Scientific Forecaster Discussion

000 fxus63 kmpx 221756 afdmpx Area forecast discussion National Weather Service Twin Cities/Chanhassen Minnesota 1156 am CST Fri Feb 22 2019 Update... issued at 1140 am CST Fri Feb 22 2019 Updated for the 18z aviation discussion below. && Short term...(today through saturday) issued at 429 am CST Fri Feb 22 2019 A couple to a few inches of snow are likely later today and tonight across the region. This snow will gradually diminish Friday night, and could see a transition to freezing drizzle as we lose ice crystals in the clouds. This could produce a light glazing of ice Saturday morning and early afternoon. By late Saturday afternoon, the powerful winter storm will lift up from the south and bring deeper saturation which will transition the drizzle back to snow. We still expect a narrow band of heavy snow across southeast Minnesota into west central Wisconsin. On Saturday night, the surface cyclone deepen across eastern Wisconsin, and surface high pressure will build across the Dakotas. This will tighten the pressure gradient, and increase the isallobaric component of the wind resulting in northwest winds of 30 to 35 mph, with gusts of 40 to 45 mph possible, and even 50 mph in the open areas of western, central, and southern Minnesota. The recent snow, together with an additional snow that falls today and tonight, will be subject to blowing and drifting with these strong winds. The one caveat is how much icing accumulates on the snow Saturday. Have expanded the Winter Storm Watch given the potential for blizzard conditions, but there is still uncertainty in the forecast. Part of the uncertainty is in the storm track and intensity. The 22.00 GFS/Gem/ECMWF are in very good agreement with a sub-980mb surface low over the up of Michigan by 12z Sunday morning. Meanwhile the 22.00 and 22.06 NAM continues to be more eastward and weaker with the surface low. The difference in a few millibars and a hundred miles in the surface track is significant, as can be seen by the forecast soundings. The NAM shows winds of 35 to 40 kts at the top of the boundary layer, while the GFS shows winds of 45 to 50 kts. If the NAM were to verify, there would still be blowing snow, but blizzard conditions would be unlikely. If the GFS were to verify (and Gem/ecmwf) then expect significant blowing snow and widespread blizzard conditions Saturday night into Sunday. Although forecast soundings off the European model (ecmwf) aren't readily available, the h925 winds show a large corridor of over 50kts, which is stronger than the GFS, so one can deduce that winds will be just as strong if not stronger than the GFS. Blended in some European model (ecmwf) into the forecast to get higher winds that should be more representative of what we can expect Saturday night and Sunday. The icing uncertainly is also a factor. If it occurs, it's tough to say how much a few hundredths of icing would inhibit the blowing and drifting. One would venture to say not that much if we get wind gusts of 40 to 50 mph for several hours. This snow pack is relatively fresh and has not been aged by the late February sun, so it still should be able to be lofted by strong winds. Plus ditches are full. From a big picture perspective, this storm is already producing significant sensible weather impacts with record snows across the Desert Southwest. Temperatures on Sunday will be falling, with overnight lows Sunday night expected to be below zero across the region. With that in mind, it seems that this storm has the potential to over-achieve in terms of sensible weather across the upper Midwest as well. This is a similar set up to the February 21,22 2014 blizzard the closed roads across much of southern Minnesota, and in fact that storm is the #2 match on the cips analog. The only difference is the 2014 storm track was about 75 miles west of where this one should be, meaning the heavy snow also occurred farther west than what we have forecast for this storm. In the 2014 storm, blizzard conditions developed all the way to the South Dakota border, so that gave a little extra confidence in expanding the Winter Storm Watch for this storm, despite hardly any snow. It is possible that the Twin Cities Metro could also see significant impacts from this storm, especially if the track shifts west, but at this time did not include those counties in the watch. Long term...(saturday night through thursday) issued at 429 am CST Fri Feb 22 2019 After this weekends storm, most of our region should be less impactful in terms of large snowfall events. However, unseasonably cold temperatures are expected which could be record cold in some areas. The record cold will be dependent on when the core of the coldest air moves overhead. At this time, the Front Range of the northern rockies has the best chance of experiencing these record cold temperatures. Our region, based on the latest guidance, has temperatures averaging on Monday/Tuesday nearly 20 to 30 degrees below normal. The normal highs and lows are the lower 30s, and upper teens respectfully. By the end of next week, another surge of cold air moves southward across the upper Midwest. This air mass is even colder then early next week. Due to the abnormally high snowpack, I wouldn't be surprised to see temperatures even colder than current models indicate. The mean upper air pattern shows this type of pattern (an anomalous upper trough centered across southern canada) which leads to colder air masses vs, more milder Pacific through the first week of March. The best chance of accumulating snowfall will occur next Tuesday as a stronger wave at mid-levels of the atmosphere, moves across the upper Midwest. This system will be moisture starved as the cold air mass remains overhead, but the mid-level forcing is near the dendritic growth zone which could overcome the lack of moisture. Currently, the best forecast is to include likely percentages with a quick 1 to 3 inches of snow possible based on the quantitative precipitation forecast amounts, and mid-level forcing. Otherwise, no major storms are expected past this weekend. && Aviation...(for the 18z tafs through 18z Saturday afternoon) issued at 1140 am CST Fri Feb 22 2019 Strong Theta-E advection ahead of the amplified trough across The Four Corners region that will begin lifting northeast will quickly saturate the lower atmosphere this evening and precipitation will begin lifting into the area as well. This afternoon should be fairly quiet, but conditions deteriorate this evening. Still expect snow accums in the 1-3" range generally, with period of IFR/LIFR overnight. Fog/haze will be a possibility again by early tomorrow morning as the precip exits. Freezing drizzle is expected for a few hours on the tail end of the precip toward morning as well. Kmsp...thinking freezing drizzle is possible tomorrow morning, but continued the prob30 as confidence isn't high yet. /Outlook for kmsp/ Sun...morning snow/IFR likely. Wind northwest 25g40kts. Mon...MVFR cigs. Winds light & variable. Tue...MVFR with -sn likely. Winds east 5-10 kts. && Mpx watches/warnings/advisories... WI...Winter Storm Watch from Saturday evening through Sunday afternoon for wiz015-016-023>028. Minnesota...Winter Storm Watch from late Saturday night through Sunday afternoon for mnz054>058-064>070-073>076-082-083-091. Winter Storm Watch from Saturday evening through Sunday afternoon for mnz077-078-084-085-092-093. && $$

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