Matt Lanza

One-sentence summary

The forecast for Hilary is basically unchanged with a severe to historic rainstorm coming for the California deserts and parts of Nevada, while the odds of lower-end Gulf of Mexico development have nudged up a little and some welcome rain heads for South Texas.

Hurricane Hilary: Major flooding potential still likely in the California deserts

We’ll start with Hilary today. Hilary is beginning to feel the influence of cooler water and begin the transition from major hurricane to moderate hurricane and major rainstorm.

Hilary’s forecast track remains basically in line with the forecast since yesterday, coming north into SoCal as a tropical storm, a status which it may maintain all the way into Nevada. There are currently tropical storm warnings across most of Southern California, a first for them in modern history. Rain will actually start today as Hilary interacts with an abnormally strong trough off the California coast. This is an event we call a PRE, or predecessor rain event. I don’t want to oversaturate you with science, but you can read about what they are here. Suffice to say, this is not a common feature in California, Arizona, and Nevada.

These types of rains can just help juice things up for the main event (tomorrow) from Hilary itself. Let’s talk things out by region to give you a sense of the impacts.

LA, Orange County, and San Diego

From what I’ve seen on social media, some people in Los Angeles think a hurricane is coming for them. While there will be impacts from Hilary in LA, the OC, and San Diego, the impacts will be worse inland in the deserts. Still, preparations for the summer equivalent of a big winter storm should probably be made, particularly along the immediate coast.

Wind gusts won’t be incredible, but they will probably push on tropical storm intensity along the coast and in the Channel Islands. Terrain in these areas means that wind will vary. With the storm going east of the mountains, winds will basically mimic a Santa Ana wind event, at least in terms of direction. So basically, areas that see the strongest winds in Santa Ana events in LA and San Diego will probably see the strongest winds with Hilary.

Rain totals in LA and San Diego will be 1 to 3 inches, enough to certainly cause some flooding, but the most serious impacts would be very limited in these areas I think to the immediate coastline and any burn scar areas, where heavy rain could lead to debris flows or mudslides. For the vast majority of metro SoCal, this will be a curiosity and inconvenient event, not a catastrophic one.

This also includes places like Riverside and San Bernardino, metro Inland Empire cities that are generally west of the north-south mountains.

Mojave Desert, Palm Springs, Imperial Valley, Death Valley

This is the area that those of us that are meteorologists are most concerned with in terms of extreme to catastrophic flooding impacts.

This is basically San Diego County east of the mountains, Imperial County, eastern Riverside and San Bernardino Counties, and Inyo County in California. Rain totals will equal 1 to 2 years of rainfall over 1 to 2 days for some of these areas. For those of you scoffing at 5 inches of rain leading to catastrophic flooding, remember that the deserts and areas with terrain are nowhere near as equipped as many places in the East to handle this type of rain and these type of rain rates. Land composition is much different in the West than the Central and East, and that means thresholds for severe outcomes are different.

Anyway, this is the region we are most concerned with. Travel between the coastal metros and places like Death Valley, Vegas, Kingman, Phoenix, and Yuma will almost certainly be disrupted for a time due to flooding and/or infrastructure damage. Damage in some parts of Joshua Tree National Park and Death Valley could be severe and extensive. As someone who has spent time in both parks, this saddens me a bit. Hopefully it’s not as bad as it could be, but these forecast totals are definitely troublesome. Please heed the warnings of your local National Weather Service offices (most likely San Diego, Phoenix, or Las Vegas) as this event unfolds.

Las Vegas and southern Nevada and Utah

Depending on exactly how the storm unfolds, impacts across Nevada may range from modest to severe. A high risk for flooding on Sunday is posted into Death Valley, but it does not include Las Vegas or southern Nevada at this time.

A moderate risk (level 3 of 4) is posted for most of Nevada tomorrow for excessive rainfall. We expect some degree of flooding here, particular in isolated spots and in favored mountain locations. Total rainfall in Las Vegas is expected to be on the order of 1 to 3 inches, an amount that will surely cause some flooding issues, but hopefully not the severe issues we’ll see in the California deserts.

