Quantitative Precipitation Forecast
Issued by NWS
Issued by NWS
795 FOUS30 KWBC 190100 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 900 PM EDT Wed Sep 18 2024 Day 1 Valid 01Z Thu Sep 19 2024 - 12Z Thu Sep 19 2024 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF CENTRAL MONTANA... ...Montana... ...0100 UTC Update... Based on the observational trends, was able to trim the western edges of the Slight and Marginal Risk areas a bit. Deep layer instability within the TROWAL is rather weak/shallow, thus the short term rainfall rates have been lacking somewhat (Isolated 1" amounts within 3 hours). Given the persistent coverage however, along with the low FFG values (many areas 0.50" or less in 3 hours), will maintain the Slight Risk over north-central MT. Hurley ...Previous Discussion... Current WV satellite imagery is a sight this evening with a broad upper circulation engulfing much of the Inter Mountain West through Big Sky Country in MT. Height falls have been impressive with this disturbance as it pivots northeastward with an intensifying dual surface low reflection located across Central MT with pressures now down below 1000mb. The expectation is for the cyclone to continue to intensify through the evening with an occlusion phase occurring later this morning allowing for a strong, well- defined deformation axis to center itself over the Northern Rockies, east of Great Falls. PWAT anomalies approaching the upper end of climatological peak for not just this time of year, but all years according the latest NAEFS and ECENS moisture return intervals. PWAT anomaly forecast is between +3 to +5 deviations, a solid 97th to 99th percentile outcome given climatology which is impressive to say the least. The strong large scale forcing focused on the western side of the occluded cyclone will induce a focused TROWAL across the aforementioned area above, in good agreement amongst all guidance, including the CAMs and global deterministic. Regardless of which model output you assess, the prospects for 3-5" of rainfall with locally higher are being forecast in-of the terrain east of Great Falls and the I-15 corridor in MT. Latest HREF probabilities are pretty stout within both the neighborhood and EAS fields across that general area with the neighborhood probs running between 50-80% for >5" and even some 15-30% probs for the >8" totals being expressed. EAS probabilities are >80% for at least 2", a testament to both the expected rainfall and the consistency among the CAMs. This is no different from the totals expressed within the global deterministic, and even the ensemble blends are signaling over 4" in places. The bottom line is; this will be a pretty formidable storm for the Northern Rockies and adjacent Great Plains of MT with scattered instances of flooding increasingly likely as we move through the daytime, Wednesday. The storm will drift north and begin losing steam the back end of the forecast leading to the organized TROWAL to fade with lighter rates before tapering off later in the evening. The previous SLGT risk was maintained given the strong continuity from guidance. The only limitation to this being a higher risk is the lack of sufficient surface instability with most of the positive buoyancy relegated above the boundary layer. Still, rates of 1-1.5"/hr are still possible in the stronger cores, enough to solidify the SLGT risk forecast. ...Mid Atlantic... The period can be characterized by uncertainty within the guidance as a disheveled surface and mid-level reflection left over from what was Potential Tropical Cyclone 8 (PTC8) continues to meander over the Mid Atlantic with a broad PWAT anomaly positioned across Northern NC through the Delmarva. The interesting aspect of the forecast is multiple deterministic outputs have some prospects of heavy rainfall across parts of the Mid Atlantic, but there is a large disagreement on where exactly that could take place. The issue is the lack of an organized surface reflection that can help bring everything together, but still has a presence that can allow for smaller zones of opportunity to crop up. Assessing the ensemble QPF has helped curb some of the discontinuity within the deterministic leading to two areas of focus within the next 24 hrs. The first is across the coast of NJ where an axis of convective potential seems reasonable as the weak surface low analyzed over Eastern NC begins to drift northeast with some resemblance of organization as it enters over the Gulf stream. A modest jet streak is forecast to develop over the Western Atlantic with the surface low likely to lie within the left exit region of the jet allowing for some modest strengthening as it moves northeast off the coast of DE/NJ. The synoptic evolution is moving towards the "classic" axis of deformation prospects as the low begins to intensify and the western side of the cyclone exhibits a band of heavier precip close to the DE/NJ coastal plain. This was outlined within the latest HREF ensemble blended mean QPF with the matched mean QPF displaying a relatively significant heavy QPF signature within the Southern and Central NJ coast impacting areas from just north of Cape May up through Atlantic City to just to the east of Toms River. These areas aren`t as flood prone compared to some of the focused urban corridors inland, but the combination of some heavy surf with rainfall rates between 1-2"/hr could cause some localized flooding within the coast. Considering some of the uncertainty, the only risk to add was a MRGL as the probabilities