Over the next several days, Long Island will begin to see temperatures gradually rise, reaching the upper 70s, maybe even low 80s by this weekend! This will change quickly, however, as a very strong cold front will sweep across much of the Great Plains, Midwest, Mississippi Valley, Great Lakes and Eastern U.S. by Halloween. Across Long Island, initial conditions will be cooler than normal, however temperatures will generally become more seasonable thereafter, with highs struggling to get out of the 60s and morning lows in the upper 30s/low 40s.
What is causing this pattern change? By now, many of you have heard of the tropospheric polar vortex, which is an area of low pressure that contains very cold air in the mid to upper troposphere (300-500 mb). In this particular scenario, that vortex becomes a bit displaced and some of that cold, arctic/polar air is tapped into and spills into the Lower 48, which you can see in the image below. Think of the dark blues (<315K) as polar air, and the purples/whites (<285K) as arctic air. In the final frames you can see that cold air swing through the Northeast, which explains this sudden pattern change.
For an explanation of the differences between the Tropospheric Polar Vortex and Stratospheric Polar Vortex, click here.
Weekend Rain: October 14-15th
Thursday, October 12, 2023 Another round of rain will arrive this weekend across Long Island associated with an area of low pressure expected to develop southeast of the Delmarva Peninsula. There is pretty good model agreement between the GFS & ECMWF, with the biggest difference being how far north the heaviest rain falls in the frontogenetical band. There is a very small spatial difference separating how far north the heaviest rain falls between the two models. The GFS shows the heaviest rainfall occurring only ~25 miles further north than the ECMWF. This could be the difference of some locations seeing upwards of 1.50" to as low as 0.5". Another thing to watch with this forecast is how tight the north-south gradient is regardless of which model verifies, meaning one town may see 1.5", while the town just to the south sees maybe half of that.
Of note, it has rained the past five, yes FIVE, weekends on Long Island, and this would mark the sixth straight weekend! What's comical (but not really), is that medium range guidance is suggesting rain next weekend as well! There is a good chance most of Long Island finishes with above normal rainfall for the third straight month. On the year for reference, Islip is 2.44" above normal for the year. New York City is a completely different story due to the historic rain they got last month. They currently sit between 7-12" above normal rainfall for the year!