Even up into southwest Utah, rain totals of an inch or two are possible for places like St. George and even not far from Zion National Park. Flood watches are posted for all of Nevada except Reno/Tahoe and north. Much of Utah is also under a flood watch.

Flood watches extend north of Nevada into southwest Idaho and southeast Oregon as well, where remnant rain will produce localized flooding.

Arizona

Heavy rain and pockets of flash flooding are likely in western Arizona as well from Hilary.

Flood watches are posted for all of western Arizona, including Prescott, Yuma, and Kingman. Rain totals of 1 to 3 inches are likely here with lesser amounts in eastern Arizona. Flooding is not currently expected to be an issue in Phoenix.

The bottom line: Hilary will bring varying impacts across the southwest, but the most concerning ones will occur in the deserts of California where we are very concerned about flooding. Please be ready for a rough Sunday and/or Monday in those areas.

Gulf of Mexico development chances inching up

The National Hurricane Center has upgraded the potential for the Gulf system to attain tropical depression or tropical storm status early next week to 50 percent.

Looking on satellite this morning, the disturbance aiming to come into the Gulf is showing some spicy thunderstorm activity, but it lacks any real organization still at this point.

This currently is just a few blobs of heavier thunderstorms moving into the Gulf. As this comes west, it will probably encounter a little bit of a less hostile environment and may begin to slowly organize. But it’s going to be hauling west and will have less than 48 hours over the Gulf meaning even if does organize, it will likely come ashore in South Texas as depression or low-end tropical storm, at worst.

The main impact from this, besides some modest marine impacts, will be rainfall, much needed rainfall at that. However not all of Texas gets to play.

In fact, much of the Houston area may see little to no rain from this system. Perhaps a bit south of I-10 or near the coast. Corpus Christi? Brownsville? Laredo? Perhaps 1 to 3 inches of extremely beneficial rain. That will extend into Mexico, which also could use some rainfall. But drought conditions that have taken hold in much of East Texas are going to remain or worsen. As we head into early September, the persistent ridge that has been bringing day after day after day of high heat to Texas may begin to weaken some, but it does not want to let go just yet. Sadly, there is no well-defined “next best” chance of rain for drought-plagued parts of Texas after this.

Elsewhere: Multiple Atlantic waves

A quick look at the NHC fruit basket shows three Atlantic disturbances, from left to right Invests 90L, 99L, and 98L.

Of the three, Invest 98L still has the best chance of forming into something, though it will remain out over the open Atlantic. Invest 99L is a nothingburger. Invest 90L may draw in most people’s eyeballs. We’ll keep an eye on this one, but at this point development looks sluggish and lower-end. Just wanted to make sure you knew we weren’t ignoring the rest of the Atlantic!

One-sentence summary

The tropics are busy, with Hurricane Hilary likely to become a severe rain and flooding threat in the low deserts of California, while the Gulf will watch a tropical wave progress toward South Texas early next week.

Happening now: All the things

I’ve always found that it’s helpful to orient myself with a map, so let’s get oriented on what’s happening by looking at a satellite image of the East Pacific and Atlantic Oceans.

What we’ll do today is work mostly left to right from that map above and discuss Hilary, the Gulf disturbance, Invest 98L, Invest 99L, and the “wild card” area below.

Hurricane Hilary: An exceptionally rare flooding threat for the SoCal deserts

I’m going to keep this section brief this morning. We discussed Hilary’s forecast in depth in our special post yesterday evening. We will have another one for you later this afternoon. Hilary has absolutely exploded into a category 4 hurricane with 145 mph maximum sustained winds.

The general idea on Hilary is unchanged today, likely to come north into or just off the coast of central and northern Baja and eventually into California as a remnant tropical storm or depression. This leads to all sorts of potential issues in SoCal, but the primary one? Flooding.

