At the beginning of the short-range forecast period July 12 to July 14, a low pressure center was located over the Middle Mississippi Valley which will continue slowly to the north-northeast over the next couple of days, U.S. National Weather Service said.
A warm front extended east through the Ohio Valley and into the Mid-Atlantic, with a cold front extending southwestward through the Lower Mississippi Valley and the Southern Plains.
More unsettled weather is expected in the Northeast over the next couple of days as the slow moving warm front advances northward bringing anomalously high low-level moisture (dewpoints in the low to mid-70s).
Widespread heavy rain with embedded thunderstorms are likely.
A Moderate Risk for excessive rainfall is in effect for the greater New York City area Monday.
A Slight Risk is in effect for additional portions of eastern Pennsylvania and New York as well as southern New England.
Heavy rain totals of 2 inches and higher over an urbanized area which recently saw heavy rainfall, leading to saturated soils, is a significant concern for additional flooding.
Heavy rainfall will also be a concern across central portions of upstate New York east through southern Vermont and New Hampshire, where some isolated amounts of 2 to 4 inches are possible through Tuesday morning.
For Tuesday, a Slight Risk for excessive rainfall is in effect for portions of upstate New York east of Lake Ontario to the Adirondacks, where isolated rain totals of up to 4 inches are also possible.
This may lead to some potential flooding and rises on area rivers, especially over river basins that have received above average rainfall over the past several days.
Marginal Risks for excessive rainfall are in effect further west along the warm front and ahead of the cold front from the Lower Great Lakes south to the Ohio Valley for widespread rain of around 1-2 inches over the next couple of days.
Heavy rain and potential flooding is also possible from the Southern Plains to the Southeast as the cold front progresses slowly eastward.
A Slight Risk of excessive rainfall is in effect for the the Lower Mississippi Valley and the Tennessee Valley on Monday as mean flow roughly parallel to the front may lead to repeated heavy rainfall over some areas.
Rain totals of 1-2 inches are possible with some isolated amounts of up to 4 inches.
Excessive heat will continue to make weather headlines across the western third of the nation.
A stubborn upper-level high anchored over the Desert Southwest is the primary factor in the ongoing heat wave, but this is expected to slowly erode some going into the beginning of the week.
Highs in the 110s in the Desert Southwest and central California Valleys today should dip back down into the mid-100s for Monday and Tuesday.
Widespread heat will also continue over the Great Basin, with highs in the mid-100s Sunday and the upper 90s to 100 Monday and Tuesday.
However, this small relative cool down is of little relief to areas that have seen long-term oppressive and above normal temperatures.
Excessive heat warnings remain in effect for a majority of locations in the region through Tuesday.
Relative above normal temperatures look to hold on the longest over the Interior Pacific Northwest, with highs in the upper 90s to 100 through the forecast period.
It is also worth noting that overnight lows have also been impressive with this heat wave, with lows failing to drop below the lower 90s for portions of the Desert Southwest.
Some relief from the extreme heat may arrive in the form of monsoonal showers and thunderstorms each day across portions of Arizona and New Mexico, which will increase in coverage and intensity through the period.
A Marginal Risk for excessive rainfall is in effect on Monday with a Slight Risk in effect for Tuesday.
The greatest threat is for heavy downpours with rain rates of up to 2 inches per hour that may lead to the rapid onset of flash flooding, especially for arid locations where such rain rates are a more rare event.
In addition to the threat for flooding, severe weather is also possible.
The Storm Prediction Center has issued a Slight Risk for severe thunderstorms today for the threat of high winds and hail, with a Marginal Risk in effect for Monday.
Elsewhere, below normal high temperatures for July are expected for the Midwest and Central Plains Monday and the Midwest and Southeast Tuesday behind the current storm system, with highs mainly in the 80s.
The chance for showers and thunderstorms will increase across the Northern and Central Plains as well as the Central Rockies by Monday evening and into the day Tuesday as a cold front moves south across the region from Canada. ■