Good Saturday afternoon. With a bonafide winter storm on the way, we’ll keep you posted on things this weekend. Most of the morning model data is in, and we can discuss some of what is shown and lay out some thoughts for what might actually happen tomorrow.

First, the housekeeping: A Winter Storm Watch (as of 2 PM CT Saturday, still a watch) is posted to the north and northwest of Houston for the Brazos Valley, Walker, Trinity, and Houston Counties. We expect this to be upgraded to a warning before this evening, and it’s possible an advisory or watch is expanded south also.

On satellite, the two features we are most interested in stand out nicely this afternoon. We’re looking at the water vapor channel on satellite, which shows us where dry air and moisture are located. Over the Four Corners region, you can see a swirl in the clouds representing low pressure about 20,000 feet up, our upper level storm. Over northern Mexico, you see moisture gathering, which is where low pressure at the surface will begin to develop. The system in Mexico will help deliver moisture and precipitation. The upper low will help deliver cold air behind a front. The combination is why we’re talking about winter weather.

We still believe rain will develop tomorrow morning across the entire region. Everyone should begin as rain, as cold enough air for anything else won’t arrive til later. Look for snow and sleet to begin mixing in with the rain well northwest of the city tomorrow by early afternoon. Think: College Station, Madisonville, Crockett. The rain/snow or sleet line will continue to nudge south and east tomorrow afternoon and evening, arriving in places like Brenham, Navasota, and Huntsville by late afternoon. From there, how much farther southeast could the rain/snow line get? Ultimately it will depend on the exact track of the storm, but I expect it will get close to places like The Woodlands, Conroe, Hempstead, or Cleveland before all is said and done.

In addition to that, as discussed earlier this week, we could be talking about dynamic cooling (where the rain falls so hard, it pulls down enough cold air to flip over to snow or sleet for a time) driven snow or sleet virtually anywhere north of I-10 late tomorrow afternoon or evening. It wouldn’t accumulate much, if at all, but if you see snowflakes falling or hear sleet pinging, don’t be shocked.

So the question becomes: How much snow for places north of Houston? For The Woodlands or Conroe, right now we don’t expect much. Maybe a slushy coating if things break right (or wrong depending on your viewpoint). However, as you go north of there toward Huntsville west toward College Station or east toward Lufkin, it gets dicey. Here’s the current forecast from the National Weather Service as of early afternoon Saturday:

Snow forecast as of early Saturday afternoon for southeast Texas. (NOAA)You can see about 1 to 2 inches for Huntsville, less as you go south, more as you go north. It is notable that there are some weather models that show considerably higher snowfall amounts up that way, up to 4 to 6 inches of snow or even more in spots.

There are two main problems in determining how much snow will fall. One is the storm track, discussed a bit above. The second is something I made a Twitter thread about earlier today: Sleet. Sleet can destroy a snow forecast. I’ve been burned many times by that. The more the precipitation that is frozen falls as sleet, the less snow accumulation there will be.

Let's look at 7 PM Sunday. Here's the 12km NAM thermal profile for Huntsville. Definitely good for snow with the entire column except the surface below freezing. pic.twitter.com/eqHSdtHUOS

— Matt Lanza (@mattlanza) January 9, 2021

And right now, the temperature profiles north of the Houston region are a mess, with a very, very fine line between a cold rain, sleet, or snow. That could mean the difference between someone seeing a coating or 1 inch or someone seeing closer to 5 inches of snow.

The GFS model at Conroe for that same time shows a very mild temperature profile with above freezing temps from about 7,000' above down to the surface. That's a cold rain. Alternatively, the NAM has a fairly substantial subfreezing layer that allows for sleet instead. pic.twitter.com/Qm0hLlmOid

— Matt Lanza (@mattlanza) January 9, 2021

That’s a big, big deal, and it’s very difficult to pin down. So forecasting how much snow falls will be challenging, but who sees the snow? That should be a little more straightforward.

