Wednesday, April 23, 2014

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.


**Significant Severe Weather Outbreak Expected This Weekend**

A significant severe weather outbreak, possibly including strong to violent tornadoes, is expected Saturday, Sunday and Monday.

The Storm Prediction Center has outlined three days of potential severe weather in their long range outlook. In these outlooks, the SPC will mark regions it believes are at risk for enhanced severe weather. Typically, one or two days may be outlined at a time, but it is uncommon (though not unheard of) to see three different areas outlined, especially considering how much area they cover. By SPC estimates, over 47 million people may be affected by this potential severe weather outbreak.

The wording by the Storm Prediction Center is very strong, as this excerpt below shows:

Jet Stream Forecast for Saturday
The whole situation looks to unfold as a strong trough digs into the Southwest United States, developing a negative tilt in the process. A negative tilt, commonly seen how the isobars seem to push towards the southeast like you see above over Arizona, indicates the maturity of the trough in question, and thus means weather of any kind is generally stronger than it would be if the storm were not fully mature yet, or if it was beginning to occlude and weaken. In this forecast of the jet stream, valid Saturday night, we see a strong Pacific jet stream powering the trough as it digs east, and the divergence over the South Central Plains, shown as how the two jet streams (subtropical jet stream, seen along the bottom of the image, and the Pacific jet stream) separate just west of Texas. Divergence in this sense means the air is rising in monumental fashion, also a big signal for severe weather potential.

Lifted Index and Dewpoints over the Southern Plains, valid Saturday
The Lifted Index (LI), a measurement of how unstable the atmosphere is, looks to be plummeting as low as -11 or -12, indicating extreme amounts of instability. Worse yet, the sharp gradient in dewpoints (shown by the shaded colors) indicates the presence of a dryline, literally meaning a line where the air is dry to the west and very humid to the east. These drylines almost always originate and stick around in the Plains, and are a big reason why we see major severe weather down there. I have little doubt based on the projected Lifted Index that if/when a storm does break the capping inversion predicted to be in place (a capping inversion hinders thunderstorm development, and is the opposite of instability), we may very well see violent thunderstorms capable of strong tornadoes and potentially extreme severe weather. The rhetoric here is elevated, but there is quite a bit of concern here on how this will evolve.

By Sunday, the trough looks to continue to mature and eventually close off, as the full-circle contours in the 500mb wind speed image above show. That negative tilt is also evident, as we see the contours tilting towards the south-east direction. In response to this strong trough, as well as the intense Pacific jet stream we discussed above which is also evident in the image above, the mid-level jet stream will be absolutely howling over the Plains, right over the area projected to be hit on Sunday. We could see mid-level winds of over 80 knots, an astounding strength for this event.

Projected Energy-Helicity Index (EHI) values look to be heading towards double-digit territory on Saturday, a major concern for tornadic activity that evening. The EHI combines instability and helicity (spinning) to make an index that tries to predict situations where tornadoes are likely to occur. Values as elevated as the ones shown above confirm my concern for this event, which won't be just one day, but possibly three days of continuous severe weather problems.

To summarize:
• A potentially significant severe weather event is expected Saturday, Sunday and Monday.
• Potentially strong tornadoes are anticipated Saturday evening.
• Risks beyond Saturday look to focus more on other modes of severe weather.
• Those in the risk areas outlined by the SPC should review severe weather guidelines and preparations.


Sunday, April 20, 2014

Spring, Summerlike Warmth Expected in Late April, May

I'm expecting the end of April and beginning of May to have seasonal to above-normal warmth across much of the country.

Tropical Tidbits
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.

The factors for warmth don't stop there- it goes far deeper, literally and metaphorically.

The image above may look complicated, but isn't that complex when explained thoroughly. Shown above is a graphical representation of a phenomenon called the Walker Circulation, also known as the Walker Cell. The Walker Circulation is most prominent in a La Nina situation, and forms when you have cool waters towards the Eastern Pacific, and warm waters near Oceania and Australia. The trade winds across the Pacific go from east to west in the Walker Circulation on the surface, due to the presence of high pressure in the East Pacific and low pressure in the West Pacific (think of it like a ball rolling downhill from the top of the hill (high pressure in the east Pacific) to the bottom of the hill (low pressure in the West Pacific), where the ball signifies the trade winds). When the trade winds reach the low pressure area in the western Pacific, the winds converge and begin pushing up into the atmosphere, creating thunderstorms. The air from the thunderstorms shoots up to roughly the 250mb to 200mb level before the winds spread out, as the air cannot rise further. From there, the air at 200mb flows from west-to-east, opposite the direction of the surface winds. Then, the high pressure and cool/dry atmosphere in the East Pacific pulls that air down back to the surface to start the whole process over again. As I said, this typically appears in La Nina situations, as cool waters in the East Pacific that are required for the Walker Circulation typically arise in conjunction with the La Nina.

