Frost in the Forecast for Some Areas on Thursday Morning (2:30 p.m., Tue., April 3, 2018)

— Written By Barclay Poling
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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:

1. I believe conditions could be “right” for a radiation frost event on Thursday morning, April 5th, and that this event could be damaging to open blossoms, but not likely of any consequence to other stages mentioned (near ripe berries and green fruit). The risk here is primarily to open blooms. 
2. I would like to rec. row covers, but a complicating factor —> rainfall in Roper, NC, on Wed during afternoon could amount to about 2/10 inch (according to AccuWeather Pro). My experience has been that it is often very difficult to get row covers to completely dry out by evening, even with just 1/10 – 1/15th inch of rain in the morning — here we are dealing with some afternoon showers!
3. So, with rain coming the afternoon (Wed), at just the time when covers should be applied, they will likely remain wet going into the evening, and that is not a good thing.
4. Thus, sprinkling appears to be the best option ( though I am not crazy about irrigating out of an open pond this close to harvest). 
5. Plan to monitor the field closely early Thur morning and initiate sprinkling at first sign of frost (ice crystals) forming on lay flat hose and/or rolled up row cover, and follow the same procedure Sunday morning, though I think chance of frost is much much less.

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.

Good luck!

State Minimum Temperature Maps and Detailed Hourly Forecasts for Cities/Towns in Each State:

MD and VA:

Mid-Atlantic region minimum temps mapFig. 1. Mid-Atlantic region minimum temps Thur morning



Aberdeen Andrews_AFB Annapolis_Naval Baltimore_Sci_Ce
Baltimore/Wash Cambridge/Dorche College_Park_AP Cumberland_AP
Easton_MD Fort_Meade Frederick_MD Gaithersburg_AP
Hagerstown_AP Leonardtown Middle_River Oakland_MD_AP
Ocean_City_AP Patuxent_River Salisbury Stevensville_AP
St_Inigoes/Webst Washington_DC_AP Westminster_AP



Abingdon Ashland_VA Charlottesville Chesapeake-Ches
Culpeper Danville Dublin/New_Riv Emporia_VA
Farmville Fentress_Naval_A Fort_Belvoir/Da Fort_Eustis/Fel
Franklin/J_B_Ro Hillsville Hot_Springs/Ing Jonesville_AP
Langley_AFB Leesburg/Godfre Louisa-Louisa_C Lynchburg
Manassas_Muni(A Marion/Wythev Martinsville Mecklenburg/Bru
Melfa/Accomack Newport_News Norfolk_Hampton_ Norfolk_NAS
Norfolk_VA Oceana Orange_VA Petersburg_(AWO
Quantico Richlands_AP Richlands_VA Richmond
Richmond_Chester Roanoke Shannon_Arpt Stafford_AP_VA
Staunton-Shenan Suffolk-Suffolk Tappahannock_AP Virginia_Tech_A
Wakefield_AP Wallops_Island Warrenton_Fauqui Washington_Dulle
West_Point_VA Williamsburg_VA Winchester_Rgnl Wise/Lonesome_P


NC minimum temps mapFig. 2. NC minimum temps Thur morning


Ahoskie-Tri-Cou Albemarle_AP Andrews Asheboro
Asheville_Munic Beaufort-Michae Boone_NC Burlington-Burl
Cape_Hatteras_AG Chapel_Hill-Hor Charlotte Cherry_Point
Clinton_NC Concord_NC Currituck_NC Edenton-Northea
Elizabeth_City Elizabethtown_NC Erwin-Harnett_C Fayetteville_AP
Fort_Bragg/Simm Franklin/Macon_C Gastonia-Gaston Goldsboro_AFB
Goldsboro_NC Greensboro Greenville_AP_NC Hickory
Jacksonville Jacksonville_(A Jefferson_AP_NC Kenansville-Dup
Kill_Devil_Hills Kinston_AP Lexington-David Lincolnton_AP
Louisburg_NC Lumberton_AP_NC Mackall_AAF Manteo/Dare_Co
Maxton Mcalf_Bogue_Fiel Monroe_AP_NC Morganton-Morga
Mount_Airy-Moun New_Bern North_Wilkesbor Oxford_NC
Piney_Island_Bom Pope_AFB Raleigh-Durham Reidsville_AP
Roanoke_Rap_AP Roanoke_Rapids_R Rockingham_NC Rocky_Mount-Wils
Roxboro_AP Rutherfordton Salisbury-Rowan Sanford_NC
Shelby_AP_NC Smithfield_AP Southern_Pines Southport_NC
Statesville-Sta Tarboro_AP Wadesboro_AP Washington-Warr
Whiteville_NC Wilmington_AP_NC Winston-Salem


SC and Southeast Region minimum temps mapFig.3. SC and Southeast Region minimum temps Thur morning


Aiken_AP Anderson_Co_Air Barnwell_AP Beaufort_Cnty_AP
Beaufort_SC Bennettsville_Je Camden_AP Charleston_Exec_
Charleston_Muni Cheraw_AP Chester_AP Clemson-Clemson
Columbia-Columb Columbia/McEnti Columbia_Metro Conway_AP
Darlington Florence_SC Georgetown_AP_SC Greenville_Donal
Greenville_Down Greenville-Spart Greenwood-Green Hartsville_AP
Hilton_Head_AP Kingstree_AP Lancaster-McWhir Laurens_AP
Manning_AP Marion_AP_SC Moncks_Corner_AP Mount_Pleasant_A
Myrtle_Beach Newberry_AP North_Myrtle_Be Orangeburg-Oran
Pickens_AP Rock_Hill Spartanburg_Down Summerville_AP
Sumter_AP Sumter_SC Walterboro_AP Winnsboro_AP


Georgia min. temps mapFig. 4. GA min. temps Thur morning


Albany_Municipa Alma/Bacon_Co Americus_AP Athens_Municipa
Atlanta/DeKalb Atlanta/Fulton Atlanta_Intl_Ar Atlanta_Paulding
Augusta/Bush_Fi Augusta_Daniel_F Bainbridge_GA Blakely_AP
Brunswick_GA Brunswick/Glync Canton_GA Carrollton_AP
Cartersville_AP Claxton_AP Columbus_GA Covington_GA_AP
Dalton-Dalton_M Douglas_Muni Dublin/WHBarron Fitzgerald_AP
Fort_Benning Ft_Stewart/Wrig Gainesville_AP_G Greensboro_GA
Homerville_AP Hunter_(AAF) Jesup_AP La_Grange
Lakeland Lawrenceville_AP Macon_AP Marietta
Marietta/Cobb Milledgeville_AP Moultrie_Muni_AP Newnan-Newnan_C
Peachtree_City Rome_GA Savannah Statesboro_GA
Swainsboro_AP Sylvania_AP_GA Thomaston_AP Thomasville_AP
Thomson_GA Tifton_AP Valdosta Valdosta/Moody
Vidalia Warner_Robins_A Washington_GA Washington_GA
Waycross_AP Winder_AP


Kentucky min. temps mapsFig. 5. KY min temps Thur morning


Bowling_Green Covington Danville-Powell_ Fort_Campbell
Fort_Knox/Godma Frankfort_AP Glasgow_KY Henderson_City
Jackson_KY Lexington_KY London_Corbin Louisville_AP
Louisville/Bowm Middlesboro_AP Monticello_KY_AP Murray_AP
Owensboro_AP Paducah Richmond_Madison Somerset(AWOS)