Winter Storm Thor

Winter Storm Warning in effect for our area from Wednesday 12am until Thursday 9pm. 

The name is ironic enough. This storm is going to be "hammering" us with massive amounts of snow on the back end of a rainstorm Wednesday evening into Thursday evening. Snow from Winter Storm Thor has already blanketed regions of the West, Southwest, and South Central including California, Arizona, and Texas.

Models as of this morning are now learning towards the likelihood of a major snow event for the entire metro area. The GFS model for snowfall within the next 72 hours has totals of 8-10 inches. While other models have totals closer to 3-5 inches. Model output such as this is alarming, but forecasters (especially via media) need to remain skeptical in light of the situation and not be swayed easily either way.

48 hour Snowfall forecast (Intellicast)

The problem with forecasting a major snowstorm far in advance is that the some of the development features for the storm are not present within the initial conditions, meaning that some of the forecast has to be left to probability in the longer range. With all that being said, this system might prove to become the largest snowfall of the season.

This system is quite a complex frontal system with warm air pushing out ahead of the front and cold air moving in later on. The warm moist air moving in tonight into tomorrow is not as warm as once projected earlier in the week. Highs tomorrow will stay in the 40s throughout the day. A strong system of moisture will move into the region from the South Central region bringing warm saturated Gulf and Pacific air with it. By midday Wednesday, rain will begin to fall in moderate amounts. Behind it a large swath of cold air pushes southeast from the Midwest behind a very large cold front. This cold front will plummet temperatures late Wednesday and they will continue to fall into Friday morning. The active and strong jet will intercept this cold air and steer this system directly towards the Mid-Atlantic along with the sufficient moisture.

Then comes the fun part. Later Wednesday, temperatures will start to approach the freezing mark. A change over to snow requires the entire column of air to cool below freezing. If any sufficient part of the air from upper cloud levels to ground is above freezing enough to melt the ice crystals than the changeover might not happen immediately. If it does though, accumulation may start to pile up Wednesday night into Thursday morning shifting snowfall totals higher than some are projecting. Sleet may mix in within the first couple hours of the changeover.

Currently the highest totals of the storm are projected to be in the Northern and Western portions of West Virginia, Maryland and Virginia and lower totals projected to be seen in Southern and Eastern suburbs of DC.

With many different snowfall forecasts out there, I will just use NOAA's Probabilistic Snowfall forecast as my reference to likelihood of different 24 hour snowfall totals.

Starting at 18Z Wednesday (1 pm Wednesday until 1 pm Thursday):

Probability of at least 1 inch of snowfall

Probability of at least 2 inches of snowfall

Probability of at least 4 inches of snow

Probability of at least 6 inches of snow

Probability of at least 8 inches of snow

So what the maps are telling us, is that there is a good amount of confidence that there will be at least 4 inches of snow, but less confidence of totals in the higher end of the range.

Timeline for Winter Storm Thor:

Wednesday Morning: Rain will begin to pick up around 6-7:00 am and will continue throughout the morning. Temperatures at this time will hold in the low to mid 40's and will stay that way until the afternoon. It will be a steady rain at this point. No showers or downpours.

Wednesday Midday/Afternoon: Rain will continue at this point. It may die down a bit midday, but will pick back up in the afternoon. Temperatures will reach a high of 48 around 2 pm, then begin to slowly fall through the later afternoon hours. Rain intensity will pick up after 5 pm.

Wednesday Evening: Rain showers will and wind will increase as the sun goes down as temperature begins to sharply decrease. Temperatures will be in the mid 30's by 8 pm and could be below freezing by 10pm. Look for a changeover to snow to be around this point. The time of the changeover is important due to how much of the precipitable water falls and sticks as snow as opposed to rain. This could shift accumulation totals farther up or way farther down than forecasted.

Wednesday Night: All snow expected by this time. Accumulation may start to pick up after midnight, or like in situations in past storms this winter season, it may continue to accumulate by this point. Expect around 0.5-1 inch of snow on the ground by 2am.

Thursday Early: Snow will start to pick up in intensity by the early morning hours of Thursday and accumulate at rates of at least of 1 inch an hour or higher. By sunrise there could be anywhere from 2-4 inches of new snow on the ground including on the streets. Temperatures will stay consistent in the upper 20s with wind gusting out of the NNW at 20-25mph. Visibility at this time will be very low. 

Thursday Morning/Midday: Snow will continue through the morning hours at relatively high accumulation rates, though these rates may diminish after 11 am. Total snowfall by 11am should be around 3-4 inches. Temperatures will continue to fall through the day. Temperature will be below 25 by 12 pm.

Thursday Afternoon: Snow will finally start to dissipate and turn to more snow showers/flurries by early afternoon before completely ending by 6 pm. Temperatures will continue to drop as wind continues to gust out of the NNW at 20-25 mph. Additional accumulation by this time around 1-2 inches.

Thursday Evening: Skies will begin to clear by sundown on Thursday. Temperatures will drop into the upper teens and eventually mid-single digits overnight. Wind chill values will be well below 0. Record lows possible in western suburbs.

Snowfall Forecast: 4-8 inches with high variability. Tomorrow's update will help narrow that down a bit.

Impact: Medium-Medium High-with plenty of time to prepare, road crews are going to be out in full force making sure that this storm has a minimum amount of impact on the roads. That being said, look for road conditions to drastically deteriorate by early Thursday.

Notes on Delays/Cancellation: It is way too early to state any conclusion on whether there will be a delay or cancellation Thursday. Though if I were to have put odds on it right now? I will put it at 50-60 percent chance of at least a delay and 30-40 percent chance of a full cancelation.

Another update is coming tomorrow, so stay tuned. A more detailed picture will become clearer after tomorrow morning's model runs. Thank you all for checking in!

-Jamees