Sunday-Monday snow and ice on track: Timeline, amounts, impacts, FAQs

warnings** For Sunday morning to Monday morning: Winter Storm Warning west of the District | Winter Weather Advisory for District and points east **

8:45 p.m. update: A winter storm warning has been posted for locations west of the District for 1-3″ of snow followed by around 0.25″ of ice.

6:45 p.m. update: The National Weather Service has issued a winter weather advisory, generally along and southeast of I-95. This includes D.C. and Baltimore. The winter weather advisory begins at 8 a.m. Sunday and runs until 7 a.m. Monday. The advisory calls for 1-2 inches of snow and two tenths of an inch of ice. This is fairly similar to our forecast for the storm as seen below. Areas west remain under a winter storm watch and appear more likely to see winter storm warnings.

From 2:25 p.m….

Overview

team_forecastA wintry mess is on its way to the Washington area for Sunday, and it will bring a plethora of precipitation types along with sub-freezing temperatures. We start with snow developing Sunday morning, then mixing with or changing to sleet and freezing rain during the afternoon. Significant icing, with hazardous travel and some power outages, is possible Sunday afternoon and night, especially for areas west of I-95 where temperatures may not rise above freezing until Monday morning.

Several factors make this a tricky forecast, including uncertainty as to when snow will begin and change to sleet and freezing rain, and when temperatures will warm past the freezing mark. On the other hand, models have been very consistent in their overall picture of a significant winter storm with significant impacts — though not crippling in the D.C.-Baltimore metro areas — for most of the region with the exception of lower Southern Maryland and the Eastern Shore.

Storm Timeline

Sunday morning: Snow develops between approximately 7-11 a.m., generally from south to north. Temps: Near 30
Sunday afternoon: Snow mixes with or changes to sleet and freezing rain, generally from south-southeast to north-northwest. Temps: Upper 20s to near 30.
Sunday night: Precipitation becomes primarily freezing rain during the evening, then changes to plain rain between approximately 1 a.m. and 8 a.m., generally from southeast to northwest. Temps: Near 30 to low 30s.
Monday: Precipitation is plain rain everywhere by late morning, with intermittent rain during the day. Temps: Rising through 30s to afternoon highs near or above 40.

snow_chances_12-7-13-2

Frequently Asked Questions

When will the storm start? We expect snow to develop from south to north between approximately 7-11 a.m. Places to the south like around Fredericksburg are likely to see flakes start to fall during the earlier part of that window, while it may take until the later part for snow to start in places to the north like Baltimore.

How much snow will there be? Generally we see the potential for most locations to get somewhere between 0.5-2″ of snow before precipitation changes over to sleet and freezing rain. We’re not favoring any one location for the lower or higher end of the range. That’s because while snow will hold on the longest in northern areas, the heaviest precipitation may set up over southern areas. See our snowfall forecast map above for more details.

When will snow change to sleet and/or freezing rain? The timing of changeover is always tricky to predict. But models have been fairly consistent in showing this occurring between approximately noon and 4 p.m., first to the south-southeast such as in southern Prince George’s County, and last to the north-northwest such as in Frederick, Md.

GFS model has a reasonable looking simulation of precipitation type transitions.  Map above shows accumulated precipitation and type at 6 hour intervals between 1 p.m. Sunday and 1 a.m. Monday. Blue is snow, orange is sleet, purple is freezing rain, and green is rain. (WeatherBell.com adapted by CWG)

GFS model has a reasonable looking simulation of precipitation type transitions. Map above shows accumulated precipitation and type at 6 hour intervals between 1 p.m. Sunday and 7 a.m. Monday. Blue is snow, orange is sleet, purple is freezing rain, and green is rain. (WeatherBell.com adapted by CWG)

How much ice will there be? Along the I-95 corridor, we expect around 0.1-0.25″ of ice accumulation before a changeover to rain between midnight and pre-dawn Monday morning.  West of a line from Manassas to Fairfax to Rockville to Columbia, we see the potential for more than 0.2″ before a changeover to rain between pre-dawn and around 8 a.m. Monday morning.  Along and east of the Bay, a brief glaze is possible before a transition to rain Sunday evening.

Revised ice accumulation map as of 10:30 p.m. Saturday night (<a href="http://www.washingtonpost.com/blogs/capital-weather-gang/files/2013/12/ice_chances_12-7-13-2.gif">earlier version from 2:30 p.m.</a>)

Revised ice accumulation map as of 10:30 p.m. Saturday night (earlier version from 2:30 p.m.)

Could there be more or less snow/ice than forecast? If temperatures in the mid-levels of the atmosphere stay colder longer than expected, then we could see a little more snow and a little less ice, perhaps up to around 3″ in the metro area where a heavier band of snow sets up, but even so this will not be a major snowstorm. It’s also possible the mid-levels warm faster than expected which would reduce the amount of snow and increase ice totals, though this would probably not significantly change the overall impact of the storm.

When and where will conditions be at their worst? The worst conditions are expected late afternoon Sunday into Sunday night from around I-95 and D.C. toward points west, as the sun goes down and ice begins to build up more significantly. The very worst of the storm is expected from Frederick County, western Montgomery County, western Fairfax County, northern Prince William County and points west, where the risk is greatest for a prolonged period of treacherous roads and power outages late Sunday afternoon into Monday morning.

How will the roads be? Pavement temperatures will still be in the process of cooling down after the warmth earlier this week. So most roads, especially treated ones,  should be manageable through the initial period of snow, even with air temperatures a bit below freezing. Still, snow could stick on roads if it falls heavily enough, and visibilities could be reduced as well. So you’ll want to drive with caution. As the sun goes down and ice builds up during the late afternoon and evening, road conditions could deteriorate quickly. Treated highways and main roads should still be passable, but untreated roads, ramps, bridges and overpasses may turn treacherous, especially north and west of the District.

