clouds, 5/22/11

Table of Air Mass Source Region Characteristics

Courtesy of/lifted from a UCAR page to which my students may not have access.

Rain we need (but may not get)

The numerical models are at it again, predicting between 1/2 and 2 inches of rain over Minnesota. For last Tuesday's storm, the one that produced tornadoes in western MN, the models did reasonably well. Much of central Minnesota received 1-3 inches of rain, most of which merely ran off, but in any case helped at least a little bit. The recent cool conditions have probably kept some of that new moisture from escaping, which means that tonight/tomorrow's system will probably be producing more rain up there. The wet get wetter.

As of this writing, our drought is severe and expanding (click here for weekly updates):



Don't look for much change over the Twin Cities area any time soon. Any change will be nickel/dime-style, through the end of the month, and that's putting a fairly positive spin on things...we could easily make it through the rest of the month without another half-inch of rain. Let's hope not, but it certainly is possible.

Tonight and tomorrow, we have the chance for a nickel. A weak system, with accordingly poor instability, weak winds aloft, and no real fronts, is going to loll about the region. Despite the poor instability, this is actually a decent set-up for heavy rains, mostly because the weak winds will help storms sit over the same places for more time than is usual. The heavy-ish rains have already been hitting the Dakotas and now western Minnesota. More rain will fall there, and it will expand into central, northern, and southwestern Minnesota. Some of those areas will see flooding. But that's them. Those areas are not super-dry. They do not have problems with moisture convergence. We do.

The main axis of precipitation will move over eastern Minnesota later tonight and tomorrow morning. Most of the precipitation should be of the showery, thunderless variety (owing to the lack of strong instability), and thus, rainfall amounts over the Twin Cities should not be very high. But, whatever rain does fall may help moisten up the atmosphere for a bit of afternoon destabilization tomorrow (Tuesday). This could lead to redevelopment of thunderstorms fairly close to the Metro, and that could score score some areas a whole inch of badly-needed rain. Most areas though, should see less than that...maybe a quarter-inch, and that's only if round two gets going in the afternoon.

Here is 2-day precip forecast from the Hydrometeorological Prediction Center, for the period ending 7PM Wednesday:

Since the forecast begins this evening, I think for starters, the heavy amounts need to come westward some. I think the .75-1.00" amounts over the entire TC area are very optimistic, but I hope they're correct. We shall see.

5-day precip from HPC (updated!)

Updated 7/20/09: If you were wondering how the HPC did with this forecast, the six-day total (I threw in a day for them), June 22-27 at MSP, was .88"...about 29% of what they called for.

Below is the 5-day "QPF" (Quantitative Precipitation Forecast) from the Hydrometeorological Prediction Center (HPC), issued at 7 AM CDT on Monday June 22, 2009. In other words, this shows the expected total rainfall during the next five days.


These forecasts have been coming in quite a bit wet over the past week or so in the Twin Cities area. Recent heavy rains certainly have dented the regional drought, but over east-central MN and west-central WI, we've seen little improvement. Calling for 3" in five days at a time when we can barely get that amount in two months seems bold to me, but I hope it happens, mostly for selfish reasons. You can follow the changing forecast values here.

A Pretty Good Fight

I get irritated when television meteorologists dumb-down the elegant complexity of atmospheric processes and tell viewers about "clashing air masses," and "battles between warm, humid air and cold dry air." I also get irritated when they wax idiotic about climate change (see here too), but that's a topic for a day when I have given up coffee.

So what gives...me using battle metaphors?

As of this writing, the TC area and much of the surrounding region is in a drought:




It's worth noting, too, that the many of the non-drought areas have been pretty wet (though the drought area is expanding slowly):

And this weekend, our established drought is in for its first real challenge of the 2009 warm season, as a sloppy weather system attempts to squeeze out big-time rains, right over Drought Territory. That's the fight. You can go to sleep now.

If you are still awake though, the always-loved and always-trusty models have been fairly consistent, in their subtly morphing way, about advertising significant rains this weekend. As of the evening model runs (Thursday evening), here is what the GFS model, which can get carried away, is depicting:




And here is the five-day total precipitation forecast from the Hydrometeorological Prediction Center, just to show what real forecasters are thinking:



Both of these forecasts (that is, in the heads of forecasters, and also in non-heads of the computer models) put a significant dent in the drought. In order to end the drought, we would need a thorough breaking of the pattern that has been in place for several months, and it's a bit early to call for that. But, 1-2 inches of rain would put water into the ground and into the soil, and some of that water would be available for the next system, making precipitation production at least a little bit easier.

Right now though, I would encourage readers to hold off on those "Yay, the Drought is Over!" parties that you may have (or almost certainly have not) been planning. While this system is the best-yet candidate to take the drought down, a few things make me suspicious. First, the GFS is clearly suffering from some "gridscale feedback," meaning basically, that it is inventing small features that produce small pockets of intense precipitation. This might be artificially balanced by the fact that the same model will generally under-forecast precipitation associated with thunderstorms, which operate on scales too fine for it to resolve.

I am also suspicious about the placement of the precipitation. The warmest, most humid air will be to our south, in Iowa, and while the conditions do support "overrunning" precipitation, which is found on the cooler side of the warm front (where we should be), I tend to think that storms are going to build towards the higher energy content of the warmer, more humid air...even if that means just barely being on the warm side of the warm front. Thus, I am concerned that the models might be driving the heaviest rains a bit too far north.

Lastly, any intense storms that do form in or very near the warm sector, will detract from moisture streaming to the north, and thus, would reduce--perhaps dramatically--the amount of rainfall we receive.

This will be interesting to follow, at least for me. I don't know if we should consider this weekend event a fight, or just the first round of a fight. If MSP ends up with a half-inch of rain or less between Saturday and Monday night, then I think we'll know this drought means business (has a tough chin, sticking with the fight theme). A half-inch, while potentially valuable, would be a real disappointment considering what this system has going for it (namely a good feed of moisture and a hang-around-the-region, slacker-ish attitude) On the other hand, if the event evolves as advertised, then the whole area would get a good soaking, indicating that the drought can be beaten, and soon (or that it had been wobbled...again, sticking with the fight theme).


As always, we shall see.

Model Baroclinicity for Next Tuesday

Here is a simple illustration of baroclinicity. Both images from
http://www.rap.ucar.edu/weather/model/

The top image is the pressure and wind pattern from the GFS (7 AM on Weds Mar 18, valid 7 PM Tues Mar 24). Notice the center of low pressure in northwestern Minnesota, but moreover the concentric pattern of the isobars.

And above we have the temperatures from/for the same times. Notice the bulgy, almost s-shaped pattern going from Nebraska or so into Minnesota. If you overlaid the two images, you would see the isotherms and isobars intersect eachother, as in below (from an earlier model run, courtesy of College of DuPage http://www.weather.cod.edu)

Watch chances for big snow on Thurday slip away

Loop of 36-hour accumulated precipitation from four runs of the GFS model for the period ending 6 PM CST on Friday December 19. Source