Quantitative Precipitation Forecast
Issued by NWS
Issued by NWS
175 FOUS30 KWBC 170114 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 914 PM EDT Mon Sep 16 2024 Day 1 Valid 01Z Tue Sep 17 2024 - 12Z Tue Sep 17 2024 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER THE CAROLINAS AND NORTHEASTERN NORTH DAKOTA AND NORTHWESTERN MINNESOTA... 01Z Update... ...Carolinas... With drier conditions now spreading across southeastern North Carolina and redeveloping heavy rains not expected, the Moderate Risk previously centered over Cape Fear was removed with this update. However, a Slight Risk was maintained further to the north and west. Deep moisture (PWs 2+ inches) and sufficient instability (MLCAPE 1000-1500 J/kg) coincident with strong easterly inflow north of the low previously known as Potential Tropical Cyclone Eight continues to support widespread rain from eastern to central North Carolina. However, rainfall rates have come down over the past few hours. MRMS shows some pockets of 1+ in/hr rainfall rates, but these are largely disorganized. Given the available moisture and instability, periods of heavy rainfall will likely remain a threat through the overnight, and isolated to scattered flash flooding cannot be ruled out. Some members of the hi-res guidance suite, including recent runs of the HRRR, show heavy amounts and suggest a greater flash flood threat from the Crystal Coast northward overnight. But their output compared with current observations suggest these models are overproducing. Further to the west, steady moderate to heavy rain will continue to shift west along the upper center back into the western Carolinas. Strong forcing along with orographic enhancement may support some heavier totals overnight, with the HREF indicating amounts of 2-3 inches are likely. However, limited instability should hamper rainfall rates and the threat for widespread flash flooding. ...Elsewhere... Elsewhere, made smaller adjustments based on current observation trends. This includes northeastern North Dakota and northern Minnesota, where a convective heavy rainfall threat continues to develop this evening. Refer to WPC MPD #1026 for additional information regarding the heavy rain and flash flooding threat across this region. Pereira Previous Discussion... ...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 SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTHERN MID ATLANTIC... 2030Z Update... With the Southeast/Mid-Atlantic system becoming increasingly disjointed between the surface and upper feature...rainfall amounts should be tapering off...so a Marginal Risk area is still generally fine. The exception tends to be over southeast Virginia where models have started showing a signal for some enhanced rainfall...so maintained a Slight risk there. The signal for even less rainfall and a corresponding drop in the threat of excessive rainfall allowed amount of territory to be removed from the Marginal across the northern Mid-Atlantic and parts of New York. The Marginal Risk for Montana continues to be supported by the latest model runs showing low pressure forming over northeast Montana that reaches the international border during the latter part of the outlook period. Bann Previous Excessive Rainfall Discussion... ...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... 2030 UTC Update... Focus for the most organized precipitation with a corresponding threat of excessive rainfall will be in parts of the Northern Rockies...where a well-defined deformation zone sets up to the west of a well organized area of low pressure in southern Canada. Precipitation amounts in excess of 3 inches appears in most of the broader scale model QPF and even a few 5 inch amounts appear in the high resolution guidance. This amount of QPF remains worthy of Slight Risk and may require an upgrade if the guidance continues to inch up. The latest guidance shows even moderate rainfall amounts becoming scant...allowing much of the northern part of the Marginal risk area to be removed. If trends continue...more area may be removed. However there are still a couple of pieces of guidance which manage to produce locally heavy rainfall amounts in proximity to a trowal near the Mid-Atlantic so felt it was too early to remove entirely. Bann Previous Excessive Rainfall Discussion... ...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 from the Marginal Risk 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