Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Mar 24 2024 00:56:11 FOUS30 KWBC 240055 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 855 PM EDT Sat Mar 23 2024 Day 1 Valid 01Z Sun Mar 24 2024 - 12Z Sun Mar 24 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF EASTERN MASSACHUSETTS... Heavy rainfall was ongoing at 00Z across eastern Massachusetts into coastal New Hampshire and Rhode Island in association with a strengthening surface low located near TAN in southeastern Massachusetts. Observed rainfall rates since 23Z have peaked in=20 the 1.0 to 1.5 in/hr range between PVD and BOS despite little to no instability. The lack of instability has been made up for by=20 strong dynamics with low level convergence (70-80 kt low level jet) and frontogenetic forcing beneath the right entrance region of a 150+ kt upper jet over northern New England.=20 The 12Z hires models have had a relatively good handle on precipitation amounts but have been a bit slow to translate the heavy rain axis across the northeastern U.S. Even recent runs of=20 the HRRR have been about an hour slower when compared to the=20 observed radar.=20 The strong axis of low level moisture transport will soon shift east of Massachusetts and focus into Maine where temperatures have been at or below freezing supporting snow or freezing rain. Strong low level warm advection is expected to push surface temperatures above freezing from the Mid-Coast to Downeast Maine through 06Z as the surface low tracks very near the eastern tip of Maine. Hourly=20 rainfall along the immediate coast of Maine is expected to reach into the 0.5 to 1.0 inch range for a brief time, shifting fairly=20 progressively toward the east overnight, limiting additional=20 rainfall to 1-2 inches for the region. Beyond ~06Z, the axis of=20 heavy rain should have exited or nearly exited Maine into New=20 Brunswick and Nova Scotia. Localized flooding/flash flooding may=20 result across these regions of New England prior to the exiting of=20 precipitation. ....South Florida... The Marginal Risk was removed for the 01Z update as deep moisture and associated heavy rainfall has exited off to the east. Otto Day 2=20 Valid 12Z Sun Mar 24 2024 - 12Z Mon Mar 25 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTHERN CALIFORNIA... ....20Z Update... Little change in the overall forecast for Sunday as increased onshore flow across southern CA will allow for a regional maximum in precip across the Peninsular Range down towards the San Diego metro area. Guidance is fairly steady within the 1-1.5" range across the interior with HREF EAS probabilities over 80% for at least 1"/24-hrs in a small zone in-of of the Peninsular Range. Lower probabilities are located within the San Diego metro itself, but still non-zero around 10-20%. Primary concern will be terrain influences within the zone of heaviest rainfall, as well as poor drainage areas within the risk area that could be more prone to flooding. The MRGL risk was maintained. Kleebauer ....Previous Discussion... The vertically stacked low currently off the Washington/Oregon coast will have opened up into a vigorous positively tilted trough at the start of the period Sunday morning. The trough will track down the coast through Sunday evening as a northerly 150 kt jet streak approaches. The left exit region of the jet will align over southern California, while onshore west to west-northwesterly winds pump Pacific moisture into the Peninsular Ranges. Forecast rainfall Sunday afternoon and evening has been gradually increasing, with favored upslope west-facing slopes now expected to pick up around an inch of rain for the day. Thus, the Marginal remains in place with increased confidence for isolated flash flooding in the flash flood prone foothills of the Peninsular Ranges as well as poor- drainage locations in the San Diego metro. Wegman Day 3 Valid 12Z Mon Mar 25 2024 - 12Z Tue Mar 26 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE LOWER MISSISSIPPI VALLEY... ....20Z Update... The Slight Risk was maintained for the Lower Mississippi Valley for Monday as the overall synoptic pattern supports heavy rainfall thanks to improving dynamics and increased buoyancy ahead of a sharp cold front advancing east out of the plains. No major changes were made to the overall risk alignment, but a few places were added to the threat areas based off the latest ensemble trends and ML based guidance. The MRGL risk was expanded to the southwest to include the eastern section of the hill country to the east of I-35 as the latest ensemble trends show the tail end of the cold front tilting back to the southwest across south-central TX which would allow for trailing thunderstorms to propagate over the more flood-prone areas just to the east of the I-35 corridor. This also matches the GFS and ECMWF Graphcast ML models with the trailing convection idea that could spur trouble for localized flooding in that particular zone. The other area of interest was the expansion of the Slight Risk to the south over northern LA where deep-moist convergence along the advancing cold front into a well-defined instability axis will project a threat of heavy rainfall as ML and deterministic guidance paints upwards of 2" or more of precip within the corridor between I-10/20. Considering the stout meridional pattern expected over the central US, the trends for a slower moving convective scheme is favored which would allow for a greater threat of training. This is also agreed upon by the latest ML total precip depiction, as well as the ensemble mean forecast from the NBM and associated global ensembles. Kleebauer ....Previous Discussion... A positively-tilted upper level longwave trough over the West will eject into the southern Plains Monday, becoming more neutrally tilted with time. Vigorous shortwaves will lead the reorientation of the longwave trough, with the northernmost one energizing a mature low and winter storm across the northern Plains. That low's trailing cold front will provide the forcing for convection across the Marginal and Slight Risk areas. Due to a second shortwave in the subtropical jet moving the base of the trough eastward, the trailing cold front will tap into increasing Gulf moisture in the lower levels, leading to the rapid intensification of a low level jet up the lower Mississippi Valley. Moisture-rich showers and thunderstorms will develop ahead of the cold front, resulting in a wide swath of 2+ inch forecast rainfall totals in the Slight Risk area from south-central Missouri across most of Arkansas and much of the northern two-thirds of Mississippi. Since these rainfall amounts will be associated with thunderstorms, it's likely they will fall in a short-time frame, which in turn will increase the flash flooding risk. The Slight Risk was nudged a bit towards the northwest and southeast to cover more of AR and MS, while trimmed across northern LA following small changes in the latest guidance from inherited. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6rGm9cvXupKDkktYB4bK_VZXdD5k0c1dsrZU9GmQujOB= fjMkVv8j-1b_U4lTX6JLXzfTpCrA4ACFGYXkkC9TzZT8Lhc$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6rGm9cvXupKDkktYB4bK_VZXdD5k0c1dsrZU9GmQujOB= fjMkVv8j-1b_U4lTX6JLXzfTpCrA4ACFGYXkkC9TQQuwo5c$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6rGm9cvXupKDkktYB4bK_VZXdD5k0c1dsrZU9GmQujOB= fjMkVv8j-1b_U4lTX6JLXzfTpCrA4ACFGYXkkC9TzGfhWnM$=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. 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