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Quantitative Precipitation Forecast
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515 FOUS30 KWBC 121953 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 353 PM EDT Fri Jul 12 2024 Day 1 Valid 16Z Fri Jul 12 2024 - 12Z Sat Jul 13 2024 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE MID-ATLANTIC... ...16Z Update... Some minor adjustments were made to the respective SLGT risk across the Mid Atlantic and the MRGL risk over Missouri as there has been little variability within the recent guidance compared to overnight. There was no change to the MRGL risk across the Upper TX coast, including the Houston metro into the Piney Woods area to the north. Heavy rain axis is positioned over the Delmarva down into central and southern VA through portions of NC. Diurnal destabilization factors will contribute to an invigoration of convection across Eastern NC up through VA this afternoon with locally heavy rainfall rates of 1-2"/hr increasingly likely after cell initiation. Deep moisture presence within the profile is evident when assessing some of the regional 12z RAOB soundings, as well as hi-res forecast soundings within the CAMs suite. The presence of the stalled front will maintain a focus for convective development and training potential as the flow begins aligning parallel to the boundary leading to repeated impacts across the above areas. A high end SLGT risk is most prominent within NC up through the I-64 corridor with the secondary area of focus across the northern Delmarva into Southeastern PA with guidance suggesting a more organized convective cluster traversing the I-95 corridor from NoVA up the I-95 corridor and points east during the overnight period. This is likely due to a more consolidated vorticity maxima that will originate from convection over Southern VA and propagate northeast within the mid-level southwesterly flow. There is still a non-zero chance for a targeted upgrade within the Northern Delmarva given the signal, however the current forward speed of any organized convection is less favorable for a more significant impact, although if the convective cluster exits into the Philadelphia metro area, that would be more conducive for higher impacts given the urbanization factors that would be most susceptible for flash flooding. Will be monitoring closely. MCV over Southern MO will pivot east, aligned within the persistent westerly flow based over the Mid-Mississippi Valley. Modest instability within the focal area of ascent will likely yield some scattered cell clusters capable of heavy rates exceeding 1"/hr, locally as high as 2"/hr in the strongest cells. East and Southeast MO is a bit more susceptible for impacts given the still moist soils after the impacts from Beryl`s remnants. This creates a more conducive environment for localized flooding and within the lower end of the MRGL threshold necessary for the current risk area. The MRGL was maintained due to those factors, but adjusted on the western flank due to the end of the convective threat, or at least the flash flood prospects with the focal point of the MCV shifting eastward. Isolated bouts of heavy rainfall will impact the Upper TX coast through the Piney Woods area leading to a low-end MRGL risk for flash flooding in a spot that is still recovering from Beryl. Grounds will remain very wet and the combo of elevated top soil moisture and urbanization factors will allow for a spotty flash flood threat through the afternoon, ending around sunset due to the loss of diurnal heating. Kleebauer ...Previous Discussion... ...Mid-Atlantic... No major changes were made with this update, but certainly the active pattern typical of July continues. A strong front featuring an impressive moisture gradient is set up along the eastern slopes of the Appalachians this morning. Meanwhile, a shortwave in the upper levels will lift up the Ohio Valley and into the Northeast today through tonight. On the warm side of the surface front, PWATs are near their maximum for this time of year, with amounts above 2.25 inches over southeast Virginia and the Carolinas. Instability is somewhat subdued over land, but values quickly spike just off the coast to over 3,000 J/kg, with a strong southeasterly wind helping advect it northwestward. Rates with the stronger cells this morning are routinely exceeding 1 inch per hour. As the band moves northwestward, it will eventually run into the front near the I-95 corridor, resulting in a slowing and eventual stalling of the band. With the upper level shortwave shifting northeastward, expect multiple rounds of storms training northeastward along the front today into tonight. The Chesapeake and Delaware Bay breezes may help to focus that convection by locally adding forcing, with HREF probabilities