As another beautiful weekend comes to a close, the weather will begin to change during the upcoming work-week. The airmass as a whole will become much warmer and more humid, which will lead to a true summer-like feel on Tuesday and Wednesday. Today will be a transition day.
One of the great indicators of the type of atmosphere we have is the amount of precipitable water in it. Precipitable water essentially tells us how much water would result at the surface if the entire atmosphere were to be squeezed out — like a wet towel. Thus, it is a good indicator of how much moisture is in the entire atmopshere, and not just at the surface. Large-scale changes in precipitable water is often a good indicator that the airmass is changing.
Over the past couple of weeks, we have been developing some of our own computer model forecast images. The main ones we will be using in this article overlay precipitable water, CAPE (Convective Available Potential Energy), CIN (Convective Inhibition), and 1000mb-500mb wind shear (essentially the wind shear [change in wind speed and direction] from the surface up to around half way up the atmosphere).
Today looks to be a transition day.
The forecast for this afternoon shows precipitable water values still well under an inch — indicating a dry atmosphere. Thus, today for the most part should feel similar to how today felt, but perhaps a touch more humid. Skies will be sunny, but high clouds will be on the increase, and highs will be in the low 80s, with little wind.
Notice how the much more moist values are just off to our west.
Once we move forward to 8:00pm this evening, the moist atmosphere begins to make its way into our area.
What’s also important to note is that much higher CAPE values will also be advancing towards our area. This is an indicator that the atmosphere has the potential to produce thunderstorms. Not surprisingly based on what was just illustrated, a complex of showers and thunderstorms will be moving into the area late on Monday night. Fortunately, the sun setting will decrease the atmospheric instability somewhat, so the highest CAPE values may get delayed until Tuesday afternoon — meaning the showers and thunderstorms should not be severe on Monday night. Regardless, a period of rain with a rumble of thunder is possible as the predecessor of a warmer and more humid airmass.
Tomorrow will be dominated by a strong ridge. This means that despite the relatively high precipitable water values and high CAPE values, there will not be enough of a trigger for thunderstorms, as the ridge will help to place a cap on any lifting. As implied before, however, it will be hot and humid, with partly cloudy skies and temperatures in the upper 80s to around 90. Westerly surface winds will also help to prevent any seabreezes for the most part, as well. The warm, muggy airmass may prevent low temperatures from falling below 70 degrees.
The forecast for tomorrow night shows relatively high CAPE in our area, a pretty moist atmosphere — but not too moist. Too much moisture (generally P-WAT >1.8″) in the atmosphere can sometimes make for the threat to only be heavy rain as opposed to stronger thunderstorms, so the right balance of moisture is there if you want stronger thunderstorms. Our area still has significant convective inhibition, however, indicated by the lightly dashed blue lines. But there is plenty of CAPE to the west along with moderately strong wind shear of 30 to 40 knots all pointed in the same direction. This is a strong indicator for a potential line of severe thunderstorms to form in the Ohio Valley and move eastward throughout Tuesday night and into Wednesday.
Wednesday morning may start off with a line of showers and thunderstorms before clearing out for a few hours. Temperatures will once again be very hot, and humidity will be plentiful — with temperatures probably exceeding 90 in most spots. Sea breezes will also be halted by the westerly flow.
Later in the afternoon, a cold front will be moving eastward along with a shortwave trough in the atmosphere. This will help to weaken the ridging somewhat; and combined with a warm, moist atmosphere, and the factors previously discussed for a strong line of thunderstorms to form to our west, the chance for severe weather exists. The main threats other than thunder and lightning would be strong, gusty winds, and some hail.
We are still a few days away, so we cannot say with definitive confidence that there will be severe weather. But the threat does look to be there.