Thursday, January 21, 2010

Tracking a major slop-storm

Just once I'd like a winter storm that is black and white, not some nebulous, hard-to-fathom shade of gray. Here's the sad, exhilarating truth about meteorology: weather patterns may be similar, but every day is a new creation, every storm a new challenge. The truth? The following forecast should come with a disclaimer, like a pack of cigarettes. The computer models we rely on (for any forecast beyond 24 hours) are wildly divergent, one of them is definitely out-to-lunch. The GFS is hinting at a foot of snow for much of central Minnesota (see below), while the NAM model is only predicting 3-5" for St. Cloud, and 2-3" of heavy, wet, slushy snow for the Twin Cities.

More colors = more weather headaches. Check out the dizzying weather menu of watches, warnings and advisories - blizzard warnings as far south as Mexico (!) The same storm that battered California will tap moisture from the Gulf of Mexico before turning north toward Minnesota, shoving a shield of ice, rain and snow into the state (in that approximate order) tonight through Sunday. Get ready for a very sloppy weekend, everything under the sun (except the sun). For the very latest NOAA warnings click here.

I'm leaning toward the (lesser) NAM solution, a few inches of snow Sunday into Monday morning during the last half of the storm, a significant percentage of precipitation falling as rain, freezing rain and sleet, keeping ultimate, final amounts down. A few stormy headlines that are top of mind:

1). The farther west you live in the great state of Minnesota the heavier the final snow amounts. I still think parts of far western Minnesota, from Windom to Wheaton to Detroit Lakes and Moorhead could wind up with 6-12" of snow by the time the flakes stop flying Monday.

2). Rain and ice. A southerly surge of unusually mild air aloft should switch snow/ice over to mostly rain much of Saturday over all of southern and central Minnesota. The only problem? Ground temperatures may still hold below freezing, causing rain to freeze on contact with cold surfaces: trees, powerlines and highways. A period of significant icing is very possible from southwestern Minnesota (where there is still 15-22" of snow on the ground) into parts of central Minnesota Saturday. The farther north/west you travel, away from St. Cloud on Saturday, the better the odds of running into some icy patches.

3). By Sunday most of the precipitation will fall as snow, and travel conditions will get progressively worse as the day goes on. I'm nervous about all those Viking playoff games scheduled for late Sunday. Getting to the party will be tough enough, getting home could be a real mess. Leave extra time to get around, especially Sunday PM hours.

4). Final snowfall tallies are very much up in the air, but one thing is inevitable & unavoidable: colder air will get sucked back into Minnesota on the backside of the storm; we'll all be shivering and complaining about the (%$&#*%@#!!) wind chill by the middle of next week. Our 30-degree party has a definite shelf-life, within 72 hours it will feel like an old fashioned Minnesota January.

5). Temperatures run a few degrees below average from Jan. 26-31, about 4-5 numbing days (but NOT as cold as early January was). Long-range models are hinting at more 20s and 30s by the first week of February, just in time for Groundhog Day!

A tale of two weather models. The GFS model is hinting at some 8-12"+ snowfall amounts by Monday over the northwestern half of Minnesota. The farther east the storm tracks, the less warm air will reach into central and southern Minnesota, meaning less rain/ice and more snow.

Reality check? The NAM model isn't nearly as impressive in terms of final snowfall amounts, suggesting a more westerly storm track, a longer, more widespread period of rain/ice that will keep final snowfall tallies lower. The NAM solution is hinting at 3-5" for St. Cloud, maybe 1-3" for the Twin Cities. I have a hunch the NAM will be closer to reality.

Current snow cover. I'm concerned about the 18-24" of snow on the ground over far southwestern Minnesota, south/west of the Minnesota River. The deeper the snow, the greater the "refrigerant" effect: the snow cooling the air from below. Where this becomes critical is when the air temperature is close to freezing. A heavy snow pack might favor keeping temperatures a few degrees cooler, favoring freezing rain vs. (liquid) rain. Roads may stay icy (longer) over far southwestern Minnesota Saturday, especially during the morning hours. The latest DNR information is here.

We'll get a (minor) break today, a little freezing drizzle leaking out of a slate-gray sky at times, but with temperatures in the upper 20s to near 30 major roads (certainly all the freeways) should be mostly-wet, the commute not too terrible out there. A surge of heavier precipitation arrives Friday night, and it may start as a mix of wet snow, sleet (ice pellets) and some freezing rain - changing over to rain during the day Saturday as temperatures edge past 32 F. south of a line from St. Cloud to Princeton, some mid 30s are possible from the Twin Cities to Rochester and Winona, keeping roads mostly-wet. If your Saturday travels take you to Brainerd, Wadena or even Willmar, the odds of running into patchy ice will increase the farther north & west you drive. You have been forewarned. At least by Sunday precipitation will be snow - and traction on snow is an order of magnitude more manageable than trying to slip and slide around on glaze ice.

It sounds like cheap, promotional hype (not above that, btw) but we scan 4 new computer runs/day. If the storm zigs farther west more warm air will get wrapped into the storm's circulation - we'll wind up with more rain, less snow. But if the storm zags 50-100 miles farther east, then the surge of warm air will be focused on southeastern MN and Wisconsin, keeping more of our precipitation falling as snow, and suddenly the (much snowier) GFS solution could wind up verifying. Insert deep sigh here. The bottom line: prepare for a very sloppy weekend. I'd recommend zipping out to the store, getting errands done today, because I have a hunch travel conditions will slowly deteriorate as the weekend goes on. Mostly rain & ice Saturday, turning over to mostly snow on Sunday as temperatures fall through the low 30s into the 20s. Best case scenario (for snow lovers): far western Minnesota gets plastered with a foot of snow. Worst case: the storm is strong enough (and far enough west) for warm air to reach western counties, keeping amounts in the 4-8" range by Monday morning.

Check back often, the one thing that seems certain: the forecast will change as the (final) storm track crystallizes. Looks like a good weekend to hibernate, maybe watch a little football Sunday? Be careful out there...

* Icy conditions for the U.S. Pond Hockey Championship today and Saturday, changing to mostly rain for a time Saturday, before changing back to snow Sunday. Leave extra time to get between events.

* St. Paul Winter Carnival: temperatures rise above 32 by 10 am Saturday, some melting of snow/ice sculptures is probably inevitable the first half of the weekend, but the mercury should drop below freezing again by the breakfast hour on Sunday morning. No problem next week as temperatures tumble through the 20s into the teens. Yes, Boreas will kick some serious butt next week.

Paul's Outlook for the Twin Cities

Today: Overcast (windy) with patchy fog, drizzle and freezing drizzle. Roads may be icy in the outlying suburbs. Winds: East 10-20, gusty. High: near 30

Tonight: Steadier winter mix moves in, wet snow, sleet and freezing rain. Very icy conditions possible, especially in the outlying suburbs. Low: 28

Saturday: Ice quickly changes over to mostly rain - freeways/major highways become wet, but roads from southwest through central counties may be very icy in spots. High: 35

Saturday night: Mostly rain, mixing with wet snow late. Low: 33

Sunday: A changeover to snow, potentially heavy at times - accumulating by PM hours. High: 33 (falling into the 20s by evening).

Monday: Light snow tapers to flurries, total accumulations of 2-4"+ possible (a half foot possible near St. Cloud, with some 8-14" amounts possible far western MN). High: 23

Tuesday: Sunshine reappears - much better travel conditions. High: 18

Wednesday: Subzero start, bright sun. High: 17

Thursday: Partly cloudy, a few degrees cooler than average. High: 16

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