The season's first widespread frost/freeze is about 2 weeks away.
Summer's last hurrah? Raingauges (and basements) overflowed across parts of the metro this week. The north suburb trailheads were the hardest hit with 6"-8" of rain. The newly constructed Lake Rebecca trail received 4.15" of rain prior to hosting the Big Woods Classic this weekend. Most impressive was the two-day rain total of 10.16" in Waseca. Sopping wet singletrack seemed to be the theme this summer. Hopefully the rain faucet slows now that it's autumn.
Rain by the Numbers
Soil moisture remains well above normal across Minnesota. Windier conditions will develop over the weekend but expecting trails to remain tacky for quite some time.
The saturated singletrack gets another good downpour late Saturday. Thunderstorm motion, however, will be faster preventing widespread flooding but an additional inch is likely in many areas.
By early next week it will look and feel like fall. Lingering lighter showers on Monday and Tuesday are expected as the upper level low slowly drifts over northern Minnesota.
Soil Temperature Monitoring
The Minnesota Department of Agriculture is encouraging farmers to check soil temperature and delay autumn application of fertilizers until soil temperature stays below 50˚F". “Waiting until soil temperature stays below 50˚F before applying anhydrous ammonia and urea increases the availability of nitrogen to next season’s crop and decreases the amount of nitrate that could potentially leach into groundwater.” says Bruce Montgomery, manager of the MDA Fertilizer Management Section. Microbe activity in the soil slows down with temperatures below 50°F slowing the conversion of nitrogen fertilizer to nitrates therefore decreasing the probability of nitrates seeping in the water supply.
Soil temperatures typically stay below 50°F in southern Minnesota by the end of October. Across the state 48 stations measure soil moisture at a six inch depth with the data updated every 15 minutes.
Moisture leftover from Tropical Depression Paine that made landfall in Baja California on Wednesday will stream across the Rockies into the Midwest by Thursday. With the added moisture, heavy rain chances will continue through the end of the week.
While the rain won't be continuous, there will be three distinct periods of heavy rain:
The atmospheric setup is similar to the record rain event of September 23, 2010 when 5 to 8 inches of rain fell across southern Minnesota resulting in major flooding.
Stream Level Departure from Normal
Many waterways, like Minnehaha Creek, are already experiencing above normal flow prior to this heavy rain. In addition the ground is fairly saturated. In the image below, green indicates where the soil is greater than 50% of saturation.
The National Weather Service in the Twin Cities details the heavy rain and flood threat this week.
Sometimes you gotta fight through the bad weather days in order to appreciate the good ones. Friday's rain will be a distant memory this weekend. Dry air should speed up the evaporation rates making for a good, tacky MN High School League race in St. Cloud Sunday.
Late Week Rain
Next shot of rain comes late next week when amounts may be heavy again. Tropical-like moisture streaming into the Midwest will set the stage for a heavy rain event Wednesday night into Thursday. This could impact trail riding into next weekend.
Radar estimated rainfall the past 7 days shows a light to dark green shade over Twin Cities. This indicates rainfall 0.50" to 1.00" above normal. Purple shades show where a month's worth of rain has fallen in a matter of days.
Cooler Days Ahead
Almost sweatshirt weather! Almost. Looking at some cool night this week with lows potentially dipping into the 50s. Could help curb those mosquitoes! Here's a bit of context from the National Weather Service in the Twin Cities:
The last time MSP has had a low below 50 degrees was May 18th. If the low below 50 degrees occurs, this would end our streak of days where the temperature has remained 50 or greater at 118 days, which would be the second longest such stretch behind only the 124 days from the summer of 1881.
The atmosphere has become stuck. Weather systems are sluggishly moving across the country with the same weather lingering over the same areas. Minnesota just so happens to be stuck in the storm track this week.
Soggy Singletrack Rounds of thunderstorms develop through Wednesday as a front stalls over the state. Tropical moisture streaming north from the Gulf loads thunderstorms with heavy rain potential. Several inches of rain will fall on our area trails. Wouldn't be surprised to see rainfall amounts top 5 inches in spots. Opened trails will be hard to come by this week. Best to stick to pavement riding for now.
Tropical-like Downpours Precipitable water values are running high. This indicates a higher potential of thunderstorms capable of producing torrential downpours. Rainfall rates of 1-2 inches per hour are possible within any individual thunderstorm. To make matters worse thunderstorms will have a slow roll only exacerbating the flash flood threat in any one location. This will make the trails more susceptible to ruts and erosion.
Weekend Rain? A brief reprieve from the rain Thursday is followed by thunderstorms Friday night. Luckily the rain developing this weekend will not be as heavy as what we will see early this week.
Summer brings warm afternoons perfect for biking, but the summer heat and humidity can also spark thunderstorms and severe weather. Be prepared for any type of adverse weather headed your way with Aeris Pulse.
NWS Twin Cities Weather Story