August 18, 2009

Blowhard Bill and severe prospects

Tuesday August 18th, 2:27 pm: Hurricane Bill has strengthened to a Category 2 with max sustained winds of 105 mph as per the 10AM update. The current National Hurricane Center's forecast track takes the storm to the west of Bermuda and then curves to the north. This would keep the storm well away from the US but our forecast certainty for tropical systems outside of 3-4 days really drops off. A couple days ago the GFS had three consecutive models runs where it took Bill into the Gulf of Mexico, and now it has Nova Scotia as a possible target. Our best tool in forecasting the track of hurricanes has to be a spaghetti plot which shows the forecast track from multiple models. The smaller the spread, the higher the certainty. Take a look at the latest thinking : LINK

Tropics aside, tomorrow looks to be an active severe weather day around the Central US as a very potent shortwave trough digs south from Canada on the backside of a long wave trough that is drifting east. Forecasting for tornadoes with late-season events like this is exponentially harder than Spring events (as evidenced by the wording in the SPC's Day 2 outlooks, ha!) but I do see some tornado potential tomorrow. What makes these events so difficult to forecast is the widespread areas of elevated thunderstorms that seem to blow up and fade away seemingly without rhyme or reason. Our inability to forecast this early and mid-day precipitation with much confidence makes forecasting surface features a very difficult task, and without that, tornado forecasting is nearly impossible. Like Dr. Doswell says, "Chasing boils down to going out on synoptic possibilities and succeeding or failing based on unresolved mesoscale events." Tomorrow definitely has a favorable synoptic (large-scale) setup, but we likely won't know until the afternoon what the mesoscale (intermediate-scale) environment will be like.