GEARING UP FOR A COLD MAY RAIN...

This weather system that's on the way Saturday night and Sunday morning has been causing forecasters some consternation the past few days. Models have been aligned in two camps on the track. One nails most of my area with a soaking rain. The other keeps the bulk of the precipitation confined to the southern half to one third of the region. I've been waiting for greater consistency in solutions and while there has been some tightening, confidence is still not what I hoped for at this late stage of the game.


That said, rain will be knocking on the door by the end of the day Saturday so it's time to get off the fence and throw some punches. The GFS and the hi-res CAMS have been very staunch with their northern track. The EURO and Canadian have made a small but meaningful shift towards this northerly solution. I still have concerns about dry air minimizing rain totals in my northern counties and for that reason I like a a compromise of what the GFS and EURO are depicting.


That implies a healthy rain for my central and and southern counties with the heaviest amounts from roughly HWY 30 south. Rains of 1-2" have been consistently indicated, especially near and south of I-80. The WPC (Weather Prediction Center) does show a marginal risk (5-10%) of rainfall exceeding flash flood guidance in any one spot within the risk area shown below.

The official WPC rainfall guidance for the event looks like this as of Friday night. The majority of the rain falls Saturday night and early Sunday.

Of course there are differences between models and here are the choices that WPC had to pick from to make their outlook. I think WPC is in the ballpark reflecting a compromise of all solutions. Here they are for your viewing pleasure.


The GFS (by far the furthest north). Probably a bit aggressive with so much dry air being pulled in from Wisconsin.

The EURO. Probably just a bit too far south considering the strength of the dynamics. Overall I think this is a reasonable depiction and the solution that makes the most sense to me if you bump its totals up about 25 miles further north.

This is the HRRR

The 3k NAM

The 12K NAM

Whatever happens, my broad-brush thoughts are 1/2 to 1" HWY 20 to HWY 30. 1 to 1.25 HWY 30 to I-80. 1.25 to perhaps as much a 2" along and south of I-80 south. Overall, a good rain that is really needed in the NW half of my region.


The other little twist that I will mention (not because I think it is likely but because I want to cover myself in case it happens), is the possibility some wet snow could mix in for a time late Saturday night or early Sunday from approximately HWY 30 north. 850 temperatures are shown reaching freezing (about a mile up) as the storm center sweeps into Illinois. That combined with evaporative cooling with low dewpoints could mix enough cold air down to get snow to the surface, particularly if precipitation rates are heavy enough. This is a real long shot, maybe a 5-10% chance but every major model indicates some snow in the central Midwest in its output. To show you I'm not crazy, here are the snowfall depictions. Remember, these are just raw model output and not forecasts. It's the stuff forecasts would be made from. Right now nobody, including the NWS is even mentioning the snow potential. If by some quirky chance it happens in a few spots, at least I brought it up. I sure wouldn't hold my breath though.


The GFS

The 12K NAM

The HRRR

The 3k NAM

The EURO

Needless to say there's still issues to resolve with this system and there's still a little wiggle room for some things to change (or happen). Meantime, after a chilly start to Saturday, clouds will rapidly increase from west to east during the morning and early afternoon. However, in most areas rain should not begin in earnest until after 5:00 PM.


By the way, most of the rain should diminish and end during the morning of Mother's Day. Clouds on the other hand will likely be a player through much of the afternoon, especially in the east. It all adds up to a very chilly Sunday that could lead to highs holding in the upper 40s and low 50s (mid 50s at best). Those readings are roughly 15-20 degrees below average. Sorry mom, maybe next year! Roll weather...TS

ARCHIVED POSTS