Matt Lanza

Good morning. Today’s post will be fairly brief, as I went in depth rather significantly on weekend snow chances yesterday, and we will be keeping you posted this weekend. To be quite frank, not much has changed since yesterday’s post, although trends overnight were slightly warmer. But still, let’s jump in.

Today & Saturday

Not much weather to speak of over the next 24 to 36 hours. Today will see a bit of low cloud cover in parts of the area this morning give way to mostly sunny skies. Look for highs in the mid-50s. Tonight will see clear to partially cloudy skies with lows in the 30s and 40s. High clouds may dim the sun a bit tomorrow, but otherwise, it will be a mainly nice day with highs once again in the mid-50s.

Sunday

So the forecast for Sunday looks mostly on track with the big picture details being cloudy skies to start with rain developing from west to east through the late morning and afternoon. The thought right now is that basically the entire area from the coast through Conroe (Highway 105) will begin as rain. As the atmosphere cools later Sunday afternoon and evening and the rate of precipitation increases, that’s when we’ll start to likely see some snow reports begin to trickle in from places like Huntsville or Lake Livingston or College Station. It’s possible a few locations farther south closer to Houston see snow mix in if it rains hard enough Sunday afternoon.

As we go deeper into Sunday evening, watch for more snow reports to come in from places even farther south, more into Conroe or The Woodlands or Tomball out into Cleveland and Liberty. Rain may turn to mostly snow in some of those places before ending after midnight. And perhaps the rain will end as some light snow in Houston also after midnight.

Will snow accumulate? As of right now that seems very unlikely in the city of Houston and most of Harris County and surrounding counties to the west, south, and east. For Montgomery County and points north from there, yes, snow might accumulate a bit. Right now I wouldn’t expect more than an inch or two, mostly on grassy surfaces and trees, but we could see more in spots depending on how things come together. Slick road conditions are indeed possible up that way Sunday night and Monday morning.

As of Friday morning, the snowfall forecast for Texas shows a wide band of 4 to 8 inches of snow to the north and west of about Waco, with snow totals dropping into the 1 to 3 inch range east of I-35. Again, this too could change.

Once more, we have to underscore that because storm tracks matter a lot and a few miles can make a world of difference, there is still substantial uncertainty surrounding this forecast. Much may change by Sunday morning. We’ll keep you posted this weekend. I mean, can you tell that I’m excited by this? Has it shown at all?

Again, in addition to the snow aspect of things, there will be locally heavy rain (up to an inch in spots, more perhaps near the coast) but probably no serious flooding. Thunderstorms are possible. And coastal issues, such as rip currents, rough seas, strong winds, and even some tidal flooding will be possible also.

Monday and next week

Look for clouds to hang tough on Monday behind the storm, with cold and windy conditions. Clouds clear for Tuesday, Wednesday, and probably Thursday. Look for temps to rebound into the 50s and eventually 60s. Morning lows in the 30s Monday morning (and perhaps a freeze north of the city) will rebound into the 40s by midweek.

As a Jersey transplant, having a bonafide winter storm to forecast feels like a Super Bowl or something to me. So in this post, we’re going to go under the hood a bit on the ways it may or may not snow in the Houston area.

How it happens

First, let’s just tackle the big picture story. Why are we even talking about this? More often than not this time of year, we see storm systems pass by to our north and drag cold fronts through. And we get some rain or storms, and then it turns windy and cold but much drier behind the front. On Sunday and Monday, things happen a bit differently. First off, on Sunday morning, the air mass over Texas is quite chilly. This is a map of temperature anomalies 5,000 feet above our heads as forecast by the European model on Sunday morning.

The air mass isn’t super cold, but it’s a few degrees colder than normal, colder still in West Texas with an upper level storm system passing by at 20,000 feet above their heads. While this certainly keeps things appetizing, the best word to describe the air mass over Texas is marginally cold. I’d like to see it colder to be more excited about snow chances. Alas, we’ll work with this.

What do we need next? Ideally, a coastal low pressure system at the surface. And we get just that on Sunday evening. By 6 PM or so, the European model shows the surface low just off Corpus Christi.

Keeping it to our south is critical because that will limit or inhibit warm air from being drawn into our area. So at this point in the game, widespread cold rain should be breaking out across the region. Temperatures will begin to fall toward the dew point as rain cools the air, and we may have temperatures down into the upper-30s to near 40 in Houston and south by evening, with mid to upper-30s north. It’s plausible that some sleet or snow could mix in with the rain at times, mainly along or north of Highway 105; think Lake Livingston, Conroe, Cleveland, Navasota.

