Tuesday, March 31, 2015

Multi-Day Severe Weather Threat for Midwest, Ohio Valley

A multi-day severe weather event will unfold Wednesday and Thursday across the Midwest and Ohio Valley, respectively.

Tropical Tidbits (click to enlarge)
Thunderstorm development is expected over southern Minnesota, northern Iowa, and eastern Nebraska by Wednesday evening, with the most intense convection being simulated over northwest Iowa. The image above shows forecasted radar reflectivity over the Midwest area at 7PM Wednesday. In other words, this is what one forecast model thinks the radars will see unfolding at 7PM Wednesday. If it does verify, this could be a rather intense severe weather event for anyone caught in the line of fire of a particularly nasty line segment of storms, such as the one shows in northwest Iowa.

The primary threats here will be damaging winds and hail; the linear shape of these storms should cut down on the tornado threat.

Tropical Tidbits
By Thursday afternoon, thunderstorm development is renewed in Indiana, Michigan, Kentucky, and Ohio. These cells should be more individualized, raising the hail and tornado threats, but still keeping the tornado threat at a rather low level.


Monday, March 23, 2015

Long Range - Monthly Outlook: April 2015

This is the long range/Monthly outlook for late March into April 2015.

We'll first begin with a diagnosis of the atmosphere. For those who find the technical jargon a little overwhelming, summaries to the post are provided at the bottom of the page.

The above image shows relative atmospheric angular momentum anomalies on the top panel, with the global anomaly on the bottom panel. According to the latest data, we are in a globally positive-AAM (+AAM) state, mainly due to a nice surge of +AAM to the equator. This has pushed the atmosphere into an El Nino mindset, which has shown up in our atmosphere. We'll go more in-depth to that later on.

The graphic above shows the tendency anomalies of the atmospheric angular momentum on the top panel, with the net tendency on the bottom panel. Notice how we're currently recovering from a massive +AAM tendency spike, and are now in a net negative AAM tendency. The cause of this tendency is not the thing we will analyze; right now, we are noting that we're in a +AAM state with -AAM tendencies.
This fits well into the four stage descriptions of the Global Wind Oscillation (GWO), which takes those above graphics and makes them into useful data. Here's the descriptions of all four stages of the GWO; pay careful attention to the first and last sentences of each stage.

The four primary phases of the GWO are described below, along with generally cold season (November-March) probable weather impacts for the USA. The GWO recurrence interval, or "time it takes to make a circuit", ranges from a broad 15-80 days. Two of the stages project strongly on El Nino and La Nina circulation states, which are also characterized by positive (Stage 3) and negative (Stage 1) global AAM anomalies, respectively.  Stages 2 and 4 are transitional.

Stage 1 (La-Nina like) – the global relative AAM anomaly is negative. The negative anomaly is primarily due to easterly upper level wind anomalies that extend from the Eastern Hemisphere tropics to the Western Hemisphere mid-latitudes. A retracted Pacific Ocean jet stream is a key feature in the total field.  Troughs are probable across the western USA with a ridge over the southeast.  High impact weather is favored across the Plains.

Stage 2 – the global relative AAM tendency is positive. This means that negative AAM is being removed from the atmosphere by surface friction and mountains. At the same time, westerly wind anomalies are intensifying in equatorial regions of the Western Hemisphere. Fast Rossby wave dispersion events in both hemispheres are a coherent feature of this stage and Stage 4.  A cold regime is probable across the central USA.

Stage 3 (El-Nino like) – the global relative AAM anomaly is positive. Westerly wind anomalies move into the Eastern Hemisphere, broaden in latitudinal extent and link up with deep westerly flow anomalies over the mid-latitude Western Hemisphere. An extended Pacific Ocean jet stream and southward shifted storm track is observed  favoring high impact weather events along the USA west coast.

Stage 4 – the global relative AAM tendency is negative. Positive (westerly) AAM anomalies are being removed by surface friction in the Western Hemisphere mid-latitudes and through mountain torques across the Northern Hemisphere topography. The next phase of the oscillation (if there is one) is represented by easterly wind anomalies intensifying over equatorial regions of the Western Hemisphere. This stage has enhanced subtropical jets and closed lows in the subtropics favoring rainfall events over the southwestern USA.
What did you notice in each stage description? Did you notice how each description began with details on how the net (global relative) AAM or AAM tendency was positive or negative? Let's apply that here.
We saw earlier how we are in a +AAM / -AAM tendency phase right now. Using those particular sentences in each description, it appears we're in Stage 3 or Stage 4 of the Global Wind Oscillation. Let's investigate each description closer. In Stage 3, among other details, the Pacific jet stream is extended (can stretch across the Pacific), and active weather impacts the West US. In Stage 4, the subtropical jet stream is strengthened, and closed lows in the subtropics can permit heavy rainfall over the Southwest.

Do we have any of those factors?

The answer is yes. The graphic above shows 200 hPa heights and wind speeds in ~4 day periods, with the least recent panel in the top left and the most recent panel in the bottom left. Notice how the Pacific jet stream is clearly extended into the North Pacific, fulfilling our Stage 3 ideals. If you'll look back at the stage description for Stage 3, you'll notice how it says 'El-Nino Like'. This means that conditions in this stage are most likely to provoke an El Nino-like response in the atmosphere. We're already seeing that response, namely in the extended Pacific jet and the AAM charts. I expect this to continue throughout April due to ensemble agreement.

