5:47 p.m. update

This afternoon model runs have backed off further on the chance of accumulating snow Saturday and now lean more toward light rain that may briefly mix with or change to snow Saturday morning but with little or no accumulation. We want to wait for another model cycle or two before pulling the plug on accumulation chances, but trends suggest that’s what we may have to do. The National Weather Service massively scaled back its snow forecast (compare this to the forecast toward the bottom of this post):


(National Weather Service)

Original post from 2:37 p.m.


American (GFS) model precipitation simulation on Saturday morning.

The latest trend in computer model forecasts makes it appear unlikely that Washington will sit in the snowy sweet spot for Saturday’s storm. Even so, it does have a chance to see measurable snowfall for the first time in more than a decade in April.

Compared with Wednesday, the models generally show a weaker storm — and one that passes by more quickly and farther south. The latest European model suggests a very minor event with little, if any, snow. If models continue to trend this way with their forecasts, it’s not out of the question that this becomes a nonevent. However, blending the latest models together, there’s still a reasonable possibility that we’ll have some wet snow to deal with for a time Saturday morning.

After temperatures rise into the 60s on Friday afternoon, the air will steadily cool Friday night into Saturday as an Arctic front sinks south. Rain is likely to develop by 2 or 3 a.m. Saturday, and it may gradually change to sleet and then snow from northwest to southeast between about 4 and 8 a.m.

Snow or a mix of precipitation is likely across the region between 8 a.m. and late morning, with temperatures in the 30s.


NAM model precipitation simulation Saturday.

Models project the precipitation to taper off between midday and midafternoon, perhaps changing back to light rain at the event’s conclusion.

The main window for accumulating snow, if it materializes, is between around dawn and midmorning Saturday. This is when precipitation should be heaviest and temperatures lowest — possibly dropping to near-freezing. This window may expand back into the pre-dawn hours in our colder areas to the west and northwest and shrink to just a couple of hours in our east and southeast areas.

Any snow that falls after midmorning will have a hard time accumulating because of the high April sun angle, unless it is quite heavy. And models generally show the intensity of precipitation waning by then, except in our southern areas.

Our initial snowfall forecast favors somewhat heavier totals of about 1 to 3 inches in our western areas because the changeover from rain will occur there first, so they will have the longest period, starting in the pre-dawn hours, when accumulating snow is possible. For the rest of the region, a coating to a couple of inches are possible.

Generally, we expect any snow accumulation to mostly occur on grassy areas, although slush on the roads is possible during any heavier bursts early Saturday to midmorning.

 

Note that the likelihood of a snowfall bust (less snow than expected) is higher than a boom (more snow than expected), given the trend in models. Our confidence in the accumulation forecast is low, given variation between the models.

Questions and answers

What has changed about the forecast from Wednesday?

  • The models have shifted the most likely storm track, moving the snow sweet spot southwest of Washington. This was our third scenario in Wednesday’s update.
  • The models are generally less intense with the storm overall, showing a weaker and faster-moving system that produces less overall precipitation.

What could lead to more rainfall and a bust scenario for snow?

  • The changeover from rain to snow takes longer than expected as the cold air is slow to arrive. This happens more often than snow-lovers like in Washington, even in midwinter. We see this as a big risk with this event, especially given temperatures in the 60s the previous day. If the changeover is delayed, any snow that falls would happen after the sun is up high and would just melt on contact with the ground.
  • The storm track shifts even farther south, making the precipitation light, quick-hitting, and — mostly  — inconsequential. The European model shows this situation.

What could lead to more snow?

  • The changeover from rain to snow happens quickly during the pre-dawn hours, and snow starts piling up before the sun is out.
  • The zone of heaviest precipitation shifts back north, and the immediate D.C. area is back in the sweet spot.

Could this still be a historic event?

While a record-breaking event is unlikely, it won’t take much for Saturday to rank among the more notable April snows in Washington weather history:

  • It would just take one inch falling at Reagan National Airport for this to be Washington’s biggest April snow since 1924, when 5½ inches fell on April 2. That 5½-inch amount is very unlikely to be challenged, given recent model trends.
  • If more than 0.4 inches falls, it would be the biggest April snow since 0.6 inches on April 7 in 1972.
  • If any accumulating snow occurs, it would be the first instance since April 7, 2007, when 0.4 inches fell.

Top 20 April snow events in D.C. (xmACIS2)

Do I need to worry about my flight or air travel Saturday?

We don’t think so. There may be delays because of the precipitation and reduced visibility, but airports should be able to continue operations under these conditions.

Will it be safe to drive Saturday?

Snow could reduce visibility Saturday morning, and slush on the roads is not out of the question. However, main roads and treated roads should be passable with care. Even neighborhood roads may fare okay, especially from the city and to the east and southeast.

What about Saturday outdoor activities?

If you have something planned outside in the morning, it’s not looking good. Wet snow, sleet and/or cold rain are very likely.

By the afternoon, it will be raw and cold, with some lingering light rain and/or snow, but the forecast has improved since Wednesday. Maybe the Nationals can even get their afternoon game in.

What is the National Weather Service forecast?

We want to present two maps.

First is the National Weather Service snow total forecast:


(National Weather Service)

Second is its map showing the probability of at least an inch:


(National Weather Service)

We think the probabilities shown above are on the high side, given the latest trends in model forecasts.

Some fun April snow trivia

  • The last three measurable April snowfalls on record in the District have occurred on April 7 (in 2007, 1990 and 1972), which is this Saturday (hat tip: Mike Thomas, Fox 5).
  • 1924, the year of the heaviest April snow on record in Washington and the last time at least an inch fell during the month, was the last time Washington won a baseball championship. The Senators won the World Series that year (hat tip: Peter Mullinax).

Some notes about snow accumulation in the spring

  • Accumulation, if any, depends heavily on snowfall intensity and time of day.
    • If the snow is light and intermittent, it will simply melt on most surfaces, day or night.
    • Snow that falls steadily at night and in the early morning can accumulate, especially if moderate to heavy.
    • Snow that falls between midmorning and late afternoon will have a hard time accumulating, especially on pavement, unless it is very heavy. Typically, daytime April snow melts and/or compacts faster than it can accumulate.
  • Any snow accumulation will tend to be very elevation-dependent, simply because temperatures cool with altitude. Typically, during spring snowstorms, temperatures are above freezing at low elevations, which limits accumulation potential. However, snow can still pile up as you head toward the mountains.

https://www.washingtonpost.com/news/capital-weather-gang/wp/2018/04/05/it-still-may-snow-some-saturday-but-a-significant-record-setting-event-is-very-unlikely/

On – 05 Apr, 2018 By Jason Samenow

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