Saturday, September 13, 2014

September 27-October 1 Potential Storm System

The September 27th to October 1st period is being monitored for a potential storm system.

Tropical Tidbits
The image above shows 500mb height anomalies over the West Pacific area on the afternoon of September 21. In this image, blues indicate the presence of below-normal height anomalies, which can then lead to cooler than normal temperatures, as well as stormy conditions. Warm colors are associated with positive height anomalies, which generally bring warm and quiet conditions.

As we see in the image above, a trough is moving through Japan on the 21st, according to the latest GFS Ensembles forecast. We see negative height anomalies across the entire country of Japan, but the real point of interest is the depression in the contour lines. This depression indicates a trough moving through the area, and that's what may provide us with the chance for a storm system in the September 27th - October 1st period.

Forecasted teleconnections can give us an idea of where this potential storm may end up.

On the top-left, we see the forecast for the Pacific-North American (PNA) index. We see that at the end of the forecast period, on September 27th, the PNA is projected to be descending from a strong positive state. This tells us that ridging in place across the Western US may be dissipating by the time this potential storm rolls around. Typically a positive PNA means the Midwest and Ohio Valley are most favored for storm systems, but it could be more for the Plains if the PNA is descending to neutral or negative territory.

Additional updates will be provided in the near future.


Long Range Regional Outlook (Ohio Valley): September 20-October 12

This is the latest Long Range Regional Outlook forecast for the Ohio Valley area, valid for September 20th through October 12th.

CMC Ensembles
I have elected to use a combination of the CMC and ECMWF ensembles for this outlook, after finding the GFS ensembles to be in disagreement with the two aforementioned systems, which do hold a consensus.

The image above shows the CMC ensemble mean 500mb height contours on the date of September 22nd. In this forecast, depressions in the contours indicate troughing (cold and unsettled weather) while arcing motions in the contours depict ridging, which results in warm and relatively quiet weather. In the graphic shown above, we see a broad depression in the contours across the Plains and into the Midwest, though we then see slight ridging emerging over the Ohio Valley. This would tell me that the forecast period of September 20th to October 12th likely opens with some seasonable to above-normal temperatures, before colder weather moves in.

ECMWF Ensembles
Now shown above is a two-panel forecast from the ECMWF ensemble system. We see forecasted 500mb height contours and anomalies (with legend on the right) in the left-hand panel, with the ensemble 'spread' on the right image. An ensemble 'spread' indicates the degree of uncertainty among individual ensemble members on a particular area and its forecast. For example, we see deep purples on the right panel over the Great Lakes, which the left panel says will experience troughing (due to the depressed contours). This tells us that the ensembles are uncertain as to how likely this particular factor is to actually occur, and results in a wider spread of ensembles.

In the image above, valid for September 21st, we see a similar layout as that of the CMC ensembles. We see suppressed ridging over the Western US, created by some stormy weather in the Gulf of Alaska (see green shadings of below-normal height anomalies). This ridge in the West results in cooler weather for the Plains and Midwest, as the CMC ensembles depicted, but that then leads to some slight ridging over the Ohio Valley. This jives well altogether with the CMC ensembles.

Tropical Tidbits
In the image above, valid September 17th, we see a swath of negative height anomalies overtaking Japan as a rather strong upper level low scrapes the nation to the north. Seems pretty mundane, sure, but the consequences here at home are far more than mundane. As has been discussed consistently for the past couple of years, the weather in East Asia can have a significant impact on weather here in the United States. Utilizing the East Asian correlation of ridging over Japan equals ridging in the US 6-10 days later, and the same situation with negative height anomalies, we can foresee long range weather patterns weeks out at a time. This mechanism is referred to as the Typhoon Rule, and states that weather patterns found at the 500mb level can replicate themselves over North America 6-10 days later after they appear over Japan.

If we use this rule for the Ohio Valley, we might expect to see some cold weather, though I wouldn't call it a 'cold blast'. In this forecast, the upper level low looks displaced north enough that the core of the cold would likely stay in Canada. This fits in with the CMC and ECMWF ensemble projections of a weak trough moving east from the Plains and Midwest (as we discussed earlier), and is a reason why I disagreed with the GFS ensembles.

* The image used above is a forecast from the GFS ensembles, but the part I disagree with is for its forecast in the Northeast Pacific, not over Japan. Still, this part of the forecast must be monitored closely for the discrepancies described above.

Tropical Tidbits
Once again using this Typhoon Rule, we see the forecast on September 21st calling for continued deep troughing over Japan, now pushed deep into the country. Using the guidelines set forth for this rule, we might expect some chilly weather in the Ohio Valley around the September 27th - October 1st period.

After going over the factors listed above, the temperature and precipitation outlooks for the Ohio Valley over the September 20th - October 12th period are as follows:

Temperature Outlook:

The Weather Centre
Temperature Outlook
Temperatures for the Ohio Valley are expected to remain around average to slightly below-average, due to a warm start to the forecast period and a few weak cool shots.

Precipitation Outlook:

The Weather Centre
Precipitation Outlook
The precipitation outlook calls for predominantly below-normal precipitation over the forecast period, due to somewhat dry signals over the Japan region for the next week or two, among other factors.

The next Long Range Regional Outlook will be published Saturday, September 20th.