Update to the guest blog by the Ad hoc Blue Nile Forecast Group (listed alphabetically): Sarah Alexander (1), Paul Block (1), Annalise Blum (2), Shraddhanand Shukla (3), Shu Wu (1), Temesgen Yimane (2), Ben Zaitchik (2)*, and Ying Zhang (2).
- University of Wisconsin-Madison, Madison, WI, USA
2. Johns Hopkins University, Baltimore, MD USA
3. University of California Santa Barbara, Santa Barbara, CA, USA
*Correspondence can be addressed to firstname.lastname@example.org
The end of July represents the midpoint of the Kiremt rainy season in the Blue Nile basin. On average, just over half of the June-September rainfall total is realized by the end of July. This is also true for the annual total rainfall, January-December (Figure 1; shown spatially in Figure S1). This makes the end of July an opportune time to provide an update on the seasonal forecasts of 2018 Blue Nile rainfall and river flow that we posted at the outset of the rainy season.
Interestingly, however, the North American Multi-Model Ensemble (NMME) forecasts launched in July show a more mixed outlook than the same ensemble had offered in May. At the time of the May forecast, every NMME model had a mean prediction of average to above average rainfall for June-September (Figure 10 in our original post). Looking at forecast July-September rainfall in the July initialized NMME simulations, we see that there are now several models that predict below average rainfall, albeit with only a modest negative anomaly (Figure 3). This breaks the consensus that existed in May. This shift to drier forecasts might reflect the influence of a shifting El Niño outlook, as the predicted probability of an El Niño forming by the end of the season is higher now than it was in May (Figure 4; compare to Figure 9 in the original post).
Notwithstanding this somewhat drier outlook from NMME, the strong rains observed in June and early July got the basin off to a wet start. This, combined with an NMME forecast that still, across all models, points to average rainfall conditions for the remainder of the season, suggests that the Kiremt rains will be average to above average overall. The probability of a dry year is low, though some NMME models now suggest that late season rains might be lower than average.