Thursday, August 23, 2007
Radical Weather: From Cool to Hot in a Flash
From barely 70 yesterday to 95 Saturday?
Jason Samenow @ 9:00 AM
The weather pattern is about to undergo an abrupt transformation. We'll have one more day of cool 70s today, a transition day tomorrow (in the 80s), before the mercury soars well into the 90s Saturday. After we've gotten used to this rather cool regime, Saturday's heat may come as quite a shock (except for all of the weather-wise visitors here at CapitalWeather.com).
Still cool and clammy. Persistent northeast flow will once again deliver low cloud cover, some drizzle (especially in the morning and to the north and east) and below average temperatures. The skies may brighten a bit to the south and southwest late - but high temperatures will likely struggle to exceed the upper 70s in most parts of the area (note: some areas not far to the south will be well into the 80s -- so there is little room for error in this forecast). Overnight, it will be mostly cloudy and mild, with lows in the upper 60s to near 70 (suburbs-city).
Becoming partly sunny, warmer? The stalled out front to our south, which has shielded us from oppressive heat most of the week, will finally drift northeast through the area as a warm front. After it passes, some sun should emerge across the area. The exact timing of the passage of the warm front is unclear. An earlier passage will mean more sunshine and hotter temperatures (near 90, or even higher), a later passage will result in lingering clouds and temperatures struggling to hit 80. As of now, the most likely scenario calls for the front to move through during the morning, making for a warm and humid afternoon in the metro area with highs in the mid 80s -- but near 90 to the southwest and perhaps only 80 to the northeast. Overnight, it will be partly cloudy and quite muggy, with lows 70-75 (suburbs-city).
Very hot and humid. In the wake of the warm front, an oppressive airmass is going to overtake the area. Expect partly sunny skies with highs 93-98 but the humidity will make it feel as hot as 100-105 -- nearing heat advisory criteria. A late day thunderstorm can't be ruled out (20% chance) as a cold front approaches from the northwest. Overnight, the front should move through the area, with a slight chance of thunderstorms (30%). Low temperatures should range from 68-73 (suburbs-city).
Partly sunny, not as hot. Despite the passage of cold front Saturday night, cooler air will be slow to trickle into the area. While it won't be as hot as Saturday, high temperatures should still reach the upper 80s to around 90. During the afternoon hours, you may start to notice a decrease in humidity levels.
If my forecast made at the beginning of the week for the second half of the week had come true, we'd be in a heat wave by now. Instead, we'll be lucky if we see 80 degrees today and will be about 5 degrees cooler than average. What happened?
The explanation is pretty straight forward. The front, separating the hot air from the cool air, has remained just to our south whereas I thought it would have retreated to north of the Pennsylvania border by now. So while places like Richmond and Roanoke have been baking in the 90s, we've had a hard time reaching the 70s courtesy of cool high pressure to the north. See the adjacent map of high temperatures -- it's pretty clear where the front is (it forms an inverted "j" shape from western PA to north central VA). The orientation of warm vs. cool temperatures is characteristic of cold air damming, where northeasterly winds bank cool air against the eastern slopes of the Appalachian Mountains courtesy of an area of high pressure to the north.
Pictured above right: High temperatures in the Mid-Atlantic and Northeast yesterday. High pressure to the north drove a wedge of cool air down the eastern slopes of the Appalachians north of central Virginia. Image courtesy weather.com
This kind of cold air damming pattern (common in winter), where it's very difficult to "place" the frontal position, can drive forecasters crazy. Especially when different computer models provide drastically different solutions. For example, one model (the GFS) brings the warm front north of us today and forecasts a high in the upper 80s, whereas another keeps the warm front to the south with highs in the upper 70s. When two models have vastly different solutions, the best thing to do is a reality check to see which has been performing better. Clearly, the model forecasting the cooler air (keeping the front to the south) has been superior this week - hence the rationale for today's CapitalWeather.com forecast. Reality checks aren't possible though at the onset of such a pattern (as was the case early this week). You have to think back to similar situations in the past but, just like in the stock market, past performance is no guarantee of future results.
Looking ahead, confidence is high that damming will be over by tomorrow as high pressure moves offshore, the warm front lifts northward and winds turn southerly. All models agree on this although they differ on timing. The timing differences have important implications for the high temperature forecast Friday as already discussed (scroll towards the top of this post).
Saturday, we're in a more predictable not to mention more characteristic regime for summer time: hot and humid.
