Simulated radar from the NAM weather model shows waves of rain moving through. (Capital Weather Gang and Pivotal Weather)

As it stares at yet another soggy weekend in 2018, Washington is primed to sail past the record for wettest year on record.

Just over a half-inch of rain is needed for 2018 to top the previous record for wettest year in Washington of 61.33 inches in 1889.

The National Weather Service predicts 1.5 to 2.5 inches of rain this weekend, and up to four inches in some areas. So much rain may fall that it has issued a flood watch for 18 hours, from noon Saturday to 6 a.m. Sunday.

Additional rain is possible Sunday, although it should gradually ease as the day wears on and end by the evening.

(Ian Livingston/The Washington Post)

The question is not if the record for wettest year will be broken but when. The rainstorm aimed at the region is complex and will be drawn out.

While it won’t rain all weekend, showers are possible anytime, and it is likely to stay rather damp for the entirety.

The responsible weather system is massive and has cast cloud cover over much of the eastern half of the United States. It will generate multiple waves of rain as it lumbers eastward.

A satellite image shows large system casting clouds over much of the eastern half of the Lower 48 on Dec. 14. (NOAA)

Because of the size of this weather system and all the moisture it will draw north, it will probably be difficult to carry out much outdoor activity this weekend. Following each of the two main waves, however, rain may ease midday Saturday and again sometime between Sunday morning and early afternoon.

The heaviest rain is expected in two primary rounds, the first Friday evening into Saturday morning, and the second late Saturday afternoon into Sunday morning. Small shifts in this general timing are possible, especially for the second round.

Each round could put down around 0.5 to 1 inch of rain, and scattered showers could add more after each of the two rounds.

Storm-total precipitation forecast from the Weather Prediction Center. (Capital Weather Gang and Pivotal Weather)

For the entire event, which probably will end late Sunday, here are the most recent model projections for rainfall amounts in Washington:

  • American (GFS) model: 1.9 inches
  • NAM model: 1.0 inches
  • High-resolution NAM model: 1.8 inches
  • Canadian model: 3.5 inches (yes, really)
  • European model: 1.3 inches (nearly 2 inches west of Washington)

The exact timing of the rain and how much will fall in any one area is difficult to pin down, given multiple moving parts, but this is a setup that tends to produce copious amounts in the hardest-hit areas. Much of the Washington region is favorably positioned to receive a good deal of rain.

Some flooding is possible, but widespread significant flooding will probably be limited, considering the rain will fall over a relatively long duration rather than all at once. Also, the fact that no rain has fallen in 11 days, the longest dry stretch of the year, reduces the risk somewhat. However, the amount of rain predicted could easily result in pockets of flooding near creeks and streams and in poor drainage areas.