Long Range Forecast Update for Late April, Early May

This is an update to a previous long range outlook concerning the expected weather for late April and into early May.

Tropical Tidbits

For the last days of April and in to early May, I'm still looking at above normal temperatures for a decent portion of the nation. Shown above is the 500mb height anomaly pressure forecast over the West Pacific for April 26th. In this image, we see a strong ridge of high pressure stationed over Japan and East Asia, permitting the flow of above normal temperatures into the area. This event has significant implications on our weather here in the US. There is a rule, well explained by Joe Renken, that states a weather phenomenon in East Asia will be reciprocated in the United States 6-10 days later. This means that if there is a storm system in Japan on a certain day, we can expect a storm in the US 6-10 days after that. The same goes for high pressure and warm weather. So, if we use that rule, we can expect warmth over the United States around the May 1-5 period, if not for a longer time period than that. 

Tropical Tidbits

By the end of the first week of May, I anticipate that we will see another cool-down for a few days as a closed low looks to move into Japan. Since this 500mb anomaly image is valid for April 29th, we can expect this cool-down to start showing itself sometime around the May 4-8 time period, though the timeframe could probably be extended to May 4-10 as the closed low sticks around Japan for a day or two more. Despite how long that timeframe may seem, the actual cold weather should only be around for a few days, not for this whole 6-day timeframe.

Tropical Tidbits

By the time we get to the second week of May, we're looking at another bout of sustained warmth, as the GFS ensembles bring about a ridge of high pressure over Japan and the waters surrounding the island nation. Ensemble guidance looks to have rather low confidence in this event, though we should get a more accurate look at this portion of the forecast in the next few days. 

To summarize:
• I am still forecasting warmer than normal weather for the last few days of April into very early May.
• Cooler weather looks to take hold near the end of the first week of May.
• Warm weather may return for the second week of May.

One final note, I received a multitude of arrogant responses to the previous long range outlook I had posted. Remember that every comment is moderated before it's published, and arrogant comments like the ones I've been getting for the last few days will not be published.

Andrew

Seasonal to Above-Normal Temperatures Just Days Away

Though one of the harshest winters to hit this nation in recent memory is still leaving its mark to this day, with below normal temperatures continuing to persist across the country, I'm finding new, concrete data indicating that warmer weather is as little as 10 days away - and it could be here to stay.

Tropical Tidbits

There is a rule, well explained by Joe Renken, that states a weather phenomenon in East Asia will be reciprocated in the United States 6-10 days later. This means that if there is a storm system in Japan on a certain day, we can expect a storm in the US 6-10 days after that. The same goes for high pressure and warm weather. In this sense, we can use the long-term weather pattern over Japan to predict when some seasonal, warmer weather will arrive in the US. Take a look at the graphic above. We see 500mb height anomalies from the GFS Ensembles over the Western Pacific. I use the ensembles rather than the GFS model itself, as I find the ensembles to be more useful in determining the synoptic (large-scale) pattern than the one model (remember the GFS Ensembles have multiple 'members' that add credibility and accuracy to its forecast). The image here shows 500mb height anomalies valid for March 24th, and looking towards Japan, we see oranges over Japan, indicating the presence of high pressure and warmer weather. Using the 6-10 day correlation, we can thus expect warmer, quieter weather in as soon as March 30th. We're about 9 days away from that today, and it might seem like one of those passing warm shots that then leads to another cold blast. The truth of the matter is, this warm-up looks to be sustained.

Tropical Tidbits

Fast-forward to the GFS Ensemble forecast on March 27th. Once again, we're looking at 500mb height anomalies over the Western Pacific. Looking at Japan, we see sustained high pressure and warmer than normal weather still over the area. At this point, if we go out to the end range of our 6-10 day gap from March 27, we might expect this warm-up to go as far out as April 6th. Bear in mind that this would be a relatively uninterrupted warm-up, as ridging would stick over Japan during this entire timeframe. No big storm systems, only quiet, warm weather. So, using this tool, we've established a warm-up timeframe of March 30th to April 6th. That's a full week of potentially non-stop seasonal to above-average temperatures!

But the warmth just keeps coming.

