As discussed in this blog post, the precipitation forecast on the prog charts is issued by the local weather forecast offices (WFOs) throughout the country. The forecast is supposed to look seamless. However, it is not uncommon to see the resulting prog chart precipitation type forecast to follow the county warning area (CWA) boundaries when there's a difference of opinion as to the precipitation type expected. Here is such a case based on the difference between forecasters in the Mount Holly (PHI) , Sterling (LWX), and State College (CTP) forecast offices.
The precipitation type forecast shown below valid at 18Z clearly shows that for southern New Jersey, extreme southeastern Pennsylvania and the Delmarva Peninsula that rain (light green) is expected to reach the surface. This includes the city of Philadelphia. However, further west it is very clear that the precipitation type is expected to be either a mixed precipitation type (purple) or ice in the form of freezing rain or freezing drizzle (orange). This includes the city of Baltimore. But if you compare the line of demarcation between the two with the boundary of the CWAs above, you'll notice that one forecast office believes this will be rain and the other two believe it'll be some form of mixed precipitation or ice.
While there could be a meteorological reason for this abrupt change, this is not the case for this weather system. It appears that these forecast offices are not coordinating their forecasts as they should. In fact, it also shows up in the TAFs for Philadelphia and Baltimore as you can see below. The Baltimore TAF (KBWI) within the Sterling CWA shows the potential of light snow and ice pellets (-SNPL) which is a mixed precipitation forecast whereas Philadelphia (KPHL) that is within the Mount Holly CWA simply shows light rain showers (-SHRA). The good thing is that the forecasters that issue the TAFs also issue the precipitation type forecasts found on the progs...and they at least match within their own forecast office.
This is a tough forecast. There's no doubt that a degree or two aloft can make a huge difference in the precipitation type that reaches the surface. Let's drill down a bit here and take a look at the guidance that was available when the 12Z TAFs were issued. Below is the 7-hour forecast sounding for Baltimore valid at 18Z. Notice that the wind direction near the surface is from the east-northeast bringing in cool air off the Atlantic Ocean. At 2,000 feet MSL the temperature is expected to be -4.2°C (blue arrow). Winds above 3,000 feet MSL are from the south and southwest creating a warmer flow, giving rise to the temperature inversion and "warm nose" you see in the forecast sounding. There is a fairly deep warm layer with temperatures warmer than 0°C from 3,000 feet to 10,000 feet (purple arrows). The temperature maximum in this layer is forecast to be +1.7°C at 7,000 feet MSL.
If this forecast sounding is accurate and precipitation is occurring, there is a strong likelihood that snow falling into this warm layer would melt completely giving rise to rain reaching the surface and not a mixture of ice pellets and snow as was depicted in the prog chart and TAFs for Baltimore. The temperature at this time is expected to be slightly above the freezing mark, so it would be rain and not freezing rain. Again, you can see how much a degree or two can make a difference.
Below is the 7-hour forecast sounding for Philadelphia valid at the same time. Notice that the wind and temperature profile are similar except that the layer of warm air warmer than 0°C is much more shallow from about 5,000 feet to 6,500 feet MSL (purple arrows) with a maximum temperature forecast of +1.4°C at 5,300 feet MSL. Given such a shallow melting layer it would be more likely for the snow falling to this layer to partially melt while retaining a slushy core and then be allowed to quickly refreeze into an ice pellet in the subfreezing air below. Temperature at 2,700 feet MSL is -7.0°C (blue arrow). This would give rise to a precipitation type forecast at the surface for ice pellets or perhaps a mixture of snow, rain and ice pellets but not light rain showers as was forecast.
In fact, as this weather unfolded it was interesting to see that the TAFs for these two airports flip-flopped for the 18Z issuance with Baltimore changing their precipitation type forecast to rain and Philadelphia changing to a precipitation mixture of snow, rain and ice pellets as shown below. This makes more sense based on the forecast soundings above.
Shortly after 18Z, Baltimore was reporting light rain and Philadelphia reported a mixture of light rain and ice pellets. Earlier in the morning Baltimore was also reporting a mixture of rain and ice pellets and Philadelphia also reported light snow.
KBWI 241815Z 04007KT 4SM -RA BR FEW016 BKN022 OVC039 01/M03 A3044
KPHL 241810Z 04008KT 10SM -RAPL SCT060 OVC075 02/M08 A3046 RMK
There's no intention here to fault the NWS meteorologists for a very tough forecast, however, it would be nice if they were able to communicate better to create a more seamless forecast that eventually ends up in the National Digital Forecast Database (NDFD) and eventually rendered on the prog charts.
Most pilots are weatherwise, but some are otherwise™
Dr. Scott Dennstaedt
Weather Systems Engineer
CFI & former NWS meteorologist