Strong southerly winds over the weekend brought warming temperatures and increasing moisture to much of the Plains states, and has set the stage for several rounds of showers and thunderstorms through midweek. A frontal boundary has already pushed into areas the eastern Dakotas and Nebraska overnight with ongoing thunderstorms. A few storms earlier this evening were severe with the potential for damaging wind gusts given a good layer of dry air just below the cloud bases. This trend will likely continue through the next couple of days, with any thunderstorms being able to produce heavy rainfall and damaging winds, given some cooler air aloft and good instability a few large hail reports also seem plausible.

A plume of increased moisture currently across parts of Kansas will continue eastward during the morning hours, bringing a few showers and perhaps a thunderstorm to areas of Kansas City during mainly the morning hours. As this plume of moisture interacts with the heating of the afternoon, more widespread showers and thunderstorms appear possible across areas of central and southern Missouri. The main development will be back along the surface front, from central Minnesota through western Iowa and into south-central Nebraska and northwest Kansas. With dew points near 70 degrees, sufficient mixed layer instability will be present across much of the area to support thunderstorm development. Strong southwest flow will be present over the Plains, with a jet max over the Dakotas aiding to bring out the upper level trough. The highest winds will however be displaced west of the surface front, yielding only marginal deep layer shear near 30-35 knots along and ahead of the boundary. This combination of shear and instability will most likely yield multicell and broken lines of thunderstorms over the aforementioned areas. Nonetheless, fairly widespread thunderstorm development is expected across the Plains during the evening and will continue throughout the overnight as the low level jet continues to provide the forcing for thunderstorm development Tuesday night.

Showers and thunderstorms will once again be likely on Wednesday as a combination of the remnant outflow boundaries, remaining stalled surface front and good low level moisture trajectories all provide triggers for ongoing development. The picture here is a little more complicated, and will likely change with the occurrences of Tuesday night as to where boundaries will setup for Wednesday afternoon development. Nonetheless, at least a few clusters of thunderstorms will be possible over parts of the Central Plains with an attendant severe weather threat for damaging winds and perhaps some marginal hail. Based upon the latest 00z NAM it would appear that a favorable area for strong to severe thunderstorms may exist over north-central Kansas where moderate mixed layer instability and deep layer shear near 40 knots is expected to be available. An additional update regarding Wednesdays potential is possible as the days events become better defined…

The surface front will finally be pushed through on Thursday as the upper level trough is pulled out onto the Northern Plains. Bringing additional showers and thunderstorms to areas of Kansas, Missouri and Iowa during as it passes overhead. Another cool air mass will move in behind this frontal passage with temperatures once again nearing only the 80 degree mark for highs and low dropping back down into the 50s.

A cold front currently moving across the central Plains will be the focus for a few showers and thunderstorms, mainly over parts of Kansas/Oklahoma and then Iowa/Minnesota during the day on Tuesday. In between the lack of moisture and upper level support will likely keep anything more than clouds and a few sprinkles from forming. Thus the biggest story with this cold front passage is the actual change in temperature behind it. The cold front combined with a strong high pressure system will allow temperatures to be below normal for a few days, with lows falling well below normal thanks to the dry air and abundant lack of clouds.

Just take a look at the forecast temperatures for 12z Wednesday, approximately the low temperatures for Wednesday morning from the NAM (left) and GFS (right). A large area of lows in the 50s across the Plains!!

These trends of cooler temperatures continue into the daytime on Wednesday with high temperatures likely in the 70s for many areas, with a few lower 80s possible despite ample sunshine!

Another quick look at the nighttime models perspective of Thursday as well, with low temperatures once again below normal and high temperatures just starting to creep upwards thanks to the sunny skies across the region.

I will be pleased to leave the windows open and enjoy this refreshing air after some of the days we’ve had over the past few weeks. For those of you who aren’t wishing the arrival of Fall quite yet, you’ll be happy to hear that the continued sunny skies will be aided by a return of southerly flow at the surface by Friday and Saturday. This will push high temperatures back well into the 80s and lower 90s over the weekend into early next week.

