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.

Andrew

Long Range Regional Outlook (Northeast & Mid-Atlantic): September 20-October 12

This is the premiere post of the Long Range Regional Outlooks, this one valid for the Northeast over the September 20th to October 12th period.

CMC

After examining long range ensemble guidance, a split became evident between the ECMWF/CMC ensembles, as well as the GFS Ensembles. Have elected to side with the ECMWF/CMC projections, as GFS Ensembles are generally going against the grain of the other two ensemble sets, as well as my own thoughts.

The CMC ensemble mean 500mb geopotential height forecast is shown above. As a general rule of thumb, depressions in the contour lines tend to indicate troughing and cold/stormy weather, while arcing of the contour lines indicates ridging of high pressure, as well as warm/quiet weather. In the image above, we see 500mb height contours valid for September 20th. We see a rather zonal flow (non-wavy pattern) set-up over the northeast Pacific, with the jet stream pushing into the west coast of North America. This is in contrast to the GFS ensembles, which predicted deep troughing in the Gulf of Alaska. There is some slight ridging in the Southwest, leading to some weak troughing across Central and Eastern US areas.

ECMWF

The ECMWF ensembles are shown above, with the 500mb geopotential height anomalies and contours shown on the left panel, as well as the 'spread' (degree of disagreement among the ensemble members) for this forecast of September 21st on the right. This forecast is similar to the CMC ensemble projection above, as we see the Pacific jet stream pushing into the Western part of North America.

According to the height anomalies and contours, we do see some substantial troughing in the Gulf of Alaska, though it is not to the degree of the GFS ensembles. We then see some weak ridging in the Southwest, as was also seen with the CMC ensembles, though it is suppressed. Some weak troughing evolves in the Central US before weak ridging in the East.

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 Northeast, we might expect to see the northern parts of the region get some substantial cold around a September 23rd - 27th period. This fits in with the CMC and ECMWF ensemble projections, 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 continued cold in the Northeast around the September 27th - October 1st period.

After going over all of the factors above, as well as extra analyzations not mentioned in this post, the outlook for the September 20th to October 12th period over the Northeast is as follows.

Temperature Outlook:

The Weather Centre
Temperature Outlook

Precipitation Outlook

The Weather Centre
Precipitation Outlook

I expect the Northeast to remain predominantly below normal for temperatures in the aforementioned time period, while precipitation should end up a bit below normal, close to neutral.

Andrew