Sunday Weather Blog: Changing Pattern Ahead

By: Meteorologist Ray Hawthorne Email
By: Meteorologist Ray Hawthorne Email

The radars are lighting up this morning with rain showers over the Gulf of Mexico and Apalachee Bay. Water vapor imagery and upper air observations reveal a well-defined middle and upper atmospheric trough translating eastward across the lower Mississippi Valley. Lift associated with a vigorous upper jet circulation is, at least in part, responsible for the increasing rains. The present motion will take any showers over the eastern Florida Big Bend, Suwannee River Valley, and perhaps South-Central Georgia late this morning and early this afternoon. The Tallahassee balloon launch indicates that the drier airmass has already arrived in the central and western parts of our region, so little or no rain is anticipated west of U.S. Highway 319 today. The very strong forcing at the tropopause level is headed toward the Mid Atlantic coast, where I expect a new surface low to form in response. This low will deepen as it reaches New England on Monday, almost certainly resulting in a major snowstorm from New York City to Boston. A strengthening northwesterly flow of cold air will funnel in between the departing low and an oncoming high pressure system in the Rockies and Northern Plains. This sort of a pressure pattern will keep the winds up late tonight, so the statistical models are probably overdoing the amount of cooling in a strongly-mixed boundary layer environment. Thus, the mercury will probably not drop much below 40 degrees Monday morning, but a 5 to 10 mph wind will make it feel like it's in the low to mid 30s.

Sunshine will be in no short supply on Monday, but a 10 to 15 mph wind from the northwest will keep temperatures below their seasonal norms. We expect highs to reach the upper 50s or perhaps lower 60s at most.

The real forecast dilemma comes on Monday night and Tuesday morning. The high pressure ridge axis is expected to spread out from the Central Plains into the Southeastern United States. It therefore seems likely that winds will decrease to less than 5 mph (or even go calm). This sort of synoptic setup usually favors a light or moderate freeze for this area; however, the lift associated with upper jet is forecast to ramp up yet again. While the lower and middle atmosphere will be much too dry for rain, the vertical motion created by the upper jet circulation will probably cause cirrus clouds to form and move into the area Monday night and Tuesday morning. The $64,000 question is, "just how thick will these clouds be?" That's almost impossible to judge until we start to see these clouds form on satellite. For now, I've taken the path of least regret (and knowing my luck, probably the most pain) and I will continue to predict a light freeze. Temperatures are only expected to dip to 32 degrees or colder for 1 to 3 hours, on average. Highly urbanized areas (such as downtown Tallahassee) and the coast will not see a freeze at all. We will, of course, revise the forecast when and if it becomes clear that we'll see a freeze.

It looks like an abundance of cirrus or cirrostratus clouds will occasionally filter out the sunshine on Tuesday, but we ought to remain dry yet again. The large-scale pattern is then expected to undergo a major change. The mean longwave trough (or Rossby wave, as we call it in meteorology) is forecast by all of the numerical models to retrogress more toward the Central United States. Even though we typically do not think of troughs moving from east to west, it turns out that waves in the atmosphere having greater wavelengths respond differently to those with shorter wavelengths (known as the beta effect). It is this fundamental change in the global pattern that should give us multiple opportunities for rain. The first impulse will cause a surface low to form in the western Gulf of Mexico on Wednesday. There are still disagreements between the models as to how strong the low will get and how fast it will move, but the differences in the models should not change the fact that at least some rain is becoming increasingly likely starting late Wednesday afternoon and lasting into at least Thursday morning. Once the models resolve the details, we can start discussing amounts and timing in greater depth.

Some of the models (especially the Canadian and European models) are sending us yet another vigorous disturbance on Friday with yet another shot of rain. However, the GFS makes the first storm Wednesday night and Thursday much stronger, which then acts to suppress the development of another one. Conversely, the Canadian and European global models deemphasize the first disturbance and instead amplify the second on Friday. I've seen these sort of things go both ways, but the GFS has been known to overdo the lead impulse in a series of many. So if I had to lean one way, I'd say we'll get another shot of rain and maybe even strong thunderstorms on Friday, but that is a not high confidence prognostication - at least not yet.

Buckle up, folks. We might finally be in a somewhat interesting pattern!

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