Historic Winter Storm to Impact the Northern Rockies & Northwest

An early early-season winter storm is expected to bring heavy snow and near-blizzard conditions to the northern Rockies & parts of the Northwest this weekend.

A slow-moving, potent upper-level low is pulling in ample Pacific moisture into the Northwest (see Fig. 1). This deep moisture, coupled with an amplified trough (which is providing the cold air out of Canada) will setup this historic winter storm.

Fig. 1: Pacific moisture pulled into the northern Rockies (Tropical Tidbits)

Snow is already beginning to fall in the higher elevations of the northern Rockies and the Washington this morning. The snow is expected to increase in intensity and coverage throughout the day–continuing into Sunday and the snow levels will begin to fall during the overnight hours on Saturday. The local National Weather Service offices have issued Winter Storm Warnings from the Cascades of Washington east into Idaho and Montana. There are also Winter Storm Watches & Advisories surrounding the Winter Storm Warning (see Fig. 2).

Fig. 2: Current winter weather alerts (Pivotal Weather)

Within the Winter Storm Warnings, heavy snow, paired with strong winds will reduce visibilities and impact travel. The heaviest snow can be expected within the Winter Storm Warnings (high elevations of Washington, Montana, Idaho & Wyoming) where over a foot of snow is forecast to fall–much of northern Montana can expect to see widespread, heavy snow of one to three feet. Lesser amounts are forecast for the lower elevations of the Northwest (see Fig. 3).

Fig. 3: NAM snowfall accumulation forecast (Tropical Tidbits)

The snow will begin to taper off for the northern Rockies on Monday. Farther east, the Black Hills of the Dakotas should see rain transition to a rain/snow mixture by late Monday into Tuesday. Light accumulations are possible.

While snow is not rare this early in the season for the northern Rockies, the forecast accumulations of several feet of snow for the highest elevations of the northern Rockies is historic.

The Latest on Newly Formed Tropical Storm Karen

Tropical Storm Karen developed this morning near the southern Windward Islands. Karen has maximum sustained winds of 40 mph and is moving WNW at 13 mph (see Fig. 1). 

Fig. 1: Latest update on Tropical Storm Karen (NHC)

Karen is relatively disorganized due to strong wind shear (see Fig. 2) thanks to an upper-level trough to the northeast of the storm. As Karen continues its WNW (& eventually NW motion), the wind shear is forecast to gradually lessen allowing the storm to increase in intensity as the storm moves toward Puerto Rico (see Fig. 3). The intensification is expected to be gradual, however, due to dry air continuing to plague this system through mid-week. Numerical guidance indicates dry air will be pulled into the center of the storm through mid-week, which should keep Karen below hurricane status as it approaches Puerto Rico. 

Fig. 2: Current wind shear (University of Wisconsin)
Fig. 3: Forecast track & intensity of Tropical Storm Karen (NHC)

The uncertainty in Karen ensues as we head into late week into the weekend. Numerical guidance is suggesting a ridge will start to build north of Karen by late week (see Fig. 4). Guidance indicates the ridge north of Karen will continue to build into the weekend (see Fig. 5). Depending on the strength/placement of the ridge & the location of Karen, will determine if Karen goes out to sea in the Atlantic or begins to curve toward the west (possibly posing a threat to the U.S. sometime during the first week of October).

Fig. 4: Late week 500 mb Heights (Tropical Tidbits)
Fig. 5: Late week 500 mb Heights (Tropical Tidbits)

It is too early to forecast what Karen will do once we head into next weekend with so much uncertainty in key variables that will help steer the storm, but there is a realistic chance Karen *could impact parts of the U.S.. Continue to check back for frequent updates throughout the week on Firsthand Weather.

Hurricane Dorian Will Make A Turn Northward, Eventually

On one hand, it seems unbelievable that Hurricane Dorian has managed to achieve such a great strength in a relatively short amount of time. On the other hand, the surrounding environmental conditions allowed Dorian to reach its full potential. Since maximum sustained winds sit at 185 mph, as of Sunday, 2pm ET, Dorian can now be put in the same league as some of the most powerful hurricanes ever recorded in the Atlantic Ocean. Furthermore, Dorian is the northernmost category 5 hurricane ever recorded in the Atlantic, not including the Gulf of Mexico. Typically, water contains more heat throughout a greater depth within the Gulf of Mexico and Caribbean; whereas, ocean heat content tends to not be as high elsewhere in the Atlantic. All things considered, Dorian is already one for the record books.

