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.