When I saw this radar signature, I knew it was bad. I had no idea though. Absolutely awful.
Radar
Photo Of Destruction From Texas Tornadoes
Big tornadoes rolled through north Texas yesterday. I read some were two miles wide. This photo from CBS news is pretty striking.
Winter May Not Be Done Yet In The PacNW
Winter may have one parting shot for us in the PacNW next week. The models are showing a big, deep, cold low over the PacNW around the middle of next week.
Here are a few forecast graphics from the American GFS model that show the ugly truth quite well:
GFS 500 mb heights and wind, valid 22May2013
Big trough right over us. Period.
GFS 500 mb mean sea level pressure (black lines) and 36 hour precipitation accumulation (image), valid 22May2013
Precipitation, too.
GFS 1000-500 mb thicknesses (color lines), mslp (black lines) and 6-hour precipitation accumulation, valid 22May2013
And cold! Those blue lines are not great news since they show where the really cold air is. A 528 decameter thickness is cold any time, but for May? Sheesh! For reference, we’d be thinking about snow at sea-level with thicknesses of just 60 meters lower (522 dm). If this is exactly right, snow levels will be quite low – pass level at least, maybe lower.
If you’ve been following this blog for long, you know I wouldn’t post this if it was some wild, one-off solution. All the models show this and have been showing something like this for several days. Model spread is also very low for being this far out and this anomalous. In other words, it’s probably going to happen.
It’s always good to remember that this too will pass. 
First Tropical Storm of 2013 Forms In The Pacific – Meet Alvin!
Tropical Storm Alvin is developing off (way off, that is) the Pacific coast of Mexico today. We got a nice pass from one of the polar orbiters today and MODIS captured this visible image:
MODIS Visible Satellite Image of Tropical Storm Alvin, valid 15May2013/21Z
The National Hurricane Center’s forecast shows it becoming a hurricane by the end of the week (below). It will be little more than a fish storm (ie not affecting land), but it’s in our hemisphere. And thus it begins!
NHC Forecast for Tropical Storm Alvin – valid 15May2013
Why We Don’t Take Extreme Long-range Forecasts Too Seriously
Yesterday on Facebook I saw a post about a model forecast of an extreme and unusual storm over the eastern half of the country next weekend. While none of what was said in that post was technically incorrect, this is a great example of why we don’t take those things too seriously.
First, a snapshot of what the model showed yesterday:
GFS 500 mb heights and vorticity, valid 05May2013/12Z (model run was 4/26/13 at 12Z)
I love an extreme weather event as much as the next guy, and the storm shown above would probably wreak all sorts of havoc, including severe weather, heavy rain, and even snow much further south than usual. And I am inclined to drool over this solution too. But anybody who has tried to forecast weather for a while knows that the behavior of big closed lows like this are hard to predict tomorrow, much less more than a week from now.
Not surprisingly, the models are a little different today. Here is the same model forecast for the same time (next Sunday morning) produced 24 hours later:
GFS 500 mb heights and vorticity, valid 05May2013/12Z (model run was 4/27/13 at 12Z)
That may not seem much different to the average person, but to the person trying to forecast the weather in, say, Kentucky (directly under the low in yesterday’s model run) or Missouri (directly under the low in today’s model run), this is an enormous difference.
We have some tools to help us decide what is most likely to occur. I’ve talked about ensemble forecasting* before, and this is something we can turn to in situations like this. First, the mean (or average of all the forecasts) and spread (average difference between the forecasts):
GEFS Mean 500 mb heights (black lines) and spread (image), valid 05May2013/12Z
So the mean (or average) position of the big low is actually over eastern Iowa – yet another possible solution. Furthermore, the spread (average of the model differences) is large, but this is not really surprising for such an unusual solution at more than a week away. In fact, it’s not that bad, all things considered.
The product that is most enlightening in this situation is the so-called “spaghetti plot”:
GEFS spaghetti plot of 540 dm and 582 dm heights. Xs placed at approximate low centers, valid 05May2013/12Z
This graphic shows the individual members of the ensemble run – in this case, the 540 dm height lines are the ones we’re interested in. I placed Xs at the approximate center of the low in each individual member of the run. It is pretty obvious that there is a large amount of disagreement among the members. The potential low centers stretch from Louisiana north to Canada and east to West Virginia. In many places, this could mean a difference of 50 degrees in temperature or the difference between snow and a warm rain.
