Team 3 Halm, Chyba, Bard

Mesoscale Discussion ~48 Hr forecast (Tuesday) Valid 12Z THURS FEB – 00Z FRI FEB Cold front associated with surface low located in Eastern Canada is driving the cold air from the north into the Chicago area in early Thursday morning (THURS 12Z UTC). As day progresses, another surface low comes in from the west plains which will bring warm moist air to Chicago which will lift over the cold air creating precipitation (FRI 00Z UTC). Also, there will be frontogenetical circulations inducing vertical motion from an upper level front caused by ageostrophic forcing as well throughout the day. In the morning, snowfall will occur in the northern Chicago area. Mid to southern Chicago area will receive freezing rain. As day progresses, northern Chicago area will receive freezing rain while the rest of Chicago will transition to rain. Estimated accumulation will be about 0.5 inches of precipitation. Early on, conditions may be hazardous for travel due to icy roads. As day progresses, this risk will lower. Based on comparison with other models, the storm track and storm intensity appear to be in agreement. A disagreement appeared pertaining to the rain/snow line. The freezing line in other models appeared farther north than the freezing line in the first model.

Written Forecast (Tuesday) Weather Forecast for 6:00 AM February 16, 2006 – 6:00PM February 16, 2006 Winter weather conditions will begin to occur between at 6:00AM-12:00PM. Expect snowfall in northern Chicago area during morning hours with freezing rain in mid to southern Chicago area. By afternoon, northern Chicago precipitation should change to freezing rain while the mid to southern Chicago precipitation should change to rain. Snow plows should be deployed in the morning in northern Chicago and salt trucks should be deployed throughout the Chicago area to deal with icy roads. As day progresses, plows should be recalled while salt trucks should begin to concentrate in northern Chicago. About 0.5 inches of measurable precipitation to fall over the course of the day. The precipitation will continue all throughout this time period (early morning rush hour till the evening rush hour 6PM). This forecast is based on the ETA-211 and AVN models as well as sources from the Chicago Tribune and Weather Channel.

Freezing rain rain

24 hr. Forecast Mesoscale Discussion Valid for 12Z Thursday, February 16 th 2006, through 00Z Friday, February 17 th A stationary boundary located over central Illinois in combination with an approaching low pressure wave will combine to produce precipitation across the Chicago area. Frontal lifting will occur due to the advancing low; out ahead of the system southerly to southwesterly winds will prevail and force relatively warm, moist air up and over the frontal boundary. This will generate precipitation of various types as the freezing line dances around northern Illinois. Ageostrophic circulations will induce vertical motion thus creating the lifting mechanism around 18Z, again ahead of the low pressure center. At this point, we see that most of the precipitation will be in the form of rain. However, at 12Z any precipitation will be light and rain is expected in the southern portions of the city while the northern areas will expect freezing rain. Accumulations are expected to be minimal, approximately 0.1 inches across the city. Thus a light glaze of ice should form on the north side but will not pose any major hazards. Again, the south side should not experience hazardous weather since the precipitation will be in the form of rain. By 18Z, the entire Chicago area should experience all liquid precipitation with totals around 0.1 inches. For the evening, 00Z, precipitation will increase slightly and total around 0.2 inches and will be in the form of rain. Again, this will pose minor, if any, hazards. In terms of model agreement, both the AVN and ETA runs show a similar scenario for Chicago. There is a slight difference in the precipitation totals, namely the AVN model shows a slightly higher total for 00Z, 0.3 inches, but otherwise there is little discrepancy where the freezing line will be located. So virtually all of the precipitation will be in the form of rain.

Written 24 hr. Forecast This is the forecast for the city of Chicago beginning at 6am ending at 6pm Thursday, February 16 th. In the morning, freezing rain will occur in the northern part of the Chicago area with accumulations of about 0.1 inches. The southern portion of the area can expect to see rain with accumulations near 0.1 inches as well. Salt trucks without plows should be out salting the roads to ensure safety until 10am. After that time, all of the precipitation should be in the form of rain throughout the city for the rest of the forecast period. Rain totals by 6pm should range from 0.3 – 0.5 inches. No salt trucks or plows will be necessary 10am through 6pm. Our forecast was based on the ETA model. Also, the AVN model was in close agreement.

