Quantitative Precipitation Forecast
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Issued by NWS
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785 FOUS30 KWBC 160036 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 836 PM EDT Sat Jun 15 2024 Day 1 Valid 01Z Sun Jun 16 2024 - 12Z Sun Jun 16 2024 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE UPPER MIDWEST... Plains/Midwest... Across KS, a shortwave will assist in the primary ascent pattern that develop during peak diurnal heating through the overnight time frame. A generally modest environment currently will ramp up with the advection of richer theta-E`s and relevant instability as it pushes north with the aid of the nocturnal LLJ. Precipitable water values of 1.5-2" are expected across the region, which in organized convection would allow for hourly rain totals to 2.5" and local amounts to 5". Scattered flash flood signals are most likely the case with this kind of setup, but any organized clusters could promote a more significant areal extent for flooding prospects, especially if they make their way into the suburban and urban corridor centered over Minneapolis- St Paul. Given the above, the SLGT risk remains in place. The Marginal risks over North Dakota and the Southern Plains remain justified. Changes were minimal to the outlined areas, based on recent radar reflectivity trends and the 18z HREF guidance. The Southern Plains and North Dakota should see a general max of 1-3" is forecast in these locations with much of it occurring in short intervals. Roth/Kleebauer ...Washington Cascades... Upslope flow off the Pacific in combination with cool air aloft/nearby upper low will lead to showers and a few thunderstorms into western Washington over the next several hours. Additional lift from the Cascades may result in some widely scattered rainfall totals of 1-1.5 inches, which could result in localized flash flooding on burn scars and any canyons and fast- flowing creeks and streams. A Marginal Risk remains to account for this possibility. Wegman Day 2 Valid 12Z Sun Jun 16 2024 - 12Z Mon Jun 17 2024 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE NORTHERN PLAINS AS WELL AS THE CENTRAL GULF COAST... ...20Z Update... There were limited changes necessary to the previous SLGT risks in places across the Northern Plains and Central Gulf coast. Guidance remains fairly consistent in axis of enhanced QPF located over northern NE and SD, eastward into MN, northern WI and portions of the Michigan UP. Cold front will pivot southeast out of the Dakotas on Sunday before hitting a wall from the expansion of ridging that will dominate the eastern half of the CONUS. Boundary will stall in-of the Sand Hills region and points northeast acting as a broad surface focal point with a multitude of mid-level perturbations ejecting northeast out of the base of the mean trough to the west. Increasing LLJ presence across the plains will advect deep moisture to the north and northeast with the best instability residing along and south of the stalled frontal boundary. Scattered to widespread convection will develop and propagate to the east and northeast with QPF maxima relatively confined to near the front. A persistent signal for 1-3" with locally as high as 4" is forecast across the aforementioned corridor with the max being depicted over eastern SD as of the latest HREF probabilities (45-70% for >2" totals). This was plenty of confidence to maintain the previous SLGT risk over portions of the northern CONUS. Across the Central Gulf coast, anomalous moisture flux pivoting around the western fringes of a prominent surface and mid-level ridge will creep northward into the Gulf coast areas extending from far Southeast TX through southern LA/MS/AL. PWAT anomalies will be surging from +1 standard deviations at the beginning of the period to as much as +3 deviations as we advance into early Monday morning. Latest GEFS actually has a small segment of southeastern LA within the highest climatological percentile for the forecast PWAT indices on Monday, a tell tail sign of a well-defined tropical airmass with origins out of the Western Caribbean. This places the area along the Gulf coast into an environment capable of significant heavy rainfall within any convective cores that develop during the period in question. There`s some discrepancy on the exact placement of where the heaviest precip will occur, but the premise is the outlined SLGT risk area along the coastal plain up into portions of the urban corridor along I-10 will be subject to scattered tropical convective cells capable of putting down 2-3" of rain in a short period of time. Overall consensus within the ensemble output is sporadic 1-3" amounts with probability matched means closer to 5-6" within the areas hit with multiple rounds of heavy thunderstorms. Thankfully the area over LA is actually below normal when it comes to soil moisture, so the grounds will be able to soak in much of the rainfall. The areas most at risk are those urban centers where the ability for runoff is much greater due to the impervious surfaces. This was suitable for a SLGT risk maintenance with agreement from the local WFOs impacted. Kleebauer ...Previous Discussion... ...Northern Plains to the U.P. of Michigan... The cold front associated with a low over Canada will stall out as the parent low moves into northern Canada. Meanwhile a new leeside low will develop over Colorado. This will drag abundant Gulf moisture over the nation`s midsection back to the west into the low. After sunset Sunday night, the strengthening LLJ will run into the stalled out front, turning it into a warm front. The front will act as a forcing mechanism...lifting the abundant moisture and instability along it, resulting in widespread convective initiation across South Dakota and Nebraska. Due to the abundant