Frost in the Forecast for Some Areas on Thursday Morning (2:30 p.m., Tue., April 3, 2018)
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If you had a moment to read the Weather Forecast Outlook for 4 Apr–29 Apr that was posted yesterday (Easter Monday), you may remember reading for the immediate three day period, 4 Apr – 6 Apr:
“Much cooler air moves into the forecast area behind a cold front as it pushes through the region from west to east very early in the period. There is a significant chance of overnight below freezing temperatures especially in regions 1 and 3 if overnight conditions are clear and calm (Region 1 is Richmond, VA, and Region 3 is Clayton, NC).”
So, let’s take a quick look at the statewide AWIS minimum temperature map for this Thursday morning in MD and VA (same map), as well as NC, SC, GA, and KY. Basically, we are looking at a classic radiation frost condition that is shaping up for Wednesday night/Thursday morning, and you will def. want to examine the Hourly Detailed AWIS forecast for your immediate area (just below the state maps). Mark and I just had an email from a grower near Edenton, NC, and his AWIS forecast is showing “F” values for FROST on both Thursday morning this week, and again on Sunday morning Edenton-4.3.18 10-day AWIS.
What is not apparent from the AWIS Detailed forecast is the fact that RAIN is moving into the Edenton area as well on Wednesday (tomorrow), and this definitely complicates matters! Looking at AccuWeather Pro, I noted that rains will be off and on all day tomorrow, with as much as 0.16 inch of total rain between the hours of 2–6 p.m.! At around 2 p.m. is when you would ideally like to apply row covers to help build up some heat underneath the cover (for release through the night). So much for that “great idea!”
Not a great situation for row covers alone:
What may actually be shaping up in this particular NC location is a situation where the covers will likely go into tomorrow night moist/wet, and that could possibly lead to the covers doing some icing up before sunrise? If that happens, open blossoms in direct contact with the cover could be killed.
Sprinkling option in Roper, NC:
I shared these thoughts with the grower in Roper, NC, about “what to do” this Wed night/Thur morning:
Hopefully, your area is not dealing with a precipitation problem tomorrow? But, if you are, and you are concerned about the potential of frost damage to blossoms in touch with the row cover, you may wish to consider running your drip system several hours before sunrise to generate some extra warmth under the cover.
Que. How can I get frost at 38°F?
Another question that came up with the grower in Roper, NC, had to do with “how is it possible to get frost damage with a minimum temperature forecast of only 38°F?
Answer: you can’t get frost at 38°F, but you sure can get a damaging frost at just below 32°F at the strawberry canopy level! I have seen “many a time” when AWIS made a minimum temperature forecast of only 38°F, but there was also significant strawberry blossom injury at the ground level.
Keep in mind that these weather forecasts are NOT for the ground level (where the strawberry plant is located), but for a weather shelter at 2 m (6.56 ft). And, you can nearly always count on it being colder at ground level! So, if the air temp at ground level (strawberry level) drops below the freezing point (32°F) at any point, conditions will then favor ice crystal formation — provided that winds are calm, the sky is clear and you have a dew point near the freezing point.
Once frost formation on a strawberry blossom is triggered (ice nucleation), you can get freezing injury. That is why we always recommend initiating sprinkler irrigation at the first sign of ice crystal formation on nearby lay flat hoses, small weeds and rolled up row covers. Once frost formation on a strawberry blossom is triggered, you can get freezing injury.
I have seen strawberry blossom damage occur even with minimum temperatures of only 38-40 F in the forecast! So, that is why I pay particular attention to all these statewide AWIS minimum temperature forecast maps, such as you see below for MD, VA, NC, SC, GA, and KY.
You can see in Fig. 1 how central VA, for example, is potentially going to see minimums in the 30-35 range (green), and growers in these areas are definitely at risk for frost Thursday morning. But even out in the more southeastern coastal areas of VA where you see the light green, the minimum temperature could be in the range of 35-40, and that could be dangerous as well.
Where I now live, in Williamsburg, AWIS is showing a minimum of 36 F for Thursday morning, and is also showing “F” (frost) from 5–7 a.m.
One last thing: It would be a really good idea to use your digital thermometer with thermocouple inserted into the blossom on Wed afternoon to very closely monitor blossom temperatures tomorrow night and Tue morning. And, you may be shocked to see just how much colder it really is at the ground level than the air temp at 2 m.
State Minimum Temperature Maps and Detailed Hourly Forecasts for Cities/Towns in Each State:
MD and VA: