Tropical Storm Isaias Forecast Updates
POSTED ON: Monday, August 3, 2020 8:37 AM by Meteorologists A.R. Davies and Prof. B.S. Barrett
BLUF: The center of Tropical Storm Isaias is expected to pass near or just east of Annapolis on Tuesday. Impacts will be felt in Annapolis, including possible tropical storm force winds or wind gusts, and more likely heavy rainfall that could lead to localized freshwater flooding. There is also a chance for minor or moderate coastal flooding. The heaviest rain and strongest wind gusts will likely occur between 0900 EDT and 1500 EDT on 04 August. The National Weather Service has issued Tropical Storm Watch, Flash Flooding Watch in effect from 2300 EDT tonight through 2300 EDT on 04 August, and a Coastal Flood Watch from 0600 EDT on 04 Aug through 1200 EDT on 05 August.
DETAILS: As of the 0800 EDT update from the National Weather Service (NWS) National Hurricane Center (NHC), the center of Tropical Storm Isaias is located offshore/east of Jacksonville, FL and moving north at 13 mph with a maximum sustained wind speed of 70 mph. Isaias is expected to strengthen back to a Category 1 Hurricane today before making landfall between the Santee River Inlet, SC and Surf City, NC around midnight. The center of Isaias is then expected to move northeast through eastern North Carolina and the Tidewater region in Virginia, before crossing through Maryland during the day on Tuesday.
As the storm moves northward, it will interact/merge with a front that is stalled across the Mid-Atlantic region. Ahead of the approaching storm, southerly flow will pull tropical moisture northward today resulting in unsettled and stormy weather. Showers and thunderstorms are expected to develop along the stalled boundary this afternoon and evening. Some storms could become strong or severe with locally heavy rainfall, gusty winds, and intense lightning.
There are still some subtle differences between models on how quickly the storm will accelerate northward once it makes landfall. For example, the ECMWF/Euro model suggests the center of Isaias will cross through Maryland between 1300 and 1600 EDT on Tuesday while the GFS/American model is faster, between 0900 and 1200 EDT. The official NHC forecast suggests that the storms will pass through the area by 1400 EDT. In terms of storm path, the models and NHC track and cone are in general agreement. However, 50 miles to the left or right of center track could result in varying impacts.
Based on the NHC forecast, expect light and/or steady rainfall from Isaias to arrive early Tuesday morning, with the heavier rainfall expected late morning through early afternoon as the center of the storm approaches and passes. Widespread rainfall totals today and Tuesday will likely range 3.0 to 5.0 inches, with isolated totals as high as 6.0 inches. This could lead to flash flooding. The NWS Weather Prediction Center has categorized Central Maryland as having a moderate risk (20% chance) of flash flooding on Tuesday.
Light southerly flow is expected today ahead of Isaias. However as the storm approaches and passes, the winds will shift in a counterclockwise direction out of the southeast, then around to the northwest. Plan for a few hours of sustained winds between 20-40 mph with gusts between 40 and 60 mph, depending on the track. The NHC currently gives Annapolis 40-50% chance of tropical storm force sustained winds (1-minute average greater than 39 mph).
While a significant "storm surge" up the Chesapeake Bay is not likely due to the current forecast track and approach angle, minor or possibly moderate coastal flooding is possible. Sustained southerly flow ahead of the system is favorable for coastal nuisance flooding and a general build-up of water levels in the middle portion of the Bay. The actual storm path (+/- 50 miles) and timing will be important in terms of wind direction and water levels on Tuesday. High tides will occur at 0642 EDT (1.78 ft above MLLW) and 1837 EDT (1.18 ft above MLLW) on Tuesday, with low tide occurring favorably near the timing of the storm passage at 1310 EDT. There is additional uncertainty as to whether the wind will blow in a given direction long enough to impact water levels. Recent local research efforts by the Oceanography Dept suggest that water levels are impacted by constant direction and intensity winds in as short as 6 hours (even for relatively light winds), but it is unclear how water levels will respond to only a couple hours of intense wind forcing that is variable in direction. In short, uncertainty is high regarding the water level forecast. However, a major flooding evening is not expected. The most likely outcome is minor coastal flooding around high tides Monday and Tuesday (water levels between 2.6 and 3.2 ft above MLLW) with a chance for low-end moderate coastal flooding with water levels 3.3 to 3.5 ft above MLLW on during high tides on Tuesday. After the storm passes, water levels could rebound back-forth (like water sloshing in a bathtub) and result in minor or perhaps moderate coastal flooding for a few days, although this is also uncertain.
ISSUED: 03 August 2020 at 0900 EDT
FORECASTERS: Meteorologists A.R. Davies and Prof. B.S. Barrett
Category: General Interest