Monday Evening Weather Blog: Complex Storm Approaching

By: Meteorologists Ray Hawthorne and Mike McCall Email
By: Meteorologists Ray Hawthorne and Mike McCall Email

High, thin cirrus clouds are approaching us from the southwest late this afternoon and will begin to thicken tonight. Temperatures should fall quickly early this evening since the atmosphere is still quite dry, but increasing southeasterly winds in the lower troposphere will cause the temperatures to become steady or even slowly rise after midnight. The outlying areas should get no colder than the mid or upper 40s early tonight and it's likely that highly urbanized and coastal areas will not get below 50 degrees.

We have a very complex weather situation setting up for the next two days. Confidence is high on the large-scale pattern, but far lower with the details. The pun "the devil is always in the details" most certainly applies here. We originally thought we were dealing with two shortwaves (disturbances) in the middle and upper atmosphere. Upon a more thorough examination of the data, there appear to be three separate impulses in play. Water vapor imagery shows the first impulse over South Texas late this afternoon. This wave is expected to cross our area Tuesday morning, but whatever forcing it creates will go toward moistening our very dry atmosphere. Essentially, this lead impulse will be "taking one for the team", so to speak. The second shortwave is weak and ill-defined, but it has access to very deep tropical moisture. It is located near the Yucatan Peninsula of Mexico and will be spreading rain and thunderstorms across the Florida Peninsula on Tuesday. It's unlikely that it will come close enough to produce widespread rains in our area during the daylight hours, but a brief shower or two cannot be completely ruled out late in the day.

That leaves the third (and strongest) disturbance, which is presently plunging southeastward across the Northern High Plains. Increasing large-scale ascent ahead of this wave will overspread the entire region Tuesday night and Wednesday morning. As a result, at least some rain is anticipated for much of our viewing area during that time frame. However, we are not sure if the second impulse will cut off (or rob) the deep/moist inflow as the third wave approaches. At this time, we think the best bet is for pockets of rain to develop on Tuesday evening (but not lasting for any more than several minutes to at most an hour). As the surface cold front approaches from the west, a narrow but heavy line of showers and maybe thunderstorms could develop late Tuesday night or early Wednesday morning. At least this is the idea proposed by our locally-run Super Microcast high-resolution model and even the operational NAM. Meanwhile, the GFS forecasts a much weaker line as it shows much of the moisture lifting away from us because of that second disturbance. We've seen these sort of things go both ways, so it's just hard to tell which model solution will verify. If sufficient instability can develop ahead of the front, gusty winds and maybe even some hail is possible with any strong or severe storms that develop. We want to emphasize; however, the low confidence in the eventual structure and intensity of the line as it develops. Hopefully, we will know a little more tomorrow.

The weather should improve from west to east by Wednesday afternoon and fair conditions seem like on Thursday and Friday. The models are still indicating another Pacific system arriving on Sunday. With it could come the threat of showers and strong thunderstorms. We think the slower ECMWF, UKMET and GFS ensembles are more reasonable than the operational GFS model. Thus, we are favoring more of a Sunday afternoon sort of thing instead of Sunday morning. Obviously, we have lots of time to watch the model solutions evolve and we will keep you updated on those developments as the week moves along.

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