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I haven't wrote a blog post in quite some time, so I felt like the upcoming will be worth discussing more in detail rather than solely in my seven day forecasts. So If you haven't heard, it's going to turn very hot as we begin July. This is thinks to a building ridge of high pressure over the Eastern and Central US beginning this weekend, which will promote above average temperatures and dry conditions.
One tool to measure how strong this ridge will be is by taking a look at the forecast geopotential heights. This can be sort of hard to understand but basically the greater the height is, the warmer the air mass is. According to the North Carolina State University, "Geopotential height approximates the actual height of a pressure surface above mean sea-level. Therefore, a geopotential height observation represents the height of the pressure surface on which the observation was taken."
All of the model guidance indicates heights this weekend and into the start of next week will approach 594 decameters, which is very strong and at almost record-breaking levels for this time of the year. When heights reach this magnitude, it almost guarantees extreme heat.
The graphic I made below overlays the forecast geopotential heights early-next week, showing an area of widespread heights of at least 591 decameters across the Midwest, Northeast, and Mid-Atlantic. This is often known as a 'heat dome.'
So what does this mean in terms of temperatures? Well they will definitely be above average, possibly soaring to levels in excess of 10 degrees higher than normal. If the average high temperature for this time of the year is in the low 80s, add 10 degrees to that and that means temperatures in the low 90s.
Forecasting over 5 days in advance becomes difficult -- let alone less than a day in advance at times. One useful tool when conducting a mid-range forecast is by taking a look at the ensemble members of operational models. These members sample how a slightly different initial environment may affect the weather pattern downstream, giving forecasters different possible scenarios, as shown below.
For each day, the blue outline signifies the spread of these ensemble members while the number you see for the day is the mean of those members, which is how I often like to forecast temperatures. The ensemble guidance tends to be underdone looking days in advance, so when the guidance is highlighting high temperatures near 90 degrees for several days, this is a big signal for a heat wave. As a reminder, a technical heat wave is when there are at least 3 consecutive days of high temperatures at or above 90 degrees.
The hottest day of this upcoming warmup will be on Sunday. There is the potential some towns hit 100 degrees based on all of the model guidance I have been looking at. Otherwise, the trio of the hottest days, which may make it a heat wave, will be Saturday through Monday. Beyond Monday, the upper-level ridge is expected to propagate to the west but the above average geopotential heights will stick around through at least mid-July. This will allow for warmer than normal temperatures overall as well as drier than normal conditions, which is not good news due to the dry June we've had.
This heat will be dangerous. Not only will daytime temperatures be hot paired with very humid conditions, but overnight lows will only be in the 70s. When there is no relief from the heat overnight, that truly makes this a dangerous heat event. Make sure you have an air-conditioned environment to keep cool and to stay hydrated by drinking lots of water.