over the far northern Delmarva of 30% for 8 inches of rain in 24 hours as well as a 30% chance of reaching 100 year ARIs in the 40 km neighborhood probabilities. For the ERO, the area remains in a high-end Slight, but should those 8 inch probabilities realize, expect flash flooding. The guidance suggests overnight tonight is the best time for the heaviest rain for northeastern MD, northern DE, far southeastern PA, and southwest NJ where the highest threat remains. It`s possible a targeted MDT may be needed for this region, as it`s the one part of MD not in a drought as noted in the latest drought monitor. The DC-Baltimore I-95 corridor`s forecast rainfall totals have also increased with a small northwestward adjustment to the axis of heaviest rainfall in the forecast, so urban and small stream concerns have also increased if the band over eastern MD stalls there later today into tonight. There remains a high likelihood of a sharp gradient of rainfall totals on the north and western side of the front due to the rapid drop off of atmospheric moisture on the dry side of the front. Thus, much of western MD, eastern WV, and western VA will see very little if any rainfall from this event. For the Carolinas, many areas can expect 2 separate rounds of rain as the front pushes west today, then retreats back to the east overnight. This should overall reduce the flooding threat, though some areas along the Pamlico and Albemarle Sounds have seen some heavy rain in recent days which may locally increase the flash flooding threat. ...Southern Missouri... A Marginal Risk area was introduced with this update for continued convection through the morning associated with the current convection over southeastern Kansas. The storms are very slow- moving, but some of the guidance does speed it up through the morning with increasing westerly flow. Much of southeastern Missouri was recently hard hit with Beryl`s remnants, and therefore have lower FFGs and therefore somewhat more favorable antecedent conditions. ...Southeast Texas... Also due to the remnants of Beryl, the potential for typical daily afternoon convection over this hard hit area may cause isolated flash flooding in the possible event any stronger cells remain nearly stationary. A small Marginal Risk was introduced for this reason. Wegman Day 2 Valid 12Z Sat Jul 13 2024 - 12Z Sun Jul 14 2024 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE NORTHERN DELMARVA, CENTRAL AND SOUTHERN NEW JERSEY, ALONG WITH THE PHILADELPHIA METRO AREA... ...20Z Update Summary... SLGT risk across Southern New England was removed with a new SLGT risk in effect across much of Central and Southern NJ down through the Northern Delmarva, including the Philadelphia metro area. A MRGl risk remains in place from Southern New England down into the Eastern Carolinas. Previous MRGL risk over the Upper Midwest was expanded to the Southeast to include more of WI, including the local areas around Green Bay to Milwaukee. There were little to no changes necessary for the MRGL risk across the Southwest U.S, and the Upper TX coast into the Piney Woods area. ...Mid Atlantic... The previous SLGT risk across Southern New England was removed, but a new SLGT risk is in effect across the Northern Delmarva, Central and Southern NJ, along with the Philadelphia metro area. Recent trends within most, if not all major deterministic, including the CAMs have shifted the axis of heaviest rainfall across the Central Mid Atlantic with the northern extent of the frontal boundary now expected to be further east with the best surface convergence signature located over the eastern half of Long Island to just off the Southern New England coast. Further south, a consolidated mid- level vorticity maxima is expected to eject northeast during the end of the D1 period, moving overhead of the Philadelphia metro area and surrounds by tomorrow morning. Surface front is forecast to be aligned along and just to the east of the I-95 corridor from Baltimore up through NYC leading to a focused surface convergence axis within the zone above. Forecast soundings from multiple CAMs show a well aligned 925-700mb steering flow that would lead to the boundary layer moisture field running parallel to the surface front in place. Textbook signature of the parallel flow with increased mid and upper forcing will enhance a regional QPF maxima in-of the aforementioned area before being displaced to the northeast by the early to mid-afternoon time frame. The best signatures within the HREF probability fields have shifted focus from once over Southern New England to further south within the Central Mid Atlantic as of the latest 12z suite. Neighborhood probabilities for at least 2" are upwards of 50-80% within the Northern Delmarva up to north-central NJ with a bullseye for at least 3" located