Ultimately, how far north or south both the surface low and upper low (which is expected to be just east of the Caprock on Sunday evening) matter. If they shift farther south, colder air is possible and that rain/snow mix may start off farther south. The opposite is true if they’re farther north. Here’s an example. You see the European model above. The GFS model is very similar looking. The NAM model earlier this morning showed the surface low about 50 to 75 miles farther east at this same time, and it has the upper low south of Abilene, or about 175 miles farther south of where the Euro has it at this time. As a result, it would explicitly be showing snow as far south as Montgomery County at this point and pockets of a rain/snow mix closer to Houston. The most recent NAM model just came out, and it has the surface low about 25-50 miles farther north, which makes Houston warmer and keeps snow bottled up north of Highway 105. Which model is right? We have no idea right now. But this will be critical in determining how far south snow can get early on. We will watch this tonight and tomorrow into Saturday closely.

The other way to get snow

As we go into Sunday night, the storm will progress off to the east and northeast. For Houston and points south, again, mainly a cold rain with temps steady in the 30s or low-40s. North of Houston, it should be rain, periodically mixed with snow. Here’s the interesting thing about the chance of snow north of Houston, up along highway 105 or so though with this storm. I think it’s actually really legitimate. One of the hallmarks of this system showing up on the models this week is how “dynamic” it seems. There are several ways to get snow. Above, we talk about getting enough cold air via a favorable storm track. Now, let’s talk about dynamics. When you get enough vertical motion within a storm system, basically a lot of rising air, you tend to drop temperatures. We call this “dynamic cooling.” In winter storms like this, another way to get snow is for it to essentially rain so hard that the air cools enough for the rain to flip over to snow. I think we are close to having a case be made with this storm that we could see enough dynamic cooling occur that, in spots, heavy rain could suddenly flip to heavy snow at the height of the storm Sunday evening. Thundersnow can’t be ruled out either.

Let’s get even more technical. If we look at the latest NAM model from this afternoon, we can see how this might occur. The image below is what we call a time-height cross-section. It’s basically taking a specific location, in this case Conroe, and looking at how vertical velocity (VV, or rising air) and the dendritic growth zone (DGZ, or where air temperatures cool to roughly -10 to -20°C, good for snowflake production) change over time.

You can click the image to enlarge it. In that image above, one thing stands out to me. We have a lot of vertical velocity with this storm, and the strongest VVs come close to punching the heart of the dendritic growth zone at times. In English? That creates a very sloppy setup where, yes, we may see rain most of the time, but if it rains hard enough, there will be pockets anywhere (mainly north of I-10 though) that could change from rain to accumulating snow or sleet and back to rain at times.

This means that it will be difficult to near-impossible to give you a snowfall forecast right now, as those sorts of processes are very small scale. I come from the Northeast, and they’re difficult to forecast this far out up there; they’re next to impossible to forecast this far out in the South.

As the storm pulls away early on Monday morning, enough colder air may drift in to have the rain end as some light snow or flurries/drizzle across parts of the area, again mainly north of I-10. No accumulation would be expected from that. Monday will likely be a gray, cold day.

So let’s summarize what we’ve got quickly.

• Snow, and accumulating snow at that is becoming more likely across the northern stretches of Southeast Texas, particularly along and north of Highway 105.
• Accumulating snow can’t be ruled out anywhere north of I-10 just yet, but it’s more likely that some places maybe flip back and forth from rain to snow/sleet to rain with little to no accumulation. 
• Ice is not likely with this storm, though well north of Houston, some roads could be slick at times (think Huntsville or Lake Livingston areas). 
• There is still sizable uncertainty on specifics regarding the details of this storm and significant forecast changes may occur between now and Sunday.
• Locally heavy rain is possible south of Houston, along with some thunder.
• It will likely be a bit windy as well, and there is the potential for some coastal flooding along the Gulf, with rough seas and strong rip currents.

More for you tomorrow morning!

To close out 2020, Eric and I put together two posts to summarize the Atlantic hurricane season, its effects on Houston, and implications for the future. Eric wrote Part I yesterday, which discusses the overall season and Hurricane Laura in particular. In Part II, today, I will discuss Tropical Storm Beta and what we can learn from this season about future hurricane activity.

Tropical Storm Beta

While Hurricane Laura was a clear and definitive threat, Tropical Storm Beta was a more difficult nut to crack. From the outset, we weren’t so much concerned about Beta as a wind threat, but rather as a slow-moving, disjointed storm capable of producing prolific rainfall. The tropical update I wrote back on September 15th discussed future Beta as an “untagged” disturbance that probably was more a concern for South Texas. “So while we aren’t particularly worried about this area, we do feel it’s one to keep an eye on.”

The issue with Beta was rainfall, and you could see a strong signal several days beforehand that it had the capability of producing double digit totals, as seen below from our Thursday morning post.