moisture, the storms will be capable of heavy rainfall rates as they track northeastward across southern South Dakota and into southwestern Minnesota in the predawn hours Monday morning. The strong parent low over northern Canada will keep strong westerly flow into the front from the cold side. The result is despite impressive moisture influx from the south with the LLJ, the front will struggle to move. This will turn the front into a "train track" of sorts, allowing numerous rounds of convection to develop and move northeast along it, with the LLJ fostering new convective backbuilding to the west. The flow will turn northeastward upon running into the front, resulting in largely unidirectional flow, supporting training and backbuilding. This will set this stage for a more impressive flooding event possibly into Monday. More on that in the Day 3 discussion. Portions of the Slight Risk area across SD and MN will have been hit with multiple rounds of heavy rain from now through Sunday morning. Thus, the start of the potentially bigger flooding event across SD/MN may have some support of already wet soils as the storms move through, resulting in widely scattered instances of flash flooding. The Slight Risk area was largely unchanged from inherited. ...Central Gulf Coast... The other area of increasing concern will be along the Gulf Coast. A nothing short of impressive plume of very deep tropical moisture straight from the Caribbean will track northwestward across the Gulf and into the central Gulf Coast on Sunday. PWATs associated with this air mass will be between 2.25 and 2.5 inches, exceeding the 95th percentile compared with climatology and as high as 4 sigma above normal. This deep moisture plume will result in convection with daytime heating during the afternoon on Sunday, generally from New Orleans south...but the more substantive rainfall will come Sunday night as atmospheric moisture continues to build. Widely scattered convection all of which capable of very heavy rainfall will move across the Central Gulf...generally between Mobile and New Orleans, where the Slight Risk remains. The Slight was shifted eastward with this update, reflecting a slowing of the westward movement of the moisture plume. This slowing would further enhance the potential for flash flooding, but a lack of forcing should keep convection rather limited in coverage. One piece of good news is that this area has been rather dry in the recent past, with the last heavy rain episode a week or more ago. Thus, for the first part of this event Sunday night, instances of flash flooding should be isolated to widely scattered, with the greatest threat over flood sensitive urban areas from Mobile to New Orleans. Wegman Day 3 Valid 12Z Mon Jun 17 2024 - 12Z Tue Jun 18 2024 ...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF NORTHEAST SOUTH DAKOTA, CENTRAL MINNESOTA UP THROUGH THE ARROWHEAD, AND NORTHWEST WISCONSIN... ...20Z Update... A Moderate Risk has been forecast across portions of the Upper Midwest and adjacent Northern Plains for Monday into early Tuesday morning. More on this potential in the "Upper Midwest" sub-heading below... Elsewhere, the previous forecasts across the Gulf coast and MT were relatively unchanged with some minor adjustments further west with the SLGT risk into southeastern TX near Houston as recent ensemble mean QPF field has shifted slightly to the west with influence from the recent 12z deterministic suite showing the nudging of the anomalous moisture flux on the western flank of the ridge. Totals across the Gulf coast are still fairly aggressive within the confines of the 3-4 deviation above normal PWAT anomaly that advects north- northwestward out of the Gulf with origins based out of the Western Caribbean. IVT signatures from the GFS/ECMWF are both stout with rooted connections basically correlating from the ITZ located from Central America through the Caribbean. This is a profound setup to extract the deepest tropical airmass possible for the mid- latitudes creating an environment conducive for not only waves of heavy rainfall to protrude the Gulf coast areas, but to maximize efficiency given the source of the moisture in question. Despite some deviations in the outcome within the sense of the QPF footprint, ensemble means still suggest an area of 2-5" with locally higher likely situated from Beaumont along the I-10 corridor back into Lake Charles towards New Orleans. After the previous period of rainfall, this would be a secondary round that would push 2 day totals above 5" for some within the southern 1/3 of LA, a setup consistent for flooding potential. In coordination with the local WFOs over TX/LA/MS, decided against a MDT risk at this time as the prior period will play a pivotal role in the potential upgrade. Expect further evaluation in subsequent forecast periods with a possibility for an upgrade over the next 24 hours. Setup across MT maintains general continuity with no changes necessary from the previous MRGL issuance. ...Upper Midwest... A potentially high-impact flash flooding event may develop across portions of Minnesota on Monday and especially Monday night. A very impressive 130 kt jet for June will be nearly stationary across Manitoba and northern Ontario provinces. The jet exists because unseasonably cold air will be digging south across the Intermountain West. Meanwhile, an unseasonably hot and humid air mass associated with a strong upper level high over the East Coast will pump impressive amounts of Gulf moisture up the entire Mississippi River and into Minnesota. A impressively strong shortwave trough will eject out of the Intermountain West and into the northern Plains Monday night. The warm moist air mass will run into the front that will be over central and northern Minnesota Monday and Monday night. Additional lift from the right