Matt Lanza

With a fine, quiet stretch of weather beginning today for Houston, Eric and I thought it would be a good time to write a bit about the new climate normals released by NOAA on Monday night. When we compare weather to “normal” or “average,” these are the values we are comparing to, so it is useful to understand how they will be changing.

What are they? Every 10 years, NOAA and the National Weather Service, along with other government weather agencies around the globe recalculate what the typical weather has been like in the previous 30 years. Think of it like the Census, but for weather.

Why average 30 years of weather instead of, say, 50 or something? First off, the World Meteorological Organization (WMO), which is essentially the U.N. for meteorological agencies stipulates that member states use a 30 year standard. In general, 30 years is enough data to get you a good snapshot of what the current climate is, rather than a 50 or 100 year average. In other words, when we compare weather to a 30-year normal by saying something like, “Today was 15 degrees warmer than normal,” we’re comparing to the shorter-term climate we’re used to, not the one perhaps our grandparents lived through. And 30 values is generally when statistical averages become more reliable. Some users and industries prefer their “normal” to be calculated over a different timespan, so those calculations will be added to the mix from NOAA as well.

Why do they matter? When dealing with weather and making plans in business or designing buildings or planting crops, you need to have a baseline to anchor yourself to, some kind of “average” weather. Imperfect as they may be, the 30-year normals accomplish that. Having those allows us to explain weather in terms of how typical or atypical it is on a particular day or in a particular month. One thing we all know: Weather changes and can be wild, and “normal” is really just an average of a bunch of various extremes. But we need to have that anchor point to describe weather in a climatological context. I don’t know if this is originally attributable to him, but as Dr. J. Marshall Shepherd, former president of the American Meteorological Society likes to say: “Weather is your mood and climate is your personality.” It’s good to know a local area’s “personality!”

So what has changed? Let’s start big and then go small. Across the U.S., the new set of climatological normals is generally warmer, much warmer. The only areas cooler than the previous set of normals were in the Plains.

Officially, Houston’s annual average temperature (at IAH Bush Airport) rose by 0.7° versus the previous normal. At Hobby Airport that change was 1.0° warmer, and at Galveston it was 1.1° warmer.

The only months that showed cooling relative to the previous set of normals were November and December. Summers have warmed by a bit over a half-degree on average, and late winters have warmed by over 1 degree. Some other notes: Our first day with an “average” high of 90° has moved up to May 29th from June 3rd. Our last day of average highs of 90° or better is September 18th, up from the 14th. So, we’ve added roughly a week more of days with an average high of 90° in 1991-2020 versus 1981-2010. Additionally our hottest daily average maximum temperature (or “peak” of summer) is up to 95.7° compared to 95.0° in the previous set of normals. Not only that, that peak has moved up a bit to August 4-6 versus a longer, flatter peak from August 5-12 previously. I wouldn’t read too much into that last part, but it’s something to note at least.

You’ll also notice that rainfall has increased, thanks in part to average August rainfall being over 1 inch wetter than in the prior normals. This is almost certainly thanks mostly to Hurricane Harvey. Houston now sees rainfall of roughly 2 inches more than we used to each year.

Interestingly, but perhaps not surprisingly to some folks if they live there, Galveston actually saw annual average rainfall decrease in this latest set of normals. In fact, not only does Galveston see less rain now on average, it’s over 3 and a half inches less! This is likely both a function of randomness in the sporadic nature of daily thunderstorms here but also a notable change related to <waves hands> something with respect to the warming water temperatures of the Gulf of Mexico or more frequent ridging in the upper atmosphere over the Southeast. This feels like too big a change to me to be just random chance. In addition, Galveston was substantially warmer, especially during nighttime. Average lows are over 1.5° warmer now than they used to be.

Anyway, hopefully you find this interesting. How we describe weather “relative to normal” will change going forward, so we feel it’s notable to explain to you how that will change. On to the forecast.

Today through Friday

Step outside this morning, folks.

We have temperatures in the 50s inland to low or mid-60s in the city and along the coast. And the humidity has plummeted. Look for plenty of sunshine and highs around 80° or in the low-80s this afternoon. Tomorrow looks just as nice. Expect morning lows in the 50s north and west of Houston and low or mid-60s in the city and along the bay. Galveston should see upper-60s. Another day of sunshine will allow us to get into the mid-80s tomorrow. Friday looks like essentially a carbon copy of Thursday with a few added higher clouds at times.

