Trifecta of Weather Extremes Collides Friday

A trifecta of weather extremes will bombard the United States beginning tomorrow, and continuing on through the weekend.

Part I: Blizzard Buries Plains

Model guidance has finally come into agreement that a blizzard will develop out of an anomalously strong storm system in the Plains, delivering amounts exceeding the two foot mark. I outlined a 1-6 inch forecast across portions of the Dakotas and Minnesota, with such a large difference in amounts due to how sharp the cutoff may be with this snow event. South Dakota will get slammed the hardest, with 12'' to 24'' amounts possible. Latest model guidance suggests isolated pockets of 30'' totals are possible, but to anticipate any model exaggeration bias, I held down totals to the 12-24'' mark, with higher amounts in the realm of possibility. I'm not sold on anything higher than the 24'' mark due to this possible snowy model bias.

Those in South Dakota (and portions of North Dakota that may get in on these high totals) should prepare for widespread power outages, in large part due to trees that may not have lost all of their leaves yet. Snowfall will build up on trees and leaves, which can easily lead to partial or total destruction of many large tree branches, which can then cause power lines to collapse. This is expected to be a significant weather event, and anyone in line for these significant snows should be aware of the potential of this major storm system.

Part II: Severe Weather Swamps Midwest

On the warm side of this major storm system, a severe weather outbreak is expected. Severe thunderstorms are anticipated to fire from Oklahoma to Wisconsin, but supercellular, tornado-producing thunderstorms find the largest potential of formation in Iowa and southern Minnesota. High resolution model data paints a very intense severe weather event, though such forecasts cannot be taken at face value. That said, I do expect a substantial severe weather event tomorrow, on October 4th.

Those in Iowa and southern Minnesota should prepare for the possibility of damaging and life-threatening thunderstorms tomorrow. Any storms that manage to become supercellular will have the ability to produce tornadoes in this environment. With a strong storm system producing what ought to be a tight temperature gradient across the Plains and Midwest, storms should not have too rough of a time forming.

Part III: Tropical Storm Karen Likely to Landfall on Gulf Coast

Tropical Storm Karen formed earlier today in the Gulf of Mexico, and is expected to eventually make landfall on the Gulf Coast sometime Saturday. Tropical storm conditions are anticipated across a narrow band of land where Karen hits, and the rainfall amounts could reach the 10'' mark. At this time, it does not look like TS Karen is a significant threat to life and property, but with any tropical cyclone, high winds, heavy rainfall, and the threat of tornadoes does mean anyone in the potential landfall zone should keep a close eye on Karen. Model guidance is not perfect, and the cyclone's track could change.

Andrew

LRC Starting to Form; Significant Winter Storm May Cycle

The Lezak Recurring Cycle (LRC) is now beginning to set up as we enter October, and a significant winter storm may be a part of this equation.

A strong winter storm is projected to develop in the Plains in the next few days, and it is very possible this storm system is factored into the LRC. The whole concept of the LRC is that a recurring pattern develops between October 1 and the middle of November that then repeats for the heavy majority of the following 12 months. Now that we are entering the month of October, it's time to start looking for pieces of the LRC that may cycle in the next ~12 months. One significant piece of the LRC could be this major storm. Considering the storm will happen on October 5th, it's possible we see this storm happen again a few times over the winter.

A few things before the excitement grows: The LRC varies with the storm's strength, precipitation amount and various other factors. This could be a weak storm when it happens again in late-ish November, or it could be strong again. The track could also be relatively different- rather than going through the upper Plains, it might go through the Midwest, or even north into Canada. However, the jet stream drops south each winter, and this should affect the storm track. Just how much it affects the storm track is to be determined.

In the long range, the general consensus is that high pressure will dominate the Central and East US, with a very stormy pattern across the Southwest and general West US. This will lead to a period of above normal temperatures for the regions east of the Mississippi River, but this should last for a few days, at most per the latest forecasts. After the ridging begins to weaken and shift away from the East US, model guidance becomes too variable, so we cannot estimate what will happen beyond that. But the point is, already, it looks like this winter will contain some temperature swings, at least for a portion of the LRC.

Andrew

October 4-6 Potential Blizzard

There is potential for a blizzard to unfold in the Plains between October 4th and October 6th.

High resolution model guidance supports strengthening of a powerful storm system as it shifts north and east through the Plains. Lower level wind speed is shown as high as 69.1 knots on this map, with the strongest winds bombarding northwest Minnesota and the Dakotas. Sea level pressure contours in South Dakota drop down as low as 992.0 millibars, per the graphic above. With American model guidance showing this scenario consistently over the past 48 hours, it appears a Plains blizzard is certainly a possibility.

