Friday, August 1, 2014

Mid-August Unseasonable Cold Blast Appearing Likely

Yet another blast of unseasonably cold air can be expected, this time around the middle of August.

Tropical Tidbits
The first graphic we will examine shows the GFS model's forecast of 500mb height anomalies over the western Pacific. Blues indicates stormy and cold weather, while reds show warm and quiet weather. This forecast graphic, valid on August 9th, shows a typhoon moving north into the southern portion of Japan. We can identify this typhoon feature by the tight collection of isobar contours, as well as the concentrated nature of the below-normal height anomalies. This typhoon is expected to be weakening as it makes its move north into Japan.

Tropical Tidbits
This next graphic shows the same GFS forecast of 500mb height anomalies, now valid on the evening of August 9th, whereas the first graphic showed the forecast for the morning of August 9th. In this picture, we can still see our typhoon feature, now pulling itself northeast through the main part of Japan. The isobars are no longer as concentrated, and this can be contributed to both weakening of the typhoon, as well as the lower resolution forecasts this GFS model makes after forecast hour 192 (you'll notice that this graphic shows the forecast for hour 204).

Why is all of this significant? The general premise that I've discussed numerous times on this blog, particularly in the wintertime, is that the weather in East Asia can affect our weather here. To clarify, for example, high pressure over Japan would mean high pressure over the US 6-10 days later. Similarly, low pressure over Japan could indicate stormy weather in the US 6-10 days later, and so forth. We can use this rule to our advantage here. We see a strong typhoon hitting southern Japan on August 9th, and tracking through the middle of the country before shooting east into the open Pacific. By using this correlation technique, we can identify that cold weather and a general stormy pattern may be anticipated 6-10 days after this August 9th impact. Do the math, and we can expect an unseasonable cold weather event around the August 15-19 timeframe.

Details still need to be refined, as this is a long range forecast, but the chances are good that cold weather may be expected in the middle of August.