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From | To | Subject | Date/Time | |||
COD Weather Processor | wx-storm | HVYRAIN: Excessive Rainfall Discussion |
July 31, 2019 8:38 AM * |
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This is a multi-part message in MIME format... ------------=_1564562309-118394-2633 Content-Type: text/plain FOUS30 KWBC 310838 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 437 AM EDT Wed Jul 31 2019 Day 1 Valid 12Z Wed Jul 31 2019 - 12Z Thu Aug 01 2019 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF ARIZONA AND UTAH, AND THE LOWER MISSOURI VALLEY... ...Southwest to the Central Great Basin and Central Rockies... Large mid-level ridge spinning nearly in place will continue to drive Monsoon flow into the Southwest. The southerly flow on the western periphery of this ridge will continue to drive robust moist advection into the region, with PWATs climbing to around 2" in southern Arizona, and as high as 1-1.25" into central Utah. These values represent anomalies as much as 2-2.5 standard deviations above the climatological mean. Despite the ample degree of deep-layer moisture, the the amount of instability observed during the day will be the wild card. Considerable cloudiness will likely inhibit strong destabilization, at least across areas along or near the vort lobes. Moreover, areas along the periphery of the denser cloud shield will have a much better chance to destabilize, however at the same time the 850-700 mb moisture flux will be decidedly lower over these areas. Compared to yesterday's day 2 ERO, the Marginal Risk was expanded on both east and west sides, while the Slight Risk was extended through southern AZ, largely due to the current (early am) activity and with the anticipation that more breaks in the cloud cover will develop by afternoon and allow for a rebound in deep-layer instability. Although storm motions may be a bit too quick for widespread flash flooding noted by 0-6km mean wind of 10-15 kts, the latest HREF indicates scattered 40-60% probabilities of >1"/hr rainfall rates, with 10-20% probabilities for 2"/hr. These rain rates across areas with FFG as low as <1"/1hr, should support flash flooding, and will be most likely where the mid-day instability peaks in conjunction with the larger scale forcing. Additionally, some upslope enhancement is possible in portions of the terrain of northern AZ into UT and northwest CO, further expanding the potential for flash flooding. ...Central Plains into the Mid Mississippi Valley... Guidance is in very good agreement that a shortwave impulse riding atop the ridge and swinging southeast towards Missouri will spawn an MCS Wednesday night. This MCS will develop in response to a compact area of upper divergence out ahead of the vort lobe, interacting with an elevated confluence boundary on the nose of an intensifying 850mb LLJ which may exceed 35 kts. This LLJ will additionally transport ample moisture into this region, with PWATs progged to rise towards 1.75-2 inches, about 2 standard deviations above the climatological mean. This setup will be highly favorable for MCS development, likely to ride SE along the sharp instability gradient noted by MUCAPE dropping from nearly 3000 J/kg over KS, to less than 500 J/kg in eastern MO. Despite rapid motion of the MCS noted by Corfidi vectors reaching 35 kts, there is some potential for training as the mean flow becomes parallel to the elevated boundary. This is reflected by high-res guidance painting streaks of 3+ inches of QPF across the area, along with HREF probabilities of 40-60% that greater than 3 inches will fall within 6 hours. Eventually the MCS should become cold pool dominated as its heads further south, causing faster progression and a reduction in flash flood potential. However, a southward expansion of the Slight Risk (to near the KS/OK border) and especially the Marginal Risk (into northeast OK, southwest MO, and far northwest AR) was warranted based on the latest guidance trends. ...Northeast... A long wave trough will drive a cold front into New England during day 1, with forward progression slowing as the upper trough begins to tilt negatively. Deep SW flow ahead of the surface boundary will drive warm moist advection into the region, noted by PWATs exceeding 1.5 inches and MUCAPE forecast to rise as high as 2000 J/kg into New England. In addition to the low-level convergence, broad upper diffluence will drive ascent, which in the favorable thermodynamic environment will support scattered to widespread afternoon thunderstorms. Guidance shows a spotty signal for up to 3" of rainfall where any training can occur despite fast storm motions, with the best overlap occurring across NJ. There was some coordination with PHI/OKX about potentially hoisting a SLGT risk for day 2, but due to fast storm motions and that the lowest FFG is displaced a bit north of the highest QPF, a MRGL risk was strong enough to cover the isolated flash flood risk. ...Gulf Coast of TX/LA... As noted by the models (especially the CAMs), residual thunderstorms will persist on Wednesday along the TX/LA Gulf coast on the tail end of the baroclinic zone/front. Additionally, outflows from offshore convection, or residual boundaries from convection on Tuesday, may serve as a focus in a very unstable and moist atmosphere. Slow storm motions and rain rates which may exceed 2"/hr at times given such a highly-favorable thermodynamic environment support an isolated flash flood threat today, thus the continuation of the Marginal Risk. Hurley/Weiss Day 2 Valid 12Z Thu Aug 01 2019 - 12Z Fri Aug 02 2019 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER EASTERN COLORADO INTO WESTERN NEBRASKA...AS WELL AS THE CENTRAL PLAINS INTO THE MID MISSISSIPPI VALLEY... ...Southwest/Rockies... There are two areas of concern for flash flooding across the Southwest into Rockies and northern High Plains during Day 2. The first is across portions of AZ/southeast NV into UT and far western CO. The best synoptic scale forcing is expected to exit the region during the first part of Day 2, but deeper moisture and instability remain behind the short wave energy. Cloudiness early could affect how quickly instability rebounds across the area, but differential heating would suggest that the terrain could support convective initiation first during the late morning into early afternoon. Between 500-1500 J/KG of MLCAPE is expected along the terrain from central AZ across UT into western CO, peaking between 01/18z and 02/00z. Storms developing in the axis of instability tap the 1.00 inch precipitable water air in place, which could support hourly rainfall rates nearing an inch (per the 00z NAM CONUS Nest) over the terrain. These rates are high enough to support an isolated to widely scattered flash flood threat in these areas, mainly through 02/06z. Based on the above, a Marginal Risk was stretched from AZ across UT into western and central CO. The second area of concern is across eastern CO into the high plains of western NE. As a frontal boundary becomes banked up along the Front Range in CO, the low level east to northeast flow transports 1.00/1.25 inch precipitable water air (which is about two standard deviations above the mean) across eastern CO, southeast WY into far western NE. Model soundings showed a ribbon of 2000 J/KG of MLCAPE from eastern CO into western NE, and convective initiation is expected after 01/18z. The upslope flow could result in slow cell motions with the storms in the terrain, which track across the plains after cold pools become established. Before that occurs, the deepening moisture could support hourly rainfall rates nearing 1.25 inches (which is depicted by some of the 00z high resolution guidance), especially across eastern CO and western NE. The 00z NAM CONUS Nest was the most aggressive with rainfall amounts, with spotty 3.00 inch amounts in these locations (which seems plausible where cell mergers or short term training occurs). Considering that three hour flash flood guidance is as low as 1.00/1.50 inches over portions of eastern CO, there appears to be an enhanced flash flood threat here. With this in mind, a Slight Risk was extended across eastern CO into western NE for Day 2. ...Central Plains into Mid Mississippi Valley... Short wave energy tracking over the ridge into the eastern portion of the Central Plains and Mid MS Valley interacts with deepening moisture on the nose of a strengthening low level jet and instability to form an MCS that tracks across eastern KS into western MO, mainly after 02/06z. While there is generally good model agreement with the overall synoptic setup, there are some west/east differences with respect to the placement of the highest rainfall axis. In an attempt to mitigate these differences, both the WPC QPF and Excessive Rainfall Outlook were based on a multi model blend. Remnants of an MCS crossing eastern KS into western MO early on Day 2 could produce locally heavy rainfall, as the low level jet weakens and instability become exhausted. As the morning debris cloudiness exits, an axis of 2000/3000 J/KG of MLCAPE is expected to develop along and north of the front extending across KS into far eastern OK. The instability should be sufficient to support scattered convection in the instability axis before 02/00z, generally on the edge of the mid level capping. The storms are expected to be scattered enough to pose a low end flash flood threat. As short wave energy tracking from MT and the western Dakotas drops southeast, the low level jet backs and strengthens over eastern NE into eastern KS. The 35/40 knot low level southerly flow injects 2.00 inch precipitable water air into the instability axis. In the presence of synoptic scale lift associated with the short wave energy, an MCS is expected to form over eastern NE between 02/00z and 02/06z, which moves southeast along the edge of the cap. On the southwest side of the MCS, where the propagation vectors align with the 850/400 mb flow, resulting