over the Northern Delmarva to about the Philadelphia metro area. EAS probability fields also show a better defined corridor for heavier precip focus within the above area with 30-40% signatures showing up for at least 1" of rainfall in-of the targeted locations. This is coincident with a better than MRGL risk forecast, especially considering the timing of the precip falling mainly within a 6 hr window between 12-18z Saturday. With coordination from the local Philadelphia WFO, have upgraded much of Central and Southern NJ down into Northeast MD and Northern DE within a SLGT risk of excessive rainfall. Marginal Risks ...Upper Midwest... Shortwave trough moving into the Northern Midwest will provide sufficient large scale forcing within an axis of marginally favorable moisture and instability to create a multi-round segment of thunderstorms across the northern half of MN down through WI through much of Saturday into Sunday morning. Progressive nature of the storms will limit significant flash flood potential, although a few cells may backbuild across WI during the second period of convection as a nocturnal MCS is forecast to develop late Saturday evening and moving southeast from western Lake Superior to near or overhead of the Milwaukee metro area. A large extent of >50% neighborhood probabilities for at least 1" and 2" can be found from the western portion of the Arrowhead down into Southeast WI, correlating well with a reasonably high EAS probability (30-50%) for at least 1" as of the 12z HREF. This is sufficient for a MRGL risk area with a broader scope of impact, especially with some deterministic output being quite bullish for the prospects of an MCS developing upwind of Lake Michigan with chance at embedded 1-1.5"/hr rainfall rates in its path. Considering the enhanced QPF footprint off the ensemble means and First Guess fields being focused further southeast with the MRGL risk coverage, have opted to expand the previous risk area to match the latest trends. ...Southeast Texas... Another afternoon of diurnally driven convection in-of the Houston metro and points to the north and east where saturated soils and urbanization characteristics will entice another low-end threat for flash flooding in the heavier cell cores. The threat is on the lower end of the risk threshold, but a overachieving cells cannot be ruled out considering a favorable thermodynamic environment and wet antecedent conditions. The previous MRGL risk was maintained. ...Southwest... Another round of Monsoonal activity is expected across much of AZ into Northwest NM. The primary concerns are for locally heavy rain to impact remnant burn scars, slot canyons, and smaller urban corridors within the terrain. Best threat will lie north of I-10 around the Mogollon Rim into the Sangre de Cristos in NM. Totals generally less than 1" with a few stronger cores capable of 1-2" if persistent. Kleebauer Day 3 Valid 12Z Sun Jul 14 2024 - 12Z Mon Jul 15 2024 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE GREAT LAKES AS WELL AS FOR ARIZONA AND PORTIONS OF ADJACENT UTAH, NEW MEXICO, AND COLORADO... ...20Z Update... Only very minor adjustments were made to the previous forecast MRGL risk areas across the Upper Midwest, Great Lakes, and Southwest U.S for the period. Latest ensemble output was very much in line with the previous forecast with only some minor fluctuations in the position of relevant QPF maxima considering the less organized nature of the convective potential within each area. Best areas of interest for an upgrade in future updates would be over the Great Lakes if future guidance indicates a more organized complex of thunderstorms that could be capable of a more prolific, local QPF footprint, along with the portions of AZ and Southern UT considering an increasing Monsoonal signature over heavily prone regions of slot canyons and complex terrain. Continuity was preferred given the above, but check back for updates in upcoming forecasts for those potential upgrades. Kleebauer ...Previous Discussion... ...Great Lakes... A series of weak shortwave troughs will move across the Great Lakes and southwesterly flow of Gulf moisture advects into the region. Expect widely scattered showers to impact the area, resulting in isolated flash flooding. ...Four Corners... Increasing moisture with the monsoon will advect northward into the Mogollon Rim of Arizona. Meanwhile storms that develop over the mountains of Mexico may advect northwestward across the border near Nogales, where there`s good agreement on over an inch of new rainfall. While coordination with the associated offices resulted in agreement to remain a Marginal, it`s likely a Slight Risk will be needed for portions of Arizona with future updates for Sunday afternoon. Wegman 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