Saturday and Sunday

Look for onshore flow to kick back in this weekend, driving our dew point temperatures back up to 70° or so, along with highs well into the 80s. Saturday morning will probably see 60s for lows, while Sunday should see 70s in the morning. Along with that, there is at least a chance of a shower or storm Saturday. For Mother’s Day, we expect a slightly better chance of showers, however despite being better, those chances still remain fairly low right now in most of the Houston area. As you go north of Highway 105, the chance for some more robust rain and storms may increase somewhat. We’ll hone in on details more tomorrow and Friday.

Next week

It’s too early to get too into the weeds next week, but with a cold front “in the neighborhood,” suffice to say it will probably be a somewhat unsettled period ahead. More on this for you tomorrow and Friday as well.

Today has worked out rather well so far for most of the area. South and east of Houston was minimally impacted as expected, with under an inch in most locales. North and west of Houston saw roughly 1 to 3 inches on average. The jackpot occurred in Waller County, as Eric discussed this morning. Threemile Creek at FM 362 just northeast of Prairie View was today’s champion with nearly 6.5 inches of rain.

While that’s certainly a lot of rain, thankfully flooding has so far been restricted to some sporadic street flooding and some lowland flooding in rural Waller County. If you do need to travel between Magnolia and Prairie View tonight, use caution.

So as of 8:30 PM the radar is quiet in our area. There are two clusters of rain right now: One is just moving east of I-35, east of Austin and San Antonio. The other is sitting offshore between Corpus Christi Bay and Matagorda Bay. Everything looks fairly innocuous now, but after midnight tonight, storms are likely going to explode to our southwest. Significant, widespread heavy rain is likely, well to the southwest of Houston. The area between Corpus Christi and Victoria seems to be the most likely to see significant 4 to 8 inch totals, well away from the Houston area. The HRRR model depiction of what radar could look like overnight is shown below.

Folks in Wharton or Matagorda Counties will want to pay attention to things overnight I think, but even they may be a bit far enough northeast to avoid the real heavy stuff. This area of rain is likely going to creep closer to the Houston area on Saturday morning. Thankfully, it should also begin weakening around the same time, so while I would plan on some rain for your Saturday morning activities, if everything goes according to forecast, we should see minimal issues on area roadways.

As always with upper lows, there is a tinge of uncertainty underlying everything. However, there is very good model agreement on the general theme of things tonight and tomorrow, so we aren’t especially worried. With that said, we will maintain a Stage 2 flood alert. But honestly, that’s mainly for areas that have already been hard hit, like Waller County, although the rains don’t look too serious there tonight and the first half of Saturday.

Also, folks near Galveston Bay have been removed from the NWS Flash Flood Watch as of late this afternoon. Depending on how things go tonight and early tomorrow, Eric and I will consider pulling back to a Stage 1 alert on Saturday. Fingers crossed.

Saturday afternoon and evening

As of this evening we think most Saturday afternoon plans should be fine. There may be a few showers or some light rain around, so have an umbrella handy. But you might be able to swing most plans without issue. Saturday evening may see conditions go downhill with a line of thunderstorms coming out of Central Texas and sweeping across Houston after 9-11 PM or so. Conditions would improve greatly on Sunday. We’ll update you on that tomorrow morning.

Here’s a quick breakdown of how Saturday looks:

6 AM-Noon: Heavy rain possible, especially west of I-45 and south of I-10. Scattered showers and storms possible elsewhere.

Noon-6 PM: Scattered showers or some light rain, but intensity and coverage of rainfall is expected to diminish.

6 PM-Midnight: Increasing chances of rain and thunderstorms as you get closer to midnight.

We will freshen things up for you again tomorrow morning. Have a good night!

The forecast situation across the Houston area is walking a fine line between very beneficial rainfall and “alright, you can stop now.” Based on model data this morning, it would seem most of the area will see the beneficial aspects, but there remain concerns that some parts of the area may get a bit more than hoped, and because of this, we are going to shift from stage 1 to stage 2 on our Space City Weather Flood Scale.

Flood watch

A flash flood watch was hoisted by the National Weather Service yesterday evening, right about the same time we posted about calling for a Stage 1 flood event on our Space City Weather Flood Scale. Our reasons for escalating to stage 2 are laid out here.

What does it all mean? You should interpret all of these things to mean that street flooding is a very good bet today and at times through Sunday. It would be a good idea to adjust your plans around the weather the next couple days.

