Quantitative Precipitation Forecast
Issued by NWS
Issued by NWS
177 FOUS30 KWBC 161559 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1159 AM EDT Mon Sep 16 2024 Day 1 Valid 16Z Mon Sep 16 2024 - 12Z Tue Sep 17 2024 ...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER SOUTHEASTERN NORTH CAROLINA... ...Carolinas... Potential Tropical Cyclone Eight continues to churn off the Carolina coast with an ill- defined...north to south elongated center of circulation that has largely become exposed with broad shield of rain to the north. The first push of higher rainfall rates has started to impact mainly coastal areas of the North Carolina near the border with South Carolina...with a few ground- truth in excess of 6 inches. Current thinking is that the potential for intense rainfall rates will continue to be focused along the convergent flow of east/northeast flow off the Atlantic and the flow of drier air wrapping around the southern/eastern flank of the system. The steepening lapse rates and the added low level convergence between the two streams should be working in tandem to support isolated areal coverage of additional 4 inches to 8 inches of rain mainly confined to areas along the immediate coast. Farther inland across North Carolina and adjacent South Carolina...rainfall amounts are not likely to be as high given lower dewpoint temperatures/precipitable water values but still enough rainfall that can result in flooding/flash flooding. The amount of dry air getting pulled eastward/southeastward on the southern flank of Potential Tropical Cyclone Eight has allowed confining the Moderate Risk area along and north of the North Carolina/South Carolina border. The latest HREF signal for >5" remained very robust with probabilities between 70-90% located across southeastern North Carolina. As previously noted...greatest concern for flash flooding remains the urban areas. ...Northern Plains and Upper Midwest... Aloft, a shortwave will track from the Central Plains to the Northern Plains later tonight. This should help renew convection over portions of the Dakotas to northwestern Minnesota later this evening. Given ample mid-level ascent conducive for convective initiation, a pre-frontal surface trough that is expected to develop over the southern Canadian Prairies that builds into the Dakotas, and a formidable corridor of high theta-E that bisects the area, some enhancements of rainfall rates could result in instances of flooding. ...Western U.S... 12Z upper air analysis indicates a broad closed upper circulation positioned over northern California with an expectation that it will continue to strengthen throughout the period. Increasing moisture on the eastern flank of the mean trough will allow for a better convective environment as the region enters a strong upper forcing regime with a correlated instability maximum located over the Great Basin. The approach of the upper low will initiate a blossoming of convection across the eastern and northern Sierra, extending into Nevada and eventually Idaho through this evening. MUCAPE between 500-1000 J/kg will be most common within the interior, plenty to induce a swath of enhanced convection over an area not typically acclimated to see a decent amount of rainfall. The current forecast calls for a multi-region impact of convectively driven rainfall that could easily exceed 1" in any area located from the Great Basin, Four Corners, and portions of the Southwestern U.S. ...Deep South... Lingering low-level moisture with the proximity of a quasi- stationary front across the Deep South will allow for another round of locally enhanced rainfall that could cause some flash flood concerns, generally over the Lower Mississippi Valley into the Central Gulf coast. The signal is more diffuse than previous days but the probability fields still indicate a formidable potential for >3", especially along the Gulf coast where the surface based instability is most prominent. Maintained the low end Marginal risk area with little change...especially given some of those areas were hit hardest the past several days from Francine and its remnants. Bann Day 2 Valid 12Z Tue Sep 17 2024 - 12Z Wed Sep 18 2024 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE MID ATLANTIC, FLORIDA PANHANDLE, SOUTHERN ROCKIES, AND WESTERN MONTANA... ...Mid Atlantic... The remnant cyclone will be perusing through the Mid Atlantic come Tuesday with its moisture flux steadily pushing northward through the Central Mid Atlantic, carrying a potential for locally enhanced rainfall and flash flood prospects. As of this time, the consensus is still muddled on exactly where the remnant circulation will be progressing, as well the forecast instability positioning on the eastern flank of the circulation. PWAT anomalies will be creeping closer to 1-2 standard deviations above normal, a historical characteristic for heavy rain and flash flood concerns within the Mid Atlantic. Ensemble instability fields are generally more defined in-of the Blue Ridge in VA and across Southeastern VA. These would likely be the areas with the most interest when it comes to any higher risk. The shifts within the ensemble mean QPF add too much variance to conclude the prospects for a higher risk, at