Last Build Date: Sun, 26 Mar 2017 23:45:41 -0500
Continuing with the theme that very cold air has been difficult to come by for most of this winter season, the featured map displays IEM computed percentage of average for amount of time spent below 0 F. The general pattern this winter has been cold over the northwestern US and mild for the eastern. The blob that appears over and near Oklahoma is simply having a few more hours than a low average number of hours, so their percentage is high! Significant cold air continues to look unlikely as we'll mostly see springlike storm systems rolling by.
For Ames, this March we had one stretch of five days with the high temperature only maxing out at 31 degrees. Has this cold of a period become more of less common? The featured chart attempts to answer that question by looking at the distribution of max five day high temperatures comparing the 30 year period before 1987 to the same number of years afterwards. The answer to the question comes with the split violin charts on the left hand side. The later year period shows a shift upward in distribution and a remarkable vacating of the lowest temperature frequencies. The meaning being that the periods of very cold temperatures are less frequent than they have been.
With morning temperatures in the 40s, one may not expect to see much snow with a temperature well above freezing. Dew point temperatures were in the 20s at the time. So how did locations like Des Moines and Audubon receive snow Tuesday morning? The reason has to do with a process called the wet bulb effect. When snow showers moved over the area, the drier air near the surface cooled and moistened as evaporation happened. Eventually the temperature cools to low enough temperatures to support snow! The featured chart compares air and dew point temperatures for three sites on Tuesday. Both Audubon and Des Moines received snowfall, Marshalltown did not. The bottom panel shows the difference between the air and dew point, known as dew point depression. You can see the wet bulb effect as the snow fell late morning and the return to pre-snowfall levels after the precipitation departed. As an aside, thunder snow was reported with this event!
The afternoon high for Ames and Ankeny based on their respective airport sensors was 66 degrees on Monday. While only separated by 20 some miles, the typical afternoon high temperature difference between the two sites is about 3-4 degrees with Ankeny (the southern most station) being warmer. The featured chart compares the difference in afternoon high between the sites by day of the year and also average wind speed. The data is presented as a heat map with a simple average computed over the individual bins. It is kind of interesting to see a small annual signal in the plot with the immediate explanation being micro-climate differences between the sites likely due to subtle terrain and soil differences.
The end of March is a good time of year in Iowa as days are getting longer, the sunshine feels warmer, and temperatures are higher. The featured chart displays the change in average high temperature over the past three week period. Iowa is shown within the region with the largest change in average high temp. Of course, there is plenty of time yet in the season for snow to fall, but it certainly now feels more like spring than winter!
The latest depiction of the US Drought Monitor continues to depict abnormally dry conditions over much of southern Iowa. With today's tradition to adorn green, this area instead is shaded in tans and yellows. This is a good time of year to make up any long term deficits of rainfall as crop planting season is still a month or so away and a warm winter should have area soils receptive to absorb rainfall. The near term forecast is not very bullish on major rainfall events
The featured map displays IEM estimated precipitation departures for the past 365 days. A remarkable contrast is depicted over the state with northeastern Iowa indicated to be 15-25 inches above average while parts of southern Iowa are 5-15 inches below average! The big snowfalls this winter season have only enhanced this difference. The US Drought Monitor, which tracks drought conditions for this country, has this area in southern Iowa highlighted with D1 "Moderate Drought" depicted.
The high temperature for Ames today is expected to remain slightly below freezing making for the sixth straight day with a high temperature below 32F. The featured chart looks at the frequency of such streaks partitioned by week of the year. For mid March, the frequency is rather rare at less then 5%. The frequency shown is for an individual day having a temperature below 32 and its most recent five days below 32 as well. It is interesting to see the frequencies during winter not being all that extreme. Prolonged freezing weather is difficult to maintain without significant snow pack or sustained shunting of warmer air masses to our south.
Winter returned this past weekend to Iowa with two rounds of snow impacting the state. The first on Friday evening left just a dusting to about an inch and quickly melted away on Saturday. The second round on Sunday and into Monday packed a much heavier punch with totals approaching a foot of snow in a few limited areas. The featured chart displays an analysis of snowfall reports received thus far. It is still lightly snowing in the state this morning, so totals over southern Iowa may be increased when this map is updated Tuesday morning. This storm system is expected to merge with another system over the northeastern US and create a blizzard for the I-95 corridor.
