D.C. area forecast: Blissfully warm today, icy threats loom for Sunday
FORECAST IN DETAIL
Were it not for the weekend worries, we would be spending a lot of time talking about how nice today is. However, many computer models are conspiring against us on Sunday, with a multitude of scenarios for snow, sleet, and freezing rain. There is still time for those models to come to their senses, but not right now. Some are giving the city a couple of inches of snow and plenty more north and west of town. However, what really worries me is the ice potential with some showing up to a half inch of ice or sleet. Thankfully, Sunday is the problem and by the time Monday commuting comes around we will back to liquid precipitation.
Today (Thursday): Overcast dominates the morning. As moisture streams northward a few sprinkles could be squeezed out. By afternoon, the clouds should lighten and a few breaks are even possible. What is breath-taking is the warmth! With highs in the mid-to-upper 60s, get out and enjoy. Light winds from the south are not a problem. Confidence: Medium-High
Tonight: Hopefully, clouds are broken enough to give glimpse of a crescent moon balancing above the brightest Venus of the year in the western sky. Clouds thicken through the night and very light showers will scatter across the area with a 50% chance for any particular area to get damp. Lows are pleasantly stuck in the mid-50s with only light south winds. Confidence: Medium-High
For related traffic news, check out Dr. Gridlock. Keep reading for the forecast through the weekend
Tomorrow (Friday): The main surge of moisture spreads across the region through the day. Initially thickening clouds but by later in the afternoon showers will increase significantly in number and intensity (90% chance). Temperatures are still respectable with highs mainly in the mid-to-upper 50s. Light winds initially out of the west shift to the north late in the day. That signals the beginning of the cold air infiltration. Confidence: Medium-High
Tomorrow night: Showers continue to be frequent through much of the night (90% chance). Many areas could end up with as much as an inch or rain out of this initial charge. The real story though is cold air slowly but steadily surging across the land with lows ending up in the mid-to-upper 30s. Confidence: Medium-High
A LOOK AHEAD
Saturday is definitely the calm before the storm. Well maybe not so calm in the bread and milk aisles, but outside clouds should break enough for some sun. Meanwhile, the real cold is now gaining the upper hand. Highs only make the lower 40s. The evening should still be fine for being out and about. However, after midnight all bets are off as temperatures across the region fall below freezing and the next moisture surge begins to arrive from the southwest. Chances of snow and sleet are only 40% before dawn. Lows drop to the mid-to-upper 20s. Confidence: Medium
Sunday has plenty of time to morph into a better or worse scenario but the one good bet is precipitation will fall (80% chance). The chances of it being all rain are becoming quite low but the chances for a significant snow event are also very low. Being a poker player, I will give you my best bet for the city proper (Jason’s zone 2): A quick inch of big fluffy sticky flakes could pile up at sunrise but sleet quickly mixes in and becomes prevalent. This is likely to then mix with freezing rain by late morning and then shift over to just rain early afternoon. (But as cold air sometimes is tough to dislodge, icy conditions could last longer). As is almost always the case, we see more snow (up to 4 ?) and ice (up to 1/2 ?) north and west of town with temperatures struggling to get above freezing. Highs will only make the low-the-mid 30s in most areas. Precipitation remains active overnight and we could end up with yet another 0.5-1 of rain before tapering near dawn. Temperatures hold steady overnight or rise a few degrees. Confidence: Low-Medium
Monday remains showery (80% chance) especially in the morning but of a lighter variety. Highs climb to the mid-50s in most areas thanks to south winds back in control. Confidence: Medium
Redskins vs. Chiefs weather forecast: Icy conditions a growing concern
PM Update: Cloudy through a warm Thursday; central U.S. cold, snow and ice
With the sun angle so low this time of year, cloudy days are extra gloomy. But on the “bright” side, highs again reaching the mid-50s across most of the area felt pretty pleasant. We warm up some more tomorrow, though we stick with largely gray skies.
Through Tonight: Clouds, a south wind, and rising dew points on that south wind help keep us fairly mild. As the night wears on, some fog or drizzle may form in a few spots. Winds are light from the south.
Tomorrow (Thursday): Welcome back to summer? Not quite, but definitely a day to open the windows and such! Despite sunshine being pretty hard to come by throughout the day, southerly winds around 5-10 mph help boost temperatures into the mid-and-upper 60s. There could be a late day or evening shower.
Wintry mess: A pair of storm systems will continue to impact the central United States over coming days. The first is pushing through Minnesota, Wisconsin and Michigan, having already dropped up to 1-2 feet of snow in parts of the northern Plains and upper Midwest. Snow totals in that region may approach 3 feet in spots. The next system is moving through the Rockies and dropping heavy snow in the mountains. It is poised to emerge into the southern Plains tomorrow, where areas of moderate snow and potentially heavy ice accumulation are possible from Texas/Oklahoma and off to the northeast in coming days.
It’s also pretty ridiculously cold out there, as seen by a temperature analysis of the country this afternoon. Most of Wyoming and Montana have seen temperatures hovering near or below zero during the day today. Cold air is draining south and east nicely out ahead of the next storm as well.
Threat of wintry mess Sunday grows; ice storm potential western areas
Since Monday, we’ve been reasonably confident the region would experience a wintry mix of precipitation. As of Tuesday, we said the event in the immediate metro area may just be a conversation starter or nuisance. But today, the data are hinting at a more significant winter storm, especially for areas west and northwest of the District.