Nowcast 5pm Valid for 2pm to 5pm Thursday, February 16 th Rain will continue across the Chicago area for the rest of the forecast period. Total expected precipitation will total around 0.1 inches over the next three hours with the presence of local rain showers and thunderstorms. The rain should diminish in intensity near the end of the forecast period, so the rush hour will not be as sloppy. Snow should kick in after the forecast period as the freezing line will cross the city and drop the temperatures significantly. A tornado watch is in effect for areas south of the city, but the severe thunderstorm threat will not be an issue for Chicago. No watches or warnings are needed for the forecast period at this time. As a result, no snow or salt trucks will be needed before 5pm. Afterwards, that is out of our jurisdiction. Simply expect rain for the early part of the rush and none for the main rush hour. However, still beware of slippery road conditions and possibly gusty winds after the low passes to the east. At O’Hare, expect low ceilings, low visibility, and moderate wind shear. Rain will be a factor before 5pm, so delays are possible up to 120 minutes. Snow should not be a factor until later in the evening.

Critique of Forecast This first forecasting lab assignment was a tremendous learning opportunity. Considering our group did not contain any graduate students who have at least had experience with GARP, forecasting, and/or other information, we feel that we completed this assignment to the very best of our ability. Granted, we made some errors, ones that are usually inevitable, but we hope to learn from these mistakes in the future to become better weather forecasters. Our team learned many things while working on this assignment. First and foremost, we learned how GARP is utilized in making weather predictions. Although the class had a crash course at the beginning of the semester on how to use the program, this lab was the first time we applied the information provided by GARP to a realistic situation. We needed to understand which variables were appropriate to plot on a map or cross-section in order to visualize what happening and what was about to happen for a particular location, in this case Chicago. After spending six hours in the computer lab on the 48-hour forecast, our heads hurt but we walked away with a much better understanding of how GARP is used. As to which variables we conferred upon to locate the frontal boundary, that decision is debatable. However, plotting just about any variable would have indicated the location of the front. On the 850mb map, we chose to plot dew point and wind barbs because they matched up better together than a plot of equivalent potential temperature. We may have not been following textbook rules, but we were able to recognize and locate the front. In terms of the forecast models, we chose the ETA model out of divine inspiration. Jokes aside, the ETA model forecasted the freezing line rather accurately, within 10 – 20 miles, and that was starting with a 48-hour forecast. The AVN model had the line much farther to the north. Precipitation-wise, the ETA model predicted mostly rain for the area, which was true. Hence the frontal boundary and location of the low pressure center were more accurately predicted by the ETA model.

Critique Cont’d One issue we did not confront is the fine scale at which GARP does not operate. The tool is only as good as the person who's using it. GARP may have only forecasted one-tenth of an inch of rain for a three-hour period, as shown in our 24-hour forecast and nowcast, but it does not take into account the brief, heavy downpours associated with thunderstorms, possible training of storm cells, etc. This is where the human brain must be activated to recognize that if conditions are right for thunderstorm activity, some locations may experience significantly greater rainfall totals than their surroundings. Additionally, we did not fully utilize GARP's precipitation totals function consistently or properly. We forgot to take into account the accumulation of precipitation over time, i.e. a storm total, as well as additional localized totals put out by thunderstorm cells. Also, we excluded a number of components from the mesoscale discussion. However, no one in our group has ever written a detailed analysis like the ones provided by the NWS. While this may seem like a cheap excuse, we are still learning what to look for on weather maps and cross sections which will lead to a plausible forecast and thus a discussion of such events. The mesoscale discussions will only improve with experience in combination with what we learn in future lectures and labs.

Cross Section at 5pm

Actual Obs at 5pm

Surface Temps at 5pm

Storm Precip Total as of 00Z