Tuesday, January 27, 2015

Milder Temps and Moist Weather Ahead

Public weather briefing

The large trough centered over the northeastern US has already begun pushing off into the northern Atlantic. Upstream, ridge building in the Great Lakes region will increase the surface pressure over the Appalachians and help direct surface winds into our region from the South.  Winds will begin to shift to a more southerly orientation as the day progresses today and through most of Thursday.  This wind will bring anomalous moisture and mild temperatures to the Madison area, ultimately increasing the chance for mixed precipitation and potentially icy conditions later this week. 


Tonight. Cloudy. Light winds shifting from easterly to southeasterly. Low 20.

Wednesday.  Partly sunny. Southerly-southeasterly winds 15 mph. High around 35.

Wednesday night.  Cloudy with chance of freezing drizzle. Low 31.

Thursday.  Cloudy with mixed precipitation possible. Wind northwesterly 20 mph. High 35. Low around 20.

Friday.  Colder and clear, northerly winds around 10 mph. High 24. Low 13.

Outlook for Saturday through Monday.  Chance of snow on Saturday especially in the evening, high in the upper 20s. Chance of snow continues into Sunday, high in lower 20s.  Colder temperatures begin Sunday night and into Monday, Monday’s high around 10. [CONFIDENCE: MEDIUM]

Forecast Discussion

A large upper ridge currently sits over much of the central US and into southern Canada.  As the associated surface high migrates southeastward from the upper Mississippi valley to the Appalachians, a zonally-oriented pressure gradient in our area will strengthen.  Winds will consequently shift to near-southerly tonight and into tomorrow, advecting high theta-e air from the south into the Madison area.  Resulting conditions tonight and tomorrow will be mild and moist. With most of the ground still snow-covered, fog may develop during the late hours of the night when the high theta-e air arrives and warmer 850mb temperatures cap the boundary layer.

Both the GFS and NAM show the development of a shortwave trough on the lee side of the Rockies tomorrow.  Both models show some confidence that the trough will migrate over the Madison area late Wednesday night.  During the night hours, temperatures may hover around freezing up through 850mb, so a potential for freezing drizzle exists.  On Thursday, as the shortwave continues to migrate eastward, expect the snow to liquid equivalent to be under 10:1 with average temperatures being in the low 30s.
Forecast confidence in the medium range is medium due to the uncertainty in both the lee shortwave trough’s development and the magnitude of moisture advection over the next day.

Scott T. Trevorrow

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