Spring Heads to a Toasty Finish across the U.S.

The final week of northern spring will have a summery feel across the heart of North America, including much of the U.S. Rockies, Plains, Midwest, and Deep South. In most places, the heat won’t be smashing daily records, but it may persist or recur into next week, adding to its cumulative impact on people and ecosystems. Temperatures are likely to top 100°F from western TX to western KS later this week, with readings from 95°F to 100°F widespread from Arkansas and Louisiana across the South to Georgia by Thursday/Friday. Ample Gulf moisture will push heat indexes well into dangerous territory across large parts of the nation’s midsection and into the Southeast later this week (see Figure 1).


Figure 1. Maximum heat index values are projected to exceed 105°F on Thursday, June 16, 2016, over a large area from the Missouri and Arkansas Valleys of OK, KS, and NE across the mid-Mississippi Valley and parts of the Deep South to the lower elevations of North and South Carolina.. This outlook takes model ensemble data and statistical temperature forecasts into account. There is a chance of heat-index values exceeding 115 on Thursday over parts of the Missouri and Mississippi Valleys. The NWS calculates heat index values by incorporating the effects of both temperature and relative humidity on bodily comfort. Values above 105°F are considered dangerous during periods of prolonged exposure and/or strenuous activity. Image credit: NOAA/NWS/WPC.


Figure 2. An extremely strong upper-level high will will bring very hot temperatures this coming weekend across the Southwest U.S. Shown here are high and low centers averaged across the 22 members of the GFS Ensemble Forecast System (GEFS), for runs produced at 06Z Monday, June 13, 2016, and valid at 18Z (2:00 PM EDT) Sunday, June 19. The labeled lines show the height of the 500-millibar pressure level, expressed in decameters (tens of meters). The higher the pressure level, the warmer the air below it. Values greater than 600 dm, as shown here for the Four Corners area, are only observed during the most extreme heat waves; it’s impressive to see such values showing up in an ensemble average. The orange and blue colors show how much the pressure levels at a given location are above or below the seasonal average, again in decameters. Image credit: www.tropicaltidbits.com; thanks to Richard Grumm, NWS/State College, for calling attention to this model result.


Southwest deserts will be scorching by the weekend
Some of the worst heat in many years could take shape this coming weekend in cities like Phoenix, Yuma, and Tucson, Arizona. Already this month, Phoenix has seen four consecutive record highs (6/3 through 6/7), including the earliest 115°F reading (6/4) since record-keeping began there in 1895. The coming week should bring only garden-variety heat to southern Arizona, which means daily highs within a few degrees of the century mark Fahrenheit, but high pressure building aloft will sharpen the heat dramatically by week’s end. As of Monday morning, the WU forecast for next Sunday, June 19, is calling for highs of 117°F in Phoenix, 116°F in Yuma, and 113°F in Tucson. If it verified, the Phoenix reading would be the warmest on record on any date prior to the summer solstice, and these temperatures are not too far from the all-time records of 122°F in Phoenix (June 26, 1990), 124°F in Yuma (July 28, 1995), and 117°F in Tucson (June 26, 1990). An excessive heat watch is already in effect for much of southwest Arizona and far southeast California, including the Phoenix area, for this coming Friday morning through Monday evening.

Record highs are far outpacing record lows so far this year
Last week’s national climate roundup for May showed that 2016 for the period January through May was running fourth hottest in U.S. history behind only 2012, 2000, and 2006. Statistics from NOAA on daily record highs and lows back up this picture of a very warm year in the U.S. so far. The period from January through May 2016 saw 11,065 daily record highs and only 1,820 record lows (either tied or broken), according to the Daily Weather Records site maintained by NOAA/NCEI. This ratio of around 6 to 1 is very high for a five-month-long period. The lopsided ratio has continued into June: the first nine days of the month produced a preliminary total of 850 daily record highs and 44 daily record lows, according to NOAA/NCEI.

The oddly cool years of 2013 and 2014 both ended up with more daily record lows than record highs, but otherwise there’s been enough heat in this decade to produce a total of 194,467 record highs and 96,651 record lows, according to independent meteorologist Guy Walton, who has tracked daily record highs and lows for many years using NOAA/NCEI data. As shown in Figure 3, the ratio since 2010 of just over 2 record highs to each record low is now outpacing the ratio of just under 2:1 for the previous decade (2000-2009).


Figure 3. The ratio of daily record highs to daily record lows across the United States for each decade since the 1920s, expressed as an increment beyond the 1:1 ratio that one would expect in a stationary climate (one not being warmed by added greenhouse gases). The 2000s produced nearly twice as many record highs as record lows, and thus far the 2010s have been even more heat-skewed. Image credit: Guy Walton.


A journey from severe weather in the Plains to low pressure off the East Coast
A shot of upper-level energy now located over the Desert Southwest is destined to take an interesting path over the next week. As the impulse encounters very warm, unstable air across the Plains, NOAA’s Storm Prediction Center is calling for an enhanced risk of severe weather on Monday across parts of the central High Plains on Monday, shifting to the Missouri and Mississippi Valley on Tuesday. The first half of June is peak tornado season in eastern Colorado, and the Monday setup favors potentially tornadic supercells. Later on Monday night, a large thunderstorm complex packing high wind may rumble eastward across the Central Plains, with a similar complex possible Tuesday night further east. The upper-level impulse and associated severe weather should track onward into the Ohio Valley on Wednesday.


Figure 4. WU depictions of severe weather risk areas for Monday and Tuesday, June 13-14, 2016, as issued by NOAA’s Storm Prediction Center on Monday morning. Areas of enhanced risk (the third highest of five SPC threat levels) were in place for both days.


Toward Friday and the weekend, the upper-level energy will dive south, feeding into the base of a large North Atlantic upper low. This tail-end circulation may be enough to generate a surface low somewhere off the mid-Atlantic or Southeast coast. Several weekend runs of the GFS and ECMWF models suggested that the eventual surface low could linger for a day or two near the Gulf Stream with a piece of the tail-end upper circulation parked over it. Sunday night’s 06Z GFS run backed away from this scenario, moving the upper-level and surface lows eastward more quickly, while the 12Z Monday GFS brings a surface low into the Southeast coast early next week. This setup is worth monitoring for the potential of at least weak subtropical development, keeping in mind the large amount of model uncertainty and long time frame.

It was a quiet weekend across the globe’s tropics, with no systems being tracked by operational agencies on Monday morning. In the eastern North Pacific, we could see a center of low pressure emerge from a large, persistent area of disturbed weather south of the Gulf of Tehuantepec. In their outlook issued at 8:00 AM PDT Monday, the NOAA National Hurricane Center gave this area 10% odds of tropical development by Wednesday morning and 50% odds by Saturday morning, with the center of action drifting west-northwest and remaining off the Mexican coast.

We’ll be back with a new post by Wednesday at the latest.

Bob Henson