Tropical Tidbits

The image above shows 500mb geopotential height anomalies over the Western Pacific, now valid on April 1st. We are still seeing strong ridging over Japan, meaning our warm-up here in the United States could extend as far out as April 11th, the end range of our 6-10 day correlation tool. Beyond this timeframe, the ensembles get too spread out and we begin to see less and less confident forecasts in terms of projecting 500mb height anomalies in above or below normal categories. This is natural, as the ensemble members typically diverge in more noticeable features as the forecast period goes on. Despite that, the agreement of ensembles on continuous ridging over Japan is remarkable, and would give us at least 2 weeks of sustained warmth.

That's not the only item arguing for a end to winter in the next several days- the stratosphere is also doing its part.

The image above shows two panels of observed temperatures over the North Pole. The left image gives you observed temperatures at the 10 millibar level of the stratosphere, with the average temperature line in gray. The right panel depicts observed temperatures at the 30 millibar level of the stratosphere, with the average temperatures for that level shown in gray. The 10mb and 30mb levels are pretty high up in the atmosphere, when you consider that we live at the surface level of 1000 millibars. They seem even higher up when you consider most airplanes only fly up to the 250 millibar level or so, which is around that 35,000 foot mark. You can see on both panels how temperatures have been really bouncing around in recent months, until the last several days, when temperatures have skyrocketed to well above normal levels. After a relatively calm November through January in the stratosphere, what's going on up there?

The polar vortex is collapsing.

In my post the other day, I discussed how we were seeing a reversal of zonal winds in the upper levels of the stratosphere, indicating the collapse of the upper stratospheric polar vortex, and the significant weakening of the overall polar vortex. I talked about how this phenomenon was likely the result of the Final Warming, which is a massive stratospheric warming event in the end of winter that dethrones low pressure and instates high pressure in the stratosphere, a natural occurrence that happens each year. These Final Warmings usually signal the end of the winter in the stratosphere, and typically result in the end of winter 2-4 weeks later here at the surface.

The image above shows a six-panel graphic, each marking a day's observation of zonal wind anomalies by latitude, as the legend on the bottom shows, and by height in millibars, as the legend on the left shows. Going from the top row of panels to the bottom row, from least to most recent, we see a notable and rather sudden drop in positive zonal wind anomalies in the upper right-hand corner. Looking closer, we find this drop in anomalies over the 1 to 10 millibar level, in the highest reaches of the stratosphere (remember that we are based at roughly the 1000 millibar level, so 1 millibar is far higher than even what planes cruise at). Now, this transition from orange colors to blue colors in the top right hand corner of the most recent observation, marked under March 19 2014, means that the winds have reversed. In the Northern Hemisphere, positive zonal winds are also known as 'westerlies', as they move to the east in a counterclockwise formation. That is why we look for areas of positive zonal winds to identify the polar vortex in the stratosphere, because the polar vortex is essentially just one big low pressure system. In the same sense, negative zonal wind anomalies define 'easterly' winds, as they blow towards the west. Recall that high pressure winds spin to the west in a clockwise motion, providing the reason why we look for negative zonal winds to tell if the polar vortex has weakened. In this case, rather than the positive zonal winds just weakening a bit, it looks like they completely reversed in the far upper stratosphere, marking a collapse of the polar vortex at that level. We saw that reversal of winds in the upper stratosphere a few days ago, but it's only now that we see a rapid decline in oranges and reds across the middle and lower stratosphere, as well as the staying power of the negative zonal winds in the upper stratosphere. This all tells me that we are indeed in the process of the Stratospheric Final Warming, and we should see the end of wintry weather in the next 2-4 weeks, when the effects of occurrences in the stratosphere usually impact the surface. There may be a chill in that timeframe, as this is still, to some degree, a stratospheric warming event, which can result in chillier than normal weather. However, this would likely be short-lived, as our East Asian tool tells us warmer than normal weather is the new norm in coming days and weeks.

To summarize:
• There are increasing amounts of data that suggests we will see sustained warmth to start off April.
• The stratosphere indicates we'll be seeing an end to wintry weather in just 2-4 weeks.
• Other indications suggest sustained warmth may be just 10 days or so away.

Andrew

Long Range Outlook: Late February, March 2014 Forecast

This post will detail the expected weather conditions for the end of February into most of the month of March, 2014.

