Tropical storm Karen in Washington D.C.: how much rain and when?

Tropical storm Karen or, more likely, its remnants may pass close to the Washington, D.C. area next Monday or Monday night. Exactly how close will determine how much rain falls, but – right now – around one inch of rain seems like a reasonable possibility, or a good “first guess” forecast.

Related: Weakened tropical storm Karen heading for Gulf coast, landfall likely Saturday night

(National Hurricane Center)

(National Hurricane Center)

As always, whether it’s a winter storm or one of the tropical variety, forecasting precipitation amounts more than 48 hours in advance has “bust” potential, so other outcomes – with more or less rain than my “first guess” are certainly possible. Future adjustments to the forecast are close to inevitable.

In terms of timing, some showers could move into the area as soon as Sunday night, but the bulk of the rain may hold off until Monday or even as late as Monday evening. By midday Tuesday, the region should be drying out.

Compared to yesterday, the computer model tracks for Karen are actually more spread out which certainly doesn’t help to improve confidence in the forecast. Whereas several models bring it very close to D.C., a few do not and are not even that close.

Compilation of computer model forecasts for track of Karen and its remnants (WeatherBell.com)

Compilation of computer model forecasts for track of Karen and its remnants (WeatherBell.com)

There’s some possibility the bulk of the rain we get early next week comes from the cold front pushing through the region, with some enhancement from the moisture of Karen, rather than Karen’s circulation itself.

My cautious optimism that we’ll at least get some meaningful rain is predicated on the idea that when tropical systems move inland, their moisture spreads out.   So even if the storm center isn’t that close to the area, the combination of its expanding moisture plume and the front coming in should give us some rain.

For those who want a lot of rain, root for what’s left of Karen to move just east of the District.  Typically, the heaviest rain falls just west of the center in inland tropical systems.  In that scenario, 1.5 inches of rain or more would be very likely.

What are the models forecasting?

The GFS model simulates about one to one and a half inches of rain in the D.C. area, mostly between Monday afternoon and Tuesday morning.

GFS model simulation of rainfall in the D.C. area through midday Tuesday (WeatherBell.com)

GFS model simulation of rainfall in the D.C. area through midday Tuesday (WeatherBell.com)

The European model simulates rainfall totals in the same ballpark as the GFS, or just slightly less.

European model simulation of rainfall from Karen or its remnants (StormVistaWxModels.com)

European model simulation of rainfall from Karen or its remnants (StormVistaWxModels.com)

Two tropical weather models, the HWRF and GFDL, also project rainfall in that range (unfortunately – zoomed-in versions of these maps aren’t available and they’re challenging to eyeball), although we’re near the edge of getting less in both simulations.

HWRF model rainfall simulation (WeatherBell.com)

HWRF model rainfall simulation (WeatherBell.com)

GFDL model rainfall simulation (Florida State University)

GFDL model rainfall simulation (Florida State University)

The National Weather Service, for its part, is forecasting 1-1.5 inches of rain for the region.

National Weather Service 5-day rainfall forecast

National Weather Service 5-day rainfall forecast

Considering all available information, here are the likelihoods of different rainfall totals, the way I see it:

Less than 0.75″: 40 percent chance

More than 0.75″: 60 percent chance (0.75″-1.5″: 30 percent chance, more than 1.5″: 30 percent chance)

Given that soils are dry, I do not anticipate widespread flooding problems although localized flooding could occur if the core of the heaviest rain cycles through the region (in the high-end rainfall scenario of more than 1.5″).

The Middle Atlantic River Forecast Center has highlighted the possibility (but not likelihood) of river flooding in the region.

(National Weather Service)

(National Weather Service)

We will have another detailed update tomorrow.

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