Quantitative Precipitation Forecast
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513 FOUS30 KWBC 272138 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 538 PM EDT Mon May 27 2024 Day 1 Valid 2135Z Tue May 28 2024 - 12Z Tue May 28 2024 ...A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PORTIONS OF THE NORTHERN AND CENTRAL MID-ATLANTIC STATES... 21z Update: Main change was to add a Marginal risk to portions of central TX up into far southern OK. Isolated convection is developing this afternoon along the dryline and an east/west oriented stationary front. Enough instability is present to support a localized flash flood risk with this activity. Late tonight into Tuesday morning we may see a more organized convective cluster develop near the Red River, which could also pose some flash flood risk. Chenard 16Z Update... Expanded the Slight Risk farther north into New York where area VWP have been showing southerly 850 mb flow on the order of 35 to 45 knots. This was a bit higher than shown by earlier model runs...which has resulted in higher precipitable water values getting drawn northward than earlier forecast. In addition...the 12Z HREF probability values were pretty bullish for 1 inch per hour accumulations near the border of northeast PA with NY state. Saw little reason to make too many changes elsewhere. Bann ...0830Z Excessive Rainfall Discussion... ...Mid Atlantic... Surface low currently analyzed over Lake Michigan will continue to move northeast into Ontario/Quebec with a cold front progressing eastward through the Ohio Valley along its tail end. Pre-frontal trough axis will also be located just ahead of the cold front with a ridge of higher theta-E`s advecting poleward along the confines of the pre-frontal trough. The tongue of elevated theta-E`s will promote an area of generally favorable instability with higher PWATs anomalies (2-2.5 deviations above normal) co-located within the axis of favorable instability. The combination of thermodynamic favor and increasing large scale ascent with the incoming trough will help initiate a round of convection during the midday time frame across the Mid Atlantic with locally heavy rainfall likely within any cells that develop. 00z HREF probability fields depict a heightened threat for 1-2"/hr rates across portions of central MD up through much of eastern PA to the Lower Hudson Valley. Totals generally 1-2" will be common within the hardest hit areas, but neighborhood probabilities for at least 3" are relatively high across parts of northeast MD up through the Poconos with values upwards of 50-60% encompassing parts of northeast PA. Considering the lower FFG indices in place over the urban corridor down by Baltimore to Philadelphia, and for areas across northeast PA where the relative forecast maximum is expected, the SLGT risk inherited was maintained with a minor southern expansion based on the latest probabilities and QPF forecast within the zone encompassed. Flash flood prospects will drop off precipitously with the passing of the cold front expected between 00-06z which will end any further threat and push area precip off to the northeast. A secondary maximum is also popping up across hi-res for the area around the VA Tidewater where a combination of strong mid-level ascent from a vortmax swinging northeast out of NC will interact in tandem with the approach of the surface trough to the west. A line of heavy thunderstorms is increasingly likely late-afternoon to early evening time frame with totals forecast between 2-4" within a span of 2-3 hrs. The prospects are fairly consistent across most guidance leading into the threat, so the localized maximum has merit. Despite relatively higher FFGs for the area, the urban corridor down by Hampton Roads and VA Beach will probably have the greatest threat for localized flooding. The southern end of the MRGL remained over the area for the threat. ...Southeast... Locally heavy rainfall is possible over the Southeast through the period due to the remnants of a complex ejecting out of the Tennessee Valley initially, followed by cold front convective initiation over the southern half of the MRGL risk area. Complex out of the Tennessee Valley will continue to push southeast into the Deep South with relative QPF maximums positioned across northern GA through east-central AL. As the cold front progresses into the region in wake of the complex remnants, more thunderstorms will fire from the central Gulf coast up through southern GA into southern SC before the front clears the region. Considering the enhanced moisture and instability component in place over the above areas, strong thunderstorms capable of 2-3"/hr rain rates could cause some issues within urbanized areas in those general locales. The areas in question have been hit recently in the past 