Wednesday, May 22, 2019

May 28 - June 1 Potential Storm System & Cooler Weather

*Note: This post is using a forecasting technique that, while similar to the 6-10 day storm forecasting method I was using previously on this blog, is somewhat different and subject to more uncertainty. As such, treat this forecast with a higher level of caution.

I am expecting a storm system and simultaneous influx of cooler than normal air into the contiguous United States to close out May and introduce June. Click on any image to enlarge it.

Observed 500 millibar geopotential height anomalies over Asia as of 7am central, Monday, May 20th.
Source: Tropical Tidbits.
As of this past Monday morning, an unusually-strong upper level low was observed over northeast China, with an individual disturbance rotating around the base of the low, seen here as entering western South Korea. This upper level low is situated between a pair of ridges, the stronger of which is placed east of Japan and near the Aleutian Islands of Alaska.

In this somewhat-different forecasting method, I am using a swath of eastern Asia (namely parts of China, Mongolia and Russia into Japan) to anticipate weather patterns over different parts of the United States in about a 10-day timeframe. To allow for the aforementioned uncertainty, I have widened this to an 8-12 day band centered around May 30th, ten days after the observed graphic above. This is quite similar to the idea that weather in and around Japan correlates to a similar weather pattern in the United States 6-10 days later, but this venture tries to identify where these weather events may occur, instead of broadly anticipating a storm system, cold wave or heat wave over the broad country (which, to be sure, has proved very effective). This is merely an attempt to build on that, and of course has the potential to end in failure. For now, though, there's only one way to hold it (and myself) accountable: by placing a forecast out using it.

Forecasted 500 millibar geopotential height anomalies over the United States as of 1am central, Thursday, May 30th.
Source: Tropical Tidbits.
By early May 30th, current model guidance sees a similar situation developing, with an unusually-strong upper level low placed over Canada with an individual disturbance rotating around the base of the low: here, it is seen crossing from the western Great Lakes into Canada. In this forecast, colder than normal air is again seen over the northern Plains, Midwest and Great Lakes to end May and just begin June. Also matching with the May 20th observed conditions over Asia, a pair of ridges are seen surrounding this upper level low.

Indeed, model guidance sees below-normal temperatures a few thousand feet off the ground by the morning of May 30th over those same areas:

Forecasted 850mb temperature anomalies over the United States as of 7am central, Thursday, May 30th.
Source: Tropical Tidbits.

As such, this forecast is as follows:

- A storm system is expected to cross the central United States during the May 28th - May 30th period, with cooler than normal weather following over the northern portion of the country.
- Ridging over the Eastern Seaboard should keep that area relatively warmer to end the current month.