** Winter storm warning Tuesday evening through Wednesday afternoon for Montgomery, Loudoun, Carroll, northern Fauquier counties and points north with freezing rain advisories for most other areas except southern Maryland **

A storm forecast to track into the Ohio Valley threatens to spread freezing rain across portions of the mid-Atlantic.

While temperatures are not particularly cold, the pattern is favorable for iciness in parts of the D.C. area, especially west and northwest of I-95 during much of the night, possibly into tomorrow morning.

With that in mind, the National Weather Service has issued a number of winter storm warnings and advisories for freezing rain throughout much the region.

Quick overview



Midnight temperature forecast from this morning’s hi resolution NAM. (Weatherbell.com)

Temperatures: Upper 20s to near 30 overnight north and west, to around freezing inside the Beltway and most spots southwest. Near or above freezing to the south and east of the city. Temperatures may begin to rise before dawn in spots.

Arrival: Precipitation moves into the west and southwest counties during the late evening (8-10 p.m. or so), spreading across the whole region by or a little after midnight.

Transition to rain: Freezing rain risk lasts longest north and west (Loudoun, northern Montgomery, Frederick), perhaps through the hours after sunrise. All rain arrives earlier in the city south and southwest, where we may not see much icing at all. Mostly rain south and east parts of the area.

Main concerns: Possible small limb damage in the north and west suburbs from ice, with some messy untreated roads in those areas as well. Potential for more isolated slick spots into and around the city, especially on untreated roads or sidewalks.

Technical Discussion

Initially light precipitation is expected to start spreading into the western portions of the area by late this evening (8-10 p.m. or so) with it overspreading the entire region by around midnight. Temperatures are forecast to start off near or a little below freezing in the city and in the upper 20s across areas well to the north, where the more serious icing conditions are possible.

For areas east and south of the city, this storm is likely to mostly be a wet one. For those in the far northern and western suburbs the icing could cause downed limbs or even a few power outages if a worst-case scenario pans out.

However, like most potential winter weather events across the area, this one has some uncertainty concerning how long the cold air might last. Inside of the city there are question whether temperatures will actually fall below freezing. Subtle differences in the model forecasts end up causing larger differences in how long the cold air hangs in across the region as shown below.



(Author adaptation of NOAA/NCEP)

This morning’s NAM holds onto the cold air longer than the GFS which pretty much erodes it across the area by 4 a.m. Also, the GFS would suggest no real icing problems at all inside the Beltway.

One possible reason for the difference is that the NAM (top panel above) starts developing a a coastal trough (zone where winds converge) by 1 a.m., and then develops a weak low along it. The coastal trough and secondary low development helps maintain a northerly component at the surface longer than forecast by the GFS. Therefore it holds onto the cold air longer.

I’ve ended up opting slightly towards the NAM model because it has higher vertical and horizontal resolution, so it should theoretically handle cold air damming situations better than the GFS. Also, last night’s European model maintained the cold longer than the GFS. My confidence in how long the cold air might hold in is low. The current air is just not that cold.

The easiest way to describe the impacts of the storm is to divide the areas into different zones and then describe how the storm might affect each area.

Zone 1

Precipitation, mostly light to start, is expected to be spreading across the zone as early as 8-10 p.m. tonight. The precipitation could start briefly as sleet, but the bulk of it could be freezing rain into Wednesday morning. Icing could lead to a build up of 0.20-0.35 of an inch on trees and wires in the high end scenario — possibly enough to result in some limb damage and even a few power outages. Numerous icy spots could develop, especially on untreated road surfaces and sidewalks.

Temperatures across the region are expected to drop into the upper 20s to around 30 once the precipitation starts. The icing could continue into the morning but should end over most of the area before 10 a.m., when temperatures are expected to rise to above freezing. Precipitation should wind down during the late morning and end by noon.

My confidence in this area getting some freezing rain is high but my confidence in the ice accumulations being in the ball park is not particularly high. The warmer GFS look has me spooked, and the new European model keeps overnight precipitation relatively light until temperatures start to warm. Focusing on the low end of the ice risk may be the way to go in general here.

Zone 2

Temperatures are expected to be in 30 to 33 degree range during the night. Colder areas outside of D.C., especially to the north and west, will probably be cold enough to support ice accreting on trees, bushes and wires, but most road surfaces will probably be fine if treated.

In the more developed urban areas, streets and sidewalks should be mostly okay as well, but people keep an eye on the thermometer. If you note temperatures falling a few degrees below 32, a few slick spots could develop on walkways and untreated driveways.

By 8 or 9 a.m., possibly before especially in eastern parts of the zone, temperatures are expected to rise above freezing. Ice accumulations are expected to generally be light, 0.05-0.15 inches or less most spots, with the heaviest icing taking place west of the city and probably only a light coating at best bet for areas to the east. Precipitation is expected to end across the zone by 1 p.m. or so.

This area could easily end up with little or no ice accumulation if the GFS and newest European solutions are correct, so my confidence in the forecast above is low.



Ice forecast from NWS Baltimore/Washington as of this morning. More or less in line with CWG thoughts. (NOAA)

Zone 3

Plain old rain is expected across this region.

Temperatures are expected remain above freezing through the event except perhaps in isolated pockets very briefly. No icing problems are expected on road surfaces or walkways. Precipitation should end by early to mid afternoon.

My confidence in the forecast for this area is higher than for the other two, probably about average.

Briefly, this weekend’s storm threat

Nothing much has been resolved concerning this weekend’s potential storm. A storm still appears likely, but whether it will bring us rain, snow or a mix is still a big question mark. All we can say now is the probability of a storm is high sometime this weekend, probably towards Sunday.

Below I’ve attached the GEFS plume diagram from last night. Remember that ensemble members are just the GFS model run at a coarser resolution, with each run having slightly different initial conditions. The initial conditions are tweaked because we know we cannot accurately measure the atmospheric variables accurately at every point around the globe. The upshot is that when all the members are clustered together with the same precipitation type, storms are easier to forecast.



GEFS plume diagram.

In the figure above, where the green (rain) and blue (lines) increase in height on the diagram, that’s indicative of when rain or snow is being forecast by a member. The steeper and higher the lines rise, the heavier the precipitation. When the lines are parallel to the x-axis, no precipitation is falling.

In this case, green, blue lines are intermingled during weekend with the big spikes in precipitation but with different timing of the start of each spike.

A quick glance at the surface pattern associated with the various members indicated some taking the low into the Ohio Valley, or even great lakes, before reforming the storm off the East Coast (rain solutions). Others track the low to our south, suggesting that we would get some snow.

This morning’s GFS track took the initial low so far to our north and west that we’d see only rain locally. Essentially, the ensemble members and wild gyrations between operational model runs are screaming, YES a storm is likely, but we have no clue about the track of the storm. That includes where the heaviest precipitation associated with it will end up, or what type of precipitation the storm might yield across the region.

The bottom line on the weekend storm… A storm is likely to track to the East Coast sometime this weekend. All I can say is there is a chance of rain, snow or a mix with the storm.

Ian Livingston contributed to this post.