entrance region of the upper level jet streak, the strong shortwave, and the stationary front will result in nearly continuous convection across this region from Monday morning straight through Monday night. While convection typically wanes through the day due to competing heating sources, the front and ample additional lift will keep the rainfall going. Then as the LLJ intensifies to over 50 kt after sunset Monday night, the associated storms will also increase in coverage. Since the front, acting as the primary forcing mechanism will be either stationary or very slow moving, the convection will continuously train and redevelop over Minnesota into Monday night. The combination of all the above ingredients could result in a high-impact flash flooding event across Minnesota. Guidance is in above average agreement that this will occur over central to northern Minnesota with the latest guidance now coming into better agreement on the placement of the stalled frontal boundary that will be the focal point of where the heaviest precip will occur. With a growing consensus on the downstream ridge pattern blossoming east of the Mississippi and the progression of a stout shortwave trough across MT, there is more confidence in the overall synoptic evolution that will create a prolific convergence pattern centered across the Upper Midwest and Northern Plains. Latest ensemble bias corrected output for the area encompassing eastern SD up through central and northwest MN is fairly prolific with the QPF output a general 2-5" with a max of 6.5" located across west-central MN near the SD border. This is consistent with the current ML output from both the Graphcast ECMWF/GFS and the ECMWF AIFS with heaviest QPF footprint located that aforementioned vicinity. This is likely due to the increasing influence of the nocturnal LLJ with the nose of the 850mb jet core aimed right into the frontal boundary in that general location maximizing boundary layer convergence. This is textbook for some of the more significant flash flood episodes involving this type of regime, so the prospects have merit. Widespread totals of 3-6" with maxima of 10" will likely be the range of outcomes based on the latest synoptic and thermodynamic evolution with the evolving pattern. According to the GEFS climatological percentile for mean QPF, this would rank towards the 99th percentile of feasible outcomes for the date relaying the type of precedence for this pattern. The potential is there for a flash flooding event unrivaled in the area for at least the last 10 years. Considering all the data at hand, increasing confidence, correlating factors for significant rainfall, and in coordination with a multitude of WFOs (DLH/MPX/FGF/ABR) a Moderate Risk for excessive rainfall has been issued for the corridor extending from far Northeast SD through western and central MN, up into the Arrowhead and the Northwest section of WI. This is an evolving scenario for significant flooding, so please stay tuned for updates over the next several days. Wegman/Kleebauer ...Previous Discussion... ...Mississippi Delta Region... Concurrent with the potential flooding in the Upper Midwest, the plume of moisture moving into the central Gulf Coast from the Gulf will continue to intensify through the day Monday. PWATs will be nothing short of impressive, as they eclipse 2.5 inches along the Louisiana coast. Storms are likely to be ongoing across the area from the overnight period as the Day 3 period starts at 12Z/7am CDT Monday morning. Continuous southerly flow of deep tropical moisture will support continuous development and redevelopment of storms capable of very heavy rainfall rates across Louisiana and Mississippi on Monday. Corfidi Vectors will be both weak (under 10 kts, and out of the northwest, against the flow. This will support persistent training convection and backbuilding when Corfidi vectors are against the prevailing southerly flow. A subtle and weak disturbance in the upper levels may further support convection, but forcing should largely be low levels driven from outflow boundaries and cold pools that are constantly recycled with deep tropical moisture out of the Caribbean. In coordination with LIX/Slidell, LA; LCH/Lake Charles, LA; and MOB/Mobile, AL forecast offices, the Slight was greatly expanded with this update to central Louisiana and Mississippi, with a higher-end Slight introduced for southern Louisiana and the Mississippi Gulf Coast. This area is the most likely to be upgraded to a Moderate Risk with future updates, especially if Day 2 rainfall over this area overperforms or increases in future model guidance. Given rivers in some of these areas of far eastern Texas and western Louisiana are still in flood from events over more than a month, soils and reservoirs have not had sufficient time to recover from those events for another rainfall event of this magnitude. Further east into New Orleans and the Mississippi Gulf Coast, FFGs from urbanization are already lower. With potential for 3 inch/hour rainfall rates from convection, perhaps repeatedly, even flood resistant bayous will be unlikely to handle a deluge of that magnitude, which will be possible. ...Montana... Much of Montana will be on the "cold side" of the system, with high-elevation snow expected. The rainfall across the state will be much more reminiscent of a cold-season event, with ample upper level forcing involved and some Gulf moisture. Essentially some of the Gulf moisture from further east into Minnesota will curl back to the west as another leeside low that forms over CO early in the period tracks north into the northern Plains. Rainfall amounts should remain generally under 1 inch, but will be widespread enough across Montana that the inherited Marginal remains in place with no changes. 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