A Red Flag Warning was in effect yesterday for limited parts of western Iowa. These warnings are issued when dry and windy conditions create spreading fire concerns. The featured map presents the IEM computed average number of days per year with at least one hour with sustained wind speeds at or above 25 mph and relative humidity below 25%. This combination of conditions gets more rare the further east you go in the Midwest. Dry air masses originating over the desert southwest are slowly modified as they are transported east by vegetation transpiration and evaporation. Also these events are typically with strong southwesterly winds, which means the further east you go, the more likely air originated from the Gulf of Mexico (which will have high humidities).

The featured chart displays the combination of recent monthly precipitation departures for Ames along with the monthly El Nino 3.4 index value. Many scientists believe that the El Nino index, which determines if we are in La Nina or El Nino condition, is correlated with longer timescale temperature and precipitation in the United States and elsewhere. The chart nicely shows the drought periods of the past two years and the wet periods of 2008 and 2010. This October looks to turn wet for the rest of the week with showers arriving late this evening.

The weather is going to be very interesting over the next few days with chances of severe weather, including perhaps tornadoes, in the forecast. October is not a favored time of year for tornado warnings as shown by the featured chart. The chart shows the frequency of having at least one tornado warning issued for the given week of the year. For comparison, values for four states are plotted. Of course Texas is a very large state in Tornado Alley which helps its numbers. It is kind of interesting to compare the annual cycle for Iowa vs Oklahoma and even Alabama. Tornado Warning frequencies in Iowa drop to zero during the winter season while Alabama has the occasional event thanks to its proximity to the Gulf of Mexico. Please note that this type of comparison does not account for the total number of warnings issued but just attempts to diagnose annual climatology.

The Storm Prediction Center has a moderate risk of severe weather over much of Iowa today including the threat of tornadoes. Having such a threat is somewhat rare for Iowa and even more so when you consider the calendar says October! The powerful storm system bringing this threat is also expected to dump many inches of snow just to our northwest in South Dakota. The air on backside of this system will be a sensible change with highs only in the 50s for the weekend and more chances of precipitation.

Des Moines received just over two inches of rainfall on Friday, which was almost larger than the individual monthly totals for each of the past three months! The featured chart presents the July, August, and September monthly totals for Des Moines along with the single day maximum precipitation for October during the same year. Both 2012 and 2013 have come close to exceeding a previous monthly total in just one day in October. The only year to accomplish this feat since 1880 was 1947!

The forecasted tornado threat on Friday materialized with significant tornadoes over eastern Nebraska and northwestern Iowa. The preliminary count for Iowa is five, which is many more than the long term average number of tornadoes in October as shown by the featured chart. The chart shows the monthly average using data back to 1980 and the monthly totals for the past two years. Needless to say, these past two years have seen many fewer tornadoes than on average. Dry weather is in the forecast with very pleasant temperatures.

This is about the time of year by which much of Iowa has experienced its first freezing temperature. The featured chart presents the year to date lowest temperature after 1 July climatology for Ames. While not strictly the same metric, the intersection of the black line (average lowest temperature to date) and dashed line (freezing temperature) around this time approximately matches the average first freezing temperature of the fall. The minimum to date temperature for this year shows that we are well on the warm side of long term average. The shaded areas represent the percentile ranges of 50% and 95% of the daily data. The red line should remain flat for at least the next seven days with low temperatures well above 40 degrees.

Temperatures warmed nicely on Tuesday thanks to sunshine and warm winds. The featured chart presents the frequency of having a 3 PM temperature 5 or more degrees warmer than average at 3 PM, or at least 5 degrees cooler than average and wind speeds at least 10 mph. The overall frequency of having wind speeds over 10 mph is shown as well. The significant difference among the three lines is during the warm half of the year with above average temperatures having ten plus mph winds more frequently. This shows that when temperatures are warm in Iowa, there is more often a breeze as well.