This does not look to be a major snowstorm, but -- due to a healthy supply of cold air as the storm moves in -- some accumulating snow does seem possible before changing over to mixed precipitation.
Next accumulating snow chance: Sunday morning
Probability of more than 1 of snow: 30% (40% western suburbs)
Probability of more than 4 : 5% (15% western suburbs)
Ice may be the bigger story, from along and west of a line from Manassas to Fairfax to Rockville to Columbia (Md.), and particularly along the I-81 corridor from Harrisonburg to Hagerstown.
Arctic air is building in the western and central U.S. On Friday morning, lows in parts of Montana and the Dakota may dip below -20 F. We won’t see anything like that, but -- starting Saturday -- cold air will ooze into the Mid-Atlantic as an area of arctic high pressure is anchored to our north. Along a front draped over the region, low pressure will develop bringing precipitation to the region starting Sunday morning.
Overview of how the storm may play out
Precipitation may begin as snow and/or sleet between around 4 and 8 a.m. Sunday morning, with some accumulation possible before a changeover to sleet and freezing rain late morning into the early afternoon.
Then, as warmer air moves in high altitudes late Sunday morning into early Sunday afternoon, a period of sleet and freezing rain is likely with some ice accumulation possible.
By late afternoon and early evening, precipitation may change to rain close to and east of I-95. But in our far western suburbs, icing may continue for several more hours, with a chance of more significant ice accumulation.
By late at night, even the coldest areas (towards the Shenandoah Valley) should rise above freezing and precipitation will taper off.
“The cold air looks like it will hold on a longer than I was thinking yesterday as the models are holding the [arctic] surface high to our north in longer putting it in a very favorable position for cold air damming [draining south over area] during the day on Sunday,” says Wes Junker, Capital Weather Gang winter weather expert. “This is now definitely a system to monitor as there is potential for a period of snow and for icing problems lingering into Sunday night especially west of the city.”
Here’s a general idea of how we see the storm playing out by zone. But, first, several things to keep in mind:
1) This event will largely focus on a Sunday when there is no rush hour, reducing the impacts on commuting and schools.
2) Sunday morning religious schools/services may be impacted as well as to travel to Sunday NFL homes games for the Redskins and Ravens.
3) Temperatures will be close enough to freezing that treated roads should be ok to travel on. The problem may be untreated roads, bridges, ramps and overpasses.
4) Most precipitation tapers off by Monday morning and temperatures should rise above freezing most places by then.
Zone 1 impacts: Potential for light to moderate accumulations of snow and/or sleet, followed by significant icing. Precipitation may change to rain late Sunday night. Significant ice build-up could pose a power outage risk.
Zone 2 impacts: Potential for brief period of snow/sleet (light accumulations), and some icing, before likely changeover to rain in the late afternoon or evening Sunday.
Zone 3 impacts: Potential for brief wintry mix before likely changeover to rain late Sunday morning or in the afternoon.
We won’t be able to nail down specifics on exactly how much snow and ice falls in each zone and how disruptive the storm will be until Friday into Saturday. Nor will be able to narrow down the timing of the precipitation and precipitation-type changes.
Those specifics depend on temperature details which are hard to pin down right now. If the storm comes in later in the morning and temperatures are not quite as cold as models are currently simulating and/or precipitation isn’t as heavy, we would see less snow and ice.
On the other hand, models often underestimate the amount of cold air at the surface in these kinds of cold air damming events, which may allow icing to last longer than forecast.
We will have another detailed update tomorrow.
Unhealthy air quality in December? What’s going on?
As of 10 a.m. this morning, the Air Quality Index in Baltimore stood at 122, passing the code orange threshold, indicating unhealthy air for sensitive groups (children, older adults, and individuals with respiratory illness). (In D.C., it was 97, just below code orange levels). In fact, since Monday, parts of Maryland and northwest Virginia have experienced compromised air quality in the code orange range. We’re accustomed to code orange days on sweltering summer days. But why now?
In short, the uptick in air pollution is due to an absence of wind. We’ve been in between weather systems, with hardly any “flow” over the region.
The stagnant air has allowed pollutants in the region to gather and stay put -- particularly in sheltered valleys. The map below shows highest forecast pollution levels today in mountain valleys west of the D.C. and Baltimore.
When there’s wind, it mixes the air pollutants so it’s not too high in any one place and, like a fan, wind pushes pollution in and out so it doesn’t linger anywhere long.
Sunil Kumar, senior environmental engineer at the Metropolitan Washington Council of Governments, offers some more detail on the causes of this bad air in valleys to our west today (e.g. around Hagerstown):
Limited vertical mixing of air mass due to inversion, light surface winds that recirculate within the area, and warming temperatures. There may also be some smoke in the region that may be mixing down toward the surface too. Fine particles produced from previous days are also getting added to the current fine particle level.
Right now, the pollution we’re seeing is mostly from fine particles -- which is not unusual in winter time. Ozone pollution -- which is exacerbated by heat and most common in summertime -- is relatively low.
The good news for individuals sensitive to elevated particle pollution is that increasing winds from the south should reduce its concentration by tomorrow. Whereas today’s AQI is around 100, tomorrow it should be closer to 60, which is moderate.