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Hurricane Ian’s remnants mean a prolonged rain event for D.C. area

Rain will fall intermittently through early next week, with the heaviest coming Friday night. We should expect at least 1.5 to 3 inches.

Hurricane Ian is making its second U.S. landfall Friday in South Carolina, spreading rain over much of the Mid-Atlantic, including in the Washington and Baltimore areas through the weekend and even into early next week.

By Tuesday, the greater region could see at least 1.5 to 3 inches of rain. The rain is generally expected to fall slowly enough to prevent widespread flooding.

Hurricane Ian bathed the Mid-Atlantic in a spectacular sunset Thursday

Storm at a glance

The heaviest rain of the storm may occur Friday night into early Saturday. Much of Saturday could lack steady rain, but it will be damp, nonetheless. As the storm drifts back to the east, rainfall may increase again Sunday. It could even linger to some extent, until late Monday or even Tuesday.

  • Most widespread rain: Friday night and Sunday.
  • Total rainfall: 1.5 to 3-plus inches (about an inch tonight).
  • Temperatures: Near 60 to mid-60s during the day. 50s at night.
  • Storm departs: Late Monday into early Tuesday.

The National Weather Service forecast shows rainfall increasing from north to south across the region, from about 3.5 inches near Fredericksburg to 1.5 inches north of Baltimore. These numbers have come down a little since midweek.

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The latest computer model simulations show about 2 to 3 inches in the immediate Washington area by the end of Tuesday. Some of the higher-resolution models simulate localized amounts up to 5 inches.

What we expect, in detail

Hurricane Ian is making its second landfall along the South Carolina coast Friday as a hybrid Category 1 hurricane. The remnant vortex aloft and at the surface will slowly drift northward along the Appalachians over the next two to three days. Afterward, it will move east and eventually exit the coast.

Given this storm track, the region should buckle down for an extended period of largely beneficial, soaking rain into Monday. The outer edge of Ian’s rain shield will spread over the Washington region late Friday afternoon into early evening. A stiff breeze from the northeast also will develop, and temperatures will hover near 60.

In all, it will often feel like a long duration nor’easter through the weekend. Some exceptions may be noted further south and east, where a brief window for damaging winds or an isolated tornado may present itself in the Virginia Tidewater.

As the sets of forecast weather maps show, the progression will be slow indeed, and the surface low will assume a double-barrel configuration over time. The eventual inland surface low — former Ian — is an expression of the vortex and energy aloft, while a separate low along the Delmarva coast eventually takes shape. A dry slot of air at upper levels will work across the region at some point this weekend — probably Saturday — and may turn otherwise steady rain to more showery episodes for a time.

A lot of the moisture feeding into the system during the early weekend will be sourced as a deep, tropical plume extended into the Caribbean, what is termed and “atmospheric river.”

By Monday, strengthening winds aloft will draw the remnant upper level low toward the east and offshore, causing rains to slowly taper through the day. Some weather modeling keeps this process going into Tuesday.

Flooding potential

As noted earlier in the week, freshwater flooding risk should not be widespread thanks to relatively dry conditions ahead of this rain event. While some isolated flooding is possible, and a flood watch may be issued, the biggest water concern from Ian’s remnants locally is more likely to come from unusually high tides along the Potomac River and Chesapeake Bay.

Fueled by strong winds out of the east and northeast, a coastal flood warning is up for St. Mary’s County, as well as beaches further south. In Southern Maryland, tides of two to two and a half feet above normal are anticipated through the weekend. This will lead to flooding of some roads and other locations.

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Coastal flood advisories are also in effect for much of the bay shore and Potomac River, including the District. These areas can expect tides running above normal, as well, leading to some flooding in places like the Tidal Basin.

Forecast risks

The timing of the dry air intrusion into the remnant Ian vortex, and also exact placement of both low-pressure systems in coming days, adds some uncertainty to when and where the heavy rains will fall. Additionally, stiff easterly low-level winds impinging on the mountains may shift the heaviest amounts over that region through what is termed “orographic enhancement.”

Moderate to heavy rain that is expected to fall Friday night is a fairly certain part of the forecast. While there is an anticipated lull Saturday, it’s not a given. Timing and amounts after that are subject to potentially larger revision, although given general model agreement, we can feel reasonably sure that the scenario will play out as imagined in broad strokes.