Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Dec 17 2023 00:59:54 FOUS30 KWBC 170059 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 759 PM EST Sat Dec 16 2023 Day 1 Valid 01Z Sun Dec 17 2023 - 12Z Sun Dec 17 2023 ...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS MUCH OF FLORIDA... 01z update: No changes made to the 16z-based ERO, based on the latest observational trends as well as latest HREF/HRRR runs. Instability remains a limiting factor for more robust rainfall rates across FL (MUCAPEs have nudged between 500-1000 J/Kg across S-SE FL). Overnight the 18Z HREF probs of >1"/hr rainfall rates peak between 60-80% along NE FL, however are rather transitory. Meanwhile, notable probabilities (20% or greater) of at least 2"/hr rates remain offshore. Hurley Previous discussion below.. 16z update: Small cosmetic changes were made to the risk areas with this update, with some northeast expansion given a tad faster progression of the axis of training showers/weak thunderstorms along and left of the strengthening surface low crossing the Big Bend of Florida later this evening into very early Monday morning. Guidance remains on track for the slightly weaker but longer duration training showers with generally 3-6" toward the Okefenokee Swamps of SE GA by 17.12z, while strengthening southeasterly fetch band off the northeast Bahamas starts to flux into SE GA increasing Hi-Res CAM totals (both in the end of the D1, as well as, start of D2 period). Further south in the warm sector, the potential for stronger thunderstorms, potentially rotating supercells, will provide stronger isallobaric wind responses and increased moisture flux into the cells allowing for the potential for higher intensity rain-rates from late evening through the early overnight period.=20 Hi-Res CAMS including HRRR 15 minute totals suggest rain-rates of 2"+/hr (15-30 minute totals of 1.5-1.75"). However, forward motion appears to be quick enough reduce duration and overall totals reducing the potential for larger areal coverage flooding to higher focused localized typically urban flooding. If training/repeating occurs the potential for much higher localized totals and potential for rapid inundation flooding would be expected; however, this is high uncertain though a model or two do suggest this potential after 00z. The greatest potential for this to unfold is generally across the southern and eastern Peninsula (including the Keys), where steering flow gradient is sharpest and the linear features can flatten to the flow increasing the overall training duration/potential. Gallina ---Prior Discussion--- Surface low pressure is expected to develop over the Gulf of Mexico today and deepen as it approaches the Southeast U.S. today with low level winds accelerating across the Florida peninsula by this evening and across portions of the Southeast U.S. later tonight. The resulting moisture transport boosts precipitable water values to 1.75 inches or greater across most of Florida east of the Big Bend region...with values exceeding 2 inches across the southern part of the peninsula. With anomalously high precipitable water values...some 2 to 3 standardized anomalies above climatology across the peninsula for this time of year...the environment should support some downpours as shown by the isolated 20 percent probabilities for 2 inches of rain in 1 hour that develops late this evening in the 16/00Z HREF runs. With the exception of the model guidance continuing to favor the area around the Big Bend region with 24-hour rainfall amounts in excess of 3 inches...model guidance was still struggling with the details about the surface low...its track...and the resulting implications for where the heaviest rainfall will fall. As a result...maintained the broad Slight risk without too many changes. Bann Day 2 Valid 12Z Sun Dec 17 2023 - 12Z Mon Dec 18 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PARTS OF THE EASTERN U.S. FROM THE CAROLINAS TO THE NORTHERN MID-ATLANTIC REGION... 