NOAA Long-lead Forecast Predicts Excessive Rainfall for August in Virginia

NOAA/CPC long-lead forecast predicted the excessive rainfall risk in Virginia this summer season from last February. Next? Wet pattern shifts to Southeast.

Fig. 1: NOAA/CPC long-lead precipitation probability forecast for JUN/JUL/AUG 2020 from last February identifies wet risk centered on Virginia.

Discussion: On February 20, 2020 NOAA/CPC issued a long-lead probabilistic precipitation forecast identifying a zone of increased risk of above normal rainfall across Virginia (Fig. 1) during meteorological summer (JUN/JUL/AUG 2020). Specifically, the forecast read >40 in a green (wet) zone across the Mid-Atlantic. There are 3 climate possibilities each with 33% chance of occurring: Below normal, above normal or normal precipitation. The >40 value means that in Virginia the climate risk was tilted wet…about 45% chance of a wet climate, still a 33% chance of normal rainfall and 22% chance of a dry climate. A 45% risk does not sound very confident but in climate forecasting that value is quite high especially for 3.5 to 6.5 months ahead.

June rainfall ranked 93 of 125 years (125 wettest/1 driest) but July was somewhat drier ranking 46 of 125. In August rainfall amount will likely break 125-year records. To date (in August) rainfall totals exceed 20 in. in parts of east-central Virginia (Fig. 2). The 3-month (JUN/JUL/AUG) period will average well above normal rainfall in Virginia and the robust 45% risk forecast issued last February actually DID identify this risk.

Fig. 2: Rainfall so far in August across Virginia.

The upper air pattern generating the heavy rainfall across Virginia during August has featured a low pressure trough over the Midwest U.S. located south of an amplified upper ridge over Quebec (Fig. 3). The combination of these two semi-permanent upper air features induced a moist fetch off the warmer-than-normal western North Atlantic into the Mid-Atlantic region made unstable and therefore convective (heavy rain) by the steep temperature lapse rates in the upper atmosphere associated with the cold Midwest U.S. upper trough. Hurricane Isaias certainly contributed to the rainfall but most of the excessive rains was created by the upper air pattern in-place. The NOAA/CPC climate forecast for JUN/JUL/AUG was projecting this upper air pattern to cause the rainfall and not influence from direct-hot tropical cyclones.

Fig. 3: Upper air pattern across North America in August so far.

Does the NOAA/CPC raw precipitation forecast for the next couple months provide any clues as to the trend in Virginia rainfall? The AUG/SEP/OCT 2020 raw data forecast indicates the wet risk shifts to the Southeast U.S. in SEP/OCT. Given the buoyant seasonal tropical cyclone forecasts there is risk of several tropical events striking Florida, Georgia, Alabama and the Carolinas the next 2 months (Fig. 4).

Fig. 4: Raw data rainfall climate forecast for AUG/SEP/OCT from NOAA/CPC suggests the wet pattern in Virginia shifts to the Southeast U.S. for peak of tropical cyclone season.

Conclusion: Occasionally, NOAA/CPC long-lead forecasts will produce relatively bold probabilities in extended-range forecasts. If so, risk managers interested in possible adverse climate conditions should take note. The NOAA/CPC outlook for meteorological summer (from last February) indicated an unusually wet probability (given the 3.5 to 6.5 lead time) across Virginia which has certainly verified given the historic rainfall observed in August. Raw data forecasts from NOAA/CPC indicate the wet core shifts from Virginia to the Southeast U.S. into meteorological autumn which highlights that zone as a direct-hot tropical cyclone target areas especially given the very active seasonal tropical cyclone forecasts.