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Hurricane Ian’s remnants will douse parts of the Mid-Atlantic

Precipitation forecast for Ian and Ian remnants over the next several days. (weatherbell.com)

Hurricane Ian is making a devastating landfall in southwest Florida on Wednesday. Over the next few days, it will take a winding path toward the D.C. region as it transforms. For the most part, we’re on the cool side of the storm, but that still portends a prolonged soggy forecast.

Rainfall odds are up as soon as Friday night as the remnant Ian circulation throws rain this way. By the time raindrops wind down Tuesday or even Wednesday, several inches of rain are possible.

The duration of the event may help lessen the risk of flooding, but that is still far from guaranteed. It also keeps the forecast a little uncertain.

What Ian is most likely to throw at us

The basic process behind our heavy rain forecast is the transition of purely tropical Hurricane Ian to an extratropical storm that will drift north to around the North Carolina-Virginia border over the weekend, before slowly exiting into the Atlantic around Tuesday. This setup favors a prolonged period of rain over the Southeast into parts of the Mid-Atlantic.

As shown in the graphic above, Ian will begin interacting with a stationary front draped across Florida later today, as part of its landfall. While the storm moves across the peninsula, its wind circulation will draw in very warm and humid air off the Gulf Stream toward the Carolina coast, further enhancing a warm front.

On the back side of the storm, cooler and drier air will circulate down from the north, generating a cold front. This is the opening act of the extratropical transition phase that is in full swing early Friday morning.

To the north, a sprawling high-pressure area parked over the Northeast and Mid-Atlantic will keep Ian’s remnants from moving too far or too easily north. The combination of the low to the south and high to the north will draw in a very moist airstream off the Atlantic, allowing periodic rain to develop and spread north of the warm front. For much of this region, the rainfall will act more like a prolonged nor’easter than a tropical rainstorm.

How much rain in total?

With rainfall potential lasting from roughly Friday night through Tuesday, the potential to pile up totals is certainly there. The long duration will make or break the forecast, in more ways than one.

The National Weather Service is forecasting 2.5 to 5.5 inches of rain across the broader area, from the low end in northern Maryland to the high end in southern parts of that state. Around Washington, D.C., the forecast is for about 3 inches.

Recent weather model run details through the weekend (and total for the storm), below:

  • American GFS: 0.5-1.5 inches (0.5-2.5 inches total)
  • European ECMWF: 1-2 inches (1-3 inches total)
  • British UKMET: 1.5-3 inches (doesn’t run to end of storm)
  • ICON: 2-4 inches (doesn’t run to end of storm)

Flood risk isn’t extreme

Given that it has been relatively dry in recent weeks, flash flood risk is not extremely high, but some localized flooding is possible. It would require 3 to 4 inches or more in six hours to cause flash flooding over most of the region, with values closer to 2 inches in the urban centers (as more water runs off than seeps into soils).

It’s improbable but not out of the question we see rainfall rates that high.

Although flooding does not appear to be a significant widespread risk locally, it would not be surprising to see flood watches issued and some small streams approach flood stage. Some minor coastal flooding could also be an issue along parts of the Tidal Potomac and Chesapeake Bay, given prolonged winds from the same direction.

Uncertainty remains

We’ve mentioned it already, but a primary devil in the forecast is the storm’s slow speed. Rain may not begin in earnest for another 72 hours, and then periodic drops could fall through at least Tuesday.

There will also probably be a sharp cutoff in heavy rain, seemingly north of the Mason-Dixon Line, due to the very dry air mass associated with the high-pressure area to the north. The exact placement of that cutoff cannot be determined yet and is part of the uncertainty in predicting rainfall amounts, as it could sink southward.

The sluggish movement of the post-Ian vortex is due to slack “steering” winds, as shown above. The vortex is labeled with a red “L” and remains cut off from fast jet stream flow well to the north over eastern Canada. In fact, the jet stream is contorted into a large ridge, with the vortex literally floundering in an overall weak flow pattern.

This type of extratropical transition pattern is notably different from many other landfalling U.S. storms, in which the jet stream typically dips far enough south to “pick up” the tropical remnant and scoot it rapidly toward the northeast.

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