© 2019 Terry Swails

BIG STORM, EVEN BIGGER TEMPERATURE FLUCTUATIONS...

April 11, 2018

The next 3 days will be mild ones around the central Midwest and most of my area. A powerful storm is spinning up over the Pacific that will send the warmest air of the year northward. However, what looks to be a basic forecast could be problematic as a lake enhanced front sinks into the region. These (back-door) fronts are known to have extremely sharp gradients as easterly winds pull cold air off the snow covered grounds of Wisconsin and frigid waters of the Great Lakes. Look at what the EURO shows Friday across Iowa.

Where southerly winds are found near the front, temperatures in southern Iowa are in the 70s and 80s. North of the front near HWY 20, easterly winds are cranking and readings drop rapidly into the 30s and 40s. The range in Iowa goes from 84 to 37 degrees at noon Friday.

 

That's where the forecast gets tough. If the position of the front is off by 50 miles you can be in the 30s instead of the 70s. This really comes into play Friday as the front slowly starts to make its way south.

 

While the difference is subtle, the EURO has the front and cold air as far south as I-80 Friday evening.

The GFS on the other hand has it in northeast Iowa. 

Look at the difference that makes in 7:00pm temperatures. Here's the EURO

The GFS at the same time.

That 150 mile range in frontal positioning between the 2 models makes a massive difference in places such as Charles City, Iowa where at 7:00pm the EURO has 37 and the GFS 65. Since this disparity occurs over the heart of my area that makes me very leery of being too optimistic on temperatures. If you are south of I-80 you've got a shot at 70 but north of there the potential exists for a bust. My gut feeling is the front will end up further south than the U.S. models think and that could rob the northern half of my area of what would otherwise be a very mild day. I guess we'll see. 

 

There is still a chance of some thunderstorms Friday (certainly some showers) as the big storm starts working its way across the Midwest. There's still quite a bit of doubt how energy will interact with the system so confidence is low with regard to the sensible weather that results. Currently the severe weather threat looks lower than what it did on Monday. The heavier rains appear north of I-80 through Saturday.

 

As I see it now temperatures will begin dropping Friday night and especially Saturday as a strong upper low evolves over Missouri. Here it is.

 

The EURO EPS ensembles have trended deeper and further south with the energy and actually develop a surface wave that wraps around the 500mb low aloft. It spreads accumulating snow into eastern Iowa. The GFS is further east and far less impressive on the intensity of the system. It does generate some snow but with little if any accumulation except in the area north of HWY 20

 

Due to all the variability in the models I'm going to stop for now. This will be a significant weekend storm with the potential to produce heavy rain, some snow, and gusty winds that grow colder as the weekend unfolds. Lots to figure out and data will get better in the next 24-48 hours. More then. Roll weather...TS

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