* Flash Flood Watch late Thursday night to early Friday afternoon *

The Mid-Atlantic is in for a real soaking and the possibility of another round of flooding.  A slowly-moving, energetic wave disturbance in the upper atmosphere and at the surface will usher in a tropical plume of moisture starting Thursday evening, continuing into Friday morning.

The setup

This weather system already produced heavy rain over the Plains and Mid-South.   As the ribbon of moisture is lifted by both the wave disturbance and the Appalachians, a widespread strip of 2-4 inches of rainfall is expected to accumulate from the Carolinas northward to  Pennsylvania (Figure 1 below).

Figure 1. National Weather Service Weather Prediction Center forecast of where the heaviest rain will fall, for the time period ending Saturday morning.

There is a concern for widespread flash flooding, and perhaps flooding of larger tributaries and mainstem rivers in the Potomac Highlands.   Let’s look at the key ingredients that are expected to coalesce over a 12-18 hour period, for what NOAA’s Weather Prediction Center (WPC) is calling a “long duration, high impact flood” for portions of the Mid-Atlantic.

Abundant Tropical Moisture.    The models are portraying copious water vapor, approaching 4-5 standard deviations above climatological “normal” for this time of year, sourced from the tropics.    This deep ribbon of moisture will feed into the low pressure system as part of a fast-flowing “warm conveyor belt” from the south (Figure 2).

Figure 2. A pipeline of tropical humidity feeding into our region, valid 2 AM Friday morning, derived from the ECMWF model. (WeatherBell.com)

Vigorous Synoptic Uplift.   The main storm system aloft is expected to become “negatively tilted”, that is, the wave axis will assume a configuration that draws up large amounts of air from below, like smoke drawn out of a chimney.    Rising motion will be assisted by a jet streak in the flow, which will position a “sweet spot” of vigorous ascent right over the Mid Atlantic.

A Significant Orographic Component.   This means that the deep, moist air stream will experience additional, forced uplift as it flows over the Appalachian cordillera.   The heaviest rain, in fact, will likely concentrate along the eastern slopes of the mountains.

The Possibility of Local Convective Enhancement.   This is always a wild card in this type of large-scale setting.   The instability overnight Thursday and Friday morning is not expected to be excessive, but enough will be present to trigger multiple rounds of convective showers and thunderstorms.

Anytime there is a setup involving a slow-moving boundary, with strong moist flow blowing parallel to the boundary, the potential exists for echo training.   That is, multiple rounds of convective storms are likely to stream over the same locations, repeatedly.   Trying to pinpoint where and when this “convective conveyor” will initiate 36-48 hours from now is not possible;  this is a nowcasting (day-of) activity.

A Second Low Pressure Center.   A separate low pressure center is expected to initiate along the Appalachian east slopes, along the cold front, Friday morning (Figure 3), passing over or close to the D.C. region.   This will create additional convergence and uplift of moist air, leading to enhanced pockets of heavy rain.

Figure 3. Surface synoptic chart, as predicted for 8 a.m. Friday morning, showing the cold front overlying the Appalachians, along with a new surface low over NOVA. (National Weather Service)

 There is an additional consideration with this second low.   It may enhance the wind shear across part of the region, by causing the low-level flow to “back” (turn out of the north).   Given sufficient instability, the curvature added to the wind profile could create a low-end tornado threat.   As of now, the Storm Prediction Center (SPC) thinks any threat may be to our south, over southern Va. and N.C.   But this is a configuration that we will have to monitor carefully. 

Instability for any strong or severe thunderstorms will likely be greatly limited by thick cloud cover, the cooling effect of heavy rain (via evaporation), and the expectation that the most vigorous activity will occur before Friday’s sun angle gets too high.

How much rainfall and where?

The latest model runs (NAM, GFS, ECMWF) place the rainfall bullseye to the southwest of the D.C. region, over high terrain. This morning’s WPC thinking reflects this concept, as forecasters there have placed their 2-4 inches maximum in that general location, with embedded pockets of 6-8 inches depending on convective enhancement.

The three figures below illustrate the model-predicted rain totals, through 8 p.m. Friday, based on the latest runs available at the time of this writing.

Figure 4. GFS model total rain accumulation ending Friday evening. (WeatherBell.com)


Figure 5. ECMWF model total rain accumulation ending Friday evening. (WeatherBell.com)


Figure 6. NAM model total rain accumulation ending Friday evening.

 In the model runs, the D.C. region lies in a rainfall gradient region, with fairly low totals along the Eastern Shore.   For the immediate CWG region, a widespread 2-3 inches of rain seems like a reasonable bet.   But as we learned on April 30, the devil may lurk in the details;  if one or more narrow corridors of convective, training cells sets up, local amounts could easily climb into the 4-6 inchesrange.

This morning’s NAM and its previous run have been somewhat of an outlier, pushing the heavy rain well to the south and west, along the western slopes of the Appalachians.   This places our region in a rain shadow.  This runs counter to the climatologically-favored heavy rain region, which lies along the eastern slopes, from these types of systems.

The axis of the moisture plume and strong ascent should shift east of our region Friday afternoon.   However, given the sluggish nature of this system, shower activity (albeit tapering) could linger into Friday evening.

CWG’s Winter Storm Expert Wes Junker, a veteran forecaster of the former Hydrometeorological Prediction Center, echoed our concern for a significant rain event this morning:

 The models are showing a slow moving front crossing the area with 40 kt southerly 850mb winds and 850 and 925 mb moisture flux departures of greater than 5 standard deviations. That often leads to someone in the VA to PA region getting hammered with heavy rain and possible flash flooding over the next couple of days.

The safest course of readiness is to plan on a difficult Friday morning commute, with the very real likelihood of ponding water, low driving visibility in heavy rain, and flooded, low-lying streams. Those with flood-prone basements should make necessary preparations.

CWG will issue an update on the evolving setup tomorrow, and we will be closely monitoring the event during the day on Friday.