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.

ESRL

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.

Andrew

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

Multiple Arctic Blasts Projected to Hit North America During Coming Weeks

The remainder of September into October looks to feature a series of Arctic cold shots, potentially record-breaking at times.

Tropical Tidbits
Click to enlarge

The first cold threat looks to arrive only in a matter of days after our ongoing cold shock. Shown above is the GFS ensemble forecast over the Western Pacific, showing expected 500mb geopotential height anomalies. Reds indicate above normal height anomalies, consequentially warmer and quiet weather. Blues depict negative height anomalies, which often result in cold and stormy weather. 

In the image above, valid September 11th, 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 on the model image above, a rather strong upper level low with the accompanying threat of some cold Arctic air may begin affecting the US around September 17-21st, within the six-to-ten day timeframe discussed above.

Tropical Tidbits

The image above once again shows forecasted 500mb height anomalies over the West Pacific, now valid on September 18th. Again, we see another swath of negative height anomalies over Japan, apparently due to another strong upper level low pressing south into the northern part of the country.

Using the aforementioned Typhoon Rule, it appears our second cold blast may impact the country around a September 24-28th timeframe. This one may be more significant than the last, as ensembles are already highlighting well below normal height anomalies this far out, and the upper level low appears deeper and more pronounced than the one in our first Arctic air threat.

Plymouth State

Our third threat at Arctic air follows closely behind our second threat. The image above, from Plymouth State University, shows the ECMWF model's forecast of 500mb heights (shaded colors and white contour values) and mean sea level pressure (MSLP) values (black lines). In this image, valid September 13th, we see a strong upper level low placed over the eastern Bering Sea. Just as we saw in Japan, this appears rather mundane, but actually plays a significant role in our upcoming weather.

By observing trends in the Bering Sea, weather conditions in the US can be predicted 17 to 21 days in advance. For instance, if the Bering Sea experiences a deep storm system, cold weather may be expected 2.5 to 3 weeks later. The same goes for high pressure in the Bering Sea. In this case, with the forecast above valid on September 13th, we may expect a rather significant cold blast over the United States for the end of September and into October, in a general September 30th - October 4th time period.

Plymouth State

It's this last Arctic air threat that has me concerned. The image above once again shows the ECMWF's outlook of 500mb heights and MSLP values, now valid on September 17th. In this image, we see decidedly below-normal heights spread across nearly the entire Bering Sea, with the core of this messy trough centered just north of the Aleutian Islands.

This scenario worries me a bit. Even though this is definitely a long range forecast (and thus shouldn't be seen as high-certainty), the premise is concerning. The ECMWF's proposition of a cold air mass across the entire Bering Sea would suggest more of a long-term cold weather period, across a big chunk of the nation. Extrapolating the forecast time period of September 17th, we might expect some cold weather around an October 4th - October 8th period, possibly for longer.

A short-term warm-up can be expected in the last week or two of September before the cold continues.

Stay tuned for continuing updates on these threats.

Andrew

September 10th Potentially Significant Severe Weather Event

A severe weather event looks to unfold on Wednesday, September 10th that may hold the potential for significant damaging winds, as well as tornadoes.

SPC
Click to enlarge

The Storm Prediction Center's severe weather outlook for tomorrow shows a wide swath of the Midwest and Ohio Valley under the gun. States outlined include nearly all of Illinois, all of Indiana, eastern Missouri, much of Michigan, and a heavy majority of Ohio.

SPC
Click to enlarge

A probabilistic approach to the outlook for tomorrow can better define who's in line for the worst weather. In the image above, we see the SPC has delineated the severe weather risk into four categories. The lowest category, the 5% chance of severe weather area, indicates thunderstorms can be expected, but shouldn't reach severe limits. The 15% delineation describes severe thunderstorms as being possible, but probably not widespread. The 30% outlook indicates severe thunderstorms may form en masse, with modes of significant severe weather possible. Those in the 45% outlook may want to prepare for widespread significant severe weather, which may threaten life and property. This 45% outlook zone is focused across eastern Illinois into Indiana, northwest Ohio, and southern Michigan.

PSU

High-resolution radar forecast from the 4km NAM model shows thunderstorm formation around 7:00 PM Central time / 8:00 PM Eastern time on Wednesday. This ragged squall line originally is composed of individual cells, which tells me tornadoes may be a threat. This potential tornado threat is highlighted by the incredibly dynamic wind fields aloft, owing to the very strong low pressure system passing to the northwest. These cells then congeal into a squall line, which could be capable of producing large areas of potentially damaging winds.

A good deal of uncertainty remains with this event, but the potential is there. Make sure you're prepared.

Andrew