Thursday, December 11, 2014

Cool, dismal weather persists

Public weather briefing

Weak southerly to southwesterly flow around a high pressure area centered over Tennessee and Kentucky will continue to provide weak subsidence over southern Wisconsin through Friday. While clearing skies today teased us with the prospect of warmer weather, the presence of a strong inversion will keep the atmosphere from mixing warmer air aloft down to the surface. The continued southerly flow will gradually moisten and provide increasing clouds and the increasing threat of drizzle  or light rain later in the weekend.

The weather system bringing much need rain to the western US will head eastward across the Rocky Mountains and emerge into  southern Plains on Sunday morning before heading northeast toward southern Wisconsin later Sunday. Initially the precipitation associated with the weather system will be rain.  As the system tracks over then northeast of the state on Monday night, colder air will return - changing the rain to snow early Tuesday morning. Clearing will follow Tuesday afternoon with northwesterly winds. Colder, but likely dry weather is expected for Wednesday


Tonight. Mostly cloudy and cold. Light southwest winds 5 mph. Lows 23.

Friday. Partly to mostly cloudy and cool. Light southwest winds 8 mph. High around 37.

Friday night. Cloudy.  Low 27.

Saturday. Partly to mostly cloudy. Milder. Light winds. High 45. Low around 33.

Sunday. Cloudy with light rain possible. Rain developing late. High near 47. Low. 38

Outlook for Monday through Wednesday. Rain Monday High around 40. Rain changing to snow late Monday night and early Tuesday. Snow and snow showers on Tuesday, high around freezing early with steady then slowly falling temperatures Tuesday afternoon. Partly cloudy and breezy Wednesday, high 30, low 20. [CONFIDENCE: MEDIUM]

Forecast Discussion

An upper ridge crests over the upper Midwest late week providing subsidence while further strengthening the stubborn inversion over the area. With the ridge firmly in place, surface temperatures will be slow to rise;  while at 850 hPa, temperatures will rise from 4C this evening to 11C by Saturday afternoon. Advection of warm and an increasingly moist air from the southern Plains will increase clouds and the threat for light rain on Sunday.

A potent shortwave trough will approach the area on Monday. The arrival of the sharp trough will be accompanied by significant upper tropospheric divergence and concomitant ascent. With significant northward moisture transport from the Gulf, a descent rainfall is possible late on Monday.  The 0000 UTC 12 December  GFS tracks the surface cyclone associated with the upper trough more to the north than in earlier runs.  This track will hasten the transition of rain to snow relative to earlier forecasts and, with subsidence behind the upper trough, potentially clear the area sooner on Tuesday.  

Forecast confidence in the medium range is medium due to the uncertainty in the track of the shortwave from the southwestern US this evening to the midwest Monday night.  This uncertainty is manifest in the changing model evolution seen in recent runs. 

Michael C. Morgan

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