Precipitation type and 3 hour precipitation amounts shown above project the development of a sub-freezing air mass across the western half of South Dakota, leading to very intense snowfall amounts and rates. High resolution guidance indicates there is potential for 10'' of snow in 3 hours (just over 3'' of snow per hour, on average), but considering this hi-res model guidance is notorious for over-estimating snowfall, one should use this map and the one below with caution.

The high resolution NAM model shows incredible amounts of snow across western South Dakota and southern North Dakota. Again, this very well could (and most likely is) be exaggerated, considering the past history of NAM model forecasts. Regardless of its exaggeration bias, the other American model (the GFS) agrees with the projection of significant snowfall, but it has totals in the range of 12-18'' maximum. Model guidances will most likely remain unresolved until the event actually happens, but I project very isolated pockets of 20'' snow totals in western South Dakota, with generalized 12-16'' across western South Dakota.

Andrew

October 4-6 Potential Winter Storm

There is potential for a winter storm to hit the Plains during the timeframe of October 4-6.

The energy that will make up this storm system is currently in the Gulf of Alaska, shown as a dip in the jet stream in those waters. It is anticipated that the energy will follow the jet stream and shift southeast into the Pacific Northwest. Un-needed rainfall will accompany this storm system as it enters the Pacific Northwest, along with some heavy snowfall in the mountainous regions of the northwest Plains and Pacific Northwest.

By the time the storm system enters the Central Plains, it will have bottomed out to the Rockies, as the jet stream is pictured to buckle as it carries the storm system east and eventually north. The flow out ahead of the system pictures where the storm is expected to move towards, with a northeast track most likely.

As this storm moves northeast, it now appears that a winter storm, possibly a blizzard, may hit portions of the Plains in the next 5 days. Latest model forecasts encourage a very strong system to develop, with an intense snow band to the northwest of the storm system. The GFS snowfall forecast is shown below, with the NAM forecast immediately below it.

GFS snowfall forecast through Hour 108

NAM snowfall forecast through Hour 84

Models tend to agree that if this is a big snow event, it will hit the Dakotas the worst. The NAM and GFS agree that amounts may approach or even surpass the one foot mark. Based on the composition of the storm system, I wouldn't be surprised to see scattered reports of amounts over 12'', with this being a very strong and well-set-up storm system. What is worth mentioning that the ECMWF model is not on board with this being a big snow producer. The latest maps I have seen have significant mountain snows in the Rockies, but nothing in the Dakotas from the ECMWF. As of right now, I'm naturally skeptical of such a big storm, but if it does happen, this would have significant implications on the winter pattern this year.

Andrew

Special Long Range Lookout: September 28, 2013

This special Long Range Lookout will address the expectation of a colder trend in the weather in the 20+ day forecast range, and the effects of the current weather pattern on the winter ahead.

The current weather pattern involves a highly meridional (wavy) jet stream across North America, with a deep trough stretched across the West and strong ridging spread over Canada and the Great Lakes. The deep troughing is likely due to an increased stream of storms that have been shifting across the northern Pacific. The 500 millibar anomaly chart on the left displays a strong system currently stationed over the Bering Sea and into the Gulf of Alaska, and I suspect that system will move south towards the West Coast, possibly also into the West US. This train of storm systems has changed the sea surface temperature pattern across the northern Pacific as well, as the image below shows.

There is a large body of below normal sea surface temperatures stretched from the Aleutian Islands to waters well offshore of the Pacific Northwest. This development has occurred only very recently, and it was only a few weeks ago that the entire northern Pacific was well above normal in SST anomalies. The continuing storm train over the northern Pacific, aided by a favorable Madden-Julian Oscillation (which we will discuss more in-depth later), should assist in further lowering sea surface temperatures across the waters in and southeast of the Bering Sea. A potential long term implication of this new SST anomaly could mean an Aleutian Low, which would be favorable for harsh winter prospects in the East US. Despite that potential, a decisively negative PDO pattern has been present recently, and this could discourage such cold weather in the East and Central US.

We're going to dive right into some hardcore weather with an analysis of the Relative AAM tendency. The relative AAM tendency shows what its name says; the chart helps predict the tendency of the AAM index. Right now, the AAM tendency is positive, and this fits well with the current observation of a slight positive AAM. In the medium range, this should lead to a tendency for the positive phase of the East Pacific Oscillation, which encourages the formation of above normal temperatures in much of the United States, as illustrated by blizzardof96 on the AccuWeather forums.

