are of the CFS v2 weekly forecasts. The top image shows temperature anomalies for the timeframe of December 22-27, a frame that includes Christmas. The forecast shows warmth prevailing across the Central US, with a frigid West Canada in the forecast. The bottom image shows maximum snow depth from 4 CFS v2 ensemble members from the Dec. 22-27 period, the same timeframe as the temperature anomaly forecast. The ensemble members hint at some good snows along the Northeast and North Plains, something I can't say won't happen. But in all honesty, the CFS v2 weekly forecasts aren't exactly the most reliable either. However, I think you should watch that snow chart if you're in the Northeast- all 4 members are showing it, so snow on the ground for Christmas doesn't seem all that far-fetched.

Below is my forecast for a White Christmas potential I issued a while back. I will have a new forecast out in coming days.

Andrew

December 26-28 Potential Winter Storm

The GFS model has now been showing a big Northeast snowstorm for at least 4 model runs, meaning it's time to take a look at this potential.

We'll start off with the oldest model forecast, the 0z model run from last night. This forecast of the GFS shows a wide swath of snow ongoing in the Plains and Midwest, with heavy rain in the Southeast and Mid Atlantic. The system in question is in the southern Midwest, and the way the precipitation and freezing line is angled tells me that, for many in the Northeast, such a solution would be a solid rain-to-snow event. Frankly, I'm going to discount the 0z GFS because of how different it is from the 6z GFS.

This is the second-oldest GFS model run forecast, the 6z forecast, which was made early this morning. For roughly the same timeframe, the morning of December 27th, we see a fairly strong storm system in New England with a wide swath of precipitation. The solid blue line is the rain/snow line, and its take on this forecast gives inland locations of New York, Pennsylvania and similar states a good batch of sticking snow. Coastal cities receive a nice dumping of rain per this forecast. In the wake of the storm, a  big blast of Arctic air flows into much of the nation, no doubt bringing the lake effect snow machines to  maximum power.

We now turn our attention to the second-newest forecast, the 12z GFS, which was made in the morning hours, about 6 hours later than the 6z. The 12z GFS isn't all that different from the 6z, with a system in the vicinity of the Northeast and a wide precipitation shield. Two main differences include a much weaker precipitation shield and the storm system isn't even onshore with this forecast. Coastal locations now see chances of snow, and Arctic air still flows south into the Plains and Midwest, cranking up the lake effect snow machines. Note the heavy snow ongoing in the Virginias.

And now the newest, and most startling GFS model forecast- the 18z. The 18z GFS, valid for the evening of December 26th, brings an epic storm system up the coast. The storm system bombs out to a minimum central pressure of 968 millibars- not your everyday low pressure system strength. The precipitation shield is wild, with up to 1.5 inches of rain hitting New Jersey. You weather enthusiasts know that average snow-to-rain ratios is 10:1 (inches), so this could mean 15 inches of snow for New Jersey! Heavy snow falls in the immediate coastal regions, with relatively lighter snow farther inland. Again, the arctic air continues to come on down into the Plains and Midwest, Ohio Valley and Mid-Atlantic behind this system to provide a base for lake effect snow and enhanced chances for snow in the Northeast.

It's obvious that we are dealing with something on our hands, the question is exactly how strong could this system be, where could it go, what type of precipitation could it bring, and the most important question: Will it even happen??

We still have a lot to learn on this potential, but it looks like the potential could be there.

Andrew

December 20-22 Potential Blizzard Event (Updated 12/15)

(Originally posted earlier today) I have upgraded my thinking of this winter event from a 'Significant Storm' to a 'Blizzard' event. I'll now explain why.

This is the ECMWF forecast for Hour 144, a.k.a. the evening of December 20th. We see a very strong storm system in northeast Illinois at this time with a minimum central pressure of 986 millibars- pretty strong for an onshore system. Let me first start off discussion of the model by saying I trust the ECMWF more than other models at the moment, as although all models have been having troubles, the ECMWF has historically outperformed most models in the past. The ECMWF shows a high pressure system in the Southeast, off to the east of Florida.

Moving quickly along, we now visit the ECMWF Ensemble prediction system, or the ECMWF EPS. This forecast is for the same timeframe as above- the evening of December 20th. We see our storm system is on the Michigan/Indiana/Ohio border, with a central minimum pressure of 998 millibars. Considering the ECMWF EPS is composed of 51 separate forecasts, and we are 144 hours away, the strength of this system that is being projected is astounding. It's not that common to see an average of 51 forecasts, 6 days out projecting such a strong storm system. The ECMWF EPS believes the system will be submerged in cold air, as shown in 850mb temperatures (colors). This, combined with tightening of isobars, makes me believe that a blizzard-like solution could evolve in the Upper Midwest and South Canada into Michigan.

