Quantitative Precipitation Forecast
Issued by NWS
Issued by NWS
330 FOUS30 KWBC 200821 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 421 AM EDT Fri Sep 20 2024 Day 1 Valid 12Z Fri Sep 20 2024 - 12Z Sat Sep 21 2024 ......THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHERN UNITED STATES...... ...Southern California/Western Arizona Deserts... An upper low moving down the coast early today will move inland over the Desert Southwest later today. Increasing coverage of showers and thunderstorms are expected as moisture will accompany the low into the usually dry areas of southeast California, southern Nevada, and northwestern Arizona..with greatest coverage tied to the steepest low- to mid-level lapse rates at the center of the upper low where upper level temperatures will be coldest. That is also where steering flow becomes weak...leading to slow moving storms. With any more persistent showers and storms, expect isolated flash flooding with any burn scars, dry washes, and other flood-prone areas.PWATs between 0.5 and 0.75 inches in these areas will peak at about 1.5 sigma above normal for this time of year. ...New Mexico... Broad upper level diffluence ahead of the positively tilted low over the Desert Southwest will cause a somewhat narrow 50-100 mile wide plume of training showers and possibly embedded thunderstorms to develop over eastern New Mexico and into portions of the far western Texas and Oklahoma Panhandles. Given the sensitivities of some of these areas to prolonged rainfall, the Marginal risk remains in place, especially for any burn scars or flood prone areas. ...South Florida... Today looks to be the final day of heavy rainfall potential is expected in South Florida...especially in the urban corridor from West Palm Beach south through Homestead. Highly efficient rainfall producing thunderstorms should be slow-moving on the leading edge of an advancing cold front. The front will clear South Florida by early Saturday turning the predominant flow more northeasterly. This will cut down on the precipitable water values some, while also greatly limiting, though not eliminating the coverage of storms by Saturday. Thus, the Marginal flash flooding threat should end once the convection simmers down late this afternoon or this evening. Bann Day 2 Valid 12Z Sat Sep 21 2024 - 12Z Sun Sep 22 2024 ......THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS MUCH OF THE CENTRAL ROCKIES AND CENTRAL PLAINS...... A broad upper trough with evolution of a closed low over the West will translate eastward on Saturday with an evolving setup capable of showers and thunderstorms across the Rockies through the Central Plains, especially once the trough axis picks up forward speed. A combination of favorable ascent and increased moisture advection thanks to a developing LLJ pattern east of the Front Range will lead to thunderstorms capable of producing several heavy cores with locally enhanced rainfall. The 20/00Z HREF neighborhood probabilities...available only during the first half of the day 2 outlook period on the overnight shift...focuses brief periods of 30 percent or greater probabilities of exceeding 1 inch per hour developing out over the plains of eastern New Mexico that ease into West Texas by early evening...with the threat continuing into Saturday night in response to the development of a low level jet. Farther north...the signals for locally heavy rainfall also appear across portions of the Plains states as thunderstorms form along and ahead of a cold front tied to an amplifying upper trough building across the northern US. As mentioned in a previous discussion...antecedent conditions and the fact that much of the area is rural will help mitigate some of the flash flooding concern due to amount of infiltration possible. Bann Day 3 Valid 12Z Sun Sep 22 2024 - 12Z Mon Sep 23 2024 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM THE SOUTHERN GREAT LAKES SOUTHWESTWARD ACROSS PORTIONS OF THE CENTRAL PLAINS... An upper trough initially over the central and southern Rockies on Sunday morning will move eastward on Sunday into early Monday morning...with showers and thunderstorms developing ahead of the front in an airmass characterized by precipitable water values generally in the 1.6 to 1.8 inch range. Given the moisture in place and enough mid- and upper-level shear to support some convective organization...the possibility exists for excessive rainfall due to some intense downpours. Tended to favor the cluster of models guidance led by the ECMWF/GFS/CMC with respect to the potential for heaviest amounts somewhere from southeast Kansas into Missouri...but did not go as far east as suggested by the aggressive UKMET. Suspect there is a non-zero threat of excessive rainfall farther to the southwest where storms may also be able to organize but for now favored the better moisture and the region of better upper difluence. Bann Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt