Cold wave of 2010 wanes; major jet stream pattern change coming

The worst of the cold wave of 2010 is over for the Southern U.S. Temperatures this morning across the southern tier of states rebounded substantially from the lows observed Saturday through Monday, and the citrus growing region of Florida did not receive sustained periods of temperatures below 28°--the critical threshold for fruit damage. Only one record low for the day has been reported so far this morning by the National Weather Service, a 25°F low at Melbourne, Florida, beating the old record of 26° for the date set in 1982. For comparison, 11 low temperature records were set Monday morning in Florida. Temperatures will continue to recover throughout the week as a major re-orientation of the jet stream takes place. By next week, a significant January thaw will occur over the Eastern U.S., and a period of stormy weather and heavy precipitation will impact the Western U.S.

Key West's temperature bottomed out at 51° this morning, breaking its string of 5 consecutive days with temperatures below 50°--the second longest such stretch on record there. However, the damage is done in the citrus growing regions of Florida, where four nights of extremely cold temperatures over the past week may have killed 7.5% of this year's citrus crop.

Here's a list of the number of daily minimum low temperature records set at major airports during the cold wave of January, 2010, in the South:--Texas, Louisiana, Arkansas, Mississippi, Alabama, Georgia, South Carolina, and Florida:

January 2: 1 FL
January 3: 1 SC
January 4: 4 FL, 2 GA
January 5: 1 AR, 1 FL, 1 TX
January 6: 5 AL, 1 AR, 17 FL, 1 GA, 1 LA
January 7: 15 FL, 1 GA
January 8: 2 FL, 2 TX
January 9: 2 FL, 17 TX
January 10: 7 FL, 4 TX
January 11: 11 FL, 1 TX
January 12: 1 FL (data not all in yet)


Figure 1. Ice encases tangerines in Altoona, Florida. Image taken Sunday, January 10, 2010 by wunderphotographer CAVU.

A major jet stream pattern shift coming
As I noted in my post on Thursday, a sharp kink in the jet stream and a strong negative phase of the North Atlantic Oscillation was responsible for this winter's cold blast over eastern North America and Europe. However, the ridge of high pressure that has been blocking the west-to-east motion of weather systems over the past ten days is weakening and progressing eastwards, which will allow a jet stream pattern more typical of an El Niño winter to set up next week. The jet stream will dive southward over California, bringing a strong flow of moist, Pacific air to the West Coast. A series of powerful storms is expected to begin battering California Sunday, and these storms should bring significant drought relief--and flooding rains--to most of California and portions of Arizona. The stormy period will likely last at least a week.

A strong low pressure system will also bring heavy rain to the Gulf Coast this Friday and Saturday. It currently appears that the large amount of cold, stable air this weekend's cold wave has delivered to the Gulf will prevent the storm from generating a significant tornado outbreak. Some isolated severe thunderstorms are possible with this storm, and we may see the Storm Prediction Center issue a "Slight" risk area of severe weather for this storm, late this week.


Figure 2. Surface pressure and precipitation forecast for next Wednesday, January 20, from this morning's 00Z run of the GFS model. A strong low pressure system is expected to impact the West Coast and Desert Southwest, bringing large areas of rainfall in excess of 1/2 inch (green colors) during a 12-hour period. While the timing and areal coverage of this precipitation event will no doubt be different than this, given that 8-day forecasts are pretty unreliable, we can expect a very wet and stormy period of weather for the Western U.S. next week. In the Eastern U.S., a surface high pressure system will set up over the East Coast, pumping warm air northwards and bringing a significant January thaw to the Midwest and much of the Eastern U.S.

I'll have an update on Wednesday.

Jeff Masters