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Posted at 9:30 AM ET, 10/29/2010

Freeze watch for north and west suburbs tonight

By Jason Samenow

* Autumn blows back into town: Full Forecast | Weather sorcery *

Freeze dates for the last 10 years at Reagan National (DCA) and Dulles (IAD) airports. Source: National Weather Service Presto

The National Weather Service has issued a Freeze Watch for a large portion of the north and west suburbs tonight into tomorrow morning. Areas (map) under the watch include: Frederick, Montgomery, and Howard counties in Maryland, and Loudoun, Prince William, Fauquier and Fairfax counties in Virginia.

Lows tonight will dip into the low 30s in these areas. A freeze watch means sub-freezing temperature are possible that could kill crops and other sensitive vegetation.

For areas not under the freeze watch (such as areas inside the beltway and the District), low temperatures will drop into the mid-to-upper 30s resulting in scattered frost.

The first freeze in the north and west suburbs is coming about a week later than the average over the last 10 years. The average date of the first freeze at Dulles Airport - a reasonable representative of these areas - has been around October 20.

By Jason Samenow  | October 29, 2010; 9:30 AM ET
Categories:  Alerts, Extreme Cold  
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Posted by: rosilandjordan | October 29, 2010 10:23 AM | Report abuse

Good timing for this post, I've been wondering about these stats.

You mentioned the average first freeze at Dulles - what is it at DCA? Isn't it somewhere around November 15? Also, fast-forwarding 4-5 months to spring, when is the average date of the last freeze at both DCA and IAD?

Posted by: meteorolinguist | October 29, 2010 10:55 AM | Report abuse

FWIW to gardeners, I think a lot of deciduous shrubs/trees are going to be at risk this winter, due to the extended dry spell followed by rain and late warmth. I noticed yesterday that my serviceberries (Amelanchier, for those who care) are putting out new growth, even as the old foliage is turning orange. No bloom next spring, and significant dieback, would be my guess for this and other early bloomers.

Posted by: fsd50 | October 29, 2010 11:28 AM | Report abuse

I guess I better grab many leaves from my three basil plants (that I nursed since April!!!) as I can to make pesto before they perish. :-(

Posted by: SanDieganLostinDC | October 29, 2010 11:37 AM | Report abuse

Is this really a freeze???

You need temperatures BELOW 32 for that...I've been hearing 39. Some frost but nothing near a "cricket killer" is what I'm seeing.

Posted by: Bombo47jea | October 29, 2010 1:01 PM | Report abuse

What does it take to create a killing frost, i.e., a frost slays the late-blooming raspberries, tomatoes, etc? Temps near the ground at or below 32 F, enough moisture in the atmosphere, calm night? Anything else?

Or if conditions are right, can you have a killing frost when the temp near the ground is a couple of degrees above 32?

Posted by: JerryFloyd1 | October 29, 2010 1:31 PM | Report abuse

@meteorolinguist and @JerryFloyd1

Didn't want you guys to think I was ignoring you... I'd have to research some of this some more... I'll try to do a more in-depth post on frost/freeze dates next week and will consult some gardening contacts on plant/crop sensitivity.

Posted by: Jason-CapitalWeatherGang | October 29, 2010 3:23 PM | Report abuse

Jason, thanks. I grew up in Florida where frost and freezing temperatures were always a concern. But I've seen some very dramatic overnight fall killing frosts hereabouts that can wipe out entire gardens in a few hours, save for hardier plants. (Collards and fall/winter root vegetables can LOVE heavy frost, even snow.)

Posted by: JerryFloyd1 | October 29, 2010 4:10 PM | Report abuse

thanks, Jason. Look forward to a later post on this.

Posted by: meteorolinguist | October 29, 2010 4:50 PM | Report abuse

@jerryfloyd1, the temp at ground level is not the issue, unless you're talking about total loss of a tender plant. The ground is probably still holding some heat at this point, but a nice calm cold night can freeze tomatoes at 4 feet up with no damage to plants that hug the ground.

Moisture in the air should mitigate the effect on plants by condensing on the plants and then giving up heat as it actually freezes, just as growers spray citrus groves to protect them during a freeze.

SanDiegan, you might be able to save your basils (if you're in DC proper they may be OK tonight anyway) by just setting up some sort of frame (lawn chair on its side??) so you can drape an old sheet or curtain over your basils without breaking them. If they do get frosted, you can probably still make pesto if you catch the plants before they thaw fully.

Posted by: fsd50 | October 29, 2010 7:46 PM | Report abuse

thanks, FSD; now that I think about it, I have seen the tops of tall tomato plans nipped by frost while closer to the ground the plant was undamaged.

A general heavy frost, though, and it's sayonara.

Posted by: JerryFloyd1 | October 30, 2010 9:48 AM | Report abuse

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