Quantitative Precipitation Forecast
Issued by NWS
Issued by NWS
162 FOUS30 KWBC 180050 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 850 PM EDT Tue Sep 17 2024 Day 1 Valid 01Z Wed Sep 18 2024 - 12Z Wed Sep 18 2024 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER SOUTHEAST VIRGINIA... ...Mid Atlantic... Occluded low wrapped up over South Carolina will continue to funnel moisture off the Atlantic on easterly flow into the Mid- Atlantic. Sizable area of >4" rainfall has already fallen over northeastern NC but has largely moved northward to the VA/NC border as of 0030Z. Still, with a saturated column and PW values >2" the convergent flow could still yield some heavier rain totals through the overnight hours, so the Slight Risk outline remains (but focus a bit more toward and east of I-95. Broader Marginal Risk area covers the region westward to the Appalachians/Blue Ridge where additional rainfall is likely, though not as heavy as near the coast. ...Florida Panhandle... With the loss of daytime heating and diminishing activity, removed this Marginal Risk area for the overnight hours. ...Southern Rockies and High Plains... Trimmed the Marginal Risk to just northeastern NM into the TX/OK Panhandle with a small expansion to the northeast, given recent trends. With the lead line of rainfall already through the region, upstream echoes could creep toward now lowered 3-h FFG values in the SW flow. Most sensitive areas will still be around terrain areas of northern NM but also into far SW KS this evening/overnight should any convection flare up enough again. ...Northern Rockies and High Plains... Robust closed upper low over the northeastern Great Basin will pivot into MT overnight and continue to deepen, favoring strengthening of the system. Some rainfall has fallen over much of the region, but more vigorous dynamics should support some higher rates and an expanse of the precip shield overnight, and especially into D2 (Wednesday). Will keep the Marginal Risk for the region with a slight eastward shift into the warm sector to cover any northward-moving and potentially training cells this evening. Fracasso Day 2 Valid 12Z Wed Sep 18 2024 - 12Z Thu Sep 19 2024 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF MONTANA... Negatively tilted mean trough and closed low over the Northern Rockies will become vertically stacked over Big Sky Country leading to a prolific axis of deformation developing in central MT on Wednesday. A TROWAL wrapping around the western side of the deepening surface low will promote increase lift within a region of anomalously high IVT (above the maximum climatological percentiles for all hours per the 12z GEFS). PWATs are also forecast to reach above the 95th percentile as actual values peak in the 1.00-1.25" range. Ensemble mean QPF is quite impressive given the synoptic regime has very little surface based instability to work with and low MUCAPE (less than 200 J/kg). Precip totals off the latest deterministic are gaudy with a wide swath of 2-4" with locally as high as 5-8" located within the terrain to the east of Great Falls. There has been some adjustments within the ensemble means for the heaviest axis of QPF with a small displacement to the east. When assessing the 500mb height anomalies between ensemble runs and the current deterministic, there is a subtle, but noticeable difference in the closed upper reflection being a bit more into East-Central MT leading to a heavier QPF distribution being a bit further east than previous iterations. This also follows with the ML outputs recently with the trend having the axis of deformation a bit further east of Great Falls, a general marker for where the heaviest precip will focus. Ensemble bias corrected QPF is also a touch east with totals relatively similar in the 2-4" range with a max of just over 5". This will be something to monitor going forth, but the signal is still very much present for a locally impactful rainfall thanks to a strong mid- latitude cyclone entering the interior Northern U.S. A SLGT risk was maintained with an additional small extension eastward to reflect the recent trends in guidance. Kleebauer/Snell Day 3 Valid 12Z Thu Sep 19 2024 - 12Z Fri Sep 20 2024 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Snell 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