Massive changes are underway for the Southern Plains and Southwestern US. An uncanny weather pattern will soon bring weather conditions typical of December in New York City to the region,  spreading winter-weather as far south as the US – Mexico border! The culprits behind this bizarre weather are a series of three potent shortwave troughs spanning from Canada to Mexico. Each trough will help arctic air penetrate deeper into the continental subtropics. Threats from this winter blast include hard freeze, snow, sleet, freezing rain, and flash flooding.

Three troughs are visible in the RAP upper air analysis that began early Sunday afternoon. One was centered over southern Manitoba, another over the Great Basin, and another over Arizona and the southern California coast. The shortwave over Manitoba is the strongest of the three and contains the core of the arctic air, but the jet stream is strongest rounding the shortwave over the Great Basin. The cold front associated with its surface low will slump southward across almost the entire continental US, dragging cold air with it. This cold front, in conjunction with the two northern shortwaves, has geneated an early season snowfall for the Plains and Rocky Mountains, including Des Moines, Lincoln, and even Denver.

The cold front will reinforce seasonably cool air in the East, but it will prompt a remarkable 40°F – 50°F degree temperature change in the deserts and plains of the southwestern US. Temperatures ahead of the front in Texas and southeastern New Mexico were in the 70s through middle 80s Sunday afternoon. Behind it, they will plummet into the lower through upper 30s. This transition from summer to winter will be far from gradual.

A crash and a bang will quite literally be the result as the cold front sweeps southeastward through the plains into Texas and New Mexico. Frontal forcing and strong positive vorticity advection ahead of the Great Basin shortwave will work in tandem to eradicate a capping inversion and release the instability brewing underneath. Dew points in the low 70s with surface temperatures in the low 80s and modest mid-level lapse rates will support ample instability which will be released with the approach of the cold front Sunday evening. Damaging wind gusts will be the primary threat, but large hail and perhaps a tornado or two will be possible with some of the supercells that inevitably develop late in the afternoon in central Texas before evolving into a linear system. Dallas, Arlington, Austin, and San Antonio are just some of the cities in the line of fire on Sunday night’s storms.

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After the last few flashes this evening, the dramatic seasonal change will commence. Cold-core high pressure centered over the Rocky Mountains will immediately funnel frigid air into the Southwest behind the cold front. Temperatures will tumble into the 40s in northeastern Texas, upper 30s in central and southwestern Texas and eastern New Mexico, and 20s to low 30s in the Texas Panhandle, Rocky Mountains and Great Plains. Where temperatures dip into the 20s in the Southern Plains, the growing season will immediately end as a hard freeze will be unavoidable.

Temperatures will make little recovery Monday. The cold front will stall at the mid-levels Monday afternoon, keeping the Southern Plains through central Texas in the cold pool. Temperatures are only expected to reach the middle 40s in northern and central Texas. According to the Dallas National Weather Service forecasting office’s climate page , the average high temperature for October 15 is 79°F, more than 30° warmer than Monday’s max temperatures. The records from that same source show that Monday’s forecast high temperatures are as cold as Dallas’ January minimum temperatures! It therefore comes as no surprise that the Dallas forecasting office anticipates that this incredible cold spell  may set both a daily record for lowest maximum temperature and annual record for the earliest date for which temperatures fail to rise to 50°F.

Temperature departures from normal will be even more extreme further south and west. The ECMWF is forecasting 850 mb temperature anomalies more than 20°C below normal in southwestern Texas, at the very far end of climate extremes.  At the surface, this translates to temperatures 20 – 30 F below normal. Temperatures will fail to climb out of the 50s in lowland cities like El Paso and Las Cruces Monday. Like Dallas, the average high in El Paso is 79°F according to that office’s climate page. Their record minimum temperature for October 15th is 61 F.