20z update: ....Carolinas into southern Mid-Atlantic... A full suite of Hi-Res CAM solutions (through day 2 period 17.12-18.12z), did not alleviate the uncertainty with respect to the longitudinal axis of heavy rainfall expected across the Carolinas into the Mid-Atlantic. Stronger, slightly faster solutions anchored by the NAM/NAM-Conest favor the secondary northern shortwave into pivot into the deepening trof further north and west. This draws higher moisture from the increasingly distant warm conveyor belt back west into the foothills of the Appalachians and western Piedmont regions of the Carolinas and dries out the Coastal Plains. In fact, this westward shift in this camp (incl typically favorable HRRR and ARW2 solutions) combined with a slight eastward shift in the remaining guidance (ECMWF/GFS/UKMET/ARW/FV3CAM) results in the main axis of heavy rainfall to be within the dry slot of the former, making placement highly uncertain even at this short of a time period. So it will be key to track the placement of the shortwave and axis of steepening mid-level lapse rates intersecting the western branch of the warm conveyor belt's TROWAL (generally along the western edge of the dry slot developing across GA into the Carolinas). The eastward shifting guidance being closer to the coastal source of the enhanced moisture and heating is slightly more efficient as well increasing in rainfall totals across the coastal Plains of E South Carolina, eastern NC into VA and maybe S MD/DC before instability and displacement has increased too much from the warm conveyor belt late in the day 2 forecast period (Tuesday morning). As such, rain-rates of 1.5-2"/hr are possible early across the Carolinas but diminish into the .75-1"/hr range after 18.03z across VA into MD. Rates may perk back up later over night as the core of the shortwave rotates over the top of the Mid-Atlantic with steeper lapse rates, but cells will be much more scattered/isolated aggravating any ongoing flooding issues from the prolonged rain from the TROWAL axis earlier in the morning.=20 An increase in coastal totals including some suggestions of 5-7" provide some concern toward increasing risk category level; however, higher soil absorption along the Coastal Plain may alleviate some of this increased burden, but the increase in spread/uncertainty and placement contingency remains sufficient to keep a higher end Slight Risk of Excessive Rainfall in place for the time being. Even though they are less preferred, the western camp of guidance does intersect with more complex terrain and lower FFG in the foothills of the Appalachians resulting in a slight expansion of the Marginal and Slight Risks at this time in the possibility that trend continues. It is expected that a tightening of the risk areas will occur as observational trends clarify the clear outlier OR newer guidance converges to a common solution. ....Northeast/Northern Mid-Atlantic... Prolonged moderate showers as the TROWAL veers westward ahead of the deepening mid-level cyclone will strengthen throughout the day peaking Tuesday morning just before the end of the forecast period. As mentioned above, increasing distance from the core of the higher theta-E air and reduced instability relative to the core of the digging upper-low to the south should limit rates. However, favorable orographic enhancement along terrain of E PA/N NJ as well as S NY very late, will allow for 2-3" totals by 18.12z, mainly in that favored southeast exposed enhancement. So little adjustments were required at this time with only small westward shift in the Marginal Risk area based on uncertainty. ....Northwest California... A slight faster trend was noted in the 12z guidance slightly increasing moisture flux/IVT transport into terrain mainly north of the mouth of the San Francisco Bay. The orientation of the flow continues to be very oblique to the overall terrain with winds backing more southeast to east-southeast reducing orographic ascent generally more parallel to the coast. Still some 400-500 kg/m/s IVT can be expected so some minor nuisance flooding is possible and is covered by a low-end Marginal Risk in the day 2 period with minor adjustments onshore and north. Gallina ~~~Prior Discussion~~~ ....Eastern U.S... A very dynamic system will continue to ramp up over portions of the Southeast U.S. on Sunday...spreading widespread rainfall northeast along the eastern seaboard throughout much of the day. With precipitable water values increasing to 1.5 inches along the coast from the Carolinas into the Mid-Atlantic region and on-shore winds at 850 mb on the order of 60 to 75 knots developing in a tightening gradient north of the surface low...potentially heavy to excessive rainfall is expected to be embedded within the broader shield of rainfall. The spread within the model guidance has closed somewhat...although the 16/00Z NAM was given less consideration based on how it races the surface low northward so quickly and takes it on a more westerly trajectory. While it probably should not be entirely ruled out...more weight was given to the GFS/ECMWF solutions due to the support they had from the ensembles. A broad area of 2.5 to 4+ inches is expected for the eastern