in training. Hourly rainfall rates near 2.00 inches are possible where training occurs, before the MCS moves southeast and becomes more outflow dominated. There is a multi model signal for local 2.00/3.00 inches rainfall amounts with system. Three hour flash flood guidance values are generally above 2.50 inches, but heavy rainfall during Day 1 could pre conditional the soils, resulting in a higher flash flood threat. Based on the above, a Slight Risk was placed over eastern KS into western MO. Much of the high resolution guidance indicated the highest rainfall totals over eastern KS, but this may be too far west, depending on the placement of the best instability axis. ...Mid Atlantic... Deep moisture and instability pool along a frontal boundary across the Mid Atlantic states during Day 2. As the best mid level support for the frontal shears out to the north across New England, the front is expected to slow, and could become quasi stationary across portions of NC into after 01/18z. The moisture and instability should be sufficient to support at least scattered storms along and south of the front, where MLCAPE values are expected to range between 1500/2500 J/KG. Increasing warm cloud depths could allow storms to be fairly efficient rainfall makers, and given the 2.00+ inch precipitable water axis along the front should support storms capable of producing hourly rainfall rates near 2.00 inches, especially over far southern VA into much of central NC. Storm motions could be less than 10 knots, especially across western VA/western NC, which may foster an environment for short term training and cell mergers. A weak long wave trough extending from the Ohio Valley to the central Gulf Coast would result in steepening lapse rates, but overall, there may not be sufficient shear for cell maintenance. There is some regional/global model support for local 3.00 inch rainfall amounts, particularly where training occurs. Three hour flash flood guidance values are generally between 2.50/3.50 inches, and training in the higher moisture airmass could allow some locations to approach these values. Based on this, a Marginal Risk was placed over much of southern VA and NC for Day 2. Hayes Day 3 Valid 12Z Fri Aug 02 2019 - 12Z Sat Aug 03 2019 ...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF EASTERN KANSAS AND WESTERN MISSOURI... ...Central Plains into the Mid Mississippi Valley... Eastern KS and western MO appear to be in the path of multiple days of MCSs along and north of a front boundary extending from KS into eastern OK, posing an increasing threat for significant flash flooding here during Day 3. There was generally good model agreement concerning the overall synoptic setup, though there was some longitudinal differences in the placement of the highest rainfall amounts. In an attempt to mitigate some of the differences, the WPC QPF and Excessive Rainfall Outlook was based on a multi model blend. An MCS is expected to be in the weakening phase across eastern KS into western MO early on Day 3 as the low level jet weakens, during the diurnal instability minimum. After the debris cloudiness clears, an axis of 1500/2500 J/KG of MLCAPE is expected to develop near and just north of the frontal boundary across KS into OK (though there is some spread on when this occurs because of the morning cloudiness). Scattered convection develops in the axis of instability through 03/00z. After that time, the low level jet increases to 30 knots and back to the south in advance of short wave energy dropping southeast down the mid level ridge located over the Rockies into the Southwest. The combination on increased moisture with the strengthening low level jet (as precipitable water air jumps to 2.00 inches, which is about two standard deviations above the mean) and some synoptic scale ascent is expected to induce the development of an MCS over eastern NE before 03/06z. Once the MCS forms, it will track along the best instability axis over eastern KS and western MO in the 03/06z and 03/12z time frame. On the southwest side of the MCS, where the best threat for flash flooding is expected (as the propagation vectors align with the mid level flow), hourly rainfall rates could exceed 2.00 inches. There is a solid model signal for 3.00/5.00 inch rainfall amounts along this axis, with the 00z NAM showing amounts closer to 9.00 inches over southeast KS. As mentioned earlier, there is some longitudinal differences with the placement of the highest rainfall amounts, but two day rainfall totals ending 03/12z could top 6.00 inches over portions of eastern KS into western MO. Since soils will be compromised by MCSs during Day 1 and 2, the additional rainfall poses a significant flash flood threat in these areas. After collaborating with WFOs OAX/TOP/EAX/ICT/SGF/TSA, a Moderate Risk was placed here for Day 3. ...Southern and Central Rockies... Deepening moisture on an upslope flow across portions of the Central and Southern Rockies combines with marginal to moderate