How will airports be affected? There’s a good chance of delays and possibly some cancellations Sunday afternoon and night, but conditions shouldn’t be bad enough to close any of the area’s major airports (National, Dulles, BWI). Dulles will probably see the most significant issues since air temperatures will be coldest there (upper 20s).

TravelCast: 2.5 airplanes
(Minor to major airport delays likely; we expect worse conditions at Dulles compared to Reagan National and BWI)
Will there be power outages? Scattered power outages are certainly possible locally, especially Sunday night as ice begins to build up and weigh down trees and power lines. But we don’t expect this storm to leave large portions of the metro area without power for multiple days. Power outages are likely to be more numerous and last longer to the west, especially from Frederick County, western Montgomery County, western Fairfax County, northern Prince William County and points west.

How will the storm affect the Redskins and Ravens games? Snow is likely to be falling on the drive to either game, but roads should be manageable if you drive with care. Precipitation probably mixes with or changes to sleet and/or freezing rain during the games and it will be very cold with temperatures in the upper 20s to near 30, though not too windy. After the games, treated roads should be passable if you take it easy. But untreated roads, ramps, bridges and overpasses could be very slick and you should drive with extreme caution.

Will there be school delays or cancellations? Because of uncertainty as to exactly what time in the morning snow will begin and how soon it will accumulate, decisions on Sunday schools/services will probably be a judgement call early Sunday morning. As we’ve said above, most roads and especially treated ones should be OK through the initial period of morning snow and probably manageable even a bit into the afternoon. But school administrators often err on the side of caution.  Because icy conditions are likely to linger into Monday morning, some school delays and cancellations are very much possible for Monday, especially but not necessarily limited to north and west of D.C.

Monday SchoolCast for points north and west of D.C. (Montgomery, Howard, Fairfax, Loudoun, Fauquier, Prince William): 2 apples
(50/50 chance of school. Good chance of a delayed opening)

Monday SchoolCast for the District and south and east (D.C. schools, Prince George’s, Anne Arundel and Calvert counties): 1.5 apples
(50/50 chance of a delay.  Closing unlikely)

When do conditions improve and what’s the forecast after the storm? Indications are that temperatures will be fairly stubborn across much of the area and not budge above freezing until between approximately 1 a.m. (southeast) and 8 a.m. (northwest). While intermittent rain may linger through much of the day Monday, road conditions should improve pretty much everywhere by late morning as temperatures rise at least to the mid-30s, and to near 40 or above for afternoon highs. By the way, there’s also a chance of light snow or rain on Tuesday. See Ian Livingston’s forecast into the coming work week.

Will the Federal Government be affected on Monday? Closure is unlikely, but a delay is within the realm of possibility.

FedCast: 2 capitol domes
capitol-black.jpgcapitol-black.jpgcapitol-gray.jpgcapitol-gray.jpg (50/50 chance of unscheduled leave policy and/or delay)

What does our Winter Weather Expert Wes Junker have to say?

“Nothing much has changed from yesterday. Tomorrow through Monday morning is likely to be a mess with snow starting tomorrow morning then changing to sleet and freezing rain during afternoon with the freezing rain continuing into the night even in and around the city before changing to rain. In the far western suburbs, the freezing rain could continue into the early rush on Monday. How long it lingers Sunday night into Monday in those western areas is still up in the air. Nevertheless, significant ice storm still looks like a distinct possibility especially north and west of the city.”

Where will this storm rank on the new Sperry-Piltz Ice Accumulation Index?

The Sperry-Piltz Ice Accumulation Index is a new 0-5 scale for ranking ice storms based on the amount of ice predicted, wind, and the potential damage. A category 0 is a minor ice storm with a few inconveniences, while a category 5 is a catastrophic ice storm with widespread, prolonged power outages. Here’s a graphic showing the different categories:

indexIce

Based on the predicted winds (expected to be relatively light) and ice accumulation, this storm should rank relatively low on the scale along the I-95 corridor and points east, likely around category 0 or 1. But west of I-95, a category 1 or 2 ice storm is possible, signifying the potential for hazardous roads, and isolated to scattered power outages. Towards the I-81 corridor, isolated areas could experience category 3 conditions in a worst-case scenario – suggesting the possibility of tree damage and multi-day outages.

What are other outlets forecasting?

The National Weather Service: 1-2″ of snow (map link), and the potential for 0.25″ or more of ice before a changeover to rain late Sunday night into Monday morning (link)

NBC4: On its Facebook page, it writes: “The clouds return tonight and snow is likely Sunday morning. The snow could add up to an inch or so in the Metro area, maybe 2-3″ farther out to the North and West of DC. The real trouble starts late Sunday morning. Warmer air moving up from the South will gradually change the snow over to a mix of freezing rain and sleet during the afternoon. Locations East and South of I-95 are likely to see mostly rain on Sunday afternoon, while areas West of the Blue Ridge could see a prolonged period of ice.”

WUSA: It has issued a Red Alert for ice Sunday after around an inch of snow. Excerpt: “Cold air will be in place, leading to a wintry mix of snow, sleet and freezing rain through Sunday night. The mix will eventually change to rain in southern Maryland in the late afternoon or evening but elsewhere the surface temperatures remain at or below freezing allowing ice to accumulate on trees and power lines. ” More information on its Web site.

ABC7: 1-2″ of snow, followed by ice, and then rain. See more details in its blog post: D.C. Weather Sunday: Wintry mix still expected

Jason Samenow and others contributed to this post

Also on Capital Weather Gang

D.C. area forecast: Enjoy today because Sunday brings a wintry wallop