It was covered on nearly every Northern Plains local television network, each national network’s news broadcast and even featured on sites such as ESPN! Copious amounts of rainfall occurred within a very short period in early August just north of Ames, IA and near Des Moines, IA causing rapid rises in local streams and rivers. These rises were so quick that water rose from well below flood stage to over flood stage in a matter of hours, with several homes and businesses going underwater without any time to attempt sandbagging or defensive procedures against the water.

The overall events have been covered by numerous sources and there are copious amounts of images of the flooding from Ames, Iowa State University and numerous other towns within central Iowa. Below you can see the past 30 days of rainfall across Iowa (left image) and the percentage compared to normal (right image), much of which came in only a few short days.

A quick graphic to show how rapid the rises were and to what level they occurred you can see the three hydrographs below. These are from the Squaw Creek in Ames (left), South Skunk River in south Ames (center) and the South Skunk in Colfax, IA (right). The record level is also shown on the hydrographs, with the highest levels coming in Colfax where the upstream rain allow the river to crest higher and sustain its’ levels for a long period of time.

I realize the discussion here is not too in depth, and it is not intended to be. I just wanted to be sure to touch on the Ames, IA flooding and give the city and Iowa State University it’s much deserved post regarding the situation.

This past week began quite calm after the passing of our previous storm system that brought the damaging winds to many areas across the Central Plains. Temperatures were fairly seasonable in the upper 80s and humidity was much lower than the previous weeks oppressive heat and heat index values. We slowly warmed up through the week and reached into the 90s by Thursday and Friday for parts of the Central Plains. These warming temperatures and increasing humidity aided in producing a fairly unstable atmosphere, which was promptly breached by a weak cold front and upper air disturbance(s) by Friday afternoon.

Thunderstorms were ongoing during the morning hours ahead of an upper level disturbance over parts of northern Kansas and Missouri. A weak disturbance moving to the northeast across parts of Missouri prompted several clusters of morning thunderstorms over central and eastern Missouri including a few severe storms. One of these storms impacted areas of St. Louis with winds nearing 60 mph, the lightning associated with the storm was responsible for disabling the St. Louis NWS radar and also injuring a mail carrier in St. Charles, MO. Further to the west a stronger upper air disturbance was moving across northern Kansas and initiated additional thunderstorms during the late morning hours. The thunderstorms encountered an increasingly unstable air mass and began to go severe over the Kansas City Metro with severe thunderstorm warnings being issued for mainly northern portions of the KC Metro by early afternoon. In addition to the moderately unstable air mass, wind shear values were sufficient given the increased upper level winds with the disturbance. Deep layer shear near 40 knots favored multicell and near supercell storms, with enhanced areas of wind shear along outflow boundaries from ongoing or previous storms. One tornado warning was issued during the afternoon for a thunderstorm near the town of Holt, MO as radar indicated winds of 80+ mph and some small areas of rotation. The town of Holt, MO received substantial tree and roof damage due to the straight line winds of ~80 mph with the entire town losing power.

Thunderstorms continued to develop throughout the afternoon in the unstable air mass, including additional thunderstorms only hour(s) after the initial storms as the atmosphere destabilized again. These thunderstorms were severe for a short time but quickly lost strength as the evening hours began, although once again the lightning with the evening and early overnight storms was quite impressive. Further east to areas that did not see as widespread thunderstorms earlier in the afternoon had the instability and shear to warrant several tornado warnings from near Jefferson City, MO to near St. Louis, MO. No tornadoes were reported with any of the thunderstorms…

The severe weather threat was once again damaging winds thanks to dry air located aloft, although a few large hail reports were received near Topeka with a few thunderstorms that initially took on supercellular characteristics. A few areas also received heavy rain due to thunderstorms traversing the same locations, most notably the southern Kansas City metro and eastward as well as much of central Missouri near Columbia and Jefferson City.

View the Local Storm Reports from yesterday’s severe weather.

Looking ahead to the next few days, the cold front only drew in a couple of degrees cooler air and really did not hamper any sort of humidity. In fact due to some of the heavy rains quite a few areas are likely even more humid than before the frontal passage. The next cold front is forecast for Tuesday, with little excitement expected due to the lack of surface convergence and no upper level support. The bigger story will be the cooler and drier air filtering in behind this cold front, some may say it is a pre-autumn type air mass with hints of the cool and crisp air to come!

