The remnants of Tropical Storm Debby are set to bring significant rainfall and flood risks to parts of the Northeast as the system moves through the region over the next few days. As of Wednesday morning, high pressure to the north is providing dry and pleasant weather, but conditions are expected to change dramatically as the storm’s tropical moisture advances northward.
What’s in the forecast today?
Currently, expansive high pressure centered to the north is delivering sunny skies and comfortable temperatures across most of the region. However, some lingering clouds are expected in the western Southern Tier, and lower clouds may push into the Southern Tier and Finger Lakes region tonight as Atlantic moisture starts to move in. Overnight temperatures are expected to remain comfortable, with lows in the 50s to near 60 degrees.
By Thursday, the southeast flow around the high-pressure system will deepen, allowing tropical moisture from Debby to surge northward across the Mid-Atlantic and into the Northeast. Clouds will thicken throughout the day, with scattered showers possible by the afternoon. While most areas, particularly east of Lake Ontario, should remain dry during the day, humidity levels will rise as dew points jump into the mid to upper 60s, especially in the western counties.
When will Debby arrive in the Finger Lakes?
The focus of the short-term forecast is the potential for significant rainfall as tropical moisture from Debby interacts with a mid-latitude trough over the Great Lakes. This interaction is expected to push the storm’s remnants eastward, with precipitable water (PWAT) values reaching near 2.25 inches by Friday, exceeding the 90th percentile compared to climatology. The greatest risk of heavy rainfall will be east of the Genesee Valley, particularly from the central Finger Lakes to the North Country, where 24-hour rainfall amounts of 2 to 4 inches are expected, with localized amounts as high as 6 inches.
These rainfall amounts pose a risk of flash flooding and could lead to rises in local waterways. The Weather Prediction Center (WPC) has issued a Moderate Risk of Excessive Rainfall for areas from Cayuga County eastward, with a Slight Risk extending westward and a Marginal Risk for far western New York.
Showers and periods of steady rain, some potentially heavy, are expected across western New York, with rainfall amounts increasing from west to east. A sharp gradient in rainfall amounts is anticipated, with higher totals east of the Rochester Metro area. The exact location of this cutoff remains uncertain, but current projections place it mostly east of Rochester.
The mid-latitude trough will continue to impact the region into Friday night, driving a strong cold front through the area and pushing the primary surface low and associated moisture out of the region. Heavy rainfall is likely to persist east of Lake Ontario through the first half of the night, while western zones should begin to dry out.
Long-Term Outlook: Saturday Through Tuesday
By Saturday, the longwave trough will move across the Great Lakes, pushing the last of the tropical moisture associated with Debby out of the area. While lingering showers may persist east of Lake Ontario, most areas can expect a mostly dry day. Temperatures will be cooler and less humid, with highs ranging from the upper 60s in the hilltops to the mid-70s in lower terrain areas.
Saturday night into Sunday, the upper-level trough will establish itself over the region. This could create conditions conducive to lake effect rain showers east and northeast of the lakes, transitioning to scattered thunderstorms by Sunday afternoon. Drier air will build in behind this activity for the start of the new work week. Temperatures next week are expected to average below normal, with daytime highs in the mid-70s.
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