A windy, fast-moving storm system will bring the risk of severe weather, including tornadoes, as it barrels across the Great Plains on Monday afternoon and evening. The severe storms will be generated by a very large upper-level trough swinging east across a broad area of fairly warm, moist air feeding into a surface low in the Dakotas. The NOAA/NWS Storm Prediction Center highlighted an enhanced risk of severe weather covering most of Missouri in its 10:30 am CST outlook on Monday. A slight risk extends from eastern Oklahoma and Arkansas northward to central Minnesota.
Today’s setup is a fairly typical one for March, with a very strong late-winter-style polar jet stream encountering an early-spring air mass. While a large swath of the Plains could experience a fast-moving severe storm, the most potent juxtaposition of unstable air and upper-level forcing will evolve near and just northwest of the Ozarks. Surface dew points in this area were already approaching 60°F at midday Monday, with temperatures near 70°F. A dry line now over western Kansas and Oklahoma will sharpen this afternoon, serving as a focal point for thunderstorm development. Supercell thunderstorms over far eastern Kansas and southwest Missouri ahead of the dry line have the potential to generate very large hail and significant tornadoes, according to SPC. As a cold front overtakes the dry line and sweeps through, this evening’s storms should eventually morph into a squall line, possibly extending from the upper Midwest all the way to Texas.Figure 1.
Convective outlook issued by the NOAA/NWS Storm Prediction Center at 10:30 am CST Monday, March 6, 2017. Figure 2.
Visible GOES satellite image from 1645Z (10:45 am CST) Monday, March 6, 2017. A large field of low-level moisture is evident in the cumulus streaming north across Texas into the Central Plains, while a ribbon of higher clouds angling from southwest Texas to Illinois corresponds with strong upper-level winds overtopping the moisture across parts of Oklahoma, Missouri, and far southeast Kansas. Image credit: NASA MSFC Earth Science Office
Model depiction of surface and upper-level features produced by the 12Z Monday run of the GFS model, valid at 00Z Tuesday, March 7, 2017 (6:00 pm CST Monday). A cold front across the Plains, evident in the kinked black lines (isobars, or lines of constant surface pressure) from Minnesota to Texas, will be shoved eastward by the strong upper level low evident in the blue colors (these show the height of the 500-millibar surface, shown in tens of meters). Image credit: www.tropicaltidbits.com
.Extremely critical fire danger over parts of Southern Plains
Powerful upper-level winds and the surface low wrapping up over the Dakotas are teaming up to generate a swath of strong, warm, and very dry southwest winds behind the dry line across the Southern Plains—bad news for fire risk. SPC has issued an “extremely critical” fire risk area, its most dire, along a band from east-central New Mexico to eastern Nebraska. While drought conditions are not nearly as widespread or intense across this region as they were last fall, moderate drought has persisted
across large parts of Oklahoma, western Kansas, and eastern Colorado through the winter. Figure 4.
Fire weather outlook issued by the NOAA/NWS Storm Prediction Center at 9:00 am CST Monday, March 6, 2017.The 2017 tornado season is running far ahead of average
The highly progressive pattern that’s kept midlatitude storms shuttling across the United States all winter has given an early boost to the U.S. tornado season of 2017. Such a progressive pattern is important but not sufficient for producing early-season severe weather. Even if upper-level conditions favor severe weather, there may not be enough time between midlatitude storm systems for unstable air to flow back into place from the Gulf of Mexico. This year, however, the Gulf and Caribbean have been consistently warm, which has helped generate plenty of warm, moist air for midlatitude storms to access. In records back to 1981, this is the first year in which sea surface temperatures averaged across the entire Gulf never dropped below 73°F
, as discussed by Eric Berger at ArsTechnica and shown in Figure 5 below (thanks to WU member Mark Cole for this tip). As of Monday,
SSTs over the western Gulf were running 1°C - 2°C (1.8-3.6°F) above the seasonal norm.
Thus far in 2017, NOAA/SPC has logged 268 preliminary tornado reports. This is roughly double the average of 133 reports racked up by March 5 over the preceding 11 years (2005-2015). This year also stands out in a longer-term perspective, even after you “inflation-adjust”
the data back to 1954 to account for the increased tendency since then for a given tornado to be spotted, chased, photographed, videotaped, etc. After the inflation adjustment, this year’s preliminary total of 228 tornado reports (see Figure 6 below) compares to a long-term average of just 69 reports by this point in the year. Figure 5.
In data extending back to 1981, sea surface temperatures averaged across the Gulf of Mexico were the warmest on record this winter. Image credit: Michael Lowry, @MichaelRLowry
“Inflation-adjusted” cumulative tornado totals for the period 1954 - 2016 (various colors) and 2017 to date (black). The red trace shows the highest adjusted value observed on each date through the year, with the lowest value on each date in magenta. Image credit: NOAA/SPC
.Enawo churns toward MadagascarTropical Cyclone Enawo
continued to chug toward the east coast of Madagascar late Monday local time. Satellite imagery suggests that Ewano is now at least a Category 3 storm—the first to be reported anywhere in the Southern Hemisphere since its 2016-17 season began last July. Enawo is also only the third hurricane-strength tropical cyclone to develop this season, which marks the latest appearance on record of the Southern Hemisphere’s third hurricane, according to Phil Klotzbach (Colorado State University)
. See Jeff Masters’ post earlier today
for more on Enawo.