Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Feb 14 2025 08:08:41 FOUS30 KWBC 140808 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 308 AM EST Fri Feb 14 2025 Day 1 Valid 12Z Fri Feb 14 2025 - 12Z Sat Feb 15 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS=20 OF THE MID-SOUTH... Models continue to show ingredients coming together late in the period to support a developing heavy rain that will evolve into a much larger threat on Day 2. Deepening moisture coincident with a southwesterly 50 kt low level jet, interacting with mid-level energy, is expected to support showers and storms developing=20 across the region Saturday morning. There is some signal in the hi-res models showing storms beginning to train, raising an increasing=20 threat for heavy accumulations near the end of the period. While=20 widespread heavy amounts are not expected prior to 12Z, this=20 continued signal in some of the guidance for training storms with locally heavy amounts beginning to develop, warranted leaving the=20 Marginal Risk in place across portions of the region. However, the=20 areal extent of the risk was reduced some from the previous,=20 confining the area mostly to where the 00Z HREF showed the highest=20 neighborhood probabilities for accumulations of an inch or more.=20 Pereira Day 2 Valid 12Z Sat Feb 15 2025 - 12Z Sun Feb 16 2025 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF=20 THE TENNESSEE AND OHIO VALLEYS AND THE CENTRAL APPALACHIANS... ....Ohio and Tennessee Valleys... Confidence remains high in significant heavy rainfall and flash=20 flood event across the region on Saturday and Saturday Night. Models=20 continue to depict a swath of 3-6 inches of rainfall in the 24-hour=20 period, although there continues to be some variability on the exact=20 placement. Therefore, the maximum risk level was held at a Moderate=20 Risk for this outlook, and the biggest change was to shift the=20 overall footprint slightly further to the northwest (by 25-50 mi)=20 based on trends in the 00 UTC model guidance. The Moderate Risk area=20 was also trimmed a bit in the Central Appalachians where model QPF=20 has been trending lower and the region will be removed from the area=20 of greatest instability (and thus likely to see lower rain rates.) Based on cluster analysis, a primary source of uncertainty seems to=20 be related to the position of the surface low. This makes sense as a=20 more northward track of the low would allow for a more significant=20 poleward push of the warm front and overall warm sector. Although=20 the 00 UTC NAM, NAM Nest, and GFS have trended the swath of rainfall=20 further northwest, this is far from unanimous. In fact, AI-based=20 versions of the ECMWF and GFS have been fairly consistent in their=20 depiction of the overall scenario and are among the furthest south=20 with the position of the surface low. Therefore, although the=20 overall risk areas were trended slightly to the northwest out of=20 deference to the overall ensemble envelope, the changes were=20 incremental at this point and still include a reasonable chance of=20 some of the southern scenarios unfolding. An emerging point of consensus among many models, though, appears to=20 be a focus of the heaviest rainfall generally between 85W and 90W=20 longitude, or likely somewhere in W/C KY or NW TN. This is an area=20 that would be favored to receive both the early round of training=20 and backbuilding convection along the developing warm front in the=20 morning and early afternoon, as well as more vigorous convection as=20 the surface low tracks nearby with increasing surface-based=20 instability. This would potentially maximize both the peak intensity=20 of rain rates as well as the overall duration of heavy rain. It also=20 happens to coincide with the most significant above-normal precip=20 departures over the past 30 days, with precip in that period along=20 an axis from DYR-LEX generally 1.5 to 2.0 times the normal amount.=20 Therefore, this is an area that will continue to be monitored for=20 any potential risk upgrades to a High Risk if it becomes apparent=20 that the duration of heavier rain rates will last for at least=20 several hours. More transient bursts of heavy rain would be less=20 likely to lead to severe impacts, and it is the uncertainty of both=20 the placement of the heavy rain and the extent to which it will be=20 concentrated in a sustained period that preclude a risk upgrade at=20 the present moment. The environment will be quite unusual for mid-February. The=20 combination of MUCAPE of 500-1000 j/kg and PWATs reaching around 1.5=20 inches has only rarely been observed in KY/TN at this time of year,=20 and would be supportive of 1-2 inch per hour rain rates in the most=20 organized and intense convection. A powerful southwesterly LLJ over=20 60 knots, oriented at an acute angle to the developing warm front,=20 will favor backbuilding into the region of stronger instability. And=20 the overall mean flow should favor some periods of training=20 convective clusters or bands near the warm front and surface low=20 track. All of these ingredients are fairly classic for impactful=20 flash flooding cases, and significant impacts are possible in this=20 one as well. Lamers Day 3 Valid 12Z Sun Feb 16 2025 - 12Z Mon Feb 17 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL IN PORTIONS OF THE=20 MID-ATLANTIC... ....Mid-Atlantic... A Marginal Risk was maintained across portions of the Mid-Atlantic=20 region on Sunday, primarily due to the potential for briefly heavy=20 rain rates with convection along an advancing cold front, and fairly=20 low flash flood guidance values. However, the speed of the cold=20 front should reduce the duration of any heavy rain, and thus any=20 flash flood threat should remain rather isolated. Lamers Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_KOS5jIHWJ1iDdm1eCLAZLomanfhb6JfeYCm2He0duGH= XbN_ju3Zvlk3kSmr3UFPJEFx__fQPE29ueTZ3hbIfwlHiG0$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_KOS5jIHWJ1iDdm1eCLAZLomanfhb6JfeYCm2He0duGH= XbN_ju3Zvlk3kSmr3UFPJEFx__fQPE29ueTZ3hbIH3UufdQ$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_KOS5jIHWJ1iDdm1eCLAZLomanfhb6JfeYCm2He0duGH= XbN_ju3Zvlk3kSmr3UFPJEFx__fQPE29ueTZ3hbIRPkYSh0$=20 =3D =3D =3D 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. --- SBBSecho 3.20-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .