Isaias Becomes Extratropical Over Southern Quebec

747 

WTNT44 KNHC 050246

TCDAT4

Post-Tropical Cyclone Isaias Discussion Number  32

NWS National Hurricane Center Miami FL       AL092020

1100 PM EDT Tue Aug 04 2020

Satellite and radar data, along with surface observations, indicate

that Isaias has lost its tropical cyclone characteristics as it

merges with a weak baroclinic zone over New England and

southeastern Canada.  Thus, it is being designated a post-tropical

cyclone.  The initial intensity is reduced to 40 kt based mainly on 

a recent observation at the Isle of Shoals, New Hampshire, and 

these winds are occurring over the Gulf of Maine well to the 

southeast of the center.  The post-tropical cyclone is expected to 

weaken further, with winds dropping below 35 kt early tomorrow.  

After that, the system is forecast to be absorbed into a large 

baroclinic low over southeastern Canada between 36-48 h.

The initial motion remains north-northeastward or 020/33 kt.  A

continued general north-northeastward motion with a decrease in

forward speed is expected until the cyclone is absorbed by the

larger low.  The new NHC track forecast is similar to the previous

forecast and is in the center of the tightly-clustered track

guidance models.

While Isaias is now post-tropical, the National Hurricane Center

will continue to issue advisories until the Tropical Storm Warning

can be discontinued along the coast of the United States.

Key Messages:

1. Tropical storm force winds that could cause tree damage and power

outages will continue over portions of New England for a few more

hours.

2. As the rainfall exits the United States, scattered minor to 

moderate and isolated major river flooding is ongoing or forecast 

across portions of the Mid-Atlantic with most rivers falling below 

flood stage Wednesday.  Quick-responding rivers in the Northeast 

will also be susceptible to minor and possible moderate river 

flooding.

3.  The threat of tornadoes will continue over portions of Maine 

for a few more hours.

FORECAST POSITIONS AND MAX WINDS

INIT  05/0300Z 45.3N  72.5W   40 KT  45 MPH...POST-TROP/EXTRATROP

 12H  05/1200Z 49.1N  71.1W   30 KT  35 MPH...POST-TROP/EXTRATROP

 24H  06/0000Z 52.6N  70.0W   25 KT  30 MPH...POST-TROP/EXTRATROP

 36H  06/1200Z 54.0N  68.2W   25 KT  30 MPH...POST-TROP/EXTRATROP

 48H  07/0000Z...DISSIPATED

$$

Forecaster Beven