Thursday, June 13, 2019

June 21-25 Potentially Strong Storm System

It appears a potentially strong storm system will move through the country in the June 21-25th time period.

Forecasted 500-millibar geopotential height anomalies valid for 7pm June 15th.
Source: Tropical Tidbits
By the evening of June 15th, model guidance sees a formidable trough sweeping across Japan, negatively-tilted (oriented from a northwest-to-southeast direction) with substantial negative height anomalies. In general, when an upper-level trough is oriented in that negative-tilt direction, it implies that the storm system it is associated with has reached its mature phase, and is broadly at or near its strongest point in the storm's life cycle.

Regular readers of this blog in the past will know that I frequently use a teleconnection whereby weather phenomena occurring over and near Japan correlate to similar phenomena in North America roughly 6-10 days later. I am employing this again here, and extrapolating the above model graphic gives us a potential storm system occurring in the United States in the June 21-25 period.

Forecasted MSLP and 6-hour average precipitation rates valid for 7pm June 15th.
Source: Tropical Tidbits
At the surface, we see why the 500-millibar geopotential height anomalies are so negative. Indeed, at the same time as the first image, a surface low with a minimum sea level pressure of 986 millibars is forecasted to be placed right over Japan. While it is not unusual for surface lows to plumb minimum pressure values firmly below 1000 millibars in the Pacific Ocean, particularly in the open waters near the Aleutian Islands, the substantial negative 500-millibar height anomalies at the top of this page and the substantial negative MSLP normalized anomalies below illustrate that even around Japan this storm system is somewhat strong.

Forecasted MSLP normalized anomalies valid for 7pm June 15th.
Source: Tropical Tidbits
So, what can we expect for this June 21-25 timeframe?

Forecasted MSLP  anomalies via the GFS Ensembles valid for 1pm June 24th.
Source: Tropical Tidbits
We can get a sense as to the general pattern shaping up, particularly since it's too far out to try and pick out individual model runs. Above, the forecasted MSLP anomalies from the GFS ensembles on the afternoon of June 24th are shown, and it provides a glimpse at what could be on the way.

Naturally, given these images are the average of the model's ensembles, we aren't able to identify the exact strength or placement of the storm - as stated, we're aiming to identify the broad pattern. The ensembles see below-normal sea level pressure anomalies developing over the central U.S., maximized over the central and northern Plains. This would likely be the storm that would be correlated with the storm expected to impact Japan over the next few days, but it will still take some time for model guidance to reliably latch on to (or even lose) the storm system.

Forecasted 850-millibar temperature anomalies valid for 1pm June 17th.
Source: Tropical Tidbits
In the wake of the storm system, an airmass featuring substantially below-normal temperature anomalies looks to move in over Japan, which should then correlate with below-normal temperatures again invading the United States to round out the month of June.

To Summarize:
- A potentially strong storm system looks to impact the United States in the June 21-25 period.
- Notably cooler than normal weather is expected to impact the country to end June, after the storm system moves through.