9 July 2022

Hot dry weather since early June has turned the grass brown and caused low water in Pittsburgh area streams. By the Fourth of July leaves were wilting in Schenley Park and Little Sewickley’s creek bed was exposed at Sneed’s.

Precipitation had changed in only five weeks from 1.20 inches above normal at the end of May to -1.58 inches below normal on 4 July. A thunderstorm on 6 July reduced the deficit to -1.24 inches below normal this morning. (See statistics at the NWS Pittsburgh Local Climate page.)

Are we in a drought?

Not really. Despite wilting leaves this week’s U.S. Drought Monitor map puts most of Pennsylvania in the normal range. (S=short-term impacts, L=long term impacts)

The Drought Severity Index (Long Term Palmer) map, below, is both predictive and more fine grained. It shows a hint of drought in four southwestern Pennsylvania regions (pale yellow) and extreme drought in the U.S. West and Southwest (orange). The surprise to me is the extreme long term drought in eastern North and South Carolina, evident on both drought maps. Northeastern North Carolina needs 12 inches of rain to get back to normal (click here for the map).

It’s reassuring to know that Pittsburgh is not heading for severe drought. I feel bad for all the areas colored scary red and orange.

(photos by Kate St. John and Karyn Delaney, maps from US Drought Monitor at UNL and NOAA Climate Prediction Center)

22 June 2022

Pittsburgh is 300 miles from the Atlantic Ocean so the hurricanes that pass over us are not even tropical storms by the time they reach southwestern Pennsylvania. We usually don’t know the names of the remnant hurricanes that bring us rain but there is one that lives in infamy. Fifty years ago this month Hurricane Agnes devastated Pennsylvania, New York, Maryland and Virginia.

In June 1972 I lived with my parents and worked in a restaurant in the South Hills of Pittsburgh. I remember the heavy rain that soaked me during my few steps to the car when my mother picked me up at work. Our neighborhood was not flooded but Downtown Pittsburgh was.

“The Agnes flood crested at 35.85 feet in downtown Pittsburgh, eleven feet above flood stage. The US Army Corps of Engineers estimated it would have topped 47 feet if not for the flood control dams and reservoirs, built since 1938, that held back much of the water. East Branch on the Clarion River was more than full. Kinzua Dam in Warren, PA was within three feet of the top. Tygart Reservoir in West Virginia was 85% full. Other dams stored water at 90% capacity.” — paraphrased from Flood of 1972 at brooklineconnection.com.

Pittsburgh was fortunate to receive a fraction of the rain that fell in the Susquehanna River watershed. More than 10 inches fell in much of east-central Pennsylvania, reaching a maximum of 19 inches in western Schuylkill County.

Many Pennsylvania towns were ravaged, including Harrisburg.

That September I returned to college in Geneva, NY and joined a crew of volunteers doing flood relief work in Horseheads, NY, six miles from Elmira. Our job was to remove mildewed interior walls in a house flooded by Newtown Creek during Agnes. The owners were living in a disaster-relief trailer in the backyard and hoped to rebuild their house.

It was a lesson in empathy. The walls were plasterboard on studs, just like those in my parents’ house. We broke the plasterboard and levered it off with crowbars. It could have been my own home that I was taking apart and my own life upended by the flood. I felt very lucky.

Read more about Hurricane Agnes at:

• Pittsburgh Post-Gazette: When Hurricane Agnes slammed soggy Pittsburgh
• Pennlive.com: Tropical Storm Agnes devastated PA in 1972: “Battered, lashed, flooded and paralyzed”
• NOAA: 50th Anniversary of Hurricane Agnes

(photos and maps from the NOAA / NWS, Pittsburgh flood photo embedded from brooklineconnection.com. Click on the captions to see the originals)

17 June 2022

Severe thunderstorms were predicted for 6:00pm yesterday in the upper Ohio Valley. By 2:00pm the Severe Thunderstorm Watch called for an inch of rain in 1 hour — definitely flash flood material — but at 5:00pm the storm line split. Some went north toward I-80, the rest went south to West Virginia. Pittsburgh had no lightning, no strong winds, no rain. Nothing happened. But the sky got weird.

At sunset the last of the storm clouds left our area with a flourish of rare mammatus clouds, dramatically lit from below. Their name is derived from the Latin word for breast or udder.

As Wikipedia explains, mammatus are formations that hang from the base of rain clouds. The distinct lumpy undersides are formed by cold air sinking down to form pockets. Usually composed of ice, each lobe averages 1/2 to 2 miles across and 0.3 mile deep. Alone a lobe can last 10 minutes but a cluster may last several hours.

Mammatus are an indication of a severe thunderstorm in the vicinity. In Pittsburgh often show up after the storm has passed.

The clouds started out as lines and gave way to stratus clouds and a gleam at sunset.

Next time you see these weird clouds, remember their name describes their shape.

p.s. Steve Tirone left a comment with a link to his video of the clouds.

(photos by Kate St. John)

16 June 2022

If you live in a windy place, the trees lean away from the prevailing wind.

To see this effect on paper meteorologists create a wind rose that graphs the wind’s direction and speed over time. The petals indicate the percentage of wind from each direction. The colors show the speed. The center is calm. Let’s look at some wind roses from Iowa State’s Iowa Environmental Mesonet.

Though Pittsburgh is not a particularly windy place our 50-year wind rose (1970-2022) indicates our prevailing wind from the southwest. 10.9% of the time there is no wind at all.

In places where wind is not obstructed, such as the coast and the Great Plains, wind roses are lopsided. This map shows the locations of three extreme wind roses displayed below.

Miami, Florida windrose from Iowa Environmental Mesonet

Wichita, Kansas windrose from Iowa Environmental Mesonet

Los Angeles, California windrose from IEM

Here are some more from Iowa Environmental Mesonet. Click the links.

• Provincetown, MA
• New York, NY
• Chicago, IL (Midway airport)
• Phoenix, AZ (Sky Harbor Airport)
• Yellowstone National Park at Yellowstone Lake RAMOS weather station
• Seattle-Tacoma International Airport, WA
• Seattle, WA (Boeing Field)
• Honolulu, HI (extremely lopsided!)
• Anchorage, AK
• Nome, AK

What’s your wind rose?

Retrieve it here.

1. Visit Iowa State’s Iowa Environmental Mesonet Site Locator.
2. Select by Network: Drop down to pick your state ASOS network, for example: “Ohio ASOS”. Click on [Switch Network]).
3. Then Select by Station (drop down to pick your local weather station, for example “Toledo” and click on [Select Station].
4. Click on [*Wind Roses], the 9th of 13 buttons.

(images from Wikimedia Commons and Iowa Environmental Mesonet; click on the captions to see the originals)