Upper Low, Look Out Below!
A massive, cutoff upper level low pressure system will bring an extended period of cloudy, cool, wet and windy weather to many areas for the week ahead. We call these systems cutoff low pressure systems because they get cut off from the main steering currents, or jet stream. They are so big, they define where the jet stream goes, which is around them! This makes them very difficult to forecast and they can be full of surprises.
The headline above is an old saying forecasters use. Another is simply, "Closed lows are trouble". The image below (left) shows a forecast of what the upper low will look like Wednesday morning of this week. Note how huge it is! Except for the far east and west coasts, the rest of the country will see impacts from this system. The image to the right is the satellite view of the storm Sunday afternoon, April 27, 2014. It is winding up.
Closed upper low Wednesday morning
Satellite of the upper low
The upper low drives what happens down at ground level. The low pressure system at the surface will look a little different, but you can see from the surface charts below, low pressure will be with us right into next weekend!
We will likely see quite a bit of rainfall from this weather system. Typically, it occurs in waves, or bands, as energy slowly rotates around this huge system. The image below shows total rainfall across the United States over the next 7 days. Look at the national impact this storm will have!
If this amount of rain occurs, many of our rivers and streams will rise close to their banks. Many rivers are still swollen from our earlier snowmelt and spring rain. River gauges colored green and blue in the USGS map below are locations where streamflow is at or above normal for this time of year.
So, in summary, look for an unsettled week ahead with plenty of rainfall and very little sunshine. Temperatures won't be too horrible, mostly ranging from the upper 40s to mid 50s, though cooler near Lake Michigan.