Multi-State Round-Up – What a Crazy Day Its Been! (5:49pm, Tue., April 15, 2014)

— Written By

strawberryDear Growers, Agents, Agronomists and Others,

What a busy day it has been! I am very glad that I chose to file for an extension on my taxes, as the only thing I have managed to do over the last last few days is talk to growers and post advisories! The most remarkable development of the day for us in NC has been the change from a windborne freeze forecast for tonight to what I refer to as a frost/freeze. The important thing to know about frost/freezes, which have winds from 5 mph to 10 mph, is that it is possible to use sprinkler irrigation. I don’t know how many growers out there wish they had known this earlier today, or yesterday. Many, many growers throughout NC and the Mid-South have applied row covers – thinking this would be a windborne freeze tonight! Well, here is what I know as of 5 pm Tuesday night, and I can assure you that this has been a very confusing day!

If you need to see why it has been so confusing, just look at the wind speeds being forecast at this hour (5 pm) for Asheville at 5 am tomorrow (4/16):

ash awisTable 1. AWIS is showing winds of 17 mph at 5 am tomorrow morning. They are calling for 25 min tomorrow, and AccuWeather is calling for 27 F? I would go with the lower number.

accu ashTable 2. AccuWeather is now forecasting winds of 9 mph at 5 am, or 8 mph less than AWIS. At winds from 5-10 mph we fall into what is technically classifed as a ‘frost/freeze’ and sprinkler irrigation can be used. In a windborne freeze event it cannot be safely used unless there is a row cover on the crop as well.

1. NC – two significant developments this afternoon…

At 2:35 pm today, I got my first hint that the various weather services are on different pages when SkyBit sent out an alert that we could now have a frost a Clayton on Wed morning – previously there was going to be too much wind for a frost! As I compose this advisory, I am waiting for Skybit’s evening update for Clayton, and I will post that here. But, while I am waiting for that update, I want to share some very good news. This morning I studied the AWIS duration product for NC, and I asked their computer to tell me how many hours various parts of the state would be below 28 F. What I got back at 7 am this morning was pretty frightening – locations like Louisburg were possibly going to have several hours below 28, and now just 1 hour (Table 3)

durationTable 3. The number of hours below 28 F are indicated by location in NC for 4/16 and 4/16, and you will note that Louisburg will now only accumulate 1 hr < 28 tomorrow morning (4/16) and one hour Thur morning (4/17). This product was revised at about 4 pm today (EST), and I see some positive things happening here – it will not be quite so cold over the next 2 days as we thought this morning at 8:30 am. I have also alluded to the disagreements between weather services on winds, and let’s just leave it that there is potentially now going to be less wind to deal with, and this could be some very good news for the sprinkler irrigation folks.

My SkyBit for Clayton – just in (5:20 pm). This is reasonably good news as they are not calling for any colder temps tomorrow morning, but in the Skybit hourly table they are still showing relatively high winds? This is in contrast to SkyBit frost alert at 2:35 pm saying our highest winds would be 3 mph, and frost would occur? I guess they have all the bases covered!

eveing sky clayt

I am providing here the latest state min temp update from AWIS at about 4 pm our time, and it shows a promising improvement in temperature minimums from this morning’s forecast! NC AWIS_EVENING? Apr 16 to 22

Anyway, at least we’re making news today:

Also, here are the latest 5-day detailed forecasts for some 64 locations in NC (done at 4 pm)… these are very valuable tables for identifying when the wet bulb will likely fall below 31 F in your general area


2. VA – VA AWIS EVE Apr 16 to 22

va eve update for wedFig. 1. VA minimums in forecast for Wed morning

VA eve update for ThurFig. 2. VA Minimums for Thursday, but the thing to be concerned about Thursday morning are not so much the absolute minimums, but the potential for FROST.

Detailed houry forecasts for VA:



Anderson_Co_Air Beaufort_SC Charleston_Muni Clemson-Clemson
Columbia-Columb Columbia/McEnti Columbia_Metro Darlington
Florence_SC Greenville_Down Greenville-Spart Greenwood-Green
Hilton_Head_AP Myrtle_Beach North_Myrtle_Be Orangeburg-Oran
Rock_Hill Sumter_SC

4. TN – TN AWIS Eve Apr 16 to 22


5. KY – KY AWIS EVE Apr 16 to 22

AWIS Weather Services, Inc. Western Kentucky Frost/Freeze Forecast
Produced at  304 PM CDT on Tue Apr 15 2014

... Forecast for Tonight ... Southwest Kentucky                      
NWS Fcst Zones 1-13                               