If you live in the city of Houston, keep hope alive (if you want snow), but the odds are going to be quite low. If we do see snow in the city, I would target the 9 PM Sunday to 2 AM Monday window to catch a fleeting glimpse.

If you want to see legitimate snow and feel comfortable traveling: Huntsville, Crockett, or College Station are as good a bet as any right now, though not quite a full guarantee. Just keep this in mind if you do plan to “snow chase” tomorrow: You will have to come back at night, with temperatures getting colder. While it should be fine in Harris County and most surrounding counties, there may be difficult, slippery travel up north. So just make sure you exercise wise judgment, are prepared in case it gets bad, and please be safe.

Eric will have more for you tomorrow!

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!

After Wednesday evening’s line of storms, the Houston region will see several days of placid weather before another front arrives later on Sunday. This will bring with it a stronger shot of colder air and, for areas just north of Houston, the potential for snow. Cold weather will prevail throughout the period.

Thursday

Temperatures have dropped into the 40s across much of the area this morning, and in concert with northwesterly winds it feels brisk outside. Winds should slacken this afternoon, but even with mostly sunny skies our high temperatures are unlikely to climb above the mid- to upper-50s. Lows tonight will drop down to around 40 in the city of Houston with partly cloudy skies and light northerly winds.

Friday

This will be another mostly sunny and cold day, as northerly winds continue to bring dry air into the region. Expect highs generally in the mid-50s, and with clear skies overnight temperatures may be a few degrees cooler on Friday night than Thursday night.

Saturday

This will be our third chilly, sunny day in a row with highs again in the 50s. As winds shift to come from the east clouds may begin to build during the afternoon or overnight hours, but lows should still get down to around 40 in Houston overnight.

Sunday and Sunday night

An approaching front will combine with surface low pressure over the northwestern Gulf of Mexico to produce healthy rain chances on Sunday and Sunday night. We can be pretty confident this will be a cold and gray day, during which highs will struggle to climb above 50 degrees. Rain chances will increase as the day goes on. For now, it appears as though the greater rain accumulations will remain to the south of Houston, where totals could be on the order of 1-2 inches. However, north of a line from Wharton to Alvin to League City will probably see less, perhaps 0.5 to 1 inch of rain with this front. We’ll have to iron out those details in future forecasts.

We think things will be warm enough in Houston to keep this precipitation rain throughout the day and overnight hours, but there will be a dividing line—perhaps somewhere in Montgomery County?—where this precipitation turns into snow or sleet on Sunday evening. Matt will provide an update on the region’s snow chances with a post later today.

Next week

Our region will be cold and sunny next week in the wake of the front, with highs generally in the 50s and lows in the 30s. The urban core of Houston may remain above freezing, but there will almost certainly be freezing temperatures in the vicinity of the city, if not the entire metro area itself.

Virtual Houston Marathon

Due to the pandemic, the Houston Marathon is a virtual event this year. So instead of being run as a single event on January 17, participants can run their half and full marathons on any day between Friday, January 8 and Sunday, January 17. If you have some flexibility in your schedule, you will have options for weather. (My ideal conditions are temperatures in the 40s, clouds, and light winds, but we’re all different!) Both this Saturday and Sunday look chilly, and if you complete your run before noon on Sunday you should miss the rain. The absolute coldest temperatures in the week ahead should come on Tuesday morning, January 12, when I expect a freeze for most of Houston.

Then we’ll see a slight warming trend through Friday, with mostly sunny skies throughout. Next weekend looks a bit warmer, but basically every morning during the virtual marathon period is likely to see lows in the 30s or 40s for Houston. If you prefer warmer weather, then the coming Friday, Saturday, or Sunday afternoons—January 15-17—may be your best bet as highs get into the upper 60s, perhaps. Rain chances seem fairly low throughout the running period, aside from the Sunday, January 10th mess described in the forecast above.