A new winter storm is developing and its’ effects will be felt over much of the United States east of the Rockies.
The Tuesday morning NASA/GOES satellite image shows the continuing development of the storm over the Plaines states. At the same time, plenty of moisture is being transported northward from the Gulf of Mexico all across the eastern half of the country as indicated by the cloud cover.
The National Weather Service Advisories, Watches and Warnings map shows how extensive the effects of this storm will be. Locally a Winter Storm Watch will be in effect through Wednesday morning over the northern tier of Maryland counties into Pennsylvania. Freezing rain with potential ice accumulations of a quarter inch or greater is the main threat.
The NOAA forecast map above (updated Tuesday morning) depicts expected ice accumulations possible into the predawn hours Wednesday morning. The highest values are north of the Maryland/Pennsylvania line.
The map above was generated by the National Weather Service at Sterling, Va. and depicts estimated ice accumulation Tuesday and Wednesday. This map does not include data for Cecil and Garrett counties in Maryland or counties in Pennsylvania and Delaware where ice conditions are also expected. Ice accumulation estimates have gone down slightly. This map was updated Tuesday morning.
The reason for the icing threat is high pressure to the north holding dense, cold air in place at the surface over northern Maryland. As the storm moves toward the Ohio River Valley, warm air will be pushed northward. This “less dense” air mass will ride up over the denser cold air at the surface. Rain falling into that shallow layer of cold air will likely freeze on contact with various surfaces, resulting in a coating of ice.
The graph above is a forecast profile (updated at 11:30pm Monday) of what the atmosphere above BWI/Marshall Airport will look like Tuesday morning. The blue and red lines rising from the bottom of the graph depict temperature and dew point at various altitudes, starting near the surface. Temperature is red. The fact that the blue and red lines are are together at the lowest levels of the atmosphere is a sign that the air is saturated. The red line slants to the right up to 825mb (5,700 feet), indicating rising temperatures with altitude. Beyond 6,500 feet, the line slants leftward as temperatures fall. Upon close inspection, the surface temperature at this forecast time(Tuesday 7am) should be 28 degrees whereas the temperature at 5,700 feet is expected to be considerably warmer at 39 degrees. This fits the formula for freezing rain. Colder surface temperatures or a deeper layer of cold air might result in sleet.
The right side of the graph depicts winds. The wind barbs at the surface show an east-northeast wind fetch while the barbs a few thousand feet higher show a southwest wind fetch. This is a signature for the more easterly wind “holding in” the cold temperatures at the surface with the more southerly winds higher up pushing warmer, less dense air over the top of the colder air. Again, this fits in with the formula for freezing rain.
The NOAA forecast map above is for Wednesday morning. At this stage in the storm, the weather will be awful in the Great Lakes, including Detroit and Chicago westward into Iowa. Airports in Chicago and elsewhere may be shut down at some stage in this storm, causing air travel disruptions nationwide. In the Mid Atlantic region, freezing rain will likely be transitioning to or already be rain as warmer air squeezes out the remaining pocket of sub-freezing air east of the mountains. The southern limit of the cold air pocket is delineated by the warm/stationary front reaching eastward from West Virginia, then curving up to the DELMARVA Peninsula.
The bottom line, freezing rain should be the main issue with this storm. It should start some time late Monday or early Tuesday and fall intermittently into early Wednesday before becoming all rain. The rainfall should end by the end of the day Wednesday.
Check back with wbaltv.com for storm updates and watch Tom, Tony and Sandra for the latest information on WBAL-TV 11.
By the way, it is summer in the southern hemisphere and while the La Nina weather pattern has contributed to numerous winter storms over the U.S., Australia and New Zealand have been hammered by a series of cyclones.
The latest tropical cyclone in the south Pacific is “Yasi”. As of early Tuesday morning (Baltimore time) the storm is 230 miles northeast of Cairns in northeast Australia with winds clocked at 115 mph. Yasi is expected to make landfall near Cairns by Wednesday morning (Baltimore time) with winds close to 145 mph.