The featured chart is a called a Hovmoller Diagram. It displays the time evolution of some averaged field. In this case, each row represents the meridonal (north-south) average daily high temperature departure for the midwestern US. Time increases as you go down the chart. The two vertical lines represent the areal average that includes Iowa. Values on the left are to the west of Iowa and likewise values on the right are to the east of Iowa. A neat aspect of this chart is that you can visually see the movement and modulation of air masses. For example in September, you can see movement from upper left to lower right. This represents air masses going from the western part of the domain to the eastern part, with the slope representing the speed of movement. The plot also shows a difference during the summer months to what has happened since early September. The difference is due to having storm tracks every week or so as opposed to persistent air masses during the lazy days of summer. The very dark blues (cold departures) showing up in October are where the snow fell in western Nebraska and South Dakota.

A week ago Friday severe weather visited the state, including a recently rated EF-4 strength tornado near Correctionville. This event was the first severe weather warnings in 10 days prior. The featured chart looks at the frequency of follow up severe weather events after an event that was the first in ten plus days. The chart attempts to answer the question of how frequent are days with severe weather after an initial day that ended a recent drought of it. The data is partitioned by three periods of the year. Frequencies are higher during the summer season as severe weather is more common. There is some hint at an elevated frequency 5-7 days after the event, but nothing significant. The severe weather expected today will mostly miss us to our northwest.

The drought situation in the state remains very complex. The featured chart shows time series of trailing 30, 90 and 365 day trailing departures for statewide areal averaged precipitation. The current drought analysis for the state has a small portion in D3 designation and another small portion in no drought with the rest of it in D0 to D2. The chart shows that the year long departure is now well above average while the 90 day is well below. This is due to the wettest spring season on record helping the 365 day departure while the 90 day no longer includes that period. This year's growing season is now done, so any rain now will go toward helping the soil moisture situation for next year. Some rain is expected later today, but then the forecast turns dry again.

October has seen both stretches of dry weather to help with crop harvest progress and rainfall events to help the drought situation. The featured map is of accumulated precipitation estimates from NOAA for the first fourteen days of October. Some parts of the state have missed out with totals less than an inch. After the rain moves out this morning, another stretch of dry weather is expected with pleasant fall temperatures.

October 1 is the start of the water year. Since our growing season ends roughly around this time, any precipitation that falls afterwards will go toward recharging soil moisture for the next growing season. The featured chart displays the yearly cycle of root zone soil moisture as estimated by the Iowa Daily Erosion Project. The black line represents the simple average of all the years plotted. Moisture levels start to increase this time of year showing the recharge of soil moisture taking place as there is no longer a crop depleting it. The most substantial recharge takes place during the spring time as rainfall events are more intense and the soil is not frozen.

Much of Iowa has been struggling to see the sun these past few days and so temperatures have been struggling as well. When the sun is seen, the ground is typically experiencing the potential maximum amount of energy received for that day and time of the year. This produces heating that drives our near surface air temperatures warmer during the daylight hours. The featured chart presents one model's estimate of daily maximum shortwave solar radiation received at the surface and what actually happened this year. You can consider the tan area showing up until today as the effect of clouds. The last two bars in the chart show the minimal amount of solar radiation experienced by Ames for the past two days. An increase in sunshine is expected today, which will help warm temperatures out of the 40s and 50s we've been stuck in.

The drought situation in the state has been much worse for some locations than others. The featured map is an attempt to illustrate this by counting the number of days this growing season (1 May to 1 Oct) that the previous 30 days had less than 33% of average accumulation. The area to the north and west of Des Moines shows up prominently in this analysis and is where the current most significant drought depiction is located for the state. Northeastern and far western Iowa shows up favorably on this map with few days meeting this arbitrary criterion.

The featured map displays the combination of morning low temperatures and infrared satellite imagery for this past Thursday morning. The white areas on the map represent the presence of clouds and the warmest low temperatures you can find on the map are underneath these clouds. Clouds play a critical role in the radiation balance by absorbing long wave radiation emitted by the surface and then re-emitting it back toward the ground. This process helps keep air temperatures warmer versus nearby areas without clouds.