Lowest Temperatures:      28 - 30
Murray Min                  30 
Durations at/below 32:     3 -  6
Durations at/below 28:     0 -  1
Range Dewpoint Temps:     23 - 30
Range Wetbulb  Temps:     26 - 35
AVG Wind Direction/Speed:   V  2     Long Periods of Calm 
AVG Sky Condition:         Clear    

Extended Forecast: Range of Lowest Min Temperatures in the Above Zones
Min Temps Valid For Morning of Given Date (May NOT include ALL cold pockets)

  04/17/14  04/18/14  04/19/14  04/20/14  04/21/14  04/22/14
  --------  --------  --------  --------  --------  --------
   39 - 41   46 - 49   44 - 47   47 - 49   57 - 58   55 - 57

Grayson County Area                     
NWS Fcst Zones 23-24,26-28,61-63                  

**** HARD FREEZE ****

Lowest Temperatures:      24 - 28
Leitchfield Min             25 
Durations at/below 32:     5 - 11
Durations at/below 28:     1 -  6
Range Dewpoint Temps:     20 - 31
Range Wetbulb  Temps:     27 - 34
AVG Wind Direction/Speed:  NW  3     Long Periods of Calm 
AVG Sky Condition:         Clear    

Extended Forecast: Range of Lowest Min Temperatures in the Above Zones
Min Temps Valid For Morning of Given Date (May NOT include ALL cold pockets)

  04/17/14  04/18/14  04/19/14  04/20/14  04/21/14  04/22/14
  --------  --------  --------  --------  --------  --------
   36 - 39   43 - 46   42 - 44   43 - 47   53 - 56   52 - 54

For more detailed information visit or call 888-798-9955. Copyright 2014 AWIS Weather Services, Inc. All rights reserved.


6. MD –MD AWIS EVE Apr 16 to 22

md 1 md 2 md 3


final pointers on when to start irrig up on a night like this – before wet bulb drops below 31 F – this will keep you from causing a ‘cold jolt’

To understand the ‘cold jolt,’ you need to know that the wet bulb temperature is the temperature air cools to when water is added. As soon as you start irrigating, the blossoms actually drop in temperature to the wet bulb temperature. Knowing this, you can appreciate the danger involved with starting at a wet bulb air temperature of even 29 F as this temperature is 1 degree away from a temperature that will kill an open blossom (to play it safe we even say that 30 F is the critical temp for the open blossom, but my work has shown that you need to reach 28). But, if you were to start at a wet bulb of say 26 F, then it would definitely cause a “jolt” and kill the open blossoms soon after irrigation is started.

You can avoid the ‘cold jolt’ entirely by starting before the wet bulb temperature of 31 F is reached.

Calculating wet bulb?


 A quick technique that many forecasters use to determine the wet-bulb temperature is called the “1/3 rule”. The technique is to first find the dewpoint depression (temperature minus dewpoint). Then take this number and divide by 3. Subtract this number from the temperature. You now have an approximation for the wet-bulb temperature.

 Here is an example: suppose the temperature is 42 degrees Fahrenheit with a dewpoint of 15 degrees Fahrenheit. The dewpoint depression is 42 – 15 = 27. Now divide 27 by 3 = 9. Now subtract 9 from the original temperature of 42. 42 – 9 = 33. If the temperature was 42 with a dewpoint of 15 and it started raining, the temperature and dewpoint would wet-bulb out to a chilly 33 degrees Fahrenheit. As dewpoint depression or temperature increase, the evaporational potential increases.

Here is an example of calculating the wet bulb for Ramsuer NC tonight:

Question – when will wet bulb reach 31 F?

Calculation: Air temp at 2 am = 33; Dewpoint at 2am = 21

  • 33-21/3 = 4
  • next, subtract 4 F from air temp at 2am (33), or 29 is the wet bulb…THIS IS TOO LOW (Must start at 31 wet bulb)
  • A second try – lets look at 1 am when air is 37 F and DP = 24; thus, 37 – 24 = 13, then divide by 13 by 3, and you get 4.33. So, air temp of 37 – 4.33 = 32.66. Thus, you would want to start at about 1:30 am to be safe.

What did the complete hourly table show for wet bulbs? It says the wet bulb at 1 am will be 32 (we calculated 32.66)

wet bulb calc

Good luck tonight!

Dr. E. Barclay Poling
Interim Executive Director, NC Strawberry Association Inc.
& Professor Emeritus (Strawberry Plasticulture Researcher)
Department of Horticultural Science
Campus Box 7609, 162A Kilgore Hall
NC State University
Raleigh, NC 27695-7609

Written By

Photo of Dr. Barclay PolingDr. Barclay PolingFormer Professor and Extension Specialist, Strawberries and Muscadines (919) 515-5373 (Office) barclay_poling@ncsu.eduHorticultural Science - NC State University
Updated on Apr 15, 2014
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