The takeaway from all this is that while there is general model agreement that there will be some kind of trough or storm in the east next weekend, there is very little agreement as to where it will be or how strong. While us weather geeks like to marvel at extreme model solutions in the long range, we rarely take them very seriously. And now you can see why.
Over the next few days, perhaps the models will zero in on a solution. When there is reason to believe one of them, we will be all over it. Until then, it IS fun to look at the wild solutions and imagine the possibilities!
*Remember that ensemble forecasting means to re-run the same model multiple times with slightly different configurations. The degree to which the individual runs agree determines the confidence you can have in the solution.
Severe Clear In The Pacific Northwest
It won’t be news to anyone who lives in the Pacific Northwest, but it was quite a nice day over the region on Wednesday. It is pretty rare for it to be so clear that you can see the shapes of the landmasses without the state boundaries overlaid on the image. But that’s exactly how it looked on Wednesday:
VIIRS visible satellite valid 24Apr2013
This image covers almost a half million square miles over the Pacific Northwest, and there is hardly a wisp of cloud on it – even over the ocean. This is truly unusual for April.
Warm-up Over PacNW Next Weekend
After a not-so-nice spat of weather over the PacNW this weekend, things are looking up for next weekend. Model consistency is pretty high in showing a nice ridge over the region by next weekend. As a result, temperatures will warm up.
Right now it looks as if Sunday will be the warmest day of next weekend with widespread 70-degree temperatures. But Monday could be the warmest day of the whole stretch with 70s perhaps all the way up to Seattle and even a few 80s around the further south you go. This depends on the exact position of the upper ridge and the flow at low levels, but the point is that we will probably see a warm day or two sometime between next Thursday and next Monday. Hopefully, it lands on the weekend, because I actually have that one off!
Here are a couple of forecast graphics for the visually inclined (and because pictures always sell better!):
Forecast for Sunday afternoon:
GFS forecast of 500 mb heights (green lines) and surface temperature (image), valid Sunday afternoon, 21Apr2013
Forecast for Monday afternoon:
GFS forecast of 500 mb heights (green lines) and surface temperature (image), valid Monday afternoon, 22Apr2013
80-degree Front On Southern Plains Tuesday
You won’t find a cold front much more dramatic than the one that blew through the southern plains yesterday. At one point, there was an 80°F temperature gradient across north Texas! Here is a map of surface temperatures and weather radar yesterday (09Apr2013) afternoon:
Surface temperatures in °F and radar base reflectivity, valid Tuesday afternoon, 09Apr2013
Temperatures ranged from 100°F around Abilene, TX to around 20°F over the far northern panhandle.
It is also interesting to note that while this front was generally dry (ie produced no rain), portions of it were visible on radar. One such section is denoted by the red arrow. This is because dust, dirt, bugs, and other crap are concentrated along the frontal boundary and show up as returns on radar.
The visible satellite from roughly the same time was also very pretty:
Visible Satellite Image with my rough guess of frontal position (blue line), valid Tuesday afternoon, 09Apr2013
I also wanted to point out that even though the portion of the front denoted by the red arrow is precipitation- and cloud-free, you can still “see” it on visible satellite. What you see as a slight difference in shading from one side of the front to the other is probably more dust and other “stuff” being picked up by the strong north winds north of the front. That’s just my guess, so let me know if you know otherwise.
The Weather Geek Holy Grail: Tornado AND Blizzard, In Progress!
Northeast Colorado forecasters have achieved what many of us weather geeks call the holy grail of weather: a simultaneous tornado and blizzard warning!
Northeast Colorado Hazard Map as of 8pm, MDT, 08Apr2013
Here is the radar and surface map at roughly the same time:
Northeast Colorado Radar and Surface Observations, valid 9pm MDT, 08Apr2013.
This amazing surface map shows cold air plunging south along the Colorado front range as expected. It was 71 degrees in Denver this afternoon, and this evening it is already in the mid 30s and snowing (northern-most red circle). Yet, just east of the front, it it still in the mid-70s (for example, in La Junta, the southern-most red circle)!
At the same time, tornadic supercells are running along the cold front over far northeast Colorado (upper right in the image).
Can you imagine how heavy the snow is under those severe thunderstorms that ran up into the cold air northeast of Denver? Yikes! What a night it must be over northeast Colorado.
