Departures from the normal range of sea surface temperatures in the central and eastern equatorial Pacific Ocean, coupled with observed patterns of convective weather in the tropics indicate a strengthening El Nino pattern that should persist at least through the end of the Northern Hemisphere winter.

Sea surface temperatures in the eastern Pacific have ranged as much as 1.5 C higher than average, while below the surface, anomalies ranging from 1.5 to 5.0 C have been observed in the upper 300 m layer, patterns that are consistent with an established El Nino event.

Temperatures at the ocean’s surface have a large influence on the weather above, as thermal energy is transferred from the water to the air. Persistent areas of higher pressure consistently form in the atmosphere above warmer ocean waters, a scenario that NOAA forecasters expect to unfold over the coming months.

Most of the forecast models used to predict the strength and duration of El Nino suggest that it could last into the March-May 2010 period, with its intensity likely peaking within the moderate range and lasting through the Northern Hemisphere winter.

That could be very good news for people who enjoy mild temperatures and less snow in the north-central United States and southern portions of the Canadian Prairie Provinces, where the potential impact of El Nino is expected to include above normal temperatures and below normal precipitation during the winter months.

Map Courtesy Unisys Weather

A very warm and moist air mass resided across southern Manitoba Thursday afternoon, with dewpoint temperatures reaching 25 C. A cold front, in association with an intense upper trough, was forecast to sweep northeastward from North Dakota into southern Manitoba Thursday evening. Ahead of the cold front, severe thunderstorms – with a primary threat of large hail – were a high probability.

I observed a large, isolated thunderstorm developing just north of the International Border around 7:00 PM, moving northeast at 30 km per hour into southern Manitoba. I opted to intercept this storm as it approached highway 75 near Morris, MB.

As I intercepted the storm, it was apparent that it was evolving from an isolated cell into a linear MCS, with intense electrical activity, wind gusts measured to 85 km/h and quarter sized hail.

In summary, this storm was well worth the short trip south of Winnipeg to witness the amazing “anvil crawlers” that lit up the sky in almost every direction. I found a nice vantage point just north of the precipitation to take some memorable pictures of this event that persisted until well after 11:00 PM. Opportunities like this are a rare treat in September.

At first glance, it looked as though Friday would have good potential for severe weather, and possibly a few tornadoes in North Dakota, with plenty of instability in place (3000+ j/kg) and good low level shear. Gulf moisture had been advecting across the central and northern Plains since early morning, aided later in the day by the development of a respectably strong low-level jet (LLJ).

I proceeded to the south part of the state, where temperatures had reached 83 F by just after noon, with dewpoints into the high sixties and low seventies. Everything looked primed for some explosive convection, especially with a 500 mb short wave trough rounding the crest of the ridge over the central CONUS. I was parked in Fargo, ND for most of the afternoon, waiting for things to pop in that blanket of low level moisture.

By around five in the afternoon, nothing was happening over Fargo, or any other part of southern North Dakota. The GR Level3 software was showing a large squall with possible embedded supercells progressing ESE just north of Grand Forks, so I opted to head north on I29 to intercept the band of thunderstorms, which was showing several tornado warned cells and TVS (tornado vortex signature) markers.

When I arrived on the outskirts of Grand Forks around six, the tornado sirens were wailing in the city. The edge of the squall did look impressive, with a prominent shelf cloud, powerful gust front and drenching rainfall. But no tornadoes were in sight, especially in the low level grunge and scud that rolled and boiled beneath the shelf cloud.

I suspect that the tornadoes sighted within the squall line were more than likely gustnadoes, or perhaps a bookend vortex or two in the mix.

All in all, an exciting and fun chase today, but the prize of a high-contrast tornado photo remained elusive.

May, a month that normally sees ample active weather across the Great Plains, may go down in history for 2009 as one of the poorest on record for supercell storms and tornadoes, which is extremely bad news for the Vortex II research project that began field operations on May 10. No storms equals no data, and wasted time for scientists trying to unlock the enigma of tornadogenesis.

Currently, GFS model forecasts portray a persistent blocking pattern over the central CONUS, and a resulting north west flow regime that will keep most of the moisture from the GOM locked up at least until the end of the month.

The annual northern migration of the polar jet means that any severe weather outbreaks to occur in June will be more likely to occur in the central or high plains areas, assuming that moisture-rich GOM air is able to once again advect northward during the period.