instability to produce storms capable of heavy to locally excessive rainfall during Day 3. An easterly to northeast low level easterly flow banks 1.00/1.25 inch precipitable water air (which is about two standard deviations above the mean) up across the terrain and plains of eastern CO into eastern NM, mainly before 03/00z. Differential heating should favor the terrain for convective initiation in the 02/18z to 02/21z time frame, and the low level upslope flow could hold storms in the terrain before they become more outflow dominated and move out of the terrain in the plains. The airmass could support hourly rainfall in excess of an inch, especially across eastern CO. While there could be some short wave energy dropping through the ridge position, the convection could be mostly scattered after leaving the terrain, with short term training and cell mergers resulting in hourly rates this high. Three hour flash flood guidance values are as low as 1.00/1.50 inches, particularly from the Front Range across southeast CO. The lower flash flood guidance opens a window of opportunity for a low level flash flood threat through about 03/03z, after which time the activity is expected to wane as the instability is exhausted. Based on this, a Marginal Risk was placed over much of the eastern portion of CO and NM for Day 3. ...Mid Atlantic/Southeast... A weak frontal boundary extending from the Mid Atlantic coast into the lower TN Valley during Day 3. The front will act as a focus for deep moisture and moderate instability to pose a low end flash flood threat across much of the Mid Atlantic into the Southeast states. A broad and weakening mid level trough extending from the Ohio Valley to the central Gulf coast will allow lapse rates to steep over a large portion of the Mid Atlantic and Southeast, mainly during the afternoon and early evening hours. Model soundings across the region showed between 1000 and 2000 J/KG of MLCAPE, peaking before 03/00z. Along and south of the front lies a large reservoir of 1.75/2.00 inch precipitable water air which is tapped by developing convection. The convection could form into clusters where outflow initiates additional activity, but for the most part, with weak forcing, the storms should be scattered to broken in coverage. In this airmass, hourly rainfall rates could approach 2.00 inches, especially where storms train or cell mergers occur. For the most part, three hour flash flood guidance values are 2.50 inches or higher, but heavy rainfall during Day 3 could lower these values in spots. Without a clear model signal favoring one location over another for flash flooding, a large Marginal Risk was placed over much of the Mid Atlantic and Southeast for Day 3. ...Florida... Deep moisture and a low level convergent flow across south FL during Day 3 will support convection that is capable of producing heavy to locally excessive rainfall. There is an increasing model signal for either a trough or weak surface circulation to develop just off the southwest FL coast during the first part to the period. The low level east to southeast flow supports convergent bands of convection across much of south FL, where steepening lapse rates in association with the weak long wave trough extending into the central Gulf of Mexico could allow the storms to be more long lived. Storm motions are expected to be fairly slow, with a general southwest to northeast motion under 10 knots. This could support cell mergers, as well as short term training. Any storms training in the plume of 2.25/2.50 inch precipitable water air (which is about three standard deviations above the mean), which could support hourly rainfall rates between 2.00/3.00 inches. Some of the regional/global models suggest that local 4.00/5.00 inch rainfall amounts are possible, and given the depth of moisture expected to be in place. these values seems plausible. Three hour flash flood guidance values are generally above 3.50 inches across much of south FL. The exception here is southwest FL, where values are as low as 2.50 inches. These numbers could be modulated by heavy rainfall during Day 2, and parts of the peninsula have seen 300+ percent of normal rainfall over the last seven days. Based on this, a Marginal Risk was placed over the southern third of the FL Peninsula for Day 3. Hayes Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.t... Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.t... Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.t... $$ ------------=_1564562309-118394-2633 Content-Type: text/plain; charset="US-ASCII" Content-Disposition: inline Content-Transfer-Encoding: 7BIT = = = To unsubscribe from WX-STORM and you already have a login, go to https://lists.illinois.edu and use the "Unsubscribe" link. Otherwise email Chris Novy at cnovy@cox.net and ask to be removed from WX-STORM. ------------=_1564562309-118394-2633-- --- SBBSecho 3.07-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) |
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