Fig. 1-2: Comparing the current U.S. drought monitor with the Evaporative Drought Demand Index.
U.S. PDSI versus EDDI: U.S. drought caused mostly by long-term precipitation deficits, particularly over California, the Great Basin and Southwest U.S. plus North Dakota are well-established according to the early June U.S. Drought Monitor (Fig. 1). Interestingly, the NEW Evaporative Drought Demand Index (EDDI) identifies (some) different results and concerns (Fig. 2).
The EDDI is defined as an early warning (for drought) tool. Examined is the anomalous atmospheric evaporative demand for a given location over a period of time ranging from 1 week to 12 months. The analysis provided is a 12-week EDDI analysis. According to NOAA, the particular strength of EDDI is “capturing signals of water stress which makes EDDI a strong tool for preparedness for both flash droughts and ongoing droughts”.
According to the latest EDDI analysis, the eastern Ohio Valley, Great Lakes region and into the Northeast U.S. Corridor (plus southern Florida) has high to extreme evaporative demand. This means these areas are susceptible to rapidly developing drought if the climate pattern stays drier than normal. The exceptional EDDI values for this area indicate potential for hazardous agriculture conditions, increased water demand and higher fire risk. Note that the areas mentioned are generally not covered by the current U.S. Drought Monitor.
Outlooks: The concern – identified by EDDI – is that developing drought risk for the northeast quadrant of the U.S. during summertime 2021 is increasing. Forecast models account for a dry climate pattern implied by EDDI trend. The upper air forecasts by NCEP CFS V2 during JUL/AUG/SEP 2021, park a dry and hot upper ridge over the Northeast sector of the U.S. (Fig. 3-5). ECM is generally similar.
Fig. 3-5: NCEP CFS V2 upper air forecasts for No. America JUL/AUG/SEP 2021.
NOAA/CPC indicates a wet climate for the East (including the Northeast U.S.) for JUL/AUG/SEP 2021 (Fig. 6). The trend of dynamic forecast models indicates the Northeast U.S. forecast is too wet. The upper ridge projection(s) by dynamic models are based on the established summertime relationship between large regions of warmer than normal ocean surface and nearby anomalous high pressure ridging in the middle/upper atmosphere (Fig. 7). In defense of the NOAA/CPC outlook, northward turning tropical cyclones can overwhelm the high-pressure ridge/dry and warm-to-hot forecasts by dynamic models.
Fig. 6: NOAA/CPC probabilistic precipitation forecast for JUL/AUG/SEP 2021.
Fig. 7: ECMWF global SSTA forecast for Augusts 2021 and the typical relationship of the upper atmosphere to regions of anomalous warm water.
Conclusion: Due to implications of potential flash drought in the Great Lakes/eastern Ohio Valley to Northeast U.S. by the Evaporative Drought Demand Index (EDDI) Climate Impact Company is issuing a drought watch for the northeast quadrant of the U.S. The upper air forecasts by NCEP CFS V2 and reasonably also implied by ECM support the drought watch. The NOAA/CPC wet climate forecast for summertime in the Northeast is overdone unless tropical cyclone activity can reach that far north. NOAA indicates summertime 2021 drought emphasis is in the western half of the U.S. Due to the long-term rainfall deficits affecting most of the West and North-central U.S. the drought should continue although forecast models are trending toward projecting upper air ridge patterns favoring the Northeast States. As an aside, dynamic models may be more resistant to producing amplified upper ridge patterns over the West U.S. due to the western displacement of the Northeast Pacific “warm blob” and cool phase Pacific decadal oscillation (cool) waters off the immediate West Coast of North America. Remember, the EDDI dry signatures and potentially drier climate ahead for summer combine for increasing risk of flash drought having great negative effects on agriculture, water supply and fire risk.