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FOUS30 KWBC 022005 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 405 PM EDT Fri Jun 02 2023 Day 1 Valid 2000Z Fri Jun 02 2023 - 12Z Sat Jun 03 2023 ...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR NORTHWEST TEXAS AND PORTIONS OF EASTERN MONTANA... ...Southern Plains... Another active day expected for the Southern Plains as a shortwave ejecting out of New Mexico overlaps with the LFQ of a subtropical jet streak and an advancing dry line to drive impressive ascent. This lift will occur within an extremely favorable thermodynamic environment characterized by PWs of +1 to +2 standard deviations above the climo mean according to the NAEFS ensemble, and a plume of 2000-3000 J/kg MUCAPE being drawn northward from the RGV. The result of this evolution is likely to be rapidly increasing convection which should intensify and expand across the High Plains of NM and organize into an MCS over the TX Panhandle. As convection expands, Corfidi vectors are progged to collapse to just around 5 kts, indicating the potential for storms to backbuilding into the greater instability and then train to the E/NE. Eventually, the complex is progged by the high-res simulated reflectivity to surge eastward this evening, but until that time rain rates of 2+"/hr training across areas could result in locally more than 3" of rain as shown by a good consensus among the HREF/ECENS/GEFS/SREF ensembles, with locally more than 5" possible. This rain will occur atop primed soils from recent heavy rains which have been 300-600% of normal the last 14 days leading to already high USGS streamflows. This has compromised FFG to as low as 0.25-1"/3hrs, for which the HREF indicates has a better than 60% chance of being exceeded. Although the ensemble probabilities have shifted just a bit east, the inherited MDT risk was adjusted only slightly/cosmetically as the antecedent conditions over the Panhandle are more susceptible than areas farther east. This did necessitate some expansion of the surrounding SLGT risk, but the area of greatest concern for more numerous flash floods remains over the TX Panhandle through this evening. ...Northern/Central Rockies and Northern High Plains... A fairly blocked mid to upper level pattern will persist today from the Northern Rockies, eastward through the Northern Plains into the Upper Mississippi Valley and Great Lakes. Very slow moving mid to upper level troughing will persist on the western side of the blocking upper high forecast to remain in place from the Upper Mississippi Valley into the Great Lakes. Within this trough, a wave of low pressure drifting along a nearly stationary boundary across central/eastern MT today will allow increased moisture to rotate into the area. 850mb inflow with a source region of the Gulf of Mexico will increase to 20-30 kts over the Dakotas and then turn eastward into MT, lifting isentropically atop the front, converging into weaker flow to the west, and also resulting in enhanced ascent through upslope. Additionally, an axis of deformation is still progged to pivot over the central part of the state, resulting in prolonged ascent to wring out moisture which will be nearing +3 standard deviations according to the NAEFS ensemble tables. AS MUCAPE rises to more than 1000 J/kg to the east, some of this will resupply into central MT, resulting in thermodynamics that are favorable for rainfall rates that have a greater than 50% chance for exceeding 1"/hr according to the HREF probabilities. The inherited rare (for this part of the country) MDT risk still appears warranted as it overlaps where all the global ensembles indicate at least a low-end risk for more than 3 inches of rain today, with the HREF also indicating a 10-20% chance for more than 5 inches. This will fall atop saturated soils from 14-day rainfall that is generally 150-300% of normal, including pockets of 2-3" of rainfall yesterday according to MRMS. The MDT risk was pulled east just a bit from inherited however to better align the overlap of heavy rates and antecedent soils, but there will likely be numerous instances of flash flooding today. ...Central and Southern Florida... Although Tropical Depression Two will remain well west of Florida, the associated mid-level trough and tropical moisture plume (PWs to 1.75 inches) will continue to plague the peninsula today. A weak surface trough will be a focus for low-level convergence, along which scattered to widespread showers and thunderstorms are once again expected. With favorable thermodynamics in place to support efficient 2"+/hr rain rates, storm motions that are expected to be just around 5 kts based on the 0-6km mean wind will support local rainfall amounts of 2-4 inches, locally higher where any mergers or collisions can occur. This could result in isolated instances of flash flooding, especially if any cells drift across the more urban southeast or southwest coasts. ...Northern New York State and New England... The backdoor cold front that will drop across New England and Upstate New York today is still progged to move through by this evening, with associated showers and thunderstorms developing along it. Overall the area has been quite dry based on 14-day rainfall departures, which has left FFG around 2-3"/3hrs. This suggests that despite a favorable overlap of instability and moisture to support rainfall rates of 1-2"/hr, these dry antecedent conditions will somewhat cap the excessive rain threat. This is also aided by storm motions that should be generally to the south at 15-20kts. However, in some places multiple rounds of heavy rain producing convection could occur, and there are also some more sensitive terrain features across this region. This could result in some isolated runoff or flash flooding instances, and the MRGL risk was broadened to account for Mesoscale Precipitation Discussion #410. See that discussion for more information. Roth/Weiss Day 2 Valid 12Z Sat Jun 03 2023 - 12Z Sun Jun 04 2023 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS THE PANHANDLE OF TEXAS... ...Southern and Central Plains