We'll begin with the weather over the North Pacific and East Asia. Shown above is a previous ECMWF model forecast, showing 500mb height anomalies over the aforementioned regions. If we take a look over at Japan, we see some negative height anomalies present, valid on February 20th. If we use the idea that weather anomalies over East Asia can "re-appear" in the United States 6-10 days later, I would expect a stormy and cool weather pattern for the February 26-March 2nd period. But if we look just to the west, we see strong ridging behind that storminess in Japan.

We eventually see that ridging fill into Japan a couple of days later, as this 500mb height anomaly forecast from the most recent ECMWF forecast shows. With ridging expected to hit Japan on February 24th, a warm-up is probable for the March 2nd-6th period. It's worth watching for another storm threat, as we see a depression in the contour lines right over Japan that indicates the presence of a weak storm system. It is possible we see a storm drag warm air north from the Gulf of Mexico to set-up a severe weather threat, but that's all speculation at this point.

Following that initial warm-up in the first week of January, model guidance (the ECMWF Ensemble set, shown above here) wants to bring in a strong ridge of high pressure right over Japan. It looks like this ridge has the potential to bring some seriously warm air into the United States when we see the correlation kick in. With the graphic here valid for February 26th, look out for a warm, quiet weather pattern around the March 4th-8th time period. It's quite possible we see a sustained warm period from March 2nd to the 8th, if these ridges hold together.

Looking out to the last day of February we see the ECMWF bringing in a suppressed ridge into Japan. This tells us to look for that warm pattern to continue in the United States through the March 10th period. Remember to keep track of that Rossby Wave in Alaska, denoted by the powerful ridge.

The weather pattern will then do a flip in the second week of March, as model guidance sees the entrance of a strong trough into Japan on the 3rd day of next month, as the image here shows. This would likely see a return of cold and stormy weather somewhere in the March 9th-13th time period, as we use that 6-10 day gap between East Asian weather and United States weather.

Moving out into the long-long range, we start to key in on the synoptic pattern, and not so much individual features like we were previously analyzing. We've got a good handle on the anticipated weather pattern for the first two weeks of March, so now we can begin forecasting the remainder of the month. This graphic shows you the predicted 500mb height anomalies over the Northern Hemisphere on March 8th. Note the ridging over the Arctic Circle indicating the presence of a negative Arctic Oscillation (-AO) phase, which heightens threats for cold weather in the lower latitudes (including the US). Also note the ridging starting to come back over the west coast of North America. This is part of the Rossby wave train, which is created when several rounds of strong high pressure are pumped poleward. In this case, the train has maintained itself over the northeast Pacific, at times shifting west into the waters just south of the Bering Sea. The way the pattern is aligned here, you'd be hard-pressed to not find a cold forecast for the Central and Eastern US. The flow across the Northern Hemisphere strongly favors a cold middle of March, with the Rossby Waves continuing to form and push northward along the West Coast, leading to cold weather for the populated areas out East. Since this is a 360 hour forecast, things will obviously change, but you get the idea: after the early March warm-up, additional cold weather is possible.

Why is all of this happening? The primary reason is the swath of above-normal sea surface temperature anomalies over the northeast Pacific, as this graphic shows. One of the best indicators of long-term weather patterns is sea surface temperatures. Apart from some sea surface temperature anomalies (SSTAs) defining various oscillations that also affect our weather, these swaths of above or below normal SSTAs correlate to positive or negative 500mb height anomalies, respectively. Thus, it's of little surprise that we are seeing strong ridging over the Northeast Pacific, where this big warm water mass resides. With the warm water still firmly entrenched in the waters south of Alaska, look for a cool spring to also be in store.

What we could end up seeing, if this warm pool becomes a long-term feature, is a northwest flow pattern for the spring season. The screenshot above, from a Storm Prediction Center publication, shows 500mb height contours in two different northwest flow situations, with the severe weather outbreak area circled. As you can see, severe weather events would be more prone to occurring further north during northwest flow severe weather outbreaks, and it's possible we see something like one of these situations evolve if that northeast Pacific warm pool continues to work its magic. I'll detail this more in my upcoming 2014 severe weather season outlook.

Andrew