Temperatures on Thursday were about what we would expect for March with highs generally in the 40s and lows in the 20s. The featured chart presents a histogram of high vs low temperature for Ames during March. The combination with the highest observed frequency is indicated on the chart. It is interesting to note the clustering of lows just below freezing. It is difficult for temperatures to keep above freezing during the night time as ground temperatures have yet to warm up and the air is generally dry allowing for good radiational cooling
After a relatively quite past few years for severe weather in Iowa, 2017 has gotten off to a fast start. The featured chart displays the accumulated year to date number of severe thunderstorm and tornado warnings for Iowa. This year's total is over three times the next highest year of 2006. It will be interesting to see if this trend continues into the spring season. Our next storm system looks to bring snow, so these totals would not appear to be added to anytime soon.
Amidst the numerous severe storms in Iowa on Monday rightly grabbing the headlines, the record high temperatures set were impressive as well. Sioux City reached 80 degrees on the 6th of March making for the earliest such 80+ degree temperature on record. Ironically, the previous earliest was just last year on the 7th. The featured chart displays the combination of first date at or above 80 and the subsequent number of days that year above that level. It is nice to think about all the warm weather that remains ahead of us as this would only be day one out of an average of 92 days!
Our eventful weather pattern continued on Monday with very warm and moist air for early March fueling thunderstorms that brought damaging winds, hail, and a few brief tornadoes to the state. The featured chart presents average dew points for March partitioned by wind direction. Our winds were out of the S and SSW, which is the favored direction for the highest dew points. Actual readings were well into the 50s, which is plenty higher than climatology shown by the featured chart.
The pleasantly warm weather this weekend was a reminder of the record warm just a while back during February. During which time we were setting a number of new daily record high temperatures. The featured chart presents the record high and low temperature set dates and beat margin. These events are computed as they are established with the passage of time. For example, records set in the 1940s may no longer still be records today, but are included in this chart. Anyway, the point of the plot is to show that recently we have certainly seen a larger share of record high temperatures set vs record lows.
For my nearly 20 year career in the meteorology field, it is difficult to remember a period with more excitement than what currently exists over GOES-R. GOES-R is a newly launched satellite by NOAA and NASA that features far superior and more diverse imaging than what is currently operational today over the United States. On Thursday, NOAA began a wider dissemination of test and evaluation products from this new satellite. The featured map is a simple plot of GOES-R ABI channel 2 over Iowa. While this image may not look impressive, the spatial and temporal detail provided by this product will be a game changer in weather forecasting and modelling. You will undoubtedly see many spectacular loops and images from this satellite this year as it prepares for operational status in late 2017. Here is a much more impressive loop over Texas produced by CIRA at Colorado State.
Based on unofficial and spatially averaged climate district data computed by the IEM, February came in as the warmest on record for high temperature for much of the eastern Mid West. Values of 1 on the map would represent the warmest February since records start in 1893. The values for Iowa are all in the top ten with the warmest ranks in southeastern Iowa. March has started out a bit on the chilly side, but warmer weather is expected back soon!
A powerful storm system brought a round of thunderstorms into the state on Tuesday prompting the issuance of a Tornado Watch and subsequently two Tornado Warnings for portions of Iowa. Both events are difficult to find analogs to in the recent archives maintained by the IEM. This would have been the first Tornado Watch in the state since at least 2006 and first Tornado Warning since at least 1986. The featured chart presents the number of tornado warnings issued by month and year for Iowa. The highlighted section shows no events for the months of January and February until the two yesterday!
The National Weather Service collects weather reports from their partners and the general public. These reports are then relayed to the world in the form of a text product named Local Storm Reports (LSR)s. The IEM processes and archives these reports to then make them available for download. The featured map displays the number of LSRs by NWS Forecast Office so far this year. Only reports of hail, tornadoes, and severe thunderstorm wind gusts and damage are shown. Significant amounts of severe weather are anticipated today and tomorrow, so areas in the central US that haven't seen much activity yet this year are expected to with this event.
Our week of late spring-like weather was abruptly shifted back to winter with storm system that brought snowfall totals approaching a foot over limited portions of north central Iowa. Blizzard conditions were also observed with many traffic accidents resulting. The featured map displays an analysis of available NWS COOP, Storm Reports, and CoCoRaHS reports. Almost all of the snowfalls this season have dumped the heaviest totals over north central Iowa.
It was a bit surreal to have record warm temperatures on Wednesday and be under a blizzard watch and now a blizzard warning for Thursday evening over portions of Iowa. A few locations reached 70 degrees on Wednesday and were under a blizzard warning just over a day later. The featured chart looks at the distribution of daily high temperatures around the issuance of a blizzard warning for Iowa counties over the past 10 years. Needless to say, the combination experienced this week is outside of the norm! The boxes shown display the inter-quartile range (IQR) and whiskers are at the 1.5 IQR value. It is kind of interesting to see the variability get knocked down immediately