Tropical downpours likely in Northeast US this weekend
Other than a few scattered passing thunderstorms on Wednesday, much of the Northeast US has been devoid of rain over the past several weeks. This is about to change, perhaps dramatically so in some spots. On Sunday, a cold front and associated potent shortwave trough with plenty of vorticity will run into a well established Southeast Ridge, and the remnants of Tropical Storm Julia — two great moisture sources. This combination of energy, dynamics, and moisture — as well as the fact that the ridge and the mid-level flow are parallel to the frontal boundary — will lead to the potential for slow-moving tropical downpours, some of which will train over each other and lead to flash flooding.
As we move towards Autumn, atmospheric troughs can deepen and amplify more than during the warmer summer season. This, combined with tropical moisture and dynamics can lead to impressive meteorological results. This particular shortwave trough is digging from the Great Lakes all the way down to the Tennessee Valley, and is even tilting neutrally or negatively on some guidance. This acts to slow down the atmospheric flow even more, leading to a prolonged fetch of moisture, lift, convergence, and thus heavy downpours.
A progressive flow upstream will flatten the trough somewhat as it heads east, but not before plenty of heavy rain falls. As this moisture propels into our region ahead of the front, precipitable water values will exceed 1.8″ and may even hit 2″ — which is well above average for this time of year, on the order of 2+ standard deviations. Marginal instability may also lead to some thunder and lightning as well and a few isolated strong wind gusts, though severe weather is not anticipated.
12z NAM showing high precipitable water values and SBCAPE by Monday AM
Sunday morning and afternoon should primarily feature scattered showers, as most of the frontal forcing will be off to the west. Precipitable water values will already be high, so some scattered downpours are possible at this time. However, it is Sunday evening, and particularly Sunday night and Monday morning when we anticipate the heaviest rains. The actual shortwave trough will be moving through at this time, providing additional lift and dynamics, via positive vorticity advection streaming along the front as well as a potent jet streak. This will greatly increase coverage of synoptic rains, which will lead to widespread bands of heavy downpours, rather than simply hit-or-miss thunderstorms.
The NAM showing strong upper-level divergence, (rising motion) underneath the right-rear entrance region of 300mb jet streak. This supports more forcing for showers and thunderstorms with heavy rainfall to form.
Nevertheless, there are some concerns and uncertainties regarding the widespread heavy rainfall or prolific rainfall totals. High pressure will be nosing from the Atlantic over the weekend. This could be more stubborn to weaken or leave the region on Sunday, keep areas near the coast the dry until at least late Sunday night or Monday. Some models also suggest more subsidence between the cold front and Julia intensifying off the coast.
The models also differ on the amplitude of the shortwave trough movingtrough the region. The GFS is somewhat less amplified this afternoon, with some southern stream energy actually breaking off over the Southeast US and interacting with Julia’s remnants left behind near the Southeast Coast. In this scenario, its would be possible to see more showers both off the coast and over the interior, nearer to the cold front, leaving much of the I-95 Northeast US Corridor relatively dry compared to its surroundings.
The 12z GFS showing 500mb vorticity, showing southern stream energy lagging behind and interacting with Julia’s remnant energy near Southeast Coast on Monday. A vert zonal and fast flow over Northern Plains, keep the flow more progressive.
Still, forecast models are likely to struggle with the intricate details of the setup. It would be unwise to use any particular operational model run, especially in a setup involving tropical moisture and dynamics. The overall atmospheric setup, supported on ensemble guidance, suggests the potential for tropical downpours throughout the Northeast US this weekend.
To reiterate, it appears the potential for steadiest rain in the Northeast will come along the cold front from Sunday Evening into Monday, as moisture and lift to develop precipitation are juxtaposed and maximized. Over the next 24 hours we will have higher confidence in pinning down exactly where this looks likely to occur, but presently it appears that much of the Mid Atlantic and Northeast will be affected — so have the umbrellas ready!