WHERE IT RAINS, IT WILL POUR...

July 19, 2017

Now that the deep tropical moisture is back in the pattern it appears that the table is set for a multi-day heavy rain event over some part of the central or upper Midwest. While models do a good job of broad-brushing the threat, it's usually hard to define the specific location until just a few hours before the storms unleash their rain. But, where it rains it will pour!

 

One of the essential factors in producing excessive rains is moisture. Here's the precipitable water vapor (PWATS) forecast by the GFS Thursday morning. Amounts of 2.75" over SC Minnesota is a tremendous reservoir of water vapor for storms to draw on.

Another way to measure moisture is dew points. A 70 degree dew points is big, an 84 dew point which the GFS depicts is monstrous!  

The heat index (how it feels with the temperature and humidity combined) is ridiculous. The GFS has values near 120 over a strip of central Iowa.

That type of energy leads to an explosive atmosphere. Here's the CAPE values (convective available energy) which depicts instability. Where that moisture is pooled CAPE over 5,000 j/kg is depicted!

The final ingredient in any significant heavy rain event is an upper level pattern that allows training where storms continually develop and move over the same general area. The ring of fire is well known for producing nocturnal convective events that meet that criteria.

 

So with some available forcing and all these ingredients in place, I'm watching southern Minnesota, SW Wisconsin, and northern Iowa for a flash flood threat over the next 4 days. Here's what the last 4 runs of the GFS are showing for rain the next 10 days. Most of this is forecast to fall between Wednesday and Sunday. Notice every run showing a focus on the area I just mentioned above and amounts of 9-10". The forecasts below go from oldest to newest and are produced every 6 hours.

The EURO is not as bullish and a bit further north. For a change I think the GFS may have the right idea taking into account outflow boundaries, the upper air flow, and cold pools generated from various convective episodes.

It's my belief there are going to be some problems up north before this pattern breaks. The Weather Prediction Center agrees showing this for 7 day total precipitation.

If it's not one thing, it's another. If you don't get the storms, you will get the heat and the extreme humidity. Yippee! Roll weather...TS

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© 2019 Terry Swails