Synoptic Discussion - October 2019
Note: This Synoptic Discussion describes recent weather events and climate anomalies in relation to the phenomena that cause the weather. These phenomena include the jet stream, fronts and low pressure systems that bring precipitation, high pressure systems that bring dry weather, and the mechanisms which control these features — such as El Niño, La Niña, and other oceanic and atmospheric drivers (PNA, NAO, AO, and others). The report may contain more technical language than other components of the State of the Climate series.
|Time Period||Key Driver||Other Drivers|
Tropical Storm Melissa
|October 18-23||WPO+||Tropical Storm Nestor,
Super Typhoon Hagibis
Tropical Storm Olga
The weather was particularly dynamic during October 2019. The month began with a summer-like ridge that set all-time October heat records across the eastern US. It ended with a winter-like surge of cold air along the Rockies that set all-time October records on the cold side. In between, three weak tropical storms brought rain from the Gulf Coast to the Northeast, and energy from a super typhoon in the Pacific helped spur the rapid transition from summer to winter. All the while, a ridge extending southward from the Gulf of Alaska prevented any moisture from alleviating the fire season in the Southwest.
The general pattern in October 2019 with a trough in the West and a ridge over the Southeast is consistent with a negative Pacific/North American (PNA) pattern. The PNA wave train usually traces back to the tropical Pacific, but the pattern during October 2019 was more tied to variations in the Pacific jet associated with fluctuations in the West Pacific Oscillation (WPO) and the East Pacific Oscillation (EPO). Meanwhile, the El Niño-Southern Oscillation (ENSO) remained on the warm side of neutral.
October began with a trough over the northwestern US and a strong ridge over the Southeast. The ridge brought record heat to the Southeast with many locations setting all-time October heat records. The trough was associated with cooler temperatures in the Northwest. A cold front on the east side of that trough brought a plume of moisture from the southern Rockies to the Great Lakes. The resulting rainfall was a primary contributor to the record-wet October for portions of the Great Lakes region.
A cold front swept across the eastern US bring needed relief from the record heat. The northwest remained cooler than average, but otherwise conditions were normal to warmer than normal for most of the CONUS.
The first significant cold air outbreak filtered down the Northern Plains. With it came 1-2 feet of snow over portions of the Northern Rockies. Meanwhile the Northeast experienced a significant nor'easter from the remnants of Tropical Storm Melissa.
Temperatures moderated to near-normal October 18-23. Tropical Storm Nestor made landfall along the Florida Panhandle bringing significant rains to the Southeast. However, the bigger impacts may have come from the remnants Super Typhoon Hagibis. The extratropical transition of this intense storm helped extend the jet stream over the North Pacific. This ultimately enhanced the trough over western Canada and set the stage for major surge of cold air along the Rockies.
Cold air surged southward along the Rockies October 24-31. Many parts of the West set all-time record low temperatures for October during this event. It also brought significant snow from the Rockies to the Midwest. The southern end of the cold front merged with Tropical Storm Olga and the resulting moisture surge contributed to record wet October for portions of the Lower Mississippi Valley.
- El Niño Southern Oscillation (ENSO)
- Description: Oceanic and atmospheric conditions in the tropical Pacific Ocean can influence weather across the globe. ENSO is characterized by two extreme modes: El Niño (warmer-than-normal sea surface temperature [SST] anomalies in the tropical Pacific) and La Niña (cooler-than-normal SST anomalies), with the absence of either of these modes termed “ENSO-neutral” conditions. These variations in SST change the locations of the Pacific's largest thunderstorms, which can in turn change circulation patterns around the globe.
- Status: The ocean and atmosphere system continued to be in ENSO-neutral during October 2019. The most common metric for ENSO is the SST anomalies in the Central Pacific, the Niño 3.4 region. These were moderately warm throughout the month after being cooler than normal in September. However, a much longer period of warming along with more substantial atmospheric coupling would be required for a return to El Niño conditions.
- Teleconnections (influence on weather): The teleconnections with ENSO are muted during near-neutral conditions. An active El Niño would have favored cooler than normal temperatures in the southwest, while these were located farther north during October 2019. It would have also favored wetter conditions over Texas and the Southern Appalachians, only the latter of which transpired.
- Madden-Julian Oscillation (MJO)
- Description: The MJO is the biggest source of subseasonal (30-60 day) tropical variability. It typically develops as a large envelope of tropical thunderstorms that develops over the Indian Ocean that then moves eastward. Like ENSO, the MJO's effects on tropical rainfall is so strong that it can alter the atmospheric circulation around the globe. The thunderstorms decay when they cross the Pacific, but the associated winds can often continue across the Western Hemisphere to initiate the next MJO in the Indian Ocean. The MJO is episodic, meaning that is not always active. Most indices for tracking the MJO identify both the MJO's amplitude and the longitude of its strongest rainfall, usually described as one of eight phases.