The Weather Prediction Center already has a high risk of excessive rainfall posted for Sunday for Palm Springs and the Coachella Valley. This is the first time on record that they’ve issued a high risk in the low deserts east of the mountains in California. High risks from the WPC are very well correlated to bad outcomes. Over 80 percent of all flooding-related damage has occurred during high risk days and near 40 percent of all flood-related deaths have occurred on high risk days. In other words: High risks mean business.

Rain totals will be exceptional for some of these areas on the east side of the SoCal mountains. This will cause significant flooding and potential damage, in addition to mudslides in some mountain areas. We’ll talk more about this later today, but as I said last night: Eric and I are quite familiar with flooding here in Houston. We encourage anyone in Hilary’s path, particularly in the deserts of California and Nevada to take this thing very, very seriously.

Gulf Wave: South Texas rains & potential development

The forecast for the upcoming Gulf system has not changed a whole heck of a lot unless you take the operational European model as gospel. Again, this thing will be hauling west. Look for it to emerge off Florida’s west coast on Sunday and make land in Texas on Tuesday. This limits how much time it will spend over the Gulf, and it’s likely why the NHC is still only giving it about a 30 percent chance of developing.

The big story since yesterday is that the operational European model has shifted to get more in line with the GFS and the Euro ensemble mean, which aims the bulk of the precipitation toward South Texas.

The good news? South Texas needs rain. The bad news? So does East Texas and Houston. So it appears this will help ease things a bit for Corpus and perhaps the Valley, with isolated 2 to 4 inch totals, depending on how well organized it is, exact track, etc. Eric has more over at our Houston-focused site, Space City Weather.

In terms of tropical development, this continues to look as if the speed will hinder it a bit. That being said, some modeling has shifted a bit toward this thing trying to make a last minute effort to organize. And it may be that this is what occurs. It blobs its way across the Gulf, and then in the 18 hours prior to arriving in Texas it makes a go at depression or tropical storm status. Interests along the Texas coast should continue to monitor the progress of this, just to be safe. We’ll keep you posted throughout the weekend on any forecast developments.

Invests 99L and 98L: Most likely for the fish

Both Invest 99L and Invest 98L are expected to probably remain out at sea over the next 4 or 5 days, with slow development possible from either or both area. The NHC has given 99L a 40 percent chance to develop and 98L a 70 percent chance. Invest 98L has the better chance to get a name, but Invest 99L is probably the more important of the two to watch, as it has a chance to at least graze the northeast Caribbean islands as it comes west. So should it organize, impacts would be possible in, say, the Virgin Islands or Puerto Rico. That is not the likely case however.

So interests in Puerto Rico and Virgin Islands should monitor 99L, but this doesn’t seem as if it will be a serious threat.

The medium range (days 6 to 10): The wild card wave

During the 6 to 10 day period, we’ll take Hilary and the Gulf wave off the board. Invest 98L and 99L, whatever they become should both be heading out to sea. That leaves us with the “wild card” area. Why am I calling it that? Because it has low odds of developing, but it is growing some model support. What is expected to happen is a piece of the monsoon trough, which Invest 99L is attempting to break from now may break off and sneak into the southern Caribbean. For the Windward Islands, this probably will just bring an increased chance of showers this weekend or early next week.

There are a couple options here. If it starts to organize, it will begin to feel the tug of a deep trough over the western Atlantic and get yanked north across the Greater Antilles and into the open waters, maybe grazing Bermuda on the way out. If it struggles, it may just sort of percolate in the southern Caribbean and/or dissipate.

Either way, I wouldn’t expect serious impacts from this system, should it develop. For now. However, given the time of year, warm water, etc., you should continue to monitor the progress of this through next week.

Fantasyland (beyond day 10): The western Caribbean may gyrate

We’ll keep this brief a.) because this post is long enough and b.) we really can’t say with any confidence what will happen here. But, it continues to look as if a Central American gyre may develop and that will be worth watching. There is some chance for Gulf or Caribbean development in that pattern, but any details or specifics are impossible to pinpoint right now. But the final days of August will probably not be exceptionally quiet ones.

We’ll be with you all weekend, so if anything changes, you’ll know.