Mild ridge reluctant to leave 

Posted: 3:00 am Thursday, September 27th, 2018

By Kirk Mellish

As per my previous posts an upper-level ridge over the Southeast was responsible for our end of summer-start of Fall heat wave. It has weakened and moved at times slightly off to the east allowing for the 90s to back off.

However, per my analogs and with model support it looks like no big push of true autumn air anytime soon.

And we’re back in the elevated humidity pattern that has been so persistent and frequent since Spring. We only cool off from increased cloud cover and or rain or backdoor wedge fronts.

Supporting the mild signal in the Southeast seems to be the Phase of the Madden Julian Oscillation (MJO) [phase 8 and 1] and low Global Angular Atmospheric Momentum (-GLAM or -AAM) aka Global Wind Oscillation (GWO) these large-scale hemispheric patterns have provided a background state to keep strong cold fronts away from the Southeast.

These atmospheric and oceanic states combined with lots of typhoon activity in the West Pacific goosing jet stream energy seem to be playing havoc with modeling in the medium to long-range. Thus they keep overdoing cool air penetration into the U.S.

MJO centered on October:

Screen Shot 2018-09-26 at 5.59.35 PM

The AO (Arctic Oscillation) is projected to go briefly and slightly negative in early October but the NAO (North Atlantic Oscillation) is near-neutral to weakly positive.

ecmwf_ao_bias

ecmwf_nao_bias

This can favor a central U.S. trough and below normal temperatures there, but again mostly to our Northwest. The PNA (Pacific North American) Index pops a bit positive as well so that is supportive.

The American GFS Model Ensemble is not supportive.

gefs_t2anom_16_noram_1

KATL_2018092612_min_max_16

The European ECMWF Model Ensemble is more favorable and consistent with its index teleconnection signals on the jet stream pattern and surface temperature anomalies:

Week ending Oct 11th:

eps_z500a_168h_nh_4(168)

Week ending Oct 18th:

eps_z500a_168h_nh_6(171)

WEEK ENDING OCT 11:

eps_t2m_168h_northamer_4(249)

WEEK ENDING OCT 18:

eps_t2m_168h_northamer_6(220)

WEEK ENDING OCT 25:

eps_t2m_168h_northamer_8(228)

KATL_2018092612_eps_min_max_15

6-WEEK AVERAGE DEPARTURE FROM NORMAL:

eps_t2m_1104h_east_185__1_(1)

15-DAY RAINFALL DEVIATION FROM NORMAL:

eps_tprecip_a_east_61__1_(3)

6-WEEK RAINFALL DEVIATION FROM NORMAL:

eps_qpf_1104h_east_185__1_(2)

Even if we split the difference there’s not exactly anything truly “cool” coming for at least the next couple weeks and anything that does make it probably will not hold for long.

I have to be skeptical of any real cool off given persistence until I see multiple cross-method signals (computer and non-computer techniques). That’s not in evidence as of now.

Follow me on Twitter @MellishMeterWSB.

 

 

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