The past few days have yielded numerous thunderstorms across parts of the Central Plains, including both Kansas and Missouri. The thunderstorms on Wednesday (8/11) and Thursday (8/12) were both in response to a well-mixed boundary layer that allowed surface based storm parcels to reach their level of free convection (LFC). Essentially, due to the very hot and humid air mass at the surface and generally dry air just above the surface that as the lower atmosphere mixed it was able to create parcels of air that freely convected to create thunderstorms. This layer of dry air just above the surface that allowed the strong mixing was also responsible for increasing the strength of the downdrafts within these thunderstorms, thus the several damaging winds reports that in some cases were in excess of 75 mph on all three days!

An observed sounding from Topeka (left image) on Friday evening (7pm) can give you an idea of the drier air at the lower levels. Keep in mind that thunderstorms had already passed over the area at this time, thus before thunderstorms the low level temperatures were likely even warmer than shown. You can see the large difference in the dew point and temperature in the lowest 1km, this drier air allows rain to evaporate and cool faster. Cooler air is more dense, thus falls to the ground faster, and in turn causes higher winds as the air reaches the surface. Another way of viewing this potential is through the use of Downdraft CAPE, similar to the regular CAPE the larger values in this case mean higher energy for downdrafts of thunderstorms. On Friday you can see the Downdraft CAPE is 1500-1800 J/kg over the area of severe thunderstorms just before than initiated (right image). Values of downdraft CAPE nearing 1500 J/kg were present on Wednesday and Thursday as well, thus confirming the likelihood of strong downdrafts.

Wednesday storm’s affected areas of eastern Kansas and central Missouri, where reports were received of severe winds and wind damage. Notably, a severe thunderstorm near Osage City, KS produced numerous reports of 70 mph winds; and a thunderstorm that trekked through the Lees Summit, MO area produced a wet microburst that was estimated in the 70-80 mph range. You can read more detailed information about the microburst in Lees Summit from a NWS Kansas City Page, and view all of the reports in the area via the IEM LSR App.

A similar situation occurred the following day on Thursday, with hot surface temperatures nearing or above 100 degrees and some areas of small convergence located in the 850-700hPa layer contributing to severe thunderstorm development. Thunderstorm were more numerous in eastern Missouri, where several rounds of thunderstorms near St. Louis also prompted a few flash flood warnings as thunderstorms were slow moving and traveling over the same locations. Notably on this day were the numerous damage reports near the St. Louis area, and also damage reports with 75-80 mph winds on the northeast portion of the Kansas City metro. You can see all of the reports from Thursday via the IEM LSR App.

Thunderstorms were widespread throughout areas of Kansas, Missouri, southeast Iowa and Illinois among other places due to a weak cold front and upper air disturbance moving through the area. Two complexes of thunderstorms initialized over parts of southeast Iowa, western Illinois and northeast Missouri; as well as parts of central Kansas. Thunderstorms in central Kansas into far western Missouri were the most intense, likely due to the extreme dew point depressions and effectively drier air near the surface with temperatures in the lower 100s and dew points near 70 or the 60s. A wind gust of 93 mph was measured in Manhattan, KS, with several other 80+ mph reports received in northeast Kansas during the late afternoon and evening hours. You can see all of the reports from Friday on the IEM LSR App, note that several reports seemed to have been filed twice leading to a fairly cluttered map.

The final storm that pushed by the south metro locations of Kansas City was also quite electric, easily with 5+ cloud-to-ground strikes per minute with the storm. My wife (Alyssa) was able to capture a few of them with a small point-and-shoot with just 3-second exposures while hand-holding the camera. Two of those shots are shared below:

Thought I would provide a brief update regarding some of flooding that is ongoing over areas of Iowa. You can look back at a previous post regarding a recap of July and its’ flooding woes due to copious amounts of rain that fell over much of the Central and Northern Plains. While we saw a brief break with the hot weather, several thunderstorm complexes have trekked across the state in the past few days and have once again wreaked havoc on local streams and rivers. Flooding occurring in parts of central Iowa, including Ames and Iowa State University as well as parts of the Des Moines metro saw rapidly rising waters last night and through today. Several locations in Ames are approaching or have already exceeded the record levels, many of which occurred during the devestating floods in 1993. Numerous roads, including I-35 and Highway 30 near Ames have been closed and there is potential that I-80 could also be closed later due to rising flood waters; along with dozens of local highways and rural roads. You can view several images and videos of the flooding by going to any local media outlet in the Des Moines area, and see the latest regarding Iowa State University and its’ flooding at their website (www.iastate.edu). The Cyclones football team held its’ practices today by lifting sandbags to try and hold off the water from entering a few campus athletic facilities. You can see a few of there pictures and others, including the flooding of Hilton Coliseum at the athletics website (www.cyclones.com).

