Having grown up in the Midwest and living there until my mid twenties, I always considered myself extremely fortunate in the sense that, as a future meteorologist, I was able to experience all four of the yearly seasons, and a variety of most if not all weather phenomenon. After graduating from college, I entered the world of Broadcast Meteorology. I had to move to where a position was available, which a year ago took me to Baltimore, Maryland where I was offered a position as the Weekend Weather Anchor. I began my career in Baltimore, Maryland during the early summer season of 2007, and trying to adjust to the new geographical area and its weather, I had more than my share of busted forecasts, and inaccurate predictions which was a disappointment. The main forecasting concern for the Baltimore and Washington D.C area is the profound impact of the Appalachian Mountains to the West and the Chesapeake Bay and Atlantic Ocean to the east. These two geographical surface land features have a major impact on the weather in this location, specifically the amount of rain and cloud cover that Baltimore typically receives. There are two atmospheric set ups of this challenging predicament. The first one is associated with a normal mid-latitude cyclone that has already become well organized in the Great Lakes Region. Accompanying the area of lowest pressure in the center of the mid-latitude cyclone is a warm front and a cold font. The warm front region is bringing in warm air advection, moisture from the Gulf of Mexico, and southwesterly winds. The trailing cold front of the mid-latitude cyclone brings a change in wind direction to the northwest, cold air advection and subsidence with high pressure building in behind and replacing the warm less dense air associated with the warm front. When looking on the computer radar system, the Mid-Latitude system is producing a lot of rain showers and thunderstorms to the West of Baltimore. The precipitation in the warm sector of the mid-latitude cyclone is forming due to the process of isentropic lifting which happens when “The warm less dense air rises gradually in the vertical as it overrides the sloping cold dense air. It must do this to stay at the same density. This is why warm fronts tend to bring widespread light to moderate precipitation. The uplift is at a lower angle than uplift that is generally associated with cold fronts and thermodynamic thunderstorms” (Theweatherprediction.com). The heavier rains showers and thunderstorms that are associated with the cold front due to the contrasts of the two air masses in temperature and moisture. Ahead of the cold front the air mass is warm and moist and behind dry and cool thus producing a strong temperature and moisture gradient. The approaching cold front moves at a faster pace than the warm front, which in turn produces a steeper slope. “The two differences in steepness of slope and rate of movement, largely account for the more violent nature of cold-front weather, compared to the weather generally accompanying a warm front”( Tarbuck. The Atmosphere 9th Edition). The steepness of the slope helps provide the lift that is needed to raise the warm moist air into the upper levels of the atmosphere to produce latent heat release, clouds and showers. When putting together a daily forecast, I reference all the models (GFS, MOS, ETA) as well as the National Weather Service and Unisys website. When looking at the upper level charts I look for some key features, such as how the surface low formation compares to the upper levels. If I notice that the low become occluded and vertically staked, then I know the low has reached its maximum intensity and will not intensify. If the jet streak on the 200 MB level is to the right of the trough, this tells me that the low pressure is still developing and will be moving to the northeast thus giving Baltimore and Washington D.C. a better chance for rain. When the mid-latitude system moves to the east from the Great Lakes, it comes into contact with the Appalachian Mountains and the processes of orographic lifting takes place and is defined “when an air mass is forced from a lower elevation to a higher elevation as it moves over a rising terrain. As the air mass gains altitude it expands and cools adiabatically. This cooler air cannot hold the moisture as well as warm air can, which effectively raises the relative humidity to 100%, creating clouds and frequent precipitation” (www.wikipedia.com). Thus, all the moisture that was associated with the mid-latitude cyclone drops all it’s rain on the windward side of the Appalachian Mountains. When the mid-latitude system makes its way past the mountains and officially arrives in the Baltimore and D.C. area (Leeward side), it becomes a wind event and then a rain event and is termed as a Leeward wind area. This is very challenging and frustrating because the system looks so impressive on the computer radar system off to Baltimore’s West in Kentucky and Ohio, and once that massive system hits the Appalachian Mountains it loses all of its moisture. The other factor that makes is difficult and occurs when looking at all the model data is when they keep the POP chances high, the models have a hard time handling the influence the mountains have on the precipitation and cloud cove Baltimore and Washington D.C. receives. In addition, atmospheric forecasting set up for the Baltimore and Washington D.C. area is when an area of High Pressure or Low pressure moves out into the Atlantic Ocean and becomes stationary for a period of time. Depending upon where each system stalls in the Atlantic Ocean will decide the wind direction and how much moisture is being advocated into the forecast area. An area of high pressure moves anti-cyclonic (clockwise) in the Northern hemisphere and diverges away from the center of highest pressure. An area of Low pressure moves cyclonic (counterclockwise) in the Northern Hemisphere and the air flow is towards the center of lowest pressure. When the area of High pressure or Low pressure comes to a standstill in the Atlantic Ocean, it’s critical to take note on their exact locations. If the area of High pressure is off the Eastern Shore of the Delmarva Peninsula, the clockwise rotation around that High Pressure brings in Southeasterly winds. Accompanying the Southeasterly winds is all the moisture from the Atlantic and Chesapeake Bay. The same moisture advection situation happens with an area of Low Pressure except the wind flow is counterclockwise and it’s the Northeasterly wind flow that supplements the moisture. The moisture that is brought into the Baltimore and Washington D.C. area rises over the Atlantic ocean cools, becomes saturated and then condenses into low clouds, fog and drizzle and added with the wind flow around the stationary high pressure of low pressure (NE or SE wind flow) keeps the Baltimore and Washington D.C. area dismal, cool and cloudy. The Mid-Atlantic is also influenced by a marine layer which is “an air mass that develops over the surface of a large body of water such as the ocean or large lake in the presence of a temperature inversion” (www.Wikepedia.com). With the colder, denser air below and the warmer less dense air above helps produces fog and low clouds in the Baltimore area. Both of these events occur fairly often and it’s very important to understand the geographical area you are forecasting in. Mountains, lakes, oceans, ponds, friction and other important surface features can have a huge impact and influence on forecasting cloud cover, temperate, wind direction and precipitation. These particular events are hard to forecast and prepare for, because you really have to look at all the variables before you can put together an accurate forecast. The models can be wrong and tend to cause a meteorologist to have a hard time forecasting certain mid-latitude cyclones, and at times I had to go against what they are forecasting. The largest consequence of this forecasting problem is getting a busted forecast, either you forecast rain associated with mid-latitude cyclone and it doesn’t rain, or vice versa. In all reality, I wouldn’t call it a forecasting problem, but mastering your forecast area to become more proficient at forecasting the problem. Forecasting is something that you get better at and improve at with time. The more time you experience and see how a particular weather pattern and event unfolds because of mountains or an ocean, the next time you will have a better handle on how to forecast for that particular situation. Citations: 1.www.theweatherprediction.com 2.www.wikipedia.com 3.Lutgens Fredrick. Tarbuck Edwards. The Atmosphere 9th Edition. Pearson Prentice Hall. 2004. Pg. 251. |