Weekend Severe Weather Outlook - June 6th-8th

It's looking like we'll see some marginal chances for severe weather this upcoming weekend.

FRIDAY DISCUSSION---

Forecast instability (contours) and stability (blue shading) for Friday evening

Short range model guidance indicates we will begin seeing a reservoir of instability be pulled up en  masse from the Gulf of Mexico, a rather common occurrence for this time of year. Instability/CAPE values in the neighborhood of 2000 j/kg over the Upper Midwest and north Plains tell me we could see some light convective activity, though the lack of lower and upper-level wind support means any convection here would likely be of the pulse-storm variety. These pulse storms would likely have the ability to put down some gusty winds and small hail. The relatively-high precipitable water values over these regions would also allow for torrential downpours in these pulse storms.

Attention will then focus to the central and southern Plains for a much more dynamic environment. NAM instability projections above have CAPE exceeding 5000 j/kg in northern Oklahoma by Friday evening, an incredibly high amount of instability for any time of the year. Despite this impressive statistic, upper and lower level wind support again looks marginal, though slightly improved over the environment in the Midwest and northern Plains. The blue shading inching its way across the Plains on that instability projection above tells me there will be a limited window for convection in Oklahoma, and the capping inversion (which suppresses thunderstorms) will rapidly take over in the coming hours after sundown in Oklahoma. A closer look at forecasted sounding analyses (see below) over Oklahoma for Friday evening actually shows considerable low-level veering of winds, which could allow for some more formidable convection, if any is able to fire before the cap sets in. Right now, I'm pessimistic on opportunities for large-scale convection due to the lack of wind support and capping inversion.

Forecast sounding in north-central Oklahoma for Friday evening

SATURDAY DISCUSSION---

Instability and stability forecast for Saturday evening

By Saturday evening, the sector of instability has been shunted east into the states of Louisiana, Arkansas and Mississippi, among others. We still see the capping inversion eager to take over the Gulf Coast, once again limiting the timeframe for thunderstorms to fire. Instability exceeds 4000 j/kg in some parts of this region, and a formidable mid-level jet streak rounding the base of an upper level low in southern Illinois may provide for the opportunity for some organized convection among expected pulse storms. To the north of the Gulf Coast, CAPE values are less opportunistic, despite the increased wind shear. For now, I would feel better calling for pulse storms across the Gulf Coast, with the potential for some more organized convection if we see more wind support in the upper and lower levels.

Andrew

What Are The Models Saying This Winter? - CMC2 Model

This is a continuation of this series of posts, where we are analyzing long range climate model projections for the beginning of the winter of 2014-2015. This post will feature the CMC2 model.

The temperature forecast from the CMC2 model is a toasty one. We see warmer than normal conditions prevailing across the heavy majority of the country, especially around the Midwest and Northeast regions. There is a slight cooler than normal projection in the Southern Plains, but warmth definitely triumphs here. This forecast is very reminiscent of a typical El Nino temperature pattern, where warm weather stays in the North US and cooler than normal temperatures prevail in the South US. It would definitely be a welcome turn-around from what we experienced last winter.

The precipitation forecast for December 2014 from the CMC2 model is not as similar to an El Nino precipitation pattern. We see more or less neutral precipitation anomalies across the West Coast, but the real story is the shifting of drought-esque precipitation anomalies from the West to the Central US, as the CCM2 model predicts would happen to kick off the winter of 2014-2015. Typically, these dry anomalies would be centered in the Ohio Valley for an El Nino winter, not so far west. The El Nino pattern does briefly show itself as a wet start to winter in the East, but i'm not sold. I believe this model needs more time to solidify its forecast; I am not investing much confidence in it.

Lastly, we see the forecast of z200 anomalies for December 2014 from the CMC2 model. We're going to focus on the contour lines over the northern Hemisphere. In this forecast, we see the contour lines arching northward over the west coast of North America, signifying ridging/high pressure in that area. Typically, this would mean a chilly winter for the East US, but the arching of these contour lines extends east into the Central US, meaning high pressure would be favored over low pressure for that part of the country. This map does support the warm temperature forecast we saw earlier in this post, and such a forecast would be nearly a complete-180 from this past winter.

