Subj : HVYRAIN: Excessive Rainfa To : All From : Mike Powell Date : Tue Jul 11 2023 15:47:00 FOUS30 KWBC 112012 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 411 PM EDT Tue Jul 11 2023 Day 1 Valid 16Z Tue Jul 11 2023 - 12Z Wed Jul 12 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE MID-MISSOURI BASIN AND LOWER MISSISSIPPI VALLEY... 16Z Update... Overall, made minor changes to the previous outlook based on the 12Z HREF guidance and current trends. A small Marginal Risk was added to portions of southeastern Arizona and southwestern New Mexico to reflect increasing PWs and weak steering flow which may support slow-moving storms capable of producing locally heavy amounts during the afternoon into the evening hours. Pereira Previous Discussion... ....Northern High Plains through the Mid-Missouri Valley and Southern Great Lakes... Nearly zonal flow this morning will gradually transition more to NW flow later today as a shortwave digs into the mean longwave trough across the eastern CONUS. This evolution will help push a wavering surface front to the south while maintaining a NW to SE axis parallel to the increasing flow aloft. A 300mb jet streak coming out of the Pacific Northwest will flatten at the same time, leaving favorable LFQ diffluence atop the front and into the Northern Plains. The overlap of this diffluence with subtle height falls ahead of shortwave impulses and their associated PVA, will impinge into the weak baroclinic boundary to drive deep layer ascent from eastern MT through the Mid-Missouri Valley. This ascent will occur into favorable thermodynamics characterized by PWs surging on return flow ahead of the front to 1.25 to 1.5 inches, approaching +2 standard deviations from the climo mean according to NAEFS, and MUCAPE surging northward within a corridor ahead of the front reaching 2000 J/kg. This impressive overlap of ascent and moisture/instability will result in scattered to widespread showers and thunderstorms with rain rates exceeding 1"/hr likely according to HREF neighborhood probabilities. 0-6km mean winds of 15-25 kts suggest progressive storms, but with the wind aligned to the front, some short term training is possible which could result in isolated instances of flash flooding necessitating a continuation of the MRGL risk with only subtle cosmetic adjustments. Although the overall flash flood risk is modest today, there is an area of higher probabilities from eastern SD/NE into IA and far northern MO. Here, both the baroclinic and instability gradient will be most intense, supporting an axis of greater risk for an MCS (or possibly two) to drop southeast into the higher moisture and along the 500mb thickness gradient tonight. The guidance is in good agreement that an MCS will form as a shortwave digs along this gradient and interacts with 0-6km effective bulk shear of 40-50 kts. However, timing and placement is still uncertain which is reflected by modest HREF EAS probabilities for 1" of rainfall. While parts of NE/IA have been wetter than normal the past 14-days, soils appear generally hydrophilic as reflected by low streamflow anomalies, so only the longest duration of heaviest rainfall would likely result in scattered instances of flash flooding. The greatest risk area for this according to the HREF and GEFS is across western IA, so the SLGT risk was tailored to best match these probabilities with some extension to account for lower FFG/more sensitive soils. Farther to the east towards Chicago, IL and Detroit, MI, some aftn/evening showers and thunderstorms may develop along the front as it continues to sag southeast into the area. There is a narrow corridor both temporally and spatially for an overlap of convergence for ascent along the front interacting with a rich theta-e plume surging northeastward. Confidence is generally low in evolution as any slowing of the front could reduce the available moisture considerably. However, HREF probabilities have increased for 1" of rainfall due to convection, and the mean flow appears aligned to the boundary during this time. This could produce some short duration training/backbuilding into the higher theta-e plume to support an isolated flash flood risk. ....Southern Plains through the Gulf Coast... A stationary front aligned west to east from the Southern Plains through South Carolina will begin to lift north as a warm front and weaken today. This evolution will be driven by an expanding high pressure ridge centered over the Desert Southwest which will elongate to the east to drive bulging heights along the western and central Gulf Coast. The longwave trough that has been plaguing the region the past few days will be displaced eastward during this evolution, helping to additionally weaken the surface boundary. The slow height rises combined with the weakening trough and associated displacement of the upper jet streak will result in weakened ascent today for the Gulf Coast region. Still, a corridor of enhanced PWs of 2-2.25" overlapping MUCAPE of around 2000 J/kg should allow for at least scattered thunderstorms today, with storms moving slowly within the general weakly forced environment. HREF probabilities suggest a good chance for 1-2"/hr rates in thunderstorms that develop today, which through slow motions could produce locally more than 3 inches of rainfall in some areas. Although in general this area will see only an isolated risk for flash flooding, there appears to be a subtle maximum in flash flood risk due to higher convective coverage across northern Louisiana. Here, some better backbuilding is likely as the weak warm front lifts northward allowing storms to regenerate into higher instability, which will enhance the duration of heavy rain rates. Additionally, these aftn/eve thunderstorms could precede an MCS (or possibly two) that is progged by the high-res to sweep southeast along the increasing CAPE gradient tonight. In areas that receive both leading thunderstorms and these