215 FOUS30 KWBC 230058 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 856 PM EDT Thu Oct 22 2020 Day 1 Valid 01Z Fri Oct 23 2020 - 12Z Fri Oct 23 2020 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE UPPER MIDWEST AND UPPER GREAT LAKES... ...Northeast Iowa to Wisconsin and Michigan... Only modest changes were made to the excessive rainfall outlook with some local adjustments to the Slight Risk area to account for the latest radar trends and HRRR guidance. The threat of heavy showers and thunderstorms continues across areas of northeast IA and extending northeast across central WI and into northern lower MI for the overnight period. The combination of approaching trough and increasingly-curved upper jet (both along-stream and cross-stream components) will continue to result in a dynamic as well as thermodynamic environment conducive for heavy rainfall across portions of the Upper Midwest and Upper Great Lakes, with a focus from northeast IA across south-central to eastern WI, and into northern lower MI, where areal average rainfall totals of 2-3 inches are forecast going through early Friday morning. MUCAPEs from the CAMs get as high as 500-1000 j/kg, while PWs peaking between 1.25-1.50" average around 2 standard deviations above normal for late October. Training will be possible, particularly by late this evening and overnight, as the strong southwesterly low-level inflow (50-65 kt at 850 mb) aligns nearly parallel while nearly of the same magnitude of the mean 850-300 mb layer wind. This will result in a weakening of Corfidi vectors through the evening, which the CAMs show turning northeasterly (under 10 kts) after 02-06Z from south-central to eastern WI, then into northwest Lower MI. The increased upwind propagation will allow for an enhanced threat for cell-training for at least a few hours late this evening into the overnight, and while the aforementioned thermodynamic profiles (deep-layer instability and PW) are modest at best, there remains a threat of some FFG exceedance given the repeating nature of the rainfall already seen earlier this afternoon and this evening. Orrison/Hurley Day 2 Valid 12Z Fri Oct 23 2020 - 12Z Sat Oct 24 2020 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS SOUTHEAST FLORIDA... ...Southeast Florida... ...20Z Update... Based on the 12Z model guidance, it appears most of the solutions are signally a suppression of the axis of heaviest rain/moisture convergence which will now reside across the Florida Keys. This will reduce the potential for heavy rain and isolated flash flooding across the mainland of southern FL. Therefore, the Marginal Risk was adjusted south. ...Previous Discussion... Deep moisture and instability focused on a low level east southeast flow is expected to support convection capable of heavy to locally excessive rainfall across a portion of southeast FL during Day 2. Tropical waves in the low to mid level easterly flow across south FL interact with deep moisture (with precipitable water values between 2.00/2.25 inches, which is between two and three standard deviations above the mean) and 1000/2000 J/KG of MUCAPE to spark convection in the low level convergent flow that crosses southeast FL. mainly from Miami south. While there is not a strong high resolution model consensus, given the depth of the moisture and instability, local rainfall amounts of 4.00+ inches are possible in this area (which is shown by the 00z NAM CONUS Nest). Should the tropical waves focus the convection, this amounts are certainly possible. However, if the focus is not enhanced by the tropical wave, the convective coverage could be considerably less. For the most part, the best convective threat in this flow regime is expected between 23/18z and 24/00z (during the best synoptic scale lift with short wave energy in the mid levels crossing south FL), and the timing is typical across FL. There is some indication from the WRF ARW runs that the threat could extend until 24/06z, and in this flow, that cannot be discounted. In any event, the greatest threat appears to be from Miami south, and this is shown in much of the high resolution/regional guidance. Portions of southeast FL (generally south of Miami) received 3.00/5.00 inches of rainfall Wed, and has generally been wet over the past 7/14 days (with rainfall amounts in that period over 300 percent of normal). Wet antecedent conditions could play a role in the flash flood process, and despite the high resolution guidance spread, a Marginal Risk was placed here for Day 2 (which was collaborated with WFO MFL). ...Lower Mississippi Valley into the Ohio Valley... ...20Z Update... After a detailed assessment of the 12Z guidance, also decided to not introduce a Marginal Risk area at this update given the progressive nature of convection moving along/ahead of a surface cold front. And while heavy rain may move over saturated soils, the flash flooding impacts will likely be very isolated. However, if the front slows and/or training occurs, then we will need to reevaluate the risk potential. ...Previous Discussion. Moisture and instability ahead of a cold front crossing the Ohio Valley into the Lower MS Valley will fuel storms that could produce heavy rainfall during Day 2. For the most part, given the mid level flow, the front is expected to remain progressive, and the convection should follow suite. In fact, simulated radar from virtually all of the high resolution model solutions showed cell motions greater than 10 knots. It is possible that 1.00 inch rainfall rainfall could occur due to the 1.25 inch precipitable water air in the column, but the storm motions should preclude much higher values. Some of the high resolution guidance shows local 1.50 inch rainfall amounts extending from northeast AR into northern KY/far southern IN, mainly in the 23/18z to 24/00z time frame. Normally this would not be a concern, but portions of far southern IN/far northern KY did receive between 3.00/4.00 inches of rainfall during the last event, and the wet antecedent conditions could make this area more vulnerable to even a short period heavy rainfall. Based on the progressive nature of the convection, and after collaborating with WFO LMK, it was decided to forego the Marginal Risk option this forecast cycle. However, if there is a greater model signal for heavy rainfall here, a Marginal Risk could be needed in subsequent forecasts. Hayes/Pagano Day 3 Valid 12Z Sat Oct 24 2020 - 12Z Sun Oct 25 2020 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS SOUTHEAST FLORIDA... ...20Z Update... Multi-model agreement on the potential for another resurgence of heavy rain into portions of southern FL, especially along the southeast coast with the better surface/frictional convergence and onshore advection of moisture/instability into the region. Some model guidance is quite robust with QPF totals, but given some uncertainty with respect to the QPF coverage/amounts, decided to forgo an upgrade to a Slight Risk. However, if trends continue, given the wet antecedent conditions, this may need to be considered at subsequent forecast updates. ...Previous Discussion... Much like Day 2, deep moisture and instability in a low to mid level southeast flow could supply convection capable of producing heavy to locally excessive rainfall across portions of southeast FL during Day 3. Within the more typical convection during the best instability (between 24/18z and 25/00z), scattered storms could produce hourly rainfall rates in excess of 2.00 inches, especially from Miami south. These areas saw heavy rainfall Wed, and have been much wetter than normal during the past 7/14 days. Though the convection may not be focused enough during the abovementioned time frame to pose a more bonafide flash flood threat, the 00z non-NCEP guidance (particularly the UKMET/ECMWF) have become more robust with the next wave in the easterly mid level flow. Both models suggest that the next influx of deep moisture with the wave could occur at the tail end of Day 3 (or possibly early Day 4), but timing of the wave is still in flux. Should it arrive just a bit earlier than these solutions indicate, a larger area of convection capable of producing hourly rainfall rates of 2.00 inches or greater could arrive on the southeast FL coast between 25/06z and 25/12z. With these areas having been heavy rainfall recently, a Marginal Risk was placed across southeast FL for Day 3 (mainly for areas south of the Mia metro area), which was collaborated with WFO MFL. Hayes/Pagano 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 $$