for >3" and >5" are still favorable (40-60% and 15-30% respectively), but with only modest agreement amongst guidance, it would be hard to issue anything higher. The second area of interest lies within much of Northern and Eastern VA where the closed 500mb low will continue to spin south of the region prompting some weak ascent focused over a region of relatively modest instability and deep layer moisture. This is another scenario where the guidance has precip, but the focus is anywhere from Southern PA down to Hampton, VA. HREF probabilities signal a broad axis of 40-60% probs for >3" somewhere from the WV Panhandle down to Virginia Beach with some low end >5" probs lingering within the same corridor. Ensemble QPF footprint depicts an areal average of 1-1.5" located across the above zone with a maxima centered over Northern VA. The prospects for heavy rainfall also include areas along the Potomac, including the DC metro which has notoriously lower FFGs thanks to urban sprawl. There is too much uncertainty again for anything more than a MRGL risk, however the environment is favorable for somewhere to see a heavy cell or two that could cause issues. For that, have opted to add a second MRGL risk within the Mid Atlantic with points between the two risk areas void from the threats due to a greater model consensus pointing at a relative QPF min within that area of the Delmarva. ...North-Central Sierra and Western NV... Another broad closed upper low will come ashore over Northern CA from the Pacific leading to increased moisture and favorable upper forcing across the terrain of CA/NV beginning later this afternoon. Despite cooler surface temps, steep low and mid-level lapse rates with a decent outline of positive buoyancy within the forecast soundings out of KREV signal the potential for some heavier convective cell initiations across the North-Central Sierra over into Western NV. There has been an uptick in the ensemble QPF across this area the past succession of runs leading to a bit more potential than what was thought prior. Historically, including with the past system, we`ve seen these strong upper lows tend to overachieve at times with the magnitude and spatial coverage of the convection downstream. In this case, it`s favoring the prospects for locally heavy rainfall in the complex terrain and over an area of lower FFGs like Reno which carry the urbanization factor. The latest UFVS First Guess Field has also introduced a recommended MRGL risk for the first time in its runs as of late with the uptick in the signal. A MRGL risk was added to account for the threat with the threshold still on the lower half of the risk spectrum. Kleebauer Day 2 Valid 12Z Thu Sep 19 2024 - 12Z Fri Sep 20 2024 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE TRANSVERSE RANGES OF CALIFORNIA AS WELL AS FOR THE URBAN CORRIDOR OF SOUTHEAST FLORIDA... ...California... A strong slow-moving cutoff low will drift southeastward down the coast of California on Thursday. Limited, but still notable amounts of Pacific moisture will advect into southern California on southwesterly flow ahead of the low, where the forcing will be maximized. Rainfall into central California today will drift southward towards the Santa Barbara region by Thursday. Burn scars in this area will greatly enhance the flood risk locally should persistent upslope flow result in long-duration rainfall. For this reason, the Marginal Risk was hoisted for the Transverse Ranges north of Oxnard. Resultant flash flooding should be isolated, due to limited moisture and burn scar support, but the threat will be high enough that local impacts from any flash flooding are probable. ...Florida... A stalled out front over Florida will interact with moisture with a tropical wave on Thursday. This will result in more widespread and heavier storm coverage when the usual afternoon storms form. South Florida has seen several afternoons of storms this week, so a more active day on Thursday could result in isolated flash flooding. The threat will be greatest along the urban I-95 corridor from West Palm Beach south to the Keys. In addition to timing of storms and urban concerns, flooding will be most likely at the time of high tide, when drainage from the adjacent beaches and urban areas will be hindered due to the higher sea levels. In coordination with MFL/Miami, FL forecast office, a Marginal Risk has been added. Wegman ...Previous Discussion... A non-zero flash flood threat does exist across portions of the Upper Midwest, however the antecedent dry conditions will allow for much of this rain to be beneficial over the area of interest (Northern IA, Southeastern MN, Western WI, and the Arrowhead of MN). Urban corridors will be the most favorable spots for flash flood prospects. Rates generally capped around 1-2"/hr, falling shy of the current FFG forecast in the area. If the signal becomes more impressive in the next 24 hours, a targeted MRGL is possible. Kleebauer Day 3 Valid 12Z Fri Sep 20 2024 - 12Z Sat Sep 21 2024 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR THE URBAN CORRIDOR OF SOUTHEAST FLORIDA... The stalled out front over south Florida will remain in place again on Friday. Once again the convection which will be capable of flash flooding will be generally during the afternoon and evening, over the urban I-95 corridor from West Palm Beach south, and at high tide. Otherwise, the weather pattern remains status quo with very little changing day to day. Wegman 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