The temperature anomaly charts in the US came back very cold for the next week or so. Each individual member is shown on those smaller boxes, while the large box contains the mean temperature departure. There is very good agreement on this cold, as well, valid on March 28th.

Lastly, let's go over the tropics. We're currently seeing a very strong tropical forcing episode over Phase 8 of the Madden-Julian Oscillation, a signal that has been created thanks to a powerful convectively coupled Kelvin Wave (CCKW). The long range ECMWF ensembles believe we see this current wave dissipate before we dip back into the circle of the unknown, and this proposal is backed up by the GFS model. This renders tropical forcing useful until just about March 28th, before the wave weakens into the Circle of Death on the MJO chart. Phase 8 and Phase 1 events in this time of year routinely create warm and cold conditions, respectively, in the Midwest, Central US, and East US. I have a good feeling that the Phase 8 activity will far outpace the Phase 1 activity, keeping us in the El Nino mindset well into April.

To summarize the forecast:

- The last days of March (24-30) may see sustained cooler than average temperatures. This may persist even longer in the Northeast.
- Early April will see continued threats to the West for heavy rainfall and moisture. A warm-up is expected for this time period.
- Mid and Late April can expect more El Nino conditions, with warm weather in the Morth and stormy/cool weather in the South.


Sunday, March 22, 2015

Sunspot Numbers Plummeting, Stratosphere Warming

The number of sunspots has been anomalously low as of late, the lowest trend since roughly July 2014.

The above image shows sunspot numbers from NOAA over the past many months. Higher sunspot numbers (red) indicate that the sun is more active than usual. In the last couple of months, the number of sunspots has been dropping steadily from average values earlier in the year into 2014, interestingly enough in the midst of the strongest solar storm unleashed on the Earth in this solar cycle, per some reports.

What does this mean for our weather? The influence of the sun on our weather is still somewhat murky, but one big derivation we can make is that the stratosphere tends to cool down when the sun is active, and warm when the sun is quieter.

We are already seeing the stratosphere warm up, with more warming expecting in the future. The panels above show observed temperature values at different parts of the stratosphere in color, with the forecasted values in dashed lines. Notice how the ECMWF model expects the 10hPa level (top panel) to heat up notably, with a similar story in the 30hPa level (2nd panel from top). Although this warming won't be sustained, it's quite possible we see additional warming as the stratosphere exits its winter phase and enters its summer phase.

To summarize:

- Sunspot numbers have been steadily decreasing in the last few months, as we begin to end the current solar cycle.
- Stratospheric temperatures are responding by warming up, signaling and end to the winter phase.


March 24, 2015 Severe Weather Outlook

The Storm Prediction Center has issued a severe weather outlook for March 24th, 2015.

The Storm Prediction Center is expecting potential severe weather to develop in much of Missouri, northeast Oklahoma, northwest Arkansas, eastern Kansas, southern Iowa, and extreme western Illinois. The current risk is listed as a Slight Risk, which is not a significant threat outlook, but should be monitored.

WPC forecast, valid Wednesday, March 24th
The severe weather set-up will include a strong low pressure system moving into the Upper Midwest. A warm front well displaced from the severe weather outlook area (shown in Michigan and Ohio above) will allow for a narrow corridor of potentially severe weather to develop. These storms should develop along the cold front that will shift eastward through the area. But is this risk actually legitimate?

The above image shows projected radar reflectivity at 7 AM Central Time on Tuesday. We see a large complex of showers and thunderstorms moving across the corridor highlighted for severe weather on this day. This ongoing complex likely will hinder any severe weather prospects for later on in the day, which would end up ruining this severe weather chance. The best risk for severe weather is likely with that morning's band of showers and storms, portrayed in Missouri and southern Iowa on this NAM model forecast.

To summarize:

- Missouri, Iowa, Illinois, and Oklahoma are at risk for severe weather on March 24, 2015.
- This severe weather potential could be ruined by ongoing showers and storms early in the morning.


Wednesday, March 18, 2015

Warmer Weather Returns After Early April Cold Blast

It looks as if warm weather will make a comeback after a stark cold shot hits to close March and open April.

Tropical Tidbits
In the image above, we see 500mb geopotential heights over the West Pacific, where reds indicate above normal values typically associated with high pressure and calm weather. Blues indicate toughing, symptomatic of colder and stormier weather on the surface. In this chart, valid on March 27th, we can see the strong ridge over Japan and areas just off to the north, with a slight trough to the south.

Using this, we can predict what the first week or so of April will be like. The Typhoon Rule tells us we can expect weather phenomena in Japan to 'happen again' here in the US about 6-10 days later. If the image above is valid on the 27th, we should expect some warmer weather around the first couple days of April. However, it's very possible this warmth is actually delayed, due to the aforementioned cold weather hitting the country at the end of March and start of April.

To summarize:

- After cold weather hitting in late March into the first couple days of April, warmer weather should make a comeback.