Quantitative Precipitation Forecast
Issued by NWS
Issued by NWS
000 FOUS30 KWBC 140809 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 409 AM EDT Sat Sep 14 2024 Day 1 Valid 12Z Sat Sep 14 2024 - 12Z Sun Sep 15 2024 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHEASTERN U.S... ...Southeast... Current radar/satellite composite depicts a broad cyclonic gyre situated across the Southeastern U.S with a north-south band of convection bisecting the a good portion AL into Western TN around the northeast periphery of the remnant cyclone. The surface reflection from what was Francine will become more diffuse over the next 6 hrs with a degrading surface signature, leaving much of the flow tied to the leftover mid and upper lows associated with the storm. That said, the cyclonic spin from the tandem 850-500mb low(s) will allow for a continued favorable moisture stream around the circulation with small mid-level perturbations stuck over the same areas allowing for locally enhanced ascent within the confines of AL/GA/TN. PWATs generally residing within the +1 standard deviation anomaly across the Deep South will be sufficient enough within the pattern to yield another round of scattered to widespread convection across similar areas that were impacted today. The difference is the magnitude of the convective pattern will be a bit less vigorous thanks to the weakening mid and upper low centers as denoted within all the recent guidance height fields. Still, the combination of the moisture anomaly and semi- favorable ascent under the remnant mid and upper lows will be plenty to offer another round of convection across the Deep South. To the northeast, a stout surface ridge will nose down further into the interior Southeast with a formidable easterly regime focused in-of Northern GA down into the Northwest side of AL. The combination of the cyclonic flow from the mid-level remnants and wedging associated with the surface ridge will create a tightened axis of convergence within the state of AL down into Western GA. The consensus this evening within the 00z CAMs suite was very well represented within the HREF mean QPF and associated probability fields, highlighting the semi-narrow corridor of higher rainfall potential located just to the south and west of Huntsville, bisecting much of North-Central AL down into the Southwest side of GA. This area has been the beneficiary of some significant rainfall the past 24-48 hrs leading to a degraded FFG footprint where flash flood prospects will be greater, relative to average. Signals for 1-2"/hr rainfall rates would be sufficient given the antecedent conditions across the area, and with the setup likely to garner some locally enhanced rainfall rates >2"/hr as indicated within the prob fields, there`s a higher likelihood for continued flash flood concerns within the confines of those areas in AL and GA mentioned above. A higher end SLGT risk is the forecast for those areas extending from Northwestern AL near the AL/MS line, southeast through Columbus, GA, an area that includes the Birmingham metro. SLGT risk extends further northwest and southeast, respectively with a broad MRGL extension around the periphery as heavy rain concerns exist from as far north as Western KY to as far Southeast as Northeastern FL. ...Arizona... A surge of deep tropical moisture from now Tropical Storm Ileana will make progress into the southern portion of AZ by the middle to end of the forecast period. With sufficient diurnal heating anticipated this afternoon, the environment across Southern and Southeast AZ will destabilize allowing for isolated to scattered thunderstorms to initiate in-of the terrain situated within Pima/Santa Cruz counties to possibly as far east as Cochise county in proxy to the Huachuca mountains. MUCAPE forecast is relatively favorable for convective development with the ensemble SBCAPE forecast between 500-1250 J/kg off the 00z HREF mean. The combination of increasing moisture anomalies and environmental buoyancy adds favor to an isolated flash flood risk, especially within the terrain and adjacent locales. A MRGL risk was maintained from previous forecast with only minor adjustments based within the HREF blended mean QPF footprint, and aligning with the edges of the 10% or great probabilities within the neighborhood probability output for >2" rainfall potential. This area will have more favorability later on D2. ...South Florida... Surface trough located in-of South FL will maintain a prevailing westerly flow across the Southern part of the Peninsula, placing the mean steering pattern pointed towards the population centers along the Southeast FL coast in time for the sea breeze convective initiation this afternoon. Area PWATs are very elevated this evening with the 00z sounding out of KMFL depicting a solid 2.40" PWAT reading placing the area within the +1-2 standard deviation anomaly for the period. This environment will be conducive for convective generation just inland along the penetrating sea breeze in the afternoon with locally strong convective cores producing rainfall rates between 2-3"/hr on initiation, moving towards the coast thanks to the prevailing steering flow. HREF probabilities for >3" are solidly between 50-80% along the population centers extending from PBI down through MFL with the highest probs situated between PBI and FLL. Lower end 5+" probabilities are also highlighting the area of interest, so the threat is within the middle grounds for flash flood concerns leading to an addition of a MRGL risk situated along the Southeast FL coast encompassing the population centers. Kleebauer Day 2 Valid 12Z Sun Sep 15 2024 - 12Z Mon Sep 16 2024 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTHEAST U.S... ...Southeast... Despite the remnant mid and upper circulation from Francine degrading in its presentation, another period of enhanced convective activity will spawn underneath the primary upper forcing lingering over the Southeastern U.S. There`s a bit more emphasis on an axis of convection developing across portions of the Lower Mississippi Valley in-of AR and MS, extending southeast into parts of the Deep South, mainly Southern MS and AL. Areal PWATs will remain