Fort Collins included in threat of severe weather, tennis-ball size hail, tornado possible

Update: Here's why Fort Collins didn't get hail Thursday (and the Fourth of July weather forecast)

There's a growing risk of severe storms, including damaging wind, heavy rain and tennis-ball size hail for the Fort Collins area, Denver, Interstate 25 corridor and Colorado's Eastern Plains on Thursday.

The National Weather Service is advising people to be prepared to seek shelter or avoid outdoor activities and protect vehicles under shelters if severe weather approaches, as expected after 2 p.m. along the Front Range. These storms tend to happen quickly, so be aware of conditions and have multiple ways to receive weather alerts.

Here's what we knew as of midday Thursday:

DIA included in thunderstorm warning

The National Weather Service has issued a thunderstorm warning for Denver International Airport and surrounding area.

The warning continues until 5:15 p.m. Thursday.

The weather service said 50 mph wind and quarter-size hail are possible.

Before heading to catch a flight Thursday, check with your airline carrier.

DIA has seen 632 flight delays and 173 cancellations for various reasons Thursday, according to flightaware.com/live/cancelled/today/KDEN.

−Miles Blumhardt

Denver metro area reporting flooded streets after strong storm

Repeated reports of flooded streets across the Denver metro area following a strong thunderstorm's passage has National Weather Service officials in Boulder advising people to stay off the streets if they can. The heavy rain from the storm is impacting interstate traffic before the Thursday rush hour, so motorists can expect delays on busy routes.

A flash flood warning for northwestern Denver County and northeastern Jefferson county is in effect until 6 p.m. Thursday.

–Eric Larsen

Strong storm drops large hail over west Denver metro area

A "strong and rapidly intensifying storm" was dropping hail of up to 1 inch in diameter over the west Denver metro area shortly after 3:30 p.m. Thursday, according to the National Weather Service. The storm developed over the foothills before radar showed it dropping quarter-sized hail over Red Rocks Park west of the metro area.

The NWS advises people in the severe thunderstorm warning area over the Denver metro area to stay inside, as hail could become larger and the storm more severe.

–Eric Larsen

Fort Collins included in tornado watch area

The National Weather Service at 1:15 p.m. Thursday issued a tornado watch for 13 counties, including all of Larimer County.

The watch is in effect until 9 p.m. Thursday for Larimer, Weld, Boulder, Denver, Adams, Broomfield, Arapahoe, Jefferson, Elbert, Logan, Douglas, Washington and Morgan counties.

Fort Collins, Loveland and Windsor face a major risk of severe weather 4 p.m. to 6 p.m.

− Miles Blumhardt

Here is the latest update from the National Weather Service and why you should be prepared

• The weather service has upgraded the severe weather risk three times Thursday morning

• Just after 11 a.m. Thursday, the weather service upgraded to an enhanced severe weather risk an area including Greeley, Fort Morgan and Cheyenne. Fort Collins is just outside the enhanced risk area.

• Earlier Thursday, the weather service shifted its slight risk of severe weather area west to include Fort Collins. Previously, Fort Collins was in the marginal risk area.

• A slight risk is rated 2 and enhanced risk 3 on a scale of 1 to 5, with 5 being high risk.

Why the National Weather Service is upgrading the threat from marginal to slight

• David Barjenbruch, senior forecaster at the weather service in Boulder, said severe storms are possible for the area as conditions are favoring storms intensifying as they move from the foothills to the plains.

• Ingredients for severe weather possibility include moisture, instability, lifting and wind shear, which are forecast to be present Thursday.

• The threat in the warned area includes the possibility of large hail from golf ball size to tennis ball size (medium-high probability), damaging wind up to 70 mph (medium probability), tornadoes (medium probability) and locally heavy rain, causing flash flooding (medium probability). Lightning also will be a threat.

Timing of the potential severe weather Thursday according to the weather service

• Fort Collins, Loveland, Windsor: 4-6 p.m. major risk

• Denver: 4 p.m. major risk

• Eastern Plains: 6 p.m. major risk

Here is a look at the severity of Wednesday's large hail storm in eastern Colorado

• Russ Schumacher, professor in CSU's Department of Atmospheric Science and Colorado's state climatologist, said on his Twitter account Wednesday's late night large hail event is "very rare'' in Colorado.

• Schumacher said Wednesday, mostly late night, there were 11 reports of 3-inch-plus size hail and four reports of 4-inch-plus hail. Before Wednesday's storm, there were only five reports of 3-inch-plus hail between 11 p.m. and 11 a.m. in 65-plus years of or records.

• A report near Akron showed baseball-size hail. Iliff, near Sterling, and Brush had reports of 4-inch hail.

• Schumacher said June will likely end up among the biggest hail months in Colorado records with the risk of large hail again Thursday.

• The area where Colorado, Nebraska and Wyoming meet is referred to "Hail Alley'' because it usually leads the nation in most hailstorms, averaging seven to nine hail days per year, according to the weather service.

Colorado's largest hailstone and a rare hail death in Fort Collins

• The largest hailstone recorded in Colorado of 4.83 inches occurred Aug. 13, 2019, near Bethune, which is just north of Interstate 70, 150 miles east of Denver.

• That hailstone eclipsed the record of 4.5 inches that occurred July 30, 1979 and resulted in the death of a 3-month-old in Fort Collins. According to NOAA records, before this year since 2000 only four people have been killed by hail nationally.

More: Colorado natural disasters What to know about Colorado's deadliest natural disasters, including key safety tips

Here is the National Weather Service guide to measuring hail

• When reporting hail, estimates comparing the hail to a known object with definite size are good, but measurements using a ruler, calipers or a tape measure are best. Here are object sizes as they relate to hail size:

• Pea size: 1/4 inch diameter

• Mothball: 1/2 inch diameter

• Penny: 3/4 inch diameter

• Nickel: 7/8 inch

• Quarter: 1 inch, quarter size or larger is considered severe

• Ping pong ball: 1 1/2 inches

• Golf ball: 1 3/4 inches

• Tennis ball: 2 1/2 inches

• Baseball: 2 3/4 inches

• Tea cup: 3 inches

• Softball: 4 inches

• Grapefruit: 4 1/2 inches

This article originally appeared on Fort Collins Coloradoan: Weather update: Fort Collins included in tornado watch area