Thursday, January 29, 2015

January 31 - February 2 Potentially Significant Winter Storm

A potentially significant winter storm is taking aim at a large swath of the country this weekend into the start of next workweek.

Tropical Tidbits
Click images to enlarge
The image above shows the latest GFS forecast for precipitation, mean sea level pressure (MSLP) values, and 1000-500mb thickness values (dashed blue and red lines) on the evening of February 1st. The prognosis calls for energy coming from the Southwest, moving eastward where interaction with additional energy out of Canada may occur. The more interaction these two systems have, the more snow & overall precipitation may fall. The caveats associated with this storm include storm track, strength, snow amounts, interaction between the two pieces of energy, among other items. We'll address some of these here today.

I'll start off right away and say that I think we see this storm either maintain its current position, or even shift south a bit more. Why? I'm glad you asked.

Tropical Tidbits
Here's a look at 500mb vorticity values from January 22nd, over the West Pacific. Notice how we see a positively-tilted piece of energy making its way eastward over Japan, with another piece of energy sliding southward from higher latitudes. Does this set-up look familiar? It should, because the Typhoon Rule says this will be how our storm will evolve. After this point in time, the two pieces of energy approached each other, but did not phase/interact until they were well east of Japan. To me, this means our storm coming up this weekend may not phase as easily as model guidance is saying. It is known that weather models tend to phase storms too readily, and this could be a prime example of that bias showing through. If that is true, this storm would likely stick further south.

Additionally, and perhaps most importantly, this storm will occur under predominantly northwest flow, with a strong lobe of the tropospheric polar vortex pressing southward from Canada. In this 500mb height contour map, valid for February 1st, we see that lobe of the vortex creating for a very tight gradient in the North US. This could very well act to suppress the storm further south, possibly more than what is currently being advertised. Second, notice how the height contours in Canada seem to be diving southeast into the US. This is what is called 'northwest flow', where the air in the United States is coming from the northwest. Because this is happening, I find it rather plausible we see the storm pushed further south, as strong surface high pressure dropping southward from Canada will also be playing a role here. In case you haven't guessed yet, I'm not finding many factors favoring a north shift.

Instant Weather Maps
One positive thing I am seeing is that model guidance is likely under-forecasting precipitation for this storm. Above, we see the GFS model forecast for low-level relative humidity on February 1st. You can see just how moist this storm will be, with high RH values stretching from the eastern Rockies all the way to the waters offshore the East Coast. This moisture will be driven from the Gulf, hence why I find it plausible, if not likely, more precipitation occurs in this storm than what is being modeled right now.

The question then arises, could this favor prove to stab the wintry portion in the back? Thunderstorms in the warm sector of winter storms have been known to rob the wintry sector of moisture, leading to more rain than forecasted and less snow than forecast. This will be something we'll need to watch when the storm commences.

Tropical Tidbits
Storm track forecasts should really begin to solidify by Sunday(!!), when both pieces of energy relevant to this event move into the United States. Right now, the energy from the Southwest is slowly moving east onshore California, which should aid in model accuracy, gradually, in coming forecasts. An interesting question being posed is, what if latest model guidance inching north in the last day or so has something to do with that energy near California coming onshore? Could it spell a further north track down the road? Anything's possible, sure, but I'll stick with a southern track for now.

Here's my forecast graphic for this event.
The Weather Centre

To summarize:

- Model guidance is in agreement on a potentially significant snowstorm impacting the North US this weekend.
- Amounts in the hardest-hit regions may exceed 12", but this may need revision in later updates.
- High uncertainty still exists.