April 2023 Monthly Synoptic Discussion

April 2023 Synoptic Discussion | National Centers for Environmental Information (NCEI)

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.

Overview

Key Drivers

- Month
- Negative Arctic Oscillation (AO)
- Negative North Atlantic Oscillation (NAO)
- Negative
- April 1-8
- Negative Pacific/North American (PNA) pattern
- Negative
- April 9-16
Madden-Julian Oscillation (MJO) over the Western Pacific (phase 6)
- Positive East Pacific Oscillation (EPO)
- April 17-23
- Negative AO
- Negative NAO
- Negative EPO
- April 24-30
- Positive PNA
- Negative AO
- Negative NAO

Monthly Mean

The April mean circulation for North America was strongly influenced by the negative phases of the AO and NAO during the second half of the month. These were associated with a trough near the Central U.S. that was related to below normal temperatures. The trough was connected with several severe weather outbreaks and anomalously wet conditions for the Southeast.

The cool anomalies associated with the negative AO and NAO were partially offset by warmer temperatures earlier in the month. In particular, the positive EPO during April 9-16 was related with record warmth from the Great Lakes to New England.

Submonthly Evolution

April 1-8

The beginning of April featured a negative PNA pattern with a trough along the West Coast and a ridge over the East Coast. The trough was associated with cool and wet conditions across the northwestern U.S., and the ridge was related with anomalous warmth across the Southeast. A strong cold front caused severe weather around the Upper Mississippi Valley April 4-5.

April 9-16

During April 9-16, a ridge became anchored near the Great Lakes. It was consistent with the MJO over the Western Pacific at that time along with the positive EPO. The ridge was associated with warm and dry conditions for most of the country. A notable exception was South Florida where Ft. Lauderdale experienced record rainfall on April 13 in association with a frontal boundary from a low-pressure system in the Gulf of Mexico.

April 17-23

Both the AO and NAO became negative during April 17-23 as ridging dominated the polar region. A broad trough extended from British Columbia to the Great Lakes, which was associated with cooler than normal temperatures for the northwestern U.S. Conditions were drier than normal over much of the country, but 29 tornadoes formed along a cold front near the Central Plains on April 19.

April 24-30

The negative AO and NAO patterns continued through the end of April, which maintained widespread colder than normal conditions. However, a positive PNA pattern was associated with a ridge and warmer than normal temperatures along the West Coast. On the south side of the trough, an amplified subtropical jet was connected with multiple severe weather events along the Gulf Coast.

Atmospheric Drivers

ENSO: El Niño Southern Oscillation

- 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: ENSO neutral conditions continued during April. The most common metric for ENSO is the SST anomalies in the equatorial Pacific. These continued to warm, consistent with an expected transition to El Niño this summer. The Southern Oscillation Index, which measures the atmospheric response, remained neutral during April. Together, the coupled ocean-atmosphere system was consistent with ENSO-neutral.
- Teleconnections (influence on weather): El Niño favors cooler and wetter conditions for the southern U.S. It also favors drier than normal conditions for the southern U.S. and wetter than normal for the Northwest and the Tennessee Valley. However, the weak ENSO signal this month precluded any significant impacts.

MJO: Madden-Julian Oscillation

- Description: The MJO is the biggest source of subseasonal (30-50 day) tropical variability. It typically develops as a large envelope of tropical thunderstorms 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 it 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 index circumnavigated the globe in April—beginning and ending the month over the Maritime Continent (phase 4).
- Teleconnections (influence on weather): The most significant impacts from the MJO during April are typically in MJO phase 6, which is associated with warm and wet conditions near the Middle Mississippi Valley. The temperature pattern was observed April 9-16, but the precipitation pattern was not.

PNA: Pacific/North American 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 negative at the beginning of April and positive towards the end. The monthly mean was near zero.
- Teleconnections (influence on weather): The positive PNA favors warmer conditions in the Pacific Northwest and cooler in the Southeast. The negative PNA temperature pattern was observed April 1-8 with the opposite pattern during the positive PNA April 24-30.

AO: Arctic Oscillation

- 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 daily AO index was weak at the beginning of April, positive near April 11, and strongly negative during the second half of April. The monthly mean was negative.
- Teleconnections (influence on weather): The negative AO is typically associated with cool anomalies for most of the contiguous U.S. It also favors dry conditions for the Ohio Valley and the Northeast. The cool anomalies were observed April 17-23 and April 24-30, but the precipitation patterns were not.

NAO: North Atlantic Oscillation

- 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 index positive around April 9 and then negative during the second half of the month. The monthly mean was negative.
- Teleconnections (influence on weather): During spring, the negative NAO is associated with cooler temperatures for most of the U.S. and wetter conditions for the Southeast. The temperature patterns were observed April 17-23 and April 24-30. The enhanced precipitation for the Southeast was observed

WPO: West Pacific Oscillation

- 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 daily WPO index oscillated between positive and negative in April with the strongest positive values around April 7 and the strongest negative values around April 21. The monthly mean was negative.
- Teleconnections (influence on weather): The positive WPO is typically associated with warmer temperatures near the Northern Mississippi Valley and cooler temperatures over the Southwest with the opposite in the negative phase. The negative WPO pattern was somewhat apparent April 17-23.

EPO: East Pacific Oscillation

- 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: The daily EPO index was strongly positive during the first half of April, negative around April 20, and positive again around April 26. The monthly mean was positive.
- Teleconnections (influence on weather): The positive phase of the EPO favors warmer than normal temperatures across the Upper Midwest. The related negative phase of the EP-NP is also associated with cooler temperatures along the West Coast. The positive EPO was most apparent April 9-16. The negative EPO pattern was observed April 17-23.

Citing This Report NOAA National Centers for Environmental Information, Monthly Synoptic Discussion for April 2023, published online May 2023, retrieved on May 12, 2023 from

https://www.ncei.noaa.gov/ access/ monitoring/ monthly-report/ synoptic/ 202304.