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Special Update (Wednesday 12/31 11:30 pm CST)
Precipitation is entering North Texas (use the radar links above to track the precipitation). Temperatures will fall below freezing after the precipitation begins. As I posted earlier today, the critical line is the 29º wet-bulb temperature. The following map shows the 11 pm wet-bulb temperatures and the 11 pm position of the actual freezing line. The freezing line is already responding to the precipitation southwest of the metroplex. This trend will continue into D/FW as the precipitation moves northeast.
- The greatest ice amounts will be WEST of a line from McKinney to DFW Airport to Fort Worth (west of the 29º wet-bulb line)
- Areas EAST of this line (including Dallas) will see less (perhaps none at all, especially in the urban heat island of Dallas)
Previous Forecast Discussion Posted at 3 pm CST 12/31
Since Monday, I have been discussing the potential for freezing rain and sleet over North Texas tonight and tomorrow morning (New Year’s Day). This is an extremely complicated forecast, and in general, I’ve made no changes. The most difficult aspect of the forecast is determining how much precipitation we will receive. Some of the atmospheric model data suggest moderate to heavy amounts are possible, while other models show much less. I do have growing concerns that ice accumulations may occur west of a line from McKinney to DFW Airport to Fort Worth, with lighter accumulations (if any at all) east of that line (see the section below about the 29º wet-bulb temperature, which I am using to determine that demarcation line). I expect all rain east of a Greenville to Dallas to Cleburne line.
The following map shows my forecast created on Monday, December 29, depicting areas where freezing rain and sleet may occur. I still believe that precipitation may begin as sleet here in North Texas, but it should change to freezing rain overnight as a warm layer of air aloft deepens.
This map is still a valid forecast, with no changes at this time.
Synoptic Pattern & Upper-Level Storm System
As discussed earlier this week, a vigorous upper-level disturbance will approach the Southern Plains in the wake of a strong Arctic front that moved through last night (Tuesday night). The following map shows the 500-mb geopotential height and vorticity valid at 5 am CST tomorrow (Thursday, January 1, 2015)
As the upper-level disturbance approaches from the west tonight, precipitation will begin to develop over southwest Texas, and spread rapidly to the northeast. Precipitation should enter the Dallas/Fort Worth Metroplex in the midnight to 2 am time frame. The precise time depends on your location in the metroplex, because it will be moving in from the southwest. Therefore, it will begin earlier in the western/southwestern counties.
Wet-bulb Temperatures and Evaporative Cooling
At 2 pm CST Wednesday, temperatures were generally in the low to mid 30s across North Texas, with the warmest temperatures located from Dallas County south and eastward. A critical parameter for forecasting freezing rain is the wet-bulb temperature. The wet-bulb temperature is the temperature to which the air can cool via evaporative cooling. The drier the air is, the more the air can cool when rain falls through it. At the time of this forecast update (2 pm CST, Wednesday), wet-bulb temperatures were generally below 32º across the metroplex (lowest to the north and west). While 32º might seem like the reasonable wet-bulb temperature to use when predicting freezing rain, research and my forecasting experience have shown that a better indicator is 29º or 30º, particularly for significant ice accumulations. Freezing rain is a self-limiting process, because latent heat (energy) released when water freezes can cause temperatures to rise. Furthermore, heavy rain falling through a warm layer of air aloft can “drag” some of that warmer air to the surface. Therefore, I prefer to use the 29º wet-bulb temperature when forecasting freezing rain, because it gives a little bit of wiggle room to account for those processes.
The following map shows the observed wet-bulb temperatures across North Texas at 2 pm CST today.
In the map above, the darker shade indicates where I expect more significant ice accumulations. In fact, ice amounts in excess of 1/4 inch may occur in these areas. Further east, in the lighter shade (including downtown Dallas), it is a bit more marginal (due to the processes discussed above and the slightly warmer wet-bulb temperatures). In the solid green areas east of Greenville/Dallas, I am currently indicating rain only, with temperatures above freezing.
Atmospheric Instability May Lead to Thunderstorms Thursday Morning
An aspect of this event that concerns me is the potential for convection (thunderstorms). Forecast soundings for Thursday morning indicate some conditional instability aloft. This could cause enhanced areas of precipitation, with lightning, and rapid precipitation rates. I will monitor this very closely. Obviously, areas that receive thunderstorms while temperatures are below freezing could see a substantial amount of ice accretion.
This is the forecast sounding for Thursday morning (using 7 am CST data)
Further evidence of approaching atmospheric instability is apparent in recent infrared satellite imagery. This image is from 5:30 pm CST Wednesday (December 31). Very cold cloud tops in northern Mexico (-65ºF to -75ºF) may be indicative of atmospheric instability and may be precursory to thunderstorm formation when this energy moves northeast into Central and North Texas late tonight and Thursday morning. This is is a signal that I often use when forecasting thunderstorms during winter weather events in Texas.
Summary
This has the potential to be a significant event, with large amounts of ice accretion, for areas north and west a McKinney to DFW Airport to Fort Worth line. Areas just east of this line will likely see freezing rain, but I have more uncertainty regarding amounts given the warmer wet-bulb temperatures. It’s important to note, however, that even though the wet-bulb temperatures at 2 pm may be a bit more marginal in some areas, they are still subfreezing. Furthermore, this is a nocturnal event (i.e., lacks the benefit of solar insolation to work against ice accretion) and it’s possible that some drier surface air could still advect in from the north causing the wet-bulb temperatures to drop a degree or two prior to the precipitation onset (I am currently not expecting this to happen, however).
Precipitation will begin late this evening, most likely in the midnight to 2 am time frame (onset time) and lasting through noon tomorrow (Thursday). Rain/freezing rain may be heavy at times, with a few thunderstorms possible by sunrise.
- The greatest ice amounts will be WEST of a line from McKinney to DFW Airport to Fort Worth (west of the 29º wet-bulb line)
- Areas EAST of this line (including Dallas) will see less (perhaps none at all)
- I expect just rain east of a Dallas to Greenville line.
Temperatures should gradually warm during the day Thursday, and should rise above freezing by Thursday afternoon.
Tips/Recommendations
- Please pay attention to all future forecasts.
- I will try to update on my Twitter and Facebook pages as often as possible.
- If you are traveling, check road conditions on your route, especially if you are traveling to the north and west.
- If you are flying in/out of any metroplex airports, call ahead to check your flight’s status, especially tomorrow morning.
- Remember, bad weather in other parts of the country can also affect your flight’s status here in Texas.
- Track road conditions in Texas using the DriveTexas DOT Website
–Chris Robbins, Meteorologist, M.S., B.S.,