Carolinas...tapering to 1.5 to 3 inches in parts of the Mid-Atlantic region and southern New York. Expanded the Slight Risk area a bit westward in South Carolina where the HREF showed 3-hour QPF that exceeded 3 hour Flash Flood guidance and where there was a greater than 15 percent probability of 2 inches of rain in an hour on Sunday afternoon. Farther north...nudged the Slight Risk a bit farther to the north and east. Models show limited instability despite precipitable water values exceeding an inch along the coast from the Mid-Atlantic towards Southern New England late...but some minor urban and small stream flooding can not be ruled out. Otherwise...the changes to the risk areas were changed little from the previous ERO. ....California... Maintained a Marginal Risk area for excessive rainfall...although shifted a bit northward in concert with the model QPF showing a continued northward trend. Bann Day 3 Valid 12Z Mon Dec 18 2023 - 12Z Tue Dec 19 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE NORTHEAST USA... 20z Update: ....Northeast... For all the increased uncertainty for day 2, the overall moisture, rainfall (snowfall) further west reduces with increased distance from the environmental parameters required for excessive rainfall. However, the strength of low level flow from the Western Atlantic combined with deep layer moisture are well off the scale in terms of statistical anomalies (7-9 standard anomaly compared to CFSR), with 1250+ kg/m/s IVT values. Combined with nearly orthogonal intersection of the northern Appalachians will produce heavy rainfall and atypical hour-rain rates but still at or just below ..5"/hr. Eastward propagation of the warm conveyor belt will be fairly quick (by 19.03z for main core) with diminishing moisture values as the low's connection to the sub-tropical source region severs limiting overall rainfall totals. Overall, 3-5" of rain is expected near the ridge line with 1-2" closer to the coasts of ME. As such, will maintain the broad Slight Risk across the White Mtns into much of the lower 2/3rds of Maine. A small westward expansion of the Marginal was to account for any lingering/ongoing issues near the lower Hudson Valley as the upper-level crosses the area early in the day 3 forecast period. ....Northern California... Small cosmetic adjustments were made for slight changes in rain shadowing, snow levels in the Sierra Nevada and northern Central Valley. An small expansion to coastal ranges north and south of Monterrey Bay, where slightly faster (tad east) shift of the eastern side of the closed low/warm conveyor increased rainfall totals. Gallina=20 ~~~Prior Discussion~~~ A broad shield of rainfall associated with a deep area of low pressure will continue to spread north and eastward during the period. Maintained a Slight Risk from the previous Day 4 outlook over portions of New England where combination of deep layer moisture transport and strong low level jet combine forces with the upper support of a 90 kt to 120 kt upper level jet that takes on an increasing S shape should continue to support briefly heavy rainfall rates and areas of rainfall amounts in excess of 1.5 inches. Despite precipitable water values that climb above an inch over New England....model guidance shows little instability to help enhance rates which suggests more of minor urban and small stream flooding concerns although heavy rainfall in the past week over parts of the area may result in hydrologic concerns on other tributaries or in regions of poor drainage. ....California... Rainfall becomes better organized and makes its way onshore across northern California a persistent ridge begins to break down.=20 Model guidance has greater than 50 percent chance of some northern California basins receiving more than 1.5 inches on Monday.=20 Trimmed a bit of the Marginal risk area over the Sierras where areas above 7000 ft or so should be snow...and confined the risk area where precipitable water values were low level winds transport in an airmass with precipitable water values upwards of 0.75 inches.=20 Bann Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7B0DkbDd_FZroQVMmYs9sYjH8ewsUaH1wv2wpoCOH6eI= 7AJCnHlmAVc8aKlV1GKbN5pBrBcQHvmGkTopnxjKBDH2gA0$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7B0DkbDd_FZroQVMmYs9sYjH8ewsUaH1wv2wpoCOH6eI= 7AJCnHlmAVc8aKlV1GKbN5pBrBcQHvmGkTopnxjKPbZJ5yE$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7B0DkbDd_FZroQVMmYs9sYjH8ewsUaH1wv2wpoCOH6eI= 7AJCnHlmAVc8aKlV1GKbN5pBrBcQHvmGkTopnxjKAXkfAzQ$=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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