The image above shows a view of surface pressure and surface wind vectors across the tropical regions of the world. We can use this image to identify the presence, or lack thereof, of the Walker Circulation. Taking a glance at surface pressures and winds from April 6th to April 10th, we do see surface winds flowing from east-to-west, a trait typically observed with the Walker Cell, and the low pressure center in Oceania also shows a characteristic of that circulation. This is rather odd, as we have been expecting an El Nino to arise, but the Walker Cell appearance here tells me that the El Nino has not hit the atmosphere yet, though it may start appearing in the surface temperatures. In recent days, the surface winds have been showing signs of weakening, indicating that the Walker Circulation may be weakening, which would herald the arrival of an El Nino if the winds reverse to a west-to-east stature.

For all intents and purposes, let's say that the Walker Cell's presence in the Pacific still indicates the presence of a La Nina (which is true in respect to the Walker Cell actually appearing, even though sea surface temperatures may argue otherwise). We can correlate sea surface temperatures in the Pacific to surface temperatures here in the United States. In the image above, the warm colors indicate a positive correlation of SSTs to surface temperatures (for example, warm Pacific SSTs would then correlate to warm temperatures in the US, or cool Pacific SSTs would lead to cool temperatures in the US). Blues in the above image then depict a negative correlation, where warm Pacific SSTs would lead to cool surface temperatures, and vice versa. In this case, with negative correlation signals across the US above, we can determine that below normal SSTs in the Pacific (La Nina) can then lead to above normal surface temperatures in the US, and vice versa. Thus, even though the SSTs in the Pacific indicate an El Nino, the atmospheric pattern is more supportive of a La Nina, which would then support warmer than normal conditions in the April-May-June timeframe. Despite this, since the Walker Circulation may be fading, this portion may just be a moot point in just a few weeks time.

This image here actually is pretty complicated, but I'll do my best to explain it. This graphic shows the layout of the Global Wind Oscillation (GWO), and what factors indicate the presence of the GWO in one of a possible eight phases. For example, if the GWO is to be in Phase 1, we would look out for negative mountain torque (MT) values, as well as enhanced tropical convection in the Indian Ocean. An additional description of the GWO comes from the ESRL, as shown in the screenshot below.

I outlined the Stage 1 description, as not only does it fit in with the current conditions that we're experiencing, but it is indicative of a La Nina, like we talked about earlier in this post. How do we know for sure that we're in the Phases 1-3 of the GWO (which are traditionally La Nina-esque phases)? Let's take a look at the indicators that depict Phases 1 through 3 of the GWO.

Observed Mountain Torque

Observed Relative AAM
If we look at the first GWO image we discussed, as well as the screenshot description of the GWO, we can see what defines these key phases. We see that negative mountain torque and convection in the Indian Ocean is a key definition of these first three GWO phases. Looking at that observed MT chart shown above, we see that the net MT is now below average, as the black solid line shows. While the convection in the Pacific is more displaced in Oceania instead of the Indian Ocean, the description highlights that the relative AAM is negative, and that relative AAM image above shows a negative anomaly. Overall, this does indicate the La Nina phases, also known as GWO phases 1-3, strengthening that correlation image, as well as the Walker Circulation discussion. Additionally, in the same sense that positive East Asian mountain torque can lead to cooler than normal temperatures in the US, the current negative East Asian mountain torque anomaly in the graphic above (red line two images above) indicates warm weather is more likely.

Importantly, in that screenshot description image, we see that the La Nina-like stages indicate a trough in the Western US. Typically, when we see a trough/stormy weather in the West US, we get a consequential ridge of high pressure in the East US, which is a textbook negative PNA pattern.

The image above shows observed sunspots in accordance with the 30-day sunspot cycle. As I've mentioned on this blog before, there is a known inverse correlation between the 30-day sunspot cycle and the Pacific-North American index (PNA). When the 30 day sunspot number is anomalously high, we tend to see the PNA in its negative phase. In the same sense, when the 30 day sunspot number is anomalously low, the PNA tends to reside more in its positive phase. Composite images of both the positive PNA and negative PNA are shown above, courtesy of NCSU.

500mb height anomalies of a negative PNA on the left (warm colors= high pressure, cool colors = low pressure)
Surface temperature anomalies of a negative PNA on the right

500mb height anomalies of a positive PNA on the left
Surface temperature anomalies of a positive PNA on the right
If we look back at the sunspot image, we see that the 30 day sunspot cycle is just off the charts, indicating a very high number of sunspots, or at the very least, elevated solar activity. If we use our solar correlation with the PNA, we may be expecting the emergence of a negative PNA regime in the near future. If we look at the composite image above for a negative PNA, we see that warm weather tends to prevail in that sort of situation. This fits in well with the GWO description we went over earlier, the presence of the La Nina Walker Circulation, and the East Asian correlation tool.

