Tuesday, October 13, 2015

Small Rollercoaster Ahead (and frost!!!)

Public weather briefing

After a nice cold frontal passage and winds gusting over 40 mph yesterday, cool air surged in from the northwest for a cool Tuesday. Tonight the coolness will persist as clouds move out during the evening, leading to a clear and cold night. However, expect a warm up for Wednesday and Thursday as a high pressure system tracks from the Northern Plains into the Midwest, bringing warmer and drier air into our region. Another low pressure system and its associated cold front will move through Thursday morning. A high pressure system sliding in from Canada will lead to clearing as well as our first two autumnal frosts both Friday and Saturday overnight. Yet again, our cool weather will be transient, with a warm up expected Sunday as a warm front associated with a low pressure system over northern Manitoba passes through the region.


Tonight. Clouds clearing this evening. Cold. Low 38. Light winds northwest decreasing from 10 to 5 mph.

Wednesday. Sunny and warmer, with a high around 65. Winds west at 10 to 15 mph.

Wednesday night. Mostly clear and cool. Low near 43. Light winds from the northwest at 5 to 10 mph.

Thursday. Showers possible early morning. Mostly sunny and cooler. High of 58 and a low near 35. Northwest winds increasing from 10 to 15 mph to 15 to 20 mph during the afternoon.

Friday. Sunny yet cool during the day. Clear and cold with widespread frost overnight. High near 49 with a low around 27. Northwest winds at 5 to 15 mph.

Outlook for Saturday through Sunday. Widespread frost Saturday morning and again Saturday overnight. Sunny skies and a clearing overnight, with highs around 50 and lows in the upper 20's. For Sunday, frost will linger into morning, with sunny skies and highs in the mid-50's and lows overnight in the lower 40's. Southerly winds will increase from 15 to 20 mph during the day, to 20 to 25 mph overnight. [CONFIDENCE: MEDIUM]

Forecast Discussion

A high pressure system associated with strong zonal flow will move from the Northern Plains into the Midwest, leading to drier and warmer air over the region due to the air mass originating from subsidence down the lee-side of the Rockies. Early Thursday morning, positive advection of vorticity at 500mb by the thermal wind associated with a shortwave trough over Wisconsin will lead to upward vertical motion across southern Wisconsin. This shortwave is associated with a low pressure system in northern Manitoba The return of moisture to the low levels of the atmosphere will be vital regarding whether precipitation occurs.
A 1033 MB Canadian high which tracks from the Northern Plains into the Upper Midwest will dominate the weather Friday and Saturday, and will drive cold air advection over the region, helped by a broad planetary trough stretching from Manitoba to the Labrador Sea. Aided by radiational cooling, lows will be below freezing both Friday night and Saturday night, leading to our first two autumn frosts. High Pressure will move out of the region Sunday and a low pressure system will track towards the Midwest from the Plains, leading southerly warm air advection and increasingly stronger winds through the day.

Robert Kelnosky

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