265 FOUS30 KWBC 241544 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1144 AM EDT Tue May 24 2022 Day 1 Valid 16Z Tue May 24 2022 - 12Z Wed May 25 2022 ..THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE PLAINS, LOWER MISSISSIPPI VALLEY, AND SOUTHEAST... 16Z Update... Hi-res guidance struggles this morning with picking up on the convection that fired off through areas like southern Texas among other places. Our Marginal Risk area provided coverage for this region this morning and with lingering showers this morning followed by a brief break then more at the tail end of this forecast period, have opted to leave the Marginal risk area to cover southern Texas for this update. The Slight Risk area was shrunk a bit on the northern extent through Nebraska, but remains for much of Oklahoma into Arkansas, northeast Texas into central Texas and portions of Louisiana. Hi-res shows periods of 2-3"/hr rates with HREF probs sitting near 70% for 3-hr QPF >2" and 90% for >1". Greatest instability sits through this region this afternoon before dying off soon after sunset. Minor updates were made for our eastern risk areas through the Carolinas based on the latest guidance with the main difference being a more isolated pocket of heavier QPF through eastern North Carolina into Virginia. Chiari Previous Discussion... ...Middle Mississippi Valley through East Texas... Complex forecast scenario expected to evolve with multiple areas of moderate to heavy rainfall expected, both with the upper low and with its attendant surface low and fronts. Some heavy rainfall that has been occurring overnight which could linger into the early morning hours across Deep South TX, which led to their inclusion into the Marginal Risk as a precaution. Otherwise, the main upper trough begins to move east into the Front Range Tuesday morning with an associated area of surface low pressure organizing in the Plains. Ahead of the system, southeasterly to southerly flow will draw up anomalously high moisture on the order of 1.75" PWs, which approaches 2-3 standard deviations above normal. The main conditional factor will be the available instability because of the multiple areas of convection. Instability will generally be limited to areas of low level convergence north of the advancing warm front. MLCAPE will be highest from central Texas through the Arklatex with values ranging between 1500-2000 J/kg. Low-level inflow/effective bulk shear up to 50 kts will organize convection into line segments, with mesocyclones also expected which could temporarily hold up line segments. The 12Z HREF shows widespread moderate to high probabilities for 2-3" hourly totals and some signal for 3-5" totals. The continued model spread, uncertainties with available instability, and only pockets of higher than normal soil saturation lowered the confidence such that a Moderate Risk was considered but left out for now but still expecting isolated to scattered instances of flash flooding through the period. The Slight Risk area kept reasonable continuity. The eastern side of the Marginal Risk area was extended eastward to account for heavy rain signal in an area where heavy rain fell yesterday, which was coordinated with the TAE/Tallahassee FL forecast office. ...Carolinas/North GA... Low pressure will slowly track across NC during the period, interacting/reinforcing a cold air damming pattern over the Mid-Atlantic, and will work with overriding flow to produce a narrow axis of moderate to locally heavy rainfall, with some chance of repeat convection near and ahead of the front in the coastal plain/marshes/sounds. There remains some north/south spread in the model guidance but the consensus has been for the heaviest rain to fall from northeast NC to southeast VA, with the stronger signal across NC. The environment will remain conducive for heavy rainfall due to the anomalous PWs, convergent low level flow and interactions with the frontal boundary. The HREF probabilities are quite high for 3-5" totals and some potential for 2"+ hourly totals across eastern NC into far southeast VA, so maintained the Slight Risk but shifted it southward to where the best model signal lies. The signal was less clear in the western Carolinas, but sufficient to extend the Marginal Risk area back towards the southern Appalachians. Roth/Taylor Day 2 Valid 12Z Wed May 25 2022 - 12Z Thu May 26 2022 ...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL IN AND NEAR WESTERN TENNESSEE... Central Plains, Lower MS River Valley to Gulf Coast... A closed mid to upper level level low slowly tracks eastward through the period across the southern Plains. There is a growing signal in the guidance that the upper cold low itself could instigate convection as moisture returns via the poleward end of the warm conveyor belt circulation, and if sunny enough, develops under the upper center itself where storm motions would be slow. This led to a new Marginal Risk area across in and near eastern KS and southeast NE. Local amounts of 3-4" are advertised by some of the guidance in this area. Return flow drawn northward ahead of the feature will bring anomalously high PWs reaching 1.75-2.0", particularly along and ahead of an advancing frontal boundary. With MUCAPE values approaching 2500-3000 J/kg present and nearly 30-40 kts of 850 mb inflow oriented nearly parallel to storm motions, convection with the potential of backbuilding, training, and multiple rounds will be possible, increasing the possibility for higher totals both on an hour and multi-hour basis. Hourly rain totals up to 2.5" are expected with local 24-hour amounts of 3-7" likely. Some of this rainfall could fall on areas that have been a bit wetter recently, especially portions of eastern TX through AR. Due to recent heavy rains, a Moderate Risk area was added to western TN per coordination with the MEG/Memphis TN forecast office. Further north, the instability gradient will limit intense rain rates but could still lead to a prolonged moderate rain event into portions of southeast MO, southern IL, and southwest KY. Wisconsin/Michigan... The surface low track through the Upper Midwest during the period will bring periods of showers and thunderstorms, some of which will interact with building instability and increasing moisture characterized by CAPE values around 1000 J/kg and PWs of 1.5" or so. The storm motions are expected to be fairly progressive which should limit hourly rain totals 1-2" but some isolates instances of flash flooding will be possible. A Marginal was extended a little farther west for a second day as the guidance continues to slow down the upper trough progression some due to building ridging upstream. Roth/Taylor Day 3 Valid 12Z Thu May 26 2022 - 12Z Fri May 27 2022 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHERN APPALACHIANS... ...Ohio Valley and Southeast... A deep layer cyclone presses eastward with time. Heavy rainfall is possible under its cold but increasingly moist upper center, where a few pieces of guidance advertise local 3" amounts. Any convection that forms there during the afternoon and evening hours would be slow moving, so saw no reason to reject the potential seen in the wetter guidance in and near portions of MO, which caused a western extension of the Marginal Risk area from the Monday experimental day 4 ERO. Otherwise, the risk areas from continuity looked good, as the best signal for heavy rainfall within the warm conveyor belt circulation is in the vicinity of the southern Appalachians. Precipitable water values of 1.5-2" should inhabit the warm conveyor belt, with 850 hPa inflow of 30-45 kts and CAPE of ~2000 J/kg near the Gulf Coast and 1000+ J/kg as far north as the Ohio Valley. Heavy rain of up to 2.5" would be possible where short training bands form within the unidirectional south-southwest flow and wherever mesocyclones can form and hold up said bands. There is a growing signal for heavy rainfall (3"+) closer to the Gulf Coast across portions of the western FL Panhandle, where heavy rains fell Sunday night from the tropical disturbance, but for now, have left the risk as Marginal per coordination with the TAE/Tallahassee FL forecast office. Roth 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 $$