Wednesday, December 10, 2014

Unseasonably Warm Temperatures on tap for the Weekend

Public weather briefing

Cloudy and rather calm conditions will prevail for the remainder of the week with slowly warming temperatures as an area of high pressure moves off to our east. The persistence of light winds into the weekend will encourage the development of patchy fog during the overnight hours. The next chance of precipitation will be during the latter half of the weekend as light showers may develop in conjunction with warm front pushing north through southern Wisconsin on Saturday night. A developing low pressure system brings a renewed, and more likely chance, of rain on Monday with a transition to light snow showers and cooler temperatures around normal (32) on Tuesday.


Tonight. Cloudy with areas of fog, particularly in low-lying areas. Low temperatures around 22 with light and variable winds.

Thursday. Mostly cloudy with light west-northwesterly winds around 5 mph. High around 31.

Thursday night. Mostly cloudy with light westerly winds. Areas of patchy fog.  Low around 26.

Friday. Mostly cloudy with west-southwesterly winds 5-10 mph. High around 39. Patchy fog developing overnight with lows around 30.

Saturday. Mostly Cloudy with a chance of showers developing Saturday night. West-southwesterly winds 5-10 mph. High around 43. Low around 39.

Outlook for Sunday through Tuesday. Slight chance of rain showers on Sunday with greater chances of rain on Monday. Rain should taper off Monday night and transition over to light snow showers. Mostly cloudy skies should prevail on Tuesday with a lingering chance of flurries.  Temperatures should rise as high as the mid-to-upper 40's on Sunday and Monday before falling back into the lower 30's for Tuesday. [CONFIDENCE: MEDIUM-HIGH]

Forecast Discussion

The persistence of a 500 hPa ridge axis to the west of Wisconsin has positioned a region of mid-tropospheric subsidence firmly over southern Wisconsin for the past couple of days, producing a rather strong temperature inversion around 850 hPa in the 12Z soundings at GRB and DVN. With a surface anticyclone located over Illinois, light winds have struggled to erode the inversion, indicating that cloudy conditions are likely to continue overnight into Thursday. With the lower-troposphere near saturation, areas of patchy fog are likely to develop.

Throughout the day Thursday and Friday, the surface anticyclone over Illinois is forecasted to slide slightly off to the east, allowing for winds to increase slightly, though by not much, towards the end of the week and aiding in the erosion of the strong temperature inversion above the surface. West-southwesterly winds will also advect slightly warmer temperatures into southern Wisconsin into the day on Friday. With winds remaining light during the overnight hours, patchy fog will once again become a forecast issue.

The passage of a warm front on Saturday will help foster the chance for light rain showers on Saturday evening in conjunction with an area of Qs convergence. At the moment, areas north and west of the Madison area are most likely to experience these rain showers. A greater chance of rain will occur Sunday night and into Monday with a cyclone forecasted to develop over the Central Plains. The GFS depicts the deepening and tightening of a trough now located over the central Pacific as it propagates eastward into the central US. Strong PVA associated with this trough will encourage the development of a surface cyclone and active lower-tropospheric frontogenesis along the warm front stretched over southern Wisconsin. Temperatures will likely be above freezing throughout much of Monday so rain is expected over southern Wisconsin at this point. A transition to light snow showers is expected as the cyclone slides to our south and east on Monday night into Tuesday with temperatures falling back to around normal in the lower-30's. 

Andrew Winters

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