Who gets drenched? Possibilities for late this week

It’s getting interesting out there, and the tropics are the main player here but a strong subtropical ridge over the southern US and a trough to the northwest are also role players. The big picture, however, is challenging for the Northeast. The location of the stalled frontal boundary, which is currently over the inland mid-Atlantic and expected to only oscillate north and south (rather than depart completely) will be crucial.

Although it is probable that Tropical Storm Bill will form (NHC gives it a 90% chance), the impact and situation will be the same regardless on whether or not a closed low-level circulation becomes well-defined near the Texas coast.

Scenario 1: Low dissipates just inland, moisture remains trapped in the south
GFS_3_2015061512_F144_PCPIN_96_HR

12Z GFS rainfall trends (Wednesday to Sunday). Notice the rain gets dispersed, as the ridge breaks down.

This is supported by the 12Z GFS, but quite frankly that model appears to deviate greatly from previous trends. In this scenario, the subtropical ridge weakens or dissipates and the low also dissipates in Texas or Oklahoma. The moisture then gets dispersed across the southern US and largely leaves alone areas north of the Mason-Dixon Line. The heavy rain over the Great Lakes is mostly unrelated, but may be partially due to tapped moisture as well. (Not totally sold on this scenario yet, it is a new possibility).

Scenario 2: Ridge holds strong, moisture rotates around it
NAM_221_2015061512_F84_PCPIN_72_HR

NAM rainfall forecast from tonight to Thursday (only goes out 84 hours). Extrapolating the forecast agrees with a major Northeast rainfall event this weekend.

p168i

NOAA WPC’s precipitation forecast (QPF) through Sunday night. Parts of the inland mid-Atlantic may see over 6 inches of rain (locally higher?).

This is the scenario most models had been agreeing on. Extrapolating out the NAM and Euro runs and also the WPC’s QPF forecast bring this scenario into play as the most likely result. However, the GFS changed its views on the ridge, so it isn’t a slam dunk at this time. The strength of the ridge will dictate where the front is positioned – a stronger ridge would move the moisture more into upstate New York and New England, while a weaker ridge would allow the front to drop southward and position the moisture into the Mid-Atlantic.

Other scenarios

Those two scenarios are the most likely possibilities, but not the entire enchilada of possibilities. Other possibilities include a second low developing inland and grabbing the moisture forcing it even farther north, or the ridge expanding and building northwest which would keep the initial low trapped south (much like with Tropical Storm Allison in 2001).  Those alternate scenarios may keep this out of the Northeast entirely, but with models not really sold on them, we can’t assume they will happen.

It will surely be an interesting weather week regardless!

Forecaster Craig Ceecee – Twitter: @EternalWeather1

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