Arctic blast: Five weather hazards you should be aware of

The coming cold air mass is unusually chilly even by mid-January standards in Washington.

Here is a brief description of five hazards related to the arctic blast:  Extreme cold, strong wind, wind chill, wildfire potential and upslope snow.

(1)   An extreme chill in the air – unfolding in two acts

The surface weather map at 8 p.m. Friday night (below) shows the arrival of the arctic blast in two waves.  The first was a cold front that swept through our region Friday night.   This dropped the temperatures across the region from a high near 60 °F to a low of 43 °F by sunrise.

Surface weather map, 8 p.m. Friday, showing waves of cold sweeping eastward across the U.S. (Unisys Corp)

Surface weather map, 8 p.m. Friday, showing waves of cold sweeping eastward across the U.S. (Unisys Corp)

Today, temperatures will not break 50 °F.  They will continue to rock slide downward as a second cold front, a genuine arctic front, pushes through during the overnight hours.  In the image above, you can see the arctic front nosing across the northern Plains last night.

Sunday’s high temperatures will not climb out of the mid-30s.   In the high elevations to our west, i.e. the Potomac Highlands, expect highs to remain below 20 °F.   The core of the cold will come Sunday night, area-wide, with lows near 20 °F over Washington, and single digits in the mountains.

(2)   Strong northwesterly winds

The surge of cold air is being propelled by an unusually intense high pressure cell, with its clockwise spiral of air, depicted in the figure below.   As a meteorologist, I rarely see a forecast high pressure of 1050 mb, given that average sea level pressure is 1013 mb.

Forecast surface weather map, for 1 PM today, showing a massive high pressure cell building down from the Plains.  (Unisys Corp)

Forecast surface weather map, for 1 PM today, showing a massive high pressure cell building down from the Plains. (Unisys Corp)

In the image, the light blue lines are contours of equal pressure, called isobars.  The spacing of the isobars indicates wind intensity.   The gradient in pressure between the arctic high over the Plains, and a retreating storm over New England, is a whopping 60 mb.   That difference will drive sustained northwesterly winds of nearly 20 mph with occasional gusts in the 30-40 mph range, during the overnight and much of the day on Sunday.

The peak gusts are expected early Sunday morning, and will be maximized over the high elevations to our west.   The image below shows the predicted maximum wind gusts at 7 a.m. Sunday (shown in knots;  multiply by 1.15 to get gusts in mph).    Note the region of 35-45 kt (45-50 mph) gusts along the ridgelines to our west.  There, a Wind Advisory is possible for travel difficulties and isolated power outages.

Predicted maximum wind gusts, in kts, for Sunday morning at 7 a.m. (NWS)

Predicted maximum wind gusts, in kts, for Sunday morning at 7 a.m. (NWS)

The winds are stronger at altitude because the effect of surface friction is reduced.  Surface friction is also considerably lower as air blows across a large body of water, such as the Chesapeake Bay.  In the image above, note how winds howl near 45 mph across the Bay.  Accordingly, special marine advisories for Sunday are likely.

(3)   Cold air + strong winds = WIND CHILL

This effect is easy to understand.   When air remains stagnant around your skin, it warms from contact with your body.   This warm boundary layer is stripped away by the wind.   Your rate of heat loss accelerates.   If skin is wet from perspiration (say, you were out for a fitness run), then evaporation – which is greatly enhanced by wind – further boosts heat loss by many factors.

The expected wind chills tomorrow are just downright dangerous, and can rapidly lead to frostbite on face and hands, and hypothermia during prolonged exposure.   Outdoor pets need ready access to warm shelter, out of the wind.

Here are the sobering numbers:   Wind chills in the low-mid teens for Washington, and down to -10 °F (!) in the western mountains.  A Wind Chill Advisory has been issued for the high elevations overnight.

Simulated wind chills from GFS model 7 a.m. Sunday morning (WeatherBell.com)

Simulated wind chills from GFS model 7 a.m. Sunday morning (WeatherBell.com)

I’m trying to remind myself it’s still only November…

(4)   Up in flames – Let’s all make sure this one doesn’t happen

The combination of very dry air (the source region for this arctic air mass is deep, inland Canada, far from the ocean) and high wind spells enhanced fire danger across our region. This is especially true in back yards and along roadsides, due to the recent heavy leaf fall and lack of rain – creating a tinderbox for brush fires.

The National Weather Service has thus issued a Fire Weather Watch for the region Sunday.

(5)   Upslope snow

Aside from a few flurries, this is not a Washington-region impact, but nevertheless a concern for our western mountains – specifically, the western slopes and ridge crests.   Tonight is the time of concern, as the arctic front sweeps across the ridgelines.   Uplift along the front, combined with forced ascent of air up the mountain slopes, may promote snow showers.

High resolution NAM model simulates snow along the western slopes of the Appalachians, mostly tonight into Sunday morning (WeatherBell.com)

High resolution NAM model simulates snow along the western slopes of the mountains in eastern West Virginia, western Maryland, and western Pennsylvania, mostly tonight into Sunday morning (WeatherBell.com)

Amounts could range from a dusting to an inch or two.   The other hazard is blowing snow which could lead to locally poor visibility, and the associated travel hazard. Winter weather advisories have been posted for parts of western Maryland, southwest Pennsylvania, and northern West Virginia.

Related: Abnormal cold to close November (tracking stats)

Also on Capital Weather Gang

D.C. area forecast: Mid-winter chill on our doorstep, and a storm on the horizon