Monday, October 27, 2014

Another cold Halloween in store for Madison

Public weather briefing

After a beautiful Monday in the 70's, mother nature will bring back normal fall temperatures continuing through the week, with high temperatures dropping into the 50's. A surface cyclone currently centered over Madison will propogate eastward through the forecast area later tonight. The cold front associated with this cyclone will pass through Madison around 9pm tonight, with winds shifting from nearly southerly to westerly over a three hour period. This front will bring a chance for scattered thunderstorms before midnight, with precipitation becoming lighter and more scattered through the early morning hours. Higher winds will advect cold air out of the west starting on Tuesday, dropping the low Tuesday night to around 35 degrees. A broad anticyclone dominates the region for most of the rest of the week. Temperatures will be a bit lower than average, with highs only getting into the high 40's for Wednesday and a low Wednesday night in the mid 30s. Thursday warms up a bit with a high in the low 50s, but for Halloween on Friday a cold front pushes through that could bring low temperatures Friday night down into the 20s.


Tonight. Chance of showers, winds shifting to westerly around midnight at 10 mph. Temperatures dip with a low around 47.

Tuesday. Mostly sunny with winds out the west at 10 to 15 mph. Gradually dipping temps with high around 55.

Tuesday night. Partly cloudy with winds out of west at 10 to 15 mph.  Low in the high 30's.

Wednesday. Sunny, winds shift to northwesterly at 10 mph. High of around 47, low around 35.

Thursday. Partly sunny, temperatures stay fairly steady with high in low 50's, low in mid 30's. Light winds out of the west at 5 mph.

Outlook for Friday through Sunday. Clear skies and sunny on Friday with a high of 43 and low of 27 with winds out of the north at 10 mph. Clear conditions continue through Saturday, high in mid 40's again and low around 33, winds out of the south at 10 to 15 mph. Showers possible on Sunday, steady temps with high near 50, low around 40.  [CONFIDENCE: MEDIUM]

Forecast Discussion

The surface cyclone located over Madison will track northeastward between 6Z and 12Z Tuesday and strengthen considerably as it crosses into eastern Canada. 850 hPa frontogenesis has a local maximum at 0Z just west of Madison, indicating significant strengthening of the cold front associated with this cyclone as it passes through the forecast area. This is backed up by the GFS 700 hPa vertical motion between 0Z and 12Z Tuesday (6Z shown below), which clearly shows the development of a thermally direct circulation in the vicinity of the front. Since this development occurs largely after the front passes Madison, we will be in a broad region of subsidence Tuesday into Wednesday, clearing skies significantly after a potentially stormy night tonight due to precipitation associated with the front. The GFS has Madison getting about a tenth of an inch tonight, while the NAM is dry. Overall the models agree well on the development of the baroclinic zone in the next 24 hours, but the NAM puts the front east of our area before strengthening. So, our forecast confidence is medium, due to this disagreement. 

After the passage of this cyclone, a broad anticyclone will be over the region for much of the week. The 300 hPa jet is quite weak throughout North America, but there is a strong ridge bringing north-northwesterly flow to the area. The GFS 850 hPa temperature shows localized cold air advection over the forecast area all day Tuesday and into Wednesday morning, giving us clearer skies than earlier on Monday due to weak subsidence. Flow will then shift to be roughly zonal for the latter half of the week, with little likelihood for cyclogenesis upstream of the area indicated in either the GFS or the NAM. The final feature of interest is the development of a trough and coincident baroclinic zone in southern Canada Thursday night, which could result in some precipitation Friday morning due to differential vorticity advection induced by the intense northerly flow downstream of a ridge over the central plains. However, this term of the classical QG omega equation is opposed by very localized cold air advection over the same region, so any precipitation forecasted as a result of this is up to interpretation. At this time Madison is right around the 5400m 1000-500 hPa thickness line, so areas just north of the area have a good chance of seeing some flurries, but likely no accumulation.

Alex Haugstad
Conner Hardesty

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