Long term AAM projection from Nicholas Schiraldi shows a gradually lowering AAM over the next 360 hours, to the point that we see a neutral or slightly negative AAM at the end of the forecast period. This fits in well with ensemble projections of a lowering EPO in the long range, which could lead to chillier temperatures for the United States. 2-3 week lag time between the AAM suggests not only an incoming warm trend at some point in October, but also a cooler trend towards the end of next month. Exact timing is to be determined and will depend on the strength of the AAM at its low point.

On to the more interesting concept of this special Long Range Lookout, the Madden Julian Oscillation, or MJO. The MJO involves the anomaly of tropical convection along certain regions of the Pacific and Atlantic. A certain placement of this tropical convection indicates a different phase, with the MJO having 8 phases in all. Each phase has different implications on the weather here in the United States. Model guidance projects the MJO meandering around in a weak-moderate Phase 6 for the next several days. The Canadian model guidance set prefers to retrograde the MJO back into Phase 5, but agreement between the American, European and United Kingdom guidance systems says we will most likely stay in Phase 6.

Now, the MJO at Phase 6 helps to enhance tropical cyclone activity over the West Pacific, which we certainly have seen in the last couple of weeks. This above normal tropical activity in the West Pacific has greatly helped in the negative SST anomaly trend in the north central Pacific, which we discussed earlier. Should the MJO continue to work its magic with the West Pacific, there is little reason to think that this cooling trend in the waters of the North Pacific will not continue.

A key point of interest with Phase 6 of the MJO includes the persistent blocking pattern that sets up in the 10-30 day timeframe following the MJO entering Phase 6. The circled area shows these positive height anomalies as the blocking regime that would present itself between the 90W and 30W longitude areas of the 70N and 55N latitude lines. In other words, the blocking would be present across eastern Canada and much of Greenland. The positive height anomalies present in the same region in the 15-0 days prior to the MJO entering Phase 1 have verified in the last couple of weeks, with the current weather pattern image at the top of this post confirming that suspicion and strengthening the argument for this long range idea of a blocking pattern that may resemble a negative North Atlantic Oscillation.

On another note, if you look closely, you can see a swath of positive height anomalies between the 150W and 120W markers between the numbers 10 and 20 on the left side of the image. This represents a trend of ridging across the Gulf of Alaska, which appears to eventually weaken and shift west. Depending on if this Gulf of Alaska ridging does set up, the East US may have some trouble with maintaining more seasonal temperatures.

Another composite image for the weather pattern 20 days prior to a significant upper latitude blocking event nearly guarantees this idea that we will see some long term blocking in the northeast region of North America. This graphic shows 500 millibar anomaly contour lines as well as OLR anomalies in color (OLR= tropical convection, where blue indicates enhanced tropical convection and yellow portrays below normal tropical convection). The blue anomalies stationed due south of the Bering Sea suggests the MJO is supposed to be in Phase 6 twenty days prior to the significant blocking event. Model guidance confirms that we are already seeing this play out. 500mb anomalies show steadfast high pressure across the far upper latitudes, with negative height anomalies in the northern Pacific. The September 24th 500mb height chart below shows how good of a match the two images are, further helping the argument for blocking in the next 10-30 days. The 'Slow MJO' phrase on the top of that graphic above just tells that the MJO should be moving slowly through Phase 6 in order for this situation to work out, and model guidance assures us that the MJO is in no hurry to get through Phase 6.

September 24, 2013 500mb pattern

So, what can we draw from all of this?

Well, in the short term, expect this very wavy jet stream to continue, due mainly to the continuing storm train across the northern Pacific. Inconsistent but strong bouts of ridging will occur in the Central and Eastern US in response to any storm systems that bottom out in the Southwest and enhance the meridional flow of the jet stream. In the short range, warmth looks to be the buzz word. But we are just a week or two away from what could be a sizable shift in the temperature trend. If the MJO composite works out as I explained above, we should see a tendency for a decisive cool weather trend in the Plains and portions of the western Midwest, with some iffy temperature patterns in the eastern Midwest, Ohio Valley and much of the East US. Depending on how influential the ridging in Canada and Greenland is on the United States, we could see above normal temperatures in portions of the eastern half of the US. A general stormy pattern may evolve over the Great Lakes in the next 7-14 days, but I'm a little skeptical due to low confidence from some other composites I've looked at. Probably won't see any long-term wintry weather potential in much of the Central and Eastern US outside of expanding frost zones for portions of that area. The West should keep an eye out for additional wintry weather possibilities in coming days.

Andrew