Pictured: the NAM-model forecast for high temperatures on Saturday. The model predicts highs in the mid 90s for much of the area. So does the other major U.S. forecasting model, the GFS. Image courtesy Penn State Department of Meteorology
Today
Still cool and clammy. Persistent northeast flow will once again deliver low cloud cover, some drizzle (especially in the morning and to the north and east) and below average temperatures. The skies may brighten a bit to the south and southwest late - but high temperatures will likely struggle to exceed the upper 70s in most parts of the area (note: some areas not far to the south will be well into the 80s -- so there is little room for error in this forecast). Overnight, it will be mostly cloudy and mild, with lows in the upper 60s to near 70 (suburbs-city).Friday
Becoming partly sunny, warmer? The stalled out front to our south, which has shielded us from oppressive heat most of the week, will finally drift northeast through the area as a warm front. After it passes, some sun should emerge across the area. The exact timing of the passage of the warm front is unclear. An earlier passage will mean more sunshine and hotter temperatures (near 90, or even higher), a later passage will result in lingering clouds and temperatures struggling to hit 80. As of now, the most likely scenario calls for the front to move through during the morning, making for a warm and humid afternoon in the metro area with highs in the mid 80s -- but near 90 to the southwest and perhaps only 80 to the northeast. Overnight, it will be partly cloudy and quite muggy, with lows 70-75 (suburbs-city).Saturday
Very hot and humid. In the wake of the warm front, an oppressive airmass is going to overtake the area. Expect partly sunny skies with highs 93-98 but the humidity will make it feel as hot as 100-105 -- nearing heat advisory criteria. A late day thunderstorm can't be ruled out (20% chance) as a cold front approaches from the northwest. Overnight, the front should move through the area, with a slight chance of thunderstorms (30%). Low temperatures should range from 68-73 (suburbs-city).Sunday
Partly sunny, not as hot. Despite the passage of cold front Saturday night, cooler air will be slow to trickle into the area. While it won't be as hot as Saturday, high temperatures should still reach the upper 80s to around 90. During the afternoon hours, you may start to notice a decrease in humidity levels.Pattern Discussion: I'll Be Dammed
If my forecast made at the beginning of the week for the second half of the week had come true, we'd be in a heat wave by now. Instead, we'll be lucky if we see 80 degrees today and will be about 5 degrees cooler than average. What happened?
The explanation is pretty straight forward. The front, separating the hot air from the cool air, has remained just to our south whereas I thought it would have retreated to north of the Pennsylvania border by now. So while places like Richmond and Roanoke have been baking in the 90s, we've had a hard time reaching the 70s courtesy of cool high pressure to the north. See the adjacent map of high temperatures -- it's pretty clear where the front is (it forms an inverted "j" shape from western PA to north central VA). The orientation of warm vs. cool temperatures is characteristic of cold air damming, where northeasterly winds bank cool air against the eastern slopes of the Appalachian Mountains courtesy of an area of high pressure to the north.Pictured above right: High temperatures in the Mid-Atlantic and Northeast yesterday. High pressure to the north drove a wedge of cool air down the eastern slopes of the Appalachians north of central Virginia. Image courtesy weather.com
This kind of cold air damming pattern (common in winter), where it's very difficult to "place" the frontal position, can drive forecasters crazy. Especially when different computer models provide drastically different solutions. For example, one model (the GFS) brings the warm front north of us today and forecasts a high in the upper 80s, whereas another keeps the warm front to the south with highs in the upper 70s. When two models have vastly different solutions, the best thing to do is a reality check to see which has been performing better. Clearly, the model forecasting the cooler air (keeping the front to the south) has been superior this week - hence the rationale for today's CapitalWeather.com forecast. Reality checks aren't possible though at the onset of such a pattern (as was the case early this week). You have to think back to similar situations in the past but, just like in the stock market, past performance is no guarantee of future results.
Looking ahead, confidence is high that damming will be over by tomorrow as high pressure moves offshore, the warm front lifts northward and winds turn southerly. All models agree on this although they differ on timing. The timing differences have important implications for the high temperature forecast Friday as already discussed (scroll towards the top of this post).Saturday, we're in a more predictable not to mention more characteristic regime for summer time: hot and humid.
Pictured: the NAM-model forecast for high temperatures on Saturday. The model predicts highs in the mid 90s for much of the area. So does the other major U.S. forecasting model, the GFS. Image courtesy Penn State Department of Meteorology
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