MDR storms that hit U.S. as majors: + correlation to NAO on hit date NOT formation date

[GaWx]

Since 1950, 29 storms formed east of 50W and later hit the U.S. The NAO on the date of the hit averaged +0.20 and 19 of the 29 hit dates had +NAO's. So, there appears to be a partial positive correlation with the hit date's NAO. When I dissect these #'s further, I find some interesting tendencies. Of these 29, eight hit as major H's. Regarding just these eight (I know it is a fairly small sample but just large enough to make it worth analyzing), an impressive seven (88%) hit with a +NAO and the one negative was only modestly negative (-0.327 for Allen of 1980). The average NAO for these eight hit dates was solidly positive at +0.6. When I exclude the eight major hits, I'm left with 21 that have no +NAO tendency on the hit date....their NAO's averaged near 0 with 12 being pos. and 9 being neg.

Next, I looked at the NAO's as of the date of formation. For the 29 as a whole, there was no real positive correlation found with 16 positive and 13 negative and an average that was actually slightly negative ( -0.1) . Now, when looking at just the eight that later hit as a major, six (75%) had a -NAO at formation and the eight averaged -0.2. Five of the six that had a -NAO at birth hit with a +NAO. When I exclude the eight major hits, I'm left with 21 whose NAO averaged near 0.

Summary for MDR storms that later hit U.S. as a MAJOR:

Although they don't show a preference for a +NAO when they first form and may even slightly prefer a -NAO then, they appear to have a strong preference for a +NAO when they hit. Whereas they averaged -0.2 when forming out in the MDR, they averaged +0.6 when hitting the U.S. So, it seems that the conditions that may give the best shot at an MDR storm later hitting the U.S. as a major are when the NAO rises significantly from wherever it is at date of formation (could even be negative) to a positive when it hits. Five of the eight experienced an NAO increase >1 from birth to hit and seven of the eight had an increase.

[matthewweatherwatcher]

Great Post gawx. I agree with you solidly. More ridging building north of a cyclone is always the best way to get a tropical cyclone to hit the United states--- rather than a blocking trough over the western Atlantic when dealing with mdr formations.

[Ellinwood]

Could you break down the landfall locations of said systems? (Gulf, EC, EC without FL)

[rainstorm]

great info. it makes sense that as the NAO is rising it would cause ridging to build and force a more westward path.

[GaWx]

Great Post gawx. I agree with you solidly. More ridging building north of a cyclone is always the best way to get a tropical cyclone to hit the United states--- rather than a blocking trough over the western Atlantic when dealing with mdr formations.

Thanks, Matthew. Not only that, but that also gives them the best shot at getting there as very strong and/or strengthening TC's. NW Atlantic and/or solid Bermuda ridging ftw or ftl depending on one's perspective.

By the way, I just looked more closely at Lli of 2002 and now am leaning toward it not being a major when the center actually crossed the coast even though it was a cat. 3 within about 50 mies of the shore. So, although I had included it as a major, one could argue it wasn't officially a major hit. Here are the hits from the five highest NAO's on the day of the hit:

- Hugo cat. 4; NAO = +1.466

- Bertha cat. 2; NAO = +1.123

- Lili cat 1 hit/cat. 3 offshore; NAO = +1.116

- Able cat. 2; NAO = +1.072

- Frederic cat. 4; NAO = +1.025

[GaWx]

Could you break down the landfall locations of said systems? (Gulf, EC, EC without FL)

I'll break down the eight majors, including Lili. Note that 7 of 8 of these had a rise and 6 of 8 had a significant rise. The only one without a rise, Andrew, was at the most + of the 8 as of formation and still was + as of hit date.

A) EC w/o FL: storm, NAO when formed, NAO when hit U.S.

- Hugo, +0.355, +1.466

- Fran, -0.364, +0.614

FL:

- Donna, -0.492, +0.663

- Andrew, +0.639, +0.288

C) Gulf without FL:

- Frederic, -0.037, +1.025

- Allen, -0.861, -0.327

- Lili, -0.687, +1.116

- Ivan, -0.03, +0.22

[GaWx]

great info. it makes sense that as the NAO is rising it would cause ridging to build and force a more westward path.

Thanks, rainstorm, and welcome back. I have to give you credit for inspiring me to analyze the NAO and its effect on tracks in more detail since you never seem to stop talking about its importance. The rising NAO also seems to tend to lead to a powerful and/or strengthening storm in the western part of the basin.

[WxMidwest]

Preference goes to -NAO when Tropical Entities initialize and when major hits happen on the East Coast a +NAO is most likely. This is the Archambault Event of the Tropical season going in 1 direction (NAO - to +) instead of just a Massive change from one pole to the opposite pole during Major East Coast Winter Storms. Good Deal..

[Riptide]

Preference goes to -NAO when Tropical Entities initialize and when major hits happen on the East Coast a +NAO is most likely. This is the Archambault Event of the Tropical season going in 1 direction (NAO - to +) instead of just a Massive change from one pole to the opposite pole during Major East Coast Winter Storms. Good Deal..

Excellent way to describe MDR climo and I agree completely.