The GFS model has not been mentioned, but for those wondering, it is similar to the ECMWF EPS solution above as far as placement of the system goes.

Going back to the ECMWF model, we can see that it is projecting very heavy snow to fall over some Midwestern states. Snow is shown as precipitation north of the dark blue 32 degree line:

ECMWF Precip/Temp forecast for the Morning of Dec. 20

ECMWF Precip/Temp forecast for the evening of Dec. 20

ECMWF Precip/Temp forecast for the morning of Dec. 21

I don't have access to snow depth forecasts from the ECMWF, but if I were to give a rough estimate of snow amounts from the above few images, I would predict the following accumulations for the following locations:

-Northeast IL: 6 inches+
-South WI: 5-8 inches
-Michigan: 5-8 inches+


(Begin updated data) Now, I bring to you the updated 12z model suite and some information you'll find pretty darn interesting.

The ECMWF Ensembles are farther south than their 0z counterparts this morning, as shown in the below images:

As I explained, to have the ensemble system so confident on such a strong storm system this far out is very encouraging to those wanting snow. Now, in this scenario, Wisconsin, Iowa and maybe north Illinois would get the heaviest snow before the system moves off to the east. When that happens, we see a massive cold air dump from the heart of Canada into the Northeast. If this happens, heavy lake effect snow is likely. Not possible, LIKELY. This would be air that doesn't force you to wear a heavy jacket, it chills you to the bone- and then some. The warm Great Lakes will take this and, if such a solution happens, vigorously expel potentially high amounts of lake effect snow. The fact that this is the ECMWF EPS showing this happening is great news for the states I mentioned above for snowfall, because when you have the ECMWF Ensembles on your side, you're looking pretty good as far as probability of a forecast happening goes.

Now onto the interesting part. As I previously stated, the ECMWF is my trusted model at the moment. I will show you why I trust it's 0z solution just below:

These are North America model track errors. Basically, the higher the model is on this chart, the worse it has verified. If we look in the long range, we see there is one blue line that actually goes down in the long range, meaning it gets more accurate as time goes on (not a common sight at all). This blue line is none other than the ECMWF model. The Hour 120 forecast (just about when this storm gets going) is looking very well according to this chart above- all the more reason to trust the ECMWF. If you're looking for the GFS, it's that red line that is too high up to see at hour 96. (For those of you wondering about the 12z models, the GFS is about the same as the red line above, and the ECMWF has not come in yet.)

Forecast Preference: 12z ECMWF EPS / 0z ECMWF
Confidence: 45%

I'll have my first call out tomorrow morning. Until then, updates on this system may be found on the Facebook page at www.facebook.com/TheWeatherCentre

Andrew

December 20-22 Potential Blizzard Event

See the updated version HERE

I have upgraded my thinking of this winter event from a 'Significant Storm' to a 'Blizzard' event. I'll now explain why.

This is the ECMWF forecast for Hour 144, a.k.a. the evening of December 20th. We see a very strong storm system in northeast Illinois at this time with a minimum central pressure of 986 millibars- pretty strong for an onshore system. Let me first start off discussion of the model by saying I trust the ECMWF more than other models at the moment, as although all models have been having troubles, the ECMWF has historically outperformed most models in the past. The ECMWF shows a high pressure system in the Southeast, off to the east of Florida.

Moving quickly along, we now visit the ECMWF Ensemble prediction system, or the ECMWF EPS. This forecast is for the same timeframe as above- the evening of December 20th. We see our storm system is on the Michigan/Indiana/Ohio border, with a central minimum pressure of 998 millibars. Considering the ECMWF EPS is composed of 51 separate forecasts, and we are 144 hours away, the strength of this system that is being projected is astounding. It's not that common to see an average of 51 forecasts, 6 days out projecting such a strong storm system. The ECMWF EPS believes the system will be submerged in cold air, as shown in 850mb temperatures (colors). This, combined with tightening of isobars, makes me believe that a blizzard-like solution could evolve in the Upper Midwest and South Canada into Michigan.

The GFS model has not been mentioned, but for those wondering, it is similar to the ECMWF EPS solution above as far as placement of the system goes.

Going back to the ECMWF model, we can see that it is projecting very heavy snow to fall over some Midwestern states. Snow is shown as precipitation north of the dark blue 32 degree line:

ECMWF Precip/Temp forecast for the Morning of Dec. 20

ECMWF Precip/Temp forecast for the evening of Dec. 20

ECMWF Precip/Temp forecast for the morning of Dec. 21

I don't have access to snow depth forecasts from the ECMWF, but if I were to give a rough estimate of snow amounts from the above few images, I would predict the following accumulations for the following locations:

-Northeast IL: 6 inches+
-South WI: 5-8 inches
-Michigan: 5-8 inches+

Forecast Preference: ECMWF
Confidence: 55%

Andrew