elevated within +1 standard deviations, enough of an anomaly to spur up some potential heavier convective cores that could result in some flash flood prospects, especially over any urban zones where run off potential is greatest. Ensemble bias corrected QPF and the 75th percentile of the NBM indicate a fairly stout axis of heavier precip likely to occur under the influence of the upper low and remnant energy pivoting around the general circulation. The primary concerns will continue to be those locally heavy rates that will reach between 1-2"/hr with some intra-hour potential for heavier rates that typically cause more issues in this part of the country. A SLGT risk was added to the previous forecast with an adjustment southwest to the overall SLGT/MRGL risk areas to align with the current trends and ensemble forecast consensus on the heaviest precip location. The MRGL expands into the FL Panhandle to cover the area that has been hit pretty hard in the past 36 hours as isolated storms are still forecast for those areas that have seen copious amounts of rainfall in the above time span. ...Southwest... Remnant moisture from now Tropical Storm Ileana will continue to advect northward into the rest of the Desert Southwest with a trajectory towards the Four Corners, wrapping back eastward into NM during the afternoon hours Sunday. The ensemble QPF footprint upped the total precip forecast compared to the previous periods, a trend that was noted when assessing the QPF differential from 12z to 00z. This places the convective pattern towards those areas we`ve been highlighting all summer for flash flood concerns thanks to a plethora of burn scars located within the terrain across parts of NM. The previous forecast was relatively unchanged over AZ into the Four Corners, but the MRGL risk was expanded to include much of NM with a focus into the terrain over the Sangre de Cristos and the Sacramento Mountains. General QPF average between 0.25-0.75" is expected, but maxima of >1" will be plausible anywhere within the confines of the forecast risk area. A targeted SLGT risk is still possible, but the scattered nature of the convection forecast doesn`t instill enough confidence at this time. A reassessment with the addition of CAMs guidance could sway that the other way, so stay tuned for future updates. Kleebauer Day 3 Valid 12Z Mon Sep 16 2024 - 12Z Tue Sep 17 2024 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS EASTERN NORTH CAROLINA... ...Coastal Southern Mid Atlantic... A growing consensus for developing low pressure along the Carolina coast has popped up amongst the 00z deterministic with much of the global suite indicating surface cyclogenesis off the GA/SC coast, moving northwest towards the Eastern Carolinas with emphasis on North Carolina based on the latest runs. This is a fairly complex evolution but rooted within the proposed synoptic evolution as a strong block residing over the Northeast and Northwest Atlantic and steering pattern focused towards the Eastern Carolina coast sometime on Monday. The setup would allow for a steady easterly flow to preside over Eastern NC with the approach of the surface cyclone increasing a deep moist advective pattern in-of the coastal portions of the state. The National Hurricane Center (NHC) has marked this potential disturbance with a 50% chance of developing into a tropical system within the next 72-96 hours which would invoke a growing confidence in something more organized that could promote greater tropical characteristics that would undoubtedly allow for some locally greater impacts than forecast. As of now, the SLGT from previous forecast was maintained, but adjusted to the north to account for the ensemble shift in the low pressure track and within the core of the heaviest QPF. The area from Myrtle Beach up through Cape Hatteras will have to monitor the potential closely as the chance for locally significant rainfall amounts have been scaled up since the last update. For now, the SLGT risk will suffice with the opportunity for a higher risk, pending the expected evolution of the disturbance. ...Central Gulf Coast... A weak surface low is forecast to develop in-of the Central Gulf coast by Monday morning with an increasing axis of convergence within the northern flank of the circulation. Elevated PWATs along with prominent instability located along the Gulf coast will amplify the threat of some locally enhanced rainfall prospects between Southern MS through Southern AL into the Western FL Panhandle. The main threat will continue to be urban flooding, but will have to monitor the area around Apalachicola closely thanks to significant rainfall the past few days with more anticipated prior to the period. This has allowed for a sharp degradation of the FFG indices normally in place for the area. A MRGL risk was introduced given the sneaky threat with a lot contingent on the expected development of a weak surface reflection along a quasi-stationary front situated along the Central Gulf coast. ...Western U.S... Increasing moisture nosing north ahead of an approaching deepening long wave trough axis will allow for the opportunity for widespread convective development across much of the Southwest U.S, extending north through the Great Basin and the Interior Pacific Northwest. Signals have been growing steadily for a large scale ascent pattern that would favor locally heavy precip in-of the areas above, especially across NV/UT/ID thanks to a premium diffluent upper pattern approaching the West Coast in the form of a potent trough, turned closed low by Monday morning. The setup would be conducive for totals exceeding 1" over a large area that normally does not see these types of totals very often. The PWAT anomaly forecast within the ensembles are approaching 2+ standard deviations above normal with the latest NAEFS indicating upwards of +3 deviations located across parts of NV into ID by the end of the period. If this setup maintains continuity, there`s an expectation that a risk upgrade may be in order, especially since the upper pattern is favorable for widespread convective impacts into areas with climatologically low FFG indices. 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