Monday, March 17, 2008

Cold Cutoff in the Southwest

Yet another success for the long-range models, especially theECMWF, which correctly forecast the development of the cold, midlevel low currently over the Southwest - beginning from about 5 to 6 days out.

The system produced more precipitation than I was expecting, given the very low precipitable water values. Here at the house, I measured 0.29" through 7 am this morning and light showers remain in the area. Low this morning was 33F.

Precipitation yesterday was from convective showers in the cold, unstable air. I did not hear any thunder here at the house. During midafternoon northern and western parts of the Tucson metro area had heavy showers of graupel (AKA snow pellets) and the ground was covered in some spots.

Perhaps some more of this today, before the system moves on east. It's hard to know how unstable it is this morning, since the somewhat critical Tucson upper-air sounding was missing both last evening and again this morning.

The NWS forecast this morning (see below) mentions the possibility of showers, thunder, and sleet. While showers accompanied by graupel may occur, the liklihood of sleet is extremely small, today or with any winter weather event in the Tucson metro area, since sleet results when raindrops fall through a layer of subfreezing air at or very near the surface. Definitions of graupel and sleet are also below.
Today: A chance of rain showers possibly mixed with sleet, mainly after 11am. Some thunder is also possible. Partly sunny, with a high near 53. South southeast wind 9 to 14 mph becoming west northwest. Chance of precipitation is 30%. Little or no snow accumulation expected.

From the Glossary of Meteorology:

snow pellets (Also called soft hail, graupel, tapioca snow.) Precipitation consisting of white, opaque, approximately round (sometimes conical) ice particles having a snowlike structure, and about 2-5 mm in diameter. Snow pellets are crisp and easily crushed, differing in this respect from snow grains. They rebound when they fall on a hard surface and often break up. In most cases, snow pellets fall in shower form...

sleet or grains of ice, generally transparent, globular, solid grains of ice that have formed from the freezing of raindrops or the refreezing of largely melted snowflakes when falling through a below-freezing layer of air near the earth's surface....

Wednesday, March 12, 2008


Finally looks like we may have some clouds and weather here in SE AZ during the weekend.

The ECMWF and the GFS ensemble members are all pretty much in agreement now, indicating a strong wave digging down the west coast, with the eventual cutoff affecting us from about 96 to 144 hours - from 00 UTC model runs last evening.

There are about 5 waves strung out across the northern Pacific and it's not clear which one kicks up the ridge into AK, forcing the wave down the coast. Obviously all the models have keyed on one of these to be the agent of change.

The models appear to predict about a quarter to half an inch of precip at lower elevations by late Monday afternoon. Uncertainties this far out - how long will the wave affect SE AZ - some model runs keep it more isolated from the main flow to the north; how much moisture will the system pickup (the ECMWF hints at better moisture flowing northward toward the end of the event, while - surprise - the GFS members stay drier). Only 10 of 12 GFS ensemble members predict measurable rainfall at lower elevations in SE AZ during the event.

Current fcst POPs for both TUS metro and also top of Catalinas max out at 30% for Sunday night. PHX has issued a hazardous weather outlook for this event already (see the nice early am FD from PHX). Will be interesting to watch how this event evolves.

Different topic - My total precip for Dec/Jan/Feb here at house was 5.84 inches -not bad for a la nina winter! Actually not bad for any of past ten winters - beaten only (I think) by the 97-98 el nino winter.