The latest computer models continue to suggest the odds of seeing snow Saturday morning in the Washington metro region are small, but not non-existent (especially in outlying western areas). A cold, light rain remains the most likely scenario and it’s still possible much of the precipitation misses to our east, and it’s dry. Perhaps of greater significance, many locations may experience their first freeze Saturday night.
To recap what Wes posted yesterday, in order for snow in our region during October, we need most, if not all, of the following ingredients to come together...
1) An area of low pressure from the south comes north and passes to our east so that our winds have a northerly component, tapping cold air to the north
2) High pressure resides to the north acting as a cold air source
3) Snow falls heavily enough so that it both cools the air near the ground and doesn’t melt on its descent; this requires a strong enough low pressure area to our east (but not too far to the east) to generate the heavy precipitation
4) The snow falls during the night or early in the morning before the sun can warm the air enough to melt falling snow flakes
The earlier in the fall or later in the spring it is, the more of these ingredients are required for snow. During winter, you might be able to get away without one or maybe two of these ingredients and still get snow.
Let’s look at the latest European (Euro) and NOAA GFS forecast models and take stock of what ingredients may or may not be in place Saturday morning.
To begin, both models right now have come into pretty good agreement (after conflicting yesterday) which removes a layer of complexity from this assessment (it doesn’t mean it will remain that way, however.).
Both models develop a low along a front stalled to our south and track the low to our east, tapping cold air from high pressure to the north. So the first and second ingredients are in place. Note, however, the area of high pressure to the north is not particularly strong - so the cold air supply is marginal.
Skipping to the fourth ingredient, the simulated timing of this system is pretty conducive for snow as the coldest/strongest part of the storm may coincide with pre-dawn and the early morning hours.
But the third ingredient is what we may lack to really get the snow going. Because the low is forecast to develop slowly and somewhat far offshore, steady, heavy precipitation may not materialize. Note the dark green shades in the model forecasts above, representing the heaviest precipitation (Saturday morning), are located from the Chesapeake Bay to the east. If we end up with mostly light precipitation rates as shown above, we’ll get primarily rain. The possible exception to this would be elevated areas west of D.C. where the existing cold air may be sufficient for some flakes to mix in.
Of course, the models could still shift the track of the storm west and bring some of the heavier precipitation rates back into the metro region. On the other hand, the track could shift east too, meaning no precipitation (i.e. partly sunny skies). The latest NAM model, not shown, simulates that scenario.
Based on the current information, I lean towards some light rain Saturday morning which may mix briefly with light snow in Loudoun and Frederick counties.
After the low moves away, skies clear Saturday night and with the cold air mass in place, there’s a strong chance of freezing temperatures in colder suburban areas (mainly outside the beltway) - potentially ending the growing season.