Understandably, many are concerned that Dorian will not make the northern turn at all. Well, it will, eventually. Here’s the problem though, which has inevitably led to much uncertainty over these last few days. Dorian is expected to run parallel to the Florida coastline, either onshore or offshore, once it makes the northward turn. Thus, we’re left trying to predict when the turn will occur, which depends on how quickly the ridge to Dorian’s north weakens. One small error in timing, and Dorian ends up making a landfall in Florida versus staying offshore until approaching the Carolina coast.

Can I say with high confidence that Dorian won’t make landfall in Florida? Honestly, I can’t. Can I say with high confidence that Dorian will make the northward turn quickly enough before impacting Texas, Louisiana, Mississippi, or Alabama? Absolutely, I can. It might seem that once a storm reaches a certain strength that it would be able to call its own shots without worrying about other nearby features. Actually, the opposite is true. A strong hurricane can potentially modify surrounding environmental conditions, but atmospheric steering currents will ultimately guide where Dorian will go.

But why did a storm like Katrina cut across the Gulf of Mexico, yet I can confidently say Dorian won’t do the same thing. Because, the overall pattern was entirely different. Taking a look at reanalysis data, a big mid to upper-level high was centered over Louisiana and Arkansas, which placed Katrina on the southeastern periphery of that high. Flow is clockwise around high pressure systems in the Northern Hemisphere; thus, it is was only natural for Katrina to continue on a westward path into the Gulf of Mexico until the ridge broke down. Once the ridge was out of the way, Katrina moved northward towards Mississippi and Louisiana.

Hurricane Katrina track

For Dorian, the environment is entirely different. Broad troughing over the northeast quadrant of the U.S. is sandwiched between a ridge over the Four Corners region and Bermuda ridge. Similar to how Katrina moved underneath a ridge positioned over the mid-South, Dorian has been steered by Bermuda ridging that has stretched to its north and northwest. Take away the ridge, and the storm starts pulling northward, given the troughing across the eastern U.S. In fact, Dorian has already begun to slow its forward speed.

With all of this said, the concern that Florida will suffer a landfall are valid. The ridge could stay a hair stronger, delay the northward turn by a little while, and the outcome would be completely different for Florida. But let’s take a look at the National Hurricane Center’s latest projected path. Even though their official track keeps Dorian barely offshore, the cone of uncertainty includes most of the Florida east coast through the Carolinas and even into the Mid-Atlantic. The cone of uncertainty IS NOT meant to forecast impacts. The cone is based on previous forecast track error. In fact, below is NHC’s definition of what the cone means.

The cone represents the probable track of the center of a tropical cyclone, and is formed by enclosing the area swept out by a set of circles along the forecast track (at 12, 24, 36 hours, etc). The size of each circle is set so that two-thirds of historical official forecast errors over a 5-year sample fall within the circle. Based on forecasts over the previous 5 years, the entire track of a tropical cyclone can be expected to remain within the cone roughly 60-70% of the time. It is important to note that the area affected by a tropical cyclone can extend well beyond the confines of the cone enclosing the most likely track area of the center.

NHC’s Explanation of the Cone of Uncertainty

Okay, so let’s break this down further. Based on the last 5 years of actual tracks versus forecasted tracks, 60-70% of the tropical cyclone forecasts (tropical depressions, tropical storms, hurricanes, etc.) remained within the cone. That means that 20-30% of the forecasted tracks didn’t even stay in the cone for a given forecast time. If you are within the cone, the storm’s eye and eyewall could go over your location. Again, the cone of uncertainty isn’t a forecast for impacts. It’s a forecast for where the center could go. Essentially, the National Hurricane Center is providing a realistic margin of error in their forecast. In other words, these forecasts are never perfect or go as predicted.

Around a day ago, I stated that I felt Dorian’s landfall could occur somewhere between Myrtle Beach, SC and the Outer Banks, NC. Although that’s still a completely viable scenario, I strongly encourage those in Florida to prepare for a landfall, just in case. We are dealing with a very powerful storm, and we have very little room to make a mistake in track. Thus, continue to prepare and consider evacuating, especially if it’s mandated. Most of the model guidance brings Dorian dangerously close to Florida or even projects a landfall. This has prompted the National Hurricane Center to begin issuing hurricane warnings for parts of the Florida coast.

View loop by clicking here.

Given Dorian’s current strength of 185 mph, the storm will likely begin to weaken some, especially since it’s forward speed will continue to slow with time. It takes a lot to maintain such strength. Over time, upwelling and vertical mixing of cooler waters, plus Dorian’s interaction with the Bahamas, could induce some weakening. Nonetheless, Dorian will remain a major and deadly hurricane as it nears Florida.

We will continue to provide numerous updates on Firsthand Weather throughout this event. Please to continue following our updates closely on Facebook.