Charts of the day: Cooling Northeast Pacific SSTA Foreshadows More -PNA Patterns
Discussion: Since late January, the Pacific North America (PNA) index has shifted to a (mostly) steady negative phase. A brief positive phase is with us now, but strengthening negative phase returns in the latest 15-day forecast. The -PNA pattern occurs when an upper-level troughing in the jet stream pattern occurs just off the North America West Coast. The persistence of the -PNA pattern has caused the Northeast Pacific to cool, the Northeast Pacific marine heat wave (MHW) to shift westward and reinvigorate the cool phase of the Pacific decadal oscillation (-PDO). The return of the -PNA pattern maintains a chilly West U.S. pattern in the most recent 15-day outlooks. The cool West persistence is compensated for by a warmer change in the East. The -PNA shift should maintain a stormy pattern for the West U.S. although due to seasonality highest impacts likely shift northward easing but not ending California rains. An added concern with the -PNA pattern is the presence of somewhat warmer than normal Gulf of Mexico which could lead to a wetter pattern across the eastern half of the U.S.
Week-2 Ahead Forecast valid March 27-April 3, 2023: -PNA is key to the pattern.
Discussion: ECM correctly projects a cool regime across the West inspired by the -PNA pattern heading into early April. The attendant West Coast storm track stays intact although lifting slightly northward due in-part to seasonality. In the East, the model is too cool and not wet enough. However, the Great Plains are likely drier and the southwest Great Plains to Western Texas could be warmer.
Week-3 Ahead Forecast valid April 3-10, 2023: East-central (heavy) rainfall concern.
Discussion: Forecast confidence is below average for week-3 ahead as the -PNA pattern could weaken due to a shift in the MJO regime. Cool weather hangs on across the snow-covered West U.S. The previous forecast which was wet East-central and warm Northeast may be more correct than the new ECM projection. The concern is the East-central U.S. may be wetter than indicated.
Week-4 Ahead Forecast valid April 10-17, 2023: -PNA climate could linger.
Discussion: The most likely scenario for mid-April is lingering chill in the West, a warmer than indicated pattern across the Southeast U.S., and in-between wet risk for the Great Plains to Midwest States.