MCSs, locally more than 5 inches of rain is possible as reflected by the newest HREF exceedance probabilities. While much of LA has been dry recently noted by 14-day rainfall generally 50-75% of normal, there is an axis in northern LA and far southern AR that has received more than 150% of rainfall during this same temporal window. This is also the area that has the greatest risk for the heaviest rain today, and although HREF 3-hr FFG probabilities are still only 20-25%, they have increased since the last run. After coordination with SHV/JAN/LZK, a small SLGT risk area was added. ....Northern New England... Lingering light to potentially moderate rainfall rates will persist through the morning, primarily across far northern VT, NH, and ME. This rain will be driven by a pivoting deformation axis in the vicinity of a negatively tilted shortwave lifting into Maine, and an occluded front connecting two surface low pressures also shifting northeast through the morning. This shortwave and accompanying occluded boundary will likely eject into Canada by the late aftn, allowing for much drier air advecting in from the west, noted by a rapid decrease in mid-level RH, to finally bring an end to the area rainfall. PWs in the morning will still be generally 1.25 - 1.5 inches, about +1 standard deviation above the climo mean according to NAEFS, overlapping a corridor of 500-750 J/kg MUCAPE ahead of the front. These thermodynamics will support at least brief rainfall rates of 0.5-1"/hr, but with generally fast storm motions to the north. While the total coverage of rainfall this morning appears to be limited, and total rainfall will also likely be modest noted by HREF probabilities for 1" peaking around 20-50%, this will be falling atop extremely saturated soils and likely still ongoing flash flooding from Monday's rain event. This suggests the inherited MRGL risk is still needed as any additional rainfall could quickly become runoff, but the overall threat will be isolated and ending by the evening. Weiss Day 2 Valid 12Z Wed Jul 12 2023 - 12Z Thu Jul 13 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE MID MISSISSIPPI/LOWER MISSOURI VALLEYS, OZARKS, AND SOUTHERN GREAT LAKES... ....Mid Mississippi Valley through the Great Lakes... Consensus of the CAMs shows ongoing convection early in the period across southern Wisconsin/northern Illinois. A mid-level shortwave embedded within the base of a broad upper trough, along with an upper jet couplet, are expected to help support storms continuing from late in the Day 1 period through the early hours of Day 2, with storms translating west-east into southern Michigan by the afternoon hours, and possibly continuing into the evening/overnight. Deepening moisture ahead of the wave (PWs around 1.75 inches) along with the potential for training storms are expected to contribute to a heavy rainfall/flash flooding threat. A Slight Risk was maintained from southern Wisconsin/northern Illinois through southern Michigan. Models have shown a fairly consistent signal across this region, but have shifted a bit farther to the north, which is reflected in the Slight Risk. HREF neighborhood probabilities indicate that accumulations of 2-3 inches can be expected within the Slight Risk area, especially across southeastern Michigan. Models have been less consistent farther to the southwest across the mid Mississippi/lower Missouri Valley and Ozark regions. There remains a signal for at least locally heavy rainfall to develop, but the general consensus is farther southwest than previous runs. A majority of the CAMs show the heaviest amounts developing during the evening/overnight hours as convection develops along the trailing front and propagates southeast across the region. PWs are forecast to increase to 1.75-2 inches, with training/backbuilding storms contributing to the heavy rainfall threat. While confidence in the occurence of heavy rainfall remains fairly high, the run-to-run model variability continues to contribute to lower confidence in placement. A Slight Risk was maintained for portions of the region, but was restricted to areas where the 12Z HREF showed higher probabilities for accumulations exceeding 2 inches. ....Gulf Coast States... Slow-moving thunderstorms are again expected near the Gulf Coast on Wednesday, with two areas of focus possible. The first will be across far southern Arkansas/northern Louisiana where a lingering shortwave from Day 1 may persist across this region in the first half of Day 2. Uncertainty is high as to how intense this will still be as the LLJ veers Wednesday morning and instability gets overturned, but lingering heavy rain atop soils likely saturated from Day 1 rainfall necessitated an introduction of a Marginal risk for this area. If the Day 1 amounts come to fruition and the signal remains strong for additional heavy amounts on Day 2, an upgrade to a Slight Risk may be required for portions of southern Arkansas and northern Louisiana. Farther to the south and east, from eastern LA through the northern FL peninsula, a signal exists for some very slow-moving thunderstorms as Corfidi vectors collapse to just 5 kts and align into the instability offshore into the Gulf of Mexico to suggest some training. Low-level inflow from the Gulf will keep PWs above 2 inches, with some local backing possible as the remnant shortwave/MCV from northern LA shifts southward. This could produce scattered slow-moving thunderstorms with 2+ in/hr rain rates, leading to locally more than 3 inches of rain on Wednesday. ....Southwest... A Marginal Risk was added to portions of southeastern Arizona and southwestern New Mexico. Anomalous moisture and weak steering flow will contribute to slow-moving afternoon/evening storms that may produce locally heavy amounts, with isolated flash flooding a concern, especially across burn scar and urbanized areas. Pereira/Weiss $$ --- SBBSecho 3.14-Linux * Origin: Ilink: CCO - capitolcityonline.net (454:1/105) .