As if it weren’t enough that the region will be under the grip of record cold, heavy precipitation of both liquid and frozen states will also be around. Frontogenetic forcing will generate showers overnight Sunday through Tuesday. Isentropic lift from the Gulf of Mexico will result in warmer, moist air gliding over this colder surface, resulting in heavy downpours in northern and central Texas. Meanwhile, the trough over the Great Basin will merge with that over Arizona, forced southward due to the encroaching ridge in the Pacific Northwest. Eventually the ridge will force the combined low to cut off from the jet stream and stall. Perpetual rain chances will be the result from Arizona to central Texas.

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A cutoff low is the last thing the Southwest needs. Many cities have already recorded their wettest month ever, just two weeks into October. One example is Phoenix, AZ which has already recorded 5.32″ of rain this month, making it their fourth wettest month on record, per the city’s National Weather Service forecast office. This represents more than half a year’s worth of rain falling in just two weeks. The cutoff low  will keep downpour chances until Tuesday, by which point the cut-off low will have broadened too far to the east, where rain chances will last well beyond Tuesday.

Global models like the GFS, ECMWF and their respective ensembles depict the blocking pattern to persist through at least Saturday, keeping chances for cold rain in place for New Mexico and Texas throughout that entire period. Deterministic models are in relatively strong agreement that up to 6″ of rain may fall here this week, mainly near the stalled mid-level frontal boundary in northern and central Texas. Dallas, Fort Worth, San Angelo, and Del Rio will likely fall within this bulls eye of seemingly endless rainfall. Further south and east, temperatures will be slightly warmer, but precipitation will still be heavy. Up to 3″ of rain is likely to fall in San Antonio, Austin, and Houston.

Rain won’t be the only form of precipitation possible this week. Snow and sleet will mix with rain Sunday night into Monday in Kansas, the Texas and Oklahoma Panhandles, and northern New Mexico. Snow may accumulate up to two inches in the Oklahoma Panhandle, Santa Fe, and Albuquerque, with totals up to 6″ in nearby higher elevations. Further south and east, warm upgliding will make freezing rain the more likely mode of precipitation. Sub-freezing air will channel all the way into Northern Mexico, but flow aloft from the Gulf of Mexico will create a low-level inversion. The inversion will reach the surface in central Texas northward through the Oklahoma panhandle, where a glaze of ice is likely to make for slick surfaces by Monday morning.

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Further south towards the US – Mexico border, the inversion will not reach the surface. It will instead develop near the peak altitude of the Chisos Mountains along southwestern Texas’s Big Bend. Along these mountains through the higher elevations of New Mexico and northern Mexico is where ice will be most likely to accumulate, including the mountains near El Paso and Las Cruces. A quarter to a half-inch of ice may accumulate in these higher elevations by Tuesday night.

The cutoff low responsible for the cold, unsettled conditions in the Southwest is expected to retrograde southwestward next weekend ahead of another ridge in the Pacific Northwest. A well-defined Rex Block may setup over the Western US, which would give central Texas a break from the rain but return chances elsewhere. Where the plague of cool, cloudy, and wet weather migrates towards is still uncertain, but as of this writing areas near the southern US border from southwestern Texas to southern Arizona are favored. The relief from rain for central and northern Texas will come at a cost, however.

Rex Block and strong upper-level trough at 500 mb as forecast by the 12 UTC Sunday Oct 14.  Graphics credits to Pivotal Weather.

Global models suggest that a deep trough containing a similarly cold air mass will be the cutoff low’s replacement next weekend. This airmass could be the coldest of the season so far, bringing a crop-killing freeze as far south and east as northern Georgia. Such a trough could prompt the development of a strong coastal storm along the East Coast, perhaps becoming the first Nor’Easter of the cold season.

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Author

Josh is a lifelong nature and weather enthusiast as well as the Head Meteorologist at WeatherOptics. He began regularly forecasting for New Jersey, Long Island and New York City in 2014 on social media, contributing to community pages such as SBU Weather. He holds degrees in Physics and in Atmospheric and Oceanic Sciences from Stony Brook University, from which he graduated in 2018. In the Fall of 2018 Josh will start graduate school for his M.S. in Marine and Atmospheric Sciences at Stony Brook, continuing his research on approaches to non-convective wind gust forecasting.

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