this time. With coordination from the local WFOs across the Mid Atlantic, a MRGL was maintained from the previous forecast with some adjustments based on the latest ensemble QPF footprint. ...Florida Panhandle... Lingering low-level moisture flux tied a quasi-stationary front and subsequent weak surface reflection along the Gulf Coast will allow for one more period of isolated to widely scattered thunderstorms within the Western FL Panhandle, bisecting an area that is currently under more sensitive FFG indices compared to climatological averages. The most prone location resides near Apalachicola where recent 5-day rainfall totals are upwards of 8-15" leading to a fairly water-logged position along the immediate Gulf coast. Despite the signal not being as impressive as previous periods, the instability gradient right along the coast could provide the necessary element to maintain a risk for locally heavy rainfall, especially any cells tied to the boundary running parallel to the coast. This is a lower-end MRGL risk categorically, but with forecasts within the deterministic still pointing towards cells producing >2" of rainfall, this was enough of a signal to warrant the MRGL risk inherited from the previous forecast. ...Southern Rockies... Ejecting shortwave trough out of the Four Corners will provide ample mid and upper support for a generation of scattered convection located across the Southern Rockies down into the Sacramento Mountains in NM. The setup is geared towards locally enhanced rainfall prospects tied to the terrain with the best opportunity for impacts likely occurring over burn scars located within the Sangre de Cristos and the Sacramento Mountains near Ruidoso. Those two locations are very sensitive, and the setup would certainly be conducive to providing local impacts within the two separate MRGL risk areas. Areal QPF average is between 0.25-0.5", but deterministic output still signals potential for upwards of 1.5-2" of rainfall, plenty to cause issues within the complex terrain and over any sensitive grounds. A MRGL risk was added over the above locations to account for the threat. ...Western Montana... Approaching closed upper low over the Great Basin will pivot northeastward while tilting negative as it progresses into the Interior Mountain West, eventually situating over MT by the end of the forecast cycle. Large scale ascent pattern will be maximized downstream of the mean trough with ample mid-level moisture advection being carried poleward into Big Sky Country, pushing PWAT anomalies into the +2 to +3 standard deviation range. This is a prominent signal when coupled with a strong upper forcing regime and is co-located with an increase in boundary layer instability when assessing the area MUCAPE field(s). Current QPF distribution signals a scattered array of 1-2" totals with the most precip footprint of the maxima tied closer to the terrain thanks to the orographic elements typically favored in these types of setups. The convectively driven pattern is one that maintains prominence on local flash flood prospects and still has some room for a targeted higher risk pending the convective outlook once into better range of the CAMs. For now, maintained general continuity with some extension on the southern periphery of the risk area over MT. Kleebauer Day 3 Valid 12Z Wed Sep 18 2024 - 12Z Thu Sep 19 2024 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER NORTHWEST MONTANA... ...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-of the Northern Rockies encompassing Northwest MT. Ensemble mean QPF is quite impressive given the synoptic regime with very little surface based instability to work with. The whole pattern is driven by powerful upper forcing within a well-defined axis of deformation on the western side of a broad, occluding surface cyclone. Precip totals off the latest deterministic are gaudy with a wide swath of 2-4" with locally as high as 5+" located within the terrain to the east of Great Falls. The current solution is generating pQPF 90th percentile output of 100yr ARI`s located within the Rockies to the east of the valley up in Western MT. With multi-day totals bordering 6-8" across portions of MT, a SLGT risk was added for the time being as we still have some time prior to eventual impacts. That said, it`s a period to monitor as the current consensus is pretty bullish with a general agreement to the setup within much of the 00z deterministic suite. A higher risk is not out of the question, especially if the magnitude of the forecast rainfall maintains through multiple model cycles, and/or becomes even more aggressive in the signal. ...Central Mid Atlantic... The remnants of the tropical disturbance will linger across the Mid Atlantic with some locally heavy rainfall prospects still in the forecast. There is a great amount of uncertainty on specifics, so the current forecast is based off the latest ensemble blend with a focus on the potential most likely closer to the coast where instability is highest during the period. For now, wanted to maintain some continuity with the prospects still non-zero with more time to assess and hone in more as we move closer into the period. Kleebauer 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