This morning’s GFS model simulation for the coastal system and rain right after it passes.

A coastal storm that was previously predicted to graze the Mid-Atlantic on Saturday has shifted farther west in recent computer-model forecasts.

This shift doesn’t markedly change the forecast for the beaches, where wet and windy conditions have been the expectation, but substantially alters the forecast for inland areas, such as the District. It means an increased chance of rain Saturday, and the rain could even become heavy.

Rain is likely to begin between 8 a.m. and noon Saturday and end early-to-mid evening. Outdoor plans, especially those in the afternoon, may prove difficult.

Even as the storm moves away on Sunday, a chance of showers will linger. It sure looks as if we are just getting started, with a flood risk perhaps growing into next week.

Saturday’s forecast

As the forecast surface map below shows for Saturday morning, a weak area of low pressure is expected to develop overnight in the vicinity of coastal North Carolina. From there, it will begin working slowly north along the East Coast.

Even while embedded in the mid-latitude flow, there is some suggestion that the storm will feature tropical characteristics, meaning it will feature very moist air.


Forecast surface weather map for early Saturday morning. Dark green patch over Mid Atlantic indicates most likely region of heavy, showery rains. (NOAA)

The revised track means the D.C. area will be right in the action, though the amounts of rain and winds will vary.

The closer one is to the Eastern Shore, the heavier the rain and the steadier the cool winds blowing in off the Atlantic will be. Along the beaches, from Rehoboth to Ocean City, expect steady easterly winds of 20 to 25 mph with gusts in the 30 to 40 mph range. The winds there will probably peak during the afternoon, and they should begin to calm down overnight Saturday.


Forecast wind direction and wind speed for Eastern Shore region, valid 2 p.m. Saturday. (NOAA)

In terms of rainfall, model estimates differ somewhat. A consensus estimate of 1 to 3 inches emerges from the National Weather Service, as detailed below.

Note the gradient or increase in rain accumulation as one moves closer to the beaches. The highest rain totals are likely to be found along the track of the low pressure center.

If the storm jogs a little bit to the east, that could reduce both the intensity and duration of rainfall. Especially in our western areas, there is a realistic possibility rain is lighter, more spotty and short-lived.

Based on the latest available information, the expectation for D.C. and the immediate area is at least one inch, falling mainly during the day on Saturday. This comes in waves of showers and rain, some heavy.


National Weather Service rainfall forecast through Sunday morning. (WeatherBell.com)

A widespread two inches or so across the D.C. area is not totally out of the question, but those kinds of totals will depend on the exact track and speed of the low. Both are difficult to judge until the short term. The National Weather Service is contemplating issuing a flood watch that could cover our area and extend all the way to the Eastern Shore of Maryland and Virginia.

On the Delmarva Peninsula, two to four inches of rain is likely.


Predicted rain totals for the three-day period ending this coming Monday morning. A maximum of 3 to 4 inches (red area) over Delaware and southern New Jersey is associated with the weekend coastal storm. (NOAA)

Should the rainier scenarios play out, Saturday’s high temperatures, including those at the beach, may stay stuck in the 70s and may not even exceed the mid-70s. As the storm tracks north, the main slug of rain should end during the evening.

Sunday’s forecast

Conditions here in D.C. and along the beaches seem likely to improve on Sunday, but only marginally so and by comparison to Saturday.

Clouds will become more broken, and afternoon temps will likely rise toward and past 80. The threat of lingering showers on the back side of the retreating low seems likely to remain. In fact, raindrops could become numerous by afternoon and evening as the next major weather feature begins to set up.

A pattern that could be very wet is ahead

Beginning Sunday evening, a powerful low pressure area in the upper atmosphere begins to approach from the west. At the same time, a Bermuda High pressure system over the western Atlantic strengthens and migrates closer to the East Coast.

These two features will work in tandem to pump extreme amounts of tropical moisture into the Mid-Atlantic. As described in Jason Samenow’s report Thursday, this pattern is expected to persist well into next week.

A stalled frontal boundary will help focus waves of showers and thunderstorms across the broader Mid-Atlantic and parts of New England. Meanwhile, the slowly approaching upper-level low pressure system will create a dynamic “sweet spot” of uplift near the front and associated plume of rich tropical moisture.

Wes Junker, our winter-weather expert at Capital Weather Gang, came to us with the wisdom of many years of forecasting heavy-rain events. He cautions that while next week’s setup is conducive for widespread, heavy rain, pinpointing the most susceptible region is nearly impossible several days in advance.

Several factors will be crucial heading into next week, including the exact position of the stalled front, the direction of low-level winds, subtle waves of energy that slip through the larger pattern, and the size and strength of the dynamic sweet spot aloft.

CWG will be closely monitoring the continued setup for next week’s rain event and will issue new reports as the geographical region and time frame of heavy rain becomes more certain. One concern about Saturday’s coastal storm is that a widespread 1 to 3 inches of rain will begin to soak the ground, potentially exacerbating flood issues that may follow on this weekend storm’s heels.