Forecast Discussion
Thursday, September 14, 2023 3:28PM EDT
Our most significant and dangerous flash flood threat in more than 50 years will begin by this evening. Through Monday parts of the region may see more than half-a-years’ worth of rainfall.
A Flash Flood Watch is in effect for the entire Washington/Baltimore Metropolitan region from 6PM this evening through Saturday though this will almost certainly end up being extended through at least Monday as forecast details for Sunday and Monday become clearer.
With a near record strong WAR, a deep trough over the Ohio Valley and moisture from Margot streaming north from the Caribbean and western Atlantic east of Florida, Pwats will rise to the 2.25”-2.5”+ range from this evening through the beginning of next week. Clusters and trains of tropical downpours/storms will move north over the region at all times of the day. With limited sunshine and high moisture content temperatures will probably only vary by about 5-10F from daytime highs to nighttime lows (Low/mid 80s and mid/upper 70s respectively). Dewpoints will remain in the mid/upper 70s throughout the period. As far as severe weather goes there will be a daily threat of gusty winds and even a brief tornado or two especially with any transient supercells given CAPE values of 1500+ J/kg and effective SRH of 150-200 m2/s2. Lack of downdraft CAPE will prevent a bigger wind threat with storms though there may be a couple of mesovort maxes that move from south to north over the region over the next couple of days which may enhance both deep-layer and low-level shear leading to a locally higher risk of a brief tornadic supercell as well as bowing segments allowing for damaging/near-severe wind gusts and a QLCS tornado threat.
The main threat by far, however, will be the very heavy rainfall rates with these downpours and storms. Given the aforementioned Pwats and CAPE any location that gets under a band of storms that trains over the area for an extended period of time may receive several inches of rain within a few hours leading to major flash flooding. Even areas that don’t get stuck under a train of convection as frequently if at all will see many individual storms or clusters over the next couple of days resulting in several inches of rain and flooding concerns. All of the above does not even account for when Margot will finally move north into the region by the Sunday PM/ Monday AM timeframe. More on that below.
Models are coming into much better agreement that by sometime Sunday Margot will make landfall along the Carolina coastline being pulled north fairly rapidly by the strong ridge of high pressure to the east and deep trough of low pressure to the west. While the exact track and timing of the center are still being ironed out, as this is still 3 days away from occurring, it’s now looking very likely that at least a majority of the Baltimore/Washington region may experience significant impacts from Margot. Ahead of Margot on Sunday the low-level shear that will be with us through the weekend will further increase leading to a higher likelihood of tornadic supercells. As the center approaches and moves over the region a solid shield of heavy rain will move through leading to an additional several inches of rain. The biggest uncertainty that remains with Margot is how strong the tropical system will be at landfall along the Carolina coast as anything ranging from a tropical storm to, in the worst-case scenario, a major hurricane is plausible. This will determine the difference between getting several inches of rain with gusty winds Sunday into Monday OR damaging to hurricane-force wind gusts teaming up with the heavy tropical rains leading to widespread tree and structural damage.
We will continue to monitor this potentially catastrophic situation with frequent updates over the next several days.
Forecaster Wannabe: George BM
