Thursday 10 a.m.
The upper-air feature that has been pumping warm, moist air northward through the Eastern states is getting kicked northeastward by a wider trough that is taking shape farther west. The latter feature is sponsoring the southward push of colder air. Near the core of the cold air mass, temperatures dropped as low as the teens in parts of Montana. A few places had their coldest morning since last winter. In eastern North Dakota, Fargo was hit by gale-force, icy wind gusts carrying the first snow of the season.
The main thrust of the cold air will be across the upper Great Lakes and Ontario, but the leading edge of the change to cooler weather has already reached northern Texas. At Amarillo, it was 64 degrees at midnight with the wind out of south. At 8 a.m. CDT, it was 49 with a north wind gusting past 40 mph! The change to chilly will not be as dramatic when it reaches the East Coast, but it will certainly be noticed. Temperatures will be well up in the 70s in Boston tomorrow... and near 80 degrees in Philadelphia and Washington, D.C. On Sunday, temperatures in those cities will be in the 40s and 50s.
This morning's NMM-WRF run shows the band of rain associated with the leading edge of the change to colder weather crossing the Appalachians by midday Saturday. The rain shown on the map is the predicted amount from 8 a.m. until 2 p.m. EDT.
In the six-hour period after this (2 p.m. until 8 p.m. EDT), the rain band almost totally vanishes. I interpret the maps as suggesting I may not actually need the rain jacket that I will carry along anyway to the Penn State - Northwestern game on Saturday. I always carry the jacket in my golf bag so that if it does happen to rain, it looks like I predicted it all along!
The cold front is embedded in a large area of clouds, showers and thunderstorms covering the middle of the country. It will advance past the Northeast coast by the end of the day Sunday. The frontal zone is shown on this picture from late morning Friday.
When weather systems are relatively weak, small scale variations cause forecast uncertainty because several different weather types (such as showers, sunshine, cloudiness, etc.) can coexist in the same region and change constantly. This map shows such a pattern:
Some of the thunderstorms can become severe, with damaging wind and brief cloudbursts of rain. The greatest chance for locally severe storms should be in the "S" areas highlighted on this map (based on the NWS Storm Prediction Center's guidance).
There is a slight risk for severe thunderstorms later today from north-central Tennessee up across Indiana and Ohio to Michigan and eastern Wisconsin (shown by the "S" area on the map below. Thunderstorms are not predicted for areas near the coast from Delaware to New England.
It is not going to snow any time soon, but in any type of weather the flag is a symbol of freedom. This holiday weekend we celebrate the contributions of those who were there to defend the freedoms we enjoy in these times.
Once again, the rain will miss much of central and northern New England. The region has been in a dry spell, as evidenced by its appearance on this U.S Drought Monitor map.