By that Thursday afternoon, a Tropical Depression 22 had formed, and modeling began to suggest it would come farther north. Our first real early season cold front around that time would help keep most of the rain offshore, but there were indications that the system itself could make a run at the Texas coast on its way out, moving slowly and dumping rain on the way.

Initial rain forecasts began circulating on Friday as Beta’s track came more into focus, with as much as 10 to 15 inches forecast along the coast south of Houston. We also began playing up the coastal flooding risk posed by Beta. By Friday afternoon, we had classified Beta as a Stage 2 flood risk (south of I-10) using our SCW Flood Scale.

On Saturday, we got a good handle on the general theme of how Beta would unfold, likely as a rainfall issue but not too terribly serious, some wind but not so much to cause widespread power outages, and considerable coastal flooding. Beta reached peak intensity over the Gulf on Saturday, though at the time, it was still expected to become a hurricane upon approach to the middle Texas Coast.

Beta’s rainfall was expected to still be pretty aggressive, with upwards of 4 to 12 inches expected and at least the risk of 10 to 20 inches in a worst-case scenario.

We knew it had potential to cause headaches, but we did not feel it was going to become a catastrophic storm.

By Sunday the 20th, I began to check out because becoming a second-time dad took some precedence. But the rains began as Beta tracked toward the coast near Matagorda and then along it to the northeast through Houston. On Sunday, about 1 to 2 inches fell across eastern and southern Harris County and northeast parts of Fort Bend County, mainly near Sugar Land. Beta tried valiantly to strengthen on Sunday afternoon but failed.

Monday saw Beta begin its peak assault on the Texas coast, with heavy bands of rainfall setting up over and south of Houston by evening. Close to 10 inches of rain had fallen south of Houston, causing some bayous to come out of banks. Rain continued off and on into Tuesday morning, leading to more widespread flooding that caused us to escalate things to a Stage 3 flood event south of I-10. Thankfully, the rains tapered off a good bit on Tuesday, and the heavy rain on Tuesday night setup a bit farther northeast than Monday night, allowing for less flooding problems. By Wednesday, Beta had moved away, and that was that.

From a return-period standpoint, Beta’s 48 hour total rains were mostly classified by the Harris County Flood Control District as between a 2 and 10-year event (or a storm that has anywhere from a 10 to 50 percent chance of occurring in any given year). The lone exception appeared to be portions of Sims Bayou that may have been closer to a 50-year type of storm, meaning in any given year there’s about 2 percent probability that we’ll exceed certain rainfall totals. You can see that while Beta was certainly a respectable storm, it paled in comparison to our recent major flooding events from tropical systems.

Beta flooded 20 to 25 homes along Clear Creek or Brays Bayou and a handful of homes (due to drainage) near the South Loop at Cullen. Because the maximum rate of rainfall during Beta was “only” about 2 to 3 inches in an hour, and there were breaks, the area was able to mostly avoid severe, widespread bayou and/or home flooding. Harris County Flood Control estimated that nearly 1,000 homes were spared flooding due to recent capital improvements along Brays and Sims Bayous.

As far as storm surge and coastal flooding went, the other element of this storm, it was estimated that surge levels hit 1 to 4 feet above ground level along the western shore of Galveston Bay and on the Gulf Coast, a firm moderate coastal flooding event for many areas.

Laura reminded us of our exposure and vulnerability to a major hurricane, while Beta was a good reminder of the rainfall hazards we must accept by living in this part of the world. While storms like Allison and Harvey are thankfully rare (though perhaps becoming less rare as time goes on), storms like Beta, which produce heavy rainfall and sub-catastrophic flooding, are pretty typical in Texas, which sees slow-moving tropical systems fairly frequently. Again, this may be occurring more frequently as climate change impacts become more common. It’s important to have flood insurance if you live in this area, and it’s important to have a plan if flooding from rain or bayou rises impacts your home.

For those that utilize our Space City Weather flood scale, Beta will go in the books as a lower-end Stage 3 flood. While it may have fallen in between stage 2 and 3 to some extent, given the widespread flooding we experienced and coastal impacts, we feel a low-end 3 makes sense. If you read about the historic Stage 3 examples, we would probably rank Beta just under all of those listed.

The future

The first question we expect to be asked is,  “Does the fact that this hurricane season was so active mean next season has higher odds of being more active also?” And we would say, it absolutely does not mean that.

The 10 most active Atlantic hurricane seasons on record prior to 2020 averaged 20 tropical storms, 11 hurricanes, and an accumulated cyclone energy (ACE) of 186. The years that followed averaged 15 tropical storms, 7 hurricanes, and an ACE of 106. Those years that followed saw anywhere from below average activity to borderline hyperactivity. Is it possible that next year is also a very busy hurricane season? Certainly. But it’s almost equally likely that it could be a below average season if history is any guide. One of the best examples of this is the 2006 hurricane season. Preseason forecasts were very active, coming off the absurdly destructive 2005 season with high odds for both significant storms and impactful ones. What happened? We got 10 named storms and minimal U.S. impacts.

We can begin to take a very, very wild swing at what next season may setup as in terms of La Niña or El Niño (ENSO).

If we look at the CFS model from NOAA and the European (ECMWF) model, we can see their long-range forecasts for what “state” the tropical Pacific might be in several months down the road. Per the ECMWF model, while La Niña may be with us into spring, it looks to begin to perhaps fade toward May or June 2021.

As far as the CFS model goes, it shows basically the same idea: Weakening La Niña heading toward next summer.

The huge caveat here is that these types of long-range forecasts of ENSO are notoriously fickle. A lot can and probably will change between now and next summer. However, the early read on things is that conditions may start the season in a borderline La Niña and then get less hospitable as the season wears on. There’s far more to consider besides ENSO, but those are mostly things we can’t really speculate on this far in advance. One thing we can at least look at are sea surface temperature anomalies (SSTs). Here’s a look at how SSTs have changed from 2001-2010 to 2011-2020.

Most of the Atlantic Basin has warmed during the peak months of August, September, and October, with the exception of some of the lower-latitudes between the Caribbean and Africa. This is likely due to both a combination of climate change and some cyclical background noise. Either way, it’s not a good thing.

If we want to expand further on climate change, we could talk about rapid intensification (RI), as climate change is likely leading to more RI and more significant RI. I went through a number of scientific papers on the subject recently. While there are still some mixed results about exactly which basins around the globe are more likely to see more RI and exactly how significant it will be, the general trend is pointing toward more of it in more storms. 2020 only furthered that concept.

The 2020 Atlantic hurricane season now has the most storms undergoing 36-hour intensification of at least 75, 80, 85, and 90kt pic.twitter.com/tiMBPTNa46

— Sam Lillo (@splillo) November 16, 2020

In the chart above, you can see how 2020 had the most storms with intensifications of 70 to 90 kts. in 36 hours of any prior modern hurricane season. Obviously with more storms, you should get more rapidly intensifying storms, but 2020 was sort of at some higher level than usual. And truthfully, it serves as another data point in a sea of them in recent years.

Putting all this together, you would probably expect next season to be near-average, maybe a little above, but nothing like 2020’s hurricane season. Still, if we are indeed moving into a world with more rapidly intensifying storms, it won’t take a large number of them in a hurricane season to cause problems, especially in the Gulf, as we witnessed with Hanna, Laura, Delta, and Zeta this year.

Rainfall this afternoon was quite vigorous across the Houston area, with a few places seeing as much as an inch of rain in about 15 minutes’ time.

This has added up with the gauge at Spring Creek and SH 249 for instance already over 3 inches of rain today. For most areas, this has been manageable with just some minor street flooding.

The trouble tonight is that the front is going to approach and stall west of I-45 for a time. You can see this well on the HRRR model for example, which is showing the front (roughly the boundary between warmer and cooler colors here) stalled out over Harris County tonight and into tomorrow morning. This animation runs from 7 PM Wednesday to Noon Thursday.

In this time, repeated rounds of rainfall, some heavy, will impact areas primarily west of I-45 (until you get north of Conroe). Additional rainfall during this time on top of what has already fallen, will average about an inch or so east of Houston and 1 to 4 inches west of the city (presume about 1 to 3 inches in the city itself). There will likely be some pockets of higher amounts up to 5 or 6 inches or so west of Houston. Those higher amounts would be most likely in western Harris, western Montgomery, Grimes, Fort Bend, Wharton, or Waller Counties.

This could lead to some localized flooding, and for that reason, we are going to implement the Space City Weather Flood Scale at Stage 1 for tonight and Thursday.

We will go with a Stage 1 event for the western half of the area, which generally means mostly nuisance street flooding. But there will likely be rises on area creeks as well, particularly Willow Creek, upper Spring Creek, and upper Little Cypress Creek. So we will watch and escalate if necessary, but we think this will behave for the most part in our area. Still, please exercise caution, especially if you have to be out for whatever reason tonight and tomorrow morning. A Flash Flood Watch is posted for Houston and points west through tomorrow afternoon.

Rain should rapidly end from southwest to northeast across the region tomorrow afternoon and evening.

In addition to the rainfall, we did see a tornado warning at one point for Harris and Fort Bend Counties this afternoon. We think the severe threat will be fairly minor tonight, but we can’t entirely rule out a storm or two trying to get a little excitable, particularly if it can develop east of the front. For folks in Galveston, Chambers, and Liberty Counties, this would be the primary way you would see rain tonight, from a rogue storm or shower ahead of the main action to the west. Not too serious a concern right now, but something we’ll keep tabs on.

More from Eric in the morning!