TEAM 4 POUNDER & LI

Mesoscale Discussion Valid for 1200UTC Thursday to 0000UTC Friday for the Chicago area A low pressure system is currently forming in the lee of the Rocky Mountains and is looking to propagate eastward in the next 2 days with the help of a strong jet streak that will form over the Central Plains. The associated warm and cold fronts will be propagating along with the low pressure system, though various models are in conflict as to if and when the cold front, and subsequent freezing line, will pass over Chicago, making it difficult to forecast the correct precipitation. While precipitation type is still relatively unknown, it is likely that any precipitation will not be light and possibly scattered due to the lack of frontal forcing. Cross-sections of temperature and moisture show a shallow cold front, a shallow moisture layer from the surface to roughly 700mb, and very weak frontal forcing. The winter weather that will hit Chicago is expected to be nothing beyond freezing rain, with freezing drizzle and cold rain being much more likely. Accumulation is expected to be less than 1 inch total, with the freezing rain/drizzle lasting only during Thursday morning. During the runs of the NAM and GFS models, several discrepancies were noted. First, the NAM model showed the storm propagating eastward much more quickly than the GFS model. Second, the GFS had the storm intensifying over the Great Lakes region, while the NAM model showed the storm intensify just eastward of the Great Lakes. Third, the NAM model shows Chicago directly on the freezing line, while the GFS model shows Chicago well south of the freezing line.

Chicago Transportation Department Forecast A light-to-moderate amount of freezing rain/drizzle is expected to fall over the Chicago area before and during the morning rush hour, 6:00am, on Thursday. It is recommended that salt trucks be deployed Thursday morning to offset the freezing rain/drizzle conditions. Less than 1.0 inch total of freezing rain/drizzle is expected over the Chicago are. Throughout Thursday, the freezing rain/drizzle will taper off and give way to light-to-moderate rain by the early afternoon. Precipitation will continue throughout the evening and Friday morning, although salt trucks will not be necessary again. FORECAST BASIS At this time, the NAM model is most trusted. Zonal flow across the eastern U.S. supports the quick propagation of the storm system, the shallow moisture level supports the delay of intensification until after crossing the Great Lakes region, and the quick propagation also supports the quick passage of the cold front associated with the low pressure system.

Mesoscale Discussion Valid for 1200UTC Thursday to 0000UTC Friday for the Chicago area To update the previous mesoscale discussion, the low pressure system in Oklahoma will be intensifying over the next 24 hours, due to its unique position. Here, WAA from Texas will aid in rising motion, as will upper air divergence due to a rapidly growing jet streak over the Rockies, and PVA will help move the low pressure system northeastward towards the Great Lakes region. Because of a shallow moisture layer forecast for the Chicago area, it is unlikely that frontal forcing will play a key role in storm development. Only after the cold front associated with the low pressure system has passed the Chicago area is a squall line likely to develop along the frontal boundary. However, a low level jet just ahead of the cold front will contribute to the vertical wind shear, which will enhance the rotating motion of the system. Because of the zonal flow over the eastern half of the U.S., the storm is expected to progress very quickly. Thus, any winter weather which may hit Chicago will be short lived. Light-to-moderate freezing rain/drizzle is forecast for 1200UTC Thursday. By 0000UTC Friday, rain will have replaced the freezing rain, with the last of the precipitation tapering off by Friday evening. The GFS and NAM models are still differing slightly in their predictions. The NAM model is predicting a rapidly progressing storm system and jet streak, with peak intensity occurring after passing the Great Lakes region, while the GFS model is predicting a slower moving storm system and jet streak, and a slightly more intense storm over the Great Lakes region.

Chicago Transportation Department Forecast A light-to-moderate amount of freezing rain/drizzle is expected to fall over the Chicago area before and during the morning rush hour, 6:00am, on Thursday. It is recommended that salt trucks be deployed Thursday morning to offset the freezing rain/drizzle conditions inches of freezing rain/rain is expected for the Chicago area for the entire duration of the storm. Throughout Thursday, the freezing rain/drizzle will taper off and give way to light-to-moderate rain by the early afternoon. Rain will continue throughout the evening and Friday morning, although salt trucks will not be necessary again. The last of the precipitation will taper off by Friday afternoon/evening. FORECAST BASIS At this time, the NAM model is most trusted. Zonal flow across the eastern U.S. supports the quick propagation of the storm system, the shallow moisture level supports the delay of intensification until after crossing the Great Lakes region, and the low level jet ahead of the cold front supports the growth of the system’s rotation and quick passage of the cold front, thereby limiting the amount of winter weather that Chicago can expect to receive.

Chicago Area Nowcast Valid for 0000UTC Friday for the Chicago area Rain and thunderstorms will continue to persist throughout the evening, but will not give way to freezing rain and snow until possibly late Thursday evening/early Friday morning. Rain total for Thursday evening is expected to be inches. Still be on alert for possible strong thuderstorms. For the Chicago O’hare Airport By 5:00pm, rain and thunderstorms will have persisted, but intensity will have been light-to-moderate. Visibility at the surface could be as low as 3 miles. Winds will be out of the north at 10 knots, and temps will be in the mid-to- upper 30s

Forecast Critique Misleading elements The zonal flow across the eastern half of the U.S. suggested that the storm system would propagate more quickly eastward than what it actually did. Because of the zonal flow, the storm system was expected to move out of the Chicago area by the end of the Thursday. After running several weather models, there was no consensus as to where the freezing line would be located on Thursday morning. However, it was well predicted that the freezing line would not be located south of Chicago on Thursday morning, so there was little chance of snow during the morning rush hour. However, taking the chance of predicting freezing rain and sending out salt trucks was a poor decision, since there was only rain during the day. The predicted moisture layer did not extend beyond 700mb, so it was unexpected for Chicago to receive around 1.5 inches of precipitation. Whatever type of precipitation was forecast for Chicago, it was not predicted to be anything stronger than drizzle. Neglected elements The trough in the Pacific Northwest may have contributed to the storm system not propagating as quickly as was thought. The upper-air flow only became truly zonal after passing east of Chicago. Before Chicago, the storm system was just coming off of the leading edge of the trough. To see how the storm system might track in the next hours, it would have been prudent to look back and see how it had tracked for the previous hours. This was not done as thoroughly as it should have been. The NAM model, because it agreed with our prediction which was based on misleading elements, was given more confidence than the GFS model, even though the GFS model has had a better prediction record for this winter season than the NAM model. Trusting the NAM model only furthered our trust in the misleading elements.

Forecast Critique cont. Beneficial elements The jet streak forming over the Rockies was well predicted and provided good guidance for where the general track of the storm would be, since the storm would follow the track of the upper-air divergence in the left exit region of the jet streak. The limited amount of moisture did prevent a prediction of an organized thunderstorm and subsequent warning for said storm. Issuing such a warning would have cost money.