Thankfully it currently appears that the water rises will be short-lived, with waters beginning to recede already this afternoon. However, a cold front is expected to push into the area Thursday evening which may once again bring widespread thunderstorms to the state on Thursday night, Friday and Friday night before ending once again. I’ll post an entire recap on the flooding including rainfall amounts, river levels and some links to photo albums, etc. sometime this weekend as the flooding begins to subside (hopefully).

Taking a deeper look into the meteorological parameters that aided the development of an EF4 tornado in Wilkin county, Minnesota on August 7, 2010. We’ll begin with the standard levels, from the surface to 300mb. At the surface, images below 6pm (left) and 7pm (right) with surface features added, we can see a broad area of low pressure located just west of where the tornado occurred. A warm front extends eastward across central Minnesota away from the low pressure center, with the tornado occurring nearly on or just north of this boundary; A cold front also extends southward from the low. I’ve also included the 23z surface observations from the area to give you an idea of the surface thermodynamics that were available, with mid 80s temperature and low 70s dew points at the thunderstorm location.

Looking through the rest of the standard levels, beginning with 925 hPa (far left), 850 hPa (left-center), 700 hPa (center), 500 hPa (right-center), and 300 hPa (far right). You can see throughout the lower levels (925-700 hPa) that the atmosphere is quite moist in the vicinity of the tornadic storm. Also of interest to note is the 700 hPa temperatures, which are ~10 C at the time of the tornado and indicate what is likely the edge of the cap or inhibition which likely led to this storm being the one furthest west. Through these lowest levels you can see the wind speeds turn from south to southwest to west at 15 knots, 25 knots and 15 knots respectively. This is fairly significant turning in these lowest levels, add on 30 knot westerlies and 500 hPa and 55 knot westerlies at 300 hPa and we have sufficient bulk shear for supercell storms. Looking at the 300 hPa chart again, you can see a jet streak entering South Dakota with a favorable left-exit region co-located over our tornadic thunderstorm.

Looking at a few parameters for the basis of severe and tornadic thunderstorms, first as previously mentioned the wind fields allowed for ~45 knots of bulk shear as shown by the far left graphic below. The turning in the lower levels also contributed to ~15 knots of shear in the lowest kilometer of the storm (left-center graphic below)The thermodynamic profile is also quite sufficient with 3500 J/kg of mixed-layer instability (right-center graphic below). With favorable conditions for a strong updraft and organized thunderstorms, the LCL of a mere 1000 meters also proved very favorable for a tornadic thunderstorm (far right graphic below).

A few last graphics to share to indicate just how well features came together for this tornadic thunderstorm, viewing a mid-level (700-400 hPa) vorticity plot (left graphic below) you can see a shortwave disturbance located just west of the North Dakota and Minnesota border. Being just ahead of this shortwave likely aided in upward motion for the thunderstorm to develop. Low level instability is also important to tornado development, as stronger buoyancy in the lower levels will promote the stretching of vorticity (one of the proposed keys to tornadic development). The plot of instability (CAPE) over the lowest 3 kilometers along with surface vorticity is shown on the center graphic below, once again indicating favorable conditions over the tornado location. Lastly, a full overview of the conditions is shown in the RUC analysis sounding from the approximate location of the tornado (right graphic below). Adjusting the sounding to known environmental conditions would show stronger winds in the lower levels and given buoyant actions occurring you would see increasing instability in the lowest portions of the thermodynamic profile. Essentially, some parameters as shown by the RUC analysis images shared above could be even stronger than indicated.

Overall, all parameters came together over the location to provide a tornadic thunderstorm that was very capable of producing a significant tornado. Please see previous posts for additional information regarding the storm damage survey, images, and video of the tornado and its’ damage.