Early First Freeze
by KDLT Weather
September 13, 2011 10:00 AM
After a mostly quiet period through the first part of September, the Sioux Empire weather is starting to get a bit more rocky.In a previous weather blog we discussed why we here in South Dakota see wild swings this time of the year. This week will be another great example of extreme weather swings in a very short period of time.
(Monday's High Temperatures)Monday afternoon, temperatures soared into the 90’s for the majority of us. Monday evening however a potent cold front made its way across the state. Monday’s cold front was a dry frontal passage, but the big change came in the way of wind direction. Southwesterly winds during the afternoon funneled hot air into the area. The cold front moved the winds out of the north-northwest ushering in cooler air. Within 24 hours temperatures across South Dakota had dropped between 15 and 25 degrees!
(24 Hour Temperature Change from Monday morning till Tuesday Morning)A secondary cold front will drop out of Canada Tuesday, move south through North Dakota and into South Dakota Tuesday night and early Wednesday morning. This frontal passage looks to be dry for the majority of South Dakota, as the surface is actually quite dry, any moisture aloft will fall through drier air and evaporate. However, it is possible that some places in the extreme southern portions of South Dakota could see a few raindrops. This second cold front will allow very cold Canadian air to filter in to the area, dropping our temperatures significantly. Combine cold air temperatures with high pressure building in and you get the perfect set up for a very chilly couple of evenings.
(850 mb Temperatures, Zero to negative two degree celsius isotherms over Eastern South Dakota)Tuesday morning in advance of this cold air, Freeze Watches were issued for portions of North and Eastern South Dakota. Freeze Watches are issued 24 to 48 hours in advance of conditions which would be favorable for a hard freeze.
A hard freeze, which would mean temperatures falling below freezing for at least two or more hours can kill sensitive vegetation and end the growing season. These freezes are particularly damaging to farmers.
(Freeze Warning issued from Wednesday evening through Thursday Morning)Temperatures Wednesday night and Thursday morning are forecasted to be at or below freezing for the majority of South Dakota, with many places falling very close to the upper 20’s.
(Adonis Model Futurecast Temperatures for Thursday Morning)The average first fall freeze (32°) usually occurs towards the end of September, with the first hard freeze (around 29°) at the beginning of October. Making this freeze quite early and potentially costly for many farmers.
(Average First Freeze for South Dakota is September 21st-30th)For the amateur farmers and gardeners here are some tips to keep your sensitive plants protected.