Northern Plains blizzard redux

Update: 3/1 6:00 am: The SPC has upgraded western KY to a moderate risk of severe weather although the greatest risk of tornadoes remains south and west over MS, AL and TN. As in the previous update, the key will be how much sunshine we get Thursday ahead of the front and how far north the 60+ dewpoint air will advect.

Update: 2/27 2:00 pm: The SPC site is now working and you can read their discussion here. Both the SPC and the NWS Louisville office believe the greatest instability and hence, the greatest severe weather risk will be over TN, MS and AL although there is still a slight risk for severe weather, including tornadoes, across KY. The key will be how much sunshine we get Thursday ahead of the front and how far north the 60+ dewpoint air will advect.

Original post: For over a week now I have been suggesting that there will be a “return to winter” during the first week of March (see Bottom Line point #2) and that March should come in like a lion. The strong storm currently battering the West Coast will follow a similar path as the recent weekend storm and eject from the Southwest into the central Plains Wednesday, setting the stage for more severe weather across the southern tier states as well as anothern 12″+ blizzard for parts of the northern Plains. While this storm has a similar synoptic setup as the weekend storm, there are some very important differences that will affect the mid-south.

1) Severe weather potential
Last weekend storm: As mentioned here, I predicted that the primary severe weather threat would be south and west of KY and as you can see here, most of the tornado activity did indeed occur in the eastern AR and northern MS region that I sent a WKU chase team (any luck Scott?? – if so report here). The reason KY did not see severe weather was because the warm front remained south of TN, which kept KY in a cold rain with an east wind.

This week’s storm: The lastest GFS model run shows the warm front north of KY, which means that after the warm front passes by late Wednesday night/Thursday morning the mid-south will be right in the middle of the warm sector with gusty SW winds, mixed clouds and sunshine, and temperatures in the mid 60s. As the cold front approaches Thursday afternoon/evening, the stage will be set for severe weather, as the left-front quadrant of a 150 kt jet stream and divergence aloft will be located directly over the mid-south. If the SPC server was working, I would link to their discussion of the risk of severe weather for late Thursday for the mid-south. Depending on the amount of sunshine, I wouldn’t be surprised to see severe weather as far north as southern IN and OH.

2) Cold air behind the storm
Last weekend storm: The Pacific high behind the last storm was only cold enough to lower temperatures to a couple of degrees below normal and Monday and Tuesday of this week will have highs around 50. Nothing special.

This week’s storm: Cold, arctic air will follow the cold front for Friday-Sunday as temperatures over the weekend will struggle to get much past 40 degrees for highs (normal is 53). There is even the chance for a wintry mix of rain/snow showers Saturday through Sunday as pieces of shortwave energy rotate through the longwave trough, although this is nothing that should stick to the ground.

An additional interesting factor of this week’s storm will be the development of a triple-point low along the warm front that will transfer the storm’s energy from the deep low over the northern plains on Thursday to the East coast by Friday. As with the last storm, a downstream 50-50 low will produce a blocking ridge oriented NW-SE over the eastern Great Lakes on Thursday. The northern plains low, which will produce blizzard conditions in the same areas that got over a foot last weekend, will run into the block and rapidly occlude. Note the appearance of a vorticity max over the Mid-Atlantic states Thursday night that forms directly beneach the divergence associated with the blocking ridge at 500 mb. Also note that it wasn’t there Thursday afternoon. This is a textbook example of triple point cyclogenesis. By Friday afternoon, the plains low hasn’t moved and is weakening while a 992 surface low has developed over Long Island. This transfer of energy from the plains to the coast will produce very heavy rainfall over the mid-Atlantic states, which raises the flooding risk as the heavy rain falls onto ground saturated from recent snowmelt. By Friday night, this new coastal low is producing heavy snow over northern New England and the Canadian maritimes.

Beyond the weekend is where it get tricky. I have mentioned in previous posts the possibility of a mid-March cold snap due to the southward movement of the polar vortex. All of the mid-range models show the polar vortex sinking southward, but the key will be the location of the mean 500 mb trought March 4-10. This comparison of the day 8-10 means from the European (left) and GFS (right) shows that while the European keeps the trough in the eastern U.S. (which would mean extended cold weather for KY), the GFS shows a trough along the west coast with seasonably mild temperatures across KY. While there is very strong agreement in the GFS ensembles for the cold shot this weekend you can see that by 240 hours (March 8th) there is very little agreement. The teleconnection indices have yet to play their hand, so I’ll need a few more days to sort this out.

The Bottom Line:
Tuesday and Wednesday: Progressively warmer each day.

Wednesday night – Thursday night: Showers and thunderstorms Wednesday night from the warm front. A break early Thursday, then thunderstorms likely Thursday afternoon and evening with the threat of severe weather (including tornadoes) for all of KY and TN.

Friday – Sunday: Partly to mostly cloudy, breezy and cold with the chance of rain showers Friday and a chance of a rain/snow shower mix Saturday. The best chance of snow showers will be across northern KY.

The long term: I expect next week to start out cooler than normal but it appears that by the Ides of March, Spring will have sprung.

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