A low pressure wave that had developed along a cold front continues to track into Quebec in Canada this Thursday afternoon.
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Moist southerly flow ahead of the front combined with instability from an approaching upper trough has triggered a couple of lines of heavy showers and embedded thunderstorms across upstate new York.
The heavy rain and thunderstorms are forecast to move through interior New England this evening/tonight over where the wet ground is especially sensitive to additional heavy rainfall.
A Moderate Risk of Excessive Rainfall (level 3/4) is maintained for southern Vermont and adjacent Upstate New York.
A separate Slight Risk (level 2/4) remains in effect for the upper Ohio Valley where another focused area of heavy rainfall is expected.
Strong flow aloft as well as moderate instability will also lead to a few robust, organized thunderstorms capable of large hail, damaging winds, and a couple of tornadoes for the Interior Northeast/Upper Ohio Valley.
A Slight Risk of severe weather (level 2/5) has been issued by the Storm Prediction Center for the region.
The heavy rain threat will shift slightly southeastward Friday ahead of the the slow-moving cold front with a Slight Risk of Excessive Rainfall for southern New England and the coastal Mid-Atlantic.
The prevailing upper-level pattern across mainland U.S.
will continue to feature periodic intrusions of cool air from central Canada into the northern tier states.
This pattern will tend to maintain broad troughing across the eastern two-thirds of the country with the main trough axis edges toward the East Coast with time as an anomalously strong upper ridge builds over the Southwest.
Rounds of showers and storms between these synoptic systems will likely continue across the midsection of the country.
High moisture to the south and pooling along the boundary will help promote very heavy downpours with intense rainfall rates and the risk of flash flooding.
The tendency for storms to grow upscale into organized convective systems will increase the areal coverage of rainfall and heavy rain totals.
Meanwhile, an ongoing convective system will likely promote additional heavy rain and storms inland from the central to eastern Gulf Coast region where a Moderate Risk of Excessive Rainfall remains in effect over central Mississippi into tonight.
A Slight Risk is in place more broadly across the Lower Mississippi Valley to the central Gulf Coast through Friday with the potential more organized convective systems.
Further west, more isolated, scattered storms are expected, but pooling moisture and upslope flow along the foothills of the southern Rockies will provide instability for robust thunderstorm development later Friday into early Saturday with a slight risk of excessive rainfall into the southern High Plains.
Meanwhile, a stagnant upper-level high in place over the Southwest/northern Mexico that has led to hot temperatures over portions of the Southwest and South-Central U.S.
this week will continue to build in coverage, particularly to the northwest over the Great Basin/California/Pacific Northwest.
Widespread heat-related advisories and warnings are in place across the Central/Southern Plains and Lower Mississippi Valley as well as portions of the Desert Southwest as a significant heatwave intensifies across the southern tier of the country.
Forecast highs will be in the upper 90s and low 100s for locations in the eastern half of the Plains and the Lower Mississippi Valley which, when combined with very high humidity, will lead to oppressive heat indices in the 110-115 degree range.
A frontal boundary and convective outflows sliding into northern portions of the Southern Plains/Lower Mississippi Valley will likely bring a bit of relief Friday.
To the west, while there will be less humidity, air temperatures will soar much higher into the mid 100s for west Texas and New Mexico and into the 110s for the Desert Southwest.
A few record-tying/breaking temperatures are possible.
The heat will begin to build northward into California, the Great Basin, and the Pacific Northwest on Friday, with highs into the 90s and low 100s.
Heat advisories also remain in effect for south Florida where relatively warm temperatures in the mid-90s and heat indices up to 110 continue.
In addition to the oppressive heat during the day, overnight lows will remain abnormally warm, bringing little relief from the heat overnight.
Numerous near record-tying/breaking warm lows are expected.
Unfortunately, the long term outlook through the weekend and into next week is for an increasingly significant and oppressive heat wave.
In contrast, much cooler air from central Canada will dip further into the northern Plains by early Saturday. ■
A clipper system will move quickly across the northern Plains into the Midwest Friday and the Northeast by Saturday, bringing a wintry mix of rain and snow showers ahead of a sweeping cold front.