WRAL WeatherCenter Blog

To Rain, or Not to Rain (much)???

Posted November 6, 2006

After lots of sunshine and some unseasonably cold temperatures through the weekend, we appear to be heading into a more unsettled pattern for a few days, the problem is it is they type of scenario that can make confident forecasting of details very difficult. For the past several days, long range forecasts from a few computer models indicated a full latitude trough aloft making steady eastward progress across the region, with a period of on and off rain possible Tuesday and Tuesday night. One model, the GFS, began to show the lower extent of that trough deepening and forming into a cutoff low, however, and as we have gotten closer to the event in question, other models have fallen more into line with that solution.

That sounds like good news and a source of increased consistency among the guidance products we use to help establish the 5-day forecast. However, as is often the case the "devil is in the details," such as when will rain occur, how heavy will it be, how will rain and clouds impact high temperatures each day, and when will the cutoff low depart and a more stable pattern resume? We'll do our best to answer these questions as we go along though the next couple of days, but don't be surprised to hear some words of caution as to the level of confidence we have in our outlooks, or too surprised if a forecast rainfall amount or temperature range for a given time span doesn't verify all that well. That can be the nature of dealing with deep cutoff lows, as they often move at a speed that isn't easily predicted, and their effects on the weather as they interact with nearby frontal boundaries, surface low pressure areas and passing upper level jet streaks, can be complex in such a way that areas of precipitation and extensive cloud cover can develop and dissipate rapidly, and can organize into banded structures that mean one region sees partial sunshine while another nearby is gray and rainy, all in a poorly predictable fashion.

A good example is this Wednesday afternoon, when the cutoff low aloft is expected to be a little south of us, but at which time the way it interacts with regional moisture fields, thermal fields (including a likely coastal area warm front) and pressure patterns at various altitudes are forecast by the GFS model and our WeatherScope model (a version of MM5) to anticipate partly to mostly cloudy skies, mild temperatures (mid 60s to around 70) and little more than a few sprinkles, if that, from midday into the early evening, while another model called the NAM (North American Mesoscale) indicates more clouds, much more precipitation and resulting temperatures that stay considerably cooler (upper 50s to low 60s), while a model run by the Canadian meteorological service falls somewhere in between. Our forecasts on TV, radio and the web as we go along will give you a sense of how we've weighted the evidence from analyses, satellite and radar data and the competing model calculations as to how everything will progress.

These two images show model forecasts for the height of the 700 mb surface and relative humidity at that level, which is around 10,000 feet above the ground, from the GFS and the NAM, for Wednesday evening at 7 pm. The two solutions appear roughly similar, but the GFS channels the most abundant moisture over the Atlantic into eastern SC, while the NAM indicates more moisture wrapping across the northern eadge of the cutoff low across NC and VA.

The next two maps are for the same time, from the same models, but show the sea level pressure pattern in black, the thickness of a layer from 1000 mb to 500 mb in dashed lines, and model-estimated precipitation in the 6-hour period from 1 pm to 7 pm. Again, the general features of the pressure pattern are not too dissimilar, but owing in part to the distribution of moisture at higher altitudes indicated above, the precipitation estimates differ greatly, with the Triangle and Triad areas estimated to be rain-free by the GFS, but showing around .5-1" on the NAM, with the situation almost reversed down around Charleston SC... stay tuned!


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