Additional model forecasts will be discussed in the near future.

Andrew

Could Unusual Pacific Ocean Patterns Make for Another Cold Winter?

It is possible that an unusual Pacific ocean water pattern could increase the risk of a second straight cold winter.

Shown above are two composite images. The top row shows sea surface temperature anomalies during an El Nino event (top left), and during a La Nina event (top right). The bottom row shows sea surface temperature anomalies during a positive PDO event (bottom left), and during a negative PDO event (bottom right). You may notice that the images look similar. This is because typically, when there is an El Nino, below normal waters form in the North Pacific. Similarly, during a La Nina, warmer than normal waters tend to be observed in the northern Pacific. This is not a 100% correlation, it is just what usually happens. This year, we are seeing this correlation broken.

The above image shows observed sea surface temperature anomalies (SSTAs) on May 25th. We see two items of interest. The first is the presence of significantly above normal water temperatures in the northeast Pacific. This mechanism is what created sometimes-relentless bouts of ridging over the west coast of North America this past winter, greatly aiding in the cold weather that hit North America. This body of warm water still remains here this summer, and if it stays, may once again produce bouts of ridging resulting in a chilly winter for many in the United States.

Going by the composite images at the top of this post, we would typically watch out for below normal water temperatures in the Equatorial Pacific, more specifically the waters west of Ecuador. However, that is not the case. We have been discussing the incoming El Nino for months now, which is allowing SSTAs in that area to rise. This means that we have warmer than normal waters in both the North Pacific and Equatorial Pacific, breaking the correlation we saw at the top of this post.

So, what could that mean for winter? Let's find out.

The image above shows correlation values between surface temperatures in December-January-February (the winter season), and the observed Pacific-Decadal Oscillation (PDO). In other words, if the PDO is positive (see example in composite image at top of post), areas in positive correlation areas (warm colors) would see warmer than normal temperatures, since a positive PDO is positively correlated with surface temperatures. Inversely, a positive PDO being negatively correlated with surface temperatures means a positive PDO would result in colder weather for those in the negative correlation areas/cool color shades. Since these warm waters in the Northeast Pacific appear to be proclaiming a positive PDO regime right now, we could be in for a warm West US and very cold South/East US this winter if this correlation comes to fruition.

This image above shows correlations between the current El Nino or La Nina state, and surface temperatures across December-January-February. If we take positive water temperatures to mean El Nino, areas in the North and West US would see warmer than normal temperatures this winter, as an El Nino is positively correlated with surface temperatures. Similarly, a La Nina would bring colder weather to those Northern states. However, with the expected El Nino, we can expect warmth in the North and cold in the South, as the South is in that negative correlation area again. 

These two correlation comparisons can be put together to mean that a positive PDO and El Nino this winter may result in some very cold weather in the South and East US, with warmer temperatures stretched out across the North and West US.

The PDO and El Nino are not the only two indices in the atmosphere, which may mean other factors may lessen this potential cold winter foreshadowing. We will get a better handle on this situation this summer and fall.

Andrew

What Are The Models Saying This Winter? - CMC1 Model

This is a continuation of this series of posts, where we are analyzing long range climate model projections for the beginning of the winter of 2014-2015. This post will feature the CMC1 model.

The temperature forecast from the CMC1 model, valid for December 2014, shows a very chilly winter for many in this country. This model suggests we will see an abundance of warm air overspreading Alaska and the west coast of North America. This is likely the result of high pressure, which then forces mid-level troughing to the east. This mid-level troughing, which will be discussed more later in this post, results in below normal temperature anomalies over the Central and East US, extending as far north as upper Canada. If you follow the weather closely, you may recognize this pattern as a positive Pacific-North American (PNA) index pattern. Like last winter, the Pacific pattern looks to have a grip on the downstream weather here in the US, resulting in a cold start to the winter.

