Tuesday, November 4, 2014

Winter-like Pattern Setting Up

Public weather briefing

Diminishing clouds during the afternoon will lead to a cool night across the area tonight.  Temperatures drop back into the mid 30's with clear skies and light winds out of the southwest. Attention turns to a developing area of low pressure currently entering the US in northern Montana.  This system will continue to dive southeastward Wednesday night into Thursday.  The precipitation will begin as rain mainly after 6pm tomorrow, but should transition over to a wintry mix overnight, with a chance for snow showers by Thursday morning as the system passes to our east.  Highs Thursday will be significantly cooler, struggling to reach 40 with brisk northwesterly winds approaching 20mph.  A relatively dry cold front moves in Saturday morning, accompanied by well below normal temperatures, with highs in the upper 30's both Saturday and Sunday. Significant winter weather is possible as we head into the next work week.


Tonight. Mostly Clear.  Low 35.  Southwest winds at 5 to 10 mph.

Wednesday. Increasing clouds. PM rain showers.  High 55.  Southerly winds around 10 mph.

Wednesday Night. Rain, changing to a wintry mix. Low 35. South wind 5-10mph, shifting westerly during the overnight. Precipitation amounts around a tenth of an inch.  Little snow accumulation expected. 

Thursday. Morning show showers.  Cloudy skies, high around 40.  Gusty winds out of the north at 15 to 25 mph.

Friday. A few passing clouds. High around 43. Winds light and variable

Outlook for Saturday through Monday.  Slight chance of precipitation Friday night into Saturday.  Unseasonably cold Saturday and Sunday, highs around 40. A strong area of low pressure looks to impact the Great Lakes Monday into Tuesday, with accumulating snow possible. [CONFIDENCE: LOW]

Forecast Discussion

Clear skies expected tonight. Dew points sit around 36 degrees, which should allow lows to fall back into the mid 30 overnight. Winds out of the southwest allow for warm air advection during the afternoon.  Increasing clouds throughout the day should suppress daytime heating, even with a 850mb temperature around 6C. 

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 suggest upward vertical motion.  The GFS is outputting significant Q vector convergence along the track of the system.  Consequently, vertical motion can be expected and is forecasted by both the NAM and GFS.  Both models 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 upward vertical motion directly over Madison Thursday morning.  
Models generally agree on the extent of the cold for Saturday and Sunday behind a cold front expected to pass through the area Saturday morning. Cold weather can be expected with highs in the upper 30's to around 40.  The average temperature over this period is 48.
Significant forecast challenges exist at the end of the forecast period. The ECMWF shows a full blown winter storm with a sea level pressure of below 998mb and half a foot of snow.  Meanwhile, the GFS plots a similar system but passed the surface low pressure north of Madison, giving northern Wisconsin several inches of snow.  This system bears watching.

Below is a plot from The College of DuPage.  It shows the significant winter storm forecast by the EURO model Tuesday morning, and substantial cold air advection on the back side of the cyclone as the pushes east.

Furthermore, here is the 12z ECMWF snow output for the Monday possible winter storm, can we all gasp out a collect "WOW":

And the 18z GFS:

Courtesy of Weather bell.


  1. Since we're high and dry here in the west for the foreseeable future - unfortunately yet again - I was looking at your Monday storm. This is a fun lead time in the forecast process to make use of the ensemble uncertainty guidance. Example: ECMWF normalized spreads. Here's a link to the 144 hour forecast valid Tuesday 0z - http://www.ecmwf.int/en/forecasts/charts/medium/ensemble-mean-and-spread-four-standard-parameters?area=North%20America&step=144&relative_archive_date=2014110500&parameter=MSLP Definitely some enhanced spreads on the north side of the mean low pressure center - indicating variability in the track especially in latitude - perhaps suggesting the storm could track further north than currently forecast. Of course this guidance doesn't give us any sense as to how likely a northward shift is and that would be helpful. I definitely agree with your low confidence assessment for snowfall potential.

    1. True, the ECMWF is the furthest north of the major models, with the GFS further south. Will be interesting to see how this pans out, but if you take a mean of the GFS and ECMWF it puts Madison in the bulls eye for the heaviest snow!

  2. Nice forecast for the next 5-7 days. Tough call for the system for next week Monday. The 12Z run of the ECMWF still shows a storm system but now slightly warmer and further north but the GFS shows nothing but a baroclinic like light snow. Definitely low confidence.

  3. Our forecasted highs and lows were fairly accurate, with Wednesday observed highs only 1 degree warmer than forecasted. Precipitation forecasted for Wednesday night into Thursday occurred although the amounts varied. The high on Thursday was about 7 degrees warmer than predicted, but Friday's highs were
    almost spot on.

    The major winter storm tracked further north than expected. The southward extent of the cold Canadian air mass was over forecasted in prior runs on the major weather models. The European was the first model to jump on the northward trend of the system. Consequently, our thoughts for Monday did not come to fruition, as highs reached the upper 50s Monday and will be likely be the warmest day til next Spring.

  4. Tanner and Joe - Congrats on a nicely done set of blog posts for round 2! I think you guys did a particularly nice job of discussing the relevant forcing for precipitation in our area throughout the duration of your forecast period and some of the nuances relating as to whether the precipitation will be frozen or liquid. Furthermore, nice job discussing the differences between some of the model forecasts and how that translated to the realization of your forecast. These are exactly the types of things that we want you to think about over the course of these posts, so great job with that. The only thing I would suggest is to make sure that you're specific with regards to some of the sensible weather your describing. For example, instead of saying that winds will shift across the front, make sure that you describe which directions the winds will shift from. Additionally, as far as the reflection goes, you note that there was roughly a 7 degree difference in your temperatures for Thursday. It would have been nice if you included a discussion as to why this was so drastically different than what you had expected. Nevertheless, a pretty solid job overall.


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