A forecasting rule of thumb is to look at something called the "540 thickness" to delineate snow versus rain events. The technique involves subtracting the height of the 1000 mb surface from the 500 mb surface. When this depth of air, or thickness, falls below 540 decameters (5400 meters), the air is typically cold enough to support snow. The featured chart presents the combination of this thickness value vs near surface air temperature for events where rain or snow were reported by the Omaha airport weather station. There are many caveats to this chart including exact timing issues and changes in automated reporting techniques for rain + snow. The application of this chart to today's weather is that thicknesses are well below 540 over eastern Iowa and closer to 540 over western Iowa. Snow is more likely the further east into the falling precipitation you go!

Snow fell over parts of the state on Tuesday including Waterloo which reported all of an inch of snow, which promptly melted thanks to above freezing temps and very warm ground temperatures. This was the first snowfall for the location since the epic snowfall of early May making for a period of 171 days without snow. The featured chart shows the period each year since 1896 between the last spring snowfall and the first snowfall of the fall season. The two vertical lines are the simple average for the beginning and end of this period. Only two years had shorter periods with 1991 at just 153 days.

Some snow visited the state again on Tuesday, but nothing to get excited about. It was a cold and dreary day with highs only in the low 40s and low clouds. The featured chart presents the frequency of the height of the overcast cloud layer as reported by the Des Moines Airport. This is referred to as the ceiling. There are caveats galore with this plot including changes in instrumentation and reporting algorithms. These caveats explain some of the bright bands in the plot at certain levels. Putting those issues aside, you can see a clear annual signal with low ceilings much less common during the warm half of the season and the very low ceilings confined to winter. The reason being that lower atmosphere temperatures are warmer in the summer with strong surface heating, this makes it more difficult for the air to remain saturated as heating lowers relative humidities. In the winter time, fog and snow are more persistent leading to lowered reported ceilings.

The featured map displays the USDA crop progress report from this past Sunday for percentage of corn acres harvested and the departure from recent average for the third week of October. A late maturing crop and some wet stretches in October have put the pace of harvest behind schedule. Recent freezing weather over much of the state will help kill off the crop and speed up the dry down period. The weather looks dry this weekend with chances of showers arriving next week.

After a very chilly start, temperatures warmed nicely in Iowa on Sunday with some places, like Ames, having a 40 degree warmer high than low temperature. The featured chart displays box and whisker plots for the daily temperature rise partitioned by month of the year. Having a 40 degree rise in October is not uncommon, but would be very rare in the mid summer. The box shows the 25th to 75th percentile of the data (half the observations) and the whiskers show the 5th and 95th percentile. It will be a bit before another very nice day like Sunday as wet weather is expected for much of the coming week.

A few weeks ago, this chart was featured showing how the minimum temperature for Ames to date was well above average. Fast forward two weeks and a few very cold nights, we are now below long term average! The chart shows that in a week or two more, temperatures near zero will not be out of the question! Our immediate future has thunderstorms in the forecast for this week and temperatures remaining above freezing.

The big story this week is the amount of rainfall expected to fall just to our south. The featured map is forecasted five day precipitation from the Weather Prediction Center. Totals for extreme southeastern Iowa are shown approaching two inches with amounts trailing as you go northwest. The rain is expected to get started today and last into Thursday. Temperatures will be warm enough not to worry about snow from this storm system.

Only two months remain before finishing out 2013 and the precipitation totals so far have been a mixed bag across the state. The featured map shows estimated precipitation departures for the year to date period. Northeastern and far western Iowa are analyzed above long term average while the central and southeastern portion of the state are below. This map does not tell the whole story as much of the state experienced dry conditions during the summer and extremely wet conditions during the spring. Our near term rain chances look to return next week.