Wednesday, November 24, 2010

Travel Conditions Forecast To Sour This Afternoon

* Travel conditions gradually deteriorate as the day goes on. The drive home later today will be slow.

* Cold enough for mostly snow (a little freezing drizzle can't be ruled out).

* Best bet: 1-3" of snow by 9 or 10 pm tonight.

* Many interstates/freeways remain wet into the afternoon with surface temperatures ranging from 29-32, just close enough to freezing to keep (treated) roads more wet than icy. But after dark, after 5 pm or so, many of those wet/slushy roads will become icy.

Mostly Snow. The lowest mile of the atmosphere should be just cold enough for snow this afternoon into early tonight. A little ice may mix in south/east of MSP, but the predicted profile should keep precipitation falling as snow. That's good news. We were concerned about a thin layer of ice - which still can't be ruled out, especially south of the metro area. The best bet: 2", possibly as much as 3" of snow by early tonight with little accumulation after 10 pm or so.

Doppler Update: 10:30 am. Radar shows snow (mostly aloft) - evaporating before reaching the ground. Snow is now reaching the ground over far western MN - still on track to reach the metro area by early afternoon, getting heavier/steadier as the afternoon goes on. Strong upward motion overhead may spark a period of freezing drizzle by midday. We may have 1" of new snow on the ground by 4 pm, closer to 2" by 6 or 7 pm. With surface temperatures below freezing some secondary roads/bridges may become very icy between 11 am and 2 pm.

How Much? This seems like a reasonable solution: most of the metro area in the 1-3" range (more north/east metro, less south of the cities). As much as 6-10" will accumulate along the North Shore of Lake Superior. South of the metro this will be more of an icing event, a period of freezing rain/sleet likely this afternoon. With ice in the mix travel may actually be worse SOUTH of MSP than north (where precipitation will fall as all-snow).

Latest NAM Output. Hot off the wire - the latest amounts are less than the computer runs last night (which were predicting closer to .40 to .50" liquid). The latest model is printing out .23" of liquid. With a 10:1 ratio in place (temperatures in the lowest mile of the atmosphere just below freezing) that should translate into 2, possibly 3" of snow. Far northern/eastern suburbs may pick up closer to 4", less than 1" from Mankato to Red Wing (where more ice will mix in, keeping final snowfall totals down).

Reasonable Continuity. Our blood pressure drops (a bit) when all the models converge on a single solution, when all models are reasonably close in their snowfall outlook. Not sure how we see less than 1-2" across most of the metro, a few spots may pick up 3 or 4", especially from Anoka and Ham Lake to Hugo,  White Bear Lake and Taylors Falls. Yes, it still looks (barely) "plowable". It's not so much the amount - it's the TIMING of the snow & ice.

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