La Nina coming? There are indications that for the second year in a row a La Nina maybe coming.
ENSO-neutral is favored for the remainder of summer (~60% chance in the July-September season), with La Niña possibly emerging during the August-October season and lasting through the 2021-22 winter (~70% chance during November-January).
Recently, sea surface temperatures (SSTs) were near-to-below average in the central and east-central equatorial Pacific, with above-average SSTs in the far eastern Pacific. In the last week, most Niño indices were slightly negative (-0.2°C to -0.3°C) except for the Niño-1+2 index, which was +0.7°C . Subsurface temperatures cooled considerably in July, becoming quite negative (averaged from 180-100°W; reflecting the emergence of below-average subsurface temperatures east of the Date Line . Low-level wind anomalies were easterly over the east-central Pacific Ocean, while upper-level wind anomalies were westerly across the eastern Pacific. Tropical convection was suppressed over the western Pacific Ocean and enhanced over a small region near Indonesia . Given the surface conditions, the ocean-atmosphere system reflected ENSO-neutral.
Compared to last month, forecasts from the IRI/CPC plume are generally cooler in the Niño-3.4 SST region during the fall and winter 2021-22 . Recent model runs from the NCEP CFSv2 and the North American Multi-Model Ensemble suggest the onset of a weak La Niña in the coming months, persisting through winter 2021-22. The forecaster consensus continues to favor these models, which is also supported by the noticeable decrease in the observed subsurface temperature anomalies this past month. In summary, ENSO-neutral is favored for the remainder of summer (~60% chance in the July-September season), with La Niña possibly emerging during the August-October season and lasting through the 2021-22 winter (~70% chance during November-January).
This discussion is a consolidated effort of the National Oceanic and Atmospheric Administration (NOAA), NOAA’s National Weather Service, and their funded institutions. Oceanic and atmospheric conditions are updated weekly on the Climate Prediction site. A probabilistic strength forecast is . The next ENSO Diagnostics Discussion is scheduled for 9 September 2021.
During a La Nina event, the changes in Pacific Ocean temperatures affect the patterns of tropical rainfall from Indonesia to the west coast of South America. These changes in tropical rainfall patterns affect weather patterns throughout the world. These effects are usually strongest during the winter months when the jet stream is strongest over the United States. La Nina episodes in the winter months feature a wave-like jet stream flow across the United States and Canada, which causes colder and stormier than average conditions across the North, and warmer and less stormier conditions across the south. Historically for this part of the Midwest, fall tends to be warmer and drier than normal while winters tend to be wetter than normal. However, there are also many other complicated factors in the atmosphere and oceans that can also impact our weather patterns.
There have been 8 La Nina winters in the last 20 years. 4 were weak, 2 were moderate and 2 were strong. The weak ones were (17/18, 16/17, 08/09, 05/06.) The moderate ones were 20/21 and 11/12. And the strong ones were 10/11 and 07/08. The new 30 year averages for meteorological winter at Grand Rapids are mean temperature 27.3° and mean snow fall (DJF) of 60.7” for the total snow season snow fall average is now at 77.6” during the recent La Nina winters average mean meteorological winter temperature has ranged from a warm 32.2 in 2011/12 to the coldest of 23.8 in 2008/09. The snow fall meteorological winter snow fall has ranged from a high of 94.9” in 2007/08 to a low of 45.4” in 2020/21 (2011/12 was a close 2nd with 44.7”) the most meteorological winter snow was 94.9” in 2007/08. The most snow for the season was 107.0” in 2008/09 and the least was 46.1” in 2020/21. So the bottom line is that there is a large swing at to what we can expect here in west Michigan in a La Nina winter.
- Convection possible today into tonight The basic question today is if and when there will be convection. First through, a short answer, convection is possible (not likely) this afternoon near and east of US-131 associated with warm advection and some lake breeze convergence. Then we have the frontal convection that would come through tonight. It is looking more and more like that will not get much past US-131 if it gets that far. So, I am forecasting between 8 am Saturday and 8 am Sunday most of the CWA will not see measurable rain. Normally this would be easy, we have a cold front heading into very warm and humid air. The frontal timing through is that it comes through our forecast area tonight. That is not the issue through that will keep it from raining. There are many issues but the primary issue is the shortwave energy heads well north of our CWA tonight. That happens because we have an upper low that was over us but is now east of us and there is a series of strong Pacific shortwaves crossing the North Pacific that will try to make a western trough (but that trough will be shallow). Even so that process is causing rising heights over most of the central CONUS, starting now and continuing into Tuesday. We have a very sharp upper ridge over us tonight (not good for storms to reach here). The temperature rises around 2c between 700 mb and 300 mb between 8 am this morning and 8 pm this evening. That much warming at mid and high levels is also bad for convection. So I am thinking, like what the HREF from the 8 pm run shows, that most of the measurable precipitation will have a hard time making it east of US-131 early tonight. Most of the area east of US-131 will very likely see NO PRECIPITATION tonight. All of our hi-res models (RAP, NAM, NAMNEST,HRRR,HRW NSSL,HRW ARW,HRW FV3, and HREF) all support this idea. Oddly through the HREF, FV3, ARW, NSSL and NAMNEST show the warm advection will help set off a few afternoon thunderstorms near US-131 early this afternoon that will head toward US-127 by mid afternoon. These will be widely scattered so most will stay dry here too. One other odd aspect to all of this, the frontal trough stalls over eastern Lower Michigan (that closed upper low stalled to our east with HENRI in it by then) Sunday. That set up decent surface convergence over eastern Lower Michigan in the afternoon. There is surely more than enough instability around too (1000 to 1500 j/kg of MU cape) too. So we will see some thunderstorms in the I-69 area Sunday afternoon. - Warm with periodic convection into next weekend Overall we are in an weather pattern that will change little over the next week or so. As I wrote above, a series of North Pacific shortwaves dry to develop an upper trough over the Pacific Northwest into west central Canada through this coming week. That builds heights over the central CONUS or at least holds them were they are (much higher than normal/ 1.5 to 2.5 standard deviations from the mean at 700 mb and 500 mb) through the week into the following weekend. So, that being so, one would not expect much cooling. From time to time a shortwave tracking along the polar jet will set off convection and bring us the risk of thunderstorms. Timing of this with this sort of patten is questionable at best. The ensemble means on both the GFS and ECMWF show some preference for convection in the Wednesday time frame and also for next Sunday but for each day next week, at least few members of those ensemble show some rainfall. The forecast for the next week is warm and humid with periodic thunderstorms but most of the time it will be dry.