Some severe weather possible Tuesday, watching the tropics

Good evening.

It is a calm evening for the most part, albeit with a few weakening thunderstorms in inland areas. The cold front associated with a low pressure area (which is currently weakening) over northern Quebec and Hudson Bay should move closer to the Northeast tomorrow, and some models suggest that a few severe storms are possible. In addition, the tropics are active…Danny has largely dissipated, and Erika looks to be about to form.

Severe weather Tuesday
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Supercell Composite index at 2100Z (5:00 pm EDT) August 25, 2015. Not particularly strong but noticeable nonetheless.

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Low-level helicity at 2100Z (5:00 pm EDT) August 25, 2015. Very marginal in the Northeast, so not a lot of spin in the atmosphere.

As previously mentioned, a slow moving cold front is currently tracking into the Northeast. However, it is a fairly weak front, so severe weather should be limited. That said, some damaging winds and large hail are possible, as are maybe a couple tornadoes. The supercell composite index is moderate, although wind shear forecasts are not impressive at all. As a result, severe weather should be isolated, although it is always best to keep an eye on the skies! Any thunderstorm can also produce dangerous cloud to ground lightning.

Tropical Trouble?

Danny strengthened rapidly on Friday to a Category 3 hurricane, but its small size couldn’t withstand high wind shear over the weakened and the circulation dissipated last night or this morning. Redevelopment is not anticipated. Meanwhile, a low pressure area east of the Lesser Antilles is on the verge of becoming Tropical Storm Erika, as a buoy confirmed sustained winds of tropical storm force. The National Hurricane Center gives it a 90% chance of development within the next 48 hours (although it is likely to develop within the next 12 to 24 hours). It just needs a stronger low-level circulation to be named. (There is another low in the far eastern Atlantic, but given poor conditions out there, it is unlikely to develop.)

Once Erika develops, assuming it does (and it should soon), the models agree on a strong storm, possibly becoming Hurricane Erika by late this week. In fact, the HWRF model, which did very well with Danny, intensifies Erika to a Category 4 hurricane near the Bahamas by this weekend. That would obviously be an extremely dangerous situation, and it is certainly not out of the question. Dry air should be less of a problem with Erika than with Danny, and shear – although moderate – does decrease to low levels later in the week. Most other models strengthen Erika to a strong tropical storm or Category 1 hurricane by Saturday, although a few models keep it weak, possibly due to land interaction. I would personally lean to the higher end, especially if Erika can undergo a period of rapid intensification. Confidence in the intensity forecast: Low.

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Intensity forecast of many models for the low pressure system (identified as Invest 98L) expected to become Erika.

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Track forecasts of many models through Saturday for the low expected to become Erika.

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HWRF model run of what would be Hurricane Erika on Saturday afternoon. Notice the winds of over 150 knots at 900mb, which would be about 115-120 knots (130-140 mph) at the surface, a Category 4 storm.

The track forecast is straightforward in the short to medium term, as nearly all models take the low through the Leeward Islands (likely as a tropical storm) and near or just north of Puerto Rico (likely as a tropical storm or Category 1 hurricane). By late this week and into the weekend, Erika should be near the southeastern or central Bahamas, likely as a hurricane. From there it is anyone’s guess, and I see five possibilities (note this is from late this weekend into early next week):

  1. The subtropical ridge strengthens to the north and holds firm. That would block any opportunity for Erika to recurve, and likely send it westward into Florida and into the Gulf of Mexico. (Estimate: 30% chance)
  2. The ridge collapses, a modelled trough digs south enough to open up a pathway out to sea and Erika follows it, roughly along the Gulf Stream west of Bermuda. (Estimate: 25% chance)
  3. The ridge retreats and trough gets trapped, which opens up a pathway northward along the east coast. This is the least likely scenario at this time. (Estimate: 10% chance)
  4. The ridge and trough don’t move much, which result in a game of cat and mouse with Erika moving erratically near the Bahamas or just to the northeast in very weak steering currents. (Estimate: 20% chance)
  5. The storm dissipates as it comes close to land or encounters higher shear while moving slow. (Estimate: 15% chance)

In terms of track, my confidence level is Moderate to High through Wednesday, Moderate from Thursday to Saturday and Very Low thereafter.

This can be an active time of year with multiple threats from both the tropics and severe weather. It is important that you keep an eye on the skies for any short-term severe weather threats. When it comes to the tropics, we should just keep eyes out. Although it is unlikely to be (at this time) a Northeast threat, it is still worth watching Erika. If you are reading from the Leeward Islands, you should prepared for a tropical storm impact (at least) within the next 2-3 days.

Forecaster Craig Ceecee (@EternalWeather1)

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