Posted August 31, 2009
You've probably heard us talking for a few days about the short spell of unseasonably cool temperatures setting in on us today, with parts of the area likely to stay in the 60s while others creep into the low and mid 70s, all well short of our seasonal normals which remain in the mid 80s.
What may not be as obvious is the range of possible outcomes for the combination of frontal location, strength of advection of cooler/drier air near the surface, intensity and track of an upper level disturbance and low pressure surface wave on the front, and the way those all interact to produce a final result in terms of precipitation timing, amount, and coverage, and how that impacts the way our temperatures turn out.
It certainly makes for an interesting and challenging puzzle for a meteorologist to solve, not to mention leaving us with one of those "high bust potential" forecasts you hear Greg mention from time to time. So far, it appears we're reasonably on track with the forecast (knock on wood) which has indicated a high for Monday in the upper 60s for several days now. It is worth noting that for climate record purposes, the daily high is the warmest temperature that occurs during the calendar day (between midnight and midnight). However, we try to focus the highs on our 7-day forecasts on the 7am to 7pm period, with the idea that that's when a sizable fraction of our viewers will be out and exposed to the weather of the day.
For that reason, for example, we've been showing 69 for Monday the past few days (and actually reduced that to 67 in this morning's forecast) even though we could see that the high for the day would likely go down as something in the low 70s, a value that would occur in the pre-dawn hours. For that reason, even if our daytime high forecast of 67 works out (and some models indicate potential for us to actually fall into the low 60s at times during the day) we will not break the old record for lowest high temperature on August 31st, which is 68 degrees back in 2002.
It had appeared we would have a second chance at a new record of that sort for Tuesday Sep 1st, when the old record was 70. However, recent computer models are suggesting high pressure north of us may result in less precipitation and a few more breaks in the cloud cover than earlier projections, and we've nudged our forecast high for Tuesday up into the low 70s, still cool but not quite in record territory. Of course, if this "cold air damming" episode is more persistent than models now indicate, which does happen on occasion, there could still be a chance at a high below 70.
For just a sense of the range of possible outcomes we select from in making forecasts in these "interesting" patterns, I've included a couple of panels from a display of something called the Short Range Ensemble Forecast, or SREF, available from the Penn State e-wall weather site. An "ensemble" forecast uses either several different computer models, a single computer model run several times all starting with initial conditions that have been "perturbed" slightly to simulate the kinds of uncertainties that can arise from errors in observed data or even due to missing data, or some combination of both. When most of the runs agree closely, a well-behaved and reasonably predictable pattern is pretty likely. Conversely, when there is a huge variety of results, confidence in any particular solution can be quite low. Many times, the forecast confidence falls between those two extremes.
Today is a good example, and the two panels I've attached show a number of the individual model results plus an average of them all (the larger panel at the lower right), first for surface pressure and 3-hour precipitation amount ending at 11 am, and then for surface temperature at 2 pm. The pressure pattern is quite consistent across most of the model perturbations, but the interaction of that pattern with mid and upper level patterns and variations in the distribution of moisture mean that while the general idea of a good chance of rain and rain showers is indicated, you can see lots of detail variations as to where the heavier rain occurs, and where gaps in the precipitation are located, and recent satellite and radar trends also indicate that some of these rainfall projections may be on the high side. Likewise, while the "average" panel for temperature is in pretty good agreement with our forecast that the Triangle and points north and west will be in the mid and upper 60s early this afternoon, a scan of the individual results shows some with temperatures here in the 50s at that time (green shading), along with a few that have us in the 70s (orange).
How long will this "fall preview" last? For now, most projections indicate a return to near normal temperatures later this week and next weekend, although there is one "outlier" model result that suggest highs could dip back into the 60s on Thursday and Friday due to northerly winds and lots of thick low clouds. It will be interesting to see if this model turns out to be an incorrect anomaly, or a "leader of the pack" that others follow in the days to come...