Wednesday, January 28, 2015

Potentially Icy Conditions Tomorrow Morning

Public weather briefing

A small upper wave is expected to develop off of the Rockies and migrate south of the Wisconsin-Illinois border by tomorrow morning.  Fluxes of warm and moist air from the south will increase our chances of freezing precipitation during the early morning hours.  However, the type of precipitation we can expect to receive is largely a function of the upper wave’s development and track.  Precipitation amounts will be low (less than .1”).  NWS has not issued an advisory for the Madison area at this time, but practice safe driving and prepare for potentially icy road conditions during early morning commutes .


Tonight. Cloudy. Wintry mix of precipitation expected, chance of freezing drizzle. Southerly winds tapering off and shifting to northwesterly. Low 31.

Thursday. Cloudy. Chance of snow in the morning. Breezy northwesterly winds 20mph gusting around 30mph.  High 33.

Thursday night. Partly cloudy. Wind northerly around 10mph. Low 11.

Friday. Partly sunny. Wind southwesterly 6 mph. High 22. Low around 10.

Saturday.  Partly cloudy chance of snow in the evening, winds shifting to northwesterly 10mph.  High 31.  Low 12.

Outlook for Sunday through Tuesday. More snow likely on Sunday, highs will be around 20 and in the morning hours. Monday, colder and clear, highs in the low teens, wind chills near -10.  Tuesday, highs near 20, snow possible in the evening. [CONFIDENCE: LOW-MEDIUM]

Forecast Discussion

Currently, elevated subsidence in the 850mb-500mb layer associated with a passing upper ridge over Madison has built a significant dry/warm nose that is capping the boundary layer. Warm theta-e air will continue to accumulate in the lowest layers as southerly winds persist into the early morning hours.  Both the 12Z GFS and 12Z NAM agree that a lee trough will develop and reside over western Iowa around midnight. Thus, if the position of the lee trough verifies, mixing of the PBL below the 850mb inversion will be accompanied by upward QG forcing on the downstream side of the lee trough.  Resulting erosion of the dry/warm nose and near-freezing temperatures from the surface through 850mb will produce favorable conditions for the development of mixed precipitation and freezing drizzle after midnight tonight.  The amount of precipitation expected is largely dependent on the development of the lee trough, but should not exceed a tenth of an inch since precipitable water values will remain relatively low.

In order to more accurately forecast precipitation type, close attention should be paid to the development and track of the lee trough.  Its location around midnight tonight, as it is currently forecasted, arguably puts Madison in the area most favorable for mixed-precipitation and potentially icy conditions by tomorrow morning.  In the event the lee trough migrates further south, perceptible water values in our area will drop significantly along with the chance of freezing rain/drizzle. If the lee trough migrates further north, drizzle and sleet-like precipitation can be expected.

The uncertainty in the lee trough’s development and migration promote assigning a confidence value of low-medium to this forecast.

Scott T. Trevorrow

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