Sunday, November 9, 2014

Special Update: Significant winter weather event for northern WI, unseasonable cold mid-to-late week statewide

Public weather briefing

The winter weather event highlighted in last week's blog posts is emerging to be a threat for northern Wisconsin. As of Sunday evening, it appears that southern Wisconsin will not see much in the way of snow.  The northern half of the state could see snowfall approaching a foot by Tuesday evening.

Light snow and rain will develop over southern Wisconsin tonight and change to all rain in the early morning hours before tapering off in the mid-morning. With air temperatures at or above freezing and the ground still relatively warm, snow accumulation overnight in the southern half of the state is unlikely. Snow will develop in the far north by early morning and continue through most of Monday.  Driving in northern Wisconsin will likely become difficult with time by early afternoon. 

As a cyclone develops to our southwest on Monday afternoon and moves northeast - passing just south of Madison, another round of precipitation is possible Tuesday morning.  At that time, with falling temperatures, a brief period of light snow is possible.  

Significantly below normal temperatures (15F to 20F below normal) are expected for the latter half of the week into the weekend as a cold anticyclone slowly moves into the upper Midwest.  The next chance for measurable precipitation (likely in the form of snow) is for Saturday.


Tonight. Cloudy with light rain and drizzle. Light northeast winds before midnight, becoming easterly by dawn. Lows near freezing.

Monday. Cloudy with rain and rain showers ending early. Less than 0.03" liquid equivalent during the day. Winds east-southeast 10-15 mph. High 50.

Monday night. Cloudy with rain and rain showers, especially north of Madison.  Less than 0.05" liquid equivalent. Temperatures slowly falling from the mid-40's into the 30's by dawn.

Tuesday. Rain changing over to snow during the morning with little or no accumulation. Less than 0.1" liquid equivalent. Winds southwest shifting to northwest at 10 to 15 mph with temperatures falling from the 40's back into the lower 30's by mid-afternoon.   Low 15-19.

Wednesday. Mostly cloudy with a few sunny breaks. Snow showers possible. High 25-28F. Low 15F

Outlook for Thursday through Saturday. Partly cloudy Thursday and Friday. Light snow possible on Saturday.  Highs in the mid 20's Thursday, upper 20's to near 30 on Friday and in mid 30's on Saturday. Low's in the teens through the period. [CONFIDENCE: HIGH]

Forecast Discussion

A cold front will stall just south of Madison tonight before gradually retreating northward as a warm front early tomorrow morning.  Prior to the front's northward movement, a broad deformation pattern will begin to tighten the thermal gradient associated with the front.  The thermally direct circulation about the intensifying front will be associated with a band of light rain and snow starting over the southern half of the state overnight. 

The forecast challenge for tomorrow is the position of this front during the day as a surface and lower tropospheric cyclone develops to our southwest. North of the front, cold dry air will be advected southward. This dry air and associated evaporational cooling will thwart significant motion of the surface front northward. At the surface, significant warm advection ahead of the developing cyclone will push temperatures above 50F in southeastern WI. Aloft, all of sourthern WI will be in the warm sector of the 850 hPa cyclone. As a consequence, warm advection will push the 850 hPa 0C isotherm well north and east of the Dells making the largest threat for significant frozen or perhaps freezing precipitation likely in the northwestern and north-central parts of the state. The 0000 UTC 10 November NAM and 1800 UTC 9 November GFS are similar in this scenario, with the more recent run of the NAM more aggressive in bringing the warm air into the southern half of the state.

Following the passage of the cyclone on Tuesday, an arctic anticyclone will slowly build southward into the upper Midwest. Cold advection on the periphery of the anticyclone will keep temperatures well below average for the week. Broad cyclonic flow aloft will support the development and maintenance of low clouds on Wednesday. Significant precipitation is not anticipated for the latter half of the week.

Michael C. Morgan

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