Could it get worse than this? Thankfully, we are starting off with low water levels and dry ground. So it will take a little time for grounds to truly become saturated. Remember, flash flooding is less about how much rain falls and more about how quickly it falls. Rainfall rates are expected to be 1 to 3 inches per hour at times. Initially, that can produce street flooding, hence why the flash flood watch is in effect today. Rapid rises will likely occur within bayous today, but at this point, we would expect them to remain within their banks. After today, the ground will be more saturated, and if we were to see several hours of 1 to 3 inch per hour rain rates tomorrow or Sunday morning, then we could begin talking about something a little more serious. For now, expect street flooding to be the main issue, but we are going to watch things closely today and tomorrow in case this does escalate.

So, you’re bumping to a Stage 2 flood alert? Given that this looks to be a bit beyond just nuisance street flooding (Stage 1) and will likely have some more significant localized impacts (Stage 2), we are going to upgrade our flood alert to a Stage 2 this morning. For most folks, Stage 1 impacts of nuisance street flooding are all you will see. But given the “training” aspect of heavy rain rolling over some places again and again and the excessive rates (1-3″ per hour), we do believe a few locations may see a bit more than just nuisance street flooding, more akin to a Stage 2 event. In the interest of full transparency, the risk of this escalating to a Stage 3 type event, while not especially likely, is not zero, and this is something we will be watching closely this weekend.

So how much rain are we talking? On average, we expect most places to receive 2 to 4 inches of rain by the time we get to Sunday evening.

Some may receive a bit less. With 1 to 3 inch per hour rates, yes, others will receive a good bit more than this. Most models do indicate that the maximum total rainfall in very, very localized areas could reach 10 to 12 inches or even a little more. Given the setup today, I would expect the highest chance of those max totals to be north or west of Highway 59.

Today

Radar this morning shows heavy rain lined up mostly on the west side of the city stretching on an axis from Port Lavaca through Wharton, the Katy area, Cypress, up into The Woodlands and Conroe and northeast to Lake Livingston.

Notably, it’s around Lake Livingston where the heaviest rainfall is causing almost 3 inch per hour rates this morning. Closer to Houston, Kuykendahl at Spring Creek has seen over 1.3″ in an hour as of this writing. As I was about to press “post” here, a Flash Flood Warning was just issued for parts of Waller, Montgomery, and Walker Counties.

Flash Flood Warning including Willis TX, Prairie View TX, Waller TX until 9:00 AM CDT pic.twitter.com/mBgCOai4ZH

— NWS Houston (@NWSHouston) April 30, 2021

Over the next few hours, we expect this band of rain to stay mostly in place, though it may wiggle east or west a bit at times. This will allow for repeated rounds of rainfall on the west side of town, and totals could easily jump up to 2 to 5 inches or more in some spots before this evening. East of Houston? You’re fine for now. Expect at least some rain or showers and storms today, but places like Galveston or League City or Baytown will probably fare okay today with modest rain totals.

Tonight

We may see things taper off this evening for a time, or at least become more intermittent across much of the area. But the next round of rain seems likely to develop overnight, particularly to the southwest of Houston. This batch of rain is likely to try and work into parts of the Greater Houston metro toward Saturday morning. We’ll have more on this later today.

Saturday

Many folks have Saturday plans. Between crawfish boils, the MS 150 bike ride, and other post-vaccination life fun, there is a lot planned this weekend. While I want desperately to be an optimist and tell you it’s going to be fine, the reality is that you should have backup plans in place for outdoor events tomorrow. I will say that the afternoon *might* look better than the morning. We expect that area of rain and storms to weave through the area tomorrow morning and hopefully exit to the east or die off. But there are some models that keep periods of moderate to heavy rain going west and southwest of Houston in the afternoon as well. The reality with upper lows is that forecasts beyond 12-18 hours get sloppy, and we won’t have a great idea on what tomorrow looks like until tonight.

Saturday night & Sunday

One more slug of rain and storms is possibly going to move through later Saturday night and Sunday morning. By Sunday afternoon, the focus of rain should shift into far East Texas and Louisiana and Mississippi. Hopefully by Sunday afternoon we will see some sunshine.

Temperatures all weekend look tolerable, albeit with some humidity with highs in the 70s and lows in the 60s. Sunday could see temps surge past 80° with enough sunshine, however.

Next week

Monday looks like a fine day right now. There may be just enough of a trigger left behind after the weekend storm to allow a shower or storm to pop up, but as of today we would expect the vast majority of the region to stay dry on Monday. Heat and humidity will rule, with lows in the 70s and highs in the middle or upper-80s. A cold front still seems on track to hopefully clear the area on Tuesday. It will bring a chance of showers or storms, and then follow up with hopefully a little lower humidity for Wednesday and Thursday. Right now, this still looks promising, but as noted American philosopher Justin Timberlake once quipped, “It’s gonna be May.” May in Southeast Texas is often where cold fronts come to die. Here’s hoping that’s not the case next week.

