Daily Discussion [7/17] Dog Days of Summer Are Here


  • Hot and humid today
  • Temps in the low to mid-90s, dewpoint approaching 70
  • Real feel of 105
  • Chance of Rain/thunderstorms this afternoon
  • Flash flooding increased risk
  • Severe thunderstorms capable of producing damaging winds and hail

Current Analysis

Currently we are seeing a cold front dropping down into the Great Lakes and meeting up with post-tropical cyclone Barry in the Ohio Valley. The former tropical cyclone will enhance frontal activity including thunderstorms and rain. The tropical moisture train will be a heavy influence in our weather starting later this afternoon as we increase the flood potential.

Current radar showing some showers and storms in Northern PA and NY. The models favor the heavy rainfall and thunderstorm axis north of the Philly Area. Even as the front drags south, the enhanced activity for severe weather coupled with the remnants of Barry are to the north of the region. That being said, with the warmest temperatures of the year and instability throughout the region, severe storms cannot be ruled out for Philly and even SNJ down to the shore points.

Pennsylvania Radar Loop

The remnants of Barry can be seen spinning away on the latest satellite imagery as the counter-clockwise low pressure swirl entering Ohio. The trajectory of Barry takes it pretty close to the Philadelphia area. So most of the enhanced rainfall will occur of N. Pa and NY throughout the day. However, there remains significant risk for severe weather as the upper level remnant of Barry tracks east. Should be a fair amount of sunshine this afternoon mixed with clouds to increase instability.

NAM 500mb look at initialization shows the mid-level remnants well

This afternoon those remnants track east along with the sinking cold front to the north, enhances precip in N. PA and NY. The timing of the interaction is key because all of the instability and enhancement of rain will maximize during the afternoon for those areas.

Temperatures and Excessive Heat Risk

NWS high temperature forecast for the day includes mid-90s for much of the area. From Allentown to Atlantic City, expect uncomfortable conditions. Closer to the southwest near Baltimore and D.C, temps may approach 100.

NAM heat index for this afternoon shows widespread 100s and areas approaching 105 degrees as well.

The NWS has issued an excessive heat warning for the immediate Philadelphia area. Heat indices are expected to be over 100 degrees.

Again, there remains a significant excessive heat risk for this afternoon right along I-95. The rest of the area is mostly in an elevated heat risk with temps in the low 90s and real feel of upper 90s.

Severe Thunderstorms

As stated above, with Barry’s trajectory expected to be near Philly, the enhanced area of severe weather will be to the north of the region. Below is the NWS/SPC forecast for severe weather which includes a slight risk for severe storm in Allentown, Trenton, and up into Long Island and SNE. That doesn’t mean severe weather isn’t possible for Philly down to the shore. There is more than enough instability to increase severe weather thresholds this afternoon.

NAM appears to want to bring a bow echo like segment later this evening. Plenty of storms over in PA during the day that will coalesce into this line as the instability parameters remain high in PA during the late evening.

Presently we have CAPE values already exceeding 3000 J/kg in some locations. The more sun we get, the most unstable the airmass becomes to help support stronger storms later on with remnants of Barry’s approach.

We can see in the wide view a fairly decent low level jet associated with Barry’s remnants extending into W PA and WVA. This will slide east and aid in thunderstorm development.

Flash Flooding

NAM forecast brings over 3″+ of rainfall in some spots with the heavy thunderstorms this afternoon. That seems to be a possibility with the cold front being enhanced by Barry’s remnants. There is also about 1″ of rain in the Philly area and that has to do with thunderstorms. You can see the flash flood risk and why the NWS has issued a flash flood watch.

Below is an image of low/mid-level moisture transport which is currently most enhanced ahead of the front into W. PA and W NY. There is a thumb of increased theta-e beginning in the Gulf Of Mexico (look at yellow shadings), jumping up into North Carolina, and then up into S. NY. This parameter tells us that there is warm air with moisture moving into the vicinity that will increase instability. Theta-e tells us where some of the convective areas of interest may set-up during the day.

So we have a deep moisture feed in an unstable environment aided with remnant tropical features. Hot, humid, tropical, and if you were missing summer for the last few months, you probably aren’t missing it as much today.

We have a lot to keep an eye on today, so stay weather aware and keep an eye to the sky!

Be conscious of flash flooding potential if you see storm clouds approaching.

Also, stay hydrated and limit outdoor activities. It is one of those days where you can quickly become susceptible to the conditions without even noticing it.

Images courtesy of the National Weather Service, Storm Prediction Center, Pivotal Weather Maps, Tropical TidBits, WPC, and Weather Underground

Local Warnings and Statements NWS Mt. Holly: https://www.weather.gov/phi/

Radar and more: College of DuPage

CAPE? Shear? What?!? Severe Weather Terms: https://www.spc.noaa.gov/sfctest/help/sfcoa.html

Mesoanalysis: https://www.spc.noaa.gov/exper/mesoanalysis/new/viewsector.php?sector=16#

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