Sunday, May 1, 2011

Active weather continues Monday, then a break

As we saw last week, sometimes these severe weather events can be incredibly destructive.  Last week's tornado outbreak in the south was the largest since the 1930s--around 350 have been confirmed killed so far.

And, unfortunately, there's a risk of another round of severe storms for that part of the country on Monday.  The SPC has a slight risk out for a narrow corridor of the southern US.
Fig 1 -- SPC day 2 convective outlook valid 1729Z, May 1, 2011.
Thunderstorms are ongoing over eastern Oklahoma and Arkansas tonight.  The cold front helping provide lift to these storms will translate eastward overnight in conjunction with an upper-level trough. Here's the 500mb forecast for Monday morning:
Fig 2 -- ECMWF 24-hour forecast of 500 mb winds (colors) and heights (contours) for 12Z, Monday, April 2, 2011.
The trough is positively-tilted, with a strengthening jet streak on its southeast side.  This will help to provide the divergence aloft to support continuing thunderstorm development.

By Tuesday evening, we can see in the GFS forecast for dewpoint temperature that rich moisture will only have just reached northern Mississippi and Alabama.
Fig 3 -- GFS 30-hour forecast of surface dewpoint temperature valid 00Z, Tuesday (Monday evening), April 3, 2011.
While moisture is in place, the lack of strong southerly winds and the fact that the moisture just reaches that far north by the time the cold front arrives is going to work against strong severe storm development.  However, there is still a slight risk for some powerful storms--so be aware.

For the rest of this week, the flow pattern looks more zonal in the models--east-west flow--with a few troughs staying further north.  Zonal flow can be a mixed blessing--it's not ridging, so it doesn't indicate widespread subsidence conditions and necessarily calm weather.  But, it's also not troughing, which would indicate significant storms.  Zonal flow patterns are also harder to forecast--because they don't indicate significant troughs or ridges, small differences in models can grow to become much larger differences later on.

Regardless, here's the 500mb European model forecast image for Friday:
Fig 4 -- ECMWF 120-hour forecast of 500mb winds (colors) and heights (contours) valid 12Z, Friday, May 6, 2011.
We can see two shortwaves that have backed up over the northeastern US--one off of the New England coast and the other coming through the central Great Lakes.  At this point, the amount of moisture return northward and the strength of the upper-level flow are somewhat conditional for any sort of severe weather threat.  However, chances of rain and thunderstorms will no doubt be present--only now across the northern tier of the country instead of the south.  So--after tomorrow, it looks like the south should be out of the woods for most of the week.  And, by the end of the week, most of the rest of the country should see pretty quiet weather as well.  At least with current model forecasts...

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