Rain this weekend? and technical discussion

Posted by Bret B on February 10, 2015, 4:26 pm
187.148.141.155

We continue to have slightly odd weather for February. These heavy-looking clouds we've been having the last couple days hardly ever produce rain; they are too low-altitude and warm to even show on the IR satellite loops. Just this afternoon they have gotten heavier and colder and are starting to show: http://www.ssd.noaa.gov/goes/west/epac/flash-avn.html In the last few hours there was no rain within several hundred miles of us according to the TRMM satellite: But there is a chance of rain in the next few days. I would say maybe 50% chance of light sprinkles, and somewhat less chance of any heavier rain. Anytime from Friday through Monday? Technical discussion (weather nerds only): Weather Underground says only 20%, and no significant accumulation. The rain probability peaks on Friday: http://www.wunderground.com/q/locid:MXJO0760 Of course, their forecast changes significantly hour by hour, which tells me their predictive model is very "noisy". But the GFS computer model predicts some significant showers all around us Thursday thru Sunday: http://www.hpc.ncep.noaa.gov/mike/gfs/crb_p24i_loop_12.html . If some of these showers shift just a hundred miles closer, we could get an inch or two. The Mexican SNM weather site's 96-hour technical discussion ( http://smn.cna.gob.mx/index.php?option=com_content&view=article&id=55&Itemid=58 ) is a little hard to read, but they predict a higher chance of rain for our part of the coast. As far as I can translate, they call for up to 1-2 inches of rain in Jalisco every day from tomorrow (Wed) thru Saturday, mostly along the coast. I think this is unlikely, but I thought you should know. Here is their graphical forecasts for the next 4 days. Sorry they are so tiny; I have never found bigger versions of these 4-day forecast graphs anywhere on their site: It's too bad that their model viewer is broken except for the WRF-SMN model: http://smn.cna.gob.mx/index.php?option=com_content&view=article&id=16&Itemid=87 They do have a nice narrated one-day video forecast that's non-technical and fairly easy to follow (in Spanish). Look in the lower right corner of this page: http://smn.cna.gob.mx/index.php?option=com_content&view=featured&Itemid=128 This chance of rain is being caused by conditions similar to last week's rains. You can see some of these factors in the 4 charts above: 1. An upper-level low wandering around Arizona and northern Baja the next few days. (This is causing a winter storm for those poor people up there.) I think this low (and the upper-level trough it's embedded in) helps to divert the jet stream to the south where it picks up that notorious "Pacific moisture" from the Inter-Tropical Convergence Zone (ITCZ), before heading toward us with it. 2. A strong influx of moist Pacific air carried from the southwest by a southern branch of the jet stream (as mentioned above). 3. And I think I can see a surface trough along the Pacific coast in the 4 charts; this may help to "trigger" the moisture to fall out as rain. The very interesting "earth.nullschool.net" site predicts some of these features also. This snapshot is for Friday the 13th, and shows the upper-level winds at the typical altitude of the jet streams (250hPa/10,500m/35,000ft.) You can see one or two lows spinning counterclockwise near Baja, and the strong southerly jet stream branch: http://earth.nullschool.net/#2015/02/13/2100Z/wind/isobaric/250hPa/orthographic=-106.73,18.46,1542 I'll keep you posted on the forecast, except maybe not tomorrow.

795