We will have another update on today’s rain situation before Noon today.

The main story in today’s post will be regarding the chance of strong to severe thunderstorms today. The Storm Prediction Center has most of the Houston area in a “Slight” risk (level 2/5) of severe weather today. Areas around Lake Livingston and Crockett are in an “Enhanced” risk (level 3/5) of severe weather.

Let’s walk through what to expect.

When will it rain?
We are already seeing a few showers across the area, with some localized downpours east of I-45 and lighter showers or light rain and drizzle elsewhere. Look for the coverage of showers to increase through the morning and continue off and on through at least mid-afternoon. It will not rain all day, and it may not even rain everywhere. But that chance is going to hang with us all day today, so it’s best to have an umbrella at the ready wherever you go. A second round of showers or storms is possible this evening, but confidence in what that exactly looks like is low. Rain chances will drop off and eventually end overnight after a cold front pushes through toward morning.

When is the best chance for severe weather?
There will be two chances at severe weather today. The first will be with a warm front lifting north across the area from late morning into early afternoon. That could produce individual thunderstorms capable of producing large hail or an isolated tornado. The relative highest odds of that occurring would be north of I-10 and east of I-45, particularly as you get closer to that “Enhanced risk” area northeast of Houston. The second opportunity will likely come with the “dryline” sweeping east late this afternoon or evening. Severe weather may occur in Central or North Texas as individual cells develop, eventually probably congealing into more a squall line and general strong, damaging wind threat as it sweeps east.

There is still a bit of uncertainty regarding exactly how things will play out today, but if it looks more serious closer to the Houston area we will keep you posted.

Will there be tornadoes? Hail?
As is often the case, the odds of any one location seeing significant severe weather today is quite low. But that said, hail and damaging winds will be the primary threat from today’s storms. But, yes, there is at least a chance for an isolated tornado, particularly with the late morning and early afternoon storms that develop mainly north and east of Houston. The best hail odds today will be with the storms developing in Central Texas, especially initially. As those approach the Houston area this evening, I would expect the hail threat to diminish and transition more to a damaging wind one.

When will it end?
There will probably be a lull late this evening or early in the overnight. A brief shower or storm can’t be entirely ruled out late tonight or very early Saturday morning as the cold front approaches. But for the most part, the rain chances and severe weather risks will end after this evening.

How much rain should we expect today?
It’s likely that rain totals are wildly variable across the region today. It’s possible some places see merely a few hundredths or a couple tenths of an inch of rain, while others could see 2 to 3 inches or more. The current NWS forecast is below.

The more sustained storms north and east of Houston will likely allow for heavier rain, so those areas have the highest odds of seeing 1 inch or more. Areas south of I-10, where storms may struggle to get going this morning stand the best odds of seeing a quarter-inch or less. Some of our higher resolution models show some serious rainfall gaps south of Houston, with some places seeing next to no rain and others seeing almost an inch. So it’s going to be a roll of the dice today.

Weekend

For those able to enjoy weekends, you’re in for another winner. Saturday will see any lingering clouds in the morning quickly give way to sunshine. Saturday will start off quite warm and very humid. The front should sweep through mid-morning, and that will allow the air to dry out a bit. Drier air tends to heat up faster, so we should easily reach the mid-80s Saturday afternoon. I wouldn’t be entirely shocked to see some upper-80s out there either. At least the humidity will be dropping through the day.

Drier air tends to cool off faster too, so expect temperatures to drop quickly under clear skies Saturday evening into the 60s and eventually the 50s in spots.

Sunday looks warm as well, with highs in the lower to middle 80s and plentiful sunshine again. Humidity will remain low.

Early next week

Look for onshore flow to kick back in pretty effectively on Sunday night and Monday. You’re probably going to notice the wind kick up as well later Monday into Tuesday. Winds will probably gust to 30 mph at times, especially near the coast. We should see more clouds, more humidity, and maybe some light rain, drizzle, or a few showers. Nothing really substantial is expected through Tuesday night, however.

Later next week

Our next front is coming next week — sometime. Right now, the GFS model is bringing the front through on Wednesday night, about 12 to 24 hours faster than the European model. So we’ll split the difference and pencil in a front for later Wednesday or early Thursday right now. That should set temperatures back to comfortable levels for late April once again for a couple days. Enjoy it while we have it around!