Aug 27, 2012: Cool Midweek, Hot Weekend

Forecast Highlights:

– Cool temps for Weds and Weds night; lows in upper 40s-upper 50s away from NYC
– Heat briefly returns for Fri+Sat, 90+ degrees expected near NYC
– Watching Isaac’s remnants for longer range

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Monday, August 27 Observations:

The area of showers and thunderstorms in the Mid Atlantic during the previous days drifted into the area during the morning, bringing an unexpected round of heavy thunderstorms producing heavy rain and localized flash flooding near NYC during the morning hours. Scattered storms were observed north/west of NYC in the afternoon as a cold front approached, moving through late overnight. High temperatures reached the low to mid 80s across the entire area.

Forecast Overview:

The latest radar, posted to the left, shows widespread storms to the north and west of the area. While most of the storms tonight are likely to stay north of NYC, scattered storms, locally heavy, are still expected late tonight into the first half of Wednesday. Storms may persist a bit longer into the afternoon hours for eastern areas with clearing skies from west to east. Highs will reach the upper 70s to mid 80s across the area.

Weds/Night: A trough will briefly move into the region, resulting in a brief yet significant cool down. Mainly clear sky conditions are expected with a high pressure moving through the region, with lows on Tuesday night in the mid 50s to lower 60s for most places away from NYC, a bit cooler inland (NW NJ/interior SE NY). Highs on Wednesday will reach the upper 70s to low 80s across the area, with overnight lows dropping into the low to mid 50s for most places away from NYC, with upper 50s closer to NYC and upper 40s inland.

Thurs-Sat: With the high pressure shifting south of the area, a west flow in the surface and aloft will develop with a surge of warmth moving in from the west. Mainly sunny skies will continue with highs reaching the mid to upper 80s on Thursday and the mid 80s to low 90s on Friday and Saturday. The most widespread low 90s are expected in the immediate NYC area.

Beyond Sat: The outlook after Saturday depends on what happens with Isaac’s remnants; the GFS still brings the remnants into the region with some showers for Sunday and heavy rain on Monday with the heaviest rain north of the area. The CMC is faster and weaker with the remnants, taking them over the area, while the ECM suppresses the remnants well to the south. There is still uncertainty regarding this time frame, although the potential is there for some rain on Sunday and/or Monday.

Tropical Storm Isaac: 1 Day Left Before Landfall

As noted earlier this afternoon, Isaac has been a storm that continuously defied its forecasts. Environmental conditions in the Gulf of Mexico appeared supportive for potentially rapid intensification, but other factors ended up limiting any strengthening, as Isaac continued to deal with some dry air, moderate shear as the lowest shear was still not on top of Isaac, and an increasingly large storm size, which especially prevented Isaac from rapidly intensifying. Although Isaac is still a tropical storm with 70 mph winds, its minimum pressure is 979 mb, much lower than the average minimum pressure for a tropical storm; although the minimum pressure continued to drop and is still dropping, instead of having stronger winds in a smaller radius, the winds are distributed across a larger radius, which results in weaker maximum wind speeds but a larger storm size. Isaac is a very large tropical storm, with tropical storm force winds extending 205 miles outwards from the center. For the last two days, although I expected a category 3 hurricane, I mentioned that the possibility was there that Isaac would not rapidly intensify, and this now seems to be the case; with the factors above considered, as well as the fact that Isaac has less than a day over water, my intensity forecasts from the last two days will end up busting on the high side, as Isaac never rapidly intensified as expected.

Just because Isaac’s wind speeds have not gone up much, however, it is no reason at all to take Isaac lightly. With the very large storm size, a significant storm surge is expected near the landfall region, and Isaac will also be very slow moving once making landfall, which will result in a widespread flooding potential with above 10 inches of rain expected. A similar scenario happened with Ike of 2008; the hurricane was expected to rapidly intensify in the Gulf of Mexico, but although its pressure rapidly dropped, its wind speed barely changed, and instead Ike significantly increased in its size, ending up as a very large hurricane. Although it made landfall with category 2-equivalent wind speeds, its storm surge and minimum pressures were about the equivalent of a category 4 hurricane, which resulted in more damage than what would be expected with a category 2 hurricane. While Isaac is not as strong or as large as Ike was, Isaac has some similarities to Ike; my current forecast for Isaac calls for slow yet gradual intensification until landfall as a category 1 hurricane, but with greater impacts than what would be expected from a category 1 hurricane.

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