PRESECESSOR HEAVY RAIN EVENT TUESDAY AHEAD OF HEAVY RAIN WITH DEBBY LATE WEEK

On Tuesday, a cold front will stall over northern MD or southern PA during the late afternoon into the overnight hours. Along this front, moisture will pool heavily, meaning any storm that forms along this front will drop a lot of rain in a short period of time. There is uncertainty as to where this front stalls, which will dictate where the storms will stall. If it stalls along the Mason Dixon line, then a corridor of 1-3" rainfall totals (locally as much as 4-6") could occur across northern/northeast MD and southern PA. Right now, I would say the higher chance of this occurring would be north of the state line, and its possible that northern MD doesn't have any issues. However, if the 1-3" rainfall does come to fruition, locally significant flash flooding will  be possible, especially if it occurs over northeast MD where recent days have featured heavy rain. Overall, confidence in this occurring is low, and that's probably why there is no flood watch yet. But it could be a potentially impactful event if it does occur. 

NOTE: The above graphic should say 8/5, not 8/6

There is also a low, but not 0 threat for severe weather with any storms that form on Tuesday. There will be modest amounts of instability, and a LOT of moisture, along with ample forcing from the stationary cold front draped nearby. So, a few of the storms could produce locally damaging winds gusts, or even a brief tornado (although the tornado threat is very very low) right along the front itself where shear will be maximized. The risk of severe weather is low, and SPC only has a marginal risk which is level 1/5. A higher risk will exist further north into central PA where a cluster of possibly severe storms could move through.

lets talk about debby

The 11pm update from NHC takes the center of Debby east of I-95 and over the eastern shore of MD as a low end weakening tropical depression with sustained winds of 35mph and gusts perhaps to 40mph. But this does not change our messaging or forecast issued earlier on. 

That said, catastrophic flash flooding, damaging winds, and tornadoes will continue to impact portions of the southeast, with potential for over 2 FEET of rain to fall over parts of South Carolina. Please heed all orders given by local officials, and check with the local NWS offices in those areas for specific forecasts.

As far as local impacts, they will be things we have dealt with numerous times before, and are expected to be minimal and primarily beneficial. Lets first start out by saying there is no tornado threat or severe storm threat with Debby, end of conversation. The environment simply does not support severe weather. I have seen WAY too much talk about severe weather in our region and it simply is not true that there is a risk of severe weather here. 

That said, deep tropical moisture along with a relatively slow motion associated with Debby will result in a LOT of rain for our area, with most data and the NWS forecasting a general 4-6" across the area. I think this is a bit bullish, and went with a widespread 3-5" for areas east of Rt.15, and 1-3" west of there. Because of the ongoing drought, and the fact that most of this will fall over 24-48 hours, the flood threat should be pretty minimal. This is still several days out, and some changes are possible depending on the exact track of Debby.

But that being said, some areas have seen significant rains lately, and may see another on Tuesday. So there certainly is at least some potential flash flooding but I don't expect widespread flash flooding. This will likely prompt a flood watch for most of the area. If Tuesdays rainfall is more significant than currently anticipated, then the flooding may be more widespread with Debby. This will however be quite the drought buster, especially if Debby takes a more westerly track which would bring heavier rains further west. It wont completely eliminate the drought, but it will make some big improvements! There wont be any high winds with Debby when she arrives, but some isolated gusts of 30-40mph cant be ruled out in heavier downpours especially east of I-95.

As far as timing, scattered shower and storms are likely at times on Wednesday and Thursday, but neither day look like a washout. Both days will feature a 60% chance of rain. The widespread and heavy rain wont arrive until Friday, lasting much of the day when rain chances rise to 95%. The heaviest currently looks to be from 12pm-midnight Friday. It remains uncertain if rain will last into Saturday, so I'm capping our chances at 60% for now and will adjust as needed. Sunday looks much better. 

NOTE: The above rainfall graphic should say 8/5, not 8/6

Senior Forecaster Jack