The last piece to the puzzle is the presence of enhanced tropical convection over Oceania.

The image above gives a global view of rising air in conjunction with enhanced tropical convection (blue shading), also known as negative Outgoing Longwave Radiation (OLR), as well as sinking air, exemplified by the beige shadings, which show suppressed tropical convection. The green contours highlight divergence, or air rising and spreading out due to thunderstorms, while red contours depict sinking of air. We see that there is an anomalous swath of tropical convection over Oceania, shown by the blues/tropical convection, and the wind vectors at 200mb going away from that convection. The placement of this tropical convection fits into a certain phase of the Madden-Julian Oscillation (MJO).

If we consider that the current center of the tropical convection is around the 165 East longitude line or 170 East line, we can see which MJO phase is best represented here. It looks like the MJO is currently in Phase 6 or 7, as the line on the 165 East shows splitting through the two panels on the right that are circled.

What does a Phase 6 MJO mean in the month of April?

The Phase 6 MJO looks like a typical negative PNA regime, with low pressure anomalies along the West Coast, and strong high pressure/warm weather in the Central and East US. This supports the overall warm weather idea, only adding to the rest of the factors supporting a warm weather regime in the late April and early May timeframes.

Let's summarize all of this.

• Based on a number of factors, warmer than normal weather is expected for the latter part of April, potentially well into the beginning of May.


Saturday, April 19, 2014

April 23rd Potential Severe Weather Event

The Storm Prediction Center has outlined an area of enhanced severe weather potential on April 23rd, as the severe weather season finally begins to show signs of activity.

The SPC is discussing the presence of a corridor of instability aligned just east of a dryline set up along the western Central Plains, and it is this corridor of instability that is outlined in the graphic above. In these dryline situations, it is not uncommon to see isolated storm cells fire up, which then enhance the likelihood of tornadoes and overall elevated severe weather. If enough individualized storm cells form, the cells can congeal into a nasty squall line, or may just remain as individualized cells, which would maintain a raised tornado threat.

Model guidance shows this narrow section of instability, as the GFS model graphic above shows. However, under the roughly-15000 j/kg of instability (indicating the atmosphere is decently unstable), we see a hatched gray area. This indicates the presence of a capping inversion, which just means there is warm air above the surface that is hindering the development of thunderstorms. An analysis of a sounding chart confirms a layer of warm air in the first few thousand feet above the surface. The concern here is that this capping inversion may prevent storms from forming at all, which could just wipe out the severe threat. In this case, there is a severe weather threat if storms form; the question is just if these storms will form.


Friday, April 18, 2014

Long Range Forecast for Late April, Early May

Let's examine the long range forecast for late April into the beginning days of May.

Long range analog guidance from the ESRL/PSD division, a special physics-based meteorology branch of the government weather service, indicates we will see troughing setting up in Western North America in late April as a strong upper level low drops into the Southwest, provoking high pressure out ahead of it in the Central and East US. This ridge out ahead of the upper level low will likely make for anomalously warm weather across the aforementioned sections of the country, a real treat in the face of such a nasty winter.

Beyond the last days of April, it is expected that the weather will take on a slightly cooler tone. In the wake of a Kelvin Wave currently pushing across the Pacific, enhanced tropical convection is expected to develop near the 60E Longitude demarcation, a classic Phase 1 MJO signal. When we see enhanced tropical convection in this Phase 1 signal, it typically means we can anticipate cooler than normal weather here in the United States.

I am a bit skeptical of this cold weather forecast, due to the response we're looking to see in East Asia around April 26th. 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 image above, we see projected tropopause pressures, vector winds, and wind speeds way up in the middle-upper regions of the troposphere into the stratosphere. If we look to this forecast image, valid April 26th, and find Japan in the top left corner of the image, we can make out a bulge of orange pushing towards the center of this image. That orange bulge signifies the presence of a Rossby Wave. In simple terms, this Rossby Wave will 'break' over Japan and initiate an intensive warming spell. This may continue for some time, but if it does happen in late April, we would likely see the cooling effects of the Phase 1 MJO hurt, as this East Asian development would likely overrule it.

To summarize:
• A warm end to April is expected.
• A cool start to May is possible, but there are hints that the late April warmth may just carry over into May. More time is needed to investigate this potential.
• A severe weather event is possible in the final 7 days of April, due to the upper level low in the West US.