The precipitation forecast from the CMC1 model shows a pattern almost identical to a traditional El Nino winter precipitation pattern. We first see anomalously wet conditions along the West Coast, a telltale El Nino signal due to the subtropical jet stream's natural enhancement due to the aforementioned El Nino. This enhanced subtropical jet stream then leads to more storm systems hitting the West Coast, and then more precipitation. We then see significantly below normal precipitation across the Central US, hitting the Ohio Valley the hardest. Once again, this is an El Nino signature. The El Nino is evident yet again in the CMC1 model's forecast for a very wet start to winter on the East Coast. This all strikes me as odd, since the temperature forecast does not reflect an El Nino temperature pattern, but given how this past winter went, you really have to expect the unexpected.

Lastly, we see the forecast of z200 anomalies for December 2014 from the CMC1 model. We're going to focus on the contour lines over the northern Hemisphere. In this forecast, we see the contour lines arching northward over the west coast of North America, signifying ridging/high pressure in that area. We discussed the likely presence of high pressure along the west coast earlier in the temperature forecast, and now we are seeing that theory come to fruition. We then see the contour lines plummet south over the Central US, which is how we got that cold December forecast at the top of this post. Oddly enough, we then see ridging in the Southeast US, which would typically divert storm systems north through the Ohio Valley. However, the precipitation forecast for the Ohio Valley is very dry, as shown above. This is likely the long range model up to its antics, which means confidence is similarly low.

Additional models will be discussed in coming days.

Andrew

Active Severe Weather Potential Looming for Mid-June

I am still monitoring the potential for a period of active weather in mid-June.

The image above shows a long range forecast of outgoing longwave radiation (OLR) anomalies over the equatorial region of the globe, valid on June 14th. In both of these images, the yellows represent positive OLR, or suppressed tropical convection, while the blues denote negative OLR, or enhanced tropical convection. We can see a strong active Madden-Julian Oscillation (MJO) signal on the bottom panel, with deep blues indicating abnormal convection occurring over southern India into the waters east of the subcontinent. The placement of this enhanced convection tells me we are looking at a Phase 2 MJO, as the OLR composite shows below.

OLR anomalies for each MJO phase

Now, outside of the fact that Phase 2 MJO events have been known to increase the risk of violent tornado outbreaks in the spring months, the mid-level pattern during a June Phase 2 MJO event also signals the potential for severe weather.

The image above, produced by Americanwx, shows 500mb height anomalies over the northern hemisphere when the Madden-Julian Oscillation is in Phase 2 during the month of June. In this image, warm colors indicate high pressure/calm and warm weather, while cool colors denote stormy and cooler weather. During a typical MJO Phase 2 event in June, we tend to see ridging set up from the Bering Sea to the Great Lakes, enhanced slightly by weak troughing in the West. Because of this weak West US troughing, warmth and moisture are pumped north into the Central and East US from the Gulf of Mexico. This action then feeds into the increased potential for severe weather. Based on this composite, I do feel this time period may be defined by some active severe weather.

More evidence for an active severe weather period in mid-June arises when we take a look at the long range CFS severe weather forecast. The dates on the right display the date the forecast was made on, and the dates on the bottom legend indicate when this forecast is valid for. Warmer colors indicate the higher potential for severe weather. For instance, if I want to know the severe weather potential for May 30th on a forecast made on May 5th, I go up the legend on the left to find May 5th, then scroll on the corresponding horizontal line to find the May 30th box. In this case, we have been seeing the majority of recent forecasts highlighting the June 10-25 period as the one to watch out for, as the varying-colored boxes show on the upper-right corner of this image. Sometimes, there is safety in numbers in the weather world. In this case, with the relative consistency of these forecasts for the mid-June timeframe, I feel comfortable raising the concern of a potential active period for the aforementioned timeframe.

Andrew