1-2 weeks with waves of heavy rainfall, so the FFG`s are not as high as usual, but they are high enough to limit the threat to more localized standards with emphasis on population centers. With the convective nature of the precip highlighting the mode for rainfall, the threat was just high enough within the probability fields to necessitate the continuation of the previous MRGL risk. ...Northern Michigan... Intense deformation axis on the northwest side of a maturing surface cyclone moving across the Great Lakes will induce a small window for heavy rainfall across portions of the northern MI "Mitten" up into the adjacent UP. Rates will be relatively mild considering the lower end convective threat, although totals of 1.5-3" in a shorter period of time could very well cause some issues locally. Considering the previous MRGL inheritance and forecasted totals, didn`t feel there was a need to remove completely, thus maintained continuity. Kleebauer Day 2 Valid 12Z Tue May 28 2024 - 12Z Wed May 29 2024 ...A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PORTIONS OF TEXAS AND SOUTHERN OKLAHOMA... 20Z Update... Main focus for excessive rainfall shifts from the eastern U.S. on Day 1 back to the Southern Plains on Day 2 as air with higher Thetae gets drawn back northward during the day as low level flow backs and strengthen ahead of an approaching mid-level shortwave. Model QPF has been increasing but placement has not varied much over time. Except for a minor adjustment to the southern boundary of the area based on latest guidance. There is also a low-end risk of flooding in parts of the Northwest given potential for 0.5+ inch per hour convective rates late Tuesday and Tuesday evening in complex terrain driven/focused by the passage of amid- and upper- level system with a cold front at the surface. Bann ...0830Z Excessive Rainfall Discussion.., ...Southern Plains... Extensive surface ridging to the north will aid in the advection of rich Gulf air to the northwest across much of TX into southern OK with the axis of higher theta-E`s really denoting the wedge of elevated instability over the southern plains. A weak mid-level ridge will develop across west TX and parts of the southwest US which will assist in the expected steering pattern of convection across the region. Shortwave propagating across the northern periphery of the ridge will help initiate convection across portions of northwest TX into southern OK with a secondary wave of development over west TX thanks to anticipated diurnal destabilization to the east of a dryline positioned across the Caprock down through the Texas Big Bend. Ample instability in place given by the superfluous MUCAPE forecast over across the aforementioned areas will lead to an enhanced convective environment within a moisture rich environment thanks to the poleward advancement of a Gulf- centric airmass. In fact, recent ensemble guidance is in agreement on a broad area of 2-3 deviations above normal axis of PWATs situated over the Concho Valley down through central TX which will be important in the forecast when it comes to the expected heavy rain threat. Convective initiation will blossom and eventually congeal with areal cold pool mergers moving southward due to the previously mentioned storm relative steering pattern dictated by the mid-level ridge to the west. Area thunderstorms are forecast to ride along a tight theta-E gradient bisecting northern and central TX with expected forward propagation to the south and southeast through the TX Hill Country, eventually into the I-35 corridor between Waco to San Antonio. QPF maximums of over 4" are becoming more common within the latest guidance with the ensemble bias corrected QPF driving upwards, closer to 5" in spots with the Hill country to the northeast of the Edwards Plateau. Considering the environment in place, rainfall rates exceeding 2-3"/hr is very likely within some of the stronger cells with mature mesocyclones. A general 2-4" QPF swath is in place over southern OK down through central TX with an areal extent of at least 1" going back close to the dryline positioning in west TX, and as far east as portions of the ArklaTex. Considering the magnitude of rainfall in place, the threat for flash flooding, including some considerable flash threats a SLGT risk was maintained from previous forecast with a higher-end wording for flash flooding occurring over northern and central TX with a relative bullseye across Hill Country. There is still some discrepancy on the exact placement of the heaviest precip axis with the global deterministic keying on different areas. This leads to a slight pause on any further upgrades, however, the next run of CAMs at 12z will be out through the full time frame of interest with the accompanying hi-res ensemble suite adding in references of