Another day of scattered to widespread convection is likely across the Southern/Central Plains on Saturday, but with somewhat less focus and intensity than the previous few. The mid-level pattern becomes more elongated with a trough across the Southern Plains as an expanding ridge builds to the north. This will begin to suppress the intense meridional moisture transport which has plagued the area recently, but modest 850mb inflow from the Gulf of Mexico should still produce PWs around +1 standard deviations according to the NAEFS ensemble tables, coincident with MUCAPE surging to 2000 J/kg during peak heating from the RGV into the TX Panhandle. Weak impulses aloft and some modest upper diffluence will help drive large scale ascent across the area, while residual boundaries left over from Friday's convection will also serve as a trigger for thunderstorms during the day. The guidance has little consensus in developing any organized areas of convection, which is also reflected by scattered HREF probabilities for 1"/hr rainfall rates. This area has been worked over by heavy rainfall multiple times in the past two weeks though, so any additional heavy rain could result in runoff and instances of flash flooding. The most sensitive area is again across the TX Panhandle, so the inherited SLGT risk was maintained since 3-hr FFG i still just 0.25-1 inch, and may lower from heavy rain anticipated on Friday. Some more organized convection is possible as an MCS which some high-res tries to develop in the evening near the Big Bend of Texas and then dropping southeast into the better moisture along the RGV and into South Texas or the western Hill Country. Considered a small SLGT risk for this area based on HREF EAS probabilities suggesting a local maxima in rainfall, but this area is not as sensitive as points north, and confidence is marginal as to how this will evolve. A SLGT risk may be needed for this region with later updates, but for now the MRGL risk was maintained. ...Central to Northern Rockies... Blocked mid-level pattern will cause a slow retrograde of features on Saturday as an amplifying ridge over the Northern Plains pushes shortwaves embedded within a longwave trough to the northwest. Several of these impulses are progged to lift northwest from the Central Plains through the Northern Rockies on Saturday, producing periods of enhanced deep layer ascent. This lift will occur within favorable thermodynamics noted by PWs of 0.75-1.25 inches, above the 90th percentile according to the SPC sounding climatology, and pockets of MUCAPE of 500-1000+ J/kg. This will support another day of scattered to widespread convection, especially from the Front Range of CO northwest through much of WY, ID, and the western half of MT. Rain rates within deeper convection could reach 1+"/hr as noted by HREF probabilities as high as 30%, while Corfidi vectors aligned to the mean wind will permit some short term training to offset generally progressive storms to the northwest. The large inherited MRGL risk was adjusted to account for new guidance. Considered a small SLGT risk for the Northern Rockies where HREF neighborhood and EAS probabilities feature a greater potential for an additional 2+ inches of rain as instability gets resupplied from the east into a slowly weakening deformation boundary atop pre-conditioned soils and low FFG. However, after coordination with TFX, opted to maintain the MRGL risk since rates may not be that impressive and additional rain D2 may not overlap efficiently with heavy rain from the prior few days. ...Florida... Tropical Storm Arlene should be a TD on Saturday over the eastern Gulf of Mexico as it begins to decay, with the remnant vorticity lobe becoming absorbed into the larger scale trough extended from the Southeast Atlantic Coast. This will allow the accompanying shortwave to shift towards the southern FL Peninsula by Sunday morning, leading to at least subtly enhanced ascent in a still tropical environment characterized by PWs of 1.5 to 1.75 inches. With a subtle convergence axis/surface trough still likely positioned across South Florida, coincident with the LFQ of a weak jet streak aloft, this additional forcing should result in another day of scattered to widespread showers and thunderstorms. The favorable thermodynamics should support rain rates of 2"/hr at times, with cells again moving slowly on 0-6km mean winds of just 5-10 kts. Additionally, weak bulk shear to support generally pulse convection could result in storm mergers and outflow interactions, to enhance rain rates and cause chaotic storm motions, and rainfall of more than 3" is likely in some areas as reflected by the HREF probabilities. The southern FL peninsula has received significant rain recently, with more forecast on D1. Any of these slow moving storms could produce isolated flash flooding, especially if they occur atop any more urban or sensitive areas. Weiss Day 3 Valid 12Z Sun Jun 04 2023 - 12Z Mon Jun 05 2023 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHERN TO CENTRAL PLAINS INTO THE NORTHERN ROCKIES... ...Southern Plains through the Northern Rockies... More of the same for this part of the country on Sunday, although the overall coverage and intensity of convection may continue to wane. The driver of this pattern will remain the expansive mid-level ridge over the Northern Plains, resulting in more diffuse troughing arced from the Northern Rockies through the Southeast. With still above normal PWs more than +1 standard deviation above the climo mean according to NAEFS, and elevated afternoon instability, and diurnal convection across the region could produce excessive rain rates. However, there is little signal at this range for any organized convection atop saturated soils from prior rain events during the past week. If any signals develop during the next few model cycles, it would not require a significant upward trend to drive targeted SLGT risks due to the antecedent soil conditions, but otherwise the runoff/flash flood threat appears just isolated on Sunday. Weiss 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 $$