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Tuesday 8:30 a.m.
Drier air advanced into the northeast quarter of the nation yesterday, but its tenure is temporary and will be quickly terminated. As a high pressure area moves off the East Coast and a low pressure area forms near the lower Great Lakes, more humid air will be drawn northward... and the chance of showers and thunderstorms will increase. This looks like the kind of setup that can unleash 1-2 inches of rain in some areas. However, until we see exactly where the storm forms and how the moisture becomes arranged around it, forecasting rainfall amounts for various places is risky. The video has more.
On this date in 1918, Philadelphia had a high temperature of 106, the highest official reading ever for that city. What did the weather map look like that day? NOAA maintains an archive of weather maps for each day back to 1871. You can view them here The map for Aug. 7, 1918 is below. The map time is 1 a.m., and you may be able to see the 80-degree isotherm for that hour.
In those days, weather records were kept at the Customs House in Center City, Philadelphia. In later years, the observation site was shifted to Philadelphia International Airport. Airport locations are typically somewhat cooler than the middle of our big cities.
In other words, while late summers in Phoenix have gotten wetter during the last few years, Boston has become drier. Is there anything more momentous or general that we can say about this?
This enhanced infrared satellite picture shows the cold front in the Northeast and the moisture wrapping around Odille on the southwest part of the map.
Across the Central and Northern states, thunderstorms are less common at this time of year than in late spring and summer. One area that has had more thunderstorms than usual recently is across the Desert Southwest.
Last week, I mentioned that longer range computer models were suggesting a major warmup by next weekend. More recent runs have backed off on the that idea. However, there is extreme uncertainty beyond the next 7-10 days. This can be seen by looking at the following map.
n the forecast office, we often track cold fronts with pressure maps like these. The examples are from 4AM and 10 AM today. You can see that the northern part of the front is moving more quickly than the southern end. The arrival of the front signals the start of the cooling trend that is spreading east.
The upper air flow over the East is from the west-southwest now, and some of the moisture associated with the cold front can be traced back to tropical storm activity off the Mexican west coast last week. The same moist air mass set the stage for recent flash floods in Arizona. By early next week, the upper-air flow will be coming to the Northeast region from well up in western Canada, as seen on this forecast map forecast map for next Tuesday: