Serious flood threat developing for southeastern Finger Lakes Christmas Eve Night into Christmas Day

Unfortunately Aligning Pieces

A serious flood situation will develop this afternoon and overnight as rapid snowmelt combines with prolonged heavy rain to produce widespread flooding and significant river flooding.

A powerful storm system north of the Great Lakes is pushing a cold front through the Ohio and Tennessee Valleys and Gulf States. A second low has begun to develop over Alabama and will ride north along this front, slowing its eastward progress.

Out ahead of the front, moisture is surging northward and will reach anomalously high levels for late December, as much as 3 to 5 times the climatological average.

Driving this moisture northward are gusty south and southeast winds, which will continue through the day and overnight with gusts frequently over 30 mph.

Temperatures and dewpoints will rise through the 40s into the low 50s late this afternoon and evening. This high moisture air and gusty winds on its own will cause a rapid snowmelt. Even areas that saw over 30” of snow last week will most likely see all of that melt in a short period this afternoon and evening.

The snowpack contains 2-4 inches of water from Elmira to Binghamton to just southeast of Cortland. Most or all of this will be released as runoff into area streams and rivers. For areas near and south of Wellsville to Bath, Watkins Glen, Ithaca, Moravia, and Tully, the snowpack contains 1-2 inches of liquid.

On top of this snowmelt, heavy rain will develop this afternoon with occasional downpours. This will be a widespread, prolonged heavy rain event, with 2-3 inches falling across and east of roughly a Bath to Penn Yan to Geneva line. Localized amounts over 3 inches are not out of the question.

Combined, the rapid snowmelt and rainfall will result in anywhere from the equivalent of 2 to 6 inches of rain, with an unfortunate alignment of the heaviest rain, the deepest snowpack, and the largest river basins.

Flood Expectations

There are two main types of flooding: flash flooding and river flooding.

Flash flooding occurs in urban and poor drainage areas, and along smaller streams and rivers. Rapid rises in water can cause sudden but significant flooding, closing roads and in more serious cases, requiring rescues from inundated structures.

This type of flooding may become widespread across areas that have over 2 inches of water in the snowpack, corresponding roughly to the red Level 4 zone on my map. Numerous flash flood reports are expected from southeastern Steuben, across Schuyler, Tompkins, and Cortland counties, the orange Level 3 zone.

Flooding may be exacerbated on a local scale if storm drains are blocked by ice and snow. It would be wise for municipalities and homeowners alike to spend this morning making sure these drains are open and free of debris.

River flooding takes longer to develop as the runoff from smaller streams work their way into the main stem rivers. The Tioughnioga River, Chenango River, Susquehanna River, and Chemung River are all main stem rivers that have a long history of significant flooding.

Of those four rivers, the Chemung River has the least concern with less runoff from snowmelt flowing into it. The Tioughnioga, Chenango, and Susquehanna Rivers, however, are expected to reach moderate flood stage and will come close to major flood stage, severely impacting homes and businesses on Christmas Day. Peak water levels are expected to occur early Friday evening.

While no two weather events are ever exactly alike, the event unfolding today and tomorrow bears many similarities to the flooding of January 1996, which had a similar if not more extreme rapid snowmelt, but generally lower amounts of rainfall.

Snow and Ice Concerns

While flooding is the largest and most serious threat from this system, the chances for icy travel conditions on Christmas morning need to be addressed as well.

Overnight, the cold front will stall out over western New York as low pressure passes overhead. Behind the low, the front will again press east during the hours near and just after dawn Friday.

Temperatures will quickly drop 15-20 degrees, tumbling from the 40s and low 50s to low 30s.

The cold air will initially be in just a shallow layer at the surface, opening a window for some freezing rain. No more than a glaze of ice is expected, but combined with the rapid cooling of roadways, that will be enough to make for pockets of very treacherous travel conditions.

The ice should quickly turn over to snow as the cold layer deepens, but less than an inch should fall as the precipitation winds down.

The models vary on the exact timing of this changeover, with some pushing it through before dawn Friday and others not having it clear the region until the late morning. In any case, once you see (and feel!) the temperature drop, know that travel will be hazardous for the next few hours until road crews can get a handle on the roadways.

Lastly, significant lake effect snow will impact I-90 from the Buffalo area south through Cleveland, and I-81 in the Watertown area. Travel in those areas on Friday will be difficult well into Saturday. Locally, however, the weather should be calming down.

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