Monday was the first time in roughly a week that heavy rainfall did not occur in at least some part of my area. However, the wet weather was replaced with unseasonably warm summery air that was loaded with moisture. The heat index in Waterloo reached 97 degrees. Rare indeed for the last day of September. At least 21 reporting stations in Iowa had a high of 90 or above. I put a green star by each 90 degree report and you can see the entire state was well represented.
The atmosphere was plenty unstable but no thunderstorms developed because of the capping effect of warm temperatures aloft. That put a big old lid on the pot.
Later Tuesday that's expected to change as a slow moving cool front begins its descent southeast. Ahead of it a deep moisture fetch is visible on the GOES satellite from Hurricane Narda sitting along the west central Mexican Coast which is teaming up with moisture from the Gulf. This is feeding directly into the region and will remain in place through Tuesday night.
Another satellite perspective of the moist conveyor belt.
This will provide remarkably moist air for a very strong storm system moving into the region from the Northern Plains to work with. Precipitable water across northwest Illinois, Iowa and southern Wisconsin is forecast to range from 1.50 to 2.50 inches. Below the EURO shows a max of 3.38" in SW Iowa. MAMA MIA! I'm guessing that would be an all-time record for the date. Most likely that's overdone but no matter how you cut it this is a juicy air mass.
If the EURO would verify that is more than 7 standard deviations above normal. You won't see it any higher than that.
The high water values will lead to the possibility of 1 to 2 inches per hour rainfall rates.Parts of the area are back in the cross hairs of another heavy rain event which should sweep southeast Tuesday night and Wednesday. This time around the heaviest totals should be in the northwest half of my area, especially NW of a line from Dubuque to Cedar Rapids, and on to WIlliamsburg. There could be a little wiggle room north or south depending on outflow boundaries and their impact on the speed of the front. There are flash flood watches out for the areas in green.
Heavy rain would be problematic in just about any part of my area. This graphic shows that 1 hour flash flood guidance is around an inch or less in many parts of my area. The most susceptible spots are down in far SE Iowa.
It's easy to see why the guidance is so low when you look at the rains of the past 7 days.
Add that to the previous 3 weeks and you get 30 day rainfall totals that look like this. Richard Pinkowski of Altona, Illinois, (a friend of the site) reports 16.36" for the month of September. That's a solid foot above normal.
This graphic from the NWS shows projected rainfall totals through Thursday. As noted, it would not surprise me to see a few localized spots come in with 4" totals where the heaviest band sets up...still to be determined.
The week two forecast from the Climate Prediction Center shows the wet weather continuing into mid-October, although I don't expect anything this extreme.This could become an issue for agricultural interests if it isn't already.
One of the problems I see going forward is the reluctance of the MJO to get out of phase 1. The EURO still has it stuck there October 14th...with amplitude. This suggests more of the eastern ridge western trough set-up that's been in place for a solid month.
You can see the October phase 1 correlation to both temperatures and precipitation below (temps on the left, precip. on the right). Despite the mild look of phase 1 temperatures in October, there can and will be cool downs, one of those is coming later this week. However, there is a tendency for these to be progressive meaning the duration of any dips are usually short. In the end, temperatures are likely to average warmer than normal, especially at night. Again, that's just when the MJO is in phase 1 during October which looks to be the case through mid-month.
After the rain of the next 36 hours moves out later Wednesday, temperatures will be significantly cooler to close out the week. Highs will drop to the range the upper 50s to mid 60s Thursday-Sunday . The next chance of rain returns Friday night and Saturday. Roll weather...TS