An additional factor is that this storm will likely undergo extratropical transition as it moves through Maryland. The prediction models are swinging a jet stream trough through the Great Lakes on Saturday. This is a ripple or disturbance that creates rising motion in the atmosphere. It is expected to merge with Irene's circulation over eastern Maryland and New Jersey. The additional energy source that this trough provides may help to re-energize Irene, countering some of the weakening due to cooler ocean water. The additional uplift of air will squeeze additional tropical moisture out of the storm, creating heavier rains. And Irene is also expected to interact with a weather front over southern Maryland, from which it can draw additional energy. There is precedent for this type of extratropical transition, namely, Hurricane Floyd of 1999.
Let's talk about likely impacts in the Baltimore region. The main time frame for these impacts is Saturday afternoon through Sunday afternoon, with the most dangerous conditions early Sunday morning.
Storm Surge on the Bay: Initially, strong winds from the north will blow water down and out of the Bay. However, on Sunday afternoon, the winds will shift direction, blowing strongly from the west and southwest. This may cause flooding along the Bay's eastern shore - particularly since this weekend we are experiencing spring tides (larger high tides than normal).
Heavy Rainfall: These rains will likely start falling well before the actual storm center passes by. These "rains in advance" are part of the extratropical transition process. Light rain may start to fall Saturday morning. It will intensify through the day, and will be showery in nature. The heaviest rains will fall overnight Saturday into Sunday, and may taper early Sunday afternoon. Below is the latest NWS guidance on rain accumulation. Amounts in the 3"-6" range are possible in and around Baltimore. Notice the heavy axis of 13"+ rains along the Delmarva:
Winds: With the storm potentially tracking closer to the coastline, and because of the storm's large size and intensity, the wind threat across Baltimore has increased. Being on the right side of the track diminishes the winds somewhat, but winds of tropical storm force (40 mph and greater) are possible across Baltimore. Below is a sequence of wind intensity forecasts from the NWS, showing the maximum expected wind gusts:
|Wind Gust Forecast, Knots, 2 PM Saturday|
|Wind Gust Forecast, Knots, 2 AM Sunday|
|Wind Gust Forecast, Knots, 8 AM Sunday|
|Wind Gust Forecast, Knots, 2 Pm Sunday|