- Status: The MJO was stalled over Africa (phases 8/1) during the first half of October in association with a strong negative Indian Ocean Dipole (IOD). Over the second of the month, the MJO began propagating across the Indian Ocean (phases 2/3) before ending the month over the Maritime Continent (phase 4).
- Teleconnections (influence on weather): The MJO's movement across the Indian Ocean during the middle of October would typically favor warmer than normal conditions across the Eastern US for the end of the month, but those were narrower along the East Coast than might have been expected.
- The Pacific/North American (PNA) pattern
- Description: The PNA teleconnection pattern is associated with strong fluctuations in the strength and location of the East Asian jet stream. PNA-related blocking of the jet stream flow in the Pacific can affect weather downstream over North America, especially the West and especially in the winter half of the year.
- Status: The daily PNA index was strongly negative for the first half of October. It was briefly positive before returning to a weak negative state for the final 10 days of the month. On average, the PNA was moderately negative during October 2019 due to a canonical pattern with a ridge in the Gulf of Alaska, a trough over the northern Rockies, and ridge along the East Coast.
- Teleconnections (influence on weather): A negative PNA generally favors below-normal temperatures along the West Coast and warmer temperatures over the Southeast. These patterns were both shifted slightly eastward during October 2019, but otherwise aligned well on the monthly mean.
- The Arctic Oscillation (AO) pattern
- Description: The AO teleconnection pattern generally measures the pressure difference between the low pressure over the North Pole and the higher pressures in the subtropical ridges. This pressure difference is larger during a positive AO, resulting in a stronger midlatitude jet. When the AO is negative, the jet is weaker and will have larger troughs and ridges.
- Status: The AO was positive for the first half of October and negative for most of the second half, resulting in neutral for the month as a whole. These neutral conditions resulted from the competing effects of the ridge over Greenland (negative AO) and the ridges over western Europe and the Gulf of Alaska.
- Teleconnections (influence on weather): The positive AO during the first half of October would typically be associated with warmer than average temperatures over the Northern Plains, but these only occurred during October 6-10 and even then it was a weak signal. The negative AO in the second half of the month fosters colder temperatures over the Northern Plains, which extended much farther south during October 24-31.
- The North Atlantic Oscillation (NAO) pattern
- Description: The NAO teleconnection pattern relates the pressure over the sub-polar low near Greenland and Iceland with the subtropical high over the Central Atlantic. It significantly affects the weather on both sides of the Atlantic.
- Status: The daily NAO was negative for almost the entire month, and moderately negative for the monthly mean. The negative NAO was due primarily to the persistent ridge over Greenland and the trough over the central North Atlantic.
- Teleconnections (influence on weather): The NAO's impacts on the CONUS are generally weaker during the fall months. The trough over the North Atlantic also did not extend as far east or west into Europe and the East Coast as it typically would during a negative NAO, which further limited its impacts.
- The West Pacific Oscillation (WPO) pattern
- Description: The WPO teleconnection pattern is a primary mode of low-frequency variability over the North Pacific and reflects zonal and meridional variations in the location and intensity of the East Asian jet stream in the western Pacific.
- Status: The WPO index was negative early in the month, then positive during October 14-24, and then negative for the remainder of the month. The monthly average was weakly negative, due primarily to the the enhanced ridge over the Sea of Okhotsk.
- Teleconnections (influence on weather): The negative WPO is typically associated with cooler than normal temperatures over the Northern Plains. These appeared as the negative WPO weakened October 11-17 and then even more so when the negative values returned October 24-31.
- The East Pacific Oscillation (EPO) pattern
- Description: The EPO pattern identifies variations in the strength and location of the eastern Pacific jet stream. During the positive phase, the jet is stronger and shifted southward. The negative phase is associated with an Alaskan ridge that weakens the jet. The EPO is closely related to the East Pacific–North Pacific (EP–NP) teleconnection pattern, although the two are defined with opposite signs.
- Status: Similar to the WPO, the EPO index was negative during October except for a positive period October 15-22. The negative EPO during the later part of the month was stronger than its WPO counterpart. The moderately negative EPO averaged over the month was due primarily to the ridge over the Gulf of Alaska.
- Teleconnections (influence on weather): The negative phase of the EPO favors cooler than normal temperatures across the Upper Midwest and Northeast. These signals were generally shifted farther west towards the Rockies during October 2019.