Monday, November 3, 2014

Morning Showers Clearing for your Election Day

Public weather briefing

Scattered showers move in for the overnight associated with a cold front that will pass through around 3:00am or 4:00am Tuesday morning.  A couple tenths of an inch of rain can be expected, with rain coming to an end around the time the polls open Tuesday morning.  The winds shift to a more northerly component following the passing of the front, ushering in cooler, drier air.  Winds could gust as high as 20 to 30 mph, with sustained winds around 10mph. A few breaks in the clouds can be expected during the afternoon hours.  Attention turns to a developing area of low pressure currently located just off the coast of British Columbia.  This system will dive southeastward Wednesday night into Thursday, bringing a chance for snow showers Thursday morning as the system passes to our east.  Highs Thursday will be noticeably cooler, reaching only the mid forty's with brisk northwesterly winds approaching 20mph sustained.  A relatively dry cold front moves in Friday night into Saturday,
accompanied by well below normal temperatures, with highs in the 30's both Saturday and Sunday.


Tonight. Showers, low 49.  Winds out of the south shifting to the west, 10 to 15 mph. Total precipitation 0.20 inches.

Tuesday. Decreasing clouds.  High 54.  Winds northwest at 10 to 15 mph.

Tuesday night. Mostly Clear.  Low 35.  West winds at 5 to 10 mph.

Wednesday. Increasing clouds. PM rain showers.  High 56.  Southerly winds around 10 mph.

Thursday. Morning wintery mix.  Cloudy skies, high around 45.  Gusty winds out of the north at 15 to 25 mph.

Outlook for Friday through Sunday.  Cool temperatures Friday with highs in the mid 40's.  Chance of precipitation Friday night with a better chance as we head into Monday.  Unseasonably cold Saturday and Sunday, highs in the 30's. [CONFIDENCE: MEDIUM]

Forecast Discussion

A surface cyclone will pass to our north tonight with a strong cold front pushing through southern Wisconsin around 3:00 or 4:00am tonight. A linear band of showers accompanied by a line of baroclinicity and positive vorticty advection will affect the area through about 4am, according the hrrr and hi-res NAM. As the cyclone passes to the east, cold air advection will ensue with northwesterly winds. An area of high pressure and subsidence will push into the area for Tuesday into Wednesday.

The next significant cyclone pushes in for Wednesday into Thursday as a strengthening area of low pressure drifts southeast from southern Canada. Localized warm air advection ahead of the system will warm us up into the mid 50's and suggests upward vertical motion. The GFS and NAM both have a chance of showers late Wednesday evening ahead of the passing cyclone. As the cyclone pulls away from Southern Wisconsin, cold air advection will result from the shifting of winds on the back side. While there is cold air advection, it is not localized. A significant amount of positive vorticity advection is forecasted by the NAM and GFS, which will spawn the chance of snow showers with light accumulations, as 850mb temperatures are below zero and 1000-500mb thickness values drop to 5340 meters. The GFS plots vertical motion directly over Madison Thursday morning. A dusting to maybe an inch of snow is possible early in the morning Thursday. The image below is a plot from the 18 UTC GFS initialized November 3rd, 2014, valid 06 November at 18 UTC. It shows the chance of snow showers behind clipper. The NAM lags the system relative to the GFS by about 18 hours.


Joe Nettesheim
Tanner Verstegen


  1. "The NAM lags the system relative to the GFS by about 18 hours." What causes such a substantial difference in the timing according to the two models?

  2. You did a great job explaining what is going on with first front the causes of precipitation with frontal genesis, and positive vorticity advection. Good describing the chance of snow on Thursday and why. My one question is what is going between Tuesday morning and Wednesday evening? You state that it will be clearing out but you don't state why.

  3. Hi Liz,

    That is a very good question. We are not entirely sure on the answer, however it might have to do with the NAM having different initial conditions when compared to the GFS. I think this is an active area of research that may be past the scope of this class.

    Joe and Tanner

  4. Hey Andy, We wanted to point out the main weather features in the discussion instead of hammering down every time interval. So we left out Wednesday, which will feature increasing clouds ahead of a clipper-like system with warm air advection. It will warm us up into the mid 50s, before the clouds roll into the area late in the afternoon. An increasing chance of showers works its way into southern Wisconsin after sunset, we should kick over to a wintry mix during the overnight and maybe flat out snow Thursday morning.

  5. Now that 3 models (ECMWF, GFS, CMC) are showing a winter storm for Monday into Tuesday excitement levels are on the rise!! Will be fun to see how much amplification takes place and whether or not the models are accurate.

  6. Looking forward to maybe seeing some snow! The GFS and ECMWF models are showing the 5400m thickness contour hovering right over/slightly to the south of Madison for the Monday into Tuesday forecast period, as well as the 850mb temp being right around freezing. It'll be interesting to see at what point the projected precipitation turns from rain to snow. Hopefully it'll become easier to tell over the next few days!


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