probability and mean QPF fields. This will enhance the area of focus more, likely leading to an upgrade of a MDT in the next succession of updates across parts of TX, perhaps as far north as the Red River. ...Northwest... Surface low generation with a trailing cold front will be positioned over the Pacific Northwest come Tuesday with attendant height falls occurring to the west thanks to an incoming, strong shortwave trough moving into the region. Increased upper level ascent focused within a broad axis of diffluence on the lead side of the mean trough will take aim over the interior Pacific Northwest with an area of convection forming in-of the northern ID mountains, likely along due to weak instability ahead of the surface cold front and ample large scale ascent over the terrain. Scattered thunderstorms capable of rainfall rates bordering 0.5-1"/hr will allow for localized flooding concerns within the complex terrain over central ID up through the western portions of MT. Total precip forecast is relatively modest with guidance settling between 0.75-1.5" over the aforementioned area, but that is just enough to cause some low-end flood concerns within the rugged terrain and adjacent valleys north of the Snake River. A MRGL risk was added to the above areas to account for the low-end threat with the evolving dynamical setup. Kleebauer Day 3 Valid 12Z Wed May 29 2024 - 12Z Thu May 30 2024 ...A MARGINAL RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PORTIONS OF THE SOUTHERN AND CENTRAL PLAINS, LOWER MISSISSIPPI VALLEY, MONTANA, AND NORTHWEST NORTH DAKOTA... 20Z Update... Synoptic scale becomes less favorable for excessive rainfall in both the Southern Plains and the Pacific Northwest although the risk is not expected to completely end on Day 3. See details below about expected evolution. Ban 0830Z Excessive Rainfall Discussion... ...Southern and Central Plains through the Lower Mississippi... Mid-level ridge axis across the Southern Plains will slide eastward over the course of Wednesday, but more convection will spawn upstream under the broad expansion of relatively prominent axis of theta-E`s that will advect as far northwest as the Front Range of the Rockies on the western flank of the surface ridge encompassing much of the eastern CONUS. More shortwaves will round the northern periphery of the mid-level ridge, helping to spawn another round of convection Wednesday afternoon and evening, moving south to southeast with the mean steering flow and expected cold pool propagation. This round of convection is not currently forecasted to be as robust due to lower bounds of instability compared to the previous D2 offering. Still, the threat for more flash flooding within zones expected to see impact during the previous period, as well as an extension of the flash flood threat further north and east has lead to a broad MRGL risk area extending through portions of the southern and central plains, all the way into the Lower Mississippi Valley as the steering pattern places areas downstream into a better chance for organized convection moving overhead. Ensemble guidance favors heavier precip further to the east over the ArklaTex and portions of south-central TX within the eastern flank of Hill Country through the I-35 corridor. Considering the threat being conditional on what happens the day prior, and QPF magnitudes less than appealing for a higher end risk (1-2" totals), decided against any upgrade, but will mention the likelihood of at least a SLGT is very high as we move closer and have better definition on where the heaviest precip axis will lie. For now, the inherited MRGL risk was maintained with some minor modifications along the edges of the risk area. ...Montana and North Dakota... Diffluent pattern across the Northwest will allow for a continued threat of convection within the favored ascent pattern focused within a formidable jet coupling situated over northern MT back to WA state. The enhanced upper level dynamics in conjunction with sufficient surface forcing will promote a period of scattered strong to even potentially severe thunderstorms to propagate northeast out of western MT and southeast ID through central and eastern MT with the northeast expansion through northwest ND by the end of the forecast period. QPF forecasts are sufficient for flash flood concerns within the complex terrain of central MT, especially with the environment capable for rainfall rates breaching 1"/hr at times within the stronger cell cores. The previous MRGL risk was maintained, but would not be surprised for a targeted SLGT risk introduction in later updates as we move closer to the